Excited states of highly stripped ions

Abstract

Time-dependent perturbation theory (TDPT) has been very successful in predicting the positions of the excited energy levels of atomic and molecular systems. TDPT can be associated with different formalisms to yield results of different degrees of accuracy. The method was originally used to predict structure properties connected with single excitations. However, later developments show its potentiality towards evaluation of atomic data connected with double and multiple excitations. In the present context we have used TDPT within a variational framework to calculate singly and doubly excited energy levels of several highly stripped ions of different electronic configurations and estimated their transition properties like oscillator strengths, transition probabilities, etc. A free atom or ion is subjected to a time-dependent external harmonic field and the linear response properties of the atom are evaluated. The effect of the external field is to generate single or multiple excitation of the electronic charge cloud. Depending upon the nature of the perturbing field one can get allowed as well as forbidden transitions involving spatial and spin symmetries. The positions of the excited energy levels and analytic representations of their wave functions are directly obtained from our formalism by analyzing the positions of the poles of an appropriately constructed linearised variational functional with respect to external frequency. Transition energies and other structural parameters agree well with existing data wherever available.