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Review of Scientific Instruments

Characterization of laser ablation as a means for doping helium nanodroplets

Patrick Claas, S.-O. Mende, Frank Stienkemeier

Published Aug 20, 2003• 36 citations
Publisher

Abstract

Helium nanodroplets are doped with metal atoms by means of laser evaporation. The material is evaporated directly in front of the helium nozzle by a pulsed laser. In comparison with conventional pickup from an oven we deduce that the droplets can be loaded with more than one atom per droplet on average. Conditions and efficiencies for loading are evaluated. Furthermore, metal ions can be attached to helium droplets by this method, allowing isolation of charged particles at temperatures below 1 K. In this way we can directly measure size distributions of helium nanodroplets which are compared to results using other techniques.

DOI
10.1063/1.1602943

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Helium nanodroplets are doped with metal atoms by means of laser evaporation. The material is evaporated directly in front of the helium nozzle by a pulsed laser. In comparison with conventional pickup from an oven we deduce that the droplets can be loaded with more than one atom per droplet on average. Conditions and efficiencies for loading are evaluated. Furthermore, metal ions can be attached to helium droplets by this method, allowing isolation of charged particles at temperatures below 1 K. In this way we can directly measure size distributions of helium nanodroplets which are compared to results using other techniques.
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.
We present two facile time-of-flight (TOF) methods of detecting superfluid helium droplets and droplets with neutral dopants. Without an electron gun and with only a heated filament and pulsed electrodes, the electron impact ionization TOF mass spectrometer can resolve ionized helium clusters such as He2(+) and He4(+), which are signatures of superfluid helium droplets. Without ionizing any helium atoms, multiphoton non-resonant laser ionization of CCl4 doped in superfluid helium droplets at 266 nm generates complex cluster ions of dopant fragments with helium atoms, including (He)(n)C(+), (He)(n)Cl(+), and (He)(n)CCl(+). Using both methods, we have characterized our cryogenic pulsed valve—the Even-Lavie valve. We have observed a primary pulse with larger helium droplets traveling at a slower speed and a rebound pulse with smaller droplets at a faster speed. In addition, the pickup efficiency of dopant is higher for the primary pulse when the nozzle temperature is higher than 13 K, and the total time duration of the doped droplet pulse is only on the order of 20 μs. These results stress the importance of fast and easy characterization of the droplet beam for sensitive measurements such as electron diffraction of doped droplets.
Advancements in x-ray free-electron lasers on producing ultrashort, ultrabright, and coherent x-ray pulses enable single-shot imaging of fragile nanostructures, such as superfluid helium droplets. This imaging technique gives unique access to the sizes and shapes of individual droplets. In the past, such droplet characteristics have only been indirectly inferred by ensemble averaging techniques. Here, we report on the size distributions of both pure and doped droplets collected from single-shot x-ray imaging and produced from the free-jet expansion of helium through a 5 μm diameter nozzle at 20 bars and nozzle temperatures ranging from 4.2 to 9 K. This work extends the measurement of large helium nanodroplets containing 109–1011 atoms, which are shown to follow an exponential size distribution. Additionally, we demonstrate that the size distributions of the doped droplets follow those of the pure droplets at the same stagnation condition but with smaller average sizes.
Factors affecting the size of liquid-helium droplets produced by a pulsed nozzle are described. The shape of the nozzle orifice is found to be important in allowing control of the size of the droplets. With an appropriate choice of nozzle geometry, the average droplet size is shown to be continuously variable over nearly two orders of magnitude by adjustment of the helium gas stagnation pressure and/or temperature. A scaling law similar to, but not identical with, that found for helium droplets produced by continuous supersonic expansion sources is found for the pulsed source. The pulsed nozzle described in this article has been used to make helium droplets ranging in size from a few thousand atoms up to nearly 105 helium atoms.
A new method for stable and continuous doping of superfluid helium nanodroplets (He(N)) with high-melting elements such as refractory metals is presented. The method exploits the advantages of electron bombardment heating and avoids stray fields induced by high currents or high frequency fields. It is thus especially suitable for magnetic studies of atoms and clusters in He(N). The source is characterized by means of mass spectroscopic investigations of He(N) doped with chromium atoms and clusters. Source temperatures of up to (1650 ± 50) °C were reached and Cr clusters up to Cr(9) could be formed in He(N).
Helium nanodroplets doped with polar molecules are studied by electrostatic deflection. This broadly applicable method allows even polyatomic molecules to attain subkelvin temperatures and nearly full orientation in the field. The resulting intense force from the field gradient strongly deflects even droplets with tens of thousands of atoms, the most massive neutral systems studied by beam "deflectometry." We use the deflections to extract droplet size distributions. Moreover, since each host droplet deflects according to its mass, spatial filtering of the deflected beam translates into size filtering of neutral fragile nanodroplets. As an example, we measure the dopant ionization probability as a function of droplet radius and determine the mean free path for charge hopping through the helium matrix. The technique will enable separation of doped and neat nanodroplets and size-dependent spectroscopic studies.