The mechanism for multiphoton ionization of H2S

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Abstract
The resonantly enhanced multiphoton ionization mass spectrum of H2S has been studied as a function of laser wavelength and power. Three mass peaks, due to S+, HS+, and H2S+ were observed at all resonances which were investigated. All three ionic species were found to have different dependences on laser power. The power indexes reflect the increasing number of photons required to form the species, but less than integer differences in the indexes indicate multiple paths for the formation of the various ions. A mechanism which is consistent with these data is one involving a rate-limiting three-photon absorption from the ground state, followed by rapid stepwise absorptions up an autoionization ladder of states above the ionizational potential. Results of a model calculation based on a rate equation description of the autoionization mechanism are in general agreement with the data. A mechanism involving dissociation of parent H2S+ ions by absorption of three or more additional photons could not be ruled out, however.

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