Formation and metastable decomposition of unprotonated ammonia cluster ions upon femtosecond ionization

Journal Article · Tue Mar 21 23:00:00 EST 1995 · Journal of Chemical Physics; (United States)

DOI:https://doi.org/10.1063/1.469531· OSTI ID:6451269

Buzza, S A; Wei, S; Purnell, J; Castleman, Jr, A W ^[1]

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1. Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (United States)

The formation and metastable dissociation mechanism of unprotonated ammonia cluster ions, (NH[sub 3])[sup +][sub [ital n]], produced by multiphoton ionization (MPI) at 624 nm and a nominal pulse width of 350 fs, are investigated through a reflectron time-of-flight (TOF) mass spectrometric technique. Detection of the unprotonated ions after femtosecond and nanosecond multiphoton ionization under various intensity conditions is explained. The role of the energy of the ionizing photons, and the observation of these ions after femtosecond MPI is examined. The formation of the unprotonated series is found to be a function of intensity in the case of ionization on the nanosecond time scale, but not so for the femtosecond time domain. The results can be explained in terms of ionization mechanisms and ionizing pulse durations. The findings of the present study suggest that the unprotonated ions are trapped behind the barrier to intracluster proton transfer and/or concomitant NH[sub 2] loss. The studies of metastable decomposition also reveal that the unprotonated ammonia cluster ions dissociate in the field-free region of the TOF by losing an NH[sub 2] radical rather than via the evaporative loss of NH[sub 3] as occurs for protonated clusters. Additionally, isotopic investigations of the unimolecular decay reveal a strong dependence on the conditions of cluster formation. The cluster formation condition dependence of the unimolecular decay is further investigated by altering formation temperatures and observing the consequences reflected by changes in the spontaneous metastable decay rate constant. This is a unique example of a cluster system whose metastable dissociation does not obey an evaporative ensemble model.

• DOE Contract Number: FG02-88ER60648
• OSTI ID: 6451269
• Journal Information: Journal of Chemical Physics; (United States), Journal Name: Journal of Chemical Physics; (United States) Vol. 102:12; ISSN JCPSA6; ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

664200* -- Spectra of Atoms & Molecules & their Interactions with Photons-- (1992-)
74 ATOMIC AND MOLECULAR PHYSICS
AMMONIA
CHARGE EXCHANGE
CHEMICAL BONDS
CHEMICAL REACTIONS
DECOMPOSITION
DISSOCIATION
ENERGY LEVELS
EXCITED STATES
HYDRIDES
HYDROGEN COMPOUNDS
ION PAIRS
IONIZATION
ISOTOPES
MASS SPECTROSCOPY
METASTABLE STATES
MULTI-PHOTON PROCESSES
NITROGEN COMPOUNDS
NITROGEN HYDRIDES
PHOTOIONIZATION
PRODUCTION
PULSES
RADICALS
SPECTROSCOPY
TIME-OF-FLIGHT METHOD