Cs atoms on helium nanodroplets and the immersion of Cs+ into the nanodroplet

We report the non-desorption of cesium (Cs) atoms on the surface of helium nanodroplets (He-N) in their 6(2)P(1/2) ((2)Pi(1/2)) state upon photo-excitation as well as the immersion of Cs+ into the He-N upon photo-ionization via the 6(2)P(1/2) ((2)Pi(1/2)) state. Cesium atoms on the surface of helium nanodroplets are excited with a laser to the 6(2)P states. We compare laser-induced fluorescence (LIF) spectra with a desorption-sensitive method (Langmuir-Taylor detection) for different excitation energies. Dispersed fluorescence spectra show a broadening of the emission spectrum only when Cs-He-N is excited with photon energies close to the atomic D-1-line, which implies an attractive character of the excited state system (Cs*-He-N) potential energy curve. The experimental data are compared with a calculation of the potential energy curves of the Cs atom as a function of its distance R from the center of the He-N in a pseudo-diatomic model. Calculated Franck-Condon factors for emission from the 6(2)P(1/2) ((2)Pi(1/2)) to the 6(2)S(1/2) ((2)Sigma(1/2)) state help to explain the experimental data. The stability of the Cs*-He-N system allows to form Cs+ snowballs in the HeN, where we use the non-desorbing 6(2)P(1/2) ((2)Pi(1/2)) state as a springboard for ionization in a two-step ionization scheme. Subsequent immersion of positively charged Cs ions is observed in time-of-flight mass spectra, where masses up to several thousand amu were monitored. Only ionization via the 6(2)P(1/2) ((2)Pi(1/2)) state gives rise to a very high yield of immersed Cs+ in contrast to an ionization scheme via the 6(2)P(3/2) ((2)Pi(3/2)) state. When resonant two-photon ionization is applied to cesium dimers on He droplets, Cs-2(+)-He-N aggregates are observed in time-of-flight mass spectra. (C) 2011 American Institute of Physics. [doi:10.1063/1.3624840]