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 d(0)-d(0) dimer with the ring-deuterated d(5)-d(5) dimer tests the effect of mixing with the CEH stretches and overtones of the CEH bends. The mixed OH/OD ring-deuterated d(6)-d(5) dimer shifts the frequency and changes the form of the OH stretch normal mode. The analogous OH stretch IR spectrum of the d(0)-d(0) dimer out of the S-1 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.