II. Optical Pumping of Rubidium

Rubidium D-lines

D-lines of Rubidium embedded in helium nanodroplets (LIF absorption spectrum)

Alkali-metal and alkaline earth metal atoms do not immerse fully in He_N and reside on the surface, bound by long-range Van-der-Waals forces. The interaction with the helium environment leads to a line-broadening in the laser induced fluorescence (LIF) spectra. The 5²P_1/2 (²Π_1/2) and 5²P_3/2 (²Π_3/2,²Σ_1/2) transitions of Rubidium are blue-shifted with respect to the free atom D1 and D2 lines.

Potential of the Rb-He_N system in the ground (buttom) and first excited (top) state.

Upon excitation the alkali atoms usually desorb from the droplet. Rubidium and Caesium constitute an exception as the 5²P_1/2 (²Π_1/2) forms a weakly  bound state because of the large spin-orbit interaction.  The Rb atom stays on the surface of the droplet and can be recycled [3].

Optical Pumping - Rb recycle-scheme

Helium density profiles for a droplet of 500 He atoms doped with Rb in there equilibrium states.

The helium density of the droplet is altered by the dopant. The equilibrium is reached with a dimple shape structure. After vertical excitation the helium density adiabatically follows the new rubidium potential. Fluorescence takes place from the new equilibrium, then the system relaxes back to the initial state.

This Rb recycle - scheme could be exploited for optical pumping. Although the optical spectrum is broadened the atomic polarization is preserved to the hyperfine limit.

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