Resonances and thresholds in the Rydberg-level population of multiply charged ions at solid surfaces

Abstract

We present a theoretical study of resonances and thresholds, two specific features of Rydberg-state formation of multiply charged ions ($Z=6,$ 7, and 8) escaping a solid surface at intermediate velocities $(v\ensuremath{\approx}1\mathrm{a}.\mathrm{u}.)$ in the normal emergence geometry. The resonances are recognized in pronounced maxima of the experimentally observed population curves of Ar VIII ions for resonant values of the principal quantum number ${n=n}_{\mathrm{res}}=11$ and for the angular momentum quantum numbers $l=1$ and 2. Absence of optical signals in detectors of beam-foil experiments for $n>{n}_{\mathrm{thr}}$ of S VI and Cl VII ions (with $l=0,$ 1, and 2) and Ar VIII for $l=0$ is interpreted as a threshold phenomenon. An interplay between resonance and threshold effects is established within the framework of quantum dynamics of the low angular momentum Rydberg-state formation, based on a generalization of Demkov-Ostrovskii's charge-exchange model. In the model proposed, the Ar VIII resonances appear as a consequence of electron tunneling in the very vicinity of the ion-surface potential barrier top and at some critical ion-surface distances ${R}_{c}.$ The observed thresholds are explained by means of a decay mechanism of ionic Rydberg states formed dominantly above the Fermi level ${E}_{F}$ of a solid conduction band. The theoretically predicted resonant and threshold values, ${n}_{\mathrm{res}}$ and ${n}_{\mathrm{thr}}$ of the principal quantum number n, as well as the obtained population probabilities ${P}_{\mathrm{nl}}{=P}_{\mathrm{nl}}(v,Z),$ are in sufficiently good agreement with all available experimental findings.