Laser induced fluorescence excitation spectrum of jet-cooled benzoic acid dimers

Intermolecular hydrogen bonding in gas phase dimers of benzoic acid is investigated following preparation in a pulsed free-jet expansion. Laser induced fluorescence excitation spectra are analyzed to provide information regarding the vibrational assignments of the 1Lb excited state. In agreement with studies in low temperature mixed crystals, the electronic excitation is essentially localized on one monomer unit in the dimer. A 58 cm−1 band is associated with the ν3 hydrogen bonding stretch+bend mode and is prominent in combination bands involving internal motion in the carboxyl groups. The free jet technique has been shown to be particularly advantageous in studies of intermolecular hydrogen bonding.