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arXiv:2209.15464 · The Journal of Physical Chemistry Letters

Photoinduced Molecule Formation of Spatially Separated Atoms on Helium Nanodroplets

Florian Lackner, Wolfgang Ernst

Published Jun 12, 2018• 7 citations

Abstract

Besides the use as cold matrix for spectroscopic studies, superfluid helium droplets have served as a cold environment for the synthesis of molecules and clusters. Since vibrational frequencies of molecules in helium droplets exhibit almost no shift compared to the free molecule values, one could assume the solvated particles move frictionless and undergo a reaction as soon as their paths cross. There have been a few unexplained observations that seemed to indicate cases of two species on one droplet not forming bonds but remaining isolated. In this work, we performed a systematic study of helium droplets doped with one rubidium and one strontium atom showing that besides a reaction to RbSr, there is a probability of finding separated Rb and Sr atoms on one droplet that only react after electronic excitation. Our results further indicate that ground-state Sr atoms can reside at the surface as well as inside the droplet.

DOI: 10.1021/acs.jpclett.8b01530

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Silver atoms, initially embedded in nanodroplets of superfluid helium (roughly

{10}^{4}

atoms in size), are found to move to the surface and desorb when excited to the broadened

5p

level. The absorption of a second photon within less than 10 ns leads to the population of previously unobserved Rydberg states

(20<n<60)

of free silver atoms. This is the first result showing laser induced movement of a foreign atom from the inside to the surface of a nanodroplet of superfluid helium.

Abstract Alkali atoms and small clusters are known to reside on the surface of a helium droplet rather than its inside as most other dopant species. A theoretical investigation suggested that alkali clusters (Li–Rb) exceeding a certain critical size can become submerged in the droplet, which was experimentally confirmed for sodium and potassium. Here, we report an analogous experimental study of rubidium cluster submersion by means of electron impact mass spectrometry. We recorded size distributions of Rb cluster ions at various electron energies between 8 and 160 eV. Our data suggest that Rb clusters attached to helium droplets undergo a gradual submersion transition similar to potassium, ultimately leading to the full submersion of clusters larger than

\sim 100~\hbox {Rb}

∼ 100 Rb atoms. Our findings are consistent with previous theoretical and experimental studies. Graphic abstract