1.Ultrafast Extreme Ultraviolet Photoemission Spectroscopy in the Gas- and Liquid phase

UV light is used to photoexcite molecules in the gas or liquid phase, and subsequent chemical reactions are tracked in real time by photoelectron spectroscopy using extreme ultraviolet (UV) light pulses. The extreme ultraviolet light is generated by a table-top laser using a technique called high-order harmonic generation (HHG). Its photon energy is greater than 20 eV, which exceeds the ionization energy of any transient chemical species, allowing real-time detection of all transient chemical species and products. In a study of ultrafast electronic relaxation processes of nucleobases published in 2023, we observed very different dynamics and reaction times for gas-phase isolated molecules and nucleobases in aqueous solution, revealing distinct solvent and substituent effects.

2. Infrared and Deep Ultraviolet Transient Absorption Spectroscopy of Liquid-Phase Molecules

Photoelectron spectroscopy is a powerful tool for tracing changes in electronic states in molecules, but it provides limited information on molecular structure and vibrational frequencies. Therefore, we perform infrared transient absorption spectroscopy with subpicosecond time resolution to observe the vibrational spectra of molecules during the reaction. Transient absorption spectroscopy in the ultraviolet also provides useful information for studying chemical species or states that are difficult to identify by photoemission spectroscopy. Just as photoelectron spectroscopy can observe all molecules, infrared spectroscopy has the advantage of being able to detect all transient species and products (since no molecule is free of molecular vibrations). However, the infrared spectra of solvated molecules are difficult to interpret due to fluctuations in hydrogen bonding states caused by thermal motion. Therefore, we are working on the interpretation with the help of molecular dynamics and quantum chemical calculations. Deep ultraviolet transient absorption spectroscopy is a method that has few experimental examples, and new information has been obtained by analyzing the polarization dependence.

3. Development of New Light Sources and Experimental Techniques

Since ultrafast photoemission spectroscopy of liquids is more susceptible to space charge effects (electron repulsion) than gas-phase photoemission spectroscopy, we are developing extreme ultraviolet photoemission spectroscopy using a high repetition rate light source at the 100 kHz level. In addition, we are developing extreme ultraviolet photoemission spectroscopy with unprecedented time resolution by compressing laser pulses to about 10 fs to maximize time resolution.