Kilohertz laser ablation for doping helium nanodroplets

Abstract

A new setup for doping helium nanodroplets by means of laser ablation at kilohertz repetition rate is presented. The doping process is characterized and two distinct regimes of laser ablation are identified. The setup is shown to be efficient and stable enough to be used for spectroscopy, as demonstrated on beam depletion spectra of lithium atoms attached to helium nanodroplets. For the first time, helium droplets are doped with high temperature refractory materials such as titanium and tantalum. Doping with the nonvolatile DNA basis guanine is found to be efficient and a number of oligomers are detected.

Photoionisaton of pure and doped helium nanodroplets

“…When using a nanosecond pulsed laser for evaporating the target material, the part of the He droplet beam with is doped arrives at the detector within a time interval of about 100 µs. In this way, doping with refractory metals as well as with fragile bio-molecules was demonstrated [41]. Since a significant part of the laser-induced plasma consists of charged particles, ions are also attached to the droplets [40].…”

“…This applies to direct one-photon PI of dopant atoms [58,117], to non-resonant PI using ultrashort laser pulses, to REMPI of dopant atoms, whose intermediate excited states are non-desorbing [59,82,83,118], and to resonant REMPI of molecular dopants into the vibronic ground states of their cations [119]. Note that ion-doped He droplets have also been observed when doping He droplets with preformed ions, e. g. using laser ablation or ion traps [40,41,120]. However, due to the technical difficulty in detecting these massive ions with high sensitivity, they have only rarely been used as a quantitative observable.…”

“…Laser evaporation has been established as an alternative way to produce doped He droplets by the authors [40,41]. The material is ablated from a rotating and translating rod by a pulsed infrared, visible or ultraviolet laser.…”

Formation of Metallic Nanowires by Laser Ablation in Liquid Helium

“…Although laser ablation (LA) of metals in liquid He is used in matrix-isolation spectroscopy since the 1990's [4][5][6][7][8], the ablation process is not well studied. In particular the formation of nanoparticles (NP) during the laser ablation of Cs and Rb in liquid helium was reported [4,9].…”

Helium droplets: a chemistry perspective

“…Laser ablation has also been used to add metal atoms to HeDs but in that case care must be taken to avoid the highly energetic ablation plasma from coming into contact with the HeD beam. 21 To close this section we ponder why there is such interest in adding dopants to HeDs when one might just as well try adding them to bulk liquid helium. The principal reason derives from the poor solvation characteristics of liquid helium, which makes it difficult to inject and retain dopants inside bulk liquid helium since they will rapidly coagulate at the walls of the vessel.…”