Ionization of Rydberg atoms in thermal collisions with polar molecules

Ionization collision between a Rydberg atom R and a rotationally excited polar molecule M(J) at room temperature is examined theoretically. The approximate quantum mechanical calculations carried out so far employed the dipole-Born approximation for the process eR(n, l) + M(J) → e + M(J'). In order to take into account the higher-order corrections, however, the dipole-Born approximation should be replaced by a more sophisticated one. The Glauber approximation is employed here. The dipole-Glauber differential cross section (DCS) becomes the same as the dipole-Born DCS when the molecular dipole moment is weak, and also reproduces well the results of the close-coupling calculations even when the molecular dipole moment becomes large. The author has previously proposed a new sequential cross-section formula to be applicable to the general case where the DCS for electron-neutral atom-molecule scattering depends not only on the momentum transfer but also on the relative velocity. Brief summary of the formula is given and calculated results and discussions are presented for the Xe(n) + CO, HCl, HF, and Lif systems. Experimental and theoretical ionization rates for Xe(27f) + HF and Xe(31f) + HCl at 300 K are compared with the present results. In the present treatment, the Rydberg atom is assumed to be hydrogenic and the polar molecule is taken to be a rigid rotor. (Nogami, K.)