Effect of Pressure on Raman Spectra

Abstract

THE influonce of its neighbours on the rotation and vibration states of a molecule in a gas is due partly to the general intermolecular field and partly to actual collisions. From the kinetic theory of gases, it is known that the mean frequency of collisions which a molecule undergoes is given by the ratio P/η of the pressure to the viscosity of the gas. The perturbing influence of collisions on molecular rotation may be expectod to be small so long as this frequency is low, but would become considerable when the mean frequency of collision approaches a value comparable to that of molecular rotation. By taking P/η to be equal to the frequency of rotation of the molecule in the one-quantum state, we may obtain an estimate of the pressure at which the distinctness of quantisation of the different rotations would disappear. In this way, the following estimates of pressure in atmospheres at 30° C. are obtained for the different gases: Hydrogen, 450 ; methane, 48 ; nitrogen, 29 ; oxygen, 25 ; acetylene, 10 ; carbon dioxide, 5 ; hydrogen chloride, 128; ammonia, 83; nitric oxide, 25.