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FLORIAN LACKNER, GÜNTER KROIS AND WOLFGANG E. ERNST, Institute of Experimental Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria.

We report on the investigation of mixed alkali metal (Ak) - alkaline earth metal (Ake) molecules on the surface of helium nanodroplets (He_N). These molecules have recently attracted considerable attention as candidates for the formation of ultracold molecules with a magnetic and an electronic dipole moment. In our experiments, LiCa and RbSr molecules are formed in a sequential pick-up process in their X^{2 +} ground state and cool down rapidly to the droplet temperature of 0.38 K. Excitation spectra of LiCa and RbSr were recorded by using resonance enhanced multi-photon ionization time-of-flight (REMPI-TOF) spectroscopy and laser induced fluorescence (LIF) spectroscopy. On the helium droplet, vibronic transitions in Ak-Ake molecules are broadened and show a characteristic asymmetric peak form, which is caused by the interaction between the molecule and the superfluid He_N environment. For the lower electronic transitions in LiCa and RbSr progressions of vibrational bands excited from the X^{2 +} ( '' = 0) state are observed. The LiCa spectra can be compared to molecular beam experiments, which enables the assignment of three band systems near 15260 cm^-1, 19300 cm^-1 and 22120 cm^-1 as ^{2 +}, ² and ² band, respectively. In the RbSr excitation spectrum we observe a vibrationally resolved band system near 14020 cm^-1.
Upon electronic excitation, a fraction of the molecules desorb from the droplet surface and dispersed fluorescence spectra allow to study the X^{2 +} ground state and excited states of free Ak-Ake molecules.

% S.~Fei, X.~Zheng, and M.~C.~Heaven, Chem.~Phys.~Lett. 176, 373