Ion induced snowballs as a diagnostic tool to investigate the caging of metal clusters in large helium droplets

Abstract

Metal clusters embedded in ultracold helium nanodroplets are exposed to femtosecond laser pulses with intensities of 10(13)-10(14) W/cm2. The influence of the matrix on the ionization and fragmentation dynamics is studied by pump-probe time-of-flight mass spectrometry. Special attention is paid to the generation of helium snowballs around positive metal ions (Me(z+)He(N), z=1,2). Closings of the first and second helium shells are found for silver at N(1)=10,12 and N(2)=32,44, as well as for magnesium at N1=19-20. The distinct abundance enhancement of helium snowballs in the presence of isolated atoms and small clusters in the droplets is used as a diagnostics to explore the cage effect. For silver, a reaggregation of the clusters is observed at 30 ps after femtosecond laser excitation.

Alkali-helium snowball complexes formed on helium nanodroplets

“…Taking into account the atom density for maximum doping of a single atom of 2.1 · 10 12 cm −3 (indicated as a dashed line), we deduce that the vast majority of detected Cs + 1,2 ions originate from fragments of clusters Cs N formed on the droplets. Similar behaviour has been reported for the case of silver clusters [6].…”

“…Furthermore, experiments with amplified fs pulses may induce much more extended snowball series, as observed with magnesium [5]. Fs pump-probe measurements may reveal the presence of caging-effects [6] and shed some light onto the issue whether alkali clusters are surface bound or immersed into the He droplets. Furthermore, the theoretical study of snowball formation around dimer ions Ak + 2 for the heavy alkalis Rb and Cs is desirable to interpret their element-specific appearance and stability patterns.…”

“…More recently, the stability of ion snowballs has been studied by means of abundance spectroscopy of snowballs formed upon ionization of metal atoms in He droplets [5,6,7]. Snowballs with up to 150 He atoms attached to, e. g., Ag + and Mg + ions have been observed.…”

Charging dynamics of dopants in helium nanoplasmas

“…This result is indeed confirmed by experiments, where we measure time-of-flight ion mass-over-charge spectra for He nanodroplets doped with the species Xe, Ca, and K for various sizes of the dopant clusters. Typical spectra recorded at a laser peak intensity I = 10 15 Wcm -2 are depicted in Fig. 9.…”

Ground state potential energy surfaces and bound states of M–He dimers (M=Cu,Ag,Au): A theoretical investigation

“…Although these phenomena attracted growing experimental efforts in recent years, there are no studies devoted to Cu and Au atoms or clusters in superfluid helium, while the Ag x -He n aggregates are among the most studied systems. 10,12,[18][19][20][21][22][23][24][25][26][27] The investigations conducted on silver aggregates in superfluid helium in the past decade cover a wide variety of structures, ranging from a single Ag atom, 10,12,20,27 to very small Ag x clusters ͑x Ͻ 10͒, 19,[21][22][23] up to very complex aggregates containing thousands of atoms. 18,[24][25][26] As concerns the theoretical investigations on M x -He n ͑M =Cu,Ag,Au͒, to the best of our knowledge, there are just four studies on Ag-He n available in literature, and none on Cu-He and Au-He.…”