Ultrafast Internal Conversion of Aromatic Molecules Studied by Photoelectron Spectroscopy using Sub-20 fs Laser Pulses

This article describes our recent experimental studies on internal conversion via a conical intersection using photoelectron spectroscopy. Ultrafast S(2)(ππ*)–S(1)(nπ*) internal conversion in pyrazine is observed in real time using sub-20 fs deep ultraviolet pulses (264 and 198 nm). While the photoelectron kinetic energy distribution does not exhibit a clear signature of internal conversion, the photoelectron angular anisotropy unambiguously reveals the sudden change of electron configuration upon internal conversion. An explanation is presented as to why these two observables have different sensitivities to internal conversion. The 198 nm probe photon energy is insufficient for covering the entire Franck-Condon envelopes upon photoionization from S(2)/S(1) to D(1)/D(0). A vacuum ultraviolet free electron laser (SCSS) producing 161 nm radiation is employed to solve this problem, while its pulse-to-pulse timing jitter limits the time resolution to about 1 ps. The S(2)–S(1) internal conversion is revisited using the sub-20 fs 159 nm pulse created by filamentation four-wave mixing. Conical intersections between D(1)(π(−1)) and D(0)(n(−1)) and also between the Rydberg state with a D(1) ion core and that with a D(0) ion core of pyrazine are studied by He(I) photoelectron spectroscopy, pulsed field ionization photoelectron spectroscopy and one-color resonance-enhanced multiphoton ionization spectroscopy. Finally, ultrafast S(2)(ππ*)–S(1)(ππ*) internal conversion in benzene and toluene are compared with pyrazine.