The one-body density matrix in Helium droplets

Siu A. Chin, E. Krotscheck

Matthias Veternik, Tobias Waldhütter, Lutz Schweikhard +2 more

We report on the successful trapping of micrometer-sized, multiply-charged helium droplets over periods of several seconds up to a minute-the first observation of trapped charged helium droplets. Within a meter-long multireflection mass spectrometer, the droplets complete thousands of revolutions, opening up new possibilities for precision measurements of these unique systems. Their confinement is limited by collisions with residual gas, which gradually reduce their kinetic energy through evaporation of helium atoms from the droplet and prevent extraction from the trap below a critical value. The rate of energy loss is strongly influenced not only by the residual gas density, but also by the type of species picked up by the droplets. In particular, infrared absorption by water cluster ions inside the droplets enhances energy transfer to the helium matrix, leading to accelerated evaporation of helium atoms.

Florian Lackner, Wolfgang Ernst

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.

C. J. Tymczak

Thomas Gruber, Andreas Hauser, Wolfgang Ernst

Full text: In recent experiments of our group on alkali-metal-doped helium droplets, it could be shown that the hyperfine structure of the dopants is strongly affected by the helium surrounding. A change of the hyperfine splitting of {sup 39}K is observed as a function of the nozzle temperature, indicating a dependence on the helium droplet size. Hence we started a theoretical investigation based on an open shell coupled cluster approach to study this size effect within molecular orbital theory. In our theoretical model we are restricted to very small He{sub N}X clusters (N=1- 5,X=Li,Na,K) due to the high computational demands of the chosen method. However, it is possible to identify two antagonistic effects of the helium surrounding on the electronic wavefunction of the alkali dopant: The first effect is a compression of the wavefunction which leads to higher spin densities |{Phi}{sub 0}|{sup 2} at the core, the second one is a reduction of |{Phi}{sub 0}|{sup 2} due to the Pauli repulsion, i.e. the deformation of the s-orbitals. Despite of the computational limitations we also try to give estimations for larger systems. Here we apply a combination of the quantum chemistry approach with a stochastic geometry optimization. Assuming only two-body-interactions we use the simulated annealing technique to obtain geometry predictions. (author)