O(3P)-C2F,
The Journal of Physical Chemistry, Vol. 83, No. 16, 1979
Reaction and CF2(3B,) Emission
2065
Mechanism of Oxygen (3P) Atom Reaction with Tetrafluoroethylene and Quenching Processes of the Emission of CF2(3B,) Seiichlro Koda Department of Reaction Chemistry, Faculty of Engineering, the University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan (Received March 7, 1979)
The reaction mechanism of ground state oxygen atoms with tetrafluoroethylene was elucidated in terms of kinetic analysis of the emission from CF2(3B1)produced in Nz0-Hg-C2F4 mixtures on irradiation with a low-pressure mercury lamp. The exciting lamp was operated in a repetitive pulse mode, and the emission was monitored by means of a boxcar mode photon-counting system to determine its time profile after excitation. The emission intensity was found to be proportional to the exciting light intensity. The emission showed a first-order decay soon after an initial rise. The decay constant was proportional to the pressure of CzF4, from which the bimolecular rate constant for the reaction of O(3P)with CzF4was derived to be (7.1 f 0.2) X cm3 molecule-l s-l (293-303 K). The time profile of the emission could be satisfactorily reproduced in terms of a proposed reaction model, where the principal reaction produced CFz(3Bl)and CFzO. Assuming that no long-lived intermediate was formed in the above elementary reaction, the radiative decay constant of CF2(3B1) multiplied by its quantum yield was estimated to be around 1s-l. The quenching rate constant by O2 was 4.1 X (f2070) cm3molecule-l s-l; relative quenching rates by 02,Xe, NO, C3H8,LC4H10, C2H4, C3Hs,and CzF4 were 1:~0.017:~12:0.16:0.45:2.2:5.4:0.1.If some long-lived intermediates were formed, the above quantities should be interpreted differently. Systems with CzF3Hand with NO in place of CzF4 were also investigated.
Introduction The ground state oxygen atom (O(3P))reactions with olefinic hydrocarbons have been widely investigated by numerous researchers. The rate constants1 and reaction products2i3 for many reaction systems are already well known. The details of the reaction mechanism, however, especially concerning the possibility of production of electronic-excited products or intermediates and the partitioning of the available energy in reaction products are still not fully elucidated. Previous work of S l ~ a n e , ~ M ~ D o n a l dLin,6-8 ,~ and Nakamura and KodagJosuggests that the usual relaxation process is by rapid intersystem crossing from the triplet state of the primary addition products formed by O(3P) addition to alkenes and, sometimes) alkynes, to yield the corresponding highly vibrationally excited ground state products. This rapid intersystem crossing is possibly the reason why no conclusive evidence, with the exception of the one case described below, has yet been obtained for primary formation of triplet products. The sole reaction system where spectroscopic evidence for triplet product formation has been obtained is that with tetrafluoroethylene (CZF4).l1 The intrinsic reason for this may be found in the weakness of the C=C double bond (77 kcal mol-l)12 of CzF4,which may allow a rapid dissociation of the primary adduct to the fragments difluoromethylene (CF,) and difluoroformal (CF20)before intersystem crossing can compete. In this paper, we report the kinetic behavior of the emitting species in the 0(3P)/C2F4system which has been tentatively identified as CFJ3B1) on the basis of its emission spectrum.'l The possible chemistry of CF2(3B1)will also be discussed. In the present work, we pursued the kinetics of the emission produced in a N20-Hg-C2F4 mixture on irradiation with a low pressure mercury lamp. The lamp was operated in a repetitive pulse mode, and the emission was monitored by means of a boxcar mode photon-counting system to determine its time profile after excitation. This technique) in a similar way to the modulation technique of Atkinson and others,13J4 may be applicable to measurements of the reaction rates of O(3P),as will be described in the Appendix. 0022-3654/79/2083-2065$0 1.OO/O
Experimental Section The experimental setup is shown in Figure 1. Ground state oxygen atoms were generated via mercury-photosensitized decomposition of NzO in the emission cell, through which NzO (0.4-2.0 torr) containing Hg, CzF4, and other additives in smaller amounts slowly flowed. The laboratory constructed low pressure mercury lamp was powered by a microwave generator of 2450 MHz (K. K. Ewig Shokai, MR-I11 S) which was operated in a repetitive pulse mode; the pulse shape and the repetition rate were controlled by an external function generator (Exact Electronics, Inc., Model 190). The 253.7-nm line was selected by an interference filter (transmission maximum a t 253 nm with a half-width of 23.5 nm). The intensity of this monochromatic radiation was estimated to be around 2.5 X 1014photons s-l for the effective illuminated volume (140 cm3) of the emission cell, on the basis of the N 115). Thus the mean rate amount of N2 produced (aNz of O(3P) production on irradiation might be about 2 X 10l2 molecule cm-3 s-l. The lamp emission was monitored by a solar blind phototube (H.T.V. R765). The time profile of the exciting light was almost a rectangular form, and the 90% rise and fall times were within 0.15 ms. The averaged duration of the light pulse was usually 0.6 ms and the repetition interval was 13.8 ms. The resulting emission from the mixture was introduced through a low-resolution monochromator (Nikon P-2501, or filter combinations, to a photomultiplier. The monochromator was used only for observation of the spectrum under continuous irradiation with a H.T.V. R456 photomultiplier. The filter combinations were used for measurements of the time profile of the emission. The combinations were interference filter centered at 560 nm (Toshiba KL-56)-glass filter (Toshiba Y-52)-photomultiplier (H.T.V. R456) or glass filter (Y-52)-photomultiplier (H.T.V. R585). The sensitive spectral region for the former set was approximately 540-580 nm and for the latter, 50MOO nm. No differences were found in the time profile analyses of the emission by using these combinations, and the latter set was mostly used. The photon pulses from the photomultiplier were fed into the photon counter 5C1 (Brookdeal Co.) which was operated in a boxcar mode by
0 1979 American Chemical Society
2066
The Journal of Physical Chemistry, Vo/. 83, No. 16, 1979
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P
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Figure 1. Experimental setup: Ma, MKS Baratoron pressure meter; M, microwave cavity; MG, microwave generator; PT, phototube; PM, photomultiplier; S,,synchroscope; Osc, function generator; Pc, photon counter; f, filter; I, lens; DB, dry ice-methanol bath; Hg, mercury reservoir; Add, additive gases.
use of the synchronous sampler 5C21. The periodic sampling gate of 0.2-ms width was delayed from the exciting light by a time selected to cover the interesting time region. The Pyrex emission cell had a total volume of around 400 cm3, and was equipped with appropriate light horns and two windows (CaF, or Nippon Sekiei EQ quartz showed no difference) at right angles to each other. The N 2 0 flow containing additives such as C2F4 was split into two. One stream was led to a U-type trap immersed in a dry ice-methanol bath to remove contaminating Hg and then carried onto the inner surface of the exciting window. The other flow was carried over the mercury surface in a mercury reservoir and then introduced to the cell from the opposite side to the exciting window. By use of this arrangement, it was possible to obtain relatively stable and reproducible measurements of the emission intensity for a long period. The averaged linear flow rate around the center portion of the cell was estimated from the drop in pressure upon interruption of the flow supply to be around 2-3 cm s-l when the NzO pressure was 0.4-2.0 torr. The cell pressure was measured by use of a Baratoron pressure meter (MKS Instruments, Type 210). Partial pressures of individual additive gases co e estimated from the increase of the cell pressure ca y their addition. The increment was not equal to the partial pressure itself, because the volume-pumping rate of our system was not independent of the cell pressure. The necessary correction factor was derived from the observed relationship between the pumping rate and the cell pressure. In some cases, a part of the flow was sampled and quantitatively analyzed. It was found that these two different estimations agreed with each other within f15%. The above large error range could not be improved, but the relative errors were considered to be smaller. Hydrocarbon samples (Takachiho Shoji Co., Reagent grade), 1,l-difluoroethylene, and trifluoroethylene (the latter two, PCR Inc.) were used after purification through several freeze-thaw cycles. Tetrafluoroethylene of highest grade was provided by Mitsui Fluorochemicals Co. and was purified as above. No impurities were found by gas chromatographic analysis with Porapak Q column at 150 "C. NO (Takachiho Shoji Co., Reagent grade) was passed through a cooled silica gel column to remove traces of NO2 and purified by bulb-to-bulb distillation. Cylinder NzO, Ar (Takachiho Shoji Co., Reagent grade), and O2 (Suzuki Shokan Co., Pure grade) were used without further purification.
Results Emission Spectrum and Its Identification. When a N20-Hg-CzF4 mixture was irradiated with the 253.7-nm light, a band structured emission was observed as shown
Seiichiro Koda
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500
660nm
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Figure 2. Emission spectrum from a N,O-Hg-C,F, mixture under continuous irradiation with a low pressure mercury lamp. (a) N,O pressure, 0.83 torr; C2F4 pressure, 18 mtorr; band path width, 3 nm. The exciting 254-nm light is isolated with an interference filter. A H.T.V. R456 photomultiplier is used. (b) Spectra observed under the irradiation without the interference filter; lamp lines overlap the missed spectral region. N,O pressure, 1.5 torr; CpF4 pressure, 20 mtorr; band path width, 1.8 nm.
in Figure 2. The pressure of Hg was not determined, but should be less than the vapor pressure at a room temperature (
