Two-photon-ionization mass spectroscopy of ammonia clusters in a pulsed supersonic nozzle beam

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Abstract
Two-photon-ionization (TPI) with an ArF excimer laser at 193 nm, when combined with a mass spectrometer, is found to be a very sensitive method for the detection of ammonia clusters. Ammonia cluster distributions in the pulsed beam expansion have been measured via TPI by varying the delay time between the pulsed beam and the laser firing. The relative intensity of the clusters as observed by the TPI is a sensitive function of the delay time. Furthermore, seeding effects on the ammonia cluster distribution with He, Ne, Ar, and Kr carrier gases are examined. The distribution shifts toward high mass region with increasing mass of the seeding gases. Finally, an effective competition between the one-photon predissociation and two-photon ionization at the à 1A″2 state of the ammonia monomer (which is insignificant in the clusters) is found and discussed in terms of a simple rate-equation treatment valid in this system.