J . Phys. Chem. 1993,97, 210621 12
2106
Molecular Beam Study of the Ne*(3s3Pao) + 0 , ( X 3 q Reaction: Absolute Quenching and O* (3psP,3p3P) Product Cross Sections Don Mueller and John Krenos' Department of Chemistry, Rutgers, the Stare University, New Brunswick, New Jersey 08903 Received: August 7, 1992; In Final Form October 20, I992
Emission lines from 0*(3psP) and 0*(3p3P) products of the reaction of Ne*(3P2,~)and 0 2 are observed in a beam-scattering gas apparatus. The quenching cross section UQ, as measured by the self-monitor method, is 31 i 2 A2 for average collision energies of 54 and 140 meV. The product cross sections u* for O*(5P) and O*(3P) are 1.8 and 1.6 A,, respectively. If cross sections are assumed to reflect predominantly the 3P2component of the metastable beam, then the results obtained are in good agreement with beam experiments in which ion products and electron spectra are measured. To resolve an apparent inconsistency between our cross section and the most precisely measured rate constant value in the literature, we propose that the quenching cross section rises quickly from a threshold energy of approximately 16 meV to a nearly constant value at collision energies between 54 and 140 meV. The rate constant derived from this energy-dependent cross section is in excellent agreement with values measured in stationary afterglow experiments. Differences between ionization branching fractions measured directly in a flowing afterglow experiment and those determined from afterglow rate constants and crossed-beam ionization cross sections are also resolved. .
Introdaction Similar to alkali atoms in outer electronic configuration, electron donor capability, and chemical reactivity with halogenated molecules, metastable rare-gas atoms Rg* also undergo reactions in which the large excitation energy of the atom is transferred collisionally to produce ionized' and/or neutral species.2 If the excitation energy of Rg* greatly exceeds the ionization energy of the collision partner, then ionizationpathways aredominant.I.3 Metastableatom reactions that produce ionized products have been studied extensively. Product ion cross sections, electron energy distributions, and total quenching rate constants kQ have been measured for a large number of systems.' In contrast, competing nonionization pathways that may exist are more difficult to detect. Accordingly, absolute cross sections for these channels are available for only a few systems.4 Ionization products in the reaction of N ~ * ( ~ S - ' Pwith ~ , ~0 )2 have been studied in detail by the crossed-beam m e t h ~ d . ~Both -~ ionization and nonionization pathways are possible (see Figure l), but there is disagreement in the literature concerning the origin of the latter.'"14 The exoergic ionization pathways are represented.by reactions la+ and the exoergic (
