Velocity Dependence of Rotationally Inelastic Cross-Sections

Abstract

Techniques are described for obtaining the energy-dependence of state to state rotational energy transfer (RET) cross-sections in thermal cell experiments through velocity selection by Doppler shift. The case of polarised excitation and detection is discussed in detail and expressions presented which enable the state multipoles of population, orientation and alignment to be obtained from appropriate RET experiments. It is found that the translational order present in a non-isotropic collision environment, such as that resulting from excitation with a narrow linewidth laser, causes state multipoles of different rank to become coupled. Experimental results are presented for the energy dependence of cross-sections for state to state transfer of population and of orientation in Li2-Xe collisions. The way the relative velocity vector changes as a result of a collision may also in principle be studied using a high resolution probe (e.g. a second laser) of the velocity spread following RET.