The benzene ground state potential surface. III. Analysis ofb2uvibrational mode anharmonicity through twohyphen;photon intensity

摘要

The 1501/1401vibronic two‐photon cross section ratios are reported for a series of isotopically labeled benzenes in theA˜(1B2u)←X˜(1A1g) electronic transition. Predictions derived from theB2uforce field are found to be in close agreement with the measured ratios. These ratios are shown to provide an excellent test of theB2uforce field and mode forms as evidenced by the large variation overD6hlabeled benzenes. In C6H6the 1501/1401cross 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 in13C6H6is 0.44±0.04 (equivalent to 0.36). The 13% disparity found between the measured and predicted C6H6ratio (i.e., 0.206) is attributed to anharmonic coupling between theb2umodes: 2&khgr;15,15=−9, &khgr;14,15=4, and 2&khgr;14,14=−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 bifurcatedB2uforce 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 1501and 1401vibronic cross sections. This requires knowledge of the sign of the hydrogen motion term in the tensor. However, large anharmonic effects coupling the twob2umodes mask the small harmonic hydrogen contribution.