Analysis of the stimulated Raman spectrum of the ν16ν2 + ν18 Fermi dyad of benzene

Ionization-detected stimulated Raman spectroscopy was used to obtain high-resolution recordings of the two perpendicular bands of the benzene molecule centered at 1591.327 and 1609.518 cm−1. The effective resolution was further enhanced by deconvolving the spectrum to a linewidth 0.003 to 0.004 cm−1. Fine-tuning of the ionizing radiation made it possible to record the transitions belonging to each band separately, thus greatly simplifying the spectrum in the region of overlap. The strong sS and oO branches were, for the most part, completely resolved as were many lines in the weaker oP and sR branches and even in the central oQ and sQ branches. The observed bands belong to the E2g fundamental ν16 in nearly exact Fermi resonance with the combination ν2 + ν18. A detailed rovibrational analysis of the spectrum is reported. A perturbation detected in the sSk branches of the lower-frequency band for K = 19 to 23 was identified as a quintic anharmonic resonance with the third overtone, 4ν20, of the lowest lying fundamental ν20, which is infrared- and Raman-inactive (species E2u). Deperturbed spectroscopic constants for the interacting states are reported which reproduce the observed line positions with a standard deviation of 0.0013 cm−1. The unperturbed origins of the ν16 and ν2 + ν18 states are only 1.106 cm−1 apart. The fundamental ν16 was identified with the higher-frequency state with origin at 1600.976 cm−1.