A. P. MODICA
4594
Electronic Oscillator Strength of CF, by A. P. Modica Aeco Space Systems Division, Wilmington, Massachusetts (Received June 24, 1968)
A shock tube and samples of CZF.~, CF$Cb,and CFa in argon diluent were used to prepare CF, species for spectroscopic study. Absorption spectra of these shocked mixtures, taken with a Hilger medium quartz spectrograph, show a series of absorption bands from 2250 to 3000 A corresponding to those of the CF, radical. In the region of strongest absorption (2400-2500 A) the bands appear to overlap completely. Absorption coefficients of the observed band system were determined at various wavelengths with a low-resolution radiometer. Integration of the absorption coefficientsgave from theory electronic oscillator strengths offa,,: = 0.034 (C2F4 data, 1804°K) 0.032 (CF2C12 data, 2085"K), and 0.024 (CF4 data, 2878°K). The uncertainty in thesef-number values is estimated to be 10%.
*
Introduction The 2500-A emission band system of the CF2 radical was first observed and reported by Venkateswarlu.' Absorption2-0 and microwave' spectra of CF2 have shown that the molecule is in a singlet ground state, 'AI. The electronic oscillator strength of this band system has been estimated to be 0.01 to within a factor of 2 from a measured absorption coefficient at 2488 A.a Currently, the CF2 electronic oscillator strength is particularly useful in radiative heat transfer and optical diagnostic problems associated with fluorocarbon chemistry in rocket combustion and reentry ablation. Therefore, it has become of interest to obtain an improved value of the f number from measurements of the absorption coefficient over the entire band system.
Ultraviolet Absorption Argon gas mixtures of 1% C2F4, CFzCl2, and CF4 were shocked to temperatures around 1804 f 23 (C2F4 experiments), 2085 f 45 (CF2C12 experiments), and 2878 i= 49°K (CF4 experiments). Thermochemical equilibrium calculations'o (Tables 1-111) showed that under the experimental conditions chosen for study the parent fluorocarbon molecules were virtually all dissociated to CF2 radicals and halogen atoms. The absorption spectrum (Figure 2) obtained from these shock-heated fluorocarbon molecules was seen to consist of a series of diffused bands extending from about 2250 to 3000 A. The locations of these bands are found to closely coincide with those of the well-known CF2 ultraviolet absorption bands.4 It is believed from other experimental evidence that the spectrum observed in the present study is virtually due to only CF2 species. Spectrophotometric measurements of the parent fluorocarbons before and after shock heating indicated a lack of absorption in the 2250-3000-A region. The absorption coefficient measured at 2536 A in this work is found to be similar to that deduced from ultraviolet absorption in the pyrolysis of CFSH," for which gas there is gas chromatographic evidence'2 indicating CF2 and H F are produced in splitting the molecule. Studies of the thermal dissociation of CFa
Experimental Measurements Absorption spectra of shock-heated argon mixtures containing 1% fluorocarbon were photographed on Eastman Kodak 103-0 plates with a Hilger medium quartz spectrograph (Figure 1). A xenon flash lamp was timed with the passage of the shock wave across the entrance slit (200 p ) of the spectrograph, and multiple exposures totaling 600 psec were required to obtain spectra. Photographic plates were analyzed with a Jarrell-Ash microdensitometer (Model 2310). The ratio of the per cent transmission of the absorbing gas flash lamp spectra to that of the flash lamp spectra alone was taken to be the relative transmission of the absorbing gas sample. Absorption coefficients for different wavelengths in the band system were determined from spectrophotometric measurements, using
(1) P.Venkateswarlu, Phys. Rev., 77, 676 (1950). (2) R. K.Laird, E. B. Andrews, and R. F. Barrow, Trans. Faraday SOC.,46,803 (1950). (3) D.E.Mann and B. A. Thrush, J . Chem. Phys., 3 3 , 1372 (1960). (4) A. M.Bass and D. E. Mann, ibid., 36,3501 (1962). (6) B. A. Thrush and J. J. Zwolenik, Trans. Faraday Soc., 59, 682 (1963).
where.9
(12) E.Tschuikow-Roux, ibid., 42,3639 (1965).
The Journal of Physical Chemistry
ELECTRONIC OSCILLATOR STRENGTH OF CF,
4595
Table I: Equilibrium Campasition of a 1: 100 G F r A r Mixture at 1.3 Atm Total Pressure' Temp.*K
.4r
1700 1800 1900
0.98 0.98 0.98
CIF.
CF.
0.50 X IO-' 0.18 X IO-' 0.71 X IO-'
0.20 X 0.14 X IO-" 0.13
x IO-"
Mole fraction of apmi CFa
0.20 X IO-' 0.63 X 0.23 x IO-"
CFs
F
F.
0.19 X IO-' 0.19 X IO-' 0.19 X IO-'
0.79 X IO-" 0.12 X IO-" 0.18 X IO-"
0.21 X 0.26 X IO-" 0.32 X IO-"
- 104 kcsl/mol
"JANAF thermochemical Tables," Dow Chemical Co., Midland hlich., 1966, and AHI(CE) = Chon. Phys. 48, 3283 (1968)) were used in equilibrium calculation.
(A. P. Xlodics, J .
~~
Table 11: Equilibrium Compmition of a 1: 100 CFIC12-Ar Mixture at 2.3 Atm of Total Pressure Temp.
'K
Ar
2ooo
0.97
CFICIX
0.2.5
x
10-
2100
0.97
0.74 X
22w
0.97
0.23
10-7
x
10-7
CF'
0.6.5 X 1o-'S
0.64 x IO-:' 0.21 x
2800 2900
3wo
0.97 X
10-9 0.49 X
10-3
lo-" 0.10 x lo-"
e. 1: 100
0.97 X 10-2
0.97 X 10-1
0.97 0.97 0.97
CIF~
0.12 X 100.W X IO-' 0.64 X 10'
0.19 X IO-* 0.56 X IO-' 0.16 X IO-'
0.52 X IO-' 0.31 X IO-' 0.18 X IO-*
*
IO
I ~
= e,L[CF2] = e,Lml[X] = In
x
lo-' 0.66 x 1(r' 0.37 X
0.17 X 10-1
0.18
x
10-1
0.19 x
lo-'
Mole fraction of spmi CFa CF.
CFa
radicals'* show that these species do not absorb in the 2250-3000-~ region. Finally, in accordance with the equilibrium calculations, it was observed that for each of the fluorocarbon mixtures the absorption coefficient vaned slightly (10%) with a change in temperature of 100°K. If species other than CF? had been responsible for the absorption, larger changes in the absorption coefficients would have been noticed because of the relatively large variations in concentration for a +lOO'Ii change in temperature. The absorption coefficient 6, of the CFz radical was obtained from Beer's law according to
In
0.12
CI
F,
F
0.19 x lo-" 0.16 X lo-* 0.0
0.14 X 10-10
0.49 X
lo-"
0.29 lo-'^
10-1
CFd-Ar Mixture at 2.3 Aim of Total P r s w r e
~
AI
OK
Mole lnetioo of npecic. CFz Clr
0.43 X
lo-"
Table 111: Equilibrium Compwition of Temp.
CF,
H H-h ~
(1)
0.90 X IO-' 0.94 X LO-' 0.93 X IO-'
F.
F
0.19 X IO-' 0.19 X IO-' 0.19 X IO-'
0.20 X IO0.16 X 10-
0.13 X 10-
where Io and I are the intensities of the incident and transmitted light heam, respectively; K is the optical path Rcrow the absorbing gas; e, is the absorption coefficient, in I./mol em; [CFZ]is the CF2 species concentration, is the product of the shock density ratio; and [XI is the stoichiometric concentration of the unshocked fluorocarbon to be completely disociated. Experimentally the light intensity is measured by the detector output displayed on a Tektronix 535 oscilloscope. The incident beam produces the deflection (H) immediately after the run and h is that from CFI absorption. An oscillogram record of CFz ultraviolet absorption is shown in Figure 3. Measurements were (13) A. P.Mdiea.J. Chem. Phv8..48,3283 (1868).
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4596
A. P. MODICA 1.0
0.8 2
0 u)
Z 0.6
u)
2
a a
c W
1 c
0.4
a
-I W
a
0.2
0 2300
2500
2400
2600
WAVELENGTH,
2700
2800
2900
8
Figure 2. Analysis of the absorption spectrogram from a shock-heated 1:100 CzFa-Ar gas mixture. wavelengths of known CFZabsorption bands.4
The arrows show some
Table IV : CFZAbsorption Coefficient ----lO-bv, A,
A
2150 2180 2225 2235 2250 2300 2350 2380 2390 2400 2450 2482 2537 2600 2652 2700 2750 2804 2893 2967
X, om-’
46,511 45,871 44,943 44,742 44,444 43,478 42,553 42,016 41,841 41,666 40,816 40,290 39,416 38,461 37,707 37,037 36,363 35,663 33,566 33,704
2878
l./mol cm---2085 i 45’K
1804 i 23OK
0.32 f 0.04 0.29 f 0.04 0.50 f 0.02 0.59 f 0.00 0.67 f 0.07 0.88 f 0.07 1.22 f 0.11
1.18 f 0.09
1.39 f 0.09
2.14 f 0.04 2.62 f 0.28 1.60 0.16 1.68 f 0.02 1.87 f 0.28 1.59 f 0.17 1.33 f 0.02 1.14 f 0.10 0.88 f 0.04 0.75 f 0.02 0.55 f 0.01 0.33 f 0.01 0.21 f 0.01
made from the steady absorption level where the chemical relaxation period is completed and the gas is characterized by a constant temperature and density. The shock temperature and density ratios were obtained by solving the Rankine-Hugoniot equations with the measured shock velocity and JANAF thermochemical data of the appropriate species. Absorption coefficients of the CF2 radical were determined for concentraand 1.7 X mol/l. The tions between 0.9 X experimental results of the study are given in Table IV. The Journal o j Phyeical Chemietry
49’K
2.45 f 0.18 1.24 =I= 0.18 1.03 =I= 0.06 0.53 f 0.03 0.28 f 0.01
2.99 f 0.09 2.29 f 0.09 2.38 f 0.10 1.31 f 0.03 1.08 f 0.05 0.76 f 0.01 0.45 f 0.01 0.22 f 0.01 0.10 f 0.01
Theoretical Interpretation An expression for the electronic oscillator strength of a molecular band system can be derived from considerations analogous to those for atomic line absorption. Following Herzberg,I4 the only difference between the intensity of an electronic transition of a molecule and and that of an electronic transition in an atom is that in the molecule the total intensity is distributed over ti (14) G. Herzberg in “Spectra of Diatomic Molecules,” D. Van Nostrand Co., Inc., New York, N. y., 1960,pp 381-383.
ELECTRONIC OSCILLATOR STREKGTH OF CF,
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4597
TlME
Figure 3. The ultmviolct light nl,wrptiun hehind the shock wave into a I : 100 CF4-argon gns mixturc (shock temperature, 2X;O'K; [CF,]= 9.45 X 10-6 mole/l.; writing, 20 )Isec between vertical lines).
large number of lines or over a continuous spectrum. For an electronic transition in absorption, the electronic oscillator strength is related to the integrated molar absorption coefficient (e, I./mol cm) by
(2) = 1.877 X 10-B.fc. dv
where m and e arc the electronic mass and charge (4.802 X 10-'0 esu), e is the speed of light, and N is Avogadro's number. To evaluate the oscillator strength of the CF, absorption band, the experimental absorption coefficients were least-squares fitted with a fourthorder polynomial of the form e, =
,.a
+ a,"' + a.p2 + a d + a,v'
I
I
1
(3)
This procedure was adopted in order to cope with the statistical scatter in the data and permit numerical integration of the absorption coefficients with wavelength. The solid lines in Figure 4 are the results of the fits. In Table V, the oscillator strength of this work is presented along with the value estimated by Dalby.
-
Table V:
I
Electronic Oscillator Strength of the 'Al) Band System
Figure 4. Variation of the CF, ahsolption coefficient with wavelength and temperature. The solid lines are fourth-arder least-squares fits of data.
Dimasion
and measurement of the absorption oscillograms contribute less than 1% error to the experimental oscillator strength. Light emission from the shocked gases did not occur; hence the absorption measurements were free from this interference. Data were taken from the oscillogram records after the steady-state absorp tion was reached, where the temperature and density are constant and readily calculated from the thermodynamic properties of the dissociated gas mixture. The uncertainty in the mensured shock velocity introduced a f 20" error in the calculated shock temperature and an error of less than 1% in the calculated density.
The sources of error inherent to the present type of spectroscopy study have been considered extensively in a previous paper." Uncertainties in the recording
44,3380(1WG).
CFd'B,
1.b'
R@l
Temp. OK
a
2875
0.m
2nm 1804
0.032 0,034
20R
0.014
b
+ 0.006
'This work; integration over ravelength interval 33,OMl47,000 cm-'. * F. W. Ihlhy, J . Chem. Phys., 41,2207 (1964).
(15)
J. A. Harrington. A. P. Modiea. and D. R. Libby. J . Chm. Phyr.. Volume 76. Nu*
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A. P.MODICA
4598 Since the data were analyzed for conditions close to the shock front, boundary-layer effects on the shock temperature and density would have been unimportant. Absorption by species other than CF2 in the gas mixture would seem rather unlikely, based on arguments presented earlier in the text. Absorption coefficients for a given wavelength were averaged from measurements taken within a 100" temperature interval. This averaging amounted to about a 5% uncertainty in the absorption coefficients for wavelengths of strongest absorption and less for the other wavelengths in the band system. Theoretically, the oscillator strength is independent of temperature, since the energy of the total absorption band is constant. In the present study the absorption f number was found to decrease by about 30% for a decrease in temperature of around 1000°K. Aside from the possibility of experimental error, the lower f numbers at the higher temperatures may have resulted from the broadening of the absorption band to wavelengths below 2250 A, where the radiometer and light source could not provide accurate optical measurements. The high-temperature absorption spectrum reported
The Journal of Physical Chemistry
here is believed to be that of the singlet ground state of CF2 and not its triplet state, aB,. Because of extensive overlap of rotational states which apparently takes place a t high temperatures, it cannot be resolved from the spectra obtained that splitting of the vibrational bands did occur indicating that the triplet state might be responsible for some of the absorption. However, Simons'6 has estimated that the CF2 triplet is about 45 kcal above the singlet ground state, which would make the number of molecules in the triplet state less than a factor of lo-* at the temperatures considered in this study. Hence, it can be assumed that any contributions to the absorption spectra by the tri'plet state would be insignificant.
Acknowledgment. The support of this research by the Ballistic Systems Division, United States Air Force, under Contract AF04(694)-687, ABRES program, is gratefully acknowledged. The author wishes to thank J. A. Harrington for his comments on the interpretation of the experimental data. (16) J. P.Simons, Nature, 205, 1308 (1966).
