ANALYSIS OF THE ROVIBRONIC STRUCTURE OF THE THREE-PHOTON MULTI-PHOTON IONIZATION SPECTRA OF AMMONIA

Author Institution:The theory of three-photon rovibronic spectre of symmetric top molecules will be presented. The rotational line strength factors for three-photon absorption will be presented and used to analyze the polarization dependence of the ammonia spectrum. Vibronic contributions will be analyzed from the viewpoint of irreducible representations. This leads to easily predicted rotational selection rules as a function of different vibronic symmetries and light polarizations. Thus the specific vibronic character of a transition can often be established by analysis of the observed rotational selection rules. This theory will be compared to the experimental three-photon MPI spectra of ammonia taken at 300 K and at 50 K. At the lower temperature many of the more subtle predictions of the theory are confirmed intensities of the N, O, S, and T branches and the relative intensities of the P, Q, and R branches verses the N, O, S, and T branches of both the B and $C^{\prime}$ states of ammonia