<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>l</mml:mi></mml:math>change 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 $\mathrm{Na}(\mathrm{nd})$ 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 ${n}^{\ensuremath{\beta}}$, where $\ensuremath{\beta}=5.12\ifmmode\pm\else\textpm\fi{}0.20$ when projectile and bound-electron velocities are comparable. The velocity dependence is given in the range $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{v}=0.6\ensuremath{-}4.0$, where reduced velocity $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{v}$ 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 $\mathrm{Na}(\mathrm{ns})$ states yield an $l$-changing cross section at least an order of magnitude smaller than $d$ states and appear to go most readily to $(n\ensuremath{-}1)l\ensuremath{\ge}2$ states in endothermic nondipole-allowed $\ensuremath{\Delta}n=1$ processes. The cross-section magnitude for ${\mathrm{Ne}}^{+}+\mathrm{Na}(28d)\ensuremath{\rightarrow}{\mathrm{Ne}}^{+}+\mathrm{Na}(28,l\ensuremath{\ge}3)$ at 1000 eV is $\ensuremath{\approx}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ ${\mathrm{cm}}^{2}$, in agreement with existing theories. This exceeds the geometric cross section ${n}^{4}\ensuremath{\pi}{a}_{0}^{2}$ 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.