V. Kasperovich

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

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.

Velocity distribution measurement and two-wire field effects for electric deflection of a neutral supersonic cluster beam

The basic principle of deflection of a beam of polarizable particles by an inhomogeneous electric field is straightforward, but its application to realistic beams having spatial and velocity distributions of finite widths requires special care. We present a detailed treatment of these issues as developed for a measurement of electric polarizabilities of alkali clusters in a continuous supersonic beam. We begin by describing a general technique for extracting the beam velocity distribution from the time-of-flight profile generated by two separate 50–50 gating choppers. This method yields accurate model-free information directly from the experimental profile, without errors associated with detector delays, and with high signal throughput. We then use a simple but accurate approximation for the deflecting field variation over space, and derive analytical expressions for the deflection of beams of finite width and finite velocity spread. This allows us to evaluate the magnitude of a number of corrections to t...

Ultraviolet photoabsorption spectra of silver and gold nanoclusters

Abstract It has been previously proposed that the behaviour of visible optical absorption spectra of silver and gold nanoclusters can be explained by a two-region model in which dielectric screening by the core d electrons is ineffective near the cluster surface. In this paper we investigate the implications of this model for the ultraviolet (UV) region of the cluster photoabsorption spectrum. We find that there should exist additional distinct resonances at high (20–30eV) as well as near-UV (e.g., 4–6eV for Ag n ) frequencies. The position of the near-UV peaks is shown to be a sensitive function of the cluster size and charge. This behaviour reflects an interplay between d-electron screening and valence-electron spill-out. The calculations show that measurements of the evolution of UV spectra of size-selected noble-metal clusters can provide stringent tests of the theory of surface structure and plasmon resonances of nanoparticles.

Long-range van der Waals forces between alkali clusters and atoms

We study the long-range attractive force between neutral sodium clusters Nan (2⩽n⩽20) and the alkali atoms Li, Na, and K. Absolute cross sections for the scattering of a beam of clusters by atomic vapor are measured, and are shown to arise from the van der Waals dispersive interaction V=−C6/r6. These cross sections are extremely large (up to thousands of A2 in the center-of-mass frame). Their magnitudes are in good agreement with predictions based on the London theory of dispersion forces; the large strength of the interaction is a consequence of the high cluster polarizabilities. In addition, we evaluate the contribution of the higher-order potential term −C8/r8 to the scattering cross section and show that it can become quite significant for collisions involving large clusters (n∼102–103).

Stability, evaporation, and temperature of metal clusters

The stabilities of free metal clusters in beams, as well as their physical and chemical properties, are governed by the shell structure of the quantized electronic levels. In addition, the properties of a cluster depend upon its internal thermal energy, or temperature. We describe a new method to determine the temperature of free neutral clusters. Laser-induced photodissociation of sodium clusters is used to measure the recoil pattern of fragmentation products and thereby the amount of kinetic energy release. The fragmentation process is treated by an evaporation model similar to the statistical theory of the disintegration of compound nuclei. From the kinetic energies of evaporation, we determine the temperatures of both the daughter and the mother particles.