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 ($M\mathrm{\text{−}}{\mathrm{He}}_N$ with $M=\mathrm{Rb}$). 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 ${\mathrm{cm}}^{-1}$, comes either as photon excess energy or from the barrierless formation of a $M^{*}\mathrm{\text{−}}\mathrm{He}$ exciplex. We observe that this picture does not hold when $M=\mathrm{Rb}$ 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 dichroism experiment.

Figure 1

Bottom panel: comparison between LIF (black) and BD (gray) spectra of Rb atoms on He droplets. The relative scaling of the two spectra is chosen so as to force overlap at high wave number, where one expects the spectra to be identical; laser power is 24 mW. Top panel: ratio of BD and LIF signal, and fit thereof with a sigmoidal; we interpret this ratio as a detachment probability. Inset: saturation behavior of LIF signal at the wave numbers indicated in the main plot.

Figure 2

Bottom panel: dispersed fluorescence spectra corresponding to excitation at 12 608 (solid line) and $12645{\mathrm{cm}}^{-1}$ (dashed line, instrument-limited emission from free atoms). We interpret the difference as due to emission from bound excited states of the $\mathrm{Rb}\mathrm{\text{−}}{\mathrm{He}}_N$ potential. Inset: scheme of the corresponding emission process; potentials are calculated as a function of the atom-droplet distance $z$ (from droplet’s center of mass) for $N=1000$, at the equilibrium helium density dictated by the ${}^2\Pi _{1/2}$ state; the calculated Franck-Condon factors for $v^{\prime }=0\to v^{\prime \prime }\le 20$ emission are shown in the main panel as vertical bars. Those which are visibly nonzero ($v^{\prime \prime }=4\dots 8$) are labeled. The gray bar marks the position of the gas-phase transition; relative vertical scaling is arbitrary.

Figure 3

Bottom panel: LIF spectra with applied magnetic field ($\approx 2.9\mathrm{kG}$) parallel to laser beam for ${\sigma }^{-}$ (gray), and linear (black) polarization, under power-saturation conditions (laser power: 1.5 W). Top panel: ratio of the two signals (portions obscured by noise have been suppressed). Gray bars mark the position of the gas-phase lines.

Figure 4

LIF signal at various polarization of pump and probe beam; photon wave number: $12605{\mathrm{cm}}^{-1}$, $B=1.5\mathrm{kG}$. Inset: schematic of the optical pumping process with ${\sigma }^{-}$ polarized light. Although the individual transitions cannot be resolved, those marked with $\times$ are not excited because of selection rules.