Highly vibrationally excited aromatic molecules: Infrared emission and collisional relaxation.

Abstract

Infrared fluorescence (IRF) was observed from highly vibrationally excited gas phase aromatic molecules (benzene, toluene, naphthalene, benzene-d$\sb6$)--in which the excitation is into the dense, unresolvable manifold of vibrational states at "chemically significant" energies approaching reaction thresholds. The IRF experiments involve uv laser (248 nm) excitation of gas phase aromatic molecules which undergo fast radiationless transitions to produce nearly monoenergetic (E$\sb{\rm vib}\sim 40000\ {\rm cm}\sp{-1})$ vibrationally excited electronic ground state species. Subsequent spontaneous IR fluorescence in the C-H or C-D stretching region at $\sim$3050 or 2300 cm$\sp{-1}$ is time-resolved, and decays as the emitting molecules lose energy through collisional energy transfer to a bath gas. Results using time-resolved IRF to measure energy transfer parameters in benzene and toluene are presented. Major points include relative efficiencies of various bath gases as vibrational energy acceptors and the dependence of per-collision energy transfer rates on the vibrational energy of the donor molecule. A novel extension of the IRF technique is used to determine the first two moments of the vibrational energy population distribution during deactivation of benzene and benzene-d$\sb6.$. Low-resolution ($\sim$0.035 micron) infrared emission spectra of the C-H stretch region (3-4 microns) in vibrationally excited benzene and naphthalene are presented. Anharmonically shifted components of the emission corresponding to $\Delta{\rm v}$ = -1 transitions originating in v = 1, 2, and 3 (benzene) and v = 1 and 2 (naphthalene) are identifiable. The relative intensities of the components agree at the known initial energy (40000 cm$\sp{-1})$ with those calculated assuming statistical distribution of energy among vibrational modes. The agreement constitutes one of the most supportive experimental tests of this theory to date. Implications of the energy dependent spectra reported in this work are discussed with regard to the polycyclic aromatic hydrocarbon (PAH) hypothesis, which assigns a set of infrared features observed in many astronomical objects to emission from PAH molecules closely related to benzene and naphthalene.