A. Ben‐Reuven

Light Scattering by Orientational Fluctuations in Liquids

Tensor‐algebra techniques are used to derive expressions similar to those of Pecora and Steele for light scattering by liquids of anisotropic molecues, in a more compact form. The correlation functions are cast into a form in which their geometrical meaning is made obvious and is more immediately applicable to liquid models. A generalization of the Kirkwood g factor to second‐rank tensors is defined. The theory is applied to the depolarized Rayleigh wings. It is argued on the basis of intensity and half‐width measurements that the diffuse line, which appears in the spectrum of benzene and several benzene derivatives, originates from reorientation of single molecules. This conclusion fits into a broader scheme incorporating results of dielectric, NMR, and viscosity measurements. It is suggested to relate the background, together with the narrow hole in the middle of the depolarized spectrum, to couplings of orientational fluctuations with shear waves.

Theory and Measurement of Pressure‐Induced Shifts of HCl Lines Due to Noble Gases

Pressure shifts of HCl absorption lines in both the 1–0 and 2–0 bands caused by five noble gases have been measured. An outstanding feature of these measurements is that the shifts are not the same for all lines of a band, but there is a strong dependence on the rotational quantum number J. However, there is evidence that the shifts tend to a constant limit for high J numbers. In order to interpret these results induction‐ and dispersion‐type van der Waals forces were considered and used in conjunction with existing line‐shape theories. In cases where perturbation theory can be applied, the limit shifts were accounted for fairly well, although no satisfactory explanation has emerged for the J dependence. Attention is drawn to the significance of observations of this sort as a means of obtaining information about intermolecular forces and molecular constants. Of particular interest is the possibility of obtaining such information for molecules in specific quantum states.

Light Scattering by Coupling of Orientational Motion to Sound Waves in Liquids

The fine features in the Rayleigh wings of light scattered from various liquids, observed by Starunov, Tiganov, and Fabelinskii, and by Stegeman and Stoicheff, and associated with shear waves according to the Leontovich theory, are explained from a molecular correlation‐function point of view, based on the Kubo–Zwanzig–Mori theory. These features are attributed to a “bottleneck” effect, in which the short‐lived Brownian orientational motion (“tumbling”) is coupled to a long‐lived (hydrodynamic) sound mode. Line‐shape expressions and an estimate of the magnitude of the effect are given.

Theory of Pressure‐Induced Shifts of Infrared Lines

A treatment is presented that accounts to a large extent for the j dependence of pressure‐induced shifts due to noble gases of lines in HCl bands. After a brief critical review of the theoretical work done on this subject to date, an improved phase‐shift approximation for the intermolecular collision process is worked out with a coordinate system fixed in space during the collision. The main features of the observed phenomena are accounted for using only parameters calculated directly from known molecular properties. A discussion of the limitations of this theory is given. It appears that in order to achieve further improvement, the finite probability of j transitions and the influence of short‐range forces should be taken into account.

Rayleigh‐Wing Scattering by Aromatic Liquids

Measurements are reported of the intensities of the diffuse line and background components of the depolarized light scattered from six aromatic liquids (nitrobenzene, quinoline, bromobenzene, and the three nitrotoluenes), and their variation with temperature. Intensity of the diffuse line in nitrobenzene and its temperature dependence, together with half‐width measurements, are compatible with the suggestion that it originates from orientational relaxation by tumbling of mostly uncorrelated molecules. In quinoline a more appreciable orientational correlation between neighboring molecules apparently exists. The background component of these liquids increases with temperature in a manner suggestive of fluctuations from momentary orientational equilibrium.

Pressure Broadening and Shift of 3.39‐μ Absorption in Methane Perturbed by Noble Gases

High‐resolution tuned‐laser spectroscopy was used to study pressure broadening and shift of a 3.39‐μ methane absorption line perturbed by noble gases. Measurements were carried out in the low‐pressure region where collisions influence the line shape both by pressure broadening and by narrowing the Doppler width. The Galatry line shape was found best suitable for determining the linewidth. In order to account for the observed ratio of line width to line shift, it was found necessary to modify Foleys phase‐shift theory by adding a “hard‐core” term to the collision cross section.

LINE MIXING BY COLLISIONS IN THE FAR INFRARED SPECTRUM OF AMMONIA

The effect of mixing by collisions on overlapping lines in the far‐infrared (rotation) spectrum of gaseous ammonia was studied. A merging of the inversion doublets into single lines, similar to the collapse of the microwave inversion spectrum into a nonresonant spectrum, has been observed. The line shape has been analyzed using an adapted form of Ben‐Reuvens theory and values of the damping and cross‐relaxation parameters have been obtained for the rotational lines 3 → 4, 4 → 5, and 5 → 6.

Pressure‐Induced Shifts of DCl Lines Due to HCl: Shift Oscillation

Pressure‐induced shifts of DCl lines (2–0 band) due to HCl have been observed. In addition to the expected J dependence there is a striking oscillation from line to line of the magnitude of the shift. It is suggested that this phenomenon is due to near‐resonant dipole interactions between HCl and DCl.

Pressure‐Induced Shifts of Molecular Lines in Emission and in Absorption

Pressure‐induced shifts of lines in the 2–0 band of HCl have been measured both in emission and in absorption. Within the accuracy of the measurements, no consistent difference between absorption and emission was found. The result casts doubt on the validity of some static theories of line shape in their present form.

Impact Broadening of the Oxygen Microwave Spectrum

The impact approximation is applied to the shape of the microwave spectrum of oxygen at elevated gas pressures (up to 70 atm). The absorption spectrum consists of a resonant mode comprising all unresolved resonance lines near 60 GHz, and a nonresonant mode. The appropriate 3×3 relaxation matrix includes four independent parameters related, respectively, to the widths of the two modes, the collapse of the resonant mode, and its mixing with the nonresonant mode. These four parameters, assumed proportional to pressure, are evaluated by fitting to the data of Maryott and Birnbaum.