TWO-STEP EXCITATION OF Rb AND Cs ATOMS ON He NANODROPLETS

Abstract

We present the first sequential excitation of atom-doped helium nanodroplets. Rubidium atoms on the surface of a helium nanodroplet are selectively excited to the 5$^2$P$_{1/2}$ state so as not to desorb from the droplet. From there they are excited by a laser pulse to the 5$^2$D state; a laser-induced fluorescence (LIF) spectrum is recorded by monitoring the 6$^{2}$P$\rightarrow$5$^{2}$S$_{1/2}$ emission. We find some difference in the LIF spectrum as compared to that of the two-photon one-color direct excitation spectrum 5$^2$D$\leftarrow$$5^2$S$_{1/2}$. This indicates that the system does relax vibrationally during the lifetime of the 5$^2$P$_{1/2}$ state. To model the LIF spectra we calculate the energy levels of the Rb atom as a function of its distance $R$ from the center of the droplet. The Franck-Condon factors of the resulting potential energy curves agree with the experimental findings. A similar behavior has been found for cesium. New measurements predict that it also stays bound on the surface of the droplet in its 6$^2$P$_{1/2}$ state. From there we further excited Cs monomers into their 6$^2$D state, where also the LIF spectrum is recorded by watching the 7$^{2}$P$\rightarrow$6$^{2}$S$_{1/2}$ emission. In the future these states can be used as a springboard to reach high-lying $^{2}$S and $^{2}$D states, and possibly create an artificial super-atom.