We present a series of molecular dynamics simulations on the surface deposition process of initially free silver clusters (Agn) with different sizes (n = 100-2000) and morphologies. During the whole deposition process the morphology of the clusters was studied as a function of the landing conditions. These conditions include variations of the depth and range of the substrate potential as well as the thermal coupling to the surface and a variation of the impact velocity of the free clusters. Depending on the applied conditions the clusters' final form ranges from spread out fragments via deformed and restructured heaps to quasi unchanged spherical clusters sitting at the surface. Under certain landing conditions larger clusters retain their initial multiply twinned morphology upon deposition, while smaller ones undergo structural transitions to form single domain particles. Furthermore, the occurrence of a structural transition depends on the initial structure-initially decahedral clusters tend to conserve their morphology better than icosahedral ones. The same behavior can also be observed in our experiments, where silver clusters were grown in helium nanodroplets and subsequently deposited on amorphous carbon substrates.
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).
The interaction between He and Cr is investigated by means of post-Hartree-Fock molecular orbital theory. We analyze the influence of the van der Waals forces on the complex electronic structure of the chromium atom, starting with its septet manifold and cover the first few electronically excited states up to 30 000 cm(-1). For the sake of a direct comparison with ongoing experiments on Cr-doped helium nanodroplets we extend our analysis to selected states of the quintet manifold in order to explain a non-radiating relaxation from y (7)P(o) to z (5)P(o).
The
interaction of an electronically excited, single chromium (Cr)
atom with superfluid helium nanodroplets of various size (10 to 2000
helium (He) atoms) is studied with helium density functional theory.
Solvation energies and pseudo-diatomic potential energy surfaces are
determined for Cr in its ground state as well as in the y7P, a5S, and y5P excited states. The necessary
Cr–He pair potentials are calculated by standard methods of
molecular orbital-based electronic structure theory. In its electronic
ground state the Cr atom is found to be fully submerged in the droplet.
A solvation shell structure is derived from fluctuations in the radial
helium density. Electronic excitations of an embedded Cr atom are
simulated by confronting the relaxed helium density (ρHe), obtained for Cr in the ground state, with interaction pair potentials
of excited states. The resulting energy shifts for the transitions
z7P ← a7S, y7P ← a7S, z5P ← a5S, and y5P ← a5S are compared to recent fluorescence and
photoionization experiments.
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