Tamar Raz

Cluster impact chemistry. High‐energy collisions of I2ArN clusters with a Pt surface

In this paper, we explore cluster–surface impact induced dissociation of an I2 molecule initially embedded within an I2ArN (N=11–553) cluster, which collides with a Pt surface. Molecular dynamics simulations of high‐energy I2ArN–Pt surface collisions (with initial center of mass velocities v=0.2–10 km s−1 and initial kinetic energies E0K=1 eV−1.2×104 eV) provide information on the yields and time scales for energy acquisition by the cluster and by the surface and energy deposition to the guest molecule via the formation of an intracluster microscopic shock wave, as well as on the I2 dissociation dynamics. The intracluster shock wave is characterized by a temporal peak in the cluster potential energy and in the saturation of the cluster temperature, with the sum of the yields for potential and kinetic energy deposition into the cluster being 0.5–0.6. The cluster residence time (τ=50–800 fs over our velocity and cluster size domain) coincides (within 20%) with the time scale for the cluster energy acquisiti...

Dissociation dynamics of diatomic molecules embedded in impact heated rare gas clusters

Molecular dynamics simulations demonstrate facile dissociation of halogen molecules embedded in rare gas clusters upon impact at a surface at collision velocities up to 10 km/s. Two pathways are discerned: a heterogeneous dissociation of the molecule on the surface and a homogeneous mechanism where rare gas atoms which have rebounded from the surface cause the translational–vibrational coupling. The total yield of dissociation of the clustered molecule can reach up to 100%, whereas the yield of dissociation of the bare, vibrationally cold molecule saturates below 40%. A systematic study of the role of different conditions is made possible by not accounting for the atomic structure of the surface. The role of dissipation at the surface is found, however, to be quite important and is allowed for. Larger clusters, clusters of the heavier rare gases and a more rigid surface, all favor the homogeneous mechanism. Evidence for a shock front which, upon the initial impact, propagates into the cluster; the binary ...

Internal energy dependence of the fragmentation patterns of C60 and C60

Abstract The simplest statistical theory in which the mass spectral fragmentation pattern is governed by the mean energy is applied to fragmentation of C60+ and C60. The results are compared with experimental fragmentation spectra obtained in collisions between C60+ and atomic targets and in photoionisation/fragmentation investigations. Good qualitative agreement is found for the energy dependence of the fragmentation pattern. The appearance energies of the fragments and their temperatures are in excellent quantitative agreement with the experimental results.

Fragment size distribution in cluster impact: Shattering versus evaporation by a statistical approach

The transition from intact clusters to shattered small fragments is discussed for high velocity impact of cold atomic clusters at hard surfaces. The distribution of fragment sizes is computed as one of maximal entropy subject to conservation of matter and energy. The entropy is computed using a graph‐theoretical approach for counting the number of possible isomers for a cluster of given size. As the impact velocity is increased, there is an onset of fragmentation into small fragments and the heat capacity is found to be discontinuous at that point. The results are compared with molecular dynamics simulations for rare gas clusters. Throughout the emphasis is on the special conditions that prevail during the ultrafast compression stage of the high energy cluster.

On the shattering of clusters by surface impact heating

The onset of a shattering regime when a supersonic cluster undergoes an ultrafast heating by its impact at a surface, proposed on the basis of an information theoretic analysis, has now been demonstrated experimentally for molecular clusters. It is emphasized that the sudden onset of shattering as a function of impact velocity is a robust result depending essentially only on the multitude of possible isomers of larger clusters. There is one underlying assumption of the information theoretic approach—namely that there is a rather rapid thermalization of the translational degrees of freedom of the impact heated cluster so that mean energy is the only energetic constraint. When this is not necessarily the case, e.g., for ionic clusters at lower energies, there will not be extensive fragmentation.

Collisional energy loss in cluster surface impact: Experimental, model, and simulation studies of some relevant factors

Measurements of the collisional energy transfer of size and energy-selected ammonia cluster ions (NH3)nH+, n=1–10, impacting a silicon wafer coated with p-type diamond film are reported. The transfer from translational energy of the incident cluster ions to kinetic energy of intact scattered cluster ions has been studied as a function of impact energy, surface composition, and size of the impinging cluster cations. For low impact energies (<2.5 eV/molecule), cluster ions scattered off the target surface lost most of their initial kinetic energy, while for higher impact energies the elasticity of the cluster–surface collision is surprisingly high: Typically 75% of the impact kinetic energy is retained by the scattered parent clusters. Larger cluster ions are scattered less elastically and a large fraction of them shatter to small(est) fragments. The molecular dynamics simulations examine the two energy disposal regimes, deep inelasticity and shattering. Deep inelastic scattering occurs already below the lo...

CLUSTER-SURFACE IMPACT DISSOCIATION OF HALOGEN MOLECULES IN LARGE INERT GAS CLUSTERS

Abstract Molecular dynamics simulations of the dissociation of I 2 embedded in large Ar n ( n = 319, 553) clusters, which impact at high velocities (ν = 7–15 km s −1 1 ) on Pt surfaces, result in information on heterogeneous and homogeneous dissociation mechanisms. A broad distribution of dissociation lifetimes is exhibited, which can be attributed to prompt and retarded heterogeneous dissociation and to prompt, retarded and outbound homogeneous dissociation events. The propagation of a microshock wave within a large cluster can be interrogated by the homogeneous dissociation of a chemical probe, with the velocity of the propagation of the dissociation front being close to the cluster impact velocity.

On the burning of air

Abstract Air is shown to burn (in theory, using two complementary procedures) under the unusual combination of conditions made possible within a cluster heated by a supersonic impact at an inert surface. Both clusters of neat N 2 /O 2 and clusters containing several N 2 and O 2 molecules within a rare gas envelope have been studied. The principal reaction is N 2 + O 2 → 2NO which proceeds via a four-center mechanisms. The four-center reaction N 2 + O 2 → N 2 O + O leads to N 2 O which is quite hot, as is to be expected on the basis of kinematic considerations, and is efficiently destroyed as the cluster expands. During the early, compression, stage multi- (>4) center reactions readily occur. The results of molecular dynamics simulations using a many-body potential are well accounted for by a distribution of products of maximum entropy subject to conservation of energy, matter and charge.

Kinematic model for four-center, AB+CD, reactions

Abstract Molecular dynamics simulations showing that high-barrier, four-center, reactions occur quite readily at elevated collision energies are presented. At such energies, the reaction is found to be super direct and the motion to, and down from, the barrier, is sudden-like. A simple kinematic model is shown to provide a remarkably accurate description of many aspects of the dynamics and, in particular, of the energy requirements and energy disposal in these reactions. The model emphasizes that over and above the energy necessary to overcome the high barrier to reaction, there is a strong kinematic constraint on the formation of products. Detailed results are reported for the N 2 +O 2 reaction.

The transition from recoil to shattering in cluster-surface impact: an experimental and computational study

Size and kinetic energy distributions of the products of size and energy selected ammonia clusters, (NH3)nNH4+, n = 1–11, impacting a silicon wafer coated with p-type diamond film are reported. The transition from the recoil of the intact parent cluster to the complete fragmentation into monomers has been studied as a function of the energy of impact. Two experimental methods are used to place an upper bound on the time duration of the shattering resulting in < 120 ps and < 80 ps, respectively. The experimental results are discussed in terms of the analytical theory of the shattering transition and compared also to computational output obtained by molecular dynamics. The simulations show that while shattering is practically instantaneous upon impact, evaporation of the impact heated clusters is much slower and there is a clear separation of time scales between the two processes. The shattering of a cluster, when the constituents move apart in concert, is also of interest for cluster impact induced chemistry.