Van Der Waals Clusters Research Articles

Laser-induced fluorescence spectroscopy of the Ga–N2 cluster

The first spectroscopic observation of the Ga–N2 van der Waals cluster is reported. This was formed by laser ablation of a GaAs or GaP target followed by pulsed supersonic expansion in pure nitrogen. Laser-induced fluorescence spectra have revealed two strong band systems above 30000 cm-1. The first, which has an onset at 33468 cm-1, is composed of eight observable members of a progression in the van der Waals stretching vibration. This has been assigned to the 2Δ–2Π3/2 transition correlating with the Ga 442Dâ†�442P3/2 transition. A Birge–Sponer extrapolation gives a lower limit of 1270 cm-1 for the van der Waals binding energy in the excited state. At higher wavenumbers another prominent vibrational progression is observed which is attributed to a spin-forbidden transition correlating with the 444Pâ†�442P atomic asymptote, the cluster excited state most likely having 4Σ- symmetry. The excited state undergoes relatively slow spin–orbit-induced predissociation onto the repulsive 2Σ+ potential surface followed by rapid emission from the Ga 452S dissociation product.

Energy release in benzene–argon cluster dissociation – quasiclassical trajectory calculations

Abstract Results of quasiclassical trajectory calculations of the dissociation of a highly excited argon–benzene van der Waals cluster are reported. Two intermolecular potentials were used. The average energy removed from the excited benzene molecule, 〈ΔE〉, is −886 cm−1 for Lennard-Jones potential and −436 cm−1 for ab initio potential. The average trajectory lifetimes are 36.3 and 80.4 ps, respectively. The ΔE distribution shows a supercollision tail. No correlation was found between the lifetime of the cluster and the value of ΔE. The effects of the nature of the potential on the values of 〈ΔE〉 and on the long lifetime of the cluster are discussed.

On the use of evaporation dynamics to characterize phase transitions in van der Waals clusters: investigations in aniline–(argon)n up to n=15

The evaporation process in small van der Waals aniline–(argon)n clusters (with n=3, 4, 5, 8 and 15) has been investigated by molecular dynamics (MD) simulations. MD results have been compared to three statistical theories: classical RRK, the Engelking model and phase space theory (PST). The latter gives results in good agreement with the results derived from classical trajectories. The sensitivity of the average kinetic energy release to the thermodynamic phase of the product sub-cluster is put into evidence for small aniline–Arn clusters. This finding opens new perspectives on the experimental characterization of phase transitions in atomic or molecular clusters. A complete study of the final populations in the different product isomers has been performed by MD simulations and compared to an extended phase space theory (EPST) in which the densities of states of individual isomers are explicitly taken into account. Finally the use of PST in the low energy regime has allowed to address the question of the cluster temperature resulting from evaporation down to time windows in the microsecond domain of experimental interest. The thermodynamic state of the cluster is predicted to change at a critical size.

Vibrational predissociation of the I2⋯Ne2 cluster: A molecular dynamics with quantum transitions study

The MDQT (molecular dynamics with quantum transitions) method of Tully is applied to the vibrational predissociation of a Van der Waals cluster containing a diatomic molecule and two rare gas atoms, I2⋯Ne2. The vibrational degree of freedom of the diatomic is treated quantum mechanically using DVR (discrete variable representation) while all the other degrees of freedom are treated classically. The results are in very good agreement with the experimentally measured lifetimes and product state distributions. In particular, the final vibrational state distribution of I2, which could not be satisfactorily reproduced in quasiclassical studies, is well described. Based on these results a different kinetic scheme for interpreting the vibrational predissociation in this system is proposed. In addition, this work shows that the method is very promising for the study of clusters containing more rare gas atoms.

Rotational analysis of the ground state of the Benzene·Ne van der Waals cluster cation by resolved high Rydberg spectroscopy

Series of resolved high n (40< n<110) Rydberg states of the benzene·Ne van der Waals cluster are measured by delayed field ionization after double resonance excitation with Fourier-transform limited UV laser pulses. A cross–correlation analysis yields the first rotationally resolved spectrum of an aromatic molecule noble gas cluster ion. Its analysis leads to accurate values for the adiabatic ionization energy ( AIE=74497.512(30) cm −1) of the benzene·Ne cluster and the rotational constants of the cluster ion. The average van der Waals bonding length of the Ne atom from the (benzene) +-plane is 3.320(2) Å.

Vibrational Predissociation of p-Difluorobenzene·Ar Studied by Mass-Analyzed Threshold Ionization Spectroscopy

Mass-analyzed threshold ionization (MATI) spectroscopy has already proven to be a powerful method for the study of the fragmentation energetics and dynamics of molecular clusters in the cationic state. In this study, its application was extended to the investigation of the vibrationally induced predissociation (VP) of van der Waals (vdW) clusters in vibronically excited states (S1). To verify the feasibility of MATI spectroscopy for studies of this type, the p-difluorobenzene·Ar1 (p-DFB·Ar1) complex has been chosen as a model system. The state and mass selectivity of MATI spectroscopy promise to give useful supplementary information about the VP process, which would be difficult or even impossible to obtain by conventional methods such as fluorescence spectroscopy and time-resolved resonant two-photon ionization spectroscopy. (Butz, K. W.; et al. J. Phys. Chem. 1986, 90, 3533. Jacobson, B. A.; et al. J. Chem. Phys. 1988, 89, 5624.) In accordance with the pioneering studies of Parmenter et al., who investigated the predissociation of the vibronically excited p-DFB·Ar1 complex very extensively by means of UV fluorescence spectroscopy, (Butz et al., 1986), the MATI spectra give evidence for the strong mode selectivity of the VP process. However, from the MATI results, evidence is given that additional fragmentation channels appear, which have not been observed in the fluorescence spectra. On the basis of the fragmentation thresholds observed in the MATI spectra, we also deduced upper and lower limits for the dissociation energies of the complex in the S0, S1, and ionic ground state, which differ significantly from those determined by Parmenter et al.

Hydrogenlike Rydberg Electrons Orbiting Molecular Clusters

Series of Rydberg states up to $n&gt;100$ are observed for the first time in weakly bound, polyatomic van der Waals clusters. Cross-correlation calculations of the dense spectra with simulated Rydberg series reveal a long-lived $(\ensuremath{\mu}\mathrm{s})$, hydrogenlike series of Rydberg states. The slightly different series limits are interpreted as single quantum states in the dense rovibrational level manifold of the weakly bound cluster cation.

Potential modelling of acrylic acid copolymer nanoparticles by small angle X-ray scattering

Copolymer nanoparticles of acrylic acid, acrylic amide, acrylic butyl acrylate and methyl acrylate were characterised using a parameter relevant for in vivo organ distribution: charge distribution. Usually the zeta potential is estimated with laser Doppler anemometry as an equivalent for the particle charge. In our case, the potential distribution was investigated by small angle X-ray scattering. The nanoparticles shielded by counter-ions at high ionic strength (0.15 M NaCl solution) have short-range potentials. Different potential models were calculated: Percus–Yevick potential and Hypernetted Chain potential. The potentials estimated by small angle X-ray scattering can be divided into attractive and repulsive parts. The attractive potential, indicated by a minimum, is not significant compared with the repulsive potential and can be ignored. Consequently the tendency to form Van der Waals clusters is low. The solution shows a random order of hard spheres. Therefore, surface charges on the nanoparticles should hamper such carrier cell interaction only at short distances. In comparison with the potential estimation by laser Doppler anemometry (32.8 mV), results of the same order of magnitude were obtained using the small angle X-ray scattering method. The advantage of this method is that it provides a potential curve, which allows better appraisal of clearance by the reticuloendothelial system. By interpreting the potential curves, a more detailed prediction of steric stability of particle systems is possible.

Resonance enhanced multiphoton ionization processes to study spectroscopy and reactivity of van der Waals clusters of aromatic molecules

One colour resonance enhanced two photon ionization of van der Waals complexes of (R +)-1-phenyl-1-propanol clustered with chiral and non chiral compounds have been studied. Enantiodifferentiation, different fragmentation patterns and reactivity are discussed on the basis of experiments.

Melting and Phase Space Transitions in Small Ionic Clusters

We investigate the thermodynamical behavior of small neutral sodium fluoride clusters using both Monte Carlo (MC) and molecular dynamics (MD) simulations. The three clusters studied, (NaF)n with n = 6, 7, and 13, all display thermodynamical phases of a solidlike nature at low temperature and liquidlike at higher temperatures. However, unlike van der Waals clusters, the melting process appears to be a multistage phenomenon: The clusters undergo successive transitions toward more fully fluid states. We relate this behavior to the isomerization processes occurring in these systems and the topography of the potential energy surface. As one of our indicators, we calculate the largest Liapunov exponent λ as a function of the total energy E. The slope of log λ(E) strongly drops at the onset of first isomerization. The subsequent fluid transitions are not associated with mere changes in this slope. In view of this fact, we conclude that the degree of chaos in the dynamics of the system is not sensitive to the total volume of accessible phase space, once the lowest saddle points are allowed to be crossed.

The hindering of the inversion motion in the van der Waals aniline-Arn clusters: An adiabatic molecular dynamics simulation for n=1–3

An adiabatic quantum molecular dynamics simulation has been performed for the van der Waals clusters aniline-Arn (with n=1–3) to understand the influence of the intramolecular inversion mode on the static properties (rotational constants). The effect of deuteration of the amino group in the chromophore (C6H5NH2 or C6H5ND2) on the rotational constants of the complexes has been evaluated. The results are in good agreement with recent experimental values [W. E. Sinclair and D. W. Pratt, J. Chem. Phys. 105, 7942 (1996)]. In the ground electronic state S0, calculations show that the −NH2 (or −ND2) plane of the aniline molecule tends to be blocked on the same side as the argon atom in the complex. On the other hand, for larger clusters (aniline-Ar2 and aniline-Ar3), it appears that the −NH2 (or −ND2) group is more localized on the side opposite to the argon atoms for the isomers containing at least one argon atom bound in a site near the nitrogen atom.

Influence of chaos on the ionization induced fragmentation dynamics of van der Waals clusters

Molecular dynamics simulations based on a self-consistent electronic model are performed to investigate the effect of ionization on the atomic motion of small van der Waals clusters. We find unimolecular dissociation (fragmentation) with time scales in the picosecond range. The dynamics during the relaxation process after ionization turns out to be extremely nonlinear, with fragmentation times which depend strongly on initial conditions. Our calculations show that the largest Liapunov exponent λ+ after ionization is much larger than λ0, the corresponding exponent before ionization. This indicates that the ionization process enhances the nonlinear character of the motion of small clusters. We also determined the distribution of fragmentation times as a function of the vibrational temperature of the clusters before ionization. Since the ionization process creates a state far away from thermodynamical equilibrium, a time-dependent fragmentation probability W(t) is obtained. Furthermore, W(t) reflects the ionization induced chaotic dynamics.

Short-range effects in resonant electron–molecule scattering from van der Waals clusters

A theoretical study is presented of the dynamics of resonant electron scattering from N2–Ar and N2–Ar2 van der Waals clusters. Using the results of quantum electron-molecule scattering calculations we analyze the effects of adjacent Ar atoms on the width and position of the low-energy Πg2 electron-scattering resonance of N2. The results indicate that the presence of the Ar atoms leads to positive energy shifts and an increase in the width of the resonance. The magnitude of these changes depended on the orientation of N2 relative to the Ar atoms and on the number of Ar atoms. Additionally, in some arrangements, the degenerate Πg2 resonance was split into two distinct resonances. Implications for electron scattering from N2 adsorbed on solid Ar surfaces are also discussed.

The photoionization of ArHCl

The photoionization of the van der Waals cluster ArHCl in a molecular beam has been investigated, using a synchrotron radiation light source and a photoionization mass spectrometer. The concentration of ArHCl is approximately given by the equationα(ArHCl)% = 1.79 × 10 −8 P 0 1.5,where P 0 is the stagnation pressure in Pa. The photoionization efficiency curve of ArHCl was obtained and the appearance potential of ArHCl + was determined to be12.52 ± 0.03 eV. The bond dissociation energy D 0 of ArHCl + is deduced to be0.24 ± 0.04 eV. The theoretical calculation using Gaussian-94w indicates that ArHCl + has a linear conformation like ArHCl. The auto-ionization mechanism of ArHCl and charge transfer-vibrational predissociation processes are discussed.

Density functional calculations of Van der Waals clusters: NeN+2 as a case study

Density functional calculations of Van der Waals clusters: NeN+2 as a case study

Density functional calculations of Van der Waals clusters: NeN 2+ as a case study

Hybrid density functional theory (DFT) methods have been employed to investigate the binding energy and geometry of the Van der Waals cluster NeN 2 +. The results are compared with MP2 computations. DFT predicts a large binding energy for the cluster as compared to the MP2 calculation.

Solvation of charge in aromatic/noble gas Van der Waals clusters

The results of an experimental study of the ionisation threshold in the various structural isomers of the Van der Waals clusters aniline-(argon) n ( n = 1−5) and 4-fluorostyrene-(argon) n ( n = 1–2) using resonant two-photon ionisation are reported. The data validate and generalise the site-specific modified shift additivity rule. They show an interesting influence of the localisation of argon atoms at the surface of the chromophore on the value of the net shift of the ionisation potential. Interpreatation of the results involves evaluation and balancing between the charge-induced-dipole interaction and dispersion interaction.

Diffusion and clustering of N2O molecules in argon clusters: A theoretical approach by molecular dynamics simulations

Results of molecular dynamics simulations of small van der Waals clusters composed of one argon cluster of size 147 or 125 (incomplete outer-layer cluster) and from one to four N2O molecules deposited at thermal relative collision energy on the argon cluster are presented. The potential energy is calculated through the semiempirical Claverie method. We discuss here the necessity and the practical application of fitting some of the potential parameters in order to reproduce the N2O experimental dipole moment value as well as the experimentally observed N2O⋯N2O and N2O⋯Ar equilibrium geometries. We first show that, as in the case of atomic projectiles, a very efficient capture by collision of the N2O molecules by the argon clusters is observed, independently of the initial molecular orientation. Studying trajectories over tens of nanoseconds then gives evidence that the N2O molecules move independently on the surface of the argon clusters, and that the molecules migrate randomly through jump displacements on the surface of the clusters. We observe a very high N2O mobility and we explain the influence of the argon cluster outer-layer structure on mobility. Collisions of the N2O molecules on the surface of the argon clusters result in a sequential and fast clustering. The geometries of the energetically stable (N2O)m microclusters have been characterized. Using the formula of Perrin, we calculate and interpret single N2O diffusion coefficients and (N2O)m microcluster diffusion coefficients, whose values are 1 to 100 times lower than in the liquid state. Finally, we extend our results to larger argon clusters, such as Ar1000, through a random walk model taking place on the surface of a sphere, which enables us to calculate mean encounter times between particles. We thus interpret the fluorescence quenching that occurs in chemical reactions taking place on finite-size argon clusters.

Measurement of the K a = 1 (v co = 1)stretching mode in Ar—CO using a Herriott multipass cell

A Herriott multipass cell has provided a significant increase in the sensitivity of our computer controlled diode laser spectrometer for the investigation of van der Waals clusters in a continuous slit jet expansion. The assignment was then possible of 57 new lines to the perpendicular transition from the (v co, j, K a,n) = (0,2,2,0) ground state of the Ar—CO van der Waals complex to the combination band of the CO fundamental vibration (v co = 1) and the K a = 1 (j = 1) stretching mode (1, 1, 1, 2). The band origin of 2159·88 cm-1 locates the energy of the (1,1,1,2) stretching mode at 26 ·19cm-1. This is the highest van der Waals mode observed in Ar—CO so far. An analysis of the rotational constants resulted in a level ordering of this state relative to the nearby (1, 3, 2, 1) state, which is in contradiction to existing ab initio calculations. The experimental results demonstrate that even these high lying van der Waals modes still show relatively regular behaviour, and therefore will provide a sensitive test for future ab initio and semiempirical potential energy surfaces.

Characterization of hydrocarbon cluster ion products by surface induced reactions

A recently constructed apparatus consisting of a supersonic beam for generating neutral clusters, a variable energy electron gun for ionizing the clusters, and a tandem mass spectrometer set-up for studying surface induced reactions of mass and energy selected primary ions, has been used to study several hydrocarbons including benzene, biphenyl and products generated in cluster ion reactions, e. g. C 12H x + ions ( x=9, 11); the latter ions are demonstrated to correspond to protonated biphenyl and its dehydrogenation product rather than weakly interacting van der Waals clusters of the type C 6H 6·C 6H x + ( x=3, 5).