Absorption Spectrum of Na Atoms Attached to Helium Nanodroplets

Abstract

We present an analysis of the electronic 3p←3s excitation of Na atoms attached to 3He and 4He droplets. Once the ground state structure of the doped drops has been obtained within finite-range density functional theory, we determine the time-correlation function of the Na atom evolving in the full three-dimensional 2Π1/2, 2Π3/2 and 2Σ1/2 potentials generated by its pairwise interaction with the helium atoms. The Fourier analysis of the time-correlation function determines the absorption spectrum of Na, which is compared with the experimental spectrum. We show that the existence of a redshifted shoulder in the absorption line of Na@4HeN, absent in Na@3HeN, is due to the wider vibrational motion of the dopant atom in the dimple it causes on the surface of the droplet.