G Carter

Thermal resolution of desorption energy spectra

Abstract The rate equations for desorption of sorbed gases from solids are considered where the solid surface is exposed to a time varying temperature schedule. Three cases are considered. Firstly the case of constant energy of binding sites; secondly discrete energy heterogeneous sites; and thirdly a continuum of heterogeneous sites. New techniques for the evaluation of the desorption energy, the initial site population and the order of the reaction are considered, and the analogy, in the second case above, between the optical line spectrum and desorption from discrete energy sites, is shown to be useful in determining the ability of the temperature displacement technique to resolve multiple energy peaks.

Diffusive processes in a solid during tempering

Abstract The rate at which diffusing species cross the surface boundary of a semi-infinite solid during heating of the solid at a constant temperature rise rate is calculated where the initial concentration of species is assumed to be located at a place p lattice units below the surface. The temperature at which this rate reaches a maximum, the temperature width of the rate/temperature curves and the ability to distinguish between rate data when the diffusing species may possess differing activation energies for diffusion are explored. Distributed initial concentrations are also considered and it is shown how the initial concentration distribution can be derived from diffusion data during the tempering cycle.

Ion trapping and gas release phenomena

Abstract When energetic gas ions are injected into a solid surface there is generally a high probability of their becoming trapped within the lattice and this process is of great technological importance in ion pumps and in physical applications where a gas discharge is in contact with a solid surface. In addition to the trapping process, there exist unfortunately, two mechanims via which the trapped gas can re-evolve. The first is a purely thermally activated phenomenon which occurs without operation of further ion bombardment, whilst the second occurs during bombardment itself and is some form of sputtering phenomenon. In the present review we examine the basic physical interactions involved in both the trapping and re-emission processes and survey the relevant literature which gives the magnitude of the effects. In particular we summarize the measured probability of trapping of inert gas ions at metal and insulator surfaces and show how this is related to interatomic forces. We then investigate the process responsible for thermal gas release, which is shown to be diffusion controlled, and correlate the available data for bombardment induced gas release, for which the physical phenomenon is less definable. Finally, we study the manner in which trapped atoms dissolve in the solid target and show how individual atoms can migrate and cluster to form bubbles. Our attention is directed wholly towards examination of inert gas ion—solid systems since chemical reactions can be discounted but suggest that the basic physical interaction with more active gases is similar.

Diffusive release of gas from a solid during tempering

Abstract Ficks Diffusion equation is solved for the case of an initially plane distribution of migrating species, located at a specific depth below the surface of a semi-infinite medium, whilst the medium is subjected to a heating schedule of the form (temperature)−1 increasing linearlly with time. It is shown that the rate at which migrating species cross the surface boundary increases to a maximum at a soecific temperature and then decreases with further heating. The temperature at peak rate is found to be related to the activation energy for diffusion, the initial depth of the migrating species and the heating rate. The relevance of this diffusion behaviour to outgassing of vacuum components is considered.

Investigations into the mechanism of trapping of inert gas ions in polycrystalline tungsten

Abstract The nature of trapping sites for inert gas ions injected into polycrystalline tungsten has been explored by successive bombardment of tungsten with two different ion species and observation of the subsequent desorption spectra. It is found that Kr+ ion bombardment of tungsten greatly enhances the probability of trapping of He+ ion subsequently injected and it is concluded that the Kr ions produce damage centres which act as sites for He atom retention. The change in the Ne desorption spectra after Xe+ ion bombardment further suggests that there is a continuous interchange of trapped atoms between sites of different binding energies and that saturation conditions ensue only when the competitive effects equilibrate.

Comprehensive study of the ion pumping of the noble gases

The ion pumping of five noble gases by Bayard-Alpert ionization gauges has been studied extensively, employing an ultra-high-vacuum system which could be operated statically or dynamically. The dependence of the important pumping parameters, initial pumping speed and maximum quantity of gas pumpable, upon gas composition, electrode potentials and gauge temperature have been investigated. The relationship between instantaneous pumping speed and the quantity of gas pumped has been deduced, and observations upon the recovery of gas, following sorption, at ambient and elevated temperatures has also been made. It is shown that the experimental results confirm a previously reported model of the sorption process, where gas ions enter a heterogeneous collection of capture sites of various energies of binding in the glass walls. The observed form of a static pump-down is interpreted in terms of the kinetic processes which can occur at these sites.

Trapping and thermal re-emission of helium from polycrystalline tungsten

Abstract Observations of trapping and subsequent thermal re-emission of 100 eV–2 KeV He+ ions at a polycrystalline tungsten target are reported. The trapping probability of He+ ions increases monotonically in this energy range and is slightly higher than for A+ ions. Thermal re-emission after trapping reveals peaks in the re-emission spectrum with corresponding activation energies for the major components of 1.6 eV, 2.0 eV–3.0 eV and 5–6 eV. These energies are believed to be correlated with motion of tungsten surface atoms and with vacancies.

The mechanism of ion bombardment induced release of gas from tungsten

Abstract The release of entrapped inert gas ions from polycrystalline tungsten during bombardment with a second ion species has been studied as a function of target temperature during bombardment. The form of the variation of release rate with temperature, and the preferential removal of ions trapped with low activation energy, suggests operation of a thermal spike process rather than Target sputtering as the mechanism responsible for bombardment induced desorption.

BOMBARDMENT INDUCED RE-EMISSION OF IONICALLY PUMPED INERT GAS FROM A GLASS SURFACE

Studies have been made of the release of an inert gas, argon or krypton, ionically pumped into glass surfaces, by subsequent bombardment with the other gas, krypton or argon. It is found that ion bombardment, up to energies of 1 kev, cannot completely recover sorbed gas. This, and other evidence, leads to the conclusion that gas is released, during ion bombardment, by transfer of sufficient energy to a trapped gas atom, for its desorption, rather than simultaneous release during glass sputtering. Approximate values for gas sputtering coefficients are deduced. It is also concluded that gas is held in superficial trapping sites rather than in a depth distribution into the solid.

Bistable behaviour of the Bayard-Alpert ionization gauge

It has been observed that the Bayard-Alpert ionization gauge can possess either of two completely independent and stable modes of operation. The difference appears as variations in the pressure sensitivity of the gauge and complementary changes in the electrical adsorption characteristics of different gases. This bistable behaviour has been consistently observed on three different gauges and is attributed to the electrical charging of the inner surfaces of the gauges as a result of secondary-emission processes. Confirmation of this hypothesis has been made with a metal-screened gauge in which it was possible to control the discharge.