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The Journal of Physical Chemistry, Vol. 82, No. 75, 1978
(10) C. A. Angell and E. J. Sare, J . Chem. Phys., 52, 1058 (1970). (11) C. T. Moynihan and A. Fratiello, J. Am. Chem. Soc.,89, 5546 (1967). (12) (a) W. Bol, G. J. A. Gerriis, and C. L. van Pantaleon van Eck, J. Appl. Cwstallogr., 3, 486 (1970); (b) J. N. Albright, J . Chem. Phys., 56, 3786 (1972); (c) G. Licheri, G. Piccaluga, and G. Pinna, hid., 63, 4412 (1975); 64, 2437 (1976); (d) J. E. Enderby in "Proceedings of a NATO Summer School of Structure and Dynamics of Liquids", Corsica, 1977. (13) K. Geise, U. Kaatze, and R. Pottel, J. Phys. Chem., 74, 3718 (1970). (14) R. E. Hester and R. A. Plane, J. Chem. Phys., 40, 411 (1960). (15) C. A. Angell, A. Barkatt, C. T. Moynihan, and H. Sasabe, "Proceedings
Communications to the Editor
(16) (17) (18) (19)
of the InternationalConference on Molten Salts", J. P. Pemsler, Ed., Electrochemical Society, 1976, p 195. A. J. Easteal, E. J. Sare, C. T. Moynihan and C. A. Angell, J. Solution Chem., 3, 807 (1974). I. M. Hodge and C. A. Angell, J. Non-Cryst. So/&, 20, 299 (1976). R. A. Robinson and R. H. Stokes, "Electrolyte Solutions", 2nd ed, Butterworths, London, 1965, pp 370, 504. The change in the R dependence of T at ca. 8R is not apparent when T is plotted as a function of mole percent salt. Indeed for R > 4, is a linear function of mole percent salt for the Ca(NO& system (figure 4, inset)."
e
COMMUNICATIONS TO THE EDITOR Rate Constants for ArF" Formation from Reactions of Ar( 3P,,0) with Fluorine Containing Molecules and the Pressure Dependence of the C to B State Ratios for ArF", KrF", and XeF" Publication costs assisted by Kansas State University
Sir: In recent work the rate constants for rare gas halide formation from reaction of Xe(3Pz) with fluorinel and chlorine2 containing molecules, Kr(3P2)with fluorinel containing molecules, and Ar(3P2,0)with chlorine3 containing molecules have been reported. In this communication we report the rate constants for production of ArF* from the reaction of Ar(3P2,0)with fluorine containing molecules (reaction 1). The total quenching rate con-
+
+F 5ArF(C) + F
Ar(3Po,2) R F -!?.+ ArF(B)
(1)
stants, k,, were reported earlier,l so the present measurements provide the branching fractions. These formation rate constants are of direct importance for modeling the performance of the rare gas fluoride lasers.44 In addition, we report the pressure dependence of the B and C state ratio for ArF*, KrF*, and XeF* and interpret the high and low pressure limits. The low pressure limit gives the relative rate constants for direct formation of the B and C states from the reactions above. Providing that selective quenchings of the B and C states does not occur, the high pressure ratios can be used to obtain the energy difference between the B and C states. The ArF(D) state is only -0.15 eV above the B and C states and the strong ArF(B-X) emission overlaps the ArF(D-X) band. Thus, in this work we include any ArF(D) state formation with ArF(B). For KrF* and XeF* the D-X emission is resolved but direct formation is of minor imp0rtance.l The experimental technique used to measure the ArF* formation rate constants is the same as the described previously,1,2except that the reference was the ArCl(B-X) emission from the Ar(3P2,0)+ C12reaction, which has a rate constant of 21 X cm3 molecule-l s-l for ArCl(B) f ~ r m a t i o n .This ~ value has been confirmed by Nip and Clyne7 and by ourselves during the course of this work. The ArF* and ArC1* spectra were recorded with a 0.3-m vacuum-UV McPherson monochromator equipped with an EMR photomultiplier tube (541F); the wavelength response was calibrated from 125 to 200 nm using the molecular branching ratio method3 and fro'm 180 to 340 nm using a standard D2 lamp (Optronic Laboratories). The monochromator was coupled to the flowing afterglow 0022-3654/78/2082-1766$0 1.OO/O
system via a LiF window. The experimental and reference spectra were acquired by computer control of the monochromator and were stored on tape and subsequently corrected for monochromator response and integrated to obtain relative intensities. All fluorine containing donors were carefully purified and stored in stainless steel reservoirs as dilute mixtures in argon. Flow rates were measured by monitoring the pressure rise in calibrated volumes using a pressure transducer. Figure 1 shows spectra obtained from Ar(3Po,2) F2, CF30F, and NF,. Table I summarizes the total quenching rate constants138and branching fractions, (kB k c ) / k Q ,for ArF* formation. As noted before, the B and D state ArF* emission cannot be separated and kD is included with kB. Also presented in Table I are the rate constants for other exit channels that give emission in the 125-500-nm range. From Table I it is readily evident that only Fz,NF,, OFz, and CF,OF are good donors for ArF* production. This is a general result and only molecules with nitrogen-fluorine or oxygen-fluorine bonds and molecular fluorine give appreciable formation of XeF*, KrF*, or ArF*. ClF forms ArC1* rather than ArF*; the other mixed halogens, IF, and IBrS, give mainly ArI* and ArBr*, which subsequently predissociate to I* or Br*. For all other molecules, other than those just mentioned, the ArF* formation rates are nearly zero (50.01) even though kQ is large. The branching fractions for KrF* and XeF* from Kr(3Pz)and Xe(3P2)reactions are >0.9 for OF2 and are essentially unity with F2.l However, for argon the branching fractions have dropped to 0.53 and 0.62 with Fz and OFz, respectively. This suggests that some new channel is competing with ArF* production; however, no other emission was observed from 110 to 500 nm. Formation of F(3s)* is not energetically feasible. Excitation of F2*, which emits in the 110-150-nm region, is feasible but was not observed (we did, however, observe very weak F2* for Ne(3P,) F2). Since IP(Q) 2 11.5 eV, Penning ionization is not an exit channel and the only remaining possibilities seem to be neutral dissociation or associative ionization. The pressure dependence of the ArF* emission is similar. to that observed for KrF*, XeF*, and XeC1*.2g9 The (C,n = 312 - A$ = 312) transition changes from the low pressure broad emission (extending from 200 to 300 nm) to a band with a maximum at -220 nm. The (B-X) low pressure spectra relaxes to a single strong band at 195 nm. The shift of the wavelength with pressure (red for B-X, blue for C-A) can be readily explained by vibrational relaxation in the upper states and the relative positions of the repulsive walls in the lower and upper state potential c u ~ v e s .The ~ variation of IB-X/IC-A vs. pressure from the
+
+
+
0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 75, 1978
Communications to the Editor
.-
I :71.01
1
l
1.o '
1767
2 .o P [torr)
"
#
02
04
8
'
" 3.0
0.6
~
" 4.0
08
~
~
Pitorr)
J
,
,
,
lMnm
,
,
,
,
,
,
, 3w
250
200
Figure 1. Experimental ArF" spectra from various fluorine donors at 0.8 Torr. Spectra have been corrected for monochromator response. The 140-200-nm region contains the B-X emission plus possibly a small contribution from D-X. The 200-300-nm region contains the C-A emission.
TABLE I: Rate Constant Summary for &(,Pa,,,) t R F
IPSd reagent kaasb k k f l B F ~ F eV
Fa
C1F
75 74
40 2.9
0.53 0.04
CF,OF OFa NF, N,F, CC1,F CCl,F,
43 57 14 31 55 31
11
Oc
0.15 13.3 0.62 13.6 0.30 13.2 0.08 12.0 11.8 12.0
CCIF,
22
1
12.0
35 4.2 5.7 0'
15.7 12.7
rate constantsa for other observed emissione C1*(4s), k ArCl*, k
--
9;
38
4 31
CH,F COF,
40f
0
0
14.9 13.8
0 0
12.7 13.2
less reliable for ArF*. Over the pressure ranges just mentioned, no evidence was found for trimer emissions,1° Le., ArXeF*, ArKrF*, or Ar2F*, According to a steady-state analysis which includes formation, radiative decay, and collisional transfer between ArF(B) and ArF(C) states, but not collisional quenching; the emission intensity ratio is given by
---(
IC - 7c-l
IB
ArCl*, k = 5 continuum 230-300 nm, k 0.5;ArC1, k d 0.1 continuum 190-290 nm and 430-700 nm, k 0.42 and 3.5,respectively
-
CF, CF,H
Figure 2. Plots of the XeF", KrF", and ArF" B-X and C-A emission intensity ratios vs. pressure and (pressure)-'. The symbols I and I1 represent the RgF(B-X) and RgF(C-A) emission intensities, respectively. I n each case the emission was generated from reaction of the metastable atom with F,. The low and high limits discussed in the text were taken from these graphs.
continuum 190-290 nm, k 0.32
-
weak CO(a3n-X'P), k 0.3
-
SiF, 22f 0 15.4 SO,F, 42 0 13.3 SOF, 0 12.5 33 0 12.3 SF, 0 14.0 Br*(Ss), k = 6.5 BrF, 0 13.5 I*(6s), h = 1.2 IF, 23f 0 15.5 BF, a Units of lo-" cm3 molecule-' s-l, The total quenching rate constants were taken from ref 8 and 1. ' Small The excitation energibs amounts of ArCl were observed. of &('Po) and Ar('P,) are 11.72 and 11.55 eV, respectively. Although none of the reagents in this table can give Penning ionization, associative ionization is still a possibility. e Unless otherwise indicated no other emission was observed in region from 125 to 500 nm. f Estimated from k g for Kr(,P,) or Xe(3P,), see ref 8.
reactions of Ad3P2), Kr(3P2), and Xe(3P2) with Fz are displayed in Figure 2. The flowing afterglow apparatus operates up to -50 Torr with Kr* or Xe* but only to 10 Torr with Ar*. Therefore, the high pressure limit may be
7f1
+h
}
~ ~ + ~h ~ () [ Ah r ] ~ h ~ 7 c - l+ hA:(kC + h~)[Ar]
kC7B-l
(2)
IC and IBare the total emission intensities from the C and B states, T ~ and - ~ T ~ are - ~ the inverse of the radiative lifetimes, hc and hB are the initial formation rate constants, and hhc and hhB are the transfer rate constants. The zero pressure limiting ratio for ArF*, KrF*, and XeF* from the F2reaction are all -1.4, which indicates that the halogen donor rather than the metastable rare gas atom dictates the ratio of hc/hB. This claim has not been verified for a large variety of halogen donors but, it is found for ArF*, KrF*, and XeF from OFp and NF3 and for ArCl*, XeCl*, and KrC1* from CC4. Different halogen donors do give different kc/hB ratiosa2 In constrast, the high pressure limit is constant for a given rare gas halide with different donors, but changes with the rare gas metastable atom. Combining the numerical value of the high pressure limit with the lifetime values (48 and 4.2 ns) for ArF*11J2 gives the ratio of kA: to kA:. According to detailed balance, this ratio is just the equilibrium constant and assuming that high pressure intercepts are for 300 K, the energy separations of the B and C states of ArF*, KrF*, and XeF* are 0.018, 0.016, and 0.056 eV, respectively. The unexpected feature is that the C state is lower than the B state for each case. The ab initio results predict that the B and C states are close in energy but that the B state is lower.11J2 The main assumption made in eq 1 is that quenching of the B and C states are negligible. For XeF(B) this certainly is acceptable since 100 Torr of Ar is required for 10% quenching.13 No explicit information is available for C state quenching; however, it is unlikely to be important at 50 Torr. Since the XeF(B-X) emission from low u levels is bound-bound, it can be used to inspect the
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~
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The Journal
of Physical Chemistry, Vol. 82, No. 15, 1978
vibrational distribution; at 50 Torr there definitely is a tail of high u levels remaining from the initial distribution. However, the bulk of the distribution is relaxed and the u = 0-10 populations appear to be close to a room temperature Boltzmann distribution. We conclude that the ordering deduced for the B and C states should be correct although the magnitude of the separation may be uncertain because true Boltzmann populations may not exist a t 50 Torr. Our claim that the C state is below the B state is supported by a recent analysis of the XeF* high pressure intensities,14 although Kligler et al. report a much higher XeF(C)/XeF(B) ratio than shown in Figure 2. Using the ratio of k A F / k h c and the absolute values of kc and kB (obtained from kArFand the low pressure limits of Figure 2), eq 2 was fitted to the overall pressure dependence of the Kr(3Pz)+ F2data. The collisional transfer rate constants, khc and khB, were found to be 54 f 20 and 97 f 30 X lo-" cm3 molecule-l 8,which are effectively the gas kinetic values. This estimate of kA? and kAF is uncertain because of an expected dependence of these rate constants upon u level and the variation of T with u level, which was taken from Figure 4 of ref 9. The general variation of Ic/IB with pressure is very similar for all of the rare gas halides formed by reactive qdenching and we expect kCArand kBArto be of similar magnitude for all cases.
Note Added in Proof. The transfer rate constant for the low (u' = 0-4) vibrational levels of XeF(B) have been measured using photolysis13 of XeFz to generate XeF(B). cm3 molecule-' s-'. As For these levels kArBis 5 X suspected, kA? has a very strong dependence upon the vibrational energy. This recent work also suggests that the intercepts of Figure 2 should be viewed as intensity ratios for a temperature substantially above room temperature and that the energy separations cited in the text for
Communications to the Editor
XeF(B) and XeF(C) are lower limits.
Acknowledgment. This work was supported by the Advanced Research Projects Agency of the U.S. Department of Defense (monitored by ONR, N00014-76C-0380). We thank the authors of ref 14 for a preprint.
References and Notes J. E. Velazco, J. H. Kolts, and D. W. Setser, J . Chem. Phys., 65, 3468 (1976). J. H. Koks, J. E. Vehzco, and D. W. Setser, J. Chem. Phys., submitted for publication. J. A. Gundel, D. W. Setser, M. A. A. Clyne, J. A. Coxon, and W. Nip, J . Chem. Phys., 64, 3490 (1976). M. Rokni, J. H. Jacob, J. A. Mangano, and R. Brochu, Appl. Phys. Lett., 31, 79 (1977). J. M. Hoffman, A. K. Hays, and G. C. Tisone, Appl. Phys. Lett., 26, 538 (1976). L. F. Champayne and N. W. Harris, Appl. Phys. Lett., 31, 513 (1977). M. A. A. Clyne and W. S. Nip, J . Chem. Soc., Faraday Trans. 2, 73, 161 (1977). J. E. Vehzco, J. H. Koks, and D. W. Setser, J. Chem. Phys., in press. K. Tamagake and D. W. Setser, J . Chem. Phys., 67, 4370 (1977). (a) D. C. Lorents, R. M. Hill, D. L. Huestis, M. V. McCusker, and H. H. Nakano in "Electronic Transition Lasers", Vol. 11, MIT Press, Cambridge, Mass., L. F. Wilson, S . N. Suchard, and J. I. Steinfeld, Ed., 1977, p 30. (b) R. 0.Hunter, J. Oldenettel, and C. Howton, J. Appl. Phys., 49,549 (1978). T. H. Dunning, Jr., and P. J. Hay, J . Chem. Phys., submitted for publication. P. J. Hay and T. H. Dunning, Jr., J. Chem. Phys., 66, 1306 (1977). H. C. Brashears, Jr., D. W. Setser, and D. DesMarteau, Chem. Phys. Lett., 48, 84 (1977), and additional unpublished work using this technique. D. Kligler, H. H. Nakano, D. L. Huestis, W. K. Bishel, R. M. Hill, and C. K. Rhodes, Appl. Phys. Lett., submitted for publication. Chemistry Department Kansas State University Manhattan, Kansas 66506 Received May 11, 1978
J. H. Kolts D.
W. Setser"
