lchange in sodium Rydberg atoms induced by ion collisions near the matching velocity

Abstract

Cross-section measurements for inelastic collisions of rare-gas ions with Na Rydberg atoms that change the orbital angular momentum $l$ are reported. ${\mathrm{He}}^{+}$, ${\mathrm{Ne}}^{+}$, and ${\mathrm{Ar}}^{+}$ ions at 400-2000 eV collide with laser-excited $\mathrm{Na}\mathrm{}\left(\mathrm{nd}\right)\mathrm{}$ atoms in a crossed-beam experiment. Time-resolved field ionization is used as a selective detection method. The $l$-changing cross section is found to vary with $n$ as $n^{\beta }$, where $\beta =5.12\mathrm{}\pm 0.20$ when projectile and bound-electron velocities are comparable. The velocity dependence is given in the range $\tilde{v}=0.6-4.0\mathrm{}$, where reduced velocity $\tilde{v}$ is the ratio of ion and electronic orbital velocities. Evidence is offered that the induced transitions are predominantly dipole in character under single-collision conditions but that rapid equilibration of higher $l$'s occurs by successive ion impacts. Initial $\mathrm{Na}\mathrm{}\left(\mathrm{ns}\right)\mathrm{}$ states yield an $l$-changing cross section at least an order of magnitude smaller than $d$ states and appear to go most readily to $\mathrm{}(n-1\mathrm{})l\ge 2$ states in endothermic nondipole-allowed $\Delta n=1\mathrm{}$ processes. The cross-section magnitude for ${\mathrm{Ne}}^{+}+\mathrm{Na}\mathrm{}\left(28d\right)\mathrm{}\to {\mathrm{Ne}}^{+}+\mathrm{Na}(28,l\ge 3)$ at 1000 eV is $\approx 5\times {10}^{-8\mathrm{}}$ ${\mathrm{cm}}^2$, in agreement with existing theories. This exceeds the geometric cross section $n^4\pi a_0^2$ by a factor of ${10}^3$ and indicates that passage of an ion closer to the Rydberg atom than 30 orbital radii $n^2{a}_0$ suffices to induce $l$ change in the velocity-matching region.