G. Wedler

Laser induced thermal desorption of carbon monoxide from Fe(110) surfaces

Thermal desorption of CO is induced by bombarding an Fe(110) surface with pulses of a neodymium glass laser. The maximum amplitude of the desorption signal is recorded by a mass spectrometer as a function of the laser pulse intensity and of the CO coverage for both single pulses and sequences of pulses. Since the half width of the laser pulses is only 30 ns the shape of the desorption signal is mainly determined by the time-of-flight of the desorbed particles. There is strong evidence that the latter obey a Maxwell-Boltzmann distribution of temperature Td, identical in the low temperature range with the maximum surface temperature Ts. Above Ts = 600 K, however, Td is smaller than Ts. The experimental observations are analyzed successfully with the first order rate equation for desorption.

CO2 adsorption and reaction on Fe(111): An angle resolved photoemission (ARUPS) study

Abstract Molecular CO 2 adsorption is observed on an Fe(111) surface at 85 K. For the main fraction of molecules the relative binding energies of the valence ion states as determined by ARUPS are consistent with those in the gas as well as in the condensed phase, and indicate that the electronic structure of that fraction of adsorbed molecules is only slightly distorted upon adsorption. There is a fraction of adsorbed molecules at 85 K that can be identified as bent, anionic CO 2 − species. While the weakly adsorbed, linear CO 2 molecules desorb at low temperature, the CO 2 − species is stable up to 160–180 K. The latter is proposed to be a precursor to dissociation. Above this temperature adsorbed carbon monoxide and oxygen are observed on the surface, and at room temperature the CO 2 − signals have disappeared. Heating above room temperature dissociates the CO molecules into carbon and oxygen.

Interaction of carbon dioxide with Fe(110), stepped Fe(110) and Fe(111)

Abstract The adsorption of CO 2 on single crystal surfaces of Fe(110), regularly stepped Fe(110) and Fe(111) in the temperature range between 77 and 340 K was studied by means of He(I) UPS and measurements of the change in work function. The smooth Fe(110) face proved to be completely inactive with respect to CO 2 adsorption. On a stepped Fe(110) and an Fe(111) face CO 2 is adsorbed at 77 K in the form of a linear molecule and in the form of a species the nature of which is not yet clarified. This latter form is predominant at 140 K. With increasing temperature decomposition into CO and O and finally into C and O takes place.

Oxygen adsorption and oxide formation on Co(112̄0)

Abstract Adsorption of oxygen on the (1120) surface of hexagonal cobalt at 100 and 320 K has been studied by means of low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS), change of work function (Δφ) and Auger electron spectroscopy (AES). At 100 K, adsorbed oxygen and Co 3 O 4 features are observed, as identified by the UP spectra. During heating, a conversion into CoO takes place. This process is completed at 230 K and followed by dissolution of oxygen in the bulk at temperatures exceeding 450 K. Admission of oxygen at 320 K leads to identical results, but the total amount of adsorbed oxygen is approximately 30% lower. LEED experiments show a (1 × 3) superstructure at low coverages. After higher exposures, CoO formation, as revealed by the photoelectron data, is indicated by a changed diffraction pattern. It is concluded that an epitaxial growth of CoO(100) takes place.

Adsorption of carbon monoxide on polycrystalline nickel films

Abstract The adsorption states of carbon monoxide on polycrystalline nickel films have been investigated by measuring the thermal desorption, the heat of adsorption, the change in resistivity, and the change in work function in dependence on coverage and temperature. It can be shown that there are two chemisorbed (β2, β2) and one weakly bound (γ) species. Desorption peaks appear at 170K, 310–360 K, and 460–490 K. The differential heat of adsorption is 30 kcal mole at low coverages and approximately 25 kcal mole between 0.3 and 0.6 monolayers. The resistivity of the nickel film is characteristically changed with increasing coverage, and there is a maximum of resistivity at half a monolayer. At low coverages the increase in the work function is proportional to the amount adsorbed; at a monolayer the total increase is 1.26 eV at 77 K and 1.46 eV at 273 K. The two chemisorbed species differ only in the structures they form in the adsorption phase, β2 being the species that is stable at low coverages, β1 being the species that is stable at high coverages. These results are in good agreement with those recently found for CO adsorption on single crystal surfaces.

The adsorption states of nitrogen on polycrystalline iron films

Abstract The adsorption of nitrogen on evaporated iron films at 77 K or 90 K is studied by measuring the amount adsorbed, the equilibrium pressure, the change in resistivity of the iron films, the differential heat of adsorption and the thermal desorption. Four different adsorption states were found. By means of isotopic exchange measurements it could be seen that the nitrogen adsorption is nondissociative. There is strong evidence that the adsorption of nitrogen on iron is face specific.

Zur deutung der durch die adsorption von gasen an dünnen metallschichten hervorgerufenen widerstandsänderungen

Abstract The variation of the electrical resistance of thin metal films due to adsorption of gases is discussed in regard to coverage and thickness dependence. Experimental results available for the systems Ni/CO; Au/CO; Ni/H 2 ; Cu/CO; Ni/O 2 ; Fe/N 2 and Ni/N 2 are given. For all systems the adsorption produces an increase of the resistance, which can be explained quantitatively by means of the scattering theory. Corresponding to this theory the adsorbed particles act as scattering centers for the conduction electrons similar to alloyed atoms inside the metal. A correlation is found between the increase of the resistance and the strength of the adsorption.

CO2 adsorption and reaction on polycrystalline Fe films

Thin iron films deposited under ultrahigh vacuum conditions are used as adsorbents. The adsorption and reaction of CO 2 on these films are studied by means of HeI and HeII UPS, XPS (C1s and O1s), changes in work function and adsorption calorimetry. As in the case of single crystal surfaces (Fe(111) and stepped Fe(110)) at 80 K linear CO 2 and a bent, anionic species are adsorbed. From the intensities of C1s and O1s signals it follows that this species is CO 2 − . With increasing temperature decomposition into CO+O takes place. This is also proved with the aid of calorimetric measurements. Already at room temperature a partial decomposition into C and O is observed. There are two carbon species, a carbidic and a graphitic one. The decomposition reactions set in at lower temperatures than in the case of the single crystal surfaces.

Die photoelektrischen eigenschaften aufgedampfter nickelfilme: Einfluss von schichtdicke und temperung

Abstract The photoelectric behaviour of thin evaporated Ni-films is studied in dependence on film thickness and annealing temperature. The films with thicknesses between 30 and 800 A are condensed at 77°K under uhv conditions on glass substrates. The work function and the emission constant are calculated from the photoelectric yield by means of the Fowler theory. A correlation is found between the photoelectric properties and the orientation of the crystallites of the films.

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

Abstract 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.