The ν13 fundamental band of benzene

The perpendicular band of the E1u vibration ν13 of benzene, occurring near 6.7 μm, has been measured on a BOMEM high-resolution Fourier transform spectrometer with a 246 cm optical path difference. The observed linewidth (full width at half height) of ~ 0.0048 cm−1 was reduced by deconvolution to ~ 0.0020 cm−1, which is slightly below the Doppler limit. The dense rotational structure was well resolved, and it was possible to assign over 125 subbands in the region 1466.5 to 1503 cm−1. A perturbation, identified as a third-order Jx,y-Coriolis interaction with a state located only ~ 0.055 cm−1 above the fundamental (presumably the A2u component of ν4 + 2ν20), causes distortion of the rotational structure and formation of subband heads in the low-KpQ-branches, which exhibit a striking line-like shape. The spectrum was analyzed in detail with the aid of the computer program SYMTOP for the treatment of symmetric top bands whose upper states are involved in arbitrary interactions. The Hamiltonian included, in addition to the Jx.y -Coriolis perturbation, the l-type resonance [Formula: see text]. A set of accurate spectroscopic constants was obtained for the perturbed ν13 upper state. Using these constants, 2750 lines of the ν13 band, which could be assigned nonzero weights, were reproduced with an overall standard deviation of ~ 0.00046 cm−1.