Kenneth Möller

Rate constants for cesium bulk diffusion and neutral desorption on pyrolytic graphite basal surfaces: a field reversal kinetic study

The kinetics of desorption for Cs and Cs+ from basal graphite surfaces at surface temperatures of 1220–1530 K was studied by the field reversal method. In this case, the peak ion signal after a period with retarding field, in the range 1 micros to 2 ms, was measured. This peak signal is directly related to the surface density of Cs at the end of the retarding period. An analytical description, which includes rapid diffusion into the bulk, is shown to give good agreement at relatively short times, while a normal desorption pattern applies at longer times. Under the present high temperature conditions, diffusion via the grain boundaries is generally a relatively slow process. The detailed kinetic model, which was used in our previous studies, agrees well with the results. A few rate parameters were redetermined independently and are shown to agree within error limits. The diffusion process connecting the upper and lower migrating states is shown to have a small pre-exponential factor, indicating a grain boundary coverage of 10−3. The very rapid diffusion into the bulk was once more found to have a pre-exponential of 1025 st1. It is proposed that this process involves the ionization of a Rydberg state outside the surface.

Alkali promotor function in heterogeneous catalysis: Possibility of interaction in the form of Rydberg states

Abstract The promotor function of alkalis in heterogeneous catalysis is proposed to involve excited states of alkali atoms at the surface. These states are formed during the emission of alkali ions from the bulk. During the transit out from the bulk, the ions attach electrons in high lying levels and form highly excited states, so-called Rydberg states. Such a highly state A ∗ may react easily with most molecules R on the surface, to form complexes RA ∗ or RA and finally possibly RA + . This means that the molecule R is attacked chemically in a way which resembles standard organic chemical synthesis methods. The excited states are proposed to be formed thermally, but at non-equilibrium concentrations. The theoretical and experimental evidence for this is summarized and discussed.

Cesium ion desorption from oxygen and carbon adlayers on platinum surfaces with nanosecond time resolution: Variation of desorption parameters with time available for surface diffusion and degree of surface heterogeneity

Abstract Measurements of ionic cesium desorption from surfaces at temperatures above 1100 K have been made with the so-called field reversal method. Time constants down to 30 ns have been measured with very low surface densities of Cs. The prexponential factors for desorption from clean platinum and graphite covered platinum surfaces are close to 10 12 s −1 , in agreement with simple desorption theory. However, for slightly oxygen covered surfaces and for carbon layers formed at low temperatures the preexponential and the desorption energy become significantly larger, while a strict Arrhenius form is retained. This is proposed to be due to the influence of surface inhomogeneity and surface diffusion, in line with the theoretical description of simultaneous diffusion and desorption recently given by Nordholm. In the oxygen case, the adsorbed oxygen is considered to be the origin of the heterogeneity, while in the carbon layer case the surface, which has been formed at low temperature, is disordered and may contain many strongly binding sites. The influence of bulk diffusion on the desorption is also described.

Ion formation and acceleration with pulsed surface ionization: Two modes for time-of-flight mass spectrometry

Abstract The ion pulsing and acceleration resulting from the rapid field switching outside a hot ionizing metal surface has been studied. The impurity flux from the bulk metal itself has been surface ionized and measured. Time-of-flight mass and energy analysis, coupled with crude retarding-field energy analysis has been employed to identify the various peaks in the time spectrum. Two well-separated peaks with different energies can be found for each desorbing ion mass. A simple theory based on the assumption that the ion kinetic energy is not changed during field reversal gives quite good agreement with experiment. It is shown that the high energy peaks (up to around 500 eV) formed with suck-out times of 100–200 ns are the most suitable for use in time-of-flight mass spectrometry.

Source for excited states of alkali atoms and clusters using diffusion through a thin graphite foil

A molecular beam source used for the production of excited and ionized clusters of Cs and other materials is described. This source is closed, with the emitting graphite foil as an integral part of the source enclosure. The foil is kept at 1100–1500 K by radiation heating. From the foil, excited clusters (Cs)*n with broad distributions of n, and excited Rydberg atoms Cs* are emitted. These highly excited states can be field ionized at field strengths of less than 400 V/cm. At a Cs reservoir temperature of 400 K, total field ionized flux densities up to 1015 ions cm−2 s−1 (3×10−4 A cm−2) are found.

An electron emission study of a graphite covered platinum emitter for thermionic energy conversion: Dissolution of carbon into the bulk of the metal

Abstract The formation of graphite and carbon overlayers on polycrystalline platinum emitters for thermionic energy conversion was studied by thermal electron emission and surface ionization of alkali halide molecular beams in the temperature range 1300–1900 K under UHV conditions. The overlayers were found to disappear through dissolution into the metal bulk at a surface temperature slightly above the one where the initial layers were formed by hydrocarbon pyrolysis. S-shaped curves for electron emission were measured for the bulk solution process, similar in shape to the ones found for adsorption of e.g. alkali metals on various surfaces. The removal of the surface carbon by oxygen was also studied.

Emitter tests in an open thermionic converter with vapor injection through the collector

Mo and Pt emitters and a Ni collector with 400 laser‐bored holes were used in an ‘‘open’’ thermionic converter. The alkali vapor was introduced into the converter through the array of holes in the collector from an adjacent alkali metal reservoir with separately controlled temperature. The overall results from the open thermionic converter are comparable to results from enclosed converters. The results found with a Cs plasma are encouraging, with barrier indices down to below 1.8 eV, at emitter temperatures around 1500 K in the case of a Mo emitter. The output power density was around 3.5 W cm−2. In the case of a Pt emitter, both Cs and K plasmas were used, with power densities up to 5.7 and 1.8 W cm−2, respectively close to 1800 K. The structure of the laser‐bored collector may have contributed to these results, as well as the efficient removal of impurities in the ‘‘open’’ converter.

Surface science lettersRapid diffusion of cesium through metal foils: Diffusion limiting at high current densities

Abstract The surface ionization of Cs on foil samples of Ir and some other materials was investigated at high temperatures and large atomic Cs beam flux densities. Rapid diffusion through the foil was shown to exist, at current densities up to approximately 5 mA cm-2. The size of the ion current from both sides of the sample was diffusion limited and reached a temperature and flux independent value, for both pulsed (1–10 μs) and DC current measurements. There was no indication of a space charge limited behaviour under the conditions of the experiments. The behaviour observed is not compatible with the equilibrium Saha-Langmuir description of surface ionization. Most of the results, however, agree with our recently published model for coupled surface ionization and diffusion. Diffusion in the metal crystallites and desorption above the vacuum level over a small or vanishing desorption barrier are important processes in the model.

Desorption of cesium from pyrolytic graphite basal surfaces with strongly non-equilibrium behaviour

A field reversal kinetic study of the desorption of Cs+ and Cs from a graphite basal surface (ZYB graphite) gives results that neither follow the Saha-Langmuir equation, nor fulfil the Schottky energy cycle. The non-equilibrium behaviour is due to a lowering of the rate of the ionic desorption, while the neutral desorption at low temperatures is described by normal rate parameters. At high temperatures, the neutral desorption rate constant is dominated by vertical diffusion into the bulk. The unexpected behaviour of the ion desorption is probably coupled to vertical diffusion too. It is suggested that the desorbing ions leave the surface from special positions on the surface, which are reached during the migration out from the bulk.