SummaryThe shape of a collision-broadened spectral line depends on the assumptions made about the collision processes involved. If the collision cross section varies inversely as the relative speed of the colliding molecules, the half-width to the half-power points and to the points of steepest slope should be 1/27T1' and 1/(2";3)7T1' respectively, where l' is the mean interval between line-broadening collisions. It is shown that, in a gas of "hard-sphere" molecules, the shape of the line is modified and the above half-widths are reduced by' 6·4 and 9·7% respectively. While it is not suggested that gas molecules behave entirely as hard spheres, it is suggested that some deviations from the Van Vleck-Weisskopf-Lorentz shape may occur, and that it may be possible to detect these deviations experimentally.

SummaryThe power absorption coefficient a of microwaves of wave number v = O· 333 cm-1 by methyl chloride at 33· 7°C has been studied at pressures up to 2 atm, and the results are compared with theoretical predictions. Discrepancies which have previously been attributed to the effect of transitions between low.lying rotational states are shown to be due to three· body collisions. The collision diameter for pressure broadening of the inversion transition in methyl chloride is found to be 8·6 A; this is significantly larger than the figure of 7 . 2 A reported by earlier workers who neglected the effect of three-body collisions.

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The dielectric behaviour of gases in an electromagnetic field has been studied theoretically by Van Vleck and Weisskopf (1945). They found expressions for the contributions to the absorption coefficient a and the refractive index n at a frequency v arising from transitions between quantum states j and i. If v is much less than the natural frequency Vii associated with the transition, and if the line width is also small compared with Vii' their results give:(1) (2) where Ni is the number of particles per unit volume in the state j, P-ii is the dipole matrix-element corresponding to the transition, and Xii the resulting contribution to the susceptibility. Since, at constant temperature, N j is directly proportional to N, it follows that the contribution to the susceptibility should also increase linearly with N.Methyl chloride, however, is a symmetric-top molecule and each rotational state J,K will be split into a very closely-spaced doublet for which Vii is effectively where v = v/c and Ll v is the line width, which is assumed to be independent of J, K. At the pressures used in the present work, Ll v arises almost entirely from collisionbroadening and, according to the Van VIeck-Weisskopf theory, should be directly proportional to N. It therefore follows from (4) that Xinv should rise more rapidly than N.Microwave absorption by methyl chloride has been studied by various authors. Bleaney and Loubser (1950) found that for pressures up to 4 atmospheres and frequencies up to 24 Gc/s, the absorption is described by equation (3) if the line width is related to the pressure P in atmospheres by the equation