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Alkali atoms are deposited on superfluid helium nanodroplets where they stay at the surface and form molecules in cold collisions. Through evaporation of helium atoms from the droplet, the temperature is automatically maintained at 0.4 K, the equilibrium temperature. Droplets loaded with weakly bound species, in our case the high-spin states of alkali molecules, have the best chance to survive and are available for spectroscopic investigations. Our experiments with potassium and rubidium in this cold nanoscale environment allowed the observation of weakly bound triplet states of K2, Rb2, and KRb, and the first identification of the trimers K3, Rb3, K2Rb and KRb2 in their quartet ground states. The spectroscopy has been accompanied by our own quantum chemical calculations that allow state assignments [1]. Electronic excitations of the trimers yield insight into several states that underlie both Jahn-Teller and spin-orbit coupling [2]. In preparation for experiments involving optical detection of electron spin transitions in cold molecules, we studied the electron spin relaxation in alkali atoms and molecules on the surface of a droplet. By measuring the circular dichroism in the presence of a magnetic field, the populations of Zeeman sublevels in alkali atoms, dimers, and trimers were probed. No dichroism was observed for the atomic potassium sample on helium droplets, indicating that the sublevels have not thermalized [3]. The Zeeman sublevels of dimer and trimer molecules, however, turn out to be populated according to a temperature of 0.4 K, implicitly allowing the first determination of the droplet¢s surface temperature [4]. Optical detection of spin resonance is achieved in an optical pump-probe experiment with the electron spin transition induced in a microwave cavity in a magnetic field between the pump and probe regions. The optical laser can either deplete a particular spin state by desorption of the respective atoms or molecules from the helium droplet beam or by population transfer (truly optical pumping) [5]. In both cases, the probe laser detects the successful spin flip induced by the microwave field. Applying a magnetic field of 3.4 kG and varying the intensity of the 9.4 GHz microwaves, we were able to observe up to 50 Rabi cycles of an electron spin transition in the potassium atom during the flight time of 57 µs [6]. 1. Johann Nagl, Gerald Auböck, Andreas W. Hauser, Olivier Allard, Carlo Callegari, and Wolfgang E. Ernst, Phys. Rev. Lett. 100, 063001 (2008). 2. Gerald Auböck, Johann Nagl, Carlo Callegari, and Wolfgang E. Ernst, J. Chem. Phys. 129, 114501-1-9 (2008). 3. Johann Nagl, Gerald Auböck, Carlo Callegari, and Wolfgang E. Ernst, Phys. Rev. Lett. 98, 075301 (2007). 4. Gerald Auböck, Johann Nagl, Carlo Callegari, and Wolfgang E. Ernst, J. Phys. Chem. A 111, 7404 (2007). 5. Gerald Auböck, Johann Nagl, Carlo Callegari, and Wolfgang E. Ernst, Phys. Rev. Lett. 101, 035301-1-4 (2008). 6. Markus Koch, Gerald Auböck, Carlo Callegari, and Wolfgang E. Ernst (to be published). |
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