15min:

GERALD AUBÖCK, JOHANN NAGL, CARLO CALLEGARI AND WOLFGANG E. ERNST, Institute of Experimental Physics, TU Graz, Petersgasse 16, 8010 Graz, Austria/EU.

We present the application of Magnetic Circular Dichroism (MCD) to the determination of the level structure of orbitally-degenerate electronic levels of molecules under the perturbation of a He droplet, which had so far escaped assignment. Our target system is the (1) ³ _g [(2) ³ if heteronuclear] manifold of alkali dimers. The perturbation due to the droplet breaks the rotational symmetry around the internuclear axis of the diatom and causes a splitting of this orbitally-degenerate manifold. We extend to He droplets a model previously used to interpret the matrix spectra of the NH radical. With a small number of physically reasonable parameters the model accounts for the essential features of laser-induced fluorescence (LIF) and magnetic circular dichroism (MCD) spectra of \chemRb₂ and \chemK₂. MCD spectra are not only essential to the correct assignment of the observed structure, but also allow a determination of the populations of Zeeman sublevels in the ground state and thus a measurement of the surface temperature of the droplet. The latter agrees with the accepted temperature, 0.37 K, measured in the interior of a droplet.