Desorption of alkali atoms from 4He nanodroplets

TL;DR

This study investigates how alkali atoms desorb from helium nanodroplets after photoexcitation, combining experiments and simulations to reveal the dynamics and effects of impurity mass on the process.

Contribution

It introduces a hybrid experimental-theoretical approach to analyze alkali atom desorption from helium droplets, highlighting the role of non-linear density waves and isotope effects.

Findings

Desorption occurs after excitation via the (n+1) -> ns transition.

Kinetic energy of desorbed atoms depends linearly on excitation frequency.

Desorption involves creation of supersonic density waves in helium droplets.

Abstract

The dynamics following the photoexcitation of Na and Li atoms located on the surface of helium nanodroplets has been investigated in a joint experimental and theoretical study. Photoelectron spectroscopy has revealed that excitation of the alkali atoms via the (n+1) -> ns transition leads to the desorption of these atoms. The mean kinetic energy of the desorbed atoms, as determined by ion imaging, shows a linear dependence on excitation frequency. These experimental findings are analyzed within a three-dimensional, time-dependent density functional approach for the helium droplet combined with a Bohmian dynamics description of the desorbing atom. This hybrid method reproduces well the key experimental observables. The dependence of the observables on the impurity mass is discussed by comparing the results obtained for the 6Li and 7Li isotopes. The calculations show that the desorption of the excited alkali atom is accompanied by the creation of highly non-linear density waves in the helium droplet that propagate at supersonic velocities.