Atom lithography commonly employs self-assem- bled monolayers (SAMs) of alkanethiols which act as resists to protect prepared surfaces. Metastable atomic species such as helium are used to damage the resist, enabling pattern transfer via mask lithography, followed by wet chemical etching. The damage mechanism is, however, not well understood. Here we report studies of fragmentation of dodecanethiol (DDT) molecules embedded in helium nano-droplets that have been irradiated by an electron beam. The results of the charge-transfer fragmentation process provide the first experimental data on the damage mechanisms that occur in the metastable helium/SAM interaction.

Silver atoms, initially embedded in nanodroplets of superfluid helium (roughly 10(4) atoms in size), an found to move to the surface and desorb when excited to the broadened 5p lever, The absorption of a second photon within less than 10 ns leads to the population of previously unobserved Rydberg states (20 < n < 60) of free silver atoms. This is the first result showing laser induced movement of a foreign atom from the inside to the surface of a nanodroplet of superfluid helium.

The first highly resolved electronic spectra of small non-alkali metal clusters embedded in nanoscopic helium droplets (N≈20000) are presented. The helium droplets serve as an extremely cold liquid matrix with a temperature of 0.37 K. Resonant two-photon-ionization is used for size-selective spectroscopy on a silver cluster distribution (N≤10). Results are reported for Ag2, Ag3 and for Ag8. For the latter, a very narrow absorption spectrum is resolved corresponding to the Ag8 plasmon resonance. The linewidth of the observed resonance is a factor of two smaller than theoretical zero-temperature predictions for the plasmon line width of closed-shell metal clusters. In contrast, the lifetime of the Ag8 resonance is estimated to be of the order of nanoseconds, which is inconsistent with the plasmon picture and typical for molecular transitions.

We review the technique of helium droplet spectroscopy as a tool for the study of complex molecules and cold metal clusters. Clusters of silver and europium up to mass 4000 have been produced by pickup of single atoms by a beam of helium droplets. The clusters' binding energy is removed by evaporative cooling and the system remains at 0.4 K. As an example for spectroscopic experiments, we present the absorption of cold C-60. We observe sharp absorption lines that are nearly unshifted compared to the gas phase. This is in remarkable contrast to traditional matrix spectroscopy and may open up future applications as a substitute for gas phase studies.

In this paper we discuss helium droplets as a nanoscale cryostat for a variety of fundamental experiments in condensed matter physics. Specifically, we describe our recent work on the spectroscopy of silver atoms, europium atoms and C 60 in helium droplets and compare the helium droplet, as a matrix for low temperature studies of complex systems, with traditional matrix and gas phase techniques. Further, we discuss our recent work on the production of ultra-cold metal clusters of silver, indium and europium embedded in helium droplets at a temperature (T=0.37K) two orders of magnitude lower than previously achieved in beams of free metal clusters. This work opens the door to high resolution spectroscopic studies of metal clusters and, possibly, high resolution studies of the size dependence of their superconducting properties. We further speculate on a series of experiments where we plan to use standard spectroscopic methods, developed in recent years, to exploit the helium droplet for studies of the existence, stability and dynamics of quantized vortices. Helium droplets may be the ideal system for such studies due to the complete absence of pinning sites that plague many similar experiments performed in bulk helium.

We have measured the UV-absorption spectrum (4f→5d transitions) of single europium atoms embedded in helium droplets using a beam depletion technique. The electronic levels involved in the transition lie spatially within the closed 6s2 shell leading to significant shielding from the surrounding helium environment. We observe the narrowest absorption lines ever reported in liquid helium and compare our data and experimental method, with experiments carried out in bulk liquid helium.

We have recorded the electronic absorption of cold C-60 molecules embedded in helium droplets at a temperature of approximately 0.4 K, using a beam depletion technique. We obtain highly resolved spectra in the region between 595 and 650 nm and are able to observe in detail the origin of the electronic transitions near 638 nm. In contrast to traditional matrix experiments the spectra show negligible shifts and little broadening. This remarkable feature could establish the technique as an alternative for gas-phase spectroscopy. (C) 1997 Elsevier Science B.V.