Photoelectron spectroscopy for polyatomic molecules with 22-fs time resolution

Internal conversion (IC) in polyatomic molecules is one of the most essential processes for understanding energy transfer in photochemical reactions. The IC through conical intersection is usually very fast process, <100 fs time scale. Time-resolved photoelectron spectroscopy (TRPES) is one of the best methods to investigate such photochemical reaction dynamics for isolated molecules. However, TRPES for molecules with a time resolution even better than 100 fs has not been reported so far. The reason is that it is still difficult to generate pulses with a duration of less than 50 fs in deep-ultraviolet (DUV) or vacuum UV (VUV) regions. DUV or VUV pulses are often necessary for the TRPES since ionization potentials of many simple molecules are higher than 8 eV. Recently, we have developed a DUV ultrashort pulse generation scheme by using four-wave mixing through filamentation in gases. Stable two-color sub-20-fs DUV pulses were successfully generated. In this contribution, we demonstrate the application of the DUV light source to TRPES. The generated two-color (200 and 260 nm) DUV pulses with durations of ~15 fs, which were obtained with frequency-resolved optical gating (FROG), were focused onto a supersonic molecular beam. Here we chose benzene and toluene (seeded in He) as prototype samples. Basically, the 200 nm pulse, which is resonant with S₂larrS₀ of both benzene and toluene, is the pump pulse, and the 260 nm pulse is the probe pulse to ionize the molecules in the excited state. We used imaging technology for the TRPES. The photoelectrons generated by the (1 + 1´) resonance-enhanced multiphoton ionization were projected onto a two-dimensional position-sensitive detector. The three-dimensional photoelectron speed were reconstructed from the projection images using pBaseX algorithm. The time-resolution of the system was defined by the cross-correlation between pump and probe pulses. It was measured with (1 + 1´) non-resonant photoioniz- aion signal of ethanol (ionization potential 10.6 eV). The full width at half maximum of the cross-correlation was 22 fs, which was consistent with the FROG measurement. The photoelectron signal appears at ~1.3 eV and disappear within 100 fs. Afterwards, some components stays around zero photoelectron kinetic energy (PKE). The fact clearly indicates very fast S₂larrS₁ IC, as was observed with 150 fs pulses. In toluene, the signal stays at S₂ longer than that in benzene, and larger component remains around at low PKE, which facts are also consistent with the previous experiment. In addition, the photoelectron spectrum shifts slightly lower (~0.3 eV) before the IC. Such shift of photoelectron spectrum is not possible to be observed with 150 fs pulses. In summary, we have demonstrated the application of the newly developed ultrashort DUV pulses to TRPES. To the best of our knowledge, this is the highest time resolution TRPES for polyatomic molecules so far.