Vacuum UV fluorescence excitation spectroscopy of CF4 and CF3Br in the range 45–140 nm

Volume 188, number 3,4

CHEMICAL PHYSICS LETTERS

10 January 1992

Vacuum UV fluorescence excitation spectroscopy of CF, and CF,Br in the range 45-140 nm J.C. Creasey, P.A. Hatherly I, I.R. Lambert ’ and R.P. Tuckett 3 SchoolofChemistty,

UniversityolBirmingham. Edgbaston, Birmingham 815 ZTT, UK

Received 16 September 1991; in final form 16October 1991

Fluorescence excitation spectra of CF, and CFIBr have been measured in the range 45-95 nm and 50-l 40 nm respectively using synchrotmn radiation. Both resonant and non-resonant peaks are observed, corresponding to neutral fragment (e.g. CF,) and parent ion (e.g. CF,+) emission. Analysis of the fluorescence has shown that bands from CF, are due to CF;, CFZ ,&‘B, and CF,+ e 2Tz. Bands from CF3Br are due to CR, CF, A ‘B, and possibly CF,Br+ i? ‘A,. The radiative lifetimes ofall the fluorescing bands have been measured. Some of the CF,Br bands show biexponential behaviour. These results are interpreted by comparison with previous photoabsorption and electron energy loss spectra.

1. Introduction The halogenated methanes are important compounds in a variety of chemical processes, ranging from stratospheric ozone depletion [ 1] to the dry etching of silicon wafers for semi-conductor devices [ 21. A knowledge of their interaction with UV and vacuum UV photons is important to an understanding of the physics of such processes. The absorption spectrum of CF4 in the range 60-100 nm was measured by Cook and Ching [ 3 ] and the data have been extended down to below 20 nm by Lee et al. [4]. There is extensive vibrational structure between 55 and 60 nm [ 41, but below 55 nm the spectrum is essentially continuous. More recent photoabsorption work by Lee et al. using a synchrotron as the continuum source has confirmed these earlier studies [ 51, and the bands at wavelengths greater than 60 nm have been assigned to Rydberg transitions following the assignments of the electron energy loss spectrum of CF, [ 61. Valence transitions are not observed, and the photoabsorption cross section is very small (< lo-*’ cm2) for A> 110 nm [5]. Fluores’ Present address: Department of Physics, University of Reading, Whiteknights, Reading RG6 2AF, UK. * Present address: School of Chemistry, University of Bristol, Canto&s Close, Bristol BS8 ITS, UK. ’ To whom correspondence should be addressed.

cence following vacuum UV absorption of CF, was first observed by Cook and Ching [ 31 but they were not able to identify the emitter(s). Lee et al. [ 51 observed fluorescence following photoexcitation at 5 5, 77 and 92 nm. By analysis of the dispersed fluorescence spectra and for other reasons, they assigned the emitters to the C2T2 excited electronic state of CF,+ , the A ‘B, first excited state of CF2, and two excited states of CF3 respectively. We have reported the fluorescence excitation spectrum of CF, in the range 40-60 nm [ 71, and both the threshold for fluorescence and the form of the excitation function confirms Lee et al.‘s assignment that the emission is due to the parent ion of CF,. In this paper we extend the photon range to 100 nm, and by measuring the radiative lifetime of the emitting bands confirm Lee et al.‘s assignments for the two higher wavelength bands. Our spectra are of substantially improved quality compared to Lee et al.3 but absolute values of the fluorescence cross section are not obtained. We have reported the fluorescence excitation spectrum of CF3H and CF,Cl in the range 40-l 10 nm [ 8,9], As with CF,, at higher wavelengths bands were observed which, both by the form of the excitation function and by their radiative lifetimes, could be assigned to emission from the CF, and CF2 fragments. At lower wavelengths bands could be assigned to emission from excited electronic states of

0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

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CF,H+ and CF$l+, and the lifetimes were also measured. In this paper we extend these measurements to CF,Br over the photon range 40-140 nm. The photoabsorption spectrum of CF,Br in the vacuum UV has been measured by Doucet et al. [ IO,11 ] (60-200 nm), Suto and Lee [ 121 (106-155 nm) and Lee et al. [ 131 (50-106 nm). Within experimental error the different measurements are in good agreement. Fluorescence has been observed in both of Lee’s studies with bands at 117,94,84 and 6 1 nm. By dispersion of the fluorescence these bands are assigned to emission from CF3, CF2 A, CF, A, and an excited state of CF,Br+ respectively_ We report here improved fluorescence excitation spectra for CF,Br and lifetime measurements for all these bands. Whilst we agree with the assignments for the first three bands, the assignment of the 61 nm band to parent ion fluorescence is not so certain and may be incorrect.

2. Experimental The experiments are performed at the SERC synchrotron source at Daresbury and details of the apparatus are given elsewhere [ 7,141. Briefly, the synchrotron radiation is dispersed by a 1 m Seya normal incidence vacuum UV monochromator operating in the range 140-40 nm (9-31 eV), and the photons enter a stainless-steel chamber by a glass capillary. The best attainable working bandpass of the Seya is ~0.05 nm. The VUV radiation crosses an effusive gas jet of CF, or CF,Br at right angles, and the fluorescence produced is detected undispersed by an uncooled EM1 9883 QB photomultiplier (PM ) tube (sensitivity range 190-650 nm) operating in the photon counting mode. A broad-band or cut-on tilter in front of the PM tube can isolate the wavelengths of fluorescence detected. The maximum ambient pressure during an experiment is 1O-4 Torr, and the photon flux is measured by an A1203 photocathode for normalisation of the induced fluorescence. When the synchrotron is operated in the multibunch or quasi cw mode, excitation spectra for fluorescence are recorded. The wavelength range is spanned by two gratings mounted back-to-back in the Seya on a single kinematic amount, the “high-energy” (40-80 nm) and the “low-energy” (70-140 224

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nm). Data are collected via CAMAC electronics by a dedicated LSI 1l/23 minicomputer and transferred to the Daresbury mainframe computer (Convex C220) for analysis. When the synchrotron is operated in the single-bunch or pulsed mode, the storage ring gives 200 ps pulses every 320 ns and radiative lifetimes (in the range l-100 ns) can be measured. A fast Mullard 20204 photomultiplier tube cooled to -20°C is used, and decays are collected by standard techniques using a time-to-amplitude converter (Ortec 567) and a multichannel analyser (Canberra Series 35 Plus). Lifetimes are measured at different excitation energies. Data are transferred via the Convex C220 to the University of Birmingham mainframe computer for analysis with a multi-exponential fitting programme. Recent improvements to the efficiency of the Seya meant that when these experiments were carried out the photon flux was about 5-10 times greater than when our earlier measurements on CF4 and CF3H/CF3Cl were made. It is this enhancement which has allowed us to obtain fluorescence excitation spectra for CF4 and CF3Br over a much wider wavelength range and at improved resolution than reported before. We have also observed at improved resolution the vibrational structure between 55 and 60 nm in the photoabsorption spectrum of CF,. However, whereas Lee et al. [4] observed this structure by direct absorption, we observe these transitions by autoionisation of the excited Rydberg state(s) into the CF: ionisation continuum, In this experiment the intersection of the tunable photon beam and the effusive gas jet occurs at the centre of a symmetrical time-of-flight mass spectrometer. One drift tube detects electrons, the other ions separated by their different times of flight. Although the primary aim of this experiment is to obtain photoionisation mass spectra of parent and fragment ions [ 14,15 1, the total ion and electron count rates are also recorded as a function of photon wavelength. Data collect via CAMAC electronics and the LSI minicomputer as in the fluorescence excitation experiments, and are analysed on the Convex C220.

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3. Results and discussion 3.1. Fluorescence excitation spectrum of CF., 45-95 nm Fig. la shows the fluorescence excitation spectrum of CF., between 13 and 19 eV (95-65 nm) recorded with the low-energy grating. The bandpass is 0.4 nm, the spectrum is recorded with no optical filter in front of the PM tube, and the spectrum has been normalised to the synchrotron flux. Peaks are observed at 13.74eV (90.2 nm) and 15.89 eV (78.0nm),and

Fig. 1. (a) Fluorescence excitation spectrum of CF, between 13 and 19 eV: the EMI 9883 QB photomultiplier tube is used untiltered. (b) Spectrum between 20.5 and 27.5 eV: a Schott UGS filter is now used with the photomultiplier tube.

10 January 1992

the overall shape of both peaks is characteristic of a dissociative process where an excited (usually Rydberg) state of a neutral molecule is populated followed by (pre-)dissociation to an emitting fragment. Thus for each peak the excitation spectrum increases from threshold as the photon energy scans through the Franck-Condon region, reaches a peak at the Franck-Condon maximum, and recedes to the baseline. This is especially clear for the 13.74 eV peak. These peaks have been observed previously by Lee et al. and absolute values for the fluorescence cross sections obtained [ 51, but our spectrum is at superior signal-to-noise. From our spectrum it is not possible to say whether the CF,, Rydberg states are totally repulsive or predissociated bound states. We comment that the former is likely to give a broader fluorescence excitation spectrum than the latter, where the spectrum will depend critically on the precise region where the predissociated state is crossed. Attempts to disperse the fluorescence from each peak through a small monochromator were unsuccessful, but by use of a large range of cut-on and broad band filters we have determined approximately the range of wavelengths over which each fluorescence band occurs. Thus the 13.74 eV peak gives rise to fluorescence over the whole range of sensitivity of the PM tube; there is substantial fluorescence for ;1> 400 nm, but negligible fluorescence in the range 300-400 nm (since the signal falls to near zero with a Schott UGl filter). These results are compatible with the emitter being the CF3 radical with the emission occurring in two bands, one in the UV (200300 nm) and one in the visible (400-700 nm) [ 16 1. If the UV band is isolated (using a Schott UG5 filter) the excitation spectrum is unchanged from fig. la, showing that there is negligible difference in the fluorescence excitation function near threshold for the two CF, bands. This is discussed further later. The 15.89 eV peak shows a very different behaviour with the optical filters. Now most of the fluorescence occurs in the range 190-400 nm, and this is compatible with the emitter being the A ‘B, first excited state of CF2 [ 17 1. Our results agree with the dispersed spectra of these bands obtained by Lee et al. ]13]. It is instructive to compare this spectrum of CF, with its photoabsorption and electron energy loss spectra [ 4-61. The valence molecular orbitals have 225

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symmetry ...(4a,)2(3t2)6( le)4(4t2)6( lt,)6, and all peaks in the vacuum UV are assigned to Rydberg transitions. Thus the CF; peak at 13.74 eV can be correlated with excitation of the (4t,)-‘(3s) and (lt,)-‘(3~) states, whereas the CF2 5 state emission at 15.89 eV correlates with the (4tz)-‘(36) and ( le)-’ (3~) states of CF, [ 61, Transitions to all these states from the ground state of CF4 are allowed. It should be noted that our experiment only detects those Rydberg states of CF, which dissociate specifically to a fluorescing excited state of a fragment, whereas absorption and electron energy loss spectra will observe all transitions allowed by the selection rules. The internal states of the CFf and CF2 A fragments are not known, but it is likely they have substantial rovibrational energy. This is because the thermodynamic thresholds for forming these states are much lower than the experimental thresholds; CF,-+CF: t F and CF,-+CF, A+Ft F have thermodynamic thresholds of 11.9 and 13.8 eV respectively [ 7,12,17], to be compared with experimental thresholds of 13.2 and 15.0 eV (lig. 1). Fig. lb shows the flux-normalised fluorescence excitation spectrum of CF, between 20.6 and 27.5 eV (60-45 nm) recorded with the high-energy grating. The bandpass is now 0.05 nm, and a Schott UC5 filter (transmitting x240-400 nm) is used with the PM tube. The threshold for fluorescence is 2 1.70? 0.02 eV which is in exact agreement with the adiabatic IP of the c ‘T2 state of CF: [ 181, and the form of the excitation function is now characteristic of a photoionisation process where fluorescence is observed with photons of energy greatly in excess of threshold. Such a process is non-resonant because the electron can carry away the excess energy, and the emission intensity as a function of energy is governed essentially by the variation of the partial ionisation cross section of the t? 2Tz state with photon energy. Spectra were reported at lower resolution both in our earlier work [ 7 1, and by Lee et al. [ 5 ] who measured an absolute fluorescence cross section for this emission. Weak (resonant) structure is now observed above the fluorescence threshold, with peaks at 22&, 24.00, 24.46, 24.&, 24.73 and 24.g4 eV. They are assigned to Rydberg states of CF_,lying above the f? state ionisation potential which converge on the bZA, stateoftheion (adiabaticIP=25.12eV [ 191). 226

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These states either dissociate (producing neutral fragments like Cv or CF, A which fluoresce in the same region as the UG5 filter), or autoionise into the c state of CFZ and are observed in fluorescence as CFZ C-A and c-.% emission between 200 and 400 nm [20]. ‘Ihe former mechanism is preferred because there is no peak in the electron count spectrum at the energy of the strongest Rydberg peak, 22.8 eV (see section 3.2). The first three peaks can be fitted to the usual Rydberg equation yoo- Y= RH/ ( n -8) 2 (where yDois the IP limit (25.12 eV) and R, the Rydberg constant ( 13.59 eV) ) to give n-6 values of 2.4, 3.5 and 4.5 respectively. These states arise from electron excitation from the C 2s-F 2s/2p obonding molecular orbital of a, symmetry, and only transitions to p or d Rydberg orbitals formally are allowed. These states may therefore be the 3p, 4p and 5p members of a p Rydberg series with quantum defect J-0.5. The last three peaks have n-S values of 5.0, 6.0 and 7.0, and do not fit into higher members of this p series so well. They may be members of a different (e.g. d) Rydberg series. Similar structure has been observed above the fluorescence c *Tz state of SiCl: [21]. 3.2. Absorption spectrum of CF, 55-62 nm Fig. 2 shows the variation of total electron count from CFI excited by photons with energy between 20.0 and 22.5 eV (62-55 nm) at a resolution of 0.05 nm. As explained earlier this is similar to an absorption spectrum, except that transitions to the Rydberg states of CF, are now detected by autoionisation into the CF: continuum rather than by direct absorption. Our spectrum has a factor of two better resolution than the absorption spectrum reported by Lee et al, in this energy range [4]. We observe vibrational structure with spacings ranging between 610 and 1000 cm-‘, and the peak positions are given in table 1. Lee et al. assigned this structure to sequences involving the ZJ, C-F symmetric stretching vibration in three different Rydberg states which converge on either the c ‘T2 or i) ‘A, states of CF,f _ The Y, vibration has frequency 729 and 800 cm-’ in CF: c 2T2 and b *A, [22] (not the values of 645 and 730 cm-’ respectively assumed by Lee et al.), and both the vibrational spacing and intensity envelope of a Rydberg state should mimic that of the

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80000

75000

s '~70000 E z f

65000

5 T

55000

2010

I 21.0

I 20.5

Excitation

I 21 5

energy

, 22.0

/

2 2.5

eV

Fig. 2. Absorption spectrum of CF, between 20.0 and 22.5 eV at

a resolutionof ~0.02 eV. Photon absorption is detected indirectly by autoionisation into the CF,+ continuum. Table I Wavelengths (nm) and energies (eV) of the absorption peaks in CF., between 55 and 61 nm observed by autoionisation into the CF: continuum 1 (nm)

E (eV)

55.57 55.76 55.95 56.19 56.41 56.64 56.86 57.13 57.42 57.67 57.95

22.30 22.22 22.15 22.05 21.97 21.88 21.79 21.69 21.58 21.49 21.38

58.22 58.56 58.82 59.10

21.29 21.16 21.07 20.97

59.37 59.60 59.84 60.09 60.37 60.63

20.87 20.79 20.71 20.62 20.53 20.44

Separation (cm-‘) 611 612 738 706 711 764 775 857 767 834 807 1003 754 786 778 639 687 703 750 707

ionic state to which it converges. The C2Tz state photoelectron band structure is quite extensive peaking at u, = 5 [ 18 ] (corresponding to an increase in C-F bond distance upon ionisation of 0.08 A

10 January 1992

[ 231)) whereas the D *A, state structure is narrow with ul = 0 as the most intense peak [ I9 1. The peaks between 20.4 and 21.2 eV have an average spacing of 7 15 cm- ‘, and could be a series converging on the c 2T2 state of the ion (IP = 2 1.70 eV ). Presumably the peaks with energy > 2 1.7 eV correspond to vibronic levels converging on the B ‘A, state of CF: , but the average spacing of these peaks is substantially less than 800 cm-‘, the value of Y, in CF,+ 1)*A,. There are certainly more than one Rydberg series present, but we do not believe it is possible to assign peaks to particular series with total con& dence. One possible complication is that in an autoionisation mechanism as observed here, perturbations between discrete states and the ionisation continuum may change both the expected vibrational spacing and the intensity envelope, This part of the absorption spectrum of CF, warrants investigation at very much greater resolution. 3.3. Fluorescence excitation spectrum of CF,Br SO140 nm Fig. 3a shows the fluorescence excitation spectrum of CFJlr between 9 and 25 eV (140-50 nm) recorded with the low-energy grating at a resolution of 0.4 nm. No filter is used, the background has been subtracted and the spectrum has been normalised to the photon flux. Four peaks are observed at 10.55 eV (117.5 nm), 13.11 eV (94.5 nm), 14.89 eV (83.2 nm) and 20.50 eV (60.5 nm), and the shape of all four peaks seems characteristic of a neutral dissociative process. The flux from the low-energy grating is very weak around 20 eV, and under such circumstances normalisation of the fluorescence can give spurious results. Fig. 3b shows the excitation spectrum of the 20.5 eV peak recorded with the high-energy grating which has the peak of its transmission around this energy. This latter spectrum is recorded in the ‘Ylux-normalised)’ (as distinct from “timenormalised”) mode which gives more accurate results for weak peaks [ 241. As with CF.,, all these peaks have been observed by Lee et al. but at inferior resolution and signal-to-noise [ 12,13 1. By measuring the relative intensity of all four peaks as a function of the different optical filters, we have confirmed the following: (a) The 10.55 eV peak in CF,Br behaves identi227

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-

-I

25

%

26

Fig. 3. (a) Fluorescence excitation spectrum of CF,Br between 9 and 25 eV: the EM1 9883 QB photomultiplier tube is used untiltered. (b) As (a) between 16.5 and 26.0 eV, but now recorded with the high-energy grating of the Seya and in the “flux-notmalised”mode [24].

tally to the 13.74 eV peak in CF4, i.e. the emitter is the CF3 radical, and this result confirms the dispersed spectrum of this band observed by Lee et al. (fig. 5d of ref. [ 13] ) . The resonant nature of the peak in the excitation spectrum (fig. 3a) confirms also that the fluorescence must be due to a neutral fragment. (b) The 13.11 and 14.89 eV peaks in CF3Br both behave similarly to the 15.89 eV peak in CF,, i.e. fluorescence is predominantly in the range 190-400 nm and there is negligible signal for L> 400 nm. Thus in both cases the emitter is the A ‘B, first excited state of CF,, and emission is due to CF2 A-2. From this 228

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experiment we see no evidence for the additional band between 300 and 500 nm observed by Lee et al. [ 131 and tentatively assigned to CFBr A-g. (c) The 20.50 eV peak in CF,Br behaves differently with the filters from the three peaks described above. Fluorescence now occurs over the whole sensitivity range of the PM tube (190-650 nm), and this is confirmed by the dispersed spectrum obtained for this band (fig. 8d of ref. [ 13 ] )_ By analogy with the equivalent band in CF3H [ 81 and CFJl [ 91 it is tempting to assign this band to parent ion emission, i.e. to CF3Brf, and this is the conclusion of Lee et al. [ 131. The most likely emitter would then be the E *A, fourth excited state of CF,Br+ with a vertical IP of 19.8 eV [ 251 and we note that this photoelectron band shows extensive vibrational structure. Whilst this may be the correct assignment three points argue against it. Firstly, the threshold for fluorescence should agree with the adiabatic IP of the CFSBr+ g state. The threshold is 18.8 fO.l eV (fig. 3b) whereas the best estimate of the E state adiabatic IP from the published photoelectron spectrum is 19.3 kO.2 eV [25]. Secondly, the shape of the excitation function (fig. 3b) resembles more closely a neutral dissociative process than a non-resonant photoionisation process. Thirdly, the radiative lifetime of this band shows bi-exponential behaviour (see below). It seems likely that more than one emitting species is associated with photoexcitation of CF,Br at 20.5 eV, one of which may be the E *A, state of the parent ion. The threshold of the CR peak occurs at 9.5 f 0.1 eV (fig. 3a), and we note that this is in reasonable agreement with the thermodynamic threshold for CF3Br+CF;:+Br of 9.2 eV [26,12]. A strong peak, whose assignment is uncertain, does occur in the absorption spectrum of CF3Br at 10.50 eV ( 118 nm) [ I 1,121, so it seems likely that this state is dissociative forming CFf +Br. The CF2 A peaks at 13.1 and 14.9 eV both have corresponding peaks in the CF,Br absorption spectrum [ 111, and the thermodynamic threshold for forming CF2 A+F+Br from CF3Br is 11.3 eV [ 26,17 1. There is no obvious peak in the absorption spectrum at 20.5 eV. 3.4. Radiative lifetime measurements Radiative lifetimes of the fluorescing bands in CF, and CF,Br were measured using the single-bunch

CHEMICALPHYSICSLETTERS

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mode of the synchrotron. As preliminary experiments we measured the lifetime of N: B ‘C,’ via its B-X (0, 0) band at 391 nm following photoexcitation of N2 at 25 eV. A semi-log plot is shown in fig.

4a, and a linear least-squares fit over three lifetimes yields r= 61.5 f 0.2 ns, in excellent agreement with the literature value. Fig. 4b shows a semi-log plot of CF4 excited into the CF4+ C2Tz ionic state at 22.5 eV. Again a fit over three lifetimes yields z= 8.80+0.04 ns, in good agreement with our earlier measurement [ 71. The data for the other fluorescing bands from CF4 and CF,Br are shown in table 2. The CF3 decays (from both CF, and CFjBr) give linear semi-log plots over 2-3 lifetimes with values for T between 15 and 17 ns (fig. 5a). This is in ex-

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cellent agreement both with our previous results for CR from CF3H and CF,Cl [ 8,9 ] and with two other independent measurements [ 27,28 1. As before [ 8,9 ] we are unable to differentiate the lifetime of the CF3 UV band ( 190-300 nm) from that of the visible band (400-700 nm ), and this is in agreement with the data of Quick et al. [27 1. (Dreyfus and Urbach [ 281 claim they can detect a 50% difference between the lifetimes of the two CF3 bands, but from the data shown in their paper we cannot agree with this interpretation.) The lack of difference between the two lifetimes is surprising because ab initio calculations predict that two close-lying but different electronic states of CFJ are the origins of the UV and visible band systems [ 29 1, A calculation of the electronic transition moment and hence the radiative lifetime

-

r

(a)

1

! I

100

/

200

Channel

I

300

I

400

number

Fig. 4. Semi-log plot of (a) N$ B ‘X:-X ‘Xz emission at 391 nm, (b) CF: c 2T2-A 2T2,2 ‘TI emission between 250 and 400 nm. The time resolution is 0.4648 ns per channel in (a), 0.1846 ns per channel in (b).

i

I

100

260 Channel

330

I

400

number

Fig. 5. Semi-log plot of fluorescence from CF,Br photoexcitedat (a) 10.3 eV, (b) 14.9 eV, and (c) 20.6 eV. The time resolution in all caSes is 0.4648 ns per channel.

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Table 2 Radiative lifetime of fluorescing states of parent ions and neutral fragments studied in this work

(ns)

Precursor molecule

Excitation energy (eV)

Fluorescence collection region (nm)

7’)

NZ CF4

25.0 22.5

380-400 250-400

61.5(2) 8.80(4)

N: B%+ CF: c2&

CF,

13.7 13.7 13.7 15.9

190-650 250-400 450-650 190-650

15.2(l) 15.3(l) 16.3(l) 45(11)

CFS both bands CF, UV band CF, visible band CF, A ‘B,

CF,Br

10.3 10.3 10.3 13.0 14.9 20.7 20.7 20.7

190-650 250-400 450-650 190-650 190-650 190-650 250-400 300-650

16.7(2) 17.1(2) 17.0(2) 48.4(7), 15.6(9) 50.2(S), 15.1(6) 45(l), 16.7(3) 47(2), 16.9(4) 60(4), 17.9(6)

CF, both bands CFs UV band CF3 visible band CFrA’B,t? CF2~‘B,t?

Assignment

see text

a) Errors quoted in parentheses are one standard deviation.

of these two states of CE is needed to resolve this apparent contradiction. The instantaneous rise in the CR fluorescence in the semi-log plot (fig. 5a) means that (pre-)dissociation of the Rydberg states of CF, or CF,Br+ CF; + F, Br takes place much more rapidly than the radiative decay rate of CF: (6 x 10’ s-1). The CF, A-8 fluorescence from CF, excited at 15.9 eV is weak (fig. la), and the signal-to-noise of the fluorescence decay is inferior to the CF3 decays above. However, the data can be fitted to a single exponential with ~45 + 11 ns, in agreement with the value for the CF2 A ‘B, state lifetime of 5 1 & 2 ns [ 30 1. The semi-log plot of the CF, A-2 fluorescence from CF,Br excited at 14.9 eV is shown in fig. 5b. Now there is distinct curvature in the decay, and it is not possible to fit the data to a single exponential. Fitting the data to a double exponential gives a satisfactoryfit with rI = 15.1 kO.6 ns and rz=50.2t0.8 ns. There are therefore two different emitting species, one of which is clearly CF2 A. The coefficient of the CF2 r, decay is much greater than that of the q decay, and this is compatible with the filter results described in section 3.3. The other emitter may cause the weak emission between 300 and 500 nm [ 131, tentatively assigned to CFBr A-2. However, the lifetime of CFBrA is very long, 1150 ns [31]. The 15 230

ns lifetime suggests the emitter may be CF;, but this is not compatible with the dispersed spectrum of this emission (fig. 7 of ref. [ 13 ] ). The assignment of this second emitter remains unresolved. Fig. 5c shows a semi-log plot of the fluorescence from CF,Br excited at 20.7 eV. Now the curvature is very pronounced, and again we believe there is more than one emitting species. A double exponential tit to the data yields r,= 16.7kO.3 ns and r2=45 & 1 ns, with the coefficient of the 7l decay being greater than that of rz. What one can say with certainty is that the emission is not due solely to the parent ion CF,Br+, because a linear semi-log plot (as seen in fig. 4 for N: and CFZ ) would be observed. CF A 2Xf is not a possibility for one of the emitters because its lifetime is 26 ns [ 301. It seems that the most likely possibilities for z, and T, are CR and CF2 A, Cfl and CF3Br+ E, or CF,Br+ E and CF2 A respectively, All three combinations are compatible with the results of the filter experiments described in section 3.3 and the dispersed spectra reported by Lee et al. [ 13 1. It should be noted that emission from the E2A, state of CF3Br+ to lower electronic states is predicted to occur over a wide range of the UV and visible [ 25 1.

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4. Conclusions We have reported vacuum UV fluorescence excitation spectra of CF, and CF,Br in the range 4% 140 nm. The spectra are at much improved signal-to-noise and resolution than previously reported. The shape of the peaks gives information on whether the emission is due to the parent ion or to a neutral fragment produced by photodissociation of the parent molecule. By the use of cut-on and broad band filters the range of wavelengths over which each fluorescence band is occurring has been determined. This information, together with radiative lifetime measurements, has meant that the emitter of almost all the peaks in the fluorescence excitation spectra has been unambiguously assigned.

Acknowledgement We thank the staff of the Daresbury Laboratory (especially Dr. A. Hopkirk) for help, and SERC for a research grant, a Post-Doctoral Fellowship (PAH ) , and two Research Studentships (JCC and IRL).

References [ 1] F.S. Rowland and M.J. Molina, Rev. Geophys. Space Phys. 13 (1975) 1.

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