Abstract
The butterfly vibration during the hydrogen tunneling process in electronically excited o-fluorophenol has been visualized in real time by femtosecond time-resolved ion yield spectroscopy coupled with time-resolved photoelectron imaging technique. A coherent superposition of out-of-plane C–F butterfly motions is prepared in the first excited electronic state (S1). As the C–F bond vibrates with respect to the aromatic ring, the nuclear geometry varies periodically, leading to the corresponding variation in the photoionization channel. By virtue of the more favorable ionization probability from the nonplanar minimum via resonance with the Rydberg states, the evolution of the vibrational wave packet is manifested as a superimposed beat in the parent-ion transient. Moreover, time-resolved photoelectron spectra offer a direct mapping of the oscillating butterfly vibration between the planar geometry and nonplanar minimum. The beats for the photoelectron peaks originating from the planar geometry are out of phase with those from the nonplanar minimum. Our results provide a physically intuitive and complete picture of the oscillatory flow of energy responsible for the coherent vibrational motion on the excited state surface.
Highlights
Ultrafast molecular vibration is one of the fundamental motions that characterize chemical bonding and decide reaction dynamics at the molecular level[1,2,3]
By the careful selection of probe wavelengths combined with the intrinsic molecular properties, Stavros et al and Ashfold et al established the potential of time-resolved ion yield (TR-IY) spectroscopy for probing the vibrational wave packet dynamics in several aromatic species such as catechol[27], syringol, and guaiacol[28]
As one of the prototypes of intramolecular hydrogen-bonding systems, o-fluorophenol has been the subject of numerous spectroscopic studies, including excitation and dispersed laser-induced fluorescence spectroscopy[29], two-color resonant two-photon ionization (2C-R2PI) spectroscopy[30,31], hole burning and high resolution ultraviolet spectroscopy[32], mass analyzed threshold ionization spectroscopy (MATI)[30], Fourier transform-infrared (FT-IR) and FT-Raman spectroscopy[33], infrared spectroscopy[34,35], and femtosecond TRPEI36
Summary
Ultrafast molecular vibration is one of the fundamental motions that characterize chemical bonding and decide reaction dynamics at the molecular level[1,2,3]. Several groups have employed such a technique to prepare a coherent vibrational wave packet and detect its dynamic evolution in several yet-cooled molecules[18,19,20,21,22,23,24,25,26,27,28], including the early work by Reid and coworkers that observed obvious quantum beating patterns in the studies of p-difluorobenzene[18], p-fluorotoluene[19,20], and toluene[21,22] using picosecond time-resolved photoelectron spectroscopy (TRPES). After a coherent superposition of the S1 origin and two quanta in τ is prepared by the pump pulse, femtosecond TR-IY and TRPEI are both applied to monitor the periodic variations in the ionization cross section and the PKE distribution during its periodic motion between different geometries
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