Infrared photodissociation spectroscopy of the benzoic acid radical cation in a quadrupole trap

The vibrational spectrum of the gas-phase benzoic acid radical cation is reported in the fingerprint region from 500 to 1800 cm(-1). The benzoic acid cation is produced either via (1 + 1') resonance-enhanced multiphoton ionization or via chemical ionization of benzoic acid vapor. The ions are stored and mass-selectively isolated in a quadrupole ion trap where they are irradiated with the output of a widely tunable infrared free electron laser. Upon resonance, photodissociation occurs and recording the total fragment-ion yield as a function of wavelength yields the infrared spectrum. The experimental spectrum suggests that the ion is effectively planar, although a density functional theory calculation predicts a staggered structure to be the lowest in energy. In addition, we tentatively identify the infrared spectrum of protonated benzoic acid.