MPI spectroscopy of expansion-cooled ammonia: Structure and dynamics at 50 K

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Abstract
The multiphoton ionization MPI spectrum of ammonia cooled by a supersonic expansion is recorded in the 360–500 nm region. Rotational structure is greatly simplified over that observed at room temperature, facilitating quantitative comparisons with theoretical predictions of rotational intensities for three‐photon absorption using either linear or circular polarized light. The previous assignment of the new three‐photon resonant ?′ state is supported, and the location of its origin confirmed. Small systematic deviations are found between the calculated and observed line strengths. These differences can be understood in terms of two phenomena not accounted for in the static gas line strength theory. There is the possibility of a non‐Boltzmann distribution of rotational states following the expansion cooling. Also, the effects of the competition between rotational predissociation and photoionization of the excited ? state can result in a degradation of transitions involving higher upper state rotational levels. The observed deviations suggest that both of these processes may be important in the MPI spectrum of expansion cooled ammonia.