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

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 mhomogeneous 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 aimer's predissociation lifetime at ≃ 3000 cm-1 vibrational energy exceeds ≃ 3 ps. © 1988 American Institute of Physics.