Wavepacket dynamics of the excited (S) state of 2,5-difluoroaniline by accidental resonance with the Rydberg states.

Coherent nuclear wavepacket behavior obtained by time-resolved spectroscopy is a good choice to capture the real-time evolution of molecular configuration. Using femtosecond time-resolved photoelectron imaging, we investigate the real-time evolution of the vibrational wavepacket of 2,5-difluoroaniline following the coherent excitation of an out-of-plane vibrational mode in the S state at 289.8 nm. Probed by an accidental resonance with the Rydberg states, the periodic oscillations with the frequency of 99 cm are observed from the photoelectron kinetic energy (PKE) distributions, corresponding to the energy difference between the out-of-plane mode X of C-F bond and the band origin. Moreover, phase reversal of π rad between 0.66-0.75 and 1.00-1.08 eV is also observed in the PKE region. Combined with the scan of the potential energy surface in the ground cationic D state, the observed two ionization channels corresponding to different phases are attributed to the periodic geometry changes between the planar and the non-planar structures when the coherent wavepacket evolves from the initial vertical Franck-Condon region toward the global minimum of the S potential energy surface.