Real-time visualization of the vibrational wavepacket dynamics in electronically excited pyrimidine via femtosecond time-resolved photoelectron imaging

Abstract

The vibrational wavepacket dynamics at the very early stages of the S-T intersystem crossing in photoexcited pyrimidine is visualized in real time by femtosecond time-resolved photoelectron imaging and time-resolved mass spectroscopy. A coherent superposition of the vibrational states is prepared by the femtosecond pump pulse at 315.3 nm, resulting in a vibrational wavepacket. The composition of the prepared wavepacket is directly identified by a sustained quantum beat superimposed on the parent-ion transient, possessing a frequency in accord with the energy separation between the 6a and 6b states. The dephasing time of the vibrational wavepacket is determined to be 82 ps. More importantly, the variable Franck-Condon factors between the wavepacket components and the dispersed cation vibrational levels are experimentally illustrated to identify the dark state and follow the energy-flow dynamics on the femtosecond time scale. The time-dependent intensities of the photoelectron peaks originated from the 6a vibrational state exhibit a clear quantum beating pattern with similar periodicity but a phase shift of π rad with respect to those from the 6b state, offering an unambiguous picture of the restricted intramolecular vibrational energy redistribution dynamics in the 6a/6b Fermi resonance.

Femtosecond Time-Resolved Observation of Relaxation and Wave Packet Dynamics of the S1 State in Electronically Excited o-Fluoroaniline

“…A careful review of the MATI of the o -fluoroaniline cation [ 37 ] shows that the intensity of the S l 0 0 state is maximal, and the vibrational peaks of the S l 10b 1 and S l 16a 1 states are almost invisible in the spectrum. Therefore, the transition probability of the S l 0 0 state is much higher than that of the S l 10b 1 and S l 16a 1 states in o -fluoroaniline ionization, or the FC factor of the S l 0 0 state is larger [ 15 ], making the S l 0 0 state easier to ionize, and the S l 10b 1 and S l 16a 1 states are much less likely to ionize. When the wave packet is constantly flowing, there are two scenarios with delay: (1) when the energy in the wave packet is mainly concentrated near the S l 0 0 vibration mode, it can ionize the S l 0 0 vibration mode but cannot ionize the S l 10b 1 and S l 16a 1 modes, so the signal is at maximum, and (2) when the energy in the wave packet is mainly concentrated near the S l 10b 1 and S l 16a 1 vibration modes, the signal is at a minimum because neither of the three vibration modes can be ionized.…”

“…This research showed that beat frequency was caused by a change in molecular conformation. Shuai Li’s team used femtosecond TRPEI to study the wave packet evolution of pyrimidine [ 15 ], showing that beat frequency was caused by transitions between different vibrational energy levels of the ion state. The group of Junggil Kim studied the wave packet dynamic of 2-fluorothioanisole [ 16 ] via the femtosecond time-resolved pump-probe method and showed that there were two minimal energies when the molecule was in the S 1 state.…”

Channel Competition and Control of Relaxation Pathways in S1 State of Acrolein: Role of Conical Intersection and Surface Crossing

“…The TRPEI experiment setup used has been described in detail elsewhere, , consisting of a home-built time-of-flight (TOF) mass spectrometer and a two-dimensional position sensitive detector.…”

Real-Time Observation of Fermi Resonances in the S1 State of Phenol

“…Then quantum interferences of two or more distinct time-dependent wave functions embedded in the superposition state are manifested as oscillatory time-evolution features in various spectroscopic probing schemes. These include laser-induced dispersed fluorescence, , time-resolved photoion, or photoelectron spectroscopy. − Although many examples of quantum beats among coherently excited states were reported, real-time probing of strongly coupled Fermi resonances in polyatomic molecular systems has been quite rare.…”