If the collisional ionization is chiefly due to energy transfer from the polar-molcculc rotation to an electron in a high Rydberg state of the atom, then theory predicts that the cross section averaged over a thermal distribution of rotational states should show step-like structure as a function of the energy of the Rydbcrg state. This structure has been experimcntally detected. and it can bc considered as direct evidence of the rotational effect in the collisional ionization.
Recently one of us (M-M.) [l-3]
has proposed a mechanism for the collisional ionization of an atom in a high Rydberg state with a polar molecule, in which a Rydberg electron becomes ionized by gain of energy from rotational de-excitation of the polar molecule in a rotationally excited state at thermal energies, namely,
A**(n. I) + M(J) + (A" f e) + M(J').
(1)
Here n and 1 are the principal quantum number and the angular momentum of the Rydberg electron, and J is the rotational quantum number of the polar molecule. This mechanism explains the relative dependence of the ionization cross sections for various combinations of an atom and a polar molecule [3]. The present article points out that the step-like structure in the n dependence of the cross section for process (1) is theoretically predicted from the proposed mechanism, provided the rotational states of the polar molecule are populated according to ther-*Work performed under the auspices of the U.S. Energy Research and Development Administration.