Raphael Ruppin

An optical study on vapour, microcrystal beam and film

During evaporation of powder in confined noble gas, flowing noble gas and vacuum, the optical absorption spectra of vapour zones, microcrystal beam and film on a substrate were measured as functions of the time elapsed since the beginning of the evaporation. The results show clearly the spectral evolution from molecule to solid. Also, to clarify the details of the optical excitation and relaxation in the condensed solid, the absorption, photoluminescence and photoconduction spectra were measured for films at various temperatures from 7 K to room temperature.

Optical spectra of free silver clusters and microcrystal produced by the gas evaporation technique: transition from atom to microcrystal

By setting an appropriate condition for the gas evaporation of silver, two vertically well separated zones-a transparent emission zone and a bright smoky zone-have been produced above the evaporation source in a confined helium atmosphere. Optical data were obtained by measuring time-resolved and space-resolved transmissivity spectra. The structure of the spectra and their dependence on time on the one hand and on the distance from the source on the other hand support the occurrence of atoms, clusters and microcrystals. The mechanism of the growth of silver clusters and microcrystals by the gas evaporation technique is also studied.

Polariton modes of spheroidal microcrystals

The polariton modes of spheroidal microcrystals of dispersive materials are investigated over a wide range of spheroid sizes and eccentricities by using the T-matrix method. The method is applied to phonon-polaritons in ionic dielectric solids and to plasmon-polaritons in metallic solids. The main effects of retardation, observed when the particle size increases, are: the surface polariton peaks broaden and shift to lower frequencies; subsidiary peaks owing to higher-order surface modes appear in the spectrum. The method is applied to the interpretation of three sets of previously published experimental optical data for Ag spheroids.

Spectroscopic studies of rubidium vapour zone produced by thermal evaporation in noble gas

The time evolution of the extinction spectra of the rubidium vapour zone has been measured during the process of gas evaporation. The structure of the spectra and their time dependence show the occurrence of atoms, dimers, clusters and microcrystals. Also, they show a size trend in the surface plasma resonance frequency of rubidium clusters. Also, vapour species in each zone were selectively excited by using laser light and the emission spectra were measured. Some of the observed spectra were compared with theoretical ones, and information on the following points was derived: (1) the mechanism of rubidium cluster and microcrystal growth in the gas evaporation method, (2) the size trend of the collective mode resonance of the valence electrons, (3) the excitation energy transfer between rubidium vapour species, (4) the structure of the vapour zone and (5) the mechanism of laser photofragmentation of clusters.

Optical spectra of free and supported CuCl microcrystals produced by thermal evaporation in a helium gas stream

Time-resolved measurements of the extinction spectra of free CuCl microcrystals and of CuCl microcrystals collected on a glass substrate have been performed at various growth stages after the beginning of evaporation in a low-pressure helium gas. Absorption bands due to Z1,2 and Z3 excitons were observed, which were shifted in energy from their bulk positions owing to exciton confinement effects. Initially the energy difference between the Z1,2 and the Z3 bands is considerably larger than its bulk value, and with increasing time it decreases and approaches the bulk value. The observed Z3 exciton absorption bands are compared with those calculated using an extended Mie theory, in which non-local effects as well as the size quantization of the excitonic states are included.

Substrate effects on infrared absorption in dielectric microcrystals

Infrared absorption spectra of MgO and NiO microcrystals deposited on dielectric and metallic substrates have been measured. With a metallic substrate the main absorption maximum occurred at a lower frequency than with a dielectric substrate. These effects are also calculated using the model of a small sphere near a substrate. An interpretation of the experimental results in terms of the model calculations is presented.

Optical spectra of lithium vapour zones produced by thermal evaporation in hydrogenating atmosphere

We have applied the time-resolved and space-resolved spectroscopies of vapour zones produced by gradual thermal evaporation in a noble gas to study the electronic properties and stability of hydrogenated clusters and microcrystals. Optical extinction spectra of lithium vapour zones in pure Ar gas and Ar - H2 gas mixture atmospheres confined at various pressures were obtained. Some comparisons of the observed spectra with those predicted theoretically indicate that hydrogenated clusters LinHm are to be found near the vapour source, while lithium microcrystals coated with LiH shells are to be found far away from the vapour source. Electron localization around hydrogen atoms and the hydrogenation mechanism of lithium particles are also discussed.

An optical study of alkali-metal vapour zones produced by thermal evaporation in helium gas

We have measured the time-resolved and space-resolved optical spectra of sodium and potassium vapour zones produced by thermal evaporation in helium gas. The structure of the spectra and their time dependence indicate that atoms evaporate first and later dimers and clusters grow by coalescence in the vapour zone. Information on the size trends of the clusters and microcrystals is derived from the time evolution of the surface-plasmon extinction band. For clusters the position of the band is interpreted in terms of the spill-out effect, while for microcrystals it is calculated from the Mie theory with a size-dependent correction to the dielectric constant.

Optical studies of lithium vapour zones produced by thermal evaporation in noble gas

We have produced two vertically well separated zones of lithium vapour above the evaporation source in a confined noble gas atmosphere by setting an appropriate condition for the gas evaporation. Optical extinction spectra of lithium atoms, dimers and clusters were obtained by time-resolved and space-resolved measurements after the beginning of the evaporation. Also, vapour species in each zone were selectively excited using laser light and the emission spectra were measured. Some of the observed spectra were compared with theoretical ones, and information on the following points was derived: (i) the mechanism of lithium cluster and microcrystal growth in the gas evaporation method, (ii) the size trend of the collective mode resonance of the valence electrons, (iii) the excitation energy transfer between lithium vapour species, (iv) the structure of the vapour zone and (v) the mechanism of laser photofragmentation of clusters.