Israel Schek

Intensities of high-energy molecular C–H vibrational overtones

Abstract We present an analysis of the intensities of the C–H vibrational 0-υ (υ = 2–6) of naphthalene in terms of the local bond mode model.

High-energy cluster-surface collisions

Abstract Molecular dynamics simulations of high-energy collisions (initial kinetic energies E o k = 10−10 6 eV) of Kr n , ( n =8−512) clusters with a rigid Pt surface provide a microscopic description of the formation of an intracluster shock wave (temperatures up to 3 × 10 5 K and cluster internal potential energies up to ∼ 10 4 eV) on a time scale of 10–500 fs, which is accompanied by novel ultrafast energy acquisition processes. High-energy ((10−5) × 10 3 eV) high-yield (0.5%–10%) interatomic KrKr pair repulsive excitation is exhibited, being manifested in outer-shell electronic excitations and ionization, multiple ionization and Auger processes.

Energetics and dynamics of Coulomb explosion of highly charged clusters

Ultrafast femtosecond Coulomb explosion of charged homogeneous (Xen) and heterogeneous doped (HIArn) small and medium sized clusters (n<60) 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 charge) 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 clusters), a nonvertical multiphoton ionization during the Coulomb explosion cannot be inferred.

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 ...

Random coupling model for multiphoton photofragmentation of large molecules

In this paper we present a quantum mechanical theory of multiphoton photodissociation of large, collision‐free, molecules, which rests on the notion that the radiative coupling terms between adjacent sets of congested bound molecular states in the quasicontinuum exhibit a wide variation both in terms of magnitude and of sign. Invoking the rotating‐wave approximation, neglecting spontaneous infrared decay, and assuming that near‐resonant radiative coupling prevails, the equations of motion were solved within the framework of the random radiative coupling model for the radiative interactions in the quasicontinuum. In the low energy range (range I) the equations of motion for the amplitudes are determined by the effective Hamiltonian formalism, while in the quasicontinuum (range II) the populations are governed by kinetic equations for sequential reversible decay. All the features of coherent excitation are preserved for range I, while in range II intramolecular erosion of phase coherence effects prevails. T...

Application of the magnus expansion for high-order multiphoton excitation

Abstract In this paper we present the results of a study of the multiphoton dynamics of a truncated Morse oscillator, calculating the dependence of the dissociation probability from the uppermost level and of the mean vibrational excitation energy on the intensity and the frequency of the electromagnetic field. These calculations were performed for several forms of the dipole moment operator, involving linear and exponential dipole functions. These calculations establish the validity range of the rotating wave approximation (RWA), which allows the use of the effective hamiltonian formalism to treat the dynamics. The calculations demonstrate the applicability of the Magnus expansion to the multiphoton excitation of a sparse level system for which the RWA is not applicable. This work also establishes two effects of the form of the dipole moment operator on multiphoton excitation of the oscillator. These involve the possibility of the occurrence of bottlenecks in the radiative coupling matrix elements, which reduce the efficiency of multiphoton excitation and the presence of radiative coupling between nonadjacent energy levels, which greatly enhance the efficiency of excitation. Both of these effects can lead to the breakdown of the RWA, even for moderately small multilevel systems.

Microshock wave propagation in molecular clusters

Microshock wave propagation in ArN (N=55–555) clusters generated by high‐energy cluster‐Pt surface impact (cluster velocities v=1–10 km s−1) is explored by molecular dynamics simulations. The gross features of the dynamics of the intracluster microshock wave propagation at this impact velocity range are not sensitive to the details of the repulsive potential (i.e., the Lennard‐Jones or the exp‐6 form). The propagation of the microshock within the cluster was quantified by the time dependence of the first moment of the total energy. A linear dependence between the microshock (compression) velocity us and the cluster impact velocity v is observed and for sufficiently large clusters (N≥321) us≊v. For large clusters (N≳321), the cluster Hugoniot temperature–pressure relations are qualitatively similar to those for the compression of macroscopic fluid Ar.

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.

Numerical simulations of molecular multiphoton excitation models

In this paper we report the results of numerical simulations of the intramolecular dynamics of a model system for multiphoton excitation of large molecules, where the low energy range is represented by a single discrete state, while the quasicontinuum is mimicked by two or three manifolds of molecular eigenstates. The random coupling model (RCM), where the radiative coupling matrix elements are assumed to be random functions of the level indices, yields conventional rate equations describing consecutive–reversible transitions for the populations with golden rule rates. In addition, numerical simulations were conducted for a constant coupling model (CCM) and for a separable random coupling model (SRCM), confirming the counterintuitive analytical results for these model systems. The time evolution of a RCM system is determined by the distribution function of the coupling elements and not by individual coupling terms, and the intramolecular dynamics is essentially determined by the lower moments (average and...