Vitaly V. Kresin

Collective resonances and response properties of electrons in metal clusters

Abstract The delocalized valence electron cloud in metal clusters displays strong collective behavior. This fact is reflected in a variety of cluster properties, including photoabsorption resonances and electric polarizability. Experiments have demonstrated that the response properties of cluster electrons possess a number of features unique to the microscopic size regime. In this paper the experimental picture is considered, and a theoretical discussion of the size-dependent behavior of electrons in finite systems is presented. Topics discussed include collective resonance frequencies and oscillator strengths in spherical and deformed clusters, electron distribution, and static polarizability. The theoretical analysis presents a consistent picture of cluster spectra which reflects the unique properties of these systems and is successful in explaining the experimental data for a wide variety of metal clusters. Suggestions for further measurements are made, including the prediction of a new photoabsorption mode unique to small metallic clusters.

Capture of lithium by 4He clusters: Surface adsorption, Penning ionization, and formation of HeLi+

We investigate the capture of lithium atoms by a beam of large cold helium clusters and subsequent ion production by electron impact. Li atoms are efficiently picked up by the He droplets, although with a cross section significantly lower than that for other atoms and molecules. The mass spectrum reveals the presence of Li atom and dimer ions, as well as the weakly bound complex HeLi+, confirming that capture by He-cluster beams can be used for efficient soft ionization of fragile species. The electron-energy dependence of the Li+, Li2+, and HeLi+ yield shows that they are formed primarily by Penning ionization in a collision with a metastable He atom in the droplet. This leads to the conclusion that lithium metal atoms are not submerged in the helium clusters but locate on the surface, corroborating theoretical predictions for bulk helium surfaces and spectroscopic measurements.

Photodissociation of hydrogen halide molecules on free ice nanoparticles.

Photodissociation of water clusters doped with HX(X=Br,Cl), molecules has been studied in a molecular beam experiment. The HX(H2O)n clusters are dissociated with 193 nm laser pulses, and the H fragments are ionized at 243.07 nm and their time-of-flight distributions are measured. Experiments with deuterated species DBr(H2O)n and HBr(D2O)n suggest that the photodissociation signal originates from the presence of the HX molecule on the water cluster, but does not come directly from a photolysis of the HX molecule. The H fragment is proposed to originate from the hydronium molecule H3O. Possible mechanisms of the H3O production are discussed. Experimental evidence suggests that acidic dissociation takes place in the cluster, but the H3O+ ion remains rather immobile.

Photoabsorption by volume plasmons in metal nanoclusters.

It is well known that plasmons in bulk metals cannot be excited by direct photoabsorption, that is, by coupling of volume plasmons to light. Here we demonstrate that the situation in nanoclusters of the same metals is entirely different. We have carried out a photodepletion measurement for Na(20) and Na(92) and identified a broad volume plasmon absorption peak centered slightly above 4 eV, revealing the possibility of optical excitation of volume-type collective electronic modes in a metallic system. The observed phenomenon is related to different selection rules for finite systems.

Work functions, ionization potentials, and in between: Scaling relations based on the image-charge model

We reexamine a model in which the ionization energy of a metal particle is associated with the work done by the image-charge force in moving the electron from infinity to a small cutoff distance just outside the surface. We show that this model can be compactly, and productively, employed to study the size dependence of electron removal energies over the range encompassing bulk surfaces, finite clusters, and individual atoms. It accounts in a straightforward manner for the empirically known correlation between the atomic ionization potential (IP) and the metal work function (WF),

Low-energy electron capture by free C60 and the importance of polarization interaction

\mathrm{I}\mathrm{P}/\mathrm{W}\mathrm{F}\ensuremath{\sim}2.

Critical sizes for the submersion of alkali clusters into liquid helium

We formulate simple expressions for the model parameters, requiring only a single property (the atomic polarizability or the nearest-neighbor distance) as input. Without any additional adjustable parameters, the model yields both the IP and WF within

Electrostatic deflection of the water molecule: A fundamental asymmetric rotor.

\ensuremath{\sim}10%

Unusual pickup statistics of high-spin alkali agglomerates on helium nanodroplets

for all metallic elements, simulates the concentration dependence of the WF of regular binary bulk alloys, and matches the size evolution of the IP of finite metal clusters for a large fraction of the experimental data. The parametrization takes advantage of a remarkably constant numerical correlation between the nearest-neighbor distance in a crystal, the cube root of the atomic polarizability, and the image-force cutoff length. The paper also includes an analytical derivation of the relation of the outer radius of a cluster of close-packed spheres to its geometric structure.

Scattering of neutral metal clusters: Long‐range interactions and response properties

Abstract Cross-sections for slow electron capture by neutral C 60 have been measured by beam-depletion spectroscopy in the 0–3 eV energy range. The data confirm the existence of a very strong low-energy attachment peak. The general trend of the cross-section curve follows that of the Langevin mechanism of electron capture by the long-range polarization field of the fullerene. The sticking probability of an attracted electron is less than unity and displays an energy-dependent structure, reflecting an interplay between the effects of particle symmetry, size, and polarization.