Düsseldorf 2007 – scientific programme

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MO: Fachverband Molekülphysik

MO 63: Spectroscopy in He-droplets / Ultracold Molecules I (gemeinsam mit Q)

MO 63.1: Fachvortrag

Friday, March 23, 2007, 10:30–11:00, 6B

Magnetic dichroism of alkali atoms and molecules on the surface of helium nanodroplets — Johann Nagl, Gerald Auböck, Carlo Callegari, and •Wolfgang E. Ernst — Institute of Experimental Physics, TU Graz, Petersgasse 16, A-8010 Graz, Austria

We measured laser induced fluorescence spectra of K and Rb atoms, dimers and trimers, on the surface of superfluid helium droplets, with and without a moderately strong external magnetic field (≈ 3 kG). Atomic spectra (D lines) are saturated with a few hundred mW of laser power. In a magnetic field, and under saturation, we observe a greater signal for linear polarization but no difference between the two states of (circular) polarization of the exciting laser. We take this as evidence that the two spin sublevels of the ground-state alkali atoms are equipopulated, despite a Zeeman splitting comparable in magnitude to kT (at the temperature of the droplet, T=0.38 K). We estimate that the rate of spin relaxation induced by the droplet must be < 1000/s. We thus demonstrate that by selective depletion it is possible to create a beam of He droplets doped with spin-polarized alkali atoms.

Measurements on triplet dimers do show magnetic circular dichroism, indicating that, unlike atoms, molecules do undergo fast spin relaxation and do thermalize to the temperature of the droplet.