Collisional population of ultra-high, ultra-long-living Rydberg states under zero-electron-kinetic-energy conditions

Abstract

Zero-electron-kinetic-energy photoelectron spectroscopy (“ZEKE-PES”) is based on the pulsed field ionization of long lived Rydberg states (ZEKE states); it is generally accepted that ZEKE states have large angular momentum ℓ, which quenches electron-core interactions, but how they acquire it remains a matter of dispute. We show that {nl}→{nl′} ion-Rydberg collisions are a viable and prominent mechanism for the excitation of large-ℓ Rydberg states. We elucidate the dynamics by an exactly solvable classical model which provides a transparent and intuitive picture of the excitation of high-ℓ states. By a geometric interpretation of the dynamics we are able to predict for which values of the impact parameter and reduced velocity of the incoming ion a change of the angular momentum of the state becomes possible. We pay particular attention to the influence of the quantum defect, δl, on the {nl}→{nl′} cross section and demonstrate that, for small initial angular momenta, δl is itself a major contributor to the experimentally observed scaling of the cross section as ∼n5. This classical-quantum defect model is not only able to explain the dependence of the experimentally measured fractional population of high-ℓ states on the velocity of the incoming ion, but leads to the prediction that dipole-forbidden transitions are likely to dominate ℓ transitions induced by ion-Rydberg collisions under ZEKE-PES conditions.