Jan B.C. Pettersson

Field ionizable cesium metal clusters from a foil diffusion source

Abstract Ionized clusters of Cs were studied by time-of-flight mass spectrometry and energy analysis. The typical clusters contained 4000–10000 Cs atoms, assuming singly charged clusters. The clusters were formed in the neutral state by condensation of Cs from a foil diffusion source at 1300 K. Diffusion sources have previously been shown to emit excited states, especially high Rydberg states of Cs (Pettersson and Holmlid, 1989), and the condensation is thought to be driven by the large interaction energies between highly excited states. Ionized clusters were formed by field ionization of the neutral clusters at electric field strengths of 200–400 V/cm, both with pulsed fields and at constant field strengths. The cluster size determined from the neutral transport time to the point of field ionization agrees roughly with the cluster size determined from the ionized cluster flight time.

Dynamics of cluster scattering from surfaces

Abstract Classical trajectory calculations of van der Waals cluster scattering from a rigid surface are presented and compared with recent experimental results on argon cluster scattering from graphite by Châtelet (1992). The cluster fragmentation at surface impact is studied for different incident translational energies and can at moderate energies be described as evaporation of small particles from the parent cluster. The experimentally observed dependence of angular distributions on cluster fragment size is reproduced by the model, as well as the appearance of two components in the experimental angular distributions.

Evaporation model of cluster scattering from surfaces

We present classical trajectory calculations of ArnNem (n+m=111, 859) clusters scattering from a rigid surface. The dynamics of energy transfer and cluster decomposition during surface scattering is investigated for incident velocities of 100–700 m/s. The initial translational energy is at impact effectively transferred into internal degrees of freedom of the cluster. The overall energy transfer efficiency is very high but not complete, leaving too much energy in translation. No fragmentation takes place below 200 m/s. At incident velocities below 450 m/s, evaporation of small fragments from the heated cluster takes place in thermal equilibrium with the vibrational degrees of the cluster. This thermal evaporation is also the dominating ejection channel up to 700 m/s. Above 450 m/s, the formation of a compressed zone at impact opens up a new channel with ejection of fast fragments parallel to the surface plane. This effect becomes increasingly important at higher velocities. An evaporation model where frag...

Energy transfer in water cluster scattering from solid surfaces

Abstract Classical trajectory calculations of (H 2 O) n ( n ≤ 123)scattering from rigid model surfaces are presented. Three different intramolecular water potentials are employed together with both flat and corrugated surfaces. Clusters with an internal temperature of 180 K are scattered from the surface with incident velocities of 400–2000 m/s, and energy conversion during surface interaction is followed by probing the temperatures of various degrees of freedom. Molecular translation within the cluster couples strongly to the surface potential resulting in a compression phase of the cluster and a temperature peak at impact. Energy then dissipates to molecular rotation and further on to intramolecular vibration in the bending mode. The choice of the intramolecular potential has a strong effect only on the coupling to the stretch modes. Cluster fragmentation is very low up to 1300 m/s and thereafter increases with velocity. The energy redistribution at impact depends only weakly on cluster size and the surface potential employed. The total energy transfer efficiency is largely determined by the maximum surface potential energy during the scattering event.

Rydberg states of cesium in the flux from surfaces at high temperatures

Diffusion of cesium through an Ir foil at high temperature is used to give a flux of desorbing atoms and/or ions, which is not in equilibrium at the surface temperature. The desorbing flux is studied with electrostatic fields in a grid-collector combination, to determine the charge states and the interaction of the different states with an electric field and with a surface. At low Cs flux densities to the back of the foil, the desorbed flux contains positive ions and ordinary neutrals. At large flux densities from the source, however, both neutral Rydberg states and negative ions in the form of multiply-excited states of cesium are found in the flux. The negative ion states do not dissociate (detach) readily in such weak external electric fields where the neutral Rydberg state atoms ionize. Instead, they can emit two of their outer electrons in contact with the collector surface, thus giving positive collector currents when the field accelerates the electrons away from the collector. The large flux of such states indicates that the electron affinity is large for Rydberg state atoms. The origin of the excited states leaving the hot Ir surface is discussed. It appears to be related to the close coincidence of the desorbing ionic state and the Rydberg state at low work function.

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.

Survival of noble gas clusters scattering from hot metal surfaces

Abstract We present classical trajectory calculations of Ar n ( n = 1000−400) colliding with a hot Pt(111) surface. Large cluster fragments are found to survive a surface collision, and the fraction of atoms remaining in the fragment is concluded to increase with initial cluster size and surface temperature, and decrease with incident velocity above 100 m/s. Up to 52% of the initial cluster is found to survive as one unit in the most favorable case of Ar 4000 scattering from a surface at 1500 K. The implications of the results for new experimental investigations are discussed.

Surface scattering of NO from graphite: A statistical description of energy distributions

In the present theoretical study, inelastic scattering of NO from graphite surfaces is analyzed with a statistical model. The results are in good agreement with previous classical trajectory calculations by Pettersson et al. (1988). Angular distributions and the ‘‘rotational cooling’’ effect found in experiments published by Frenckel et al. (1982), Segner et al. (1983), and Hager and Walther (1984) are successfully reproduced. The model describes a small part of the graphite surface together with a scattering diatom as a collision complex, which decomposes in a unimolecular fashion. The surface is assumed to be flat, whereby the diatom angular momentum component along the surface normal and the linear momentum parallel to the surface are conserved. Otherwise the diatom translation and rotation are allowed to exchange energy with the surface, which is characterized by a set of harmonic oscillators. The experimentally observed ‘‘rotational cooling’’ effect is clearly demonstrated to be due to the conservati...

Detection of sodium and potassium salt particles using surface ionization at atmospheric pressure

Abstract The surface ionization of sodium and potassium salt particles with diameters of 1–100 nm is studied at atmospheric pressure. Particles impact on a resistively heated and positively biased Pt filament. The particles melt and dissociate at the filament and alkali ions are emitted, giving rise to a positive current at a closely situated collector. Salt particles with diameters below 5 nm are concluded to melt and ionize completely at the filament. Larger particles melt and ionize partially, yielding an ion signal that is proportional to the particle surface area. The ionization efficiency decreases with increasing particle size and reaches values around 1% for 0.1 μm particles, with small variations depending on the stability of the alkali salt.

A classical trajectory study of inelastic scattering of NO from graphite surfaces: Rotational energy distributions

The inelastic scattering of NO molecules from graphite surfaces is studied by classical trajectory methods. The experimental results from Frenkel et al. (1982), Segner et al. (1983), and Hager and Walther (1984) are analyzed. A model using a small isolated part of the graphite surface in interaction with the NO molecule gives results in good agreement with experiment. The parameter values in the model are fixed at the values previously found to reproduce the angular distributions well [Nyman and Pettersson (1987)]. For this system, the experimental results give a ‘‘rotational cooling’’ such that the rotational temperature of the inelastically scattered molecules becomes smaller than the surface temperature. This effect is reproduced accurately by the calculations, giving a rotational temperature of 250 K, independent of the surface temperature above 300 K. The main factor controlling this inelastic rotational cooling is the low initial value of the normal component of the total angular momentum. A ‘‘rotat...