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

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.