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We study the dynamics of avalanche ionization of pure helium nanodroplets activated by a weak extreme-ultraviolet (XUV) pulse and driven by an intense near-infrared (NIR) pulse. In addition to a transient enhancement of ignition of a nanoplasma at short delay times ~200 fs, long-term activation of the nanodroplets lasting up to a few nanoseconds is observed. Molecular dynamics simulations suggest that the short-term activation is caused by the injection of seed electrons into the droplets by XUV photoemission. Long-term activation appears due to electrons remaining loosely bound to photoions which form stable `snowball' structures in the droplets. Thus, we show that XUV irradiation can induce long-lasting changes of the strong-field optical properties of nanoparticles, potentially opening new routes to controlling avalanche-ionization phenomena in nanostructures and condensed-phase systems.

We present the results of theoretical–computational studies of inter-particle energy transfer caused by elastic electron–ion collisions in expanding $$(\hbox {H}^{+})_{\mathrm {N}} +$$ electrons and $$\left( \mathrm {N}\mathrm {e}^{{8+}} \right) _{\mathrm {N}} +$$ electrons nanoplasmas, which are produced from clusters driven by near-infrared femtosecond, intense laser pulses. In these nanoplasmas, the short-range part of the electron–ion interactions is attractive for the $$\hbox {H}^{+}$$ ions and repulsive for the $$\mathrm {N}\mathrm {e}^{{8+}}$$ ions. The electron–ion collision times are ultrashort with $$\uptau = 10-20$$ as (attoseconds) for $$\left( \mathrm {N}\mathrm {e}^{\mathrm {8+}} \right) _{\mathrm {N}} +$$ electrons nanoplasmas and $$\uptau = 10$$ –50 as for ( $$\hbox {H}^{+})_{\mathrm {N}} +$$ electrons nanoplasmas. The collisional inter-particle energy transfers (CIET) are two-directional, occurring either from an electron to an ion or vice versa. The magnitude of the separate energy transfer processes depends on the kinetic energies of both colliding particles as well as on the collision geometry. While our previous studies dealt with periphery ions, the present work considers the collisional energy transfer within the entire clusters, where the relation between electron to ion and ion to electron energy transfers was found to be close to balance, providing on average a low net collisional energy transfer to ions. In the clusters’ inner cores this energy transfer presents 2–4% of the final ion energies in the $$\left( \mathrm {N}\mathrm {e}^{\mathrm {8+}} \right) _{\mathrm {N}}$$ clusters and less than 1% in the $$\left( \mathrm {H}^{+} \right) _{\mathrm {N}}$$ clusters, being lower in the clusters’ outer shells. Our findings provide strong evidence for the absence of the so called hydrodynamic expansions and for the dominance of Coulomb interactions in the explosion of the cluster nanoplasmas.

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.

Based on molecular dynamics (MD) simulations, we explore the efficiency of molecular hydrogen as a tamper material in X-ray diffraction imaging of single nanoparticles. An $$\hbox {Ar}_{55}$$ cluster serves as a model system for a nanoparticle which is embedded in a hydrogen shell of various sizes. The MD model accounts for the initial photoionization and Auger electron emission of the $$\hbox {Ar}_{55}$$ core, the secondary field and impact ionizations in the core and in the tamper shell, and the neutralization of the electron deficiency in the core by the fast migration of electrons from the tamper shell to the core. We find that a low first ionization potential is crucial for a massive electron migration and thus for the efficiency of the tamper material. Accordingly, hydrogen is a much more efficient tamper material than the isoelectronic helium, for which we have performed comparative simulations. To fully exploit the tamper effect of hydrogen, the core must be completely embedded by the tamper shell. In this context, it is encouraging that Kuma et al. (J Phys. Chem. A 115, 7392 (2011) showed that it is possible to coat a sample by hydrogen inside helium nanodroplets. While water with its even lower first ionization potential is the natural tamper material for biological samples, hydrogen could be an alternative in material science.

Background: Scientists, physicians, and the general public legitimately expect scholarly publications to give true answers to study questions raised. We investigated whether findings from studies published in journals with higher Journal Impact Factors (JIFs) are more accurate than findings from studies in less-cited journals via a meta-epidemiological approach.Methods: We screened intervention reviews from the Cochrane Database of Systematic Reviews (CDSR) and sought well-appraised meta-analyses. We defined the trueness of RCT study results as their point estimates’ relative deviation from the pooled effect estimate. In addition to the JIF we considered the open-access policy of journals, the relative size of studies, and the studies’ methodical quality.Results: In 2,459 results from 446 meta-analyses, there was a positive association between a journal’s JIF and the trueness of the study results published therein (Pearson’s r for the deviation = −0.21, 95% CI −0.24 to −0.17, P < 0.001). The mean relative deviation decreased from 1.11 (SD = 0.94) in the lowest JIF quartile to 0.80 (SD = 0.75) in the top quartile (95% CI: 0.21 to 0.40, P < 0.001). Findings from larger and methodically sound studies were also more likely to be accurate. In a multiple regression analysis, the JIF proved to be the strongest determinant of trueness, independent of the covariates.Conclusions: Our results indicate that higher-impact journals act as gatekeepers in the process of science more effectively than less-cited journals. However, the fact alone that a study result is reported in a journal with a high impact factor is only a weak and impractical indicator for its accuracy.

We present single-shot electron velocity-map images of nanoplasmas generated from doped helium nanodroplets and neon clusters by intense near-infrared and mid-infrared laser pulses. We report a large variety of signal types, most crucially depending on the cluster size. The common feature is a two-component distribution for each single-cluster event: A bright inner part with nearly circular shape corresponding to electron energies up to a few eV, surrounded by an extended background of more energetic electrons. The total counts and energy of the electrons in the inner part are strongly correlated and follow a simple power-law dependence. Deviations from the circular shape of the inner electrons observed for neon clusters and large helium nanodroplets indicate non-spherical shapes of the neutral clusters. The dependence of the measured electron energies on the extraction voltage of the spectrometer indicates that the evolution of the nanoplasma is significantly affected by the presence of an external electric field. This conjecture is confirmed by molecular dynamics simulations, which reproduce the salient features of the experimental electron spectra., Comment: 12 pages,12 figures, submitted to New Journal of Physics: Article reference: NJP-113197

The charging dynamics of helium droplets driven by embedded xenon cluster ignition in strong laser fields is studied by comparing the abundances of helium and highly charged Xe ions to the electron signal. Femtosecond pump-probe experiments show that near the optimal delay for highly charged xenon the electron yield increases, especially at low energies. The electron signature can be traced back to the ionization of the helium environment by Xe seed electrons. Accompanying molecular dynamics simulations suggest a two-step ionization scenario in the Xe-He core-shell system. In contrast to xenon, the experimental signal of the helium ions, as well as low-energy electron emission show a deviating delay dependence, indicating differences in the temporal and spacial development of the charge state distribution of Xe core and He surrounding. From the pump-probe dependence of the electron emission, effective temperatures can be extracted, indicating the nanoplasma decay.

Tabletop nuclear fusion reactions in the chemical physics laboratory can be induced by ultrafast, high-energy dynamics of multicharged nanostructures. Compelling experimental and theoretical evidence emerged for nuclear fusion driven by Coulomb explosion (NFDCE) of multicharged deuterium, containing nanostructures generated by ultraintense, femtosecond, near-infrared laser pulses. NFDCE constitutes the conversion of laser energy to nuclear energy mediated by the dynamics of molecular nanostructures. Theoretical-computational studies of tabletop laser-driven nuclear fusion of high-energy (upto 15 MeV) deuterons with 7Li, 6Li and D nuclei demonstrate the attainment of high fusion yields within a source–target reaction design. This constitutes a source of Coulomb exploding deuterium nanodroplets and a solid, hollow cylindrical target containing the second element. The fusion yields and efficiencies were maximised for the nanodroplet size and the laser parameters which accomplish optimal laser energy depositi...

Helium nanodroplets irradiated by intense near-infrared laser pulses ignite and form highly ionized nanoplasmas even at laser intensities where helium is not directly ionized by the optical field, provided the droplets contain a few dopant atoms. We present a combined theoretical and experimental study of the He nanoplasma ignition dynamics for various dopant species. We find that the efficiency of dopants to ignite a nanoplasma in helium droplets strongly varies and mostly depends on (i) the pick-up process, (ii) the number of free electrons each dopant donates upon ionization, and remarkably, (iii) by the hitherto unexplored effect of the dopant location in or on the droplet.

We present a combined experimental and theoretical study of the charging dynamics of helium nanodroplets doped with atoms of different species and irradiated by intense near-infrared (NIR) laser pulses (

The traditional description of Coulomb explosion (CE) of multicharged clusters driven by ultraintense, near-infrared, femtosecond laser fields (peak intensities of 1015−4 × 1016) W cm−2 at pulse lengths of 25−230 fs) under the initial conditions of complete and vertical outer ionization is transcended in the presence of a nanoplasma within the cluster. Unique features of the energetics of CE of many-electron elemental Xen (n = 55−6099) clusters manifest the interrelations between electron dynamics of outer ionization and nuclear CE dynamics. Molecular dynamics simulations of high-energy electrons and ions focused on the cluster size and laser parameter domains, where the contribution of electron impact inner ionization is important, the population of the persistent nanoplasma is considerable, and the effects of the nanoplasma on CE nuclear dynamics and energetics are significant. Relations were established for the cluster size and the laser parameter dependence of CE energetic attributes, that is, the ave...

Unique features of Coulomb explosion (CE) of many-electron elemental Xe(n) (n = 13-2171) clusters driven by ultraintense and ultrashort near-infrared laser pulses (peak intensities 10(15)-10(20) W cm(-2) and pulse lengths of 10-100 fs) manifest ion dynamics and energetics in the extreme, with ultrafast (5-15 A fs(-1)) velocities and ultrahigh (keV-1 MeV) energies. Relations were established between the CE attributes, obtained from molecular dynamics simulations and from electrostatic models, and the extreme cluster inner ionization levels (5-36 per ion), in conjunction with the laser parameters required for the attainment of complete outer ionization, which was approximated by cluster vertical ionization (CVI) initial conditions. Interrelationship between electron dynamics and nuclear dynamics stems from the effects of the laser pulse length on the energetics and from the characterization of the border radius for complete outer ionization. Our computational-theoretical analysis semi-quantitatively describes CE dynamics and energetics in the CVI limit and also the energetics in the presence of a persistent nanoplasma, which is in accord with experiment.

In this paper we present computational and theoretical studies of ex- treme multielectron ionization in Xen clusters (n = 55-2171, initial cluster radii R0 = 8.7-31.0 A) driven by ultraintense Gaussian infrared laser fields (peak intensity I M = 10 15 -10 20 W cm -2 , temporal pulse length τ = 10-100 fs, and frequency ν = 0.35fs -1 ). The microscopic approach, which rests on three sequential-parallel processes of in- ner ionization, nanoplasma formation, and outer ionization, properly describes the high ionization levels (with the formation of {Xe q+ }n with q = 5-36), the inner/outer cluster ionization mechanisms, and the nanoplasma response. The cluster size and laser intensity dependence of the inner ionization levels are determined by a com- plex superposition of laser-induced barrier suppression ionization (BSI), with the contributions of the inner field BSI manifesting ignition enhancement and screening retardation effects, together with electron impact ionization. The positively charged nanoplasma produced by inner ionization reveals intensity-dependent spatial inho- mogeneity and spatial anisotropy, and can be either persistent (at lower intensities) or transient (at higher intensities). The nanoplasma is depleted by outer ionization that was semiquantitatively described by the cluster barrier suppression electrostatic model, which accounts for the cluster size, laser intensity, and pulse length depen- dence of the outer ionization yield.

We present a molecular dynamics (MD) simulation code for the ex- ploration of extreme multielectron ionization and attosecond-femtosecond electron dynamics in elemental and molecular clusters driven by ultraintense (peak intensity IM = 10 15 -10 16 W cm -2 ), ultrafast (temporal pulse widths τ = 10-100 fs), near infrared (photon frequency 0.35 fs -1 ) laser fields. Validity conditions are presented for the applicability of classical MD simulations to high-energy electron dynamics, which rest on the localization of the wave packet and the distinguishability of identical particles. We also examine the cluster size domain for the applicability of the MD simulation code where the laser intensity is uniform inside the cluster.

Ultrafast cluster dynamics encompasses femtosecond nuclear dynamics, attosecond electron dynamics, and electron-nuclear dynamics in ultraintense laser fields (peak intensities 10 15 –10 20 W·cm −2 ). Extreme cluster multielectron ionization produces highly charged cluster ions, e.g., (C 4+ (D + ) 4 ) n and (D + I 22+ ) n at I M = 10 18 W·cm −2 , that undergo Coulomb explosion (CE) with the production of high-energy (5 keV to 1 MeV) ions, which can trigger nuclear reactions in an assembly of exploding clusters. The laser intensity and the cluster size dependence of the dynamics and energetics of CE of (D 2 ) n , (HT) n , (CD 4 ) n , (DI) n , (CD 3 I) n , and (CH 3 I) n clusters were explored by electrostatic models and molecular dynamics simulations, quantifying energetic driving effects, and kinematic run-over effects. The optimization of table-top dd nuclear fusion driven by CE of deuterium containing heteroclusters is realized for light-heavy heteroclusters of the largest size, which allows for the prevalence of cluster vertical ionization at the highest intensity of the laser field. We demonstrate a 7-orders-of-magnitude enhancement of the yield of dd nuclear fusion driven by CE of light-heavy heteroclusters as compared with (D 2 ) n clusters of the same size. Prospective applications for the attainment of table-top nucleosynthesis reactions, e.g., 12 C(P,γ) 13 N driven by CE of (CH 3 I) n clusters, were explored.

The ultrafast dynamics of the bimetallic cluster Ag2Au is investigated by pump-probe negative ion-to-neutral-to-positive ion (NeNePo) spectroscopy. Preparation of the neutral cluster in a highly nonequilibrium state by electron detachment from the mass-selected anion, and subsequent probing of the neutral nuclear dynamics through two-photon ionization to the cationic state, leads to strongly probe-energy-dependent transient cation-abundance signals. The origin of this pronounced time and wavelength dependence of the ionization probability on the femtosecond scale is revealed by ab initio theoretical simulations of the transient spectra. Based on the analysis of underlying dynamics, two fundamental processes involving geometry relaxation from linear to triangular structure followed by ultrafast intramolecular vibrational energy redistribution (IVR) have been identified and for the first time experimentally observed in the frame of NeNePo spectroscopy under conditions close to zero electron kinetic energy.

We present quantum-mechanical calculations for the vibrational states of anthracene⋅3HeN and anthracene⋅4HeN (N=1, 2) clusters in the ground (S0) and first excited singlet state (S1) of the anthracene molecule. The anthracene-He potential in the S0 state was described in terms of a sum of Lennard-Jones atom-atom potentials, while the potential in the S1 state also included changes in the dispersive energy and in the repulsive interactions. Variational calculations were carried out for anthracene⋅He1. For anthracene⋅He2, 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 (

Helium (He) nanodroplets irradiated by intense near-infrared laser pulses form a nanoplasma by avalanche-like electron impact ionizations even at lower laser intensities where He is not directly field ionized, provided that the droplets contain a few dopant atoms which provide seed electrons for the electron impact ionization avalanche. In this theoretical paper on calcium and xenon doped He droplets we elucidate the mechanism which induces ionization avalanches, termed ignition. We find that the partial loss of seed electrons from the activated droplets starkly assists ignition, as the Coulomb barrier for ionization of helium is lowered by the electric field of the dopant cations, and this deshielding of the cation charges enhances their electric field. In addition, the dopant ions assist the acceleration of the seed electrons (slingshot effect) by the laser field, supporting electron impact ionizations of He and also causing electron loss by catapulting electrons away. The dopants' ability to lower the Coulomb barriers at He as well as the slingshot effect decrease with the spatial expansion of the dopant, causing a dependence of the dopants' ignition capability on the dopant mass. Here, we develop criteria (impact count functions) to assess the ignition capability of dopants, based on (i) the spatial overlap of the seed electron cloud with the He atoms and (ii) the overlap of their kinetic energy distribution with the distribution of Coulomb barrier heights at He. The relatively long time delays between the instants of dopant ionization and ignition (incubation times) for calcium doped droplets are determined to a large extent by the time it takes to deshield the dopant ions.

We address recent developments in the exploration of quantum nuclear dynamics in large, finite, low-temperature systems, focusing on Bose–Einstein condensates (T≃100 nK), optical molasses (T≃10–100 μK) and aromatic molecule·Hen clusters (T = 0.4 K). The expansion of optical molasses bears a close analogy to the nuclear dynamics of Coulomb explosion of multicharged clusters. Spectroscopy of electronic–vibrational excitations of anthracene·Hen clusters manifests large nuclear quantum effects, nonrigidity, large scale confinement and permutational symmetry effects. Perspectives for the novel research area of nuclear dynamics of ultracold finite systems are advanced.

We advance an approach for the study of multistate dynamics of low Rydberg excitations in XeAr N /Xe*( 3 P 1 )Ar N /Xe + Ar N clusters on the time scale of nuclear motion, as explored by pump-probe neutral-to excited-to positive ion (NEEXPO) femtosecond spectroscopy. The dynamics of the Xe*Ar N ( N =54) cluster induced by charge expansion inferred from the calculation of NEEXPO signals at different probe energies, manifests the lifetime τ 1 ≃100 fs for the initial formation of the ‘bubble’, an intracluster vibrational energy redistribution (IVR) lifetime τ I/e ≃5.0 ps and an IVR completion time τ c ≃8–10 ps, which corresponds to the bubble equilibration.

The formation of positively charged antimony oxide clusters has been investigated as a function of oxygen partial pressure using time of flight mass spectrometry. With increasing oxygen partial pressure magic number patterns are observed, which can be attributed to the clusters of (Sb2O3)n+ and (Sb2O3)n(SbO)+ with 2⩽n

We present a theoretical study of the multistate dynamics of the Ag3 - /Ag3/Ag3 + system on the time scale of nuclear motion, as explored by femtosecond pump-probe negative ion-to neutral-to positive ion (NENEPO) spectroscopy. The dynamics of the Ag3 cluster initiated from its linear transition state involves configurational relaxation, intracluster collision, and onset of IVR, resonant and dissipative IVR and vibrational equilibration, whose time scales were determined. Our analysis reveals that theory is essential for the elucidation of the rich dynamic information regarding geometrical change, completion of IVR, and vibrational coherence effects in the NENEPO femtosecond signals.

We investigate ultrafast multi-state nuclear dynamics in a triatomic cluster. In particular, we explore how the intracluster nuclear dynamics of the Ag3−/Ag3/Ag3+ system is reflected in the femtosecond pump-probe negative ion-to neutral-to positive ion (NENEPO) signals. The nuclear dynamics is based on classical trajectories on the ground electronic adiabatic state potential hypersurfaces obtained from accurate ab initio quantum chemistry calculations. The nuclear dynamics of Ag3 initiated from the linear transition state involves distinct sequential processes of configurational relaxation to the triangular configuration, intracluster collisions, and the onset of IVR, resonant, and dissipative IVR, and vibrational equilibration. We determined the timescales for these processes and discussed their dependence on the initial cluster temperature. The Wigner representation of the density matrix was utilized to simulate the NENEPO-zero kinetic energy (NENEPO-ZEKE) signal and the total (integrated over the photo...

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 th...

Molecular dynamics simulations have been performed directly on the ab initio potential energy surface of Li4F4, which was generated within the Hartree-Fock approximation using a Gaussian basis set (split valence contraction). Trajectories at different temperatures yield the temperature dependence of the infrared spectra and the photoelectron spectra. For the infrared spectra comparison is made with MD results using a shell model ion pair potential function.

In this paper we utilize the molecular dynamics spectral density method to explore the absorption line shapes of the ‘SO - 3P~(6s[3/2]1) electronic excitation of Xe in XeArN (N = 12-206) over the temperature range T = 10-40 K, with the Xe atom being located in distinct substitutional sites of the heterocluster (at T I 30 K), or in an interior position in nonrigid and in amorphous clusters. The Xe(’So-3Pl) extravalence excitations provide a sensitive microscopic probe for the local microenvironment of the Xe atom in these elemental clusters. The electronic excitations of Xe were described by the modified tight binding scheme (Webber, S.; Jortner, J.; Rice, S. A. J. Chem. Phys. 1965,42, 1907) with reliable Xe(3P~)-Ar(1So) excited-state exp-6 pair potentials (Messing, I.; Raz, B.; Jortner, J. J. Chem. Phys. 1977, 66, 2239). The site-specific and clustersize-dependent spectroscopic observables were characterized in terms of the blue spectral shift dv of the absorption band peak, the spectral line width (FWHM) r of the band, the spectral line shape, and their temperature dependence. The simulated spectra reveal an atomic shell structure with a hierarchy of Xe occupied site-specific spectral shifts decreasing in the order &(central site) > &(interior sites) > &(substitutional surface sites) > &(top atom), providing a spectroscopic method for the interrogation of the site-specific local structure. A quantification of the site specificity and the temperature dependence of BY was obtained in terms of its (nearly size invariant) exponential dependence on the average Xe- Ar nearest-neighbor distance, with the preexponential being proportional to the number of the nearest neighbors. We provided a spectroscopic identification of three temperature induced configurational changes in XeArN clusters, Le., a center surface dynamic isomerization of Xe in XeAr12 at T > 30 K, cluster configurational dilation around the central Xe atom in XeArN (N 2 54 with T 2 13 K for N = 54 and 35 K 130) clusters allowed for the identification of the Xe-occupied site-specific excitations of the central, interior and substitutional surface sites, while no evidence was obtained for the existence of the top atom site, which is precluded by surface melting. From the simulated temperature dependence of dv for the central site in large (N = 146-200) clusters, the cluster temperature was estimated to be T = 30-35 K. For XeArl2 we were able to identify the excitations of the center and surface sites at T = 30 K. Finally, the effects of nuclear dynamics on the spectra were inferred from the analysis of the power spectra of the energy gap correlation function, which established the dominance of the stochastic slow modulation limit for the line broadening.

The LNDO/S PERTCI method for treating vertical valence electronic transitions and ionizations of chiefly organic molecules has been reparametrized for molecules containing N and O atoms and extended to systems containing F and S atoms including d AOs for sulfur. The quality of results is statistically judged on utilizing appropriate groupings of the transitions and ionizations considered.

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 t...

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 cluste...

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 Xe(n) clusters (average size (n) = 200-2171) driven by ultra-intense, near-infrared, Gaussian laser fields (peak intensities 10(15) - 4 × 10(16) 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 Xe(q+) 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.

Spectral lineshapes of the electronic origin of tetracene·Arn (n=1–3,19) clusters have been simulated by the semiclassical spectral density method [L. E. Fried, S. Mukamel, J. Chem. Phys.96, 116 (1992)]. Information is obtained concerning the spectral shifts, the homogeneous linewidths and their temperature dependence.

In this paper we report on molecular dynamics simulations of the lineshapes of the absorption spectra of perylene Arn heteroclusters (n = 2–22), which rest on the spectral density method in conjunction with the excited-state potential modelling scheme. Inhomogeneous semiclassical absorption lineshapes were calculated by averaging of microcanonical spectra over the accessible phase space region. The size dependence and isomer specificity of the spectral shifts and spectral linewidths were elucidated, with the spectral shifts providing a powerful tool for the identification of structural isomers. Some information on isomerization dynamics was inferred from the temperature dependence of the linewidths, which mark the onset of correlated surface motion for (5|0) and (5|5) heteroclusters, as well as the onset of 2D ⇄ 3D isomerization and side crossing for (22|0) heteroclusters.

RRKM theory and molecular dynamics simulations are used to evaluate the cube, ladder, ring isomerization kinetics of Na{sub 4}Cl{sub 4} clusters for individual rate coefficients.

In this paper we report on the time‐resolved dynamics of the cube → ring isomerization of the Na4Cl4 cluster, which was interrogated by constant energy molecular dynamics simulations. The isomerization was induced by several excitation modes of the nuclear motion, i.e., nonselective, bond selective, ion selective, and normal‐mode selective vibrational excitations. The nuclear excitation was conducted from a cluster equilibrated state at 600 K (total vibrational energy Ev=7930 cm−1 ) to total energies in the range Ev=10 610 cm−1 (cube temperature T=800 K) to Ev=30 730 cm−1 (T=2300 K). The reaction rates for isomerization were initially obtained from the mean first passage times for the ring formation. Concurrently, we have simulated the time evolution of the concentrations of the cube, ladder, and ring isomers by the thermal quenching method. From the time‐dependent concentrations for nonselective excitation, we have obtained the Ev dependent four rate constants for the isomerization scheme cube ⇄ ladder ⇄...

Semiempirical valence electron calculations using the CNDO/S SECI, HAM/3 Cl, LNDO/S PERTCI, MNDOC PERTCI, and MNDO/S PERTCI methods have been applied to the interpretation of the recently reported UV/vis absorption spectrum of transiento-benzyne. Detailed calculations were performed for benzene,o-benzyne, cyclopentadienylidenecarbene, cyclopentadienylideneketene, and cyclopentadienylideneketone (6-fulvenone). From these calculations and experimental data for benzene,o-benzyne, and cyclopentadienylideneketone, an interpretation of the electronic absorption spectrum ofo-benzyne is proposed. The interpretation includes the proposal that the first band and a part of the third band originally assigned too-benzyne may be due to cyclopentadienylideneketene, a common byproduct (or intermediate) of the photolyses of benzocyclobutenedione and 3-diazobenzofuranone.

We applied theoretical models and molecular dynamics simulations to explore extreme multielectron ionization in Xe(n) clusters (n=2-2171, initial cluster radius R(0)=2.16-31.0 A) driven by ultraintense infrared Gaussian laser fields (peak intensity I(M)=10(15)-10(20) W cm(-2), temporal pulse length tau=10-100 fs, and frequency nu=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 [Xe(q+)](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 (I(M)=10(18)-10(20) W cm(-2)) inner ionization is dominated by BSI. At lower intensities (I(M)=10(15)-10(16) 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 [Xe(q+)](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 Xe(2171) at tau=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 I(M)=10(15)-10(16) W cm(-2) establishes an ultraintense laser pulse length control mechanism of extreme ionization products.

This chapter addresses some of the novel physical features of the response and dynamics of homonuclear and heteronuclear clusters in ultraintense laser fields, which pertain to new mechanisms of cluster extreme multi-electron ionization, the consequences of the interaction of an ultraintense laser with a spatially confined cluster, the dramatic effects of nuclear fusion driven by cluster Coulomb explosion, the implications of Coulomb instability of highly multicharged clusters, etc. Ultraintense table top lasers are characterized by a maximal intensity of ∼10 20 Wcm –2 , which constitutes the highest light intensity on the earth. Novel features of light–matter interactions emerge from the interaction of clusters with ultrashort and ultraintense laser fields. The modem research area of femtosecond cluster dynamics on the time scale of nuclear motion is transcended by moving toward attosecond–femtosecond electron dynamics in ultraintense laser fields. The response of clusters to ultraintense lasers induces well characterized ultrafast dynamics of electrons and of ions in these large finite systems.

The time resolved dynamics of diffusionless cube → ring isomerization of the Na4Cl4 cluster was interrogated by constant energy molecular dynamics simulations, utilizing the first passage time method. The nonreactive isomerization induced by nonselective vibrational excitation is well accounted for in terms of the statistical RRK theory, opening avenues for experimental exploration of time-resolved cluster isomerization dynamics.

Bismuth oxide cluster ions are reduced by propene in exactly the same way as solid bismuth oxide (Bi2 O3 ), which is used as a catalyst. Even in the case of the smallest bismuth oxide cluster ions, such as the Bi3 O4+ isomer shown in the picture, the supposedly unreactive bridging oxygen atom can participate in the alkene oxidation.

Based on trajectory calculations of xenon clusters up to ≈6000 atoms irradiated by laser pulses (peak intensities IM = 1014–1016 W cm−2, Gaussian pulse lengths τ = 10–230 fs and frequency 0.35 fs−1), we have analyzed the interrelation between outer ionization and ion kinetic energies. The following three main categories have been identified. (A) For short pulses (τ = 10 fs) of higher intensity IM = 1016 W cm−2, the outer ionization level leads to a sufficiently high positive cluster charge, which confines the remaining nanoplasma electrons to the cluster center. In this case, ion energies can be reasonably well accounted for by a multi-charge state lychee model, according to which outer ionization is vertical and the nanoplasma can be described by a non-expanding neutral cluster interior, causing a zero-energy component in the ion kinetic energy distribution and an expanding electron-free cluster periphery. (B) For a very low outer ionization level, which is realized for short pulses of low intensity (IM = 1014 W cm−2) and/or large clusters, a slow gradual evaporation of nanoplasma electrons under laser-free conditions on the picosecond time scale is observed, making the entire outer ionization process highly non-vertical despite the short laser pulse. Accordingly, ions are accelerated only by a gradual buildup of the total cluster charge. (C) For long pulses (τ = 230 fs), the cluster expansion during the laser pulse is large and outer ionization is non-vertical. The nanoplasma electrons attain high kinetic energies by resonance heating and are distributed over the entire ion framework without a neutral cluster interior. Consequently, a zero-energy component in the ion energy distribution is missing.

The time-resolved dynamics of diffusionless cube → ring isomerization of the (NaCl)4ionic cluster was interrogated by constant energy molecular dynamics simulations utilizing the first passage time method. The isomerization was induced by several excitation modes of the nuclear motion, i.e., nonselective, bond selective, ion selective and normal mode selective vibrational excitations. The dependence of the isomerization rates on the excess vibrational energy was successfully analysed in terms of the statistical RRK theory. Concurrently, we have calculated the intracluster vibrational energy redistribution times, their dependence on the excitation mode and on the excitation energy. At high excess vibrational energies traces of the onset of mode selectivity are manifested in the nonstatistical temporal distribution of the isomerization events.

In this paper we present quantum mechanical calculations for the energetics, nuclear dynamics, spectral shifts, and vibrational level structure of anthracene⋅Hen (n=1,2) clusters in the ground (S0) and in the first spin-allowed excited (S1) electronic states. The anthracene–He potential in the S0 state was described in terms of a sum of Lennard-Jones atom–atom potentials, while the potential in the S1 state also included changes in dispersive energy and in repulsive interactions. Variational calculations were conducted for anthracene⋅He1. For anthracene⋅He2 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 S1 state and by the He–He repulsion. The anomalous...