Remarkable Site Difference of Vibrational Energy Relaxation in Benzene Dimer: Picosecond Time‐Resolved IR–UV Pump–Probe Spectroscopy

Abstract

Benzene dimer (Bz2) is one of the most fundamental molecular systems and shows intriguing properties attributed to aromaticaromatic weak interactions. The structure of Bz2 in the ground electronic state (S0) has been extensively studied experimentally and theoretically. Several experimental studies on Bz2 such as the molecular-beam electric deflection 1(b)] and Raman-UV double resonance and UV-UV hole-burning spectroscopy strongly suggested that the two benzene (Bz) molecules are not symmetrically equivalent. In addition, recent high level ab initio calculations suggested that a floppy T-shaped structure is a global minimum, although there are several possible isomers in nearly isoenergetic region. The rotational constant obtained from the Fourier-transform microwave spectrum agreed well with the predicted value for the T-shaped form. An important subject strongly related to the structure is the vibrational dynamics. In the T-shaped Bz2, the two Bz molecules are at different symmetrical conditions; one is at the Stem-site, and the other is at the Top-site. Therefore, the rate constant as well as the process of the vibrational energy relaxation of the two molecules will be different. In this sense, the study of a site-specified vibrational energy relaxation will provide us with a more detailed picture of the structure and vibrational dynamics of Bz2. Several studies on the vibrational energy relaxation of Bz2 have been reported. Lee and co-workers estimated the lifetime of the CH stretching region to be ~3 ps based on the band-width measurement of the IR spectrum. Felker and co-workers suggested that the 2 (CH stretching, Wilson’s numbering) level of the Stem-site Bz relaxes faster than that of the Top-site Bz, according to the difference in the band-width of the Raman spectra. However, in both cases the measured band-width was not that of a single rovibrational level. It is also not clear how the band-width is related to the vibrational energy relaxation dynamics, such as intramolecular (intracluster) vibrational energy redistribution (IVR) and vibrational predissociation (VP). Moreover, there has been no report on the time-resolved spectroscopy for the site-specified Bz2. Here we study the dynamics of the vibrational energy relaxation of the T-shaped Bz2 in the CH stretching energy region by using picosecond time-resolved IR-UV pump-probe spectroscopy. It is known that for the isolated Bz monomer having a D6h symmetry the IR active CH vibration (20) with the e1u symmetry is anharmonically coupled with the 8+19 and 1+6+19 Figure 1. Energy level diagram of isotope-substituted benzene dimers [h(Stem)d(Top) and d(Stem)h(Top)] and an excitation scheme. The dimers are vibrationally excited to the CH stretching region by a picosecond IR pulse, and the time evolution is probed by a picosecond UV pulse. The h*(Stem) and h*(Top) benzene components (an asterisk represents the vibrationally excited site) show different vibronic structures. Thus, the vibrationally excited dimers can be separately monitored by a tunable picosecond UV pulse. In this figure, VER means vibrational energy relaxation.