Brillouin scattering in o- and m-xylene

Brillouin

1460

scattering in o- and m-xylene

KEYWORDS:

ultrasonics,

Brillouin

scattering,

xylene

Dear Sir Ultrasonic properties of liquids are closely related to the basic physical quantities of the medium, such as adiabatic compressibility and viscosity. The spectrum of velocity dispersion obtained as a function of sound frequency can be used to study various kinds of reactions and molecular relaxations that may affect the dynamics of those macroscopic quantities. In the past five years we have carried out a series of experiments using light scattering techniques and observed uhf and hypersonic properties in many organic liquids.’ ,’ This letter presents the results of Brillouin scattering measurements made in liquid o- and m-xylene. The apparatus used for the experiment was of conventional configuration3 : the test liquid was illuminated by a multimode HeNe laser and the scattered light analysed with a flowcontrolled pressure scanning Fabry-Perot interferometer. We used a scattering cell of recent design4 which yielded good temperature control and high accuracy for determining the scattering angle. Figure 1 shows the typical spectrum of a Brillouin triplet recorded in o-xylene. The central Rayleigh peak is smaller than the other two phonon components, suggesting this spectrum is not much degraded by troublesome, unshifted stray light due to trivial scattering and reflection within the cell. The experiments performed within the scattering angle 0 = SO”-150” gave hypersonic velocities at phonon frequencies ranging between 2.9 and 6.3 GHz. The results for o- and m-xylene are represented in Fig. 2. The accuracy was estimated to be z 0.5%. The points near the lower frequency end of each curve are the values at 3 MHz, obtained by the pulse-echo-overlap method, which actually give the initial value of the dispersion curve. The test

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0041-624X/83/020091-02/$03.00 ULTRASONICS.

MARCH

1983

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Frequency CGHz 1 Fig. 2 Velocity dispersion observed in o- (open circles) and m-xylene (closed circles)

liquid was a spectroscopic grade reagent. The temperature was controlled to within f 0.05”C and was uniform to O.Ol”C over the scattering volume. The solid lines represent the theoretical dispersion curves for single relaxation bestfitted to the experimental points. The dispersion is centred at 9-10 GHz and the final velocity in the high frequency region is estimated to be larger than the initial value by z 120 m S’ . Thus, the present results reveal only the beginning of the dispersion curve; unfortunately, the major part still lies in the frequency range beyond the reach of experimental techniques available today. It may be at some risk, therefore, that we discuss the mechanism of this relaxation. However, the most probable cause of this dispersion is the vibrational relaxation effect; a phenomenon due to the lack of immediate response to energy transfer between the vibrational and translational degrees of freedom in polyatomic molecules. The magnitude of the velocity dispersion estimated above is of the same order as, but - 30% smaller than, the value theoretically predicted from the vibrational specific heat of this molecule which has been calculated for all the fundamental vibrational frequencies. This implies that, as is often the case in many other liquids,115 vibrational relaxation in xylene may have

FSR=ZO.Z7GHz

Fig. 1 Typical example of a Brillouin spectrum obtained in o-xylene. The scattering angle is 89”

1360

at 20°C

0 1983

Butterworth

& Co (Publishers)

Ltd 91

two or more forms, and that the relaxation observed here is associated with one of them, involving only some of the vibrational modes.

References

1 2

Yours faithfully K. Takagi,

Institute

H. Ozawa,

University Tokyo

P.-K.

of Tokyo,

Minato-ku,

106,

92

3

23 March

4 5

Japan Received

Choi

of Industrial Science,

1982

Takagi, K. Vibrational Relaxation in Liquid Carbondisulfide. J. Acoust. Sot. Am. 71 (1982) 74 Takagi, K., Negishi, K. Measurement of Ultrasonic Velocity and Absorption in Liquid up to 1.5 GHz by High-resolution Bragg Reflection technique, J. P/zJ’s. II: Appl. Whys. 15 (1982) 757 Fleury, P.A., Chiao, R.Y. Dispersion of Hypersonic Waves in Liquids, J. Acoust. Sot. Am. 39 (1966) 15 1 Takagi, K., Ozawa, H. A new type of Cell for Brillouin Scattering in Liquids, (ntrasonics, 18 (1980) 135 Hunter, J.L., Carome, E.F., Dardy, H.D., Bucaro, J.A. Ultrasonic and Hypersonic Studies of Vibrational Relaxation in Benzene, J. Acoust. Sot. Am. 40 (1966) 313

ULTRASONICS.

MARCH

1983