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Total Ionization Times in ShockHeated Noble GasesMati Merilo and Edward J. Morgan Citation: The Journal of Chemical Physics 52, 2192 (1970); doi: 10.1063/1.1673284 View online: http://dx.doi.org/10.1063/1.1673284 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/52/5?ver=pdfcov Published by the AIP Publishing
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THE JOURNAL OF CHEMICAL PHYSICS VOLUME 52, NUMBER 5 1 MARCH 1970
Total Ionization Times in Shock-Heated Noble Gases*
MATI MERILO AND EDWARD J. MORGAN
School oj Engineering, Case Western Reserve University, Cleveland, Oltio 44106
(Received 3 July 1969)
. Total ionization times in s?~ck-heated argon, krypt?r,t, neon, and xenon have been calculated, taking mto acc?unt atom-a.tom colliSIOns, elec~ron-atom collISIOns, and recombination. Excellent agreement is ~ound :WIth the expenmer,ttal results obtarned for argon, neon, and some experiments on xenon. The highly rnconslsten.t results obtan:ed ~y sor,ne investigators for xenon have probably been due to high impurity levels and rnadequate testrng times rn the shock tube.
I. INTRODUCTION
Ionization processes in shock-heated gases have been the subject of recent research, both from experimental and theoretical viewpoints. Tests so far have been confin~d to air, the noble gases, alkali-metal vapors, and mIxtures thereof. Of these, the easiest to analyze and work with are the noble gases because considerable physical data are available about them, relatively high temperatures can be obtained, and since they are monatomic, the ionization process is not complicated by additional factors like dissociation and chemical reactions.
The first definite study was that of Petschek and Byron! in 1957, when they tried to account for the ionization rate in shock-heated argon. They proposed that the dominant mechanism for ionization, over a large portion of the ionization process, consists of inelastic electron-atom collisions raising ground-state electrons to excited states, and then subsequent collisions liberating the electron from the atom. They were, however, unable to account for the generation of the ini tial electrons.
Although many studies were made of this problem and a number of mechanisms was proposed,2--6 it was not until 1964 that useful results became available, when Harwell and Jahn7 published the results of their experimental investigations of initial ionization rates in argon, krypton, and xenon. They concluded that the initial ionizalion proceeds by a two-step atom-atom collision process. The first and controlling step is the collision between two ground-state atoms raising one of the atoms to the first excited level. The second step consists of a further collision, or collisions, which ionize the excited atom.
This model. in conjunction with Petschek and Byron's electron-atom mechanism, was used by Morgan and Morrison8 to calculate total ionization times in argon, and good agreement was found with the high-purity results of Petschek and Byron. From this they concluded that photoemission, photoionization, and electron diffusion play very minor roles in the generation
carried out by Wong and Bershader9 and Hoffert and Lien,to with basically the same results. It is the purpose of this research to try to obtain similar agreement between the theory and experimental results for krypton, neon, and xenon.
A great deal of data obtained for the ionization of xenon can be found in the literature. Initial ionization rates have been studied by Kornegay and Johnston,2 Hacker and Bloomberg,3 Harwell and Jahn,1 and most recently by Kelly.11 Data for total ionization times have been provided by Turner,t2 Roth and Gloersen,4 Gloersen,s and Smith.13 The most striking feature of these results is the almost total lack of agreement between the investigators. A critical evaluation of their experiments will be given later.
. Results for neon have been very scarce, due to the hlgh temperatures and long times required to ionize it. The only available data consists of a few experiments by Morgan14 and one result obtained by Mullaney and Brown.1S
To the best of the authors' knowledge no work has been published on the total ionization times in krypton, though Harwell and Jahn7 did study the initial ionization rates. Calculations for krypton were performed, however, since all the data required for this computation were available.
II. BASIC CALCULATIONS
Detailed derivations of the equations required for the calculation of ionization times in noble gases can be found in several references.B-IO The approach followed is mainly that of Morgan and Morrison8 with a few minor modifications. In the present calculations, the electron-atom, atom-atom, and recombination mechanisms are included throughout the entire relaxation regime. Also, the electron-energy equation has been rewritten, following Appleton and Bray,t6 as
Qa+Q, = Re,,(E,+!kT .) +tN.k (dT./ dt)
+NekT.(du/dx) ,
of initial electrons, especially in the temperature range where Q" and Qi are the rates of energy transfer to considered. Essentially identical calculations were later electrons by elastic collisions with neutral atoms and
2192
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IONIZATION TIMES IN SHOCK-HEATED GASES 2193
TABLE 1. Data used for the calculation of ionization times.
Ne Ref. Ar
maX 1023 g 3.31 a 6.64
Eex eV 16.6 17 11.5
EieV 21.6 17 15.8
S,aX 1018 cm2/eV 1.84 b 4.9
SaaX1()20 cm2/eV 50.0} 12.0 5.0
• C. D. Hodgman. Handbook of Chemistry and Physics (Chemical Rubber Pub!. Co .• Cleveland. Ohio. 1961).
singly charged ions, respectively, T. is the electron temperature, k is Boltzmann's constant, Eo is the ionization energy, N. is the electron number density, u is the velocity, x is the distance along the axis of the shock tube, and t is the particle time. Assuming a linear cross-section dependence on electron energy, the rate at which atoms are excited from the ground state by electron-atom collisions, Rea, is given by
R = 8SeaNeNa (kT )3/2 (Eex +1) ex (_ Eex) ea (2'Irme) 1/2 • 2kT. P kTe '
where Na is the atom number density, me is the electron mass, Sea is the electron-atom cross sectional slope constant, and Eex is the excitation energy. Once the electron-energy equation is solved together with the conservation equations for mass, momentum, and energy, the electron and atom temperatures are known as a function of the degree of ionization a. The atomatom excitation rate can then be determined from
Raa= 4SaaNa2
(kT )3/2 (Eex +1) x (Eex) ('Irma) 1/2 a 2" Ta e..t kTa '
where Ta is the atom temperature, ma is the atom mass, and Saa is the atom-atom cross sectional slope constant. The recombination rate is obtained by the principle of detailed balancing, and if it is assumed that the electron-atom process is the dominant one, it simply becomes
The equilibrium degree of ionization is obtained from the Saha equation
aeq2 2Bi (2'Irm.kTe)3/2 (-Eo) ---= - exp --l-aeq Ba (Na+Ne)h3 kTe'
where Ba and Bi are the atom and ion electronic partition functions, respectively.
The total time required for equilibrium ionization in laboratory coordinates is then
NaO r w da T= 4 10 R",,+Raa+Rr.'
Ref. Kr Ref. Xe Ref.
a 13.92 a 21.8 a
17 9.915 17 8.315 17
17 14.0 17 12.1 17
19 2.32 19 3.80 19
11 14.0 11 0.60
b S. C. Brown. Basic Data of Plasma Physics (M.l.T. Press. Cambridge • Mass .. 1959).
where N aO is the atom number density immediately benind the shock. This expression has been integrated numerically.
All the calculations were performed for an initial temperature of 3000 K and an initial pressure of 10 torr. The physical data and the references from which they were obtained are presented in Table 1.
The lower atomic energy levels and the corresponding degeneracies required for the calculation of the partition functions were obtained from Moore,t7 while the momentum transfer cross sections were taken from Brode.l8
III. RESULTS OF BASIC CALCULATIONS
Since the ionization of monatomic gases takes place primarily as a result of binary-collision processes, the product of the initial pressure and ionization time should not be appreciably dependent on the initial pressure. Consequently, the results obtained from the calculations and experiments are plotted as the product PIT versus the reciprocal of the shock temperature. For purposes of comparison, the results for argon, krypton, neon, and xenon have all been presented in Fig. 1.
Over most of the temperature range considered, xenon is noted to ionize the fastest, and then in order of increasing time required are krypton, argon, and neon, a result not entirely unexpected in view of the excitation and ionization energies of these gases. At shock temperatures above 17000oK, however, several of the curves intersect, and if these trends were to continue, at temperatures greater than 500()()OK a complete reversal of the order would take place. The reason for this behavior is that the ionization rate is very strongly dependent on the rate of energy transfer by elastic collisions between the heavy particles and the free electrons. At low shock temperatures, where the ionization rate, and hence the energy loss by inelastic collisions, is fairly slow, this energy-transfer rate is sufficiently high to maintain the electron temperature close to the atom temperature. Hence, the important term in the rate equation is the excitation energy appearing in the exponential. At high shock temperatures,
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2194 1\\. :VI E R I LOA N DE. J. 1\1 0 R G A N
4 6 8 10 12 14
II Tao x 105
(OK-I)
FIG. 1. Ionization times in noble gases.
however, where the ionization rate is high, the electron temperatures are considerably lower than the atom temperatures.8 In Fig. 2 the electron temperatures are plotted against the degree of ionization for an initial atom temperature behind the shock wave of 15000oK. Looking at the expressions for the elastic energy transfer8 it is seen that for the same degree of ionization and roughly the same electron temperatures, the energytransfer rate depends inversely on the mass of the
~14
.. 13
w 0:: :l
~12 0:: w 11. :::!l w f- II z o 0:: f-&310 ...J w
9
NEON
FIG. 2. Variation of the electron temperature behind the shock.
atom. Thus the order observed in Fig. 2 is not surprising, and the crossover in the ionization times at high temperatures seems reasonable. This analysis, however, may not be valid at extremely high temperatures since multiple ionization would become important.
The results for the individual gases will now be studied in greater detail, particularly with the viewpoint of how well the calculations compare with the available experimental data.
100
CJ .,
50
~ 10
CI :I:
E 5 CJ
I-' 11.-
.5
4
•
•
• PETSCHEK a BYRON ( imp. = 7 X 10-°)
o WONG a BERSHADER • MORGAN
6 8 10
FIG. 3. Ionization times in argon.
A. Argon
Although the calculations for argon have been performed with a fair degree of success by several workers,8-10 the calculations were repeated, primarily to check the computer program, and to observe the consequences of using the most recent cross-sectional data. The results (Fig. 3) agree exceedingly well with the experimental observations of Wong and Bershader9 and fall somewhat above the high-purity results of Petschek and Byron. l This is not surprising since the Petschek and Byron results were obtained at lower temperatures than those of Wong and Bershader. Both experiment1 and theory8 clearly indicate that the effects of impurities are greater at low temperatures.
Wong and Bershader, in order to get their calculated results to match their experimentally determined ones, were forced to reduce the Harwell and Jahn atom-
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IONIZATION TI:\1ES IN SHOCK-HEATED GASES 2195
atom cross-sectional slope constant by a factor of 2.5 while keeping the electron-atom cross-sectional slope constant at the value which was first proposed by Petschek and Byron. This difficulty is resolved when the more recent atom-atom cross-sectional slope constant of 1.2X10-19 cm2/eV as determined by Kellyll is used in conjunction with the electron-atom result of 4.9X 10-18 cm?/eV obtained by Zapesochnyi and Felston.19
In a recent paper, McLaren and Hobson20 published a value for the argon atom-atom cross-sectional slope constant which was a factor of 5 lower than that obtained by Kelly. They also stated that if Kelly had taken into account the flow nonuniformities resulting from the boundary layer growth on the shock-tube wall, his results would have decreased by a factor of 3. However, a check of Kelly's run no. 945 in argon shows that this effect could not change Kelly's result by more than 50%, a change which has no appreciable effect on the total ionization times.
B. Xenon
The wide disparity between calculated and experimental results for ionization times in xenon becomes readily apparent upon examination of Fig. 4. No two sets of experiments provide the same results and the calculated curve is observed to differ from many of the experimental points by factors of up to 80, depending on whose results are used and at what temperatures
~ / 50 - ~
x /.l tol ,," CJ ~ "/.o .6,'
30 CJ / .6. ~' 20 0 /' .j- ,/ 6
0 00 x c , / '
10 /,9 / ."..~,:'; / . I
0 , / / cv / • I on
::L , ;W tf 5 o;r . I
'" / - '66 :I: . d6
E 3 _"'6g; . ,6
0
.I 0 I
I-' .I /R 0:- j I
I I
0 --'-TURNER 0
6 - - - - ROTH a GLOERSEN o ---GLOERSEN
0.5 x HARWELL a JAHN 0 SMITH
2 4 6 8 10 12 14
I/Tao x 105 (OK-I)
FIG. 4. Ionization times in xenon.
100
u ., .. ~
50
llO
E u
I-' 5 0..-
• MULLANEY a BROWN o MORGAN
FIG. 5. Ionization times in neon.
7
they are compared. To facilitate the comparison of the data, "best lines" have been drawn for each set of experimentally determined points. In the case of Gloersen,5 this is all that could be done since he has not provided the necessary information required for the plot. Specifically, he presents only the ionization times without regard for the initial pressures used to obtain them. He does, however, furnish the range of initial pressures, and if, as seems logical, the assumption is made that the highest shock temperature was achieved with the lowest forepressure, and the lowest temperature with the highest pressure, it becomes possible to plot his extreme points. This is shown in Fig. 4, together with a presentation of his "best line."
On the average, Turner's results12 are a factor of 2 higher than Roth and Gloersen's.4 Gloersen's data5 is a factor of 2 higher than Roth and Gloersen's at low shock temperatures and as much as a factor of 10 higher at high shock temperatures, thus exhibiting an entirely different slope than those obtained by either Turner or Roth and Gloersen.
The data obtained by Smith, however, is seen to give excellent agreement with the calculated results when an atom-atom cross-sectional slope constant of 6.0X 10-21 cm2/eV is used. This is a factor of 3 lower than that obtained by Kelly, but as Kelly notes, there is some doubt about his results for xenon, apparently due to impurities for these particular tests. Hence it can be concluded that Smith's results essentially agree with the calculations.
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2196 M. MERILO AND E. J. MORGAN
MIRELS THEORY
<P o
o ROTH a GLOERSEN • GLOERSEN
.2 .3:4.5 .7 1.0 2 3 4 5 7 10
NON DIMENSIONAL DISTANCE FROM DIAPHRAGM
FIG. 6. Available test times in shock tubes.
20
Harwell and Jahn,7 as a result of their experiments on the initial ionization rates in xenon, quote approximate times required for electron-atom collisions to become as effective as atom-atom collisions in ionizing the xenon. These have also been plotted in Fig. 4, and it is observed that the time required for this stage to be reached is approximately 15% of the time calculated for equilibrium ionization. This compares favourably with similar results calculated and observed for argon. The remarkable feature of the Harwell and Jahn results is that their times required for only a fraction of the ionization to occur are greater than the total times measured by the other investigators except Smith. If the observations of Harwell and Jahn, Smith, and the present calculations are correct, one is forced to the conclusion that neither Turner, Gloersen, nor Roth and Gloersen actually measured the time required for ionization equilibrium in xenon.
C. Neon
Very few experiments have been performed in shockheated neon, mainly because of the long times and the high temperatures required to ionize it. As byproducts of investigations on magnetic interaction with shockionized neon, Mullaney and Brown14 and Morgan13 provide a few data points for the ionization time. These points are shown plotted in Fig. 5, together with calculated results obtained by assuming two different atom-atom excitation cross-sectional slope constants. Since the experimental points lie between the two calculated curves, the correct cross-sectional slope constant is probably intermediate between the two assumed values.
D. Krypton
No experimental data has been published to date on the total ionization times in krypton; hence no conclusion concerning the validity of the calculated results can be drawn.
IV. DISCUSSION OF THE XENON EXPERIMENTS
The first experiments on ionization times in xenon were performed by Turner in 1956. When describing
his experiments he states that a leak rate of 5 p./min was considered to be satisfactory so that for an elapsed time of approximately 2 min between filling the driver section and diaphragm rupture, the impurity level rose to 10 J.I.. For an initial pressure of 10 torr, this is already 0.1 %. Also, when the xenon was later analyzed with a mass spectrometer it was "found to contain less than 0.1 % of air" or about the same amount that leaked into the tube. Hence for tests with an initial pressure of 10 torr the impurity level was approximately 2 in 103. Above Mach numbers of 10, initial pressures of 5 torr were used, thereby increasing the impurity level even further.
An examination of the equilibrium ionization as predicted by the Saha equation shows that below Mach 8, the equilibrium ionization of xenon is less than or equal to the impurity level achieved in the shock tube, while at the maximum Mach number of 11, the impurity level is still greater than 10% of the final degree of ionization. It thus becomes clear that the number of impurities present was so high as to totally obscure the results which Turner hoped to measure. To further complicate matters, he found that a change in supply bottles of xenon increased the observed ionization times by a factor of 5, an observation which lends support to the statement that the effect of impurities obscured the desired results.
Gloersen states in his paper that "when the shock is near the end of the shock tube, the trailing edge of the luminosity is consistently about 10 em ahead of the xenon-helium interface calculated on the basis of idealized shock theory." However, it is well known that due to viscous effects, the testing time available in a shock tube can differ considerably from the ideal value.
In an ideal shock tube, the separation distance between the shock and the contact surface, and hence the available test time, increases with distance from the diaphragm. However, experiments21 have shown that the test time asymptotically reaches a limiting value. This testing time decreases with both initial pressure and shock-tube diameter and is well understood in terms of the boundary-layer theory of Mirels.22
Using the data obtained by Roth and Gloersen and speculating that the time T which they measured is actually the available test time, their points have been converted to dimensionless distance between the shock and the contact surface L and plotted as a function of the dimensionless distance from the diaphragm X. The results are shown in Fig. 6. The agreement of these points with Mirels' theoretical curve is striking. This agreement is comparable with that obtained by Fox, McLaren, and Hobson23 in their experiments designed to test Mirels' results. It can be concluded that there was insufficient time available in Roth and Gloersen's experiments for the xenon ionization to go to completion and that the arrival of the contact surface was responsible for the observed exceedingly rapid decay of luminosity.
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IONIZATION TIMES IN SHOCK-HEATED GASES 2197
Gloersen's results are not as easily interpreted since he does not provide the necessary information as to the conditions under which his experimental points were obtained. Readings were taken at five points along the shock tube, and the shock speed variation was achieved by varying the pressure in the driven section from 0.78 to 4 torr. Hence, if the lowest shockspeed results are considered, it can be conjectured that these were obtained at the highest initial pressure and at the point farthest downstream of the diaphragm. Conversely, at the highest shock speed it seems plausible that the lowest initial pressure was used, and that the ~easurements were taken at the point closest to the dIa~hragm. Utilizing these assumptions and again speculatmg that the measu.red times are actually test times, very good agreement IS once more found with Mirels' analysis as shown in Fig. 6. Thus in this case also, it seems that the arrival of the contact surface precluded a,ny ~easurements of the time required for the ionizatIOn m xenon to reach equilibrium.
This hypothesis also provides plausible explanations for several seemingly strange observations. Gloersen reported that when a spark-coil leak tester was used to drive impurities from the shock-tube wall it was found that the luminosi ty delay times did not' change, contrary to the experiences of Petschek and Byron and Harwell and Jahn. This result is quite plausible when it is realized that impurities in small quantities should have no appreciable effect on the separation distance or the test time. Furthermore, Gloersen's inability to observe the "interface" by the addition of iron pentac~rbonyl or benzene to the driver gas is not surprising smce the peak ionization which he observed was actually the interface.
R h24' • ot mvestigated the relative temporal behavior of visible continuum emission and resonance line emission behind shock waves in xenon, and a typical result is shown.in Fig. 7. The co~tinuum emission after reaching a maXImum decays rapIdly to zero, whereas the resonance radiation attains a maximum at the time of extinction of the visible emission and then decays fairly slowly. This would at first glance tend to refute the suggestion that the decay of the continuum radiation signaled the arrival of the contact surface. The reason for this behavior is probably that hot xenon from the nonequiIibrium region is trapped in the boundary layer and can thus continue to radiate even after the passage of t~e contact ~urface. The continuum radiation decays rapIdly, but smce the gas is very nearly opaque to resonance radiation this decay would be considerably slower, hence this observation is still consistent with the proposed hypothesis.
V. SUMMARY AND CONCLUSIONS
Calculations have been performed for ionization times !n .arg?n, krypto~, neon, and xenon employing the IOnIZatIOn mechanIsm used by Morgan and Morrison.
CONTINUUM
~r-~------~----~---------iii (/)
~ w
TIME
FIG. 7. Emission profiles behind shocks observed by Roth.
The experimental results for argon are in excellent agreement with the calculations when the most recent estimates of cross section are used.
The scarce and relatively inaccurate experimental data for neon are consistent with the calculations for reasonable assumed values of the atom-atom crosssectional slope. This slope probably lies between Sx 10-20 and:SXlO-19 cm2/eV.
The experimental results obtained by Smith for xenon are in excellent agreement with the calculations assuming an atom-atom cross-sectional slope of 6X 10-21
cm2/eV, which is about three times that obtained by Kelly, a discrepancy which is not unreasonable considering the difficulties of making accurate experimental measu:emen~s. The exp.erimental results obtained by other mvestIgators are mcompatible with the present results and the results of Smith. It has been shown that the ex.cessiv~ly low ionization times obtained by these other mvestIgators are probably due to impurities and inadequate testing times.
Experimental data on overall ionization time in krypton are not yet available.
It may thus be concluded that the ionization mechanism as used by Morgan and Morrison is consistent with the known experimental data for all the noble gases at moderate shock strengths (up to Mach 20). The alternative ionization mechanisms proposed by others to explain the spurious experimental data for xenon must be regarded as being without foundation. There is a need for further careful measurements of over-all ionization times in neon and krypton.
* This .work was financially supported by the National Sciences FoundatIOn, Grant No. GK-462.
~ H. Petscheck and S. Byron, Ann. Phys. N.Y. 1,270 (1957). W. M. Kornegay and H. S. Johnston J. Chern. Phys 38 2242
(1963). " , 3 D. S. Hacker and H. Bloomberg J. Chern. Phys 39 3263
(1963). ,. , 4W. Roth and P. Gloersen, J. Chern. Phys. 29, 820 (1958). 6 P. Gloersen, Phys. Fluids 3,857 (1960). 6 L. M. Biberman and I. T. Yakubov, Zh. Tekh. Fiz. 33, 1344
(1963) [Soviet Phys.-Tech. Phys. 8, 1001 (1964)]. 7 K. E. Harwell and R. G. Jahn, Phys. Fluids 7,214 (1964). 8 E. J. Morgan and R. D. Morrison Phys. Fluids 8 1608
(1965). " °H. Wong and D. Bershader, J. Fluid Mech. 26, 459 (1966). 10 M. I. Hoffert and H. Lien, Phys. Fluids 10, 1769 (1967).
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'2198 M. MERILO AND E. J. MORGAN
11 A. J. Kelly, J. Chern. Phys. 45, 1723 (1966). 12 E. B. Turner, Armco Services Technical Information Agency
Document No. AD 86309, University of Michigan, May 1956. 13 J. A. Smith, Phys. Fluids 11, 2150 (1968). I. E. J. Morgan, Ph.D. thesis, Cambridge University, 1960. 1. G. J. Mullaney and E. A. Brown, J. Appl. Phys. 37, 3514
(1966). 16 J. P. Appleton and K. N. C. Bray, J. Fluid Mech. 20, 659
(1964) . l7C. E. Moore, Natl. Bur. Std. (U.S.), Circ. 467 (1949).
THE JOURNAL OF CHEMICAL PHYSICS
18 R. B. Brode, Rev. Mod. Phys. 5, 257 (1933). 191. P. Zapesochnyi and P. V. Felston, Opt. Spektrosk. 20,
521 (1966) [Opt. Spectrosc. 20, 291 (1966)]. 20 T. 1. McLaren and R. M. Hobson, Phys. Fluids 11, 2162
(1968) . 21 R. E. Duff, Phys. Fluids 2,207 (1959). 22 H. Mirels, Phys. Fluids 6, 1201 (1963). 23 J. N. Fox, T. 1. Mc1,aren, and R. M. Hobson, Phys. Fluids 9,
2345 (1966). 2. W. Roth, J. Chern. Phys. 31, 844 (1959).
VOLUME 52, NUMBER 5 1 MARCH 1970
Matrix-Isolation Studies of the Infrared Spectra of the Free Radicals CCla and CBra
ERNEST E. ROGERS • STANLEY ABRAMOWITZ, MARILYN E. JACOX, AND DOLPHUS E. MILLIGAN
National Bureau of Standards, Washington, D. C. 20234
(Received 25 September 1969)
CCI, has been stabilized both by the reaction of lithium atoms with CCl. in an argon matrix at 200K and by the vacuum-ultraviolet photolysis of HCCI, or of DCCI, in an argon or a nitrogen matrix at 14°K. The analogous techniques have been found to lead to the stabilization of CBr,. The product spectra obtained in the lithium-atom experiments are considerably simpler than those obtained in the previous studies of the reaction of lithium atoms with matrix-isolated CX •. In the vacuum-ultraviolet photolysis experiments, DCC!" HCBr2, and CBr2 have also been observed. The absorption frequencies and contours obtained for Va of CCla and of CBra are independent of the method used to produce these species, suggesting that lithium atoms and their reaction products do not appreciably perturb the degenerate stretching mode of either CCla or CBra. Despite yields of these species comparable to those previously reported, absorptions at 674 and at 582 cm-t, previously attributed to VI of CCla and CBra, respectively, are completely missing from the present experiments. No other absorption attributable to VI of either species has been detected. It is concluded that a pyramidal (C,.) structure for CCI, and for CBr, has not been established.
INTRODUCTION
Recently, the infrared spectrum of the free radical CCla has been reported by Andrews1 ,2 and the infrared spectrum of the free radical CBra by Andrews and Carver,a who studied the reaction of lithium atoms with the corresponding tetrahalomethane isolated in an argon matrix at cryogenic temperatures. Since two absorptions were attributed to stretching fundamentals of each of these species, it was inferred that both CCla and CBra are nonplanar, as has been established4- 6 for the related molecule CFa.
Nevertheless, there remain problems regarding the proposed assignment of the stretching fundamentals of CCla and CBra. Although the degenerate stretching fundamental Va of these species is extremely prominent, the absorption attributed to the nondegenerate stretching fundamental VI is weak. A 7 -cm-1 discrepancy was reported2 between the observed and calculated values for VI of laCCla, assuming tetrahedral valence angles for CCls. Since the infrared spectra recorded in these experiments are quite complicated, with several other products stabilized in appreciable concentration, it is possible that the weak absorption attributed to VI of the CXa radical may in fact be contributed by some other product species. Of still further interest is the electron spin resonance study of CCla by Magat, Leray, and Roncin,7
whose data are consistent with a planar structure. For a planar molecule, VI would, of course, be infrared inactive.
In the light of these difficulties, it would be desirable to study the infrared spectra of CCIa and of CBra produced under experimental conditions different from those heretofore reported. Two such approaches are described in the following discussion. The first of these, as in the earlier studies, involves the reaction of lithium atoms with the tetrahalomethane isolated in an argon matrix, but under conditions such that the complexity of the product spectrum is greatly reduced. The second approach involves the vacuum-ultraviolet photolysis of chloroform and bromoform isolated in an argon or a nitrogen matrix, a technique analogous to that by which it has previously been found that CFa can be stabilized in significant concentration.5
EXPERIMENTAL DETAILS8
CCl4 and CBr4 (Fisher Scientific Company) used ill the lithium-atom reaction experiments were freed of dissolved gases by pumping on the sample condensed at 77°K. Ar: CCI4 and Ar: CBr4 mole ratios of approximately 100 and 200, respectively, were employed. After having been washed with hexane, a sample enriched to greater than 99% lithium-7 was loaded into a stainless-
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