The benzene ground state potential surface. III. Analysis of b2u vibrational mode anharmonicity through two-photon intensity

Publisher Website
Google Scholar
Abstract
The 1501/1401 vibronic two‐photon cross section ratios are reported for a series of isotopically labeled benzenes in the Ã(1B2u)←X̃(1A1g) electronic transition. Predictions derived from the B2u force field are found to be in close agreement with the measured ratios. These ratios are shown to provide an excellent test of the B2u force field and mode forms as evidenced by the large variation over D6h labeled benzenes. In C6H6 the 1501/1401 cross section ratio is measured as 0.249±0.008 (equivalent to 0.180 for the theoretically testable ratio: 1501/1401[〈1‖Q14‖0〉/〈1‖ Q15‖0〉]2). The corresponding ratio in 13 C6H6 is 0.44±0.04 (equivalent to 0.36). The 13% disparity found between the measured and predicted C6H6 ratio (i.e., 0.206) is attributed to anharmonic coupling between the b2u modes: 2χ15,15=−9, χ14,15=4, and 2χ14,14=−4 cm−1. Two‐photon intensities are proven to be useful in determining anharmonic interactions. The relatively small effects of the hydrogen motion provide an approach for solving the bifurcated B2u force constant problem in ground state benzene. The approach utilizes the contribution of harmonic C–C–H bending motions to the two‐photon tensor controlling the 1501 and 1401 vibronic cross sections. This requires knowledge of the sign of the hydrogen motion term in the tensor. However, large anharmonic effects coupling the two b2u modes mask the small harmonic hydrogen contribution.

This publication has 27 references indexed in Scilit: