Local modes of benzene and benzene dimer, studied by infrared–ultraviolet double resonance in a supersonic beam

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Abstract
We used rotational cooling of molecules to ∼5 K by supersonic expansion and state-selective, multilevel saturation spectroscopy to obtain high-resolution spectra of the fundamental and first and second overtone transitions of C–H stretching modes in ground-electronic-state benzene and its dimer. Greatly reduced linewidths (<3 cm−1 FWHM) in the rich spectra show that previously reported spectra have suffered from inhomogeneous congestion. Our observed spectral widths indicate that the vibrational lifetimes of the C–H stretches are at least a few ps, even at the energy of the second overtone (8800 cm−1). The ‘‘local mode’’ picture appears to apply when at least three quanta of C–H stretching motion are present. Spectra of the dimer are similar to those of the monomer but show a red shift of a few cm−1, the appearance of combination bands involving van der Waals vibrational modes, some intensity changes, and a broadening of spectral features that increases with the vibrational energy. The dimer’s predissociation lifetime at ∼3000 cm−1 vibrational energy exceeds ∼3 ps.