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C. P.; Murrel, A. J.; Vernon, P. D. F. Narure 1990, 344, 319-321. (4) Jones, R. H.; Ashcroft, A. T.; Waller, D.; Cheetham, A. K.; Thomas, J. M. Card. Lett. 1991, 8, 169-174. (5) Dissanayake, D.; Rosynek, M. P.; Kharas, K. C. C.; Lunsford, J. H . J . Card. 1991, 132, 117-127.
(6) Prettre, M.; Eichner, C.; Perrin, M. Tram. Faraday Soc. 1946, 43, 335-340. (7) Choudhary, V. R.; Rane, V . H. J . Coral. 1991, 130, 411-422. ( 8 ) Ito, T.; Watanabe, T.; Tashiro, T.; Toi, K. J . Chem. Soc., Faraday Trans. I 1989. 85, 2381-2395.
High-Resolution Stimulated Brillouin Gain Spectroscopy of Liquid Benzene Shows No Evidence of a “Structural Transition” W. Tandy Grubbs and Richard A. MacPhail* Department of Chemistry, P. M . Gross Chemical Laboratory, Duke University, Durham, North Carolina 27706 (Received: June 29, 1992; In Final Form: September 8, 1992)
We have used high-resolution stimulated Brillouin gain spectroscopy to search for evidence of a “structural transition” within the liquid phase of benzene. On the basis of previous measurements of depolarized light scattering intensities and Brillouin line shifts, it has been proposed that such a transition occurs in the temperature range 42-48 O C (Rozhdestvenskaya, N. B.; Smimova, L. V. J . Chem.Phys. 1991,95, 1223). Our highly accurate Brillouin shift measurements show a linear dependence on temperature from 24 to 69 OC,with no sign of any anomaly.
Introduction It has recently been proposed that there exists within the liquid phase of benzene a “structural transition” involving different local packing arrangements of the benzene molecules.’S2 While the psibility of different favored orientational packing arrangements in liquid benzene is well recogni~ed,~-~ the idea that competition between these arrangements could lead to a well-defined transition over a fairly narrow temperature range is probably surprising to many. Indeed, developing the correct interpretation of such a transition would present a considerable challenge. Before taking on this challenge, it is important to establish through further experiments whether this structural transition in fact exists. In this Letter we contribute to this effort through a high-resolution stimulated Brillouin gain study of liquid benzene. Rozhdestvenskaya and Smirnova first suggested the existence of a structural transition in liquid benzene on the basis of measurements of the integrated depolarized light scattering intensity, a quantity sensitive to orientational correlations in the fluid.’ They observed an increase in the scattering intensity as the temperature was raised from 7 to 65 OC, with two anomalous regions: the first corresponding to a break in slope between 27 and 30 OC and the second to a significant rise and fall in the intensity between 42 and 48.5 OC. Letamendia et a1.I0 have reported significant changes in the Brillouin line shift and Landau-Plazeck ratio of liquid benzene in the same 42-48 OC region. Pinan-Lucarre et examined the second moment of the depolarized Rayleigh wing intensity of benzene and found a break in the temperature dependence near 40 OC, but they observed no unusual temperature behavior in the integrated intensity of the “forbidden” Raman band at 405 cm-I. Rhozhdestvenskaya and Smimova have also reported “transition” behavior in the integrated depolarized light scattering intensity of liquid hexafluorobenzene.* The temperature dependence they observe for hexafluorobenzene is surprisingly similar to that seen in benzene, including a break in slope between 20 and 30 OC and a “transition” between 42 and 50 OC, except that the sign of the temperature dependence is reversed (i.e., the intensity decreases with increasing temperature). The technique employed here, continuous-wave (CW) stimulated Brillouin gain (SBG) spectroscopy, has recently been shown to be an effective way to obtain high-resolution Brillouin spectra of gases,12 liquids,I3J4and s01ids.I~In this experiment, two CW laser beams,one from a tunable pump laser and one from a weaker probe laser, are crossed in the sample. When the difference in
the frequencies of these two beams 52 = w2 - wl is equal to the Brillouin frequency, acoustic waves with wavevector k = kz- k, are stimulated in the sample, and energy is transferred from the higher frequency laser beam to the lower frequency one through Brillouin-induced four-wave mixing.16 The SBG spectrum is collected by scanning the frequency of the pump beam and monitoring the gain (or loss) in the probe beam power as a function of 52. The gain can be related to the spectral density S(k,52) measured in a spontaneous Rayleigh-Brillouin experiment” through the expression16
where G is the gain or fractional change in the probe power at the laser difference frequency 52, q is the crossing efficiency for the two beams, wl and o2are the frequencies of the probe and pump beams,respectively, N is the number density of the medium, kBTis the thermal energy, da/dQ is the scattering cross section, and P(w2)is the power of the pump beam. Several features of CW SBG spectroscopy make it particularly attractive for the problem at hand. First, extremely high-resolution SBG spectra can be obtained by employing narrow line width CW lasers. Perhaps more importantly, measurements of the Brillouin shifts and line widths can be made with high accuracy and precision since the frequency scale corresponds to the difference between two stabilized laser frequencies. However, because of the additional 52 factor multiplying S(k,52) in the square brackets in eq 1, the Rayleigh peak is suppressed and the Landau-Plazeck ratio cannot be determined from the SBG spectrum. Experimental Section
Our SBG spectrometer has been described in detail previous-
ly.I3J4 A frequency-stabilized ring dye laser (Coherent CR699-29) provides a tunable pump beam with 100 mW of power at 633 nm, and a frequency-stabilized HeNe laser (Laboratory for Science Model 200) provides a l-mW probe beam with low
amplitude noise. The vertically polarized pump and probe beams are loosely focused and crossed within the sample cell. Balanced detection and double modulation are employed to enhance the ~ignal-to-noise.’~J~ For the spectra reported here, the scattering angle was 178.7”, the time constants for the pre- and postfilters on our lock-in amplifier (Stanford Research Systems SR510)were set to 0.1 and 1 s, respectively, and the dye laser scan speed was
0022-3654/92/2096-8688%03.00/00 1992 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 8689
20 MHz/s. This choice of parameters, in combination with the jitter in the pump laser, results in an effective spectral resolution of about 10-MHz half-width a t half-maximum. An initial frequency scale for our spectra was generated by the computer and wavemeter attached to the dye laser. While the software corrects for most of the nonlinearities in the dye laser's wavemeter, the remaining error can be as much as 100 MHz over a scan of many gigahertz. To correct for these errors, we simultaneously collect a set of frequency markers for calibration, using a scheme similar to that of Moosmuller, She, and HUO.'~ In our implementation of this scheme, portions of both the dye and HeNe laser beams are sent to a reference confocal etalon (TecOptics SA-300). Since in our experiment each of these beams is chopped at a different frequency,l3*I4the transmission of the two beams through the etalon can be separated with two lock-in amplifiers. The output of one lock-in (corresponding to the HeNe beam) is fed to a PID circuit that locks the etalon cavity length to the side of a HeNe transmission peak. The output of the other lock-in (corresponding to the dye laser beam) generates the frequency markers as the dye laser is scanned. This same setup was used to determine the free spectral range of the reference etalon (297.795 MHz) by calibrating the frequency markers against the known frequencies of the iodine fluorescence spectrum.'**19With this scheme, the frequency markers enable us to determine the shift of the dye laser frequency relative to the HeNe frequency to within a megahertz over scans of tens of gigahertz, providing an extremely accurate frequency scale for our SBG spectra. The benzene sample (Aldrich, HPLC grade) was distilled under vacuum into a glass sample cell, which was then sealed under vacuum. Before the sample was introduced, the cell was rinsed under vacuum several times with benzene to remove dust and any remaining impurities. The sample cell was mounted inside of a brass block for temperature control. This block has internal channels through which thermostated water was circulated and also two cartridge heaters which were used in conjunction with a PID controller (Omega CN9112) to maintain the temperature. With the temperature of the thermostated water a few degrees below the desired set temperature, a stability of better than f0.05 K was achieved. The sample temperature was measured with a type T thermocouple mounted in the brass block next to the sample cell. (The temperature reading was corrected to account for a small thermal gradient between the thermocouple and the scattering volume within the sample cell.) SBG spectra were collected in 1-deg intervals between 38 and 58 OC and in 2-deg intervals between 24 and 38 "C and between 58 and 69 "C. At each temperature two separate spectra were collected, with the total collection time for both spectra typically occupying 40 min. After changing the temperature the sample was allowed to equilibrate for at least 30 min, and in some cases much longer (up to 3 h). We observed no changes in the spectrum with the length of equilibration time; this is in sharp contrast to the results of Rozhdestvenskaya and Smirnova,* who found that long equilibration times (more than 3 h) were necessary to achieve reproducible results. Also, our spectra were collected in three sets of temperatures on three separate days, and the spectra within and between these sets were found to be perfectly consistent and continuous over the entire temperature range (seediscussion below and Figure 2).
Results and Discussion Figure 1 shows a representative SBG spectrum of liquid benzene, recorded at 51.9 OC. Also shown are the frequency calibration markers generated from the reference etalon, a fit to the spectrum, and the residuals of the fit. Nonlinear least squares was used to fit a pair of Lorentzians for the gain and loss peaks (one p i t i v e and one negative) and an adjustable base line to each experimental spectrum. The intensities and widths of the two Lorentzians were constrained to be the same. This fitting procedure used the uncorrected frequency scale generated by the dye laser computer and wavemeter and yielded uncorrected center frequencies for the gain and loss peaks. The true separation
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Figure 1. CW-stimulated Brillouin gain spectrum of liquid benzene at a scattering angle of 178.7' and a temperature of 51.9 OC. The frequency scale on the abscissa corresponds to the pump laser frequency as recorded by the dye laser's wavemeter. Trace a shows the 300-MHz frequency markers generated by the reference etalon, trace b shows the experimental spectrum of benzene, trace c shows the least-squares fit to the spectrum in trace b, and trace d shows the residuals from the fit (expanded by a factor of 2 ) .
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between these center frequencies (twice the Brillouin shift) was then determined by counting the number of frequency calibration markers between them, with linear interpolation employed for the final fractional spacing between frequency markers. This procedure yielded extremely consistent results. As noted in the Experimental Section, pairs of spectra were taken a t each temperature. After fitting and frequency correction, the largest difference between any pair of Brillouin shifts at a single temperature was 12 MHz, and the difference between pairs of shifts averaged over all temperatures was 3 MHz. On the basis of these results we estimate the 95% confidence limits for our Brillouin shifts to be i6 MHz. We note that it is essential to use the frequency calibration markers to obtain this consistency; otherwise, the differences between the Brillouin shifts can be as large as 100 MHz.
Figure 2 shows a plot of the Brillouin shifts versus temperature, along with a linear least-squares fit to the data. As can be seen from this figure, the Brillouin shifts of liquid benzene determined from our SBG measurements vary smoothly and linearly over the entire temperature range; we find no evidence of the anomaly between 42 and 48 O C reported by Letamendia et a1.I0 Note that the change in the Brillouin shift per degree (-22 MHz/OC) is larger than the uncertainty in our shift values. We have also determined the Brillouin widths as a function of temperature from
J . Phys. Chem. 1992,96, 8690-8692
8690
the fits to our data. The widths also vary linearly with temperature, from 230 f 10 MHz at 24 OC to 255 i 10 MHz a t 69 "C, with no sign of an anomaly. It is difficult to rationalize why Letamendia et a1.I0 would see a transition region in their Brillouin shift measurements and we would not. We have used a backscattering geometry with 633-nm excitation, whereas they likely used a 90° geometry with excitation from an argon laser line, so the acoustic wavelengths probed in the two measurements should be quite similar. Since our experiment is based on stimulated scattering, the results could in principle differ from those obtained by spontaneous scattering techniques. However, the laser powers employed in our experiment are quite low, and the resulting density perturbation stimulated in the system is very small; in this case the linear response result in eq 1 applies, and the Brillouin shifts measured in both stimulated and spontaneous experiments should be essentially the same. Indeed, our previous SBG measurements on liquids indicate excellent agreement with the results of spontaneous Brillouin measurements.13J4 In the present case of liquid benzene we find a sound velocity of 1490 m/s at 25.0 OC from the Brillouin shift of 7.043 GHz (assuming a value of 1.498 for the index of refraction), which compares favorably with the value of 1484 m/s predicted at this frequency from the dispersion curve determined by Sorem and Schawlow.20 Several previous Brillouin and ultrasonic measurements20-22have indicated that the sound velocity displays dispersion in the hypersonic frequency range due to the slowly relaxing specific heat, and this relaxation process can affect the Brillouin shifts and widths and their temperature dependence. Even so, the earlier data21 also show no evidence of a transition temperature region of the sort reported by Letamendia et al.IO (although it should be noted that the data points in the earlier measurements were collected at wider temperature intervals2'). The absence of the Rayleigh line in the SBG spectrum precludes our measuring the Landau-Plazeck ratio for comparison with the results of Letamendia et a1.I0 We note that at least one other group has carried out integrated depolarized scattering intensity measurements on liquid and they have not observed the anomalies reported by Rhozhdestvenskaya and Smirnova.'s2 These measurements are of course quite difficult and prone to experimental artifacts. At this
point we believe that more measurements by independent groups will be necessary in order to establish whether the structural transition in liquid benzene is real. Our own study yields a negative result and calls into question the results of Letamendia et In addition to addressing the perplexing possibility of a structural transition in liquid benzene, this study clearly demonstrates the high accuracy and precision of Brillouin shift measurements by CW stimulated Brillouin gain spectroscopy. Acknowledgment. This work was supported in part by the National Science Foundation (CHEM-8920029). References and Notes (1) (2) 1223. (3) (4) (5) (6) (7)
Rozhdestvenskaya, N. B.; Smirnova, L. V. JETP Lett. 1986,44, 165. Rozhdestvenskaya, N. B.; Smirnova, L. V. J. Chem. Phys. 1991, 95,
Lowden, L. J.; Chandler, D. J. Chem. Phys. 1974, 61, 5228. Evans, J . J.; Watts, R. 0. Mol. Phys. 1976. 32, 93. Narten, A. H. J . Chem. Phys. 1977,67, 2101. Steinhauser, 0. Chem. Phys. 1982, 73, 155. Claessens, M.; Ferrario, M.; Ryckaert, J. P. Mol. Phys. 1983,50, 217. ( 8 ) L!nse, P. J. Am. Chem. SOC.1984, 106, 5425. (9) Xiangian, S.; Bartell, L. S. J. Phys. Chem. 1988, 92, 5667. (IO) Letamendia, L.; Belkadi, M.; Voucamp, G.; Nouchi, G. First Liquid Matter Conference, 7-11 July 1990, Lyon, France; Abstract B 27. (1 1) Pinan-Lucarre, J. P.; Loisel, J.; Berreby, L.; Dayan, E.; Dervil, E. J . Raman Spectrosc. 1992, 23, 67. (12) Tang, S. Y.; She, C. Y.;Lee, S. A. Opt. Lon. 1987, 12, 870. (13) Ratanaphruks, K.; Grubbs, W. T.; MacPhail, R. A. Chem. Phys. Lett. 1991, 182, 371. (14) Grubbs, W. T.;MacPhail, R. A. J. Chem. Phys. 1992, 97, 19. (15) Faris, G. W.; Jusinski, L. E.; Dyer, M. J.; Bischel, W. K.; Hickman, A. P. Opt. Lett. 1990, 15, 703. (16) She, C. Y.; Herring, G. C.; Moosmiiller, H.; Lee, S. A. Phys. Rev. A 1985,31, 3733. ( I 7) Berne, B. J.; Pecora, R. Dynamic Light Scattering Wiley: New York, 1976. (1 8) Moosmiiller, H.; She, C. Y.; Huo, W. H. Phys. Rev.A 1989,40,6983. (19) Gerstenkorn, S.; Luc, P. Atlas du Spectre d'Absorption de la Molecule de rlode entre 14800-2oooO cm-'; Editions du C.N.R.S.:15, quai Anatole-France, 75700 Paris, 1978. (20) Sorem, M. S.; Schawlow, A. L. Phys. Lett. 1970, 33A, 268. (21) Eastman, D. P.; Hollinger, A.; Kenemuth, J.; Rank, D. H. J. Chem. Phys. 1969,50, 1567. (22) Nichols, W. H.; Kunsitis-Swyt, C. R.; Singal, S. P. J. Chem. Phys. 1969, 51, 5659. (23) Gomperts, S.; Variyar, J. E.; Kivelson, D. Private communication.
Chemlsorptlon of Nitrogen on ZSM-5 Zeolite at Hlgh Temperature Vikram S. Nayak Guelph Chemical Laboratories Ltd., 246 Silvercreek Parkway N., Guelph, Ontario, Canada N l H 1E7 (Received: April 29, 1992; In Final Form: August 17, 1992)
The present paper deals with.the chemisorption of nitrogen on ZSM-5 zeolite at very high temperature. The sorption of nitrogen was measured gravimetrically using a Cahn electrobalance. The adsorption of nitrogen, which remains almost zero at temperature in the range from ambient to about 400 "C, becomes appreciable at 450 OC. The adsorption of nitrogen on ZSM-5 at high temperature does not seem to be dependent upon the chemical composition of the zeolite. The highly activated adsorption of nitrogen on ZSM-5zeolite is attributed to the chemical bonding between partly dissociated nitrogen molecule and the zeolite surface. The adsorption of nitrogen on the zeolite also seems to be dependent on the pretreatment conditions, namely temperature and vacuum.
Introduction The adsorption studies on ZSM-5 zeolite are generally restricted to lower temperatures. This is mainly due to the fact that most hydrocarbons upon contact with ZSM-5zeolite undergo reaction at high temperature. The high-temperature chemisorption studies of only a few compounds such as ammonia and pyridine, which do not undergo transformation into other products, have only been reported.' In order to correlate the catalytic properties to the
adsorption properties, the latter have to be studied at the catalytic reaction conditions. The study of adsorption behavior of ZSM-5 at the catalytic reaction conditions especially when dealing with multicomponent adsorbate system is very difficult. As a step toward understanding the true nature of adsorption a t reaction conditions, the sorption of nitrogen a t higher temperatures is studied. The unexpected chemisorption of nitrogen on ZSM-5 zeolite at high temperature is explained in this paper.
0022-365419212096-8690$03.00/0 0 1992 American Chemical Society
