Characterization of the ?′ state of ammonia observed by three-photon, gas-phase spectroscopy

Abstract

A new state of ammonia has been characterized by means of three‐photon absorption (four‐photon ionization) spectroscopy. A comparison of the three‐photon (MPI) and one‐photon (VUV) techniques indicates that the ? excited state gives identical spectra in both cases, but that the ? state appears in the VUV while the nearly isoenergetic ?′ state appears strongly in the MPI spectra. For linearly polarized light the one‐ and three‐photon relational selection rules and intensity factors are found to be nearly identical. Rotational analyses of several bands of both NH₃ and ND₃ are presented, and these rotational constants are used to simulate the experimental spectra. By analogy with the united atom approximation, it is proposed that the ?′ and not the ? state results from a 2pa^″ ₂ to 3pa^″ ₂ excitation and that excitations to 3d orbitals should be considered in any theoretical discussion of the electronic structure of ammonia.