Tetrabutylammonium iodide (TBAI) is known to create an electric double layer at the air-water interface. In the present study, we investigated the charge-transfer-to-solvent reaction from iodide ions to liquid water near the gas-liquid interface of an aqueous TBAI solution using extreme UV (EUV) time-resolved photoelectron spectroscopy. Use of EUV radiation ensured accurate measurements of electron kinetic energy distributions by minimizing spectral distortions caused by electron inelastic scattering in the liquid and the influence of the electron transmission efficiency through the gas-liquid interface. The spectra observed for photodetached electrons exhibited a rapid energy shift and a variation of the bandwidth in subpicoseconds, and the rates of these changes clearly depended on the TBAI concentration. The results indicate that the dynamical response of solvent water changes in the presence of a high density of ions.

Previous transient absorption spectroscopy (TAS) hinted at an exceptionally rapid geminate recombination process in charge transfer reactions involving OH^- or OD^- ions in liquid water and CH₃O^- ions in liquid methanol. However, a comprehensive investigation of these dynamics using TAS has been hindered by the technical challenges stemming from the ultrafast spectral shift that spans a wide wavelength range from the mid-infrared to the visible on the subpicosecond time scale. To address these challenges, we have employed ultraviolet time-resolved photoelectron spectroscopy of aqueous solutions, enabling us to observe and analyze the complete dynamics, including electron detachment, solvation, and geminate recombination. Our findings are consistent with those of Iglev et al. (J. Phys. Chem. Lett. 2015, 6, 986−992), supporting the hypothesis that the structural diffusion of OH/OD/CH₃O induced by a presolvated electron plays a pivotal role in facilitating ultrafast geminate recombination.

We present photoelectron spectroscopy of liquid films generated in a vacuum with microfluidic devices based on liquid-liquid or gas-liquid collisions. The results are compared to those for a standard liquid microjet technique.

Time-resolved photoelectron spectroscopy (TRPES) enables real-time observation of ultrafast electronic dynamics in solutions. When extreme ultraviolet (EUV) probe pulses are employed, they can ionize solutes from all electronic states involved in the dynamics. However, EUV pulses also produce a strong ionization signal from a solvent that is typically 6 orders of magnitude greater than the pump–probe photoelectron signal of solutes. Alternatively, UV probe pulses enable highly sensitive and selective observation of photoexcited solutes because typical solvents such as water are transparent to UV radiation. An obstacle in such UV-TRPES measurements is spectral distortion caused by electron scattering and a yet to be identified mechanism in liquids. We have previously proposed the spectral retrieval (SR) method as an a posteriori approach to removing the distortion and overcoming this difficulty in UV-TRPES; however, its accuracy has not yet been verified by comparison with EUV-TRPES results. In the present study, we perform EUV-TRPES for charge transfer reactions in water, methanol, and ethanol, and verify SR analysis of UV-TRPES. We also estimate a previously undetermined energy-dependent intensity factor and expand the basis sets for SR analysis. The refined SR method is employed for reanalyzing the UV-TRPES data for the formation and relaxation dynamics of solvated electrons in various systems. The electron binding energy distributions for solvated electrons in liquid water, methanol, and ethanol are confirmed to be Gaussian centered at 3.78, 3.39, and 3.25 eV, respectively, in agreement with Nishitani et al. [Sci. Adv. 2019, 5, eaaw6896]. An effective energy gap between the conduction band and the vacuum level at the gas–liquid interface is estimated to be 0.2 eV for liquid water and 0.1 eV for methanol and ethanol.

We report ultrafast extreme ultraviolet photoelectron spectroscopy of 6-methyluracil (6mUra) and 5-fluorouracil (5FUra) in the gas phase and 6mUra and 5-fluorouridine in an aqueous environment. In the gas phase, internal conversion (IC) occurs from ¹ππ* to ¹nπ* states in tens of femtoseconds, followed by intersystem crossing to the ³ππ* state in several picoseconds. In an aqueous solution, 6mUra undergoes IC almost exclusively to the ground state (S₀) in about 100 fs, which is essentially the same process as that for unsubstituted uracil, but much faster than that for thymine (5-methyluracil). The different dynamics for C5 and C6 methylation suggest that IC from ¹ππ* to S₀ is facilitated by out-of-plane (OOP) motion of the C5 substituent. The slow IC for C5-substituted molecules in an aqueous environment is ascribed to the solvent reorganization that is required for this OOP motion to occur. The slow rate for 5FUrd may arise in part from an increased barrier height due to C5 fluorination.

Ultrafast electronic relaxation of nucleobases from ¹ππ* states to the ground state (S₀) is considered essential for the photostability of DNA. However, transient absorption spectroscopy (TAS) has indicated that some nucleobases in aqueous solutions create long-lived ¹nπ*/³ππ* dark states from the ¹ππ* states with a high quantum yield of 0.4–0.5. We investigated electronic relaxation in pyrimidine nucleobases in both aqueous solutions and the gas phase using extreme ultraviolet (EUV) time-resolved photoelectron spectroscopy. Femtosecond EUV probe pulses cause ionization from all electronic states involved in the relaxation process, providing a clear overview of the electronic dynamics. The ¹nπ* quantum yields for aqueous cytidine and uracil (Ura) derivatives were found to be considerably lower (<0.07) than previous estimates reported by TAS. On the other hand, aqueous thymine (Thy) and thymidine exhibited a longer ¹ππ* lifetime and a higher quantum yield (0.12–0.22) for the ¹nπ* state. A similar trend was found for isolated Thy and Ura in the gas phase: the ¹ππ* lifetimes are 39 and 17 fs and the quantum yield for ¹nπ* are 1.0 and 0.45 for Thy and Ura, respectively. The result indicates that single methylation to the C₅ position hinders the out-of-plane deformation that drives the system to the conical intersection region between ¹ππ* and S₀, providing a large impact on the photophysics/photochemistry of a pyrimidine nucleobase. The significant reduction of ¹nπ* yield in aqueous solution is ascribed to the destabilization of the ¹nπ* state induced by hydrogen bonding.

Charge transfer reactions from I^- to solvent water, methanol, and ethanol were studied using extreme ultraviolet time-resolved photoelectron spectroscopy (EUV-TRPES). This technique eliminates spectral broadening, previously seen in UV-TRPES, caused by electron inelastic scattering in liquids, and enables clear observation of the temporal evolution of the spectral shape. The peak position, width, and intensity of the electron binding energy distribution indicate electron detachment and subsequent solvation and thermalization processes. Geminate recombination between detached electrons and iodine atoms is discussed using a diffusion equation and a global fitting analysis based on a kinetics model.

We present condensed-phase first-principles molecular dynamics simulations to elucidate the presence of different electron trapping sites in liquid methanol and their roles in the formation, electronic transitions, and relaxation of solvated electrons (e_met⁻) in methanol. Excess electrons injected into liquid methanol are most likely trapped by methyl groups, but rapidly diffuse to more stable trapping sites with dangling OH bonds. After localization at the sites with one free OH bond (1OH trapping sites), reorientation of other methanol molecules increases the OH coordination number and the trap depth, and ultimately four OH bonds become coordinated with the excess electrons under thermal conditions. The simulation identified four distinct trapping states with different OH coordination numbers. The simulation results also revealed that electronic transitions of e_met⁻ are primarily due to charge transfer between electron trapping sites (cavities) formed by OH and methyl groups, and that these transitions differ from hydrogenic electronic transitions involving aqueous solvated electrons (e_aq⁻). Such charge transfer also explains the alkyl-chain-length dependence of the photoabsorption peak wavelength and the excited-state lifetime of solvated electrons in primary alcohols.

We present a study using extreme UV (EUV) photoemission spectroscopy of the valence electronic structures of aqueous and methanol solutions using a 10 kHz EUV light source based on high-order harmonic generation and a magnetic bottle time-of-flight electron spectrometer. Two aspects of the observed spectra are highlighted in this study. One is variation of the vertical ionization energy (VIE) for liquids as a function of the solute concentration, which is closely related to surface dipoles at the gas–liquid interface. The experimental results show that the VIE of liquid water increases slightly with increasing concentrations of NaCl and NaI and decreases with NaOH. The VIE of liquid methanol was also found to change slightly with NaI. On the other hand, tetrabutylammonium iodide (TBAI) and butylamine (BA) clearly reduce the VIE for liquid water, which is attributed to the formation of an electric double layer (EDL) by segregated solutes at the gas–liquid interface. As evidence for this, when the pH of an aqueous BA solution is reduced to protonate BA, the VIE shift gradually decreases because the protonated BA moves into the bulk to suppress the influence of the EDL. We computed the surface potentials for these solutions using molecular dynamics simulations, and the results supported our interpretation of the experimental results. Another observation is the variation of the relative energy and shape of individual photoelectron bands for solvents, which is related to alteration of the structure and constituents of the first solvation shell of ionized solvent molecules. All of the solutes cause changes in the photoelectron spectra at high concentration, one of the most prominent of which is the degree of splitting of the 3a₁ band for liquid water and the 7a' band for liquid methanol, which are sensitive to hydrogen bonding in the liquids. The 3a₁ splitting decreases with the increasing concentration of NaI, NaCl, and NaOH, indicating that Na⁺ penetrates into the hydrogen-bonding network to coordinate to a nonbonding electron of a water molecule. On the other hand, TBAI and BA cause smaller changes in the 3a₁ splitting. Full interpretation of these spectroscopic features awaits extensive quantum chemical calculations and is beyond the scope of this study. However, these results illustrate the strong potential of EUV laser photoemission spectroscopy of liquids for exploration of interfacial and solution chemistry.

We describe a magnetic bottle time-of-flight electron spectrometer designed for time-resolved photoemission spectroscopy of a liquid micro- jet using extreme UV and X-ray radiation. The spectrometer can be easily reconfigured depending on experimental requirements and the energy range of interest. To improve the energy resolution at high electron kinetic energy, a retarding potential can be applied either via a stack of electrodes or retarding mesh grids, and a flight-tube extension can be attached to increase the flight time. A gated electron detector was developed to reject intense parasitic signal from light scattered off the surface of the cylindrically shaped liquid microjet. This detector features a two-stage multiplication with a microchannel plate plus a fast-response scintillator followed by an image-intensified photon detec- tor. The performance of the spectrometer was tested at SPring-8 and SACLA, and time-resolved photoelectron spectra were measured for an ultrafast charge transfer to solvent reaction in an aqueous NaI solution with a 200 nm UV pump pulses from a table-top ultrafast laser and the 5.5 keV hard X-ray probe pulses from SACLA.

High-resolution photoelectron (PE) spectra of liquid methanol and ethanol were measured using a liquid microjet and He IIα radiation (40.813 eV). The vertical ionization energy and the ionization threshold were determined as 9.70 ± 0.07 and 8.69 ± 0.07 eV for methanol and 9.52 ± 0.07 and 8.52 ± 0.07 eV for ethanol, respectively. Individual photoemission bands observed for the liquids are well correlated with those in PE spectra of the gaseous samples also measured in the present study, except that the liquid band positions were shifted on average by −1.23 eV for methanol and −1.10 eV for ethanol as compared to the gas. The 5a' and 7a' bands of liquid methanol exhibit specifically larger broadening than other bands, for which we attempted spectral fitting with two components, similarly with the case of the 3a₁ band of liquid water. PE spectra of both liquid and gaseous ethanol are congested partly due to the presence of the trans and gauche isomers; however, the overall band positions are generally in good agreement with predictions based on quantum chemical calculations. Comparison of the measured PE spectra with experimental and simulated X-ray emission spectra indicate that spectral differences in the lowest ionization band of both methanol and ethanol originate from involvement of nuclear dynamics in the X-ray emission process.

The ultrafast formation, solvation, and geminate recombination of hydrated electrons upon vacuum ultraviolet photoexcitation of liquid water and the static and dynamic scavenging by NO₃^– are investigated using femtosecond time-resolved photoelectron spectroscopy. The solvation time constant for excess electrons is typical of that for liquid water but increases slightly with increasing excitation energy. The electron survival probability for geminate recombination is found to be much lower than the literature values owing to previously unobserved ultrafast geminate recombination in a period of 5 ps. NO₃^– induces the ultrafast (static) scavenging of photoexcited electronic states of liquid water and the dynamic scavenging of detached electrons with a reaction rate that is dependent on the excitation energy. The formation of hydrated electrons at 7.7 eV is ascribed to a H-atom-transfer process, but it is plausible that additional formation channels open at higher energies.

Photoelectron spectroscopy of a liquid microjet requires careful energy calibration against electrokinetic charging of the microjet. For minimizing the error from this calibration procedure, Kurahashi et al. previously suggested optimization of an electrolyte concentration in aqueous solutions [Kurahashi et al., J. Chem. Phys. 140, 174506 (2014)]. More recently, Olivieri et al. proposed an alternative method of applying a variable external voltage on the liquid microjet [Olivieri et al., Phys. Chem. Chem. Phys. 18, 29506 (2016)]. In this study, we examined these two methods of calibration using extreme ultraviolet photoelectron spectroscopy with a magnetic bottle time-of-flight photoelectron spectrometer. We confirmed that the latter method flattens the vacuum level potential around the microjet, similar to the former method, while we found that the applied voltage energy-shifts the entire spectrum. Thus, careful energy recalibration is indispensable after the application of an external voltage for accurate measurements. It is also pointed out that electric conductivity of liquid on the order of 1 mS/cm is required for stable application of an external voltage. Therefore, both methods need a similar concentration of an electrolyte. Using the calibration method proposed by Olivieri et al., Perry et al. have recently revised the vertical ionization energy of liquid water to be 11.67(15) eV [Perry et al., J. Phys. Chem. Lett. 11, 1789 (2020)], which is 0.4 eV higher than the previously estimated value. While the source of this discrepancy is still unclear, we estimate that their calibration method possibly leaves uncertainty on the order of 0.1 eV.

We present extreme ultraviolet (XUV) time-resolved photoelectron spectroscopy (TRPES) of an aqueous aniline solution. One-colour XUV-induced photoemission signal of aniline was observed with much greater intensity than expected from its molar fraction in the bulk solution, indicating that aniline is hydrophobically segregated on the liquid surface. The concentration dependence of the photoelectron intensity is found to be well correlated with the surface concentration of aniline estimated by surface tension measurements. Similar segregation was observed also for phenol in aqueous solution. The enhanced surface concentration of aniline makes its XUV-TRPES to be highly vulnerable to the pump-induced space charge effects (PISC). The PISC caused by a moderate pump intensity was not completely corrected using a simple mean field model, and reduction of the pump pulse intensity was necessary. The spectra measured at lower pump intensity were corrected for PISC, which enabled us to extract the information on the excited state dynamics of aniline in aqueous solution under 240 nm photoexcitation. Two components with lifetime on sub-ps and > 100 ps timescales were determined, and the former is ascribed to the solvation dynamics in the S₁ state after the ultrafast internal conversion from the S₃ state and the latter to the subsequent population decay of the S₁ state.

We report solvated electron (e⁻_solv) formation dynamics from the conduction band of liquid methanol studied using femtosecond time-resolved photoelectron spectroscopy. Liquid methanol is excited with vacuum UV (9.3 eV) pump pulses, and the subsequent electron dynamics are probed with UV pulses. The photoelectron signal exhibits a short-lived component (τ = 85 fs) without spectral evolution followed by a long-lived component with continuous spectral evolution over tens of picoseconds. We ascribe the former to a superexcited state, most likely the Wannier exciton, and the latter to the ground electronic state of e⁻_solv. In order to extract accurate energetics from the observed photoelectron spectra, we employ a spectral retrieval method to account for spectral broadening and shifting due to inelastic scattering of photoelectrons in the liquid. The electron binding energy (eBE) of the initial trap state of an electron is determined to be about 1.5 eV, and its biexponential increase up to 3.4 eV is observed with time constants of 2 and 31 ps, which are greater than 0.27 and 13 ps observed for e⁻_solv created by the charge-transfer-to-solvent reaction from CH₃O⁻ to liquid methanol. The solvation dynamics of e⁻_solv created by the electron trapping exhibit a pseudoisosbestic point at a pump-probe delay time of around 15 ps, and the peak energy of the eBE distribution rapidly changes around that time. These results indicate that there exist two trap states, both of which exhibit increasing eBE with time; however, the eBE of the shallow trap state increases only up to 2.1 eV, and transformation to a deep trap state at 25 ps occurs to reach an eBE of 3.4 eV.

The electronic energy and dynamics of solvated electrons, the simplest yet elusive chemical species, is of interest in chemistry, physics, and biology. Here, we present the electron binding energy distributions of solvated electrons in liquid water, methanol, and ethanol accurately measured using extreme ultraviolet (EUV) photoelectron spectroscopy of liquids with a single-order high harmonic. The distributions are Gaussian in all cases. Using the EUV and UV photoelectron spectra of solvated electrons, we succeeded in retrieving sharp electron kinetic energy distributions from the spectra broadened and energy shifted by inelastic scattering in liquids, overcoming an obstacle in ultrafast UV photoelectron spectroscopy of liquids. The method is demonstrated for the benchmark systems of charge transfer to solvent reaction and ultrafast internal conversion of hydrated electron from the first excited state.

Ultrafast internal conversion from the first excited state of a solvated electron in water, methanol, and ethanol is investigated using time-resolved photoelectron spectroscopy of liquid microjets and a spectral retrieval method. Photoelectron spectra corrected for inelastic scattering clearly reveal well-separated signals from the excited and ground states, and the latter enables us to analyze the solvation dynamics in the ground state after internal conversion. Measurements with 25 fs time resolution identify a rapid increase in the vertical electron binding energy of the solvated electron owing to nuclear wave packet motions in the excited state and allow us to precisely determine the internal conversion time.

We present ultrafast photoelectron spectroscopy of the charge-transfer-to-solvent reaction in a segregated TBAI (tetrabutylammonium iodide) molecular layer in aqueous solution. The reaction times and electron binding energies of transient species vary with TBAI concentration from a very low value of 1 × 10^-3 mol L^-1, which is in contrast to NaI solution exhibiting no concentration (0.01−1.0 mol L^-1) dependence. The result from soft X-ray N(1s) spectroscopy indicates that the photoelectron intensity in TBAI aqueous solution is about 70 times enhanced as compared to that in NH₄Cl aqueous solution for an identical salt concentration, and TBA⁺ drags I^- to the surface region. At high TBAI concentrations, electrons released from I^- are trapped and held in the TBAI molecular layer owing to electrostatic attraction by TBA⁺.

A liquid microjet discharged from a fused silica capillary is electrified, and the polarity of thiselectrification is opposite to that of the net charge at the solid-liquid interface of the capillary nozzle.

The unknown influence of inelastic and elastic scattering of slow electrons in water has made it difficult to clarify the role of the solvated electron in radiation chemistry and biology. We combine accurate scattering simulations with experimental photoemission spectroscopy of the hydrated electron in a liquid water microjet, with the aim of resolving ambiguities regarding the influence of electron scattering on binding energy spectra, photoelectron angular distributions, and probing depths. The scattering parameters used in the simulations are retrieved from independent photoemission experiments of water droplets. For the ground-state hydrated electron, we report genuine values devoid of scattering contributions for the vertical binding energy and the anisotropy parameter of 3.7 ± 0.1 eV and 0.6 ± 0.2, respectively. Our probing depths suggest that even vacuum ultraviolet probing is not particularly surface-selective. Our work demonstrates the importance of quantitative scattering simulations for a detailed analysis of key properties of the hydrated electron.

Angle-resolved photoemission spectroscopy of liquid water was performed using extreme ultraviolet radiation at 29.5 eV and a time-of-flight photoelectron spectrometer. SiC/Mg coated mirrors were employed to select the single-order 19th harmonic from laser high harmonics, which provided a constant photon flux for different laser polarizations. The instrument was tested by measuring photoemission anisotropy for rare gases and water molecules and applied to a microjet of an aqueous NaI solution. The solute concentration was adjusted to eliminate an electric field gradient around the microjet. The observed photoelectron spectra were analyzed considering contributions from liquid water, water vapor, and an isotropic background. The anisotropy parameters of the valence bands (1b₁, 3a₁, and 1b₂) of liquid water are considerably smaller than those of gaseous water, which is primarily attributed to electron scattering in liquid water.

The charge-transfer-to-solvent (CTTS) reactions from iodide (I^-) to H₂O, D₂O, methanol, and ethanol were studied by time-resolved photoelectron spectroscopy of liquid microjets using a magnetic bottle time-of-flight spectrometer with variable pass energy. Photoexcited iodide dissociates into a weak complex (a contact pair) of a solvated electron and an iodine atom in similar reaction times, 0.3 ps in H₂O and D₂O and 0.5 ps in methanol and ethanol, which are much shorter than their dielectric relaxation times. The results indicate that solvated electrons are formed with minimal solvent reorganization in the long-range solvent polarization field created for I^-. The photoelectron spectra for CTTS in H₂O and D₂O measured with higher accuracy than in our previous study [Chem. Sci. 2 1094 (2011)] indicate that internal conversion yields from the photoexcited I^-* (CTTS) state are less than 10%, while alcohols provide 2 - 3 times greater yields of internal conversion from I^-*. The overall geminate recombination yields are found to be in the order of H₂O > D₂O > methanol > ethanol, which is opposite to the order of the mutual diffusion rates of an iodine atom and a solvated electron. This result is consistent with transition state theory for an adiabatic outer-sphere electron transfer process, which predicts that the recombination reaction rate has a pre-exponential factor inversely proportional to a longitudinal solvent relaxation time.

We have measured the wavelength dependence (340 - 215 nm) of one-photon photoemission from the ground electronic state of solvated electrons in bulk water, methanol, and ethanol. In every case, the vertical electron binding energy (VBE) gradually increased with photon energy, indicating that the photoelectron kinetic energy diminishes as a result of electron-vibration inelastic scattering prior to emission from the liquid surface. In contrast, the VBE of the Rydberg electron in DABCO (1,4-diazabicyclo[2,2,2]octane), which has a surface-excess density, revealed no clear wavelength dependence. These results suggest that the solvated electrons are created predominantly in the bulk and that VBEs measured using UV photoemission spectroscopy of liquids generally require energy corrections to account for inelastic scattering effects. From the wavelength dependence, we have re-estimated the VBEs of solvated electrons in bulk water, methanol, and ethanol to be 3.3, 3.1, and 3.1 eV, respectively. Hydrated electrons were also identified by photoemission spectroscopy using 90 nm radiation.

We have studied ultrafast non-adiabatic dynamics of excess electrons trapped in the band gap of liquid water using time- and angle-resolved photoemission spectroscopy. Anisotropic photoemission from the first excited state was discovered, which enabled unambiguous identification of non-adiabatic transition to the ground state in 60 fs in H₂O and 100 fs in D₂O. The photoelectron kinetic energy distribution exhibited rapid spectral shift in ca. 20 fs, which is ascribed to librational response of a hydration shell to electronic excitation. Photoemission anisotropy indicates that the electron orbital in the excited state is depolarized in less than 40 fs.

The absolute values of the effective attenuation length of an electron in liquid water are determined using soft x-ray O1s photoemission spectroscopy of a liquid beam of water without employing any theoretical estimation or computationally obtained value. The effective attenuation length is greater than 1 nm in the entire electron kinetic energy region and exhibits very flat energy dependence in the 10-100 eV region.

We present time- and angle-resolved photoemission spectroscopy of trapped electrons near liquid surfaces. Photoemission from the ground state of a hydrated electron at 260 nm is found to be isotropic, while anisotropic photoemission is observed for the excited states of 1,4-diazabicyclo[2,2,2]octane and I^- in aqueous solutions. Our results indicate that surface and subsurface species create hydrated electrons in the bulk side. No signature of a surface-bound electron has been observed.

The streaming potentials of liquid beams of aqueous NaCl, NaBr, and NaI solutions are measured using soft X-ray, He(I), and laser multiphoton ionization photoelectron spectroscopy. Gaseous molecules are ionized in the vicinity of liquid beams and the photoelectron energy shifts are measured as a function of the distance between the ionization point and the liquid beam. The streaming potentials change their polarity with concentration of electrolytes, from which the singular points of concentration eliminating the streaming potentials are determined. The streaming currents measured in air also vanish at these concentrations. The electron binding energies of liquid water and I^-, Br^-, and Cl^- anions are revisited and determined more accurately than in previous studies.

Photoelectron spectra of solvated electrons in bulk liquids were obtained at energy-resolution of 60 meV using a linear time-of-flight photoelectron spectrometer and a 100 kHz ultraviolet femtosecond laser. Solvated electrons in H₂O, D₂O, methanol, and ethanol were generated by 226 nm excitation of the charge-transfer-to-solvent bands of I^- in 0.1 M NaI solutions, and the photoelectron spectra were measured using 260 nm pulses with a time delay of 2 ns. The electron binding energies and band shapes are discussed.

A charge-transfer-to-solvent (CTTS) reaction from a photoexcited iodine atomic anion, I^-(aq), in bulk water (H₂O and D₂O) was studied by time-resolved photoelectron spectroscopy using a liquid beam (microjet) of aqueous NaI solution. The ²P_3/2 CTTS state of I^-(aq) was excited by a 226 nm femtosecond laser pulse and the evolution of the nonstationary electronic state was probed using another ultraviolet femtosecond laser pulse. Global fitting of the observed time-dependent photoelectron kinetic energy distributions provided the time constants of individual reaction steps and the photoelectron spectra from the CTTS state, a contact pair, the solvent-separated state, and a hydrated electron. Most of the elementary reaction steps revealed a strong deuterium isotope effect, indicating coupling of the electron dynamics and the hydrogen atomic motion of solvent water. However, nondiffusive geminate recombination processes from the CTTS state and a contact pair were almost insensitive to deuteration. Consequently, geminate recombination processes from the CTTS state and a contact pair occurs more efficiently in D₂O, because the response of water is decelerated in D₂O. In contrast, the recombination process from the solvent-separated state in the final step of the CTTS reaction is less efficient in D₂O, presumably due to the smaller zero point energy.

High-resolution soft X-ray photoelectron spectra of liquid water (H₂O and D₂O) were measured using a liquid beam photoelectron spectrometer. The 1a₁ (O1s) band and the lowest valence 1b₁ band had single peaks, which is not consistent with the split 1b₁ → 1a₁ of the X-ray emission band of liquid water if the splitting is assumed to originate from level shifts in two different hydrogen bonding structures. The second valence 3a₁ band of liquid water exhibited a flat top implying that two bands exist underneath a broad feature, which is similar to the case of the 3a₁ band of amorphous ice. The energy splitting between the two 3a₁ bands is estimated to be 1.38 eV (H₂O) and 1.39 eV (D₂O). Ab initio calculations suggest that the large splitting of the 3a₁ band is characteristic of water molecules that function as both proton donor and acceptor. The overall result is consistent with the conventional model of a tetrahedral hydrogen-bonding network in liquid water.

Time-resolved photoelectron spectroscopy (TR-PES) of ultrafast dynamics in solution is presented. To measure the photoelectron kinetic energy distribution (PKED) that is free from inelastic scattering in solution, photoelectrons were generated with ultra-low kinetic energies (ULKE: <5 eV). Time constants of the elementary processes in the charge-transfer-to-solvent (CTTS) reaction from I^- to bulk water were in excellent agreement with those obtained by transient absorption spectroscopy, demonstrating the bulk-sensitivity of TR-PES-ULKE. The analysis suggests that the CTTS reaction proceeds via two intermediates, and that 30% of the first intermediate and 70% of the second intermediate respectively are quenched by geminate recombination between the electron and the neutral iodine atom.

Time-resolved photoelectron spectroscopy at ultralow kinetic energy was applied to liquid beams of NaI solutions in methanol and ethanol. Solvated electrons were formed from I- ions in these solutions by charge transfer to solvent reactions. The vertical electron binding energies of the solvated electrons were determined for the first time. Both of the binding energies were found to be 3.10 ± 0.1 eV. This is in reasonable agreement with the dielectric continuum model of solvated electrons using the previously measured vertical binding energy of a hydrated electron. This indicates that the cavity radii of solvated electrons in water, methanol, and ethanol are approximately the same (0.33 - 0.35 nm).

We present the first measurement of the vertical binding energy (VBE) of a hydrated electron in bulk water by the time-resolved photoelectron spectroscopy (TRPES) of the charge-transfer-to-solvent (CTTS) reaction in aqueous NaI solution. Our best estimate of VBE is 3.27 ± 0.10 eV for H₂O and 3.20 ± 0.10 eV for D₂O.

The effects of methyl substitution on the ultrafast internal conversion from the S₂(¹B_1u, ππ*) state to the S₀ state of benzene were studied using ultrafast extreme-ultraviolet photoelectron spectroscopy and electronic structure calculations. The quantum yield of the internal conversion to the S₀ state reached ∼0.69 in benzene, while lower values of 0.35 and 0.12 were obtained for toluene and o-xylene, respectively. These results indicate that methyl substitution makes the conical intersections less accessible to the nuclear wave packet.

The effect of deuterium isotopes on the internal conversion of ethylene is studied by using extreme ultraviolet time-resolved photoelectron spectroscopy. For deuterium-labeled ethylene, the time scale for ultrafast internal conversion is increased by a factor of approximately √2, in agreement with the results of ab initio multiple spawning calculations, indicating the essential role played by hydrogen motion in the conversion process. Following internal conversion, a metastable species with an electron binding energy of ∼9 eV is produced, and it decays with a time constant similar to that for both isotopologues.

Ultrafast internal conversion via a conical intersection is ubiquitous in highly efficient photochemical reactions. Internal conversion from the ¹ππ* to the ¹nπ* state of pyrazine is the paradigm for this phenomenon; however, the relaxation occurs in such a short time (<20 fs) that the nuclear motion is difficult to observe in real time. The present study precisely measures the vibrational coherence transferred from the ¹ππ* state to the ¹nπ* state using time-resolved photoelectron spectroscopy with an unprecedented time resolution of 13.3 fs and reveals the key nuclear motions that drive the internal conversion.

Ultrafast internal conversion of furan upon deep UV excitation at 200 nm is studied by using extreme ultraviolet time-resolved photoelectron spectroscopy with a time resolution of 15 fs. Ballistic nuclear wavepacket motion from the ¹B₂(π,π*) state to the ground state is fully observed using 21.7 eV probe pulses. Through the performance of a comparison with the results of electronic structure calculations at the MS(3)-CASPT2(10,10)/cc-pVTZ level of theory, the photoelectron signals from the conical intersection regions are identified.

cis-Stilbene (cis-St) is a well-known benchmark system for cis–trans photoisomerization. cis-St also produces 4a,4b-dihydrophenanthrene (DHP) in solution with a quantum yield of less than 0.19. The ring closure reaction, however, has never been identified for gaseous cis-St, and a recent computational simulation predicted the quantum yield of DHP to be only 0.04. In the present study, we identified an ultrafast ring closure reaction of gaseous cis-St for the first time using extreme ultraviolet time-resolved photoelectron spectroscopy. Surface hopping trajectory calculations at the SA3-XMS-CASPT2(2,2) level of theory reproduce the features of the observed time-resolved photoelectron spectra and predict the cis-St:DHP:trans-St branching ratio to be 0.55:0.41:0.04, in contrast with previous estimates. The results indicate that photoexcited cis-St favors ring closure over cis–trans isomerization under the isolated condition.

The photoinduced ring-opening reaction of 1,3-cyclohexadiene (CHD) to produce 1,3,5-hexatriene (HT) plays an essential role in the photobiological synthesis of vitamin D₃ in the skin. This reaction follows the Woodward−Hoffmann rule, and C₅−C₆ bond rupture via an electronically excited state occurs with conrotatory motion of the end CH₂ groups. However, it is noted that the photoexcited S₁(π,π*) state of CHD is not electronically correlated with the ground state of HT, and the reaction must proceed via nonadiabatic transitions. In the present study, we have clearly observed the nonadiabatic reaction pathway via the doubly excited state of CHD using ultrafast extreme UV photoelectron spectroscopy. The results indicate that the reaction occurs in only 68 fs and creates product vibrational coherence. Extensive computational simulations support the interpretation of experimental results and provide further insights into the electronic dynamics in this paradigmatic electrocyclic ring-opening reaction.

We studied nonadiabatic dissociation of CS₂ from the ¹B₂ (¹Σ_u⁺) state using ultrafast extreme ultraviolet photoelectron spectroscopy. A deep UV (200 nm) laser using the filamentation four-wave mixing method and an extreme UV (21.7 eV) laser using the high-order harmonic generation method were employed to achieve the pump−probe laser cross-correlation time of 48 fs. Spectra measured with a high signal-to-noise ratio revealed clear dynamical features of vibrational wave packet motion in the ¹B₂ state; its electronic decay to lower electronic state(s) within 630 fs; and dissociation into S(¹D₂), S(³P_J), and CS fragments within 300 fs. The results suggest that both singlet and triplet dissociation occur via intermediate electronic state(s) produced by electronic relaxation from the ¹B₂ (¹Σ_u⁺) state.

Previously, theoretical calculations on the non-adiabatic dynamics of benzene from the S₂ state have indicated that the S₂/S₁ and S₁/S₀ conical intersections (CIs) facilitate ballistic nuclear wavepacket motion from S₂ to S₀ (fast channel) and branching to S₁ (slow channel). In this paper, we present time-resolved photoelectron spectra of benzene and its methyl-derivatives (toluene and o-xylene) measured with a vacuum-UV laser, which clearly reveal both the fast and slow channels. The extremely short propagation time of the wavepacket between the two CIs of benzene indicates that the two are in close proximity to each other, while methyl substitution extends the propagation time and decreases the branching ratio into the fast channel. The results suggest that the quasi-degeneracy of the three states in benzene is lifted by the geometrical shifts of the CIs by methyl substitution.

Ultrafast electronic relaxation following 9.3 eV photoexcitation of gaseous acetone was investigated with time-resolved photoelectron imaging spectroscopy. An intense photoionization signal due to a transition from the 4¹A₁(π,π*) state to the D₁(π⁻¹) cationic state diminishes within 50 fs, owing to vibrational wave packet motion leaving our observation energy window. Additional photoionization signals were assigned to transitions from Rydberg states with principal quantum numbers of 3−8 to the D₀(n⁻¹) cationic state, created by strong vibronic couplings with the bright 4¹A₁(π,π*) state. The deactivation processes of the 4¹A₁(π,π*) and Rydberg states are discussed based on their decay profiles obtained from a time−energy map of photoelectron kinetic energy distributions.

Time-resolved photoelectron spectroscopy using vacuum-UV probe pulses enables observing ultrafast dynamics during and after passing through conical intersections (CIs). The ring-puckering CI plays a prominent role following the ππ* photoexcitation of furan. More than 90% of the excited molecules safely return to the original ground state, while the remaining 10% transforms into isomers after passing through the puckering CI.

The primary photochemical rezction of nitromethane (NM) after ππ* exicitation is known to be C-N bond cleavage (CH₃NO₂ + hν → CH₃ + NO₂). On the other hand, NO₂ can be formed in both the ground and exited states, and identification of the electronic state of the NO₂ product has been a central subject in the experimental and theoretical studies. Here we present time-resolved photoelectron spectroscopy using vacuum-ultraviolet probe pulses to observe all transient electronic states of NM and the reaction products. The result indicates that ultrafast internal conversion occurs down to S₁ and S₀ within 24 fs, and the dissociation proceeds on the S₁ surface (τ_diss ≲ 50 fs), leading to comparable product yields of NO₂(A) and NO₂(X). The overall dissociation quantum yield within our observation time window (< 2 ps) is estimated to be 0.29.

Ultrafast photodissociation dynamics from the ¹B₂(¹Σ_u⁺) state of CS₂ are studied by time-resolved photoelectron imaging using the fourth (4ω, 198 nm) and sixth (6ω, 133 nm) harmonics of a femtosecond Ti:sapphire laser. The ¹B₂ state of CS₂ was prepared with the 4ω pulses, and subsequent dynamics were probed using the 6ω vacuum ultraviolet (VUV) pulses. The VUV pulses enabled real-time detection of S(¹D₂) photofragments, produced via CS₂*(¹B₂(¹Σ_u⁺)) → CS(X ¹Σ⁺) + S(¹D₂). The photoionization signal of dissociating CS₂*(¹B₂(¹Σ_u⁺)) molecules starts to decrease at about 100 fs, while the S(¹D₂) fragments appear with a finite (ca. 400 fs) delay time after the pump pulse. Also discussed is the configuration interaction of the ¹B₂(¹Σ_u⁺) state based on relative photoionization cross-sections to different cationic states.

The dynamics of CO₂ excited into Rydberg states lying 0.2 eV below the ionization threshold is studied by means of time resolved photoelectron imaging. Over 3 eV broad photoelectron spectra are measured for all pump-probe delay times. Quantum mechanical calculations demonstrate that the spectral broadening is due to ultrafast electronic relaxation of Rydberg states and identify the likely relaxation pathways. Experiment and theory bracket the relaxation time between 15 and 65 fs. A weak time independent ionization signal is attributed to CO₂ trapped in near-threshold triplet states.

The ultrafast electronic dynamics of pyrazine (C₄N₂H₄) were studied by time-resolved photoelectron imaging (TRPEI) using the third (3ω, 4.7 eV) and fifth harmonics (5ω,7.8 eV) of a femtosecond Ti:sapphire laser (ω). Although the photoionization signals due to the 5ω - 3ω and 3ω - 5ω pulse sequences overlapped near the time origin, we have successfully extracted their individual TRPEI signals using least squares fitting of the observed electron kinetic energy distributions. When the 5ω pulses preceded the 3ω pulses, the 5ω pulses predominantly excited the S₄ (ππ^*,¹B_1u + ¹B_2u) state. Photoionization signal from the S₄ state generated by the time-delayed 3ω pulses was dominated by the D₃(²B_2g) ← S₄ photoionization process and exhibited a broad electron kinetic energy distribution, which rapidly downshifted in energy within 100 fs. Also observed were photoionization signals for the 3s, 3p_z, and 3p_y members of the Rydberg series converging to D₀(²A_g). The Rydberg signals appeared immediately within our instrumental time resolution of 27 fs, indicating that these states are directly photoexcited from the ground state or populated from S₄ within 27 fs. The 3s, 3p_z, and 3p_y states exhibited single exponential decay with lifetimes of 94 ± 2, 89 ± 2, and 58 ± 1 fs, respectively. With the reverse pulse sequence of 3ω - 5ω, the ultrafast internal conversion (IC) from S₂(ππ^*) to S₁(nπ^*) was observed. The decay associated spectrum of S₂ exhibited multiple photoionizations to D₀, D₁, and D₃, in agreement with the 3ω-pump and 6ω-probe experiment described in our preceding paper. The electron kinetic energy and angular distributions from S₁ populated by IC from S₂ are also discussed.

A photoexcited molecule undergoes multiple deactivation and reaction processes simultaneously or sequentially, which have been observed by combinations of various experimental methods. However, a single experimental method that enables complete observation of the photo-induced dynamics would be of great assistance for such studies. Here we report a full observation of cascaded electronic dephasing from S₂(ππ^*) in pyrazine (C₄N₂H₄) by time-resolved photoelectron imaging (TRPEI) using 9.3-eV vacuum ultraviolet (VUV) pulses with a sub-20 fs time duration. While we previously demonstrated a real-time observation of the ultrafast S₂(ππ^*)→S₁(nπ^*) internal conversion in pyrazine using TRPEI with UV pulses, this study presents complete observation of the dynamics including radiationless transitions from S₁ to S₀ (internal conversion) and T₁(nπ^*) (intersystem crossing). Also discussed are the role of ¹A_u(nπ^*) in the internal conversion and the configuration interaction of the S₂(ππ^*) electronic wave function.

Isomerization of tetramethylethylene (TME) following ultrafast internal conversion was investigated using time-resolved photoelectron spectroscopy with vacuum-ultraviolet probe pulses. The difference photoelectron spectrum at τ = 15 ps was reasonably well reproduced using a linear combination of static photoelectron spectra of TME and its isomers. The isomers were produced as a consequence of unimolecular reaction of vibrationally hot TME, created by internal conversion from the excited state.

Electron correlation can break the symmetry of photoelectron angular distribution upon two step ionization to a doubly degenerate ionic state via a doubly degenerate intermediate state. The interference between the two components of the intermediate state prepared using linearly and circularly polarized light is discussed for cyclopropane as an example.

The ultrafast deactivation process of ethylene in the ππ^*(V) state was studied using time-resolved photoelectron imaging with sub-20 fs pulses at 159 and 198 nm. The photoelectron kinetic energy distribution observed upon 159 nm photoexcitation exhibited a continuous downward shift within 20 fs, attributed to both C-C twist and pyramidalization motions. A partial revival of the vibrational wave packet was observed with the period of about 18 fs, which is attributed to the C-C twist from 0 to 180° on the ππ^*(V) potential energy surface. Signature for internal conversion from the ππ^*(V) state to a lower-lying π3s Rydberg (R) state, which has been previously suggested, was not detected in the time-dependent photoelectron kinetic energy and angular distributions.

The wavepacket dynamics of CS₂ after photoexcitation to the ¹B₂(¹Σ_u⁺) state at 198 nm are studied by time-resolved photoelectron imaging using sub-20 fs 159 nm pulses, which enable single photon ionization from the entire region of the ¹B₂ potential energy surface. The time-energy map of the photoelectron intensity reveals vibrational motions along the symmetric stretching and bending coordinates. The time-energy map of the photoelectron anisotropy parameter exhibits time-evolution within single oscillation periods of the ν₁ and ν₂ modes, which is attributed to variation of the excited state electronic character along these vibrational coordinates. The initially populated ¹B₂ state evolves with two time constants of 107 and 394 fs.

Ultrafast photoelectron imaging using a 90 nm vacuum-UV probe pulse is applied to the ring-opening reaction of 1,3-cyclohexadiene (CHD) in the gas phase, and formation of 1,3,5-hexatriene (HT) and CHD in their electronic ground states is observed in real time. The analysis of the transient photoelectron kinetic energy spectra reveals the branching ratio into HT and CHD as 3:7 upon 270 nm photoexcitation. The ratio is in reasonable agreement with the experimental values reported for the liquid phase and theoretical values for the gas phase, resolving the discrepancy.

The ultrafast dynamics of CS₂ in the ¹B₂(¹Σ_u⁺) state was studied by photoelectron imaging with a time resolution of 22 fs. The photoelectron signal intensity exhibited clear vibrational quantum beats due to wave packet motion. The signal intensity decayed with a lifetime of about 400 fs. This decay was preceded by a lag of around 30 fs, which was considered to correspond to the time for a vibrational wave packet to propagate from the Franck-Condon region to the region where predissociation occurred. The photoelectron angular distribution remained constant when the pump-probe delay time was varied. Consequently, variation of the electronic character caused by the vibrational wave packet motion was not identified within the accuracy of our measurements.

Ultrafast internal conversion of benzene and toluene from the S₂ states was studied by time-resolved photoelectron imaging with a time resolution of 22 fs. Time-energy maps of the photoelectron intensity and the angular anisotropy were generated from a series of photoelectron images. The photoelectron kinetic energy distribution exhibits a rapid energy shift and intensity revival, which indicates nuclear motion on the S₂ adiabatic surface, while the ultrafast evolution of the angular anisotropy revealed a change in the electronic character of the S₂ adiabatic surface. From their decay profiles of the total photoelectron intensity, the time constants of 48 ± 4 and 62 ± 4 fs were determined for the population decay from the S₂ states in benzene and toluene, respectively.

We performed He I ultraviolet photoelectron spectroscopy (UPS) of jet-cooled aromatic molecules using a newly developed photoelectron imaging (PEI) spectrometer. The PEI spectrometer can measure photoelectron spectra and photoelectron angular distributions at a considerably higher efficiency than a conventional spectrometer that uses a hemispherical energy analyzer. One technical problem with PEI is its relatively high susceptibility to background electrons generated by scattered He I radiation. To reduce this problem, we designed a new electrostatic lens that intercepts background photoelectrons emitted from the repeller plate toward the imaging detector. An energy resolution (ΔE/E) of 0.735% at E = 5.461 eV is demonstrated with He I radiation. The energy resolution is limited by the size of the ionization region. Trajectory calculations indicate that the system is capable of achieving an energy resolution of 0.04% with a laser if the imaging resolution is not limited. Experimental results are presented for jet-cooled benzene and pyridine, and they are compared with results in the literature.

We report resonance-enhanced two-photon ionization photoelectron spectroscopy of jet-cooled Benzene via the 6¹1ⁿ (n = 0-3) vibronic levels in S₁(¹B_2u ππ^*) using a nanosecond UV laser and photoelectron imaging. The best energy resolution (ΔE/E) was 0.7 %. The photoelectron spectrum from the S₁ 6¹1³ level (E_vib = 3284 cm^-1) in the channel three region exhibited a clear signature of intramolecular vibrational redistribution (IVR). The spectral features were consistent with picosecond zero kinetic energy photoelectron (ZEKE) spectra reported by Smith et al. [J. Phys. Chem. 1995, 99, 1768]. The photoelectron angular anisotropy parameter b₂ was found to be negative in ionization from the 6¹1ⁿ (n = 0-3) levels with photoelectron kinetic energies up to 5000 cm^-1. No influence of a shape resonance was identified.

Ultrafast photodynamics of furan has been studied by time-resolved photoelectron imaging (TRPEI) spectroscopy with an unprecedented time resolution of 22 fs. The simulation of the time-dependent photoelectron kinetic energy distribution (PKED) has been performed with ab initio nonadiabatic dynamics “on the fly” in the frame of time-dependent density functional theory. Based on the agreement between experimental and theoretical time-dependent photoelectron signal intensity as well as on PKED, precise time scales of ultrafast internal conversion from S₂ over S₁ to the ground state S₀ of furan have been revealed for the first time. Upon initial excitation of the S₂ state which has π-π^* character, a nonadiabatic transition to the S₁ state occurs within 10 fs. Subsequent dynamics invokes the excitation of the C-O stretching and C-O-C out of plane vibrations which lead to the internal conversion to the ground state after 60 fs. Thus, we demonstrate that the TRPEI combined with high level nonadiabatic dynamics calculations provide fundamental insight into ultrafast photodynamics of chemically and biologically relevant chromophores.

A nonadiabatic electronic transition through a conical intersection was studied by pump-probe photoelectron imaging spectroscopy with a 22 fs time resolution in the benchmark polyatomic molecule of pyrazine and deuterated pyrazine. The lifetimes of the S₂ state of pyrazine and deuterated pyrazine were determined to be 22±3 fs by the global fitting of the time-energy maps of photoelectron kinetic energy (PKE) distributions. The lifetime of S₃ was determined to be 40-43 fs. Two-dimensional maps of photoelectron distributions were obtained for time (t) and PKE, and individual PKE distributions upon ionization from S₂ and S₁ were extracted. Quantum beat with an approximately 50 fs period was observed after the S₂→S₁ internal conversion, which was attributed to the totally symmetric vibration v _6a in S₁.

A time-resolved photoelectron imaging using a femtosecond ultraviolet (UV) laser and a vacuum UV freeelectron laser is presented. Ultrafast internal conversion and intersystem crossing in pyrazine in a supersonic molecular beam were clearly observed in the time profiles of photoioinzation intensity and time-dependent photoelectron images.

The benchmark system of molecular photoionization dynamics, the (1 + 1') two-photon ionization of NO via the A state, is investigated using the time-energy mapping of the photoelectron angular distribution in a laboratory frame. The molecular frame photoelectron angular distribution and partial wave composition are determined from time-energy maps and compared with those obtained by Schwinger variational calculation (SVC) and state-to-state photoelectron spectroscopy. Good agreement is found with SVC. By comparison of the phase shifts of the scattering waves and the quantum defects of the Rydberg states, the l hybridization of p waves is identified.

We have constructed a photoelectron imaging spectrometer with super-resolution image processing and have applied it to the photoionization of nitric oxide and benzene in molecular beams. A field programmable gate array is employed for real-time subpixel centroiding calculations on hardware, providing 64 megapixel resolution (8192 × 8192 pixels). We examined eight different centroiding algorithms based on the center-of-gravity (COG) and Gaussian fitting (Gauss) methods and have found that the two-dimensional COG (2D-COG) and weighted mean of Gaussian center (w-Gauss) methods have the best performance. The excellent performance of the instrument is demonstrated by visualizing a 25 μm diameter pore structure of an MCP, indicating a spatial resolution of 0.03%. The photoelectron image in one-color (1 + 1) resonance-enhanced multiphoton ionization of nitric oxide using a nanosecond laser provided a photoelectron kinetic energy resolution of 0.2%. This resolution is currently restricted by charged-particle optics. The photoelectron energy and angular distributions in the one-color (1 + 1) resonance-enhanced multiphoton ionization of benzene via 6¹ and 6¹1¹ vibronic levels in the S₁ state are also presented. The results demonstrate that photoelectron angular anisotropy varies with the photoelectron kinetic energy and the vibronic state of the cation.

The charge-carrier dynamics of anatase TiO₂ nanoparticles in an aqueous solution were studied by femtosecond time-resolved X-ray absorption spectroscopy using an X-ray free electron laser in combination with a synchronized ultraviolet femtosec- ond laser (268 nm). Using an arrival time monitor for the X-ray pulses, we obtained a temporal resolution of 170 fs. The transient X-ray absorption spectra revealed an ultrafast Ti K-edge shift and a subsequent growth of a pre-edge structure. The edge shift occurred in ca. 100 fs and is ascribed to reduction of Ti by localization of gen- erated conduction band electrons into shallow traps of self-trapped polarons or deep traps at penta-coordinate Ti sites. Growth of the pre-edge feature and reduction of the above-edge peak intensity occur with similar time constants of 300–400 fs, which we assign to the structural distortion dynamics near the surface.

Time-resolved X-ray absorption spectroscopy was performed for aqueous ammonium iron(III) oxalate trihydrate solutions using an X-ray free electron laser and a synchronized ultraviolet laser. The spectral and time resolutions of the experiment were 1.3 eV and 200 fs, respectively. A femtosecond 268 nm pulse was employed to excite [Fe(III)(C₂O₄)₃]^3- in solution from the high-spin ground electronic state to ligand-to-metal charge transfer (LMCT) state(s), and the subsequent dynamics were studied by observing the time-evolution of the X-ray absorption spectrum near the Fe K-edge. Upon 268 nm photoexcitation, the Fe K-edge underwent a red-shift by more than 4 eV within 140 fs; however, the magnitude of the redshift subsequently diminished within 3 ps. The Fe K-edge of the photoproduct remained lower in energy than that of [Fe(III)(C₂O₄)₃]^3-. The observed red-shift of the Fe K-edge and the spectral feature of the product indicate that Fe(III) is upon excitation immediately photoreduced to Fe(II), followed by ligand dissociation from Fe(II). Based on a comparison of the X-ray absorption spectra with DFT calculations, we propose that the dissociation proceeds in two steps, forming first [(CO₂^•)Fe(II)(C₂O₄)₂]^2- and subsequently [Fe(II)(C₂O₄)₂]^2-.

We present femtosecond time-resolved X-ray absorption spectroscopy of aqueous solution using a hard x-ray free electron laser (SACLA) and a synchronized Ti:sapphire laser. The instrumental response time is 200 fs, and the repetition rate of measurement is 10 Hz. A cylindrical liquid beam 100 mm in diameter of aqueous ammonium iron(III) oxalate solution is photoexcited at 400 nm, and the transient X-ray absorption spectra are measured in the K-edge region of iron, 7.10-7.26 keV, using a dual X-ray beam dispersive detection method. Each of the dual beams has the pulse energy of 1.4 mJ, and pump-induced absorbance change on the order of 10-3 is successfully detected. The photoexcited iron complex exhibits a red shifted iron K-edge with the appearance time constant of 260 fs. The X-ray absorption difference spectra, with and without the pump pulses, are independent of time delay after 1.5 ps up to 100 ps, indicating that the photoexcited species is long-lived.

We present a light source capable of generating sub-10-fs deep UV (DUV) and extreme UV (EUV) pulses for use in time-resolved photoemission spectroscopy. The fundamental output of a Ti:sapphire laser was compressed using the multi-plate method and mixed with the uncompressed second harmonic in a filamentation four-wave mixing process to generate sub-10-fs DUV pulses. Sub-10-fs EUV pulses were generated via high-order harmonic generation driven by the second harmonic pulses that were compressed using Ar gas and chirped mirrors. The minimum cross correlation time between 267 and 57 nm (corresponding to 21.7 eV) was measured to be 10.6 ± 0.4 fs.

A single-order harmonic pulse in the vacuum-ultraviolet (VUV) is highly desirable for time-resolved photoelectron spectroscopy (TRPES) of polyatomic molecules. A high-power 9th harmonic of a Ti:sapphire laser (hv = 14 eV) is obtained using a UV driving laser at 270 nm (the 3rd harmonic). We describe our recent efforts to develop VUV TRPES combined with UV-driven harmonic generation, and present a few representative results from our recent TRPES studies.

A charge excitation in a two-dimensional Mott insulator is strongly coupled with the surrounding spins, which is observed as magnetic-polaron formations of doped carriers and a magnon sideband in the Mott-gap transition spectrum. However, the dynamics related to the spin sector are difficult to measure. Here, we show that pump-probe reflection spectroscopy with seven-femtosecond laser pulses can detect the optically induced spin dynamics in Nd₂CuO₄, a typical cuprate Mott insulator. The bleaching signal at the Mott-gap transition is enhanced at ~18 fs. This time constant is attributable to the spin-relaxation time during magnetic-polaron formation, which is characterized by the exchange interaction. More importantly, ultrafast coherent oscillations appear in the time evolution of the reflectivity changes, and their frequencies (1400–2700 cm⁻¹) are equal to the probe energy measured from the Mott-gap transition peak. These oscillations can be interpreted as the interference between charge excitations with two magnons originating from charge–spin coupling.

Time-resolved photoelectron spectroscopy (TRPES) of gaseous polyatomic molecules using 266-nm (4.7 eV) pump and 42-nm (29.5 eV) probe pulses is presented. A 1-kHz Ti:sapphire laser with a 35 fs pulse duration is employed to generate high harmonics in Kr gas, and the 19th harmonic (42-nm) was selected using two SiC/Mg mirrors. Clear observation of the ultrafast electronic dephasing in pyrazine and photoisomerization of 1,3-cyclohexadiene demonstrates the feasibility of TRPES with the UV pump and VUV probe pulses under weak excitation conditions in the perturbation regime.

We present a novel design of a few-cycle noncollinear optical parametric amplifier (NOPA) pumped by the second harmonic of a Ti:sapphire laser. A quasi-transform-limited sub-6 fs pulse width was realized by static dispersion compensation with commercially available chirped mirrors. The performance of the NOPA was tested by performing transient absorption spectroscopy on sensory rhodopsin II, and we observe short-lived oscillatory components that are associated with the vibrational coherence from the isomerizing molecule in the excited electronic state.

We demonstrate self-compression of deep-ultraviolet (DUV) pulses by filamentation in krypton. In contrast to self-compression in the near-infrared, that in the DUV is associated with a red-shifted sub-pulse appearing in the pulse temporal profile. The achieved pulse width of 15 fs is the shortest among demonstrated sub-mJ deep-ultraviolet pulses.

We present pump.probe photoelectron spectroscopy of Kr and NO using a high-power vacuum-ultraviolet (VUV) laser at a wavelength of 90 nm. Clear quantum beats are observed in the photoelectron angular distributions as well as in the photoelectron yields, resulting from the coherent excitation of two Kr Rydberg states by the VUV pump. The entire Franck.Condon envelope of the NO A(²Σ⁺) excited state is also successfully captured by the VUV probe.

The sixth harmonic (6ω, 133 nm) of a Ti:sapphire laser is generated using cascaded four-wave mixing in filamentation propagation of the fundamental (ω) and the second harmonic (2ω) pulses through Ne gas. The method provides the 6ω pulse energy higher than 5 nJ/pulse at 1 kHz and a pulse duration shorter than 17 fs without dispersion compensation.

Sub-20 fs pulses of the third, fourth, and fifth harmonics of a Ti:sapphire laser are simultaneously generated using cascaded four-wave mixing in filamentation propagation of the fundamental frequency and the second harmonic pulses in Ne gas. Reflective optics under vacuum are employed after the four-wave mixing to minimize material dispersion of the optical pulses. The cross-correlation between 198 and 159 nm pulses of 18 fs is achieved without dispersion compensation. This new light source is applied to time-resolved photoelectron imaging of carbon disulfide (CS₂).

A 90 nm single-order harmonic pulse with a 200 nJ on-target pulse energy at 1 kHz was realized through a harmonic generation process with a 35 fs Ti:Sa third harmonic in a Kr gas cell.

Transfer of spectral phase from near infrared ultrashort pulses to deep ultraviolet (UV) sub-30-fs pulses through four-wave mixing process is demonstrated. Micro joule UV pulses at 237 nm were generated by nonlinear mixing of second harmonic pulses of Ti:sapphire laser output and near infrared pulses from a noncollinear optical parametric amplifier. Chirp of the near infrared pulse was transfered to the UV pulse with the opposite sign. A positively chirped near infrared pulse was used for generating a negatively chirped UV pulse, which was compressed down to 25 fs by a magnesium fluoride window.