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

Doping helium nanodroplets with more than a single Mg atom leads to a shift of the atomic absorption from $279\mathrm{nm}\text{to}282\mathrm{nm}$. 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 $10\mathrm{\AA }$ 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 $20\mathrm{ps}$ as measured with a femtosecond pump-probe experiment.

Figure 1

(Color online) Cut through the calculated helium density around two magnesium atoms separated by a distance of $10\mathrm{\AA }$. A region of increased helium density is clearly visible between the Mg atoms, corresponding to a local energy minimum.

Figure 2

R2PI spectra of magnesium doped helium droplets consisting of 6000 atoms detected on the Mg monomer mass channel [15] at different doping conditions. With fewer than one Mg atom on average [panel (a)] the spectrum shows a single maximum centered at $279\mathrm{nm}$. At a higher doping with two Mg atoms per droplet, an additional structure at $282.5\mathrm{nm}$ appears [panel (b)]. Increasing the mean number of Mg atoms to 4 [panel (c)] the peak at $279\mathrm{nm}$ vanishes and the absorption around $282.5\mathrm{nm}$ becomes dominant.

Figure 3

(Color online) R2PI spectra of helium droplets consisting of 40 000 atoms doped with an average of 8 Mg atoms for different mass channels. The high similarity to the monomer spectrum recorded at the same conditions implies that all species originate from the same atomic precursor.

Figure 4

Femtosecond pump-probe signal at $800\mathrm{nm}$ with a highly Mg doped He droplet. The signal of clusters (here, ${\mathrm{Mg}}_5$) only rises after a considerable delay and stabilizes beyond about $30\mathrm{ps}$. The time scale for the cluster formation can be estimated to be roughly $20\mathrm{ps}$.

Figure 5

(Color online) Long-range Mg-Mg potential energy curves of a dimer embedded in helium (dots) and in the gas phase [9] (solid line). The result for the embedded dimer is shifted by the solvation energy in helium to enable an easy comparison. One clearly sees the modulation induced by the helium solvation layers, leading to several shallow local minima, the deepest located at about $10\mathrm{\AA }$ distance.