Analysis of the 3-μm bands of benzene

Abstract

A comprehensive rovibrational analysis is reported for the 3-μm absorption bands of the benzene molecule measured on a difference-frequency laser spectrometer and deconvolved to an effective linewidth of 0.0010–0.0015 cm −1. The four strong bands observed, centered at 3047.908, 3078.614, 3100.408, and 3101.854 cm −1, are due to the E 1 u fundamental ν 12 sharing its intensity with the E 1 u components of the combinations ν 13 + ν 16, ν 2 + ν 13 + ν 18, and ν 3 + ν 10 + ν 18 via anharmonic resonances. The bands exhibit strong perturbations due to l resonances with the inactive B 1u B 2u components of the ν 13 + ν 16 and ν 2 + ν 13 + ν 18 states, and numerous localized perturbations due to overtones and combinations of the 16 low-frequency vibrations of the molecule. Symmetry arguments and values of constants such as Cζ known for the fundamentals have been used to propose tentative assignments for many of the perturbing states. The Hamiltonian matrix used for the treatment of the 8384 assigned transitions, with all the components of the main interacting states plus 18 perturbing states, was of order 34 and contained 130 parameters of which 112 were eventually adjusted. A set of spectroscopic constants is reported which reproduces all the data with a standard deviation of 0.0012 cm −1. The analysis shows that the Fermi resonance between ν 12 and ν 13 + ν 16 combined with the other anharmonic interactions shifts the fundamental down by 16.46 cm −1 from its unperturbed position. It is found that x,y-Coriolis and anharmonic resonances of surprisingly high order have to be invoked to account for most of the localized perturbations observed in the spectrum.