Electron spin pumping of Rb atoms on He nanodroplets via nondestructive optical excitation

We measured laser-induced-fluorescence (LIF) and beam-depletion (BD) spectra of rubidium atoms (5S - 5P transition) on the surface of superfluid helium nanodroplets (M-HeN with M = Rb). It is known that when M is a lighter alkali atom electronic excitation always leads to detachment of the excited atom (M*). The dissociation energy, few tens cm(-1), comes either as photon excess energy or from the barrierless formation of a M*-He exciplex. We observe that this picture does not hold when M = Rb and the photon excess energy is small: we are able to excite atoms without detaching them from the droplet, thanks to a barrier preventing formation of the exciplex. This system is ideally suited for optical spin pumping in a He nanodroplet, whose achievement we explicitly demonstrate in a pump-probe magnetic circular dichroisin experiment.