Abstract

Investigation of optical reorientation of molecules in gases by studying the dependence of the optical Kerr effect (OKE) on the gas density can provide new information on the dynamics of molecular interactions. To ensure that the orientation of molecules in an optical field is to the least possible degree obstructed by the interactions, the density of the studied gas should be as low as possible. On the other hand, for low gas densities the OKE signal is very weak and is difficult to record. For this reason molecular orientation has been so far successfully determined only in the gases CS₂ (Ref. 1) and CO₂ (Ref. 2), whose molecules are endowed with significant natural optical anisotropy. Exceptionally advantageous physical and chemical properties of CO₂ allowed us to carry out OKE measurements and to determine the absolute Kerr constant B₀ and the molecular Kerr constant _mB, at room temperature, as functions of the gas density as it varies over a wide range. Significant changes in density affect the mean free path of a chosen molecule and thus the time between its subsequent collisions t_ic (intercollision time). These two parameters are also influenced by the mean velocity of the molecule and its collision cross section. Therefore in gases of extremely low densities a molecule has the greatest freedom of movement and is the least restricted by the interactions with other molecules. The measured values of molar Kerr constant as a function of the intercollision time of a chosen molecule are presented in Fig. 1. The unexpected character of this dependence, which is known to be a function of the thermodynamical parameters of the gas (p, γ, T), stimulated a new approach to the phenomena of electric and optical Kerr effects and the related orientation of molecules (orientational relaxation) in the medium. In the dependence shown in Fig. 1, a slight decrease in _mB corresponds to a significant change in t_ic: a pressure of 0.1 MPa corresponds to t_ic = 119.6 ps, and a pressure of 2.6 MPa corresponds to t_ic = 4 ps. This insignificant change in _mB may be due to increasing number of collisions through which the molecules, oriented along the applied optical field before the collision, become unoriented. The change in _mB is related to, among other factors, the ratio of the number of collisions in a unit volume per a unit of time to the total number of molecules in a unit volume ρ. This ratio changes in proportion to ρ since the number of binary collisions is proportional to ρ. For intercollision times lower than t_ic = 4 ps, corresponding to the range of pressures higher than 2.6 MPa, a considerable decrease in _mB is observed. This cannot be explained only by the increasing number of collisions. It takes time t_or for the molecule to be fully oriented along the optical field. This time is determined by the mechanical, electrical, and optical properties of the molecule, the intensity of the applied optical field, and the energy of the molecule’s interaction with the environment. If this time is shorter than or equal to the mean intercollision time, i.e., if t_or/t_ic ≤ 1, the molecule manages to be fully oriented and its contribution to _mB is the maximum. Otherwise, when t_or/t_ic > 1, the molecule becomes only partly oriented and its contribution to _mB is smaller, which is clearly shown in Fig. 1. Taking all this into account and using the OKE measurements as a function of number density of molecules, we can find from the condition t_or/t_ic = 1 that the time of complete orientation of the molecule in an optical field is where M is the molar mass, R is the gas constant, T is the absolute temperature, 8 is the collision cross section, and p is the number density of molecules.

© 1993 Optical Society of America