Size Evolution of Photoabsorption Spectra of Small HeN plus ${{\bf He}_N∧ plus }$ Clusters: A Computational Study

Photoabsorption spectra of HeN+ ${{\rm{He}}_N<^> + }$ clusters, N=5-9, have been calculated using a diatomics-in-molecules like electronic structure model and a path-integral Monte Carlo sampling method. A qualitative change in the calculated spectra has been observed at N=9, which has been interpreted in terms of a structural transformation in the clusters consisting in a transition from trimer-like ionic cores observed for N <= 7 to dimer-like ionic cores prevailing in He9+ ${{\rm{He}}_9<^> + }$ through an intermediate state (comparable abundances of both types of ionic cores) observed in He8+ ${{\rm{He}}_8<^> + }$ . The calculated spectra have been thoroughly compared with an earlier calculation on He3+ ${{\rm{He}}_3<^> + }$ , He4+ ${{\rm{He}}_4<^> + }$ , and He10+ ${{\rm{He}}_{10}<^> + }$ reported from our group and data available for the same cluster sizes from an experiment.