Visualization of coherent nuclear motion between different geometries in photoexcited 2,4-difluorophenol

Abstract

The coherent nuclear motion in photoexcited 2,4-difluorophenol (2,4-DFP) has been visualized in real time using femtosecond time-resolved ion-yield spectroscopy and time-resolved photoelectron imaging. A coherent vibrational packet is created by the simultaneous excitation of the out-of-plane bending modes in the first excited electronic state $({S}_{1})$ in 2,4-DFP. By virtue of the geometry change upon photoionization, the wave packets around the planar geometry are ionized exclusively with selective probe wavelengths, exhibiting the pronounced quantum beats superimposed on the parent-ion transients. Furthermore, photoionization signals of the planar and nonplanar geometries are both acquired in the photoelectron spectra but dispersed with respect to kinetic energy via resonances with different intermediate Rydberg states. The time dependences of the photoelectron peaks originating from the planar and nonplanar geometries exhibit the clear beats with similar periodicities but a phase shift of \ensuremath{\pi} rad, offering an unambiguous picture of the oscillating nuclear motion between the planar geometry and the nonplanar minimum.