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

Journal Article · Sun Aug 21 04:00:00 EDT 2011 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.3624840· OSTI ID:22038671

Theisen, Moritz; Lackner, Florian; Ernst, Wolfgang E ^[1]

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1. Institute of Experimental Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz (Austria)

We report the non-desorption of cesium (Cs) atoms on the surface of helium nanodroplets (He{sub N}) in their 6{sup 2}P{sub 1/2} ({sup 2}{Pi}{sub 1/2}) state upon photo-excitation as well as the immersion of Cs{sup +} into the He{sub N} upon photo-ionization via the 6{sup 2}P{sub 1/2} ({sup 2}{Pi}{sub 1/2}) state. Cesium atoms on the surface of helium nanodroplets are excited with a laser to the 6{sup 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{sub N} is excited with photon energies close to the atomic D{sub 1}-line, which implies an attractive character of the excited state system (Cs*-He{sub 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{sub N} in a pseudo-diatomic model. Calculated Franck-Condon factors for emission from the 6{sup 2}P{sub 1/2} ({sup 2}{Pi}{sub 1/2}) to the 6{sup 2}S{sub 1/2} ({sup 2}{Sigma}{sub 1/2}) state help to explain the experimental data. The stability of the Cs*-He{sub N} system allows to form Cs{sup +} snowballs in the He{sub N}, where we use the non-desorbing 6{sup 2}P{sub 1/2} ({sup 2}{Pi}{sub 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{sup 2}P{sub 1/2} ({sup 2}{Pi}{sub 1/2}) state gives rise to a very high yield of immersed Cs{sup +} in contrast to an ionization scheme via the 6{sup 2}P{sub 3/2} ({sup 2}{Pi}{sub 3/2}) state. When resonant two-photon ionization is applied to cesium dimers on He droplets, Cs{sub 2}{sup +}-He{sub N} aggregates are observed in time-of-flight mass spectra.

• OSTI ID: 22038671
• Journal Information: Journal of Chemical Physics, Journal Name: Journal of Chemical Physics Journal Issue: 7 Vol. 135; ISSN JCPSA6; ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
74 ATOMIC AND MOLECULAR PHYSICS
CESIUM
CESIUM IONS
DESORPTION
DETECTION
EMISSION SPECTRA
EXCITATION
EXCITED STATES
FLUORESCENCE
FLUORESCENCE SPECTROSCOPY
HELIUM
MASS SPECTRA
PHOTOIONIZATION
POTENTIAL ENERGY