Isolated-core quadrupole excitation of highly excited autoionizing Rydberg states

TL;DR

This study investigates high-lying doubly excited Rydberg states in strontium, revealing a unique quadrupole excitation phenomenon caused by electron correlation effects and their dependence on quantum numbers.

Contribution

It demonstrates the first observation of electric quadrupole isolated-core excitation in highly excited Rydberg states, supported by advanced theoretical modeling.

Findings

Most spectral lines originate from a single optically active state.

Electron correlations vanish at specific quantum numbers, revealing quasi hydrogenic behavior.

Quadrupole excitation occurs with intensity comparable to dipole transitions.

Abstract

The structure and photoexcitation dynamics of high lying doubly excited states of the strontium atom with high angular momenta are studied in the vicinity of the Sr$^+(N=5)$ threshold. The spectra recorded using resonant multiphoton isolated core excitation are analyzed with calculations based on configuration interaction with exterior complex scaling, which treats the correlated motion of the two valence electrons of Sr from first principles. The results are rationalized with a model based on multichannel quantum-defect theory and transition dipole moments calculated with a perturbative treatment of electron correlations. Together, both approaches reveal that most of the lines observed in the spectra arise from the interaction of a single optically active state, coupled to the initial state by an electric-dipole transition, with entire doubly-excited Rydberg series. The long-range electron correlations responsible for this interaction unexpectedly vanish for identical values of the initial and final principal quantum numbers, a fact related to the quasi hydrogenic nature of the high-$l$ Rydberg electron. This special situation, and in particular the vanishing interaction, leads to the surprising observation of an electric \emph{quadrupole} isolated-core excitation with a similar intensity as the neighboring electric dipole transitions.