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Physical Review A

Light-induced collapse of metastable magnesium complexes formed in helium nanodroplets

Andreas Przystawik, Sebastian Göde, T. Döppner, J. Tiggesbäumker, K.‐H. Meiwes‐Broer

Published Aug 27, 2008• 67 citations
Publisher

Abstract

Doping helium nanodroplets with more than a single Mg atom leads to a shift of the atomic absorption from $279\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}282\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. This behavior is stable against a change of the chosen droplet size. There is evidence that the interplay of the weak long-range potential between individual Mg atoms and the interaction with the surrounding quantum fluid leads to the formation of metastable very weakly bound complexes high above the global energy minimum. They resemble a bubble foam, where the individual Mg atoms are separated by a helium layer. The formation of compact Mg clusters is hindered by density variations within the superfluid solvent. The distance between the Mg atoms is substantially larger compared to the ground state. We estimate a value of roughly $10\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and the barrier, preventing the formation of compact Mg clusters to only a few kelvin. Upon laser excitation the metastable complexes collapse on a time scale of $20\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ as measured with a femtosecond pump-probe experiment.

DOI
10.1103/physreva.78.021202

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Doping helium nanodroplets with more than a single Mg atom leads to a shift of the atomic absorption from 279nmto282nm279\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}282\phantom{\rule{0.3em}{0ex}}\mathrm{nm}279nmto282nm. This behavior is stable against a change of the chosen droplet size. There is evidence that the interplay of the weak long-range potential between individual Mg atoms and the interaction with the surrounding quantum fluid leads to the formation of metastable very weakly bound complexes high above the global energy minimum. They resemble a bubble foam, where the individual Mg atoms are separated by a helium layer. The formation of compact Mg clusters is hindered by density variations within the superfluid solvent. The distance between the Mg atoms is substantially larger compared to the ground state. We estimate a value of roughly 10A˚10\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}10A˚ and the barrier, preventing the formation of compact Mg clusters to only a few kelvin. Upon laser excitation the metastable complexes collapse on a time scale of 20ps20\phantom{\rule{0.3em}{0ex}}\mathrm{ps}20ps as measured with a femtosecond pump-probe experiment.
Metal atoms have proved to be sensitive probes of the properties of superfluid helium nanodroplets. To date, all experiments on the doping of helium droplets have concentrated on the attachment of metal atoms in their ground electronic states. Here we report the first examples of metal atoms in excited states becoming attached to helium nanodroplets. The atoms in question are aluminum, and they have been generated by laser ablation in a metastable quartet state, which attaches to and remains on the surface of helium droplets. Evidence for a surface location comes from electronic spectra, which consist of very narrow absorption profiles that show very small spectral shifts. Supporting ab initio calculations show there to be an energy incentive for a metastable Al atom to remain on the surface of a helium droplet rather than move to the interior. The results suggest that helium droplets may provide a method for the capture and transport of metastable excited atomic and molecular species.
Superfluid helium nanodroplets are an ideal environment for the formation of metastable, self-organized dopant nanostructures. However, the presence of vortices often hinders their formation. Here, we demonstrate the generation of vortex-free helium nanodroplets and explore the size range in which they can be produced. From x-ray diffraction images of xenon-doped droplets, we identify that single compact structures, assigned to vortex-free aggregation, prevail up to 10810^8108 atoms per droplet. This finding builds the basis for exploring the assembly of far-from-equilibrium nanostructures at low temperatures.
Superfluid helium nanodroplets are an ideal environment for the formation of metastable, self-organized dopant nanostructures. However, the presence of vortices often hinders their formation. Here, we demonstrate the generation of vortex-free helium nanodroplets and explore the size range in which they can be produced. From x-ray diffraction images of xenon-doped droplets, we identify that single compact structures, assigned to vortex-free aggregation, prevail up to 10^{8} atoms per droplet. This finding builds the basis for exploring the assembly of far-from-equilibrium nanostructures at low temperatures.
Silver atoms, initially embedded in nanodroplets of superfluid helium (roughly 104{10}^{4}104 atoms in size), are found to move to the surface and desorb when excited to the broadened 5p5p5p level. The absorption of a second photon within less than 10 ns leads to the population of previously unobserved Rydberg states (20<n<60)(20<n<60)(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 In this introductory chapter, we begin by informing the reader about the fascinating history of superfluidity in bulk liquid helium. This is followed by relating attempts in using liquid helium as a low temperature matrix for spectroscopy. After a brief review of the thermodynamic properties of helium in Sect. 1.2, the different types of free jet expansions used in experiments to produce clusters and nanodroplets of different sizes are described in Sect. 1.3. First it is shown how they depend on the nature and location in the phase diagram of the isentropes which determine the course of the expansion. Depending on the four regimes of isentropes, different number sizes and distributions are expected. Next in Sect. 1.4, the results of theoretical and, where available, experimental results on the total energies, excited states, radial density distributions, and temperatures of clusters and droplets are discussed. Finally, in Sect. 1.5 the theoretical and experimental evidence for the superfluidity of nanodroplets is briefly reviewed. For more information on the production and characteristics of nanodroplets, the reader is referred to the chapters in this book and to the reviews in Appendix.