J. Phys. Chem. 1995,99, 15086-15092
15086
Rate Constants for the Reactions CF2(X1A1, v2 = 0 and 1) PhotodissociationLaser-Induced Fluorescence Technique
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Katsuyoshi Yamasaki,* Aki Tanaka, Akihiro Watanabe, Keiichi YokoyamaJ and Ikuo Tokue Department of Chemistry, Niigata University, 2-nocho, Ikarashi, Niigata 950-21, Japan Received: April 4, 1995@
Dibromodifluoromethane, CF2Br2, mixed with oxygen atoms was irradiated with an ArF (193 nm) laser. Difluoromethylene, CFZ(X'AI,u l V 2 u 3 = 000 and ulu2v3 = 010) was detected with laser-induced fluorescence via A'Bl %'Al 020-000 and 030-010 transitions. Rate constants for the reactions CF:! (X'AI, v2 = 0 and 1) 0 have been determined to be [ 1.63 f0.3(20)] x lo-'' and [ 1.44 f 0.3(20)] x lo-'' cm3 molecule-' s-' at 294 f 1 K, respectively. No distinct effect of vibrational excitation on the reaction can be rationalized on the basis of theoretical calculation. The rate constant for vibrational relaxation, CF2(X1A1, v2 = 1) He CF~(%'AI,u2 = 0) He, was also found to be [1.0 f 0.3(20)] x cm3 molecule-' s-I, which is smaller than that in the electronically excited state CFZ(A'BI,u2 = 1 u2 = 0) by an order of magnitude.
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1. Introduction Chlorofluorocarbon (CFC) and bromofluorocarbon (halon) have been well-known to be causes of the ozone depletion in the stratosphere.' Chlorine atoms, for example, emitted from CFC in the photolysis by solar flux in the stratosphere initiate the chain reaction called the C10, cycle. The counterproducts, halocarbon radicals, are also produced in the photolysis. Trihalomethyls or dihalomethylenes are generated by the release of one or two halogen atom(s) from CFC's and/or halons. The reactions of trihalomethyls with such stable molecules in the atmosphere as oxygen ( 0 2 ) have been intensively studied.2 A few propensities for the reactions have been found: trihalomethyls, CFmX, (X = halogen atom, m n = 3, m = 0-3), simply recombine with oxygen, producing trihalomethyl peroxide (CFmX,02);the reaction rate constants are larger for the molecules containing more fluorine atoms. Dihalomethylenes, CFmXn (m n = 2, m = 0-2), on the other hand, are nonreactive with stable molecules such as 02, Nz, NO, and N20 at around room temperature; thus, they appear to be inert. Only the upper limits of the rate constants for the reactions have been reported so far.3-6 Reactions of atomic species with dihalomethylene CFmX,, however, have recently been reported to be relatively fast, and one of the reaction products is a halogen atom originating from the dihal~methylene.'-~ The halogen atoms emitted in the reaction could also be an initiator of the C10, (or BrO,) cycle in the ozone depletion. Accordingly, precise measurements of the rate constants for the reactions of dihalomethylene with atomic species are now valuable to propose as an additional mechanism for the production of halogen atoms followed by the ozone depletion cycle. The photolysis in the stratosphere is due to the solar flux in the so-called "solar window" whose wavelength is over 185220 nm between the Schumann-Runge bands (1-= 185 nm) of 0 2 and Hartley bands (1 > 220 nm) of O3.I0 Since the dissociation energies of C-C1 and C-Br bonds are small, large excess energy can be deposited into the molecules generated in the photolysis even with a release of two atoms, e.g., EeXce,, = 45 kJ mol-' for the photolysis CF2Cl2(CFC-12) hv(200 nm) CF2 2C1.
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* To whom correspondence should be addressed.
' Present address:
Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki, 319-1 1 Japan. Abstract published in Aduance ACS Abstracts, September 1. 1995. @
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The vibrational relaxation rate is fast even in the stratosphere (
