E. Nowicka

The adsorption kinetics of weakly bound hydrogen on thin iron film surfaces

Abstract Three adsorption states of hydrogen β−s, β−w, and α−e on a thin iron film surface were distinguished on the basis of surface potential, and volumetric measurements carried out simultaneously. The sticking probability for the selected weakly bound hydrogen adsorption state β−w was determined, and its variation with coverage and temperature was shown, monitoring hydrogen population by means of surface potential measurements. Consideration of adsorption-desorption phenomena which occur under isothermal conditions leads to the conclusion that the weakly adsorbed state β−w arises due to induced heterogeneity.

Incorporation of positively and negatively polarized hydrogen and deuterium adspecies from the surface into the bulk of thin palladium films

The incorporation of hydrogen and deuterium adspecies from the surface of a thin palladium film into the bulk has been studied measuring simultaneously the surface potential changes by means of high sensitivity (± 1 mV), short response time (0.1 s) static capacitor system, and pressure by means of the ultrasensitive Pirani type gauge. It has been found that the positively polarized β+ adspecies of hydrogen incorporate quickly at 78 K and 87 K. via a tunneling mechanism, with a diffusion coefficient of 3 × 10−13 cm2s. The β+ adspecies of deuterium incorporate with slightly increasing diffusion coefficient with the temperature increase: 1 × 10−13 cm2s at 78 K and 2 × 10−13 cm2s at 87 K. The negatively polarized adspecies β−, which strongly predominate and at 160 K, slowly incorporate in the bulk of palladium.

Low temperature hydrogen adsorption on thin nickel films

The characteristics and kinetics of hydrogen adsorption on thin nickel films at 195 and 78 K, within a large pressure range of 1×10 −9 –1×10 −1 Torr were studied by measuring surface potential changes and the sticking probability. Negatively polarized atomic adspecies exist at 195 K up to a pressure of the order of 10 −2 Torr. Above this pressure the molecular weakly bound, negatively polarized form of the deposit arises. At 78 K adsorption of negatively polarized hydrogen adspecies is followed by the formation of positive surface potential transients. These transients are observed to be due to positively polarized hydrogen adspecies arising on the surface followed by their diffusion into the bulk of the film. Patches of nickel hydride are formed in this process. Hydrogen adsorbs weakly on the nickel hydride surface in the form of both positively polarized atomic and molecular adspecies.

Surface phenomena in titanium hydride formation

Surface phenomena which occur during hydrogen interaction with thin titanium films within the pressure interval 10−10−10−2 Torr, at the temperature range 78–298 K, have been studied through measurement of surface potential changes by means of high sensitivity, short response time static capacitor system, with simultaneous recording of pressure by means of an ultra-sensitive Pirani-type gauge. At the beginning of adsorption negatively polarized adspecies stable on the surface, named β+ formed. With the increase of the population they are followed by the creation of positively polarized adspecies named β−. The β− adspecies are incorporated quickly below the surface forming percipitates of titanium hydride in a thin titanium film matrix. The activation energy for β+ adspecies incorporation has been estimated to be 6kJ/mol H2. On a titanium hydride surface, positively polarized, weakly adsorbed (Ed < 20 kJ/mol) hydrogen adspecies exist.

Investigation of atomic deuterium (hydrogen) emission from the surface of some transition metal deuterides (hydrides)

Abstract Deuterium desorption from the surface of decomposing palladium deuteride was studied while monitoring the atomic component of the desorbing gas by means of an adsorption method. A thin gold film was applied as an adsorbent active for D adsorption, but inert against interaction with D2. The deuterium deposit thereby collected on the Au surface was analysed by means of thermal desorption mass spectrometry (TDMS). The atomic component of deuterium arising by desorption from decomposing PdDy was clearly detected. The experimental results obtained for PdDy are compared with those observed during the decomposition of PdHx, as well as the decomposition of other hydrides including TiHz and VHu.

Correlation between surface and bulk phenomena in the process of thin palladium hydride and titanium hydride film formation

Abstract The measurements of surface potential SP and resistance changes ΔR of thin Pd and Ti films in the process of PdHx(x = 1) and TiHy(y = 2) hydride formation were performed. SP measurements allow differentiation between various forms of hydrogen adspecies. The influence of these forms of the adsorbate on the resistance are determined and discussed.

Oxygen interaction with palladium hydride and titanium hydride surfaces

Abstract Oxygen interaction with thin palladium hydride (PdH x ) and titanium hydride (TiH y ) films obtained in situ under controlled conditions in UHV apparatus has been studied. The surface potential (SP) measurements have been carried ut to distinguish between elementary steps of surface processes during hydrides formation, oxygen adsorption on Pd (Ti) and O 2 reaction with PdH x and TiH y , Mass spectrometry has been applied to determine gas phase composition. It has been found that O 2 interaction with PdH x at 195 K leads to H 2 O formation with the hydride decomposition. O 2 interaction with TiH y within a temperature range of 78–195 K causes hydrogen replacement on the surface and in the subsurface region by oxygen. Hydrogen is removed to the gas phase.

Surface phenomena and isotope effects in the process of titanium hydrides (deuterides) formation

Abstract Surface phenomena related to the process of TiHx (TiDy) formation at 78K and 298K were studied through simultaneous surface potential (SP) and pressure measurements, by means of sensitive, short response time circuits. Thermal Desorption Mass Spectrometry was applied to compare hydrogen (deuterium) uptake on the surface of titanium hydride (deuteride) and in the bulk. All studies were performed in situ, on thin Ti films deposited under ultra-high vacuum conditions. An image of the Ti film was obtained by Atomic Force Microscopy. It has been found that TiHx (TiDy) is formed following the incorporation of positively polarized hydrogen (deuterium) adspecies from the surface into the bulk of the Ti films. An isotope effect was observed in the rate of incorporation of hydrogen (deuterium) adspecies from the surface to below the surface image plane. The rate of incorporation was found to depend strongly on the H Ti ( D Ti ) atomic ratio in TiHx (TiDy).

Atomic hydrogen desorption from thin palladium hydride films

It has been proved that hydrogen atoms desorb from the surface of a decomposing thin palladium hydride film. A thin gold film deposited and sintered in situ was used as a selective adsorbent for atomic hydrogen. The TDMS (thermal desorption mass spectrometry) technique was applied to detect the adsorption of hydrogen on gold and to determine the amount adsorbed.

Deuterium adsorption on thin nickel films

Abstract Adsorption states arising upon interaction of deuterium with thin nickel films at 298 and 78 K were distinguished by simultaneous surface potential and pressure measurements. Rapid recording, sensitive and nondisturbing methods (static capacitor and ultrasensitive Pirani-type gauge) were applied. Negatively polarized β- adspecies are formed at 298 K, while at 78 K the β- state arising at low coverage is followed by the positively polarized β+ state. The features of the β+ state are described. The isotope effect is shown.