No abstract

The shattering transition expected upon ultrafast heating has been observed in size selected (NHs)"~NH4 clusters, n = 4 to 40, upon impact at supersonic velocities on a graphite surface.As a function of the impact velocity, the transition is between the recoil of the intact parent cluster and the appearance of small, n = 1, 2, . . . charged fragments. As the cluster size increases, the transition becomes a sharper function of the impact velocity. The experimental fragmentation pattern is well accounted for by a distribution of maximum entropy subject to conservation of energy, atoms, and charge.It is an unwelcome but common experience that a china plate that drops to the floor shatters into a large number of small pieces. The same is true for other high velocity impact phenomena [1]. In this Letter we report the observation of this transition on a molecular scale under controlled conditions. Specifically, both the velocity of impact and the molecular size of the projectile can be varied. The generalization, supported by the theory [2], is that a superheated cluster does not evaporate, but shatters. By evaporation we mean the sequential loss of small (one, two, etc. ) monomeric subunits. Shattering is the limiting behavior when the cluster breaks into many small fragments. The technique of cluster impact [3 -5] enables us to vary the amount of energy supplied, on a sub-ps time scale, to the cluster. It is thereby possible to demonstrate the shattering transition.At sonic velocities of impact, the initially directed energy of the cluster, which upon impact is converted to random motion, is low. The rate of heating of the cluster is therefore moderate, and the cluster has the time to cool down by evaporation [6]. Experimentally, this is the regime where it is the intact parent cluster, or a cluster which lost one or two subunits, which rebounds from the surface. At higher velocities, energy which is sufficient to fully dissociate the cluster is provided on a time scale comparable to that of molecular motion. The cluster then shatters, and the experimental signature proposed herein is the disappearance of the charged parent cluster the simultaneous observation of small ionic fragments and hardly any ions of intermediate size. Moreover, as suggested by the theory, the onset of shattering is already a steep function of the collision velocity for fairly small (say, n = 10) parent clusters. The present experiment, which detects only charged clusters, cannot, however, establish that the neutral fragments, which must accompany the disappearance of the parent ion, are small. The theoretical expectation [2] is that the transition toshattering has the characteristics of a phase transition. This is not unreasonable because of two aspects. First, the simultaneous breaking of the cluster into its constituents is clearly a collective event. Then, the transition has to have a size dependence which will make it sharper for larger clusters. The reason is that it is due to an interplay between energetic and entropic effects as follow...

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 lowest impact energies probed by the experiment. At higher collision energies, the energy loss continues to increase but a point is reached where most clusters shatter. Those few clusters that rebound intact have lost a disproportionately low fraction of their initial energy. The simulations also explore the cluster size effects, the role of the attraction to the surface, and the importance of the anisotropic forces between the molecules in the cluster. The experimental results and the simulations are discussed using the hard cube model with special reference to collective effects.

At fertilization of the mammalian egg, resumption of the cell cycle and the cortical reaction are two events of egg activation, correlated with an increase in intracellular Ca2+ concentration and activation of protein kinase C. To evaluate the pathways leading to both events, rat eggs were parthenogenetically activated by the calcium ionophore ionomycin, or by the protein kinase C activators 12-O-tetradecanoyl phorbol-13-acetate (TPA) or 1-oleoyl-2-acetylglycerol (OAG). Cortical granule exudate was visualized by the lectin Lens culinaris and Texas Red streptavidin, using a confocal microscope. Resumption of meiosis was detected by Hoechst dye, and intracellular Ca2+ concentration by fura-2. Ionomycin triggered both a cortical reaction and resumption of meiosis, while chelation of intracellular Ca2+ rise by BAPTA-AM (1,2-bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester) revealed a segregation between these two events. A low Ca2+ transient (approximately 150 nM) induced a partial cortical reaction in half of the eggs, but the meiotic status was not affected. TPA triggered a cortical reaction with neither resumption of meiosis nor intracellular Ca2+ rise, while OAG induced both aspects of activation, as well as a significant intracellular Ca2+ rise. We conclude that in the cascade of events leading to egg activation, the initial Ca2+ rise is followed by a segregation in the pathway. A relatively low Ca2+ rise is sufficient to induce a partial cortical reaction. However, a higher level of Ca2+ is required to complete the cortical reaction and resumption of meiosis. The activation of the cell cycle is Ca2+-dependent, but protein kinase C-independent.

Emission spectra of homonuclear diatomic rare gas molecules in solid neon 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 kmls. 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 translationalvibrational 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 nature of the homogeneous dissociation process; and the absence of a dominant cage effect are discussed. A quantitative functional form of the velocity dependence of the yield of dissociation, which accounts for the size of the cluster, the rigidity of the surface and other attributes, is used to represent the data. The physics of the processes within the cluster is dominated by the novel dynamical features made possible when the duration of the atom-molecule collisions is short compared to the vibrational period. This "sudden" regime is sudden with respect to all modes of the nuclear motion and provides a hitherto unavailable tool for examination of reaction dynamics under extreme conditions. 8606

No abstract

Moment expansion approach to calculate impact ionization rate in submicron silicon devices

We address some of the unique and basic features of molecular clusters, which involve (i) surface, interior, and site-selective energetics and dynamics, and (ii) the size dependence of the energetic, spectroscopic, electromagnetic, and dynamic attributes of large finite systems. Cluster-size equations provide a unified (but not universal) description of the “transition” of different attributes of clusters to those of the macroscopic bulk material. We explored fundamental issues, e.g., the physical origins of cluster-size effects, which originate either from cluster packing or from excluded volume contributions, and discussed some applications for the quantification of the size dependence of site-specific ionization potentials, extravalence and intravalence electronic spectroscopy, collective vibrational excitations, and dynamic effects. The quantification of dynamic cluster-size effects for energy acquisition in high-energy cluster-wall collisions opens avenues for the exploration of cluster-impact thermal femtosecond chemistry.