Hydrogen adsorption on a clean palladium ribbon; Thermal desorption and work function measurements

The adsorption of hydrogen on a palladium ribbon has been studied by thermal desorption and work function measurements. It has been established that several heating cycles of the sample covered with hydrogen, up to 700 K lead to the repeated appearance of hydrogen thermal desorption peaks. Analogous experiments of adsorption and repeated heating cycles up to 700 K have shown work function changes decrease to zero as a result of heating and an increase again almost up to the initial value following cooling, in a much shorter time than that required for adsorption. The experimental results show that only a small part of the adsorbed hydrogen is desorbed in the temperature range of the thermal desorption peak. The major part of adsorbed hydrogen penetrates below the surface which leads to a nonequilibium increase of the bulk concentration.     

The effect of carbon monoxide on the work function of evaporated films of titanium and erbium

Small doses of CO reduced φ by ~50 mV for Er films but did not reduce it for Ti films. The sign and magnitude of this reduction of φ can be accounted for by a simple geometrical model. Larger doses increased φ for both metals. Each increase of φ for Er before saturation was followed by a slow decrease, but this effect was seen only occasionally with Ti. The difference between φ for the clean film and φ for the saturated film was 0.49 ± 0.05 eV for CO/Ti and 0.71 ± 0.11 eV for CO/Er. The bulk structure of both films was found to be randomly oriented hcp. The surface cleanliness of the Ti films was checked by their Auger spectrum; this spectrum changed slowly owing to beam effects and to the reaction of Ti with residual gases. The results are compatible with the dissociative adsorption of CO, provided that there is some reconstruction of the surface.     

Conductivity measurements on phthalocyanine films: evaporated vs. pressure contacts

Conductivity measurements on thick films of lead phthalocyanine in a sandwich cell with one pressure contact are reported. The results obtained at different loads are discussed and compared with those for a cell with both evaporated contacts.     

Study of adsorption states and interactions of CO on evaporated noble metal surfaces by infrared absorption spectroscopy: I. Silver

We have used infrared absorption spectroscopy to study the adsorption of CO at low temperature on evaporated silver films as a function of the coverage of CO and the deposition temperature of the silver. We observe two adsorption regimes when a cold silver film is exposed to CO gas. If the silver deposition temperature, (or the highest temperature at which the silver has been annealed), is above the threshold temperature of 150 K, then only physisorbed CO is observed. For sample temperatures below 25 K, these physisorbed molecules are oriented perpendicular to the metal surface. Films deposited at temperatures below 150 K, however, contain ≈ 0.01 monolayer of chemically active sites at which CO chemisorbs. The infrared band due to chemisorbed CO shifts to lower frequency with increasing coverage. We have analyzed this shift and separated the static and dynamic contributions. The static, chemical shift is caused in part by the change in the work function induced by surrounding adsorbates. The dynamics shift is fully explained by a dipole-dipole interaction; we find no evidence for a vibrational coupling through the metal. We have analyzed the vibrational polarizability and infrared absorption strength of the absorbed CO, and find no evidence for the infrared enhancement suggested by some theories of surface enhanced Raman scattering.     

Initial oxidation of Al films investigated by AES,work function and gravimetric measurements

The beginning oxidation of evaporated Al films was measured under UHV conditions by means of AES, gravimetric oxygen uptake and work function changes. The high resolution of our quartz oscillator microbalance (Δm = 7 × 10^-11g/cm² with a time response <1 s) allowed to separate two reaction rates, a logarithmic one for an oxygen uptake <6 × 10^-8g/cm² and an inverse-logarithmic one for Δm > 7.5 × 10^-8g/cm². At room temperature a decrease of work function with an equilibrium value of -0.75 eV (oxygen uptake 9.10^-8g/cm²) with increasing oxidation was observed. This is interpreted as incorporation of oxygen and diffusion of Al^δ+ ions to the surface. But at T = 80 K an increase of work function with a maximum value of 0.5 eV was found. This is attributed to adsorbed oxygen because the incorporation is much slower at this temperature. The AES of pure Al is characterized by the low energy 64 eV peak. The decrease of the 64 eV peak and the increase of two new peaks at 50 and 37.5 eV during the oxidation were measured as a function of oxygen uptake and attributed to the change of electron density of states in the conduction band of aluminum.     

Fundamental absorption edge of evaporated amorphous WO3 films

The fundamental absorption edge of evaporated WO₃ films is investigated. The optical gap of the virgin film is estimated to be 3.41 eV at room temperature and it decreases with increase of annealing temperature up to 200°C. Annealing at 300°C leads to change in the spectral shape, which is caused by crystallization. For the films annealed at 200°C, temperature coefficient of the optical gap is estimated to be ?2×10^?4 eV/K and the slope of Urbach's tail is found to be independent of measuring temperature up to 200°C. With electrolytic coloration, shift of the optical gap toward higher energy is observed. Magnitude of this shift is estimated to be 0.05 eV at the color center concentration of 7.5×10²¹ cm^?3 when H⁺ electrolyte is used. If Li⁺ electrolyte is used, the magnitude of this shift is about three times larger than in the case of H⁺ electrolyte. This fact is interpreted by a small change in the host matrix structure owing to the injection of proton or Li⁺ during coloration.     

Study of adsorption states and interactions of CO on evaporated noble metal surfaces by infrared absorption spectroscopy: II. Gold and copper

We have used infrared absorption spectroscopy to study the adsorption of CO at low temperature on evaporated films of gold and copper as a function of the coverage of CO and the deposition temperature of the metal. For both metals we observe two distinct adsorption regimes when the cold metal is exposed to CO gas. These regimes arise depending on whether the deposition temperature of the metal (or the highest temperature at which the metal has been annealed) is above or below a threshold temperature. Gold films deposited at temperatures below 290 K contain chemically active sites at which CO chemisorbs, while films deposited at higher temperatures do not support CO chemisorption. This behavior is very similar to that observed on silver films. Chemisorption of CO occurs on copper films regardless of deposition temperature, but the vibrational spectrum is radically different for films deposited above and below the threshold temperature of 250 K. For both metals, the shift of the vibrational frequency with CO coverage has been analyzed, and the static and dynamic contributions separated. The dynamic shift is found to be well modeled by dipole-dipole coupling, with no evidence for vibrational coupling through the metal. An analysis of infrared intensities and vibrational polarizabilities shows no evidence for any special infrared enhancement analogous to the large enhancement in Raman cross section peculiar to adsorption on rough metal films, particularly noble metal films, deposited at low temperatures.     

Exafs measurements on Nb3Ge thin films

We have measured the Extended X-ray Absorption Fine Structure (EXAFS) spectra for superconducting thin films of Nb₃Ge prepared at variable deposition temperature by sputtering and electron beam coevaporation. As the deposition temperature is varied from 1050 K to 875 K, the film structure changes continuously from the A15 phase to a quasi-amorphous phase. This amorphous phase is characterized by a mean Ge-Nb distance of 2.66 A, as compared to the A15 distance, 2.87 A. The mean coordination of the Ge sites is reduced from 12 to 8±2, and the mean square spread in Ge-Nb distances (thermal + spatial) has decreased by only 0.01 A².     

Optical absorption and electrical conductivity of flash evaporated CuGaTe2 thin films

The optical absorption edge of CuGaTe₂ thin films in the energy range 1 to 2·3 eV was studied. The characteristic band gaps were found to be 1·23 eV and 1·28 eV whereas the acceptor ionization energy was 170 meV. Electrical conductivity measurements were carried out in the temperature range 300–550 K and two acceptor states with ionization energies 400 meV and 140 meV were found. The origin of acceptor states is explained based on covalent model.     

The influence of CO adsorption on the galvanomagnetic and magnetic properties of evaporated nickel films

The influence of CO adsorption on the ordinary Hall coefficient, R_HO, the extraordinary Hall coefficient, R_HE, the perpendicular magnetoresistivity ΔR_{mag O} and the saturation magnetization, B_S, of Ni films with thicknesses between 1 and 200 nm has been studies as a function of the CO coverage at 77 and 273 K. There is a maximum in ΔR_HE at a coverage of about half a monolayer. ΔR_HO, ΔΔR_{mag O} and ΔB_S exhibit an oscillating behaviour.     

The adsorption of CO on evaporated iridium films,studied with infrared reflection-absorption spectroscopy

The adsorption of ^12CO on Ir films evaporated under ultrahigh vacuum (UHV) conditions was studied using infrared reflection-absorption spectroscopy (IRAS). Only a single absorption band was observed at 300 K, shifting continuously from the “singleton” value ~2010 cm^?1 at very low coverages to 2093 cm^?1 at saturation coverage. This band is attributed to CO adsorbed on top of the surface atoms. Synchronously with this shift the bandwidth at half maximum intensity $\text{Δv}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ decreases from ~30 to 8 cm^?1. The integrated peak area increases linearly with coverage up to a relative coverage (θ_r) of approximately 0.4, then the increase levels off and a maximum is observed. Upon continuing adsorption the intensity decreases slightly. In addition results are presented on adsorption at 300 K of ¹²CO?¹³CO isotopic mixtures. The coverage induced frequency shift is discussed in terms of a dipole-dipole coupling mechanism and it is concluded that intermolecular coupling can explain the shift (~83 cm^?1) observed. The decrease in intensity at coverages > 0.4 is attributed to the formation of a compressed overlayer with part of the CO molecules adsorbed in a multicentre position with different spectral properties. No infrared bands of nitrogen adsorbed at 78 K could be detected at pressures up to 6.7 kPa (1 Pa = 0.0075 Torr, 1 Torr = 133.32 Pa).     

Adsorption of carbon dioxide and coadsorption of carbon dioxide and hydrogen on iron: I. Volumetric measurements,adsorption isotherms,changes in resistance and work function

The adsorption of CO₂ on thin evaporated iron films is studied at 196, 273, 300 and 370 K by means of volumetric measurements and measurements of equilibrium pressures, changes in work function and electrical resistance. Preadsorbed hydrogen only slightly influences the amount of a consecutive CO₂ adsorption at 273 K, but has a strong influence on the change in resistance due to the CO₂ adsorption. An iron film totally covered with CO₂ at 273 K is able to adsorb further hydrogen. Carbon dioxide and hydrogen seem not to compete for the same adsorption sites.     

Fundamental absorption edge of evaporated amorphous WO3 films

The fundamental absorption edge of evaporated WO₃ films is investigated. The optical gap of the virgin film is estimated to be 3.41 eV at room temperature and it decreases with increase of annealing temperature up to 200°C. Annealing at 300°C leads to change in the spectral shape, which is caused by crystallization. For the films annealed at 200°C, temperature coefficient of the optical gap is estimated to be −2×10⁻⁴ eV/K and the slope of Urbach's tail is found to be independent of measuring temperature up to 200°C. With electrolytic coloration, shift of the optical gap toward higher energy is observed. Magnitude of this shift is estimated to be 0.05 eV at the color center concentration of 7.5×10²¹ cm⁻³ when H⁺ electrolyte is used. If Li⁺ electrolyte is used, the magnitude of this shift is about three times larger than in the case of H⁺ electrolyte. This fact is interpreted by a small change in the host matrix structure owing to the injection of proton or Li⁺ during coloration.     

Hydrogen adsorption on thin low temperature deposited unsintered gold films

It has been found that molecular hydrogen does adsorb at 78 K on unsintered thin gold films deposited at low temperature, while sintering of these films at 420 K precludes H₂ chemisorption. The adsorbate is characterized by a single TD peak with a maximum placed at 125 K. The rate of desorption fits neither the first nor the second order kinetic equations exactly.     

Hydrogen adsorption on rhodium: Hydride formed on the surface of thin rhodium films

H₂ interaction with thin Rh films deposited on Pyrex glass under UHV conditions has been studied by simultaneous measurement of work function changes ΔΦ and hydrogen pressure P, at selected constant temperatures: 78 and 298 K. Prior to the adsorption experiments the thin film topography was illustrated using the AFM and STM methods. The influence of hydrogen adsorption on the resistance of thin Rh film was examined in the course of an independent experiment. The number of sites accessible for adsorption on the thin Rh film surface was found determining population of oxygen adatoms within the monolayer at 78 K, when incorporation of these adspecies below the surface is negligible. It was established that at all examined temperatures hydrogen adsorption led to coverage Θ approaching 1 under equilibrium pressure below 10⁻³ Pa, increasing the work function. Under higher H₂ pressure an additional uptake of hydrogen leading to Θ ∼ 1.68 at 298 K, and to Θ ∼ 2 at 78 K is reached. On this surface at low temperatures there exist weakly bound, reversibly adsorbed, positively charged adspecies characteristic for hydrogen adsorption on transition metal hydrides. The change of thin Rh film resistance caused by hydrogen adsorption was not measurable.     

Photoelectric work function changes during heterogeneous adsorption of alkali halide molecules on alkali halide crystals

The changes of photoelectric work function during heterogeneous adsorption of alkali halide crystals had been observed. They were attributed to the non-zero effective dipole moment of the adsorbed layer, and were estimated using the Helmholtz formula. Measurements of this effect showed semi-quantitative agreement with the theoretical model. The divergences between experimental data and theoretical calculations were briefly discussed.     

The adsorption of CO on the (100) plane of tungsten; thermal and work function measurements

Thermal desorption and work function measurements indicate that a largely molecular layer, with some dissociation, is formed at 80–100 K, with an increase in work function of 0.55 eV. The coverage in this layer is 11.5 × 10¹⁴ molecules/cm², or CO/W = 1.15. On heating, equal amounts of a β precursor, possibly dissociated, and a molecular α species are formed at ≈300 K, with abundances of 5 × 10¹⁴ molecules/cm² each. The α desorption is complete at 360 K. The β precursor evolves on heating without desorption in the range 400–700 K as indicated by work function decreases, to β-CO, which is almost certainly dissociated. This change occurs at lower temperatures for low coverages. Thermal desorption shows 3 peaks, which have been traditionally labelled β_{1, β2}, and β₃ at 930, 1070, and 1375 K. Of these only β₃ corresponds to a well defined state. Readsorption after heating to 950 or 1150 K results in a doubly peaked spectrum at 1070 and 1375 K. The β₁ and β₂ peaks obey complex desorption kinetics, probably corresponding to desorption and rearrangement. The coverage of β₃ is 2.5 × 10¹⁴ molecules/cm², suggesting that the c(2 × 2) LEED pattern corresponds to occupany of every other unit cell by a C or an O atom. For coverages ? 1.5 × 10¹⁴ molecules/cm² β₃ desorption obeys second order kinetics with an activation energy of 83 ± 3 kcal/mole. For β₃ the work function decreases from the clean W value by 0.1 eV, suggesting adsorption of C and O in the center of the W unit mesh, below the surface layer of W atoms. Readsorption on β and β precursor layers leads to formation of electropositive α-CO, with a multiply peaked thermal desorption spectrum, indicating the existence of different binding sites. Adsorption-heatingreadsorption, -heating-readsorption sequences indicate that additional changes in the α desorption spectrum occur, suggesting reconstruction in the β layer.     

Investigation of the temperature dependent changes of work function induced by adsorption of Ag on W(011)

The adsorption of silver on the (011) face of tungsten has been studied at temperatures from 78 to 1200 K. Investigations were performed using LEED, AES and work function measurements. The work function changes appeared to depend on the nature of the adlayer, and its arrangement with respect to the substrate. An absolute coverage scale was determined for room temperature, and this scale was related to the work function changes observed at this temperature. An interpretation of the obtained curves of the work function changes is presented.     

Rare-earth metal adsorption on silicon: Variation of the work function

The variation of the work function of the (111)Si surface under deposition of submonolayer Sm, Eu, and Yb films was calculated with inclusion of the dipole-dipole adatom repulsion and the metallization effects. The results of the calculations agree satisfactorily with experiment.