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 ([12pt]{minimal}{document}HeN{document} He N) in their 62P1/2 (2Π1/2) state upon photo-excitation as well as the immersion of Cs+ into the [12pt]{minimal}{document}HeN{document} He N upon photo-ionization via the 62P1/2 (2Π1/2) state. Cesium atoms on the surface of helium nanodroplets are excited with a laser to the 62P 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-[12pt]{minimal}{document}HeN{document} He N is excited with photon energies close to the atomic D1-line, which implies an attractive character of the excited state system (Cs*-[12pt]{minimal}{document}HeN{document} 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 [12pt]{minimal}{document}HeN{document} He N in a pseudo-diatomic model. Calculated Franck-Condon factors for emission from the 62P1/2 (2Π1/2) to the 62S1/2 (2Σ1/2) state help to explain the experimental data. The stability of the Cs*-[12pt]{minimal}{document}HeN{document} He N system allows to form Cs+ snowballs in the [12pt]{minimal}{document}HeN{document} He N, where we use the non-desorbing 62P1/2 (2Π1/2) 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 62P1/2 (2Π1/2) state gives rise to a very high yield of immersed Cs+ in contrast to an ionization scheme via the 62P3/2 (2Π3/2) state. When resonant two-photon ionization is applied to cesium dimers on He droplets, [12pt]{minimal}{document}Cs₂⁺{document} Cs 2+-[12pt]{minimal}{document}HeN{document} He N aggregates are observed in time-of-flight mass spectra.