Thermal release of inert gases from tungsten: Dependence on the crystal face bombarded

Abstract

Flat (100), (110), and (211) faces of tungsten were bombarded in ultrahigh vacuum with inert-gas ions having energies up to 5 keV. The rate of release of the entrapped gas was then measured as the crystals were heated at 40 °K/s by electron bombardment. The spectra of release rate vs. temperature consist of a series of maxima which show distinctive variations with the type, energy, and number of the incident ions, and with the crystal face on which they impinge. Evidence is presented that the peaks at T < 1600 °K are associated with gas atoms trapped within a few lattice constants of the crystal surfaces, the binding energies being determined by characteristic arrangements of the lattice atoms about the trapping site. The gas release at T > 1600 °K is consistent with the diffusive motion of more deeply penetrating particles as impurities in the bulk of the crystal, the expected variations of penetration depth with energy and crystallographic direction being clearly evident. Evidence is also presented that for sufficiently high trapped neon atom concentrations, gas release can occur by the diffusive motion of two or three atom clusters.