Fragmentation channels of K-shell excited rare-gas clusters studied by multiple-ion coincidence momentum imaging

Multiple-ion coincidence momentum imaging experiments were carried out for K-shell (1s) excited Ar clusters containing about 130 atoms and Kr clusters containing about 30, 90, and 160 atoms. The time-of-flight spectra reveal that the major products of the Coulomb explosion are singly charged ions. With increasing the number of charges generated in clusters, the momentum of monomer ions such as Ar(+) and Kr(+) increases, while that of cluster ions such as Ar(3) (+), Kr(2) (+), and Kr(3) (+) decreases. This observation indicates the site-specific decay process that the heavier ions appear in the central part of clusters. We have also investigated the momentum distribution in various fragmentation channels and the branching ratio of each channel at the Coulomb explosion. When the number N(coin) of coincidently detected ions is four, for example, the most frequent channel from Kr clusters containing 30 atoms is to emit simply four Kr(+) ions, but Kr(2) (+) ions participate in the fragmentation from the larger Kr clusters. The fragmentation channel in which two Ar(2) (+) ions are emitted becomes dominant with increasing N(coin), and the average momentum of Ar(2) (+) ion in this channel is larger than that in the channels where only single Ar(2) (+) is emitted.     

The far from equilibrium structure of argon clusters doped with krypton or xenon

Heterogeneous clusters created by doping Ar host clusters with Kr or Xe are shown to have radically different structures from the mixed clusters of the same type created by co-expansion of Ar-Kr or Ar-Xe gas mixtures. In contrast to the co-expansion case, the doped mixed clusters can be produced with Kr or Xe on the surface and Ar in the bulk. With the doping technique it is thus possible to control the surface composition of a specific cluster. A study of the cluster properties as a function of the doping pressure is also reported for the case of Ar clusters doped with Xe. The clusters have been studied by means of synchrotron radiation based X-ray photoelectron spectroscopy.     

Study of coulomb explosion and dissociation channels in dicationic argon clusters: a study based on stochastic optimization

In this article, we explore the use of stochastic optimization technique, namely simulated annealing, in elucidating the correct cut-off limit for suppression of Coulomb explosion in dicationic argon gas clusters. We also do a detail study of cluster sizes where the clusters are not stable as one single entity, and try to find out the dissociation channels for these, namely fission or non-fission type. We compare our results with available literature results, both theoretical and experimental.     

Fragmentation channels of large multicharged clusters

We address unifying features of fragmentation channels driven by long-range Coulomb or pseudo-Coulomb forces in clusters, nuclei, droplets, and optical molasses. We studied the energetics, fragmentation patterns, and dynamics of multicharged (A+)n (n=55, 135, 321) clusters. In Morse clusters the variation of the range of the pair-potential induced changes in the cluster surface energy and in the fissibility parameter X=E(Coulomb)2E(surface). X was varied in the range of X=1-8 for short-range interactions and of X=0.1-1.0 for long-range interactions. Metastable cluster configurations were prepared by vertical ionization of the neutral clusters and by subsequent structural equilibration. The energetics of these metastable ionic clusters was described in terms of the liquid drop model, with the coefficients of the volume and surface energies depending linearly on the Morse band dissociation energy. Molecular-dynamics simulations established two distinct fragmentation patterns of multicharged clusters that involve cluster fission into a small number of large, multicharged clusters for X<1 and Coulomb explosion into a large number of individual ions and small ionic fragments for X>1. The Rayleigh instability limit X=1 separates between spatially anisotropic fission and spatially isotropic Coulomb explosion. Distinct features of the fragmentation energetics and dynamics were unveiled. For fission of n=55 clusters, large kinetic and internal energies of the large fragments are exhibited and the characteristic fragmentation time is approximately 700 fs, while for Coulomb explosion the major energy content of the small fragments involves kinetic energy and the characteristic fragmentation time of approximately 300 fs is shorter. The Rayleigh (X=1) limit, leading to isotropic Coulomb explosion, is transcended by a marked enhancement of the Coulomb energy, which is realized for extremely ionized clusters in ultraintense laser fields, or by a dramatic reduction of the surface energy as is the case for the expansion of optical molasses.     

Optimal covering of C(60) fullerene by rare gases

Putative global energy minima of clusters formed by the adsorption of rare gases on a C(60) fullerene molecule, C(60)X(N) (X=Ne, Ar, Kr, Xe; N ≤ 70), are found using basin-hopping global optimization in an empirical potential energy surface. The association energies per rare gas atom as a function of N present two noticeable minima for Ne and Ar and just one for Kr and Xe. The minimum with the smallest N is the deepest one and corresponds to an optimal packing monolayer structure; the other one gives a monolayer with maximum packing. For Kr and Xe, optimal and maximum packing structures coincide. By using an isotropic average form of the X-C(60) interaction, we have established the relevance of the C(60) surface corrugation on the cluster structures. Quantum effects are relevant for Ne clusters. The adsorption of these rare gases on C(60) follows patterns that differ significantly from the ones found recently for He by means of experimental and theoretical methods.     

Noble gas-transition-metal complexes: coordination of VO2 and VO4 by Ar and Xe atoms in solid noble gas matrixes

The matrix isolation infrared spectroscopic and quantum chemical calculation results indicate that vanadium oxides, VO2 and VO4, coordinate noble gas atoms in forming noble gas complexes. The results showed that VO2 coordinates two Ar or Xe atoms and that VO4 coordinates one Ar or Xe atom in solid noble gas matrixes. Hence, the VO2 and VO4 molecules trapped in solid noble gas matrixes should be regarded as the VO2(Ng)2 and VO4(Ng) (Ng = Ar or Xe) complexes. The total V-Ng binding energies were predicted to be 12.8, 18.2, 5.0, and 7.3 kcal/mol, respectively, for the VO2(Ar)2, VO2(Xe)2, VO4(Ar), and VO4(Xe) complexes at the CCSD(T)//B3LYP level of theory.     

The size of neutral free clusters as manifested in the relative bulk-to-surface intensity in core level photoelectron spectroscopy

A new approach for obtaining an estimate of the effective size of the free neutral clusters is proposed. The approach relies on an experimental measure of the surface and interior or "bulk" cluster atoms provided by the x-ray photoelectron spectroscopy and on a model for the attenuation of photoelectrons ejected from the bulk of the cluster as the result of the ionizing irradiation. The experimental part gives the ratio of the electron signal from the bulk cluster atoms to that from the cluster surface atoms for a wide range of cluster sizes and electron kinetic energies. The attenuated response of the bulk atoms is modeled using an exponential law with the cluster size and kinetic-energy-dependent electron escape depth as parameters. For the experimental size range, model-based calculations for Ar, Kr, and Xe clusters are presented. The cluster size estimates obtained from comparison of the model calculations and experimental results agree well with those determined from the parameters of the cluster creation process. The combination of experiment and modeling also makes it possible to estimate the effective escape depth for electron propagation in free clusters. For Ar, Kr, and Xe clusters of varying mean size, absolute determination of the surface and bulk electron binding energies of the core levels used in the experiments has also been made.     

Long‐range interactions in embedded ionic cluster calculations

The effect of long‐range Coulomb interactions on bulk properties is studied for the ionic solids NaBr and NaCl. The embedded cluster approach in the framework of density functional theory is employed. The Madelung potential is calculated with the Evjen cube summation method. To explore the effects of the long‐range interactions on the electron densities and the Madelung constant, the Evjen cube size is varied from 310 to 19650 point charges for 33 atom clusters. To study the size effect of the quantum region, all‐electron clusters with 33 to 87 atoms, embedded in Evjen cubes of 6859 point charges, are investigated. The results show that for the 81 and 87 atom clusters the Madelung potential is constant from the center up to the second neighbor shell. For the same atoms, in all clusters, the electron density at the nuclei has nearly the same value. The largest difference found for the positive ions was 0.54%, and for the negative ions, 0.14%. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Theoretical prediction of noble-gas compounds: Ng-Pd-Ng and Ng-Pt-Ng

Following our recent study on Ng-Pt-Ng (Ng=Ar,Kr,Xe) [J. Chem. Phys. 123, 204321 (2005)], the binding of noble-gas atoms with Pd atom has been investigated by the ab initio coupled cluster CCSD(T) method with counterpoise corrections, including relativistic effects. It is shown that two Ng atoms bind with Pd atom in linear geometry due to the s-d(sigma) hybridization in Pd where the second Ng atom attaches with much larger binding energy than the first. The binding energies are evaluated as 4.0, 10.2, and 21.5 kcalmol for Ar-Pd-Ar, Kr-Pd-Kr, and Xe-Pd-Xe, respectively, relative to the dissociation limit, Pd ((1)S)+2Ng. In the hybrid Ng complexes, the binding energies for XePd and Ng (=Ar,Kr) are evaluated as 4.0 and 6.9 kcalmol for XePd-Ar and XePd-Kr, respectively. The fundamental frequencies and low-lying vibrational-rotational energy levels are determined for each compound by the variational method, based on the three-dimensional near-equilibrium potential energy surface. Results of vibrational-rotational analyses for Ng-Pt-Ng (Ng=Ar,Kr,Xe) and Xe-Pt-Ng (Ng=He,Ne,Ar,Kr) compounds are also given.     

Electron and nuclear dynamics of molecular clusters in ultraintense laser fields. IV. Coulomb explosion of molecular heteroclusters

In this paper we present a theoretical and computational study of the temporal dynamics and energetics of Coulomb explosion of (CD4)(n) and (CH4)(n) (n=55-4213) molecular heteroclusters in ultraintense (I=10(16)-10(19) W cm(-2)) laser fields, addressing the manifestation of electron dynamics, together with nuclear energetic and kinematic effects on the heterocluster Coulomb instability. The manifestations of the coupling between electron and nuclear dynamics were explored by molecular dynamics simulations for these heteroclusters coupled to Gaussian laser fields (pulse width tau=25 fs), elucidating outer ionization dynamics, nanoplasma screening effects (being significant for I< or =10(17) W cm(-2)), and the attainment of cluster vertical ionization (CVI) (at I=10(17) W cm(-2) for cluster radius R(0)< or =31 A). Nuclear kinematic effects on heterocluster Coulomb explosion are governed by the kinematic parameter eta=q(C)m(A)/q(A)m(C) for (CA(4))(n) clusters (A=H,D), where q(j) and m(j) (j=A,C) are the ionic charges and masses. Nonuniform heterocluster Coulomb explosion (eta >1) manifests an overrun effect of the light ions relative to the heavy ions, exhibiting the expansion of two spatially separated subclusters, with the light ions forming the outer subcluster at the outer edge of the spatial distribution. Important features of the energetics of heterocluster Coulomb explosion originate from energetic triggering effects of the driving of the light ions by the heavy ions (C(4+) for I=10(17)-10(18) W cm(-2) and C(6+) for I=10(19) W cm(-2)), as well as for kinematic effects. Based on the CVI assumption, scaling laws for the cluster size (radius R(0)) dependence of the energetics of uniform Coulomb explosion of heteroclusters (eta=1) were derived, with the size dependence of the average (E(j,av)) and maximal (E(j,M)) ion energies being E(j,av)=aR(0) (2) and E(j,M)=(5a/3)R(0) (2), as well as for the ion energy distributions P(E(j)) proportional to E(j) (1/2); E(j)< or =E(j,M). These results for uniform Coulomb explosion serve as benchmark reference data for the assessment of the effects of nonuniform explosion, where the CVI scaling law for the energetics still holds, with deviations of the a coefficient, which increase with increasing eta. Kinematic effects (for eta>1) result in an isotope effect, predicting the enhancement (by 9%-11%) of E(H,av) for Coulomb explosion of (C(4+)H(4) (+))(eta) (eta=3) relative to E(D,av) for Coulomb explosion of (C(4+)D(4) (+))(eta) (eta=1.5), with the isotope effect being determined by the ratio of the kinematic parameters for the pair of Coulomb exploding clusters. Kinematic effects for nonuniform explosion also result in a narrow isotope dependent energy distribution (of width DeltaE) of the light ions (with DeltaE/E(H,av) approximately 0.3 and DeltaE/E(D,av) approximately 0.4), with the distribution peaking at the high energy edge, in marked contrast with the uniform explosion case. Features of laser-heterocluster interactions were inferred from the analyses of the intensity dependent boundary radii (R(0))(I) and the corresponding average D+ ion energies (E(D,av))(I), which provide a measure for optimization of the cluster size at intensity I for the neutron yield from dd nuclear fusion driven by Coulomb explosion (NFDCE) of these heteroclusters. We infer on the advantage of deuterium containing heteronuclear clusters, e.g., (CD4)(n) in comparison to homonuclear clusters, e.g., (D2)(n/2), for dd NFDCE, where the highly charged heavy ions (e.g., C4+ or C6+) serve as energetic and kinematic triggers driving the D+ ions to a high (10-200 keV) energy domain.     

Structures of small mixed krypton-xenon clusters

Structures of small mixed krypton-xenon clusters of different compositions with an average size of 30-37 atoms are investigated. The Kr 3d(5/2) and Xe 4d(5/2) surface core level shifts and photoelectron intensities originating from corner, edge, and face/bulk sites are analyzed by using soft x-ray photoelectron spectroscopy. Structural models are derived from these experiments, which are confirmed by theoretical simulation taking induced dipole interactions into account. It is found that one or two small Xe cores are partly embedded in the surface of the Kr clusters. These may grow and merge leading to a phase separation between the two rare gas moieties in mixed clusters with increasing the Xe content.     

Geometric and energetic properties of Al-doped Sin (n = 2–21) clusters: FP-LMTO-MD calculations

We have investigated the effect of aluminum impurity atoms on the geometric structures and stabilities of neutral and ionic Si_n (n = 2–21) clusters in detail by using full-potential linear-muffin-tin-orbital molecular-dynamics (FP-LMTO-MD) method. Our calculations suggest that most of the ground state structures for neutral and ionic Si_nAl (n = 1–20) clusters can be obtained by substituting one Si atom of their corresponding Si clusters with an Al atom. The neutral Si_n–1Al clusters with one Al atom have similar geometrical configurations to those of the pure Si_n clusters except for local structural distortion. But one Al impurity atom probably reverses the energy ordering of two isomers with small difference. Although, an Al heteroatom reduces the average binding energies for the mixed clusters, it would improve the bond strength between Si atoms in some mixed clusters. Our calculations also suggest that most of the ionic Si_n–1Al clusters adopt the same geometrical configurations as their neutral clusters. But for one selected mixed cluster, the charged structures probably have different energy ordering from the neutral clusters. The anionic Si_n–1Al⁻ clusters, which are isoelectronic to their corresponding pure Si_n clusters, show similar magic behavior.     

Cyclic Octamer of Hydroxyl-functionalized Cations with Net Charge Q=+8e Kinetically Stabilized by a ‘Molecular Island’ of Cooperative Hydrogen Bonds

Cyclic octamers are well-known structural motifs in chemistry, biology and physics. These include covalently bound cyclic octameric sulphur, cylic octa-alkanes, cyclo-octameric peptides as well as hydrogen-bonded ring clusters of alcohols. In this work, we show that even calculated cyclic octamers of hydroxy-functionalized pyridinium cations with a net charge Q=+8e are kinetically stable. Eight positively charged cations are kept together by hydrogen bonding despite the strong Coulomb repulsive forces. Sufficiently long hydroxy-octyl chains prevent “Coulomb explosion” by increasing the distance between the positive charges at the pyridinium rings, reducing the Coulomb repulsion and thus strengthen hydrogen bonds between the OH groups. The eightfold positively charged cyclic octamer shows spectroscopic properties similar to those obtained for hydrogen-bonded neutral cyclic octamers of methanol. Thus, the area of the hydrogen bonded OH ring represents a ‘molecular island’ within an overall cationic environment. Although not observable, the spectroscopic properties and the correlated NBO parameters of the calculated cationic octamer support the detection of smaller cationic clusters in ionic liquids, which we observed despite the competition with ion pairs wherein attractive Coulomb forces enhance hydrogen bonding between cation and anion.     

A molecular dynamics simulation of rebound,capture and diffusion in collisions at thermal energies between a rare gas atom and an argon cluster (n=125)

Molecular dynamics calculations have been performed to simulate the low energy collision (0.2 eV) of a rare gas atom (He, Ar, Xe) with a cluster of 125 argon atoms. Depending on its relative mass to argon, the projectile is either deflected (He) or captured (Ar, Xe) by the argon cluster. We have determined the deflection function of the He projectile that is scattered, and for Xe we have determined wether it stays near the surface of the cluster or migrates inside. These results have been discussed in the light of very simple models.     

Wavelength-dependent Coulomb explosion in carbon disulphide (CS2) clusters: generation of energetic multiply charged carbon and sulphur ions

A gigawatt laser-induced Coulomb explosion has been observed in carbon disulphide (CS(2)) clusters generating energetic, multiply charged [C](m+) (m = 1-4) and [S](n+) (n = 1-6) atomic ions of carbon and sulphur. The Coulomb explosion shows wavelength dependence. Comparison of these results with our earlier work shows that the polarizability and dipole moment might help in energy absorption from the laser field but they are not mandatory conditions for this low-intensity Coulomb explosion. The results show that in a field of 10(9) W/cm(2), absorption of 266 and 355 nm laser radiation by CS(2) clusters leads to multiphoton dissociation/ionization whereas at 532 nm the whole cluster explodes generating multiply charged atomic ions.     

Electronic spectroscopy of toluene-rare-gas clusters: the external heavy atom effect and vibrational predissociation

Toluene-X van der Waals clusters (where X = Ne, Ne2, Ar, Ar2, Kr, Xe) have been investigated by fluorescence excitation spectroscopy in the region of the S1-S0 transition. With the exception of Xe, for each rare-gas studied, we have assigned cluster transitions in the region of all the strong monomer vibrational bands up to 1000 cm(-1) above the origin band. We have further investigated the S1 relaxation dynamics for each vibrational level of each complex, via their fluorescence decay profiles. Clustering with neon has little appreciable effect on the vibrationless S1 lifetime. By contrast, the clusters with argon and krypton exhibit markedly shorter fluorescence lifetimes compared with the monomer. The effect is so severe in the case of toluene-Xe clusters that no fluorescence signals were observed. We interpret these results in terms of an external heavy atom effect in which the rate of intersystem crossing in toluene is influenced by the cluster partner. For clusters built upon excited S1 vibrational levels, the situation is potentially complicated by intramolecular vibrational redistribution and vibrational predissociation (VP). The majority of the fluorescence decay profiles were satisfactorily modeled using single exponential decays. The emission following pumping of the 37(1) level in the toluene-Kr cluster, however, is an exception. We have modeled the decay of this level with a simple kinetic scheme including VP and determined a predissociation rate of (1.04 +/- 0.54) x 10(7) s(-1).     

Phase changes in Lennard-Jones mixed clusters with composition Ar(n)Xe(6-n) (n=0,1,2)

We have carried out parallel tempering Monte Carlo calculations on the binary six-atom mixed Lennard-Jones clusters, Ar(n)Xe(6-n) (n=0,1,2). We have looked at the classical configurational heat capacity C(V)(T) as a probe of phase behavior. All three clusters show a feature in the heat capacity in the region of 15-20 K. The Ar(2)Xe(4) cluster exhibits a further peak in the heat capacity near 7 K. We have also investigated dynamical properties of the Ar(2)Xe(4) cluster as a function of temperature using molecular dynamics. We report the interbasin isomerization rate and the bond fluctuation parameter obtained from these calculations. At 7 K, the isomerization rate is on the order of 0.01 ns(-1); at 20 K, the isomerization rate is greater than 10 ns(-1). Furthermore, at 7 K, the bond fluctuation parameter is less than 3%; at 20 K, it is in the range of 10-15% (depending on the sampling time used). Using this information, together with Monte Carlo quenching data, we assign the 15-20 K feature in the heat capacity to a solid-liquid phase change and the 7-K peak to a solid-solid phase change. We believe this is the smallest Lennard-Jones cluster system yet shown to exhibit solid-solid phase change behavior.     

The effect of two gases forming supercritical fluids (Xe and CO2) on the spectral characteristics and analytical capabilities of microwave induced plasmas

A comparative study of the effect of CO₂ and Xe added along with the plasma gas to He and Ar microwave induced plasmas (MIPs), simulating possible conditions to be used when a MIP is employed as specific detector for supercritical fluid chromatography (SFC), has been carried out. The proportions of CO₂ and Xe to the plasma gas investigated are comparable to the typical percentages used for SFC-MIP couplings. The study has been performed with two different MIP systems: an atmospheric pressure discharge held in a Beenakker cavity TM₀₁₀ and a reduced pressure surfatron-MIP.The influence of CO₂ and Xe addition on the spectrochemical properties of the discharge has been studied by using the atomic emission of mercury and some typical non-metals (chlorine, carbon and sulfur) at different wavelengths (atomic and ionic lines). Results showed that ion line emission intensities are always reduced more significantly than atom line emissions by both dopant gases on study, whatever the pressure. In general terms, however, the effect of adding Xe is less severe, both for atom and ion lines, than that of CO₂; in most cases the detection limits (DLs) observed are better for Xe than for CO₂ as dopant gas. In fact, the DLs obtained for the selected lines of mercury measured were practically unaltered by the addition of 0.2% Xe to atmospheric pressure Ar or He MIPs. CO₂ addition (0.2%) produced about 1.5 times worsening of the observed DLs for mercury. For non-metal analyses better DLs were also obtained, in general terms, with Xe than with CO₂ as dopant gas.