Vibrational energy relaxation of benzene dimer and trimer in the CH stretching region studied by picosecond time-resolved IR-UV pump-probe spectroscopy

Kusaka Ryoji

Inokuchi Yoshiya

Ebata Takayuki

Journal of Chemical Physics

Vibrational energy relaxation (VER) of the Fermi polyads in the CH stretching vibration of the benzene dimer (Bz_2) and trimer (Bz_3) has been investigated by picosecond (ps) time-resolved IR-UV pump-probe spectroscopy in a supersonic beam. The vibrational bands in the 3000-3100 cm^<-1> region were excited by a ps IR pulse and the time evolutions at the pumped and redistributed (bath) levels were probed by resonance enhanced multiphoton ionization with a ps UV pulse. For Bz_2, a site-selective excitation in the T-shaped structure was achieved by using the isotope-substituted heterodimer hd, where h = C_6H_6 and d = C_6D_6, and its result was compared with that of hh homodimer. In the hd heterodimer, the two isomers, h(stem)d(top) and h(top)d(stem), show remarkable site-dependence of the lifetime of intracluster vibrational energy redistribution (IVR); the lifetime of the Stem site [h(stem)d(top), 140-170 ps] is -2.5 times shorter than that of the Top site [h(top)d(stem), 370-400 ps]. In the transient UV spectra, a broad electronic transition due to the bath modes emerges and gradually decays with a nanosecond time scale. The broad transition shows different time profile depending on UV frequency monitored. These time profiles are described by a three-step VER model involving IVR and vibrational predissociation: initial → bath1(intramolecular) → bath2(intermolecular) → fragments. This model also describes well the observed time profile of the Bz fragment. The hh homodimer shows the stepwise VER process with time constants similar to those of the hd dimer, suggesting that the excitation-exchange coupling of the vibrations between the two sites is very weak. Bz_3 also exhibited the stepwise VER process, though each step is faster than Bz_2.

exchange interactions (electron)

infrared spectra

isomerism

molecular clusters

molecule-photon collisions

multiphoton spectra

optical pumping

organic compounds

photoionisation

predissociation

time resolved spectra

vibrational states

visible spectra

Chemistry [ 430 ]

eng

American Institute of Physics

Copyright (c) 2012 American Institute of Physics.This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.

[ISSN] 0021-9606

[DOI] 10.1063/1.3676658

[NCID] AA00694991

[DOI] http://dx.doi.org/10.1063/1.3676658