396
J. Phys. Chem. 1980, 84, 396-401
Sonoluminescence of NO- and NO,-Saturated Water as a Probe of Acoustic Cavitation C. Sehgal, R. G. Sutherland, and R. E. Verrall” Department of Chemistry and Chemistry Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N OW0 (Received June 27, 1979) Publication costs assisted by the University of Saskatchewan
The sonoluminescence spectra of NO and NOz have been studied at an insonation frequency of 459 kHz and compared with thermal emission, chemiluminescence,and fluoroescence from NOz. The measurement of the short wavelength cutoff of the sonoluminescence spectra gives a minimum value of AHo = 72.5 kcal/mol for the reaction NOz NO + 0, in fair agreement with thermal data. There is considerable loss of structure because of the nature of the cavitation phenomenon. Evidence in support of the thermochemical origin of sonoluminescence has been obtained. Using the relative distribution of intensities, we have estimated the intracavity temperatures to be 1350 and 860 K for NO- and NOz-saturated solutions, respectively. These results support the “hot spot” theory of cavitation. Also, some experimental evidence in support of Gilmore’s mathematical model for the dynamics of bubbles is obtained.
-
Introduction Three distinct types of emission in the visible and near-infrared spectral regions have been observed from electronically excited states of nitrogen dioxide. They include thermal emission arising from collisional excitation of molecules at elevated temperatures in shock wave heated gases and f l a m e ~ ; l -chemiluminescence ~ due to radiative recombinations of 0 + NOk8 in air afterglow, and from O3 + N09J0 and Hg (A = 4358 A) fluorescence of NO2.lo Analysis shows that these transitions occur from the same electronically excited state of NOz, any difference in spectral distribution being due to a difference in the population of vibrational levels of the excited state. For example, the same low lying vibrational levels of NOz* are populated in the NO O3 chemiluminescence and in the thermal emission spectrum of NOz excited at 1200 K, whereas in NOz fluorescence and NO 0 emission spectra higher vibrational levels appear to be populated. The dissociation of NO2 to NO + 0 is very rapid1 and the equilibrium concentration of NOz is small for the temperatures at which thermal emission is observed. To circumvent this experimental problem shock tubes have been used to heat the gases over extremely short intervals such that emission can be observed before appreciable dissociation takes place and an equilibrium is achieved. The emission, therefore, is observed from a quasiequilibrium mixture of NO, 0, and NOz. Acoustic cavitation can heat cavity contents during the collapse time of cavities which is estimated to be of the order of lo* s12J3or less. Thus, the phenomenon is believed to give rise to reaction conditions somewhat like shock tube experiments. In this paper we report sonoluminescence from nitric oxide and nitrogen dioxide saturated solutions and compare it with the emission obtained from other sources. A mechanism of sonoluminescence is proposed in terms of intracavity chemical reactions and is the basis for obtaining estimates of cavitational parameters. Also information on the origin of sonoluminescence is obtained along with some experimental evidence in support of mathematical models proposed for bubble dynamics.13J4
+
+
Experimental Section Sonoluminescence was measured by means of a singlephoton counting system previously described.15 Singly distilled water was degassed and saturated with NO and NO2 separately by using the techniques described earlier.16 Research grade NO and NOz gases (Matheson) were used 0022-3654/80/2084-0396$0 1.OW0
without further purification. The gas-saturated water solutions were insonated a t a frequency of 459 f 1 kHz with a nonfocussing piezoelectric transducer17 with a rated average output E,, of 6.2 X W mM2,When cavities collapse the cavitation energy is recovered in several forms, mainly as shock waves, chemical energy, etc. Therefore, only a very small fraction of the total absorbed acoustic energy is available for chemical reactions.18 The Fricke dosimeter was used to gain some measure of the chemical energy absorbed. The procedure17was modified to include the addition of sulfamic acid to quench oxidation of Fez+ arising from secondary reactions with insonation products, e.g., HNOZ, known to be produced when air-saturated solutions are insonated. The spectrophotometric determinations of [Fe3+]at 304 nm were made by using a Cary 14 spectrophotometer. The chemical energy was determined to be 13.4 erg mL-’ min-l based on an assumed value of GFe3+= 3.9 ions per 100 eV.I9 The insonation cell and contents were maintained at a constant temperature of 285 f 2 K. An excess pressure of 0.7 atm was maintained over the solutions by use of nitrogen gas. The spectra were recorded at a slit width of 2000 pm (Ax = 40 A) with a time constant of 3 s. The area under the peak was recorded simultaneously by using an Omniscribe strip chart recorder (Model 5233-51). There is an inherent problem associated with the measurement of the absolute intensity of sonoluminescence and it arises because of the following reasons. Sonoluminescence occurs from the gaseous phase of cavitating bubbles16 which are small, have widely differing sizes, and are mobile in the liquid phase, As a consequence of their random disposition in the cavitation field they project different angles at the photomultiplier tube. Also, the number of bubbles seen by the photomultiplier tube changes with time. Therefore, because of these uncertainties, it is not possible to measure the absolute intensity of sonoluminescence. Results Figures 1and 2 show the luminescence spectra of NOand NOz-saturated solutions, respectively. The banded nature of the spectra is apparent but the structure of the individual bands is not well defined (the reasons for this are discussed later). A similar loss of band structure is observed in the emission from high-temperature flames containing NOz.5 Sonoluminescence spectra of NO-saturated water consists of two spectral bands: (i) a continuum 0 1980 American Chemical Society
The Journal of Physical Chemistty, Vol. 84, No. 4, 1980
Sonoluminescence as a Probe of Acoustic Cavitation
397
TABLE I: Intensity Distribution of Nitrogen Dioxide Sonoluminescent, Fluorescent, and Chemiluminescent Spectra
sonoluminescence,NO-saturated water sonoluminescence, NO,-saturated water chemiluminescence, NO + 0 (air after glow)" fluorescence, NO, Hg ( h = 4 3 5 8 ) excitation'' chemiluminescence, NO + 0,"
495-570
615-670
670-720
720-775
nm
nm
nm
nm
nm
2.62 2.44 0.37 0.36 0
1 1 1
0.52 0.56 0.83 0.82 2.38
0.35 0.33 0.62 0.46 3.40
10.32 10.33
1 1
390-700
-
TABLE 11: Relative Intensities of Sonoluminescence from N O and NO, Solutions in Various Wavelength Ranges
_.
495-570
615-700
1 3.41
3.44
nm
sonoluminescence,NO,/H,O sonolurninescence, NO/H,O
t VI
w E
El!-
'nm
OL
720-775
590-700
1 3.20
1 3.67
3.35
1
I
'
200
670-720 nm
i
w
___
I--
1
300
400
-_--_-__L-
500
700
600
c
z
W
k.
LA
Om0
nm
1
390-700
nm
av
1 3.40
1 3.44
3.42
nm
1
minescence and fluorescence stronger emission occurs at longer wavelengths. Table I1 shows the sonoluminescence flux from NOsaturated water as compared to that from NO2 saturated solution in the various wavelength ranges. These results show that sonoluminescence from NO-saturated water is 3.42 times stronger, on the average, than that arising from NO2 solution.
Xhm)
Figure 1. Sonoluminescence spectrum of nitric oxide saturated solution insonated at 459 kHz.
Lo
nm
-
400-540
300
I
400
I
500
I
I
600
700
I
BOO
X (nm)
Figure 2. Sonoluminescence spectrum of nitrogen dioxide saturated solution insonated at 459 kHz.
from 230 to 350 nm with a maximum at 280 nm; and (ii) a broad continuum extending from 400 nm to the nearinfrared. The first continuum has a cutoff at ca. 248 nm. The intensity of emission of the second continuum increases with a decrease in wavelength and reaches a maximum value at -4400 nm and after that decreases to the background at 395 nm. The cutoff value of 397.5 f 2.8 nm is in good agreement with the value of 397.5 nm observed for the chemiluminescence of NO + 0 in the air afterglow.' Sonoluminescence from NO2-saturated solution shows a broad continuum extending from 400 nm to the near infrared. This continuum is similar to the long wavelength continuum of the sonoluminescence spectra of NO-saturated solution. The cutoff occurs a t 402.5 f 7.5 nm and the maximum is slightly shifted to longer wavelength, 410 nm. Table I compares the spectral distribution of sonoluminescence from NO- and NO2-saturated solutions with that of chemiluminescence from NO + 0 and NO Os, and also with mercury excited NOz fluorescence. The relative emission intensities were obtained by measuring the area under the emission curve in the region of 400-700 nm. A comparison of various rows of Table I shows the spectral distribution of sonoluminescence from NO- and NOz-saturated water is similar to but different from that of chemiluminescence and fluorescence. There is a stronger emission at the shorter wavelengths in the case of sonoluminescence whereas in the case of chemilu-
+
Discussion Sonoluminescence from NO and NO2 aqueous solutions occurs in the same spectral region as NO2 fluorescence (excited by Hg at X = 4358 A),air afterglow emission, NO + O3 chemiluminescence,and thermal emission from NOz. This suggests that it occurs from the same metastable electronic excited state of NO2. Also, since the various emissions from NO2 (fluorescence, chemiluminescence, etc.) are from NO2* in the gaseous phase, it may be concluded that sonoluminescenceoccurs from the same phase, i.e., from the gaseous phase of cavitating bubbles. This is in agreement with previous result# obtained from alkali metal salt solutions. The lack of discrete structure in the sonoluminescence spectra is due to the poor resolution caused by the use of large slit widths. It is not likely, however, that improved resolution can be achieved by smaller slit widths, since the high cavitation temperatures populate a large number of rotational levels leading to spectral diffuseness. As it is not possible to regulate intracavity temperatures and compositions the only information that one can obtain about the continuum radiation is from the distribution of spectral intensity with wavelength, the wavelength of maximum intensity, and the short wavelength cutoff. The latter occurs at 397.5 f 2.5 nm (Figure 1)and gives the dissociation energy of NOz at 0 K (i.e., NO2 N0(211) + O(3P))as 72.0 f 0.5 kcal/mol. Since the kinetic energy of the products formed at room temperature is -0.5 kcal/mol, the heat of reaction at room temperature is 72.0 - 0.5 = 71.5 kcal/mol in good agreement with the thermal value of 72.0 kcal/moP and the spectroscopic value of 71.2 k~al/mol.~ Analysis of Emission. The difference in spectral distribution of sonoluminescence from that of thermal emission, NOz fluorescence, and NO + 0 and NO + O3 chemiluminescence can be explained with the aid of a two-dimensional potential energy diagram as shown in Figure 3. Curves A, B, C, and D are the potential energy curves of NO2in the various electronic states. The diagram for a triatomic molecule should be four dimensional, hut for the present discussion a two-dimensional plot is adequate. The difference in the spectral distribution is associated with excitation of different upper states of NOz by the
-
398
Sehgal, Sutherland, and Verrall
The Journal of Physical Chemistry, Vol. 84, No. 4, 1980
id .......
.....
y
1 5
I r
+
w0
Flgure 3. Potential energy curves for NO2.
-
various energy sources. The reaction NO + O3 NO2 + O2 is exothermic with a AH of reaction equal to -49 kcal/mol.10 This means the energy released by NO + O3 can excite the molecules t o the vibrational level which is 49 kcal/mol above the ground state (line ab of Figure 3). The short wavelength cutoff observed for such a system is 590 nml0 which corresponds to -48.5 kcal/mol, in agreement with the expected value. The heat of reaction NO 0 NO2 is -72 kcal/mol. The spectroscopic short wavelength cutoff is observed at 397.5 nm which corresponds to -72 kcal/mol. Thus the heat evolved by the reaction between NO + 0 produces NOz molecules in the excited state C (see line ef of Figure 3) from which they radiate directly to state A. Also, since the potential energy curve of state C intersects that of B at -64 kcal/mol, the NOz* molecules formed in state C can by a resonance transfer process populate the B state and subsequently radiate to the ground state. Therefore, the emission at shorter wavelengths (i.e., from -64 to 72 kcal/mol) occurs directly from the C state and at longer wavelengths &e., 400 nm are similar (cf. rows 1 and 2 of Table I), it is reasonable to assume that excitation in both cases proceeds by the same mechanism, Le., via the
+
same potential energy surfaces involving barriers of the same height. Thus in the case of NO-saturated solution the formation of NO2 via ultrasonic-induced oxidation of NO should precede its excitation. It has been shown that acoustic cavitation in water produces hydroxyl radicals in the ground and excited states.21t22The excited OH radicals in turn may produce O(3P)and H(2S)by predissociation. Thus there are two possible routes for NO2*formation in NO-saturated solution: (i) NO combines directly with O(3P)as in the case of air afterglow to produce emission or, (ii) NO reacts with OH to produce emission via a potential energy surface different from that of NO 0 and NO + 03. In the former case one should expect the intensity distribution of sonoluminescence to be similar to that of air afterglow emission, whereas in the latter case population of different vibration-electronic excited states of NO2 should occur. Because the spectral distribution of sonoluminescence is different from the other kinds of emission, therefore, the second mechanism seems to be more likely, i.e., NO + OH NO2 H. Since no emission from H20*,OH*, etc. is observed as in the case of argon-saturated water,21p22it follows that these species are scavenged by NO2 before they can possibly radiate, i.e.
+
+
NO2
+
+ M 5 NO2* + M
(1) where M is an energized chemical species, such as H20*, OH*, OH, etc., and collisionally activates NO2. NO2*, so formed, is removed by quenching and dissociation, i.e.
NO2*
+ M A NO2 + M
(2)
-% NO + 0 + M
NO2* + M
(3) The reverse of (3) need not be considered because oxygen atoms are removed irreversibly by fast reactions like H 0 -OH, OH O-+H02, a n d N 0 2 0 - N O 02,Using the steady state treatment, we can express the NOz* concentration as
+
+
+
+
If Z1, 22, .... Zi are the collision numbers and P I ,P2,.... Pi the proportion of energetically favorable encounters for which electronic-vibrational excitation occurs by collision with the species 1, 2, .... i, then the rate constant for excitation to the energy level by all the species 1, 2, .... i is given by the relationship (5) kl = Cki = C(PiZi)exp(-EA/RT) I
i
The factor EiPiZi= CiPiCiZi- CCiji#jPiZj.The term CCi/#jPiZj= 0 because cross terms do not occur, therefore, CiPiZi = CiPiCiZi= PZ,, where 2, and P are the
collision number and proportion of energetically favorable encounters, respectively, for which electronic excitation occurs. Substituting the relationship into eq 5, one obtains kl = Z&' exp(-EA/RT) (6) By the principle of microscopic reversibility the number of collisions effective in quenching will be equal to 2 8 (i.e., k2 = Z,,P). In a rapidly compressing cavity only a partial chemical equilibrium is achieved and, strictly speaking, the principle is not likely to hold true. To take into account the quasi-equilibrium condition, the number of collisions effective in quenching can be written as 6Z$, where 6 is a correction factor whose magnitude is a measure of the
The Journal of Physical Chemktty, Vol. 84, No. 4, 1980 399
Sonoluminescence as a Probe of Acoustic Cavitation
degree of deviation from the equilibrium distribution. When 6 = 1, the system is a t an equilibrium and the principle of reversibility holds. For the case 6 > 1, the quenching process (i.e., k2) would be efficient and [NO2*] concentrations at any instant will be less than the equilibrium concentration. Using the Kassel term, we can ‘derive the rate constant k, asz k3
RT]1 -[-4)
= %Z0[ E D - E A
ex.(
E D- E A
(7)
where E D is the dissociation energy of the NO2 molecule. Therefore substituting eq 6 and 7 into eq 4 gives the following expression for [NOz*]: p exP(-&/R?3 [NO2*] = (8)
The total emission intensity Q is given by the equation Q = N[NOZ*]/7 photon s-l L-l (9) where N is Avogadro’s number and T the radiative lifetime. An expression for the transition from the energy level E, to the ground state Eo can be obtained from Levitt’s equation (eq 2, ref 2 ) and eq 8 as follows:
2E, - Eo RT
]
photons s-l L-’ cm-’ (10) Furthermore, one can assume the observed intensity at a given wavelength (dQ/dh), is proportional to the absolute intensity (dQ/dh),, i.e. (dQ/dX), = t(dQ/dh), (11) where 5 is a proportionality constant dependent upon the experimental conditions (viz., nature of insonation cell, transducer, etc.). Equating eq 10 and 11 and taking the ratio of the resulting equation at two different wavelengths hl and hz, one can obtain the following expression:
For the case where E,, = ED and EA, = E,, eq 12 can be reduced to the form
where
- 0
1000
500
I500
2000
ED - E A ( c a l l
Flgure 4. Plot of Y vs. (E, I
- E A )for I
, 500
nitric oxide saturated solution. I
I
,
I
1000
ED
1500
- EA (cal)
2000
2500
Figure 5. Plot of Yvs. (ED- EA)for nitrogen dioxide saturated solution.
Since the cavitational temperatures are large,14*ls therefore, in the specific region where E, approaches ED higher order terms in eq 13 can be neglected and the line should approach linearity. Figures 4 and 5 show a plot of Y vs. ED - EA taken from the experimental data and this result is confirmed. The linear portion of the curve extends over a wider range of ED - E , in the case of NO- than for NOz-saturated solutions (see below) which makes the neglect of higher order terms to be less valid at relatively lower values of ED - E, in the case of NOz. Initial slopes of the curves in Figures 4 and 5 were determined to be -7.5 X and -1.18 X cal-l mol, respectively. Using a value of R = 1.98 cal K-l mol-’ we estimated the temperatures from these values to be 1350 f 50 and 860 f 100 K in the presence of NO and NOz, respectively. Emission in the Ultraviolet Region from NO Solution. The emission peak in the ultraviolet region (first continuum of Figure 1) is probably due to higher intracavity temperatures for NO- as compared to NOz-saturated solutions. The difference between the low wavelength cutoff limits of the two continua is 115.3 - 72.0 = 43.3 kcal/mol (1.877 eV), which is less than the excitation energy of 1.967 eV for the ‘D state of the 0 atom. Therefore, the first continuum (Figure 1)cannot be ascribed to the dissociation N0(211)+ O(’D) but it is probably due to dissociation into N(4S) + Oz(3Z;)23. The predissociation limit calculated from the short wavelength cutoff is at 248 f 22 nm (Le., 115.3 f 10 kcal/mol) as compared to the value of 115.6 f 0.7 kcal/mol obtained from the absorption spectrum of NOz.z3 Analysis of Cavity Properties. The study of sonoluminescence of NO- and NO2-saturated solutions provides some information on the mechanism by which the phenomenon occurs. Despite the fact that sonoluminescence was first discovered some time ago, so far there has been no unified point of view on the reason of its appearance. Several hypotheses have been suggested and They generally may be categorized into two classes: electrical and thermal. The growing experimental evidence seems to substantiate the view that the origin is thermochemical. According to the “hot spot” theory14 it is believed that a bubble undergoes an adiabatic compression and heats the intracavity contents. Due to finite collapse
400
The Journal of Physical Chemistry, Vol. 84, No. 4, 1980
Sehgal,
Sutherland, and Verrall
TABLE 111: Relative Values of RJRf of Nitric Oxide and Nitrogen Dioxide Saturated Solutions Calculated with Respect to Argon-Saturated Solutions
sonoluminescence, Ar/H,O
NO/H,O NO,/H,O a
Reference 14.
y 513 715 917
NN modela--HG modelb 1 2.6 6.2
1 2.1 4.0
These values were read f r o m Figure 1 of ref 27.
time and high temperature gradients at the bubble interface heat is lost to the surrounding l i q ~ i d ,Le., ~ ~the ,~~ bubble collapse is not completely adiabatic. Temperatures derived from the study of sonoluminescence from NO and NO2 solutions are based on the population distribution of molecules in various levels. These. estimated effective temperatures are therefore the actual temperatures within a cavity and not the adiabatic temperatures. A relation between the effective temperature T of a cavity and its compression ratio p has been derived by Young28
where a = 0.92(3R/M)‘/2,R is the gas constant per mole (8.31 J K-’ mol-’), PH is the ambient pressure in the liquid before a sound field of pressure amplitude PA is applied = 0.68 X lo5 N m-2, PA = 6.2 X lo5 N m-2 (ref 35), p = lo3 kg m-3, To = 285 K, = Rf/R,, R, is the radius at which bubble collapse begins, R f is the radius to which it collapses, and n = 3. This equation was solved by inspection for /3 for NOand N02-saturatedsolutions. The values obtained for the ratio R J R f are 5.9 and 9.7 for NO and NO2, respectively. According to the Noltingk and Neppiras modelt4 for bubble motion
where y is the specific heat ratio, q the gas pressure inside the cavity at R,, and P,, the total external pressure which is assumed to be constant because the collapse occurs very rapidly. For the case P,Jq = 100 (ref 14),the ratio R,/Rf is 8.1, 21, and 50 for y = 513, 715, and 917, respectively. If R, is the same for the three cases, it follows that absorption of energy by a molecule with additional degrees of freedom (i.e., decrease in magnitude of y) causes the bubble to collapse to a smaller radius. A similar conclusion can be deduced by using Gilmore’s theory13 based on the Kirkwood-Bethe h y p o t h e s i ~ .These ~ ~ features are illustrated in Figure 1 of ref 27 where Gilmore’s theory is used to determine an R vs. t curve for a bubble with an internal pressure ( 4 ) of 0.05 atm just before the collapse begins. A comparison of experimental and theoretical values of ( R c / R f )are given in Table 111. The experimental results show that R,/Rf increases with a decrease in y, as predicted by the theoretical models. The difference between the experimental and theoretical values is not unexpected. Noltingk and Neppiras’ model strictly speaking is only true for an incompressible liquid. Calculations, based on Gilmore’s model, take into account liquid compressibility and fluid velocity and give R,/Rf values closer to the experimental ones. A major source of discrepancy arises from the fact that
exptl
Tad, K 34OOc 2400 2000
Tg,K
_ I _ _ _ _ -
1C 1.7 2.8
245OC 1350 860
Values taken from ref 16.
the theoretical models are limited to only gaseous cavities. In reality bubbles contain vapor of the surrounding liquid and this dampens the intensity of cavitation by delaying the initiation of the collapse. Therefore, true R, values are lower than those predicted. Hence, it follows th.at the theoretical R,/Rf should be greater than the true value which is in agreement with our experimental results. Another source of discrepancy arises from the fact that, in theoretical studies, bubbles are considered to be spherical and isolated. In reality such a bubble is rarely encountered. Even at acoustic pressures not too far above the threshold values, an aggregate of cavities is formed. These bubbles interact with one another, undergoing fragmentation or coagulation before collapse. It t,herefore follows that the principles set down for the motion of an isolated cavitat,ion bubble will not be strictly true for a collection of interacting bubbles. Since, the experimental values obtained above refer to the average behavior of a bubble in a field, the difference between experimental and theoretical values is bound to occur. The fact that emission is observed from NO2* proves unequivocally that sonoluminescence is due to chemiluminescence and not due to thermal heating of intracavity gases to incandescence, as proposed by Srinivasan and H o l y r ~ y d , and ~ ~ ~Gunther ’ and c o - w ~ r k e r s Moreover .~~~~~ blackbody emission from compressing cavities would mean that there is thermal equilibrium inside the cavities, Le., the distribution is Maxwell-Boltzmann in nature and this is not the case because of the rapidly changing cavity conditions. The results of this study show that relatively high temperatures are produced inside cavities during cavitation, as .implied by the hot spot theory. Furthermore, the phenomenon of sonoluminescence occurs from the gaseous phase and is due to chemiluminescence caused by these high temperatures. Finally, the results indicate that the compression ratio /3 is directly related to y, as suggested by Noltingk and Neppiras and Gilmore.
Acknowledgment. Financial support from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. One of us (C.S.) acknowledges the award of a University of Saskatchewan Graduate Scholarship. References and Notes (1) R. E. Hoffmann and N. Davidson. J . Am. Chem. Soc., 81, 2311
(1959). (2) B. P. Levitt, Trans. Faraday Soc., 58, 1789 (1962). (3)W. E. Kaskan, Combust. F/ame, 2, 286 (1958). (4) M. L. Spealmn and W. H. Rodebush, J. Am. Chem. Soc., 57, 1474 (1936). (5) H. P. Broida, H. I. Schiff, and ‘T. M. Sugden, Trans. Faraday Soc., 57, 259 (1961). (6) A. Fontign and H. I. Schiff, “Chemical Reactions in the Lowor and Upper Atmosphere”, Interscience, New York, 1961. (7) A. Fontign, C. B. Meyer, and H. I. Schiff, J . Chem. Phys., 40,64 (1964). (8) M. A. A. Clyne and B. A. Thrush, Discuss. Faraday Soc., 33, 139 (1962);Roc. R. Soc. iondon, Ser. A , 289, 404 (1962). (9) J. G. Greeves and D. Garvin, J . Chem. Phys., 30, 348 (1959). (10) M. A. A. Clyne, 8.A. Thrush, and R. P. Wayne, Trans. Farachy soc., 60,359 (1964).
J. Pbys. Cbem. 1980, 84, 401-403
401
(11) B. P. Levitt, J . Cbem. Pbys., 42, 3, 1038 (1965). (12) Lord Rayleigh, Phil. Mag., 34, 94 (1917). (13) F. R. Gilmore, California Institute of Technology Hydrodynamic Lab Report No. 26 (1952). (14) B. E. NoRingk and E. A. Neppiras, Proc. Pbys. SOC.London, Sect. 6 , 63, 674 (1950). (15) T. Oka, A. R. Knight, and R. P. Steer, J . Cbem. Pbys., 63, 2414 (1975). (16) C. Sehgal, R. P. Steer, R. G. Sutherland, and R. E. Verrall, J . Chem. Phys., 70, 2422 (1979). (17) E. L. Mead, R. G. Sutherland, and R. E. Verrall, Can. J . Cbem., 54, 7, 1114 (1976). (18) L. D. Rozenberg, Akust. Zb., 11, 1, 121 (1965). (19) J. H. Todd, Ultrasonics, 8, 234 (1970). (20) F. D. Rossinl and D. D. Wagman, "Selected Values of Chemical
(24) H. G. Ftynn, "physical Acowtics", Vol. 18, W. P. Mason,Ed., Academii Press, New York, 1964, p 62. (25) M. G. Sirotyuk, Sov. Pbys. Acoust. (Engl. Trans/.),8, No. 3 (1963). (26) M. A. Margulis, Sov. Phys. Acoust. (Engl. Trans/.),15, No. 2 (1969). (27) R. Hickling, J . Acoust. SOC.Am., 35, 967 (1963). (28) F. R. Young, J . Acoust. SOC.Am., 60, 100 (1976). (29) J. G. Kirkwood and H. A. Bethe, OSRD, ref 588 (1942). (30) D. Srinivasan and L. V. Holyroyd, Pby. Rev., 99, 633 (1955). (31) D. Srinivasan and L. V. Hoiyroyd, J . Appl. Phys., 32, 446 (1961). (32) P. Gunther, W. Zeil, U. Gisar, and E. Heim, Z. Ekctrochem., 61, 188 (1957). (33) P. Gunther, E. Heim, and H. 0. Burgsted, Z . Electrochem., 63, 43 (1959). (34) T. F. Hueter and R. H. Bolt, "Sonics", Wiley, New York, 1955. (35) Assuming the front plate of the insonam cell to be a perfect reflector,
Thermodynamic Ropertiis", United States Government Printing m e , Washington, D.C., 1952. (21) C. Sehgal, R. P. Steer, R. G. Sutherland, and R. E. Verrall, J . Pbys. Chem., 81, 2618 (1977). (22) C. Sehgal, R. G. Sutherland, and R. E. Verrall, J . Pbys. Cbem., preceding paper in thls issue. (23) G. Herzberg, "Electronic Spectra of Polyatomic Molecules", Van Nostrand, Princeton, N.J., 1967.
I,, = 2€,, =
we estimated the pressure amplitude PAof a sinusoidal ultrasonic field by using the following equation^:^' PA2
2PC
I,, is the ultrasonic intensity. Taking p = lo3 kg m-3 and c = 1530 ms-', we estimated PAto be 6.2 X lo5 N m-'.
where
Emission Spectrum of Fluorobromocarbene in Solid Argon at 12 K John C. Miller and Lester Andrews. Department of Chemistry, University of Virginia, Charlottesville, Virginia 2290 1 (Received February 26, 1979; Revised Manuscript Received November 12, 1979)
The CFBr intermediate has been synthesized by vacuum ultraviolet photolysis of CHzFBr and CHF,Br and subsequently trapped in solid argon at 12 K. The fluorescence, following laser excitation at 424 and 428 nm, consisted of a 13-member progression in the ground-state bending mode with an average spacing of 327 cm-' and an electronic origin near 23 300 cm-'.
Introduction Recently, dihalocarbenes have been extensively studied, primarily by the technique of low-temperature, matrixisolation spectroscopy. CFZ,ldCCl,,7-13 and CBr$4-16have been investigated in this manner, as well as the asymmetric carbenes CFC117-20and CC1Br.'2J4J6 These intermediates were synthesized by reactions of carbon atoms with haloof alkali metals with tetrahalog e n ~ by , ~ reaction ~ ~ or by vacuum UV photolysis of the appropriate dihalomethane~.~~~~J~J~J~ In general, infrared studies have provided antisymmetric stretching frequencies, and optical emission spectra have given the bending modes for the ground state. Excited-state constants have been obtained from either absorption or tunable dye laser excitation spectra, and electronic lifetimes have also been measured. A useful summary of the spectroscopic constants and lifetimes has been given by Bondybey and English.16 This paper describes the laser-induced emission spectrum of fluorobromocarbene, CFBr. No previous results have appeared on this molecule, although an unanalyzed absorption band in the region 390-440 nm following flash photolysis of CHFBr, has been tentatively attributed to CFBr.*l In a related matrix study, the C-Br and C-F stretching fundamentals of CFBr have been identified in the infrared spectrum at 656 and 1157 cm-l, respectively, following argon resonance photolysis of CHzFBr.22 Experimental Section The CFBr intermediate was produced by codepositing CHzFBr/Ar mixtures (1/300 mol ratio) a t about 1-3 0022-3654/80/2084-0401.$0 1.OO/O
mmol/h with simultaneous irradiation from a windowless argon resonance lamp onto a polished copper wedge. As the argon from the lamp was also condensed on the cold surface, the final concentration is about 1/600 mol ratio. The lamp output, described p r e v i o u ~ l yconsists ,~~ mainly of 106.7- and 104.8-nm Ar resonance lines and 121.6-nm light from impurity hydrogen Lyman a emission. The light is energetic enough to photodetach hydrogen atoms, which then can diffuse away as the matrix freezes. In one experiment CHFzBr was used as the precursor and produced identical results. The synthesis of CHzFBr and CHFzBr have been reported e l s e ~ h e r e . ~The ~ , ~ substrate ~ was cooled to about 12 K by a CTI Model 21 closed-cycle refrigerator, and standard vacuum and gas-handling techniques were used. Excitation was provided by a pulsed nitrogen laser (Molectron UV 14) and a pumped dye laser (Molectron DL11) using the dye DPS (396-416 nm). The resulting emission was focused on the slit of a Spex 1401 double monochromator and detected photoelectrically with a RCA C31034 phototube and a Keithley 414s picoammeter.
Results and Discussion The emission spectrum (shown in Figure l), recorded from a sample of CH,FBr diluted in argon and subjected to argon resonance photolysis, consists of a long, 13-member progression of broad bands with an average vibronic spacing of 327 cm-'. The band positions and spacings are listed in Table I. The spacing undoubtedly reflects the ground-state bending mode vi', and the Franck-Condon intensities indicate a substantial change in bond angle between the ground and the excited state. The average 0 1980 American Chemical Society
