ABSOLUTE STATE-SELECTED AND STATE-TO-STATE TOTAL CROSS-SECTIONS FOR THE REACTION AR+(2P3/2,1/2)+O-2

Absolute spin-orbit state-selected total cross sections for the reactions, Ar+(2P3/2,1/2) +O2→O 2+ + Ar [reaction (1)],O++O+Ar [reaction (2)], and ArO+ + O [reaction (3)], have been measured in the center-of-mass collision energy (Ec.m.) range of 0.044-133.3 eV. Absolute spin-orbit state transition total cross sections for the Ar+( 2P3/2,1/2) + O2 reaction at Ec.m. = 2.2-177.6 eV have also been examined. The appearance energies for the formation of O+ (Ec.m. = 2.9 ± 0.2 eV) and ArO+ (2.2 ± 0.2 eV) are in agreement with the thermochemical thresholds for reactions (2) and (3), respectively. The cross sections for O2 +, O+, and ArO+ depend strongly on E c.m. and the spin-orbit states of Ar+, suggesting that reactions (1)-(3) are governed predominantly by couplings between electronic potential energy surfaces arising from the interactions of Ar+( 2P3/2) + O2,Ar+(2P 1/2)+O2, and O2+ + Ar. In the E c.m. range of 6.7-22.2 eV, corresponding to the peak region of the O+ cross section curve, the cross sections for O+ are ≥50% of those for O2+. The production of O+ by reaction (2) is interpreted to be the result of predissociation of O 2+ in excited states formed initially by reaction (1). The formation of charge transfer O2+ (ã 4Πu) has been probed by the charge transfer reaction O2+ (ã 4Πu) + Ar. The results indicate that in the Ec.m. range of 0.4-3.0 eV charge transfer product O2+ ions are formed mainly in the O 2+ (ã 4Πu) state. Experimental evidence is found supporting the conclusion that the vibrational distributions of O2+ (ã4Πu) formed in reaction (1) and by photoionization of O2 in the energy range between the O2+ (ã4Π u, v = 0) and O2+ (Ã2Π u, v = 0) thresholds are similar. The population of O +(4S) formed by reaction (2) has also been measured by the reaction O+(4S) + N2→NO++N. In the Ec.m. range of 3-44 eV, product O+ ions of reaction (2) are shown to be dominantly in the O+(4S) ground state. At Ec.m. ≥ 14 eV, the retarding potential energy analysis for O2+ shows that more than 98% of the charge transfer O 2+ ions are slow ions formed mostly by the long-range electron jump mechanism. Product ArO+ ions are observed only in the Ec.m. range of 2.2-26.6 eV. At Ec.m. slightly above the thermochemical thresholds of reactions (2) and (3), the overwhelming majority of ArO+ and O+ ions are scattered backward and forward with respect to the c.m. velocity of reactant Ar+, respectively. This observation is rationalized by a charge transfer predissociation mechanism which involves the formation of ArO+ and O+ via nearly collinear Ar+-O-O collision configurations at Ec.m. near the thresholds of reactions (2) and (3). © 1990 American Institute of Physics.