J . Phys. Chem. 1989, 93, 1916-1922
1916
Gas-Phase Atom-Radical Kinetics of Elementary CF, Reactions with 0 and N Atoms Cheng-ping Tsai, Susan M. Belanger, Joung T. Kim, Jeffrey R. Lord, and David L. McFadden* Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 021 67 (Received: May 9, 1988; In Final Form: August 1, 1988)
The absolute rate constants for the reactions of CF3 radicals with atomic oxygen and nitrogen have been measured in a gas flow system with photoionization mass spectrometry detection. CF3 was produced by dissociation of C2F, or CF3Br in a radio frequency discharge, and 0 and N atoms were produced by dissociation of the elemental gases in a microwave discharge. The pressure was 1.7 Torr. The rate constants at 293 K are (3.1 A 0.6) X lo-'' and (1.8 f 0.3) X lo-'' cm3 molecule-' s-I for the 0 and N atom reactions, respectively. The reaction products that were observed are consistent with a fluorine atom displacement mechanism for both reactions.
Introduction The reactions of atoms with small molecular free radicals have been the subject of recent kinetics studies in several laboratories.' Work in this area is directed in part toward a more complete understanding of radical-radical reaction d y n a m h 2 Two open shell reactants approaching one another with paired electron spins interact on an attractive potential surface, leading to the formation (rate-limiting step) and subsequent breakup of a thermodynamically stable intermediate complex. Atom-radical reactions exhibit a variety of mechanisms including atom metathesis, displacement, and addition-elimination, yet they are simple enough to provide good tests of theoretical kinetic models for reactions distinguished by complex formation and decay. In this article we present our first experimental results on fluorocarbon radical plus atom reactions. The room temperature rate constants for reaction of CF3 radicals with atomic oxygen and nitrogen have been determined under pseudo-first-order conditions, and product analysis has been accomplished, using a discharge flow method with photoionization mass spectrometry (PIMS) detection. The apparatus is described for the first time.
Experimental Section The experiments were performed in a discharge-flow system with photoionization mass spectrometry detection. A schematic diagram of the photoionization mass spectrometer (PIMS) is shown in Figure 1. The stainless steel vacuum system consists of three differentially pumped chambers. The main chamber, which has interior dimensions of 2 ft on a side, is evacuated by a 6 in. oil diffusion pump and a liquid nitrogen cooled cryotrap (8.3 L capacity). A 1 in. diameter flow tube passes vertically through the first differential pumping or flow tube chamber that is evacuated by a IO in. oil diffusion pump and a liquid nitrogen cooled cryotrap (1.7 L capacity). The flow tube is evacuated by a Stokes Microvac rotary pump (150 CFM) through a high conductance liquid nitrogen cooled trap to minimize backstreaming of pump oil. The mass spectrometer and ion detector are housed in the third chamber that is evacuated by a 4 in. oil diffusion pump. Cryo-baffles on the diffusion pumps in the main chamber and the flow tube chamber are cooled by a circulating Freon refrigeration system. The diffusion pump in the mass spectrometer chamber has a liquid nitrogen cooled baffle. There is also a liquid nitrogen trap in the region of the ion detector. Gases effuse from a sampling aperture (0.040 in. diameter, 0.015 in. wall thickness) in the flow tube wall and are modulated (100%) at a frequency of 90 Hz by a slotted disk beam chopper ( 1 ) (a) Hancock, G.; Harrison, P. D.; MacRobert, A. J. J . Chem. SOC., Faraday Trans. 2 1986,82,647. (b) Margitan, J. J. J . Phys. Chem. 1984, 88, 3638. (c) Ryan, K. R.; Plumb, I. C. Plasma Chem. Plasma Process 1984, 4 , 141, 271. (d) Hancock, G.; Ketley, G. W.; MacRobert, A. J. J . Phys. Chem. 1984.88, 2104. ( e ) Gershenzon, Yu.M.; Il'in, S. D.; Kishkovitch, 0. P.; Malkhasyan, R. T., Jr.; Rozenshtein, V. B.;Umanskii, S. Ya. Inr J. Chem. Kinet. 1983, 15, 399. ( f ) Kaufman, F. J . Phys. Chem. 1984, 88, 4909. (2) (a) Howard, M. J.; Smith, I. W. M. Prog. React. Kinet. 1983, 12, 5 5 . (b) Modern Gas Kinetics; Pilling, M. J., Smith, I. W. M., Eds.; Blackwell Scientific Publications: Oxford, 1987.
0022-3654/89/2093-19l6$01.50/0
that is driven by a hysteresis synchronous motor (30 Hz) before passing through a beam-defining aperture (0.100 in. diameter) into the main chamber. The molecular beam and photon beam intersect at 90' inside the ion source cage. The photoionization lamp is inserted through the top of the chamber with the photon beam directed vertically downward (lamp not shown in Figure 1). A distance of 1 in. separates the sampling aperture in the flow tube and the center of the ion source cage. The light source is a continuous gas flow, microwave discharge lamp that provides atomic resonance radiation at a number of different wavelengths in the vacuum ultraviolet (vacuum-UV) depending on the gases used in the lamp.3 In the present study a mixture of 13% hydrogen in helium at a pressure of 1.5 Torr gives the maximum intensity of Lyman-a radiation (10.2 eV). A small amount of radiation ( 160 nm). The relative H and D concentrations are monitored by time-resolved, atomic-resonance fluorescence spectroscopy with pulse counting ( 2 ) Marshall, P.; Fontijn, A . J . Chem. Phys. 1986, 85, 2637.
0 1989 American Chemical Society
