Ionization of low-Rydberg-state He atoms by polar molecules. II. Large rotational-energy transfers and absolute values of the cross sections

The absolute values of cross sections for the collisional ionization of low-Rydberg-state He atoms in the n ${}_{}{}^1{}_{}{}^{}P$ state (n=14) by the polar molecules ${\mathrm{NH}}_3$, ${\mathrm{SO}}_2$, and acetone (${\mathrm{C}}_3$ ${\mathrm{H}}_6$O) are measured in a crossed-beam experiment at thermal kinetic energies, for a well-defined relative velocity. Simultaneous Penning ionization of ${\mathrm{NH}}_3$ by metastable He${(2\mathrm{}}^3$S) atoms is carried out in situ for calibration. Experimental collisional-ionization cross sections ${\sigma }_i^{\mathrm{expt}\left(v_r\right)}$, for $v_r$=2400 m/s, as large as 9×${10}^{\mathrm{-}15}$ ${\mathrm{cm}}^2$ for ${\mathrm{NH}}_3$, 5×${10}^{\mathrm{-}15}$ ${\mathrm{cm}}^2$ for ${\mathrm{SO}}_2$, and ${10}^{\mathrm{-}14}$ ${\mathrm{cm}}^2$ for ${\mathrm{C}}_3$ ${\mathrm{H}}_6$O at 300 K, are obtained. These large cross sections cannot be explained by the ‘‘free-electron model’’ usually used in Rydberg-atom–molecule collisions: This model predicts no ionization of Rydberg levels as low as n=14, since the amount of energy available from rotational ΔJ=-1 ΔK=0 relaxation transitions of the molecule is insufficient. Our hypothesis of large ‖ΔJ‖ rotational transitions is strengthened by comparison of the results for the three studied molecules.