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.