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

Molecules

关键词: photoelectron spectroscopy;    femtosecond;    laser;    filamentation;    ultraviolet;    imaging;    pyrazine;    benzene;    toluene;    internal conversion;    ionization;   

DOI  :  10.3390/molecules19022410

来源: mdpi

【 摘 要 】

This article describes our recent experimental studies on internal conversion via a conical intersection using photoelectron spectroscopy. Ultrafast S₂(ππ*)–S₁(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 S₂/S₁ to D₁/D₀. 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 S₂–S₁ internal conversion is revisited using the sub-20 fs 159 nm pulse created by filamentation four-wave mixing. Conical intersections between D₁(π⁻¹) and D₀(n⁻¹) and also between the Rydberg state with a D₁ ion core and that with a D₀ 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 S₂(ππ*)–S₁(ππ*) internal conversion in benzene and toluene are compared with pyrazine.

【 授权许可】

CC BY   
© 2014 by the authors; licensee MDPI, Basel, Switzerland.