Alkali atoms attached to vortex-hosting helium nanodroplets

Light absorption spectroscopy of alkali atoms attached to 4 He droplets is investigated as a possible way of detecting the presence of vortices. To this end, we have calculated the equilibrium configuration and energetics of alkali atoms attached to a 4 He1000 droplet hosting a vortex line using 4 He density functional theory. We use them to study how the dipole absorption spectrum of the alkali atom is modified when the impurity is attached to a vortex line. Spectra are found to be shifted to the blue (higher frequencies) and broadened compared to vortex-free droplets, because the dimple in which the alkali atom sits at the intersection of the vortex line and the droplet surface is deeper. This effect is smaller for lighter alkali atoms, and all the more so when using a quantum description since in this case, they sit further away from the droplet surface on average due to their zero-point motion. Spectral modifications due to the presence of a vortex line are minor for np ← ns excitation, and therefore insufficient for vortex detection. In the case of higher n p ← ns or n s ← ns (n > n) excitations the shifts are larger, as the excited state orbital is more extended and therefore more sensitive to changes in the surrounding helium density.