Rovibronic two-photon transitions of symmetric top molecules

Abstract

Rotational line strengths of two-photon absorption (TPA) processes in symmetric top molecules in the gas phase are developed by angular momentum theory. Experimentally accessible polarization states and frequencies of the photons are considered case by case. For TPA processes of identical photon frequency, parallel linearly polarized photons and same-sense circularly polarized photons form a complete polarization study. In the absence of resonance-enhanced intermediate states, TPA of nonidentical photon frequencies normally does not give new spectral features which are forbidden in identical TPA, because of the highly unfavorable weighting factors involving the intermediate state energies. High resolution studies of TPA, especially in the Doppler-free regime where features can be further resolved, is an alternative and can be more fruitful than polarization study for excited state assignments. A more general selection rule for TPA is that the initial and final states must have identical overall rovibronic symmetry. The Jahn–Teller effect induced, j-type doubling can be probed directly by TPA. As resonance condition is approached, features of different rotational structures (ΔK selection rules) within a two-photon allowed electronic band can occur through a nonadiabatic intermediate state. The polarization ratios for near-resonant TPA, which is useful for both intermediate and final state assignments, are also presented.