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Femtosecond pump-probe negative ion-to-neutral-to-positive ion spectroscopy was used in a theoretical analysis of the multistate dynamics of the Ag(sub 3)(super -)/Ag(sub 3)/Ag(sub 3)(super +) system on the time scale of nuclear motion. Configurational relaxation, intracluster collision, and onset of intracluster vibrational relaxation (IVR), resonant and dissipative IVR and vibrational equilibration were found to be involved in the dynamics of the Ag(sub 3) cluster. Theory was revealed as essential for clarifying the abundant dynamic information on geometrical change, completion of IVR, and vibrational coherence effects.

We present a theoretical-computational study of the formation, structure, composition, energetics, dynamics and expansion of nanoplasmas consisting of high-energy matter on the nanoscale of ions and electrons. Molecular dynamics simulations explored the structure and energetics of hydrogen and neon persistent nanoplasmas formed under the condition of incomplete outer ionization by the laser field. We observed a marked microscopic inhomogeneity of the structure and the charge distribution of exploding nanoplasmas on the nanoscale. This is characterized by a nearly neutral, uniform, interior domain observed for the first time, and a highly positively charged, exterior domain, with these two domains being separated by a transition domain. We established the universality of the general features of the shape of the charge distribution, as well as of the energetics and dynamics of individual ions in expanding persistent nanoplasmas containing different positive ions. The inhomogeneous three-domain shell structure of exploding nanoplasmas exerts major effects on the local ion energies, which are larger by one order of magnitude in the exterior, electron-depleted domain than in the interior, electron-rich domain, with the major contribution to the ion energies originating from electrostatic interactions. The radial structural inhomogeneity of exploding nanoplasmas bears analogy to the inhomogeneous transport regime in expanded and supercritical metals undergoing metal-nonmetal transition. [ABSTRACT FROM AUTHOR]

We report on the calculations of kinetic energy distribution (KED) functions of multiply charged, high-energy ions in Coulomb explosion (CE) of an assembly of elemental Xen clusters (average size = 200-2171) driven by ultra-intense, near-infrared, Gaussian laser fields (peak intensities 1015 - 4 × 1016 W cm-2, pulse lengths 65-230 fs). In this cluster size and pulse parameter domain, outer ionization is incomplete/vertical, incomplete/nonvertical, or complete/nonvertical, with CE occurring in the presence of nanoplasma electrons. The KEDs were obtained from double averaging of single-trajectory molecular dynamics simulation ion kinetic energies. The KEDs were doubly averaged over a log-normal cluster size distribution and over the laser intensity distribution of a spatial Gaussian beam, which constitutes either a two-dimensional (2D) or a three-dimensional (3D) profile, with the 3D profile (when the cluster beam radius is larger than the Rayleigh length) usually being experimentally realized. The general features of the doubly averaged KEDs manifest the smearing out of the structure corresponding to the distribution of ion charges, a marked increase of the KEDs at very low energies due to the contribution from the persistent nanoplasma, a distortion of the KEDs and of the average energies toward lower energy values, and the appearance of long low-intensity high-energy tails caused by the admixture of contributions from large clusters by size averaging. The doubly averaged simulation results account reasonably well (within 30%) for the experimental data for the cluster-size dependence of the CE energetics and for its dependence on the laser pulse parameters, as well as for the anisotropy in the angular distribution of the energies of the Xeq+ ions. Possible applications of this computational study include a control of the ion kinetic energies by the choice of the laser intensity profile (2D/3D) in the laser-cluster interaction volume. [ABSTRACT FROM AUTHOR]

We applied theoretical models and molecular dynamics simulations to explore extreme multielectron ionization in Xen clusters (n=2–2171, initial cluster radius R0=2.16–31.0 Å) driven by ultraintense infrared Gaussian laser fields (peak intensity IM=1015–1020 W cm-2, temporal pulse length τ=10–100 fs, and frequency ν=0.35 fs-1). Cluster compound ionization was described by three processes of inner ionization, nanoplasma formation, and outer ionization. Inner ionization gives rise to high ionization levels (with the formation of {Xeq+}n with q=2–36), which are amenable to experimental observation. The cluster size and laser intensity dependence of the inner ionization levels are induced by a superposition of barrier suppression ionization (BSI) and electron impact ionization (EII). The BSI was induced by a composite field involving the laser field and an inner field of the ions and electrons, which manifests ignition enhancement and screening retardation effects. EII was treated using experimental cross sections, with a proper account of sequential impact ionization. At the highest intensities (IM=1018–1020 W cm-2) inner ionization is dominated by BSI. At lower intensities (IM=1015–1016 W cm-2), where the nanoplasma is persistent, the EII contribution to the inner ionization yield is substantial. It increases with increasing the cluster size, exerts a marked effect on the increase of the {Xeq+}n ionization level, is most pronounced in the cluster center, and manifests a marked increase with increasing the pulse length (i.e., becoming the dominant ionization channel (56%) for Xe2171 at τ=100 fs). The EII yield and the ionization level enhancement decrease with increasing the laser intensity. The pulse length dependence of the EII yield at IM=1015–1016 W cm-2 establishes an ultraintense laser pulse length control mechanism of extreme ionization products. [ABSTRACT FROM AUTHOR]

We present quantum-mechanical calculations for the vibrational states of anthracene· [SUP3]He[SUBN] and anthracene· [SUP4]He[SUBN] (N = 1, 2) clusters in the ground (S[SUB0]) and first excited singlet state (S[SUB1]) of the anthracene molecule. The anthracene-He potential in the S[SUB0] state was described in terms of a sum of Lennard-Jones atom-atom potentials, while the potential in the S[SUB1] state also included changes in the dispersive energy and in the repulsive interactions. Variational calculations were carried out for anthracene· He[SUB1]. For anthracene· He[SUB2], configuration interaction calculations were performed, accounting for the boson and fermion permutation symmetry. For both helium isotopes of the N = 1 cluster, tunneling splitting is negligible (<0.01 cm[SUP-1]), as an appreciable interaction of the densities was only found for highly excited states above the potential-energy barrier of side crossing (for energy eigenvalues &ges;-22 cm[SUP-1] below the dissociation limit). The two-boson anthracene· [SUP4]He[SUB2] system assumes a singlet [SUP1]A[SUB1] ground state due to zero spin of the [SUP4]He isotope. Because of the dominance of the two-particle over the one-particle interactions, the two-fermion anthracene· [SUP3]He[SUB2] system has a triplet ([SUP3]B[SUB2]) vibrational ground state. The singlet-triplet (1 [SUP3]B[SUB2]-1 [SUP1]B[SUB2]) splitting between the two lowest states of the same spatial symmetry of anthracene· [SUP3]He[SUB2] was calculated to be 10.5 cm[SUP-1]. Mass and permutation symmetry effects on the vibrational level structure of anthracene· He[SUB1] and anthracene· He[SUB2] were explored for anthracene· [SUP4]He[SUB1], anthracene· [SUP3]He[SUB1], the two-boson system anthracene· [SUP4]He[SUB2], the two-fermion system anthracene· [SUP3]He[SUB2] and for the hypothetical fermion system of mass 4. While the isotope effect on the... [ABSTRACT FROM AUTHOR]

In this paper we present quantum mechanical calculations for the energetics, nuclear dynamics, spectral shifts, and vibrational level structure of anthracene·He[sub n] (n=1,2) clusters in the ground (S[sub 0]) and in the first spin-allowed excited (S[sub 1]) electronic states. The anthracene–He potential in the S[sub 0] state was described in terms of a sum of Lennard-Jones atom–atom potentials, while the potential in the S[sub 1] state also included changes in dispersive energy and in repulsive interactions. Variational calculations were conducted for anthracene·He[sub 1]. For anthracene·He[sub 2] we carried out configuration interaction calculations with the wave functions consisting of Hartree products, accounting for boson permutation symmetry. Extensive, anisotropic, one-dimensional spatial delocalization of the He atoms on the anthracene microsurface, which originates from large-scale confinement by the aromatic molecule, is exhibited, being further enhanced by repulsive interactions in the S[sub 1] state and by the He–He repulsion. The anomalous size-dependence of the (red) spectral shifts for the S[sub 0]→S[sub 1] electronic origin arises from mutually canceling dispersive and repulsive contributions which, together with the electronic-vibrational level structure, manifest quantum effects of anisotropic spatial delocalization, confinement and He–He interaction in nonrigid clusters. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We analyze spectral absorption line shapes simulated using the molecular dynamics spectral density method. We explore three classes of line shapes: (1) the region of the 0–0 S0→S1(ππ*) transition of perylene·ArN clusters, (2) the Xe1S0→3P1 transition of XeArN clusters, and (3) the photoelectron spectrum of the Li4F4 cluster in the valence region. These spectra represent examples for weak, unresolved, and extensive vibrational progressions, which have been analyzed and assigned. Employing a simplified model for the energy gap autocorrelation function allows for an understanding of the different behaviors and for a classification of the interrelation between nuclear dynamics and spectral line shapes. With decreasing the characteristic decay time of the transition dipole autocorrelation function, the line shape passes the limiting cases of the model in the order fast modulation limit→vibrational progression limit→slow modulation limit, with the vibrational progression limit extending the limiting cases of the Kubo stochastic model of line shapes. Some simple qualitative rules have been extracted to predict the overall character of a line shape. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this paper we report on the electronic two-photon two-color near threshold spectroscopy of mass-resolved perylene·Arn (n=1–45), perylene·Krn (n=1–35), perylene·(N2)n (n=1–12), and perylene·(CH4)n (n=1–10) heteroclusters. The S0→S1 inhomogeneously broadened spectra of perylene·Arn (n=1–6) and perylene·Krn (n=1–4) exhibit resolved spectral features, which were assigned on the basis of experimental combination rules and polarizability relations to the electronic origins of distinct two-sided and one-sided structural isomers. Larger perylene·An (A=Ar, Kr; n=6–10) heteroclusters exhibit an ‘‘abnormal’’ specific size dependence of the red spectral shifts, which decrease with increasing n and reach a local minimum at n=8. Similar characteristics of the red spectral shifts are exhibited for perylene·(N2)n and perylene·(CH4)n (n=4–8) heteroclusters. This abnormal size dependence of the spectral shifts is attributed to the dominance of one-sided single-layered and double-layered structural isomers in this cluster size domain. On the basis of the comparison between the spectroscopic data and molecular dynamics simulations of the absorption line shapes we have obtained a quantitative description of isomer-specific structures for n=2–6, a semiquantitative description of the abnormal size domain for n=6–10 (due to the dominance of one-sided structures with the abundance of two-layered structures increasing at higher n), the prevalence of one-sided structures for n=16 and n=22, and the realization of two-sided multilayered structures at n=45. [ABSTRACT FROM AUTHOR]

In this paper we report on molecular dynamics simulations of the line shapes of the absorption spectra of perylene·Arn heteroclusters (n=1–45), which rest on the spectral density method. Inhomogeneous semiclassical absorption line shapes were calculated by averaging of microcanonical spectra over the accessible phase space region. We utilized a potential modeling scheme for the excited-state atom–atom potentials, which accounts well for the spectral shifts of small (n=1–6) clusters. The size dependence and the isomer specificity of the spectral shifts and spectral linewidths were elucidated. The analysis of the power spectra of the energy gap correlation function established that the dominating contribution to the line shape and line broadening of the electronic origin at finite temperatures (T≥10 K) originates from the low-frequency diffusive soft mode. For these heteroclusters the soft mode contribution can be described within the stochastic slow modulation limit, being determined by the dispersion of the soft mode and being independent of its damping. Finite-frequency intermolecular motion gives only a minor (≤10%) contribution to the line broadening. Our analysis interrelates spectral line broadening and nuclear dynamics. [ABSTRACT FROM AUTHOR]