Marcus Svanberg

Angular resolved neutral desorption of potassium promoter from surfaces of iron catalysts

Abstract The angular dependence of neutral potassium emission in the form of ground-state atoms as well as Rydberg species is studied from a fused iron catalyst. The catalyst is of the type used for ammonia synthesis in so-called pre-reduced (metallic) condition. The angular distributions observed by surface ionization detection have a more peaked shape than the cosine distribution expected for thermal equilibrium. In the case of a catalyst sample used in the industrial process even a sharp peak on top of a cosine distribution is found. Using detection by field ionization, i.e. detection of Rydberg species only, a rather sharp lobe in the normal direction is found. A theoretical description of cluster formation outside the sample surface from atoms with velocity distributions characteristic for thermal equilibrium is used to interpret the results. The cluster formation is probably due to the long-range interaction between the Rydberg atoms formed on the surface, and the clusters are at least partially formed in an excited state. The cluster sizes contributing to the distributions are estimated from fits to the experimental results. The main cluster size observed with surface ionization detection is concluded to be quite small, containing just a few atoms. There also exist contributions of larger clusters of the size around 10–30 atoms in the case of the pre-reduced catalyst. The used catalyst also gives mainly small clusters, but it does not give clusters of the size 10–30 atoms. Both types of catalyst also give a small number, less than 5%, of very large clusters, with more than 100 atoms according to the model. The field ionization data for the pre-reduced catalyst are well matched by a single cluster size of approximately 30 atoms, which indicates that such clusters have a longer lifetime in the initial excited state than the small clusters.

Classical trajectory study of argon–ice collision dynamics

Classical trajectory simulations have been used to study Ar–ice Ih collisional energy transfer, trapping coefficients and scattering distributions for initial Ar kinetic energies between 0.1 and 2.0 eV, incident angles between 0 and 70° and surface temperatures between 0 and 300 K. Collisional energy transfer is extremely efficient due to substantial transfer of energy from the Ar atom to the ice surface over typically 2–4 gas-surface encounters, and the rapid dissipation of this energy away from the collision center, preventing energy transfer back to the Ar atom. This leads to large trapping coefficients over this range of Ar collision energies, incident angles and surface temperatures. Scattered gas atoms lose most of their initial kinetic energy and have broad angular distributions. The large trapping coefficients obtained for the Ar–ice collisions are expected to be found for similar reactions under stratospheric conditions (e.g., HCl–ice, HOCl–ice and ClONO2–ice).

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.

Collision dynamics of large water clusters

Classical trajectory calculations of (H2O)n+(H2O)n collisions are carried out for n=125 and n=1000. We investigate energy redistribution and fragmentation behavior for relative collision velocities up to 3000 ms−1, impact parameters up to 4 nm, and initial cluster temperatures of 160 and 300 K. Three main scattering channels are identified; coalescence, stretching separation, and shattering collisions. For small impact parameters, low collision velocities produce coalesced clusters while high velocities yield shattering behavior. Large impact parameters combined with high velocities result in stretching separation collisions. A decreased internal temperature influences the dynamics by increasing the stability of the collision complex. The results for (H2O)125 and (H2O)1000 are comparable, although the smaller size allows individual molecules to have a larger influence on the overall behavior. We find good agreement between the cluster simulations and experimental data for water drops in the micrometer ran...

Molecular-dynamics simulations of cluster–surface collisions: Emission of large fragments

Large-scale classical molecular-dynamics simulations of (H2O)n (n = 1032,4094) collisions with graphite have been carried out. The clusters have an initial internal temperature of 180 K and collide with an incident velocity in the normal direction between 200 and 1000 m/s. The 1032-clusters are trapped on the surface and completely disintegrate by evaporation. The 4094-clusters are found to partly survive the surface impact provided that the surface is sufficiently hot. These clusters are trapped on the surface for up to 50 ps before leaving the surface under strong evaporation of small fragments. The time spent on the surface is too short for full equilibration to occur, which limits the fragmentation of the clusters. The size of the emitted fragment is roughly 30% of the incident cluster size. The cluster emission mechanism is found to be very sensitive to the rate of the surface-induced heating and thus to the surface temperature. The incident cluster velocity is less critical for the outcome of the collision process but influences the time spent on the surface. The trends seen in the simulations agree well with recent experimental data for collisions of large water clusters with graphite [Chem. Phys. Lett. 329, 200 (2000)].

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.

Ozone photodissociation in the Hartley band: A statistical description of the ground state decomposition channel O2(X 3Σ−g)+O(3P)

Ozone photodissociation in the Hartley band O3+hν→O(3P)+O2(X 3Σ−g) is simulated with a statistical model. In the model, energy is partitioned at a decoupling distance which is located at a position with nonzero potential energy on a repulsive and dissociative potential energy surface. Introduction of the repulsive potential on which dissociation takes place, and the choice of decoupling distance is shown to be of crucial importance for the final energy distributions, and in particular it determines the amount of energy left in translation. The model is shown to give good agreement with experimental vibrational and translational energy distributions, while the rotational distributions predicted by the model seem less peaked than experimental data. Vibrational state distributions are calculated for different dissociation wavelengths in the Hartley band (200–310 nm), and they are concluded to deviate substantially from distributions previously used in atmospheric modeling. The statistical approach is compare...

Work function of Rydberg matter surfaces from jellium calculations

The theoretical description of Rydberg matter as an electronically excited type of matter was given by Manykin et al. [Sov. Phys. Tech. Phys. 26 (1981) 974; Sov. Phys. JETP 57 (1983) 256]. Experimental studies of Rydberg matter of Cs with principal quantum numbers, estimated to be around n = 15 from the life-time and temperature range where Rydberg matter is stable, give work functions less than 0.7 eV. Solid-state density-functional calculations of work functions using pseudopotential techniques for Rydberg matter of Cs give values less than 0.2 eV at n > 15 (Manykin et al. [Sov. Phys. JETP 75 (1992) 440, 602]). Jellium calculations give work function values in general agreement with the more complex solid-state calculations due to Manykin et al. We now present more accurate work functions based on the Rydberg matter densities recently calculated by Manykin et al. It is shown that jellium theory gives consistently lower work functions than the more complex solid-state theory. This is not unexpected due to the larger volume per valence electron. The close agreement between the two strongly different models indicates that the true work function is close to and probably slightly higher than predicted by Manykin et al.