Photoelectric emission and phase transitions of evaporated metastable sodium, potassium and sodium-potassium alloy films

Abstract

The photoelectric emission from sodium, potassium and Na-K alloy films has been investigated for wavelengths between 270 and 530 nm (2.3–4.6 eV). The films were prepared by simultaneous vapour deposition on a molybdenum substrate cooled at 80 K and subsequently warmed in ultra-high vacuum. Layer thicknesses (up to 1 urn) and alloy compositions were derived from in situ measurements of the atomic beam densities by surface ionization on a hot platinum filament. The photoelectric yield and its spectral distribution are found to depend on the alloy composition and on the degree of heating the vapour quenched films. While the eutectic and peritectic points of the Na-K phase diagram give rise to comparatively small changes of the photoelectric emission, very spectacular effects are observed at lower temperatures. Their analysis suggests that alloys containing 20–85 at% K are condensed at 80 K in an amorphous state which upon annealing crystallizes before melting. The pure metals and the potassium-rich alloys condense in a state possibly comprising hcp and bcc crystals. In Na-rich films the crystal lattice is highly disturbed by the large K atoms resulting in an extremely high photoelectron yield. The molten and the solid alloys obtained by quenching these melts appear to have surfaces enriched in potassium which is recognizable from their spectral distribution curves.