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

Abstract

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}$${\mathrm{H}}_{6}$O) are measured in a crossed-beam experiment at thermal kinetic energies, for a well-defined relative velocity. Simultaneous Penning ionization of ${\mathrm{NH}}_{3}$ by metastable He${(2}^{3}$S) atoms is carried out in situ for calibration. Experimental collisional-ionization cross sections ${\ensuremath{\sigma}}_{i}^{\mathrm{expt}({v}_{r})}$, for ${v}_{r}$=2400 m/s, as large as 9\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$ for ${\mathrm{NH}}_{3}$, 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$ for ${\mathrm{SO}}_{2}$, and ${10}^{\mathrm{\ensuremath{-}}14}$ ${\mathrm{cm}}^{2}$ for ${\mathrm{C}}_{3}$${\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 \ensuremath{\Delta}J=-1 \ensuremath{\Delta}K=0 relaxation transitions of the molecule is insufficient. Our hypothesis of large \ensuremath{\Vert}\ensuremath{\Delta}J\ensuremath{\Vert} rotational transitions is strengthened by comparison of the results for the three studied molecules.