The study and use of beams of clusters of helium began in the early eighties and is still a growing field. As a tool, He N , called droplets if N ≥ 1000, serve as cryostats for the preparation and analysis of species that would otherwise, say by free beam expansion or conventional matrix isolation, be difficult to study. The droplets are also interesting by themselves. Clusters of 4 He are superfluid yet too small to be approximated by an infinite condensate. Mass abundance spectra of by helium clusters captured yet heliophobe species lead to intriguing problems. To grow alkali guest clusters (A k ) with He N hosts enables ultra low temperature isolation spectroscopy with these for cluster science important metal nano-particles. However, helium does not wet alkali atoms. It was thought impossible to grow sodium to sizes larger than the trimer Na k>3 .
We describe the first observation of clusters A k up to k = 13 on He N . Their surface location is proven via establishing the shape of ionization yield curves. The observed mass spectra have been interpreted with two mutually exclusive models. Each involves a configuration He N A k of considerable interest, namely on one hand highly spin polarized clusters on the surface, on the other metallically bound, ultra cold alkali clusters that may or may not reside on the surface of the helium droplet. We helped resolving the issue by deriving folded statistics of the capture, the spin dependent desorption and the evaporation of helium due to each capture event. Results are mostly exact or at least analyticity is preserved. A lot of this has by now been settled with much better experimental equipment [Bue06], though some questions seem still open.
A model applicable to usually employed cells for beam scattering and impurity pickup is presented. It predicts steady states of cluster beams that self destruct in the cloud of atoms that a beam thereby continuously replenishes. The model is used to estimate whether the mechanism responsible can enhance depletion spectroscopic signals via a “Wittig tube”. Helium droplet size distributions and their dispersion relations between average and standard distribution are also discussed.
Helium is a very mild matrix. As a noble gas, it is chemically inert. It is the chemical with the smallest liquid density. The inter-atomic distance is about 4.5Å although the atomic radius is only 31pm. Firstly, this is because the element helium (He) has the lowest dipole polarizability α = 0.205Å 3 [Rad85], even lower than the already low ones of other noble gasses (α Ar = 1.64Å 3 ). Corresponding to the low α, He-He has the smallest van der Waals attraction [Wha94,Tan03] (vdW radius: 140pm). Secondly, due to its low mass and therefore high QM zero point energy, its condensed state is only a third as dense as it would be classically.
The extraordinarily weak interactions between the atoms cause the two stable helium isotopes to have the lowest boiling points of all substances, that is 4.21K for 4 He and 3.19K for 3 He [Wil87]. For most species it provides the smallest perturbation of any matrix. Embedded guest particles may have slightly red shifted spectra due to the polarizability of helium (attractive part of the potential) and thus a lower rate of spontaneous emission (rate is proportional to the emission frequency cubed) or a slight blue shift coming from the collective Pauli repulsion of the helium’s s-electrons.
Helium has no triple point. Due to the strong zero point motion it stays in the liquid phase even at zero K. It has already condensed in momentum space and must be pressurized to 25atm for 4 He (34atm for 3 He) to solidify in real space. Even at very low temperatures, there is almost no inhomogeneous broadening due to the sampling of different possible matrix sites in a solid lattice. Other noble gas matrices at low temperatures trap in interstitial or substitutional sites. Yet even compared to the fluid phase of any other noble gas, helium shows little inhomogeneous broadening, because although homogeneity always breaks down close to an impurity, in helium, the surround is determined by the dominating interaction between the impurity and the helium rather than by the interaction between atoms of the matrix [Kan97].
by 11 orders of magnitude (from η = 3.5 mg m -1 s -1 at 4K). Therefore, the rotational relaxation of captured species can be suppressed and rotational spectra sharply resolved. This is impossible in a classical liquid, because random collisions destroy rotational coherence and the rotational spectrum collapses into a broad band whose width gives the rotational diffusion time. Impurities move frictionless in super fluid helium and quickly find each other. The super fluid has a six orders of magnitude larger thermal conductivity than helium above the lambda point. The thermal conductivity is then a 1000 times that of room temperature (R.T.) copper (30 times if at the same temperature). Thus, released binding energy
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