Quasielastic state-changing collisions of high-Rydberg atoms with rare-gas atoms

For $20\le n\le 45$, two types of quasielastic state-changing-collisional (QESCC) processes, i.e., (i) $l$-changing and (ii) $n$-and $l$-changing processes, have been theoretically investigated based on the free-electron model in which an excited Rydberg electron behaves as if it were "free" and slow with its interaction with a rare-gas atom playing a decisive role. It is shown that a parameter $u_{min}[=\frac{\nu {\nu }^{\prime }\Delta E{a}_0}{\left(\hslash V\right)}]$ is quite useful for characterizing both of these QESCC processes in a unified way where $\nu \left(\simeq {\nu }^{\prime }\right)$ is an effective principal quantum number of the initial (final) state, $\Delta E$ is an energy defect, and $V$ is a relative velocity. It is theoretically found that higher-angular-momentum states are dominantly populated in the final channel of these QESCC processes with $1<\mathrm{}{u}_{min}\lesssim \nu$ such as in the $n$- and $l$-changing collision of ${\mathrm{Rb}}^{**}\mathrm{}\left(\mathrm{ns}\right)+\mathrm{He}$. This is in contrast with the uniform distribution over the final angular-momentum states for the usual $l$-changing processes, i.e., $u_{min}\lesssim 1$. The calculated cross sections of these QESCC processes with $1<\mathrm{}{u}_{min}\lesssim \nu$ are found to be in reasonable agreement with experimental findings available at present.