Ultrafast femtosecond Coulomb explosion of charged homogeneous (Xe n) and heterogeneous doped (HIAr n) small and medium sized clusters (nϽ60) is studied resting on the picture of a vertical high-order multiphoton ionization from the ground state nuclear configuration. The final average atomic velocity ͑simulated at constant charge͒ increases with increasing the cluster size, and at constant cluster size increases linearly with the ion charge, in accord with the predictions of an analytical model. The linear dependence of the reciprocal explosion time on the charge is also in accord with the analytical prediction. From the energetics of the Coulomb explosion ͑reflecting a probable initial atomic distribution of the cluster size for small clusters͒, a nonvertical multiphoton ionization during the Coulomb explosion cannot be inferred.

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

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.

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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 acquisition, decreasing linearly with v−1 and obeying a dynamic size equation τ∝(N+2.9)1/3. The characteristic time tp for energy deposition to the I2 molecule via a local mechanism involving pair interactions is also close to τ. The initial cluster kinetic energy dependence of the dissociation yields YD of I2 reveals a gradual increase of YD towards unity above a threshold at the energy Et. For smaller (N=11,53) clusters, Et/N is close to the dissociation energy of bare I2, while for larger clusters Et exhibits an exponential N dependence. Cluster impact dissociation of I2 in I2ArN results in higher YD values (≳0.4) than the high-energy collision of bare I2 with the Pt surface for which YD saturates at 0.35. The I2 dissociation times 〈τD〉, which were characterized by averaging over the first passage times for the attainment of the turning point of the I–I intramolecular Morse potential for reactive trajectories, fall in the range 170–800 fs, exhibiting a marked inverse kinetic energy dependence, revealing an increase with increasing cluster size and obeying the rough relation 〈τD〉≂2τ, i.e., being proportional to the cluster radius. Energy acquisition and dissociation times are comparable to or even shorter than the vibrational time [τ(I2)=156 fs] of the I2 molecule, opening up a new research area of thermal femtosecond chemistry.