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

doi:doi:10.1063/1.3676658

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Vibrational energy relaxation (VER) of the Fermi polyads in the CH stretching vibration of the benzene dimer (Bz₂) and trimer (Bz₃) 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 Bz₂, a site-selective excitation in the T-shaped structure was achieved by using the isotope-substituted heterodimer hd, where h = C₆H₆ and d = C₆D₆, 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. Bz₃ also exhibited the stepwise VER process, though each step is faster than Bz₂.

Article Outline

INTRODUCTION
EXPERIMENTAL AND ANALYSIS
RESULTS AND DISCUSSION
1. IR spectra of Bz ₂
2. Transient UV spectra of Bz ₂ after the IR excitation of Fermi-polyad
3. IVR of the IR-pumped levels of Bz ₂
  1. IVR decay of hd and hh dimers
  2. Excitation-exchange coupling between Stem and Top sites of the hh dimer
4. Time profile of the bath states and VER mechanism of Bz ₂
  1. Time evolution of the bath states
  2. Origins of bath states 1 and 2
5. Time evolution of Bz monomer fragment: VP of Bz ₂
6. VER of Bz ₃
CONCLUSION

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KEYWORDS, PACS, and IPC

Keywords

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

PACS

36.40.Mr

Spectroscopy and geometrical structure of clusters
33.15.Mt

Rotation, vibration, and vibration-rotation constants
33.20.Ea

Infrared spectra
33.20.Kf

Visible spectra
33.80.Eh

Autoionization, photoionization, and photodetachment
36.40.Cg

Electronic and magnetic properties of clusters

International Patent Classification (IPC)

H01S3/091
Using optical pumping

ARTICLE DATA

Digital Object Identifier

http://dx.doi.org/10.1063/1.3676658

PUBLICATION DATA

ISSN

0021-9606 (print)

1089-7690 (online)

Publisher

$abstract.society.value is a Member of CrossRef

American Institute of Physics

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