Fluorescence-dip IR spectra of jet-cooled benzoic acid dimer in its ground and first excited singlet states
The IR spectra of three isotopomers of the benzoic acid dimer have been recorded under jet-cooled conditions using the double resonance method of fluorescence-dip IR spectroscopy. In so doing, the spectra are assuredly due exclusively to dimers in the ground-state zero-point level at a rotational temperature of 3–5 K. Even under these conditions, the three isotopomers have remarkably broad spectra, extending from 2600 to almost 3150 cm−1. The spectra show extensive substructure consisting of some 15–20 transitions where only a single OH stretch fundamental should appear in the harmonic limit. The comparison of the undeuterated d0–d0 dimer with the ring-deuterated d5–d5 dimer tests the effect of mixing with the C–H stretches and overtones of the C–H bends. The mixed OH/OD ring-deuterated d6–d5 dimer shifts the frequency and changes the form of the OH stretch normal mode. The analogous OH stretch IR spectrum of the d0–d0 dimer out of the S1 excited-state zero-point level has also been recorded. In this case, much of the closely-spaced substructure is not apparent. What remains is a set of three bands separated from one another by about 180 cm−1. Preliminary results of model calculations of the anharmonic coupling, responsible for the broadening and substructure, are presented. These calculations indicate that it is OH stretch–OH bend coupling, rather than coupling with the intermolecular stretch, that is responsible for much of the observed structure and breadth.