CF2 Emission During Vacuum Ultraviolet Photodissociation of CF2Br2 BY CHRISTOPHER A. F. JOHNSON" AND (IN PART) MRS. HILARY J. Ross Chemistry Department, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS Received 22nd June, 1978 Photolysis of CF2Br2 at wavelengths shorter than 135 nm leads to the production of electronically excited CF2. At low pressure fluorescence is observed over the range 230-340 nm. Considerable vibrational excitation of the CF2 is evident. The CF2 fluorescence is quenched by H2, D2, CO and C02with rate constants of 5.6 x lo9, 2.7 x lo9, 4 x 1O'O and 3.2 x lo1* dm3 mol-' s-l, respectively. The ultraviolet emission spectrum of the CF2 radical has been examined by a number of workers since its original observation by Venkateswarl~.~ The main emission system lies between about 240 and 340 nm, and has also been examined in absorption under high resolution by MathewsY6 the transition being assigned lB1-'A 1 .Bands at wavelengths longer than 350 nrn have been noted by some groups, and it is possible that some or all of these bands are due to the 3B1-1A1 transition of CF2.3 Gas phase emission studies of CF2 to date have involved electrical or microwave discharge excitation of low pressures of various fluorinated hydrocarbons, while matrix isolation studies employed photolysis of CF2N2 or CF2H2 as the source of CF2, followed by direct U.V. excitation of the matrix isolated CF,. We report the observation of electronically excited CF2 produced by vacuum ultraviolet photo- dissociation of CF2Br2. EXPERIMENTAL The apparatus and technique have been described previously.' Pressures (0-10 Torr, 0-1330 Pa) were measured with a capacitance manometer, and higher pressures (to atmos- pheric) with a resistance strain gauge transducer.Commerical CFzBrz was distilled before use and stored under vacuum. Owing to the weak signal and rapid accumulation of polymeric material on lamp windows, experiments were carried out under low resolution with slit widths of 0.75 mm (band pass w1.2nm) and scan rates of 5 or 10nmmin-'. During quenching experiments the slits were opened to 1.5 111111, and the monochromator set to the broad maximum in the emission (-260 nm). RESULTS AND DISCUSSION During photolysis of CF2Br2 at 121.6, 123.6, 129.5 and 130.6nm emission was observed at x230-350nm. None was observed on photolysis at 147nm (Xe/ sapphire).Although little structure could be resolved, the emission appeared to consist of many closely overlapping bands. The maximum intensity occurred at a shorter wavelength with increasing energy of the incident radiation, from x280 nm at 129.5nm to x255 nm at 121.6nm. There was also a small shift to the blue in the short wavelength limit (< 10 nm) with the higher energy radiation. We assign the emission to CF, for the following reasons. (i) The emission has all the general features of wavelength range and intensity distribution of CF2('B1-'A 1) 2930C. A . F. JOHNSON AND H . J . ROSS I l l 293 1 wavelengthlnm FIG. 1.-Lower trace: emission observed (uncorrected for system response) during the 123 nni photolysis of 0.06 Torr CF2Br2. Upper trace : emission observed during 123 nm photolysis of 0.08 Torr CF2ClN0. Some bands due to NO are also produced.The " baseline " of the upper curve is indicated. i I I I I I I I I 380 360 340 320 300 280 260 210 220 wavelength/nm FIG. 2.-Lower trace : emission observed during the 123 nm photolysis of 0.06 Torr CFzBr2. Upper trace : emission observed during the 123 nm photolysis of 0.06 Torr CF2Br2+720 Tom He. The wavelengths of some known CF2 bands are indicated.6 The intensities of the two spectra are not directly comparable.2932 emission observed by others. (ii) CF,Br, is known to give high yields of ground state CF, when photolysed at longer wavelengths.8 (iii) We obtain a very similar emission on irradiation of CF,ClNO at 123.6 nm, although in this case it is overlapped to some extent by bands due to NO.Only CF, is common to the two molecules. Spectra for comparison are shown in fig. 1. (iv) Addition of He at several hundred Torr causes some rotational and vibrational relaxation. Under these conditions some bands are more clearly resolved (fig. 2) and our estimated wavelengths are similar to literature data for CF2. There are two processes that could give rise to electronically excited CF,, reactions (1) and (2). u LTR A VIO LET P HOTODI sso CI AT ION OF CF,Br, CF2Br2 +hv -+ CF2(lB1) + 2Br[(4~~)~P;] CF2Br2 +hv + CF2(lB1) +Br,(XICT). (1) (2) Using the method and tabulations of Benson to estimate C-Br bond energies, we derive threshold wavelengths of w 135 and x 172 nm for processes (1) and (2) respectively, assuming CF2lB1 is produced in the v = 0 level.Allowing for the vibrational excitation observed by us in photolysis at 130 nm, we estimate from our data a threshold wavelength of 3 138 nm. We therefore conclude that it is reaction (1) we observe in these studies. Vibrational excitation in the bending mode is not surprising in view of the difference between the FCF angle in CF,, lB1 (122.3°)6 and that in the tetrahedral CF2Br, molecule. The short wavelength limit of the fluorescence implies vibrational levels are populated at least to v = 12 at the shorter photolysis wavelengths. 1.0 0.8 0.2 Y I I I I I I I 0.10 0.20 0.30 0.40 pressure/Torr FIG. 3.-Variation of fluorescence intensity with pressure of CF,Br,. The solid curve is a plot of the expression l2 with Pmax set equal to 0.072 Torr.This equation describes the intensity variation with pressure 4lIfmax = ( f ' / P , a x ) ~ X P (1 -f'/f'max) when quenching may be neglected. There are few literature data on the reaction of electronically excited CF2, although it is known that ground state CF2 is unreactive in comparison with, e.g., methylene. Dimerisation [reaction (3)] is the dominant process for removal of CF2 in most gas-phase studies, with k3 21 2.1 x lo7 dm3 mol-l s-l at 298 K.l0 Reaction of ground state CF2 with olefins or 0, is slow l 1 in comparison with reaction (3) CF2 +CF2 + CZF4. (3)C . A . F. JOHNSON AND H . J. ROSS 2933 We have carried out a limited number of quenching experiments using 123 nm incident radiation. For these measurements a wavelength of x260nin was used, with a band pass of w2.5 nm.Over the limited pressure range in which fluorescence was observed (pressures <0.4 Torr), no self-quenching was detected. Fig. 3 demon- strates that the data closely fit the behaviour expected on the basis of no quenching,12 the intensity variation being due entirely to absorption of radiation between the lamp window and the viewing region. Addition of helium at pressures up to 700 Torr resulted in a small reduction in fluorescent intensity at the monitoring wavelength ( ~ 3 0 % at 700 Torr). The complete spectrum obtained under these conditions indicated that the fluorescence maximum had shifted to longer wavelengths with the total intensity unchanged. We assume this to be a consequence of some rotational and vibrational deactivation of the emitting CF,.In all other quenching measurements we assume rotational/ vibrational deactivation of the same magnitude as with helium, and the derived rate constants are corrected accordingly. Quenching half-pressures were determined from Stern-Volmer plots for H2, D2, CO, CO, and N,, a constant pressure (0.05 Torr) of CF,Br, being used throughout. The experimental half-pressures are listed below. Rate constants were derived assuming a fluorescence lifetime for CF, IB1 of 31 ns. Allowance has been made where necessary for absorption of incident radiation by the quenching gas. D2 H2 co coz NZ P, /To r r 190 98 14.4 12.6 1130 k,/109 dm3 mol-1 s-l 2.7k0.15 5.6f0.3 40f2 32f3 <0.1 Quenching due to nitrogen is slow.Indeed, the observed " quenching " could be satisfactorily explained by a rate of vibrational deactivation of CF, 'B1 only about 20 % higher than that observed with helium. Therefore we emphasise that the derived k, for N, is an upper limit. Quenching by H,, D,, CO and C02 is very much faster, the calculated rate constants being two or three orders of magnitude larger than that for dimerisation of ground state CF, [reaction (3)]. The reactivity of electronically excited CF, is comparable with that of singlet methylene itself. For example, Laufer and Bass I3 have reported rate constants for reaction of lCH2 with NO, CO and CH2C0 of 2.4 x lolo, 5.4 x lo9 and 1.9 x 1O1O dm3 mol-l s-l, respec- tively. It would have been of interest to extend these quenching studies to a wider range of molecules.However the present technique limits the quenching molecules to those with small extinction coefficients at 123 nm. Besides the emission attributed to CF, lB1, we also observed weak emission centred at about 285 nm. This emission was very rapidly quenched by H2 or D,, and became sharper in the presence of He or NZ. It is probably the D(lX;)- B(3JI&J transition of molecular bromine, which has been reported in emission by Venkateswarlu and Verma,14 and in absorption by Briggs and Norrish.lS As the D state of Br, lies at 51 800 cm-l above the ground state, we estimate the threshold for process (4) CF,Br, +hv -+ CF,(lAl)+Br,(DIZl) (4) to be at about 137 nm. This value is consistent with our observations of the emission in photolysis at 121, 123 and 130 nm but not at 147 nm.This emission occurs over a relatively narrow range of wavelength, implying very little vibrational excitation of the Br,D state (the B state has a very shallow minimum). It should be noted that the incident radiation is of insufficient energy both to excite Br, to the2934 ULTRAVIOLET PHOTOD I SSOCI A T 1 0 N OF CF2Br, D state and CF2 to the lB1 state at the same time. Therefore process (4) is in competition with process (1) and is apparently of less importance. C . E. Smith, M. E. Jacox and D. E. Milligan, J. Mol. 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