Giant cross sections for L changing in Rydberg-Rydberg collisions

Abstract

State changing in thermal-energy collisions between strontium atoms in very-high-$n$, $n\ensuremath{\approx}300$, $n{\phantom{\rule{0.16em}{0ex}}}^{1}{F}_{3}$ Rydberg states is studied. In collisions between Rydberg atoms, the dipole-dipole interaction generates an effective electric field at each atom, which triggers Stark precession and the evolution of their angular momenta, $L$. Such $L$-changing collisions are examined using both quantum and classical theory. The theoretical predictions are experimentally verified by monitoring the damping of quantum beats induced by the sudden application of a DC ``pump'' field. The present work shows that, due to the long-range nature of the interactions, the cross sections, $\ensuremath{\sigma}$, for $L$ changing are considerably larger than the geometric collision cross section. For thermal collisions the cross section $\ensuremath{\sigma}$ for $n{\phantom{\rule{0.28em}{0ex}}}^{1}{F}_{3}$ Rydberg states with $n\ensuremath{\simeq}300$ is $\ensuremath{\approx}8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}$. Scaling rules derived in this paper predict that for cold collisions $\ensuremath{\sigma}$ may increase by a further factor of 4.