The Monte Carlo growth method (MCGM), a recently developed method of Potential Energy Surface (PES) exploration, is adapted to the case of molecular clusters. The choice of the relevant parameters of this method for the study of these systems is discussed. The MCGM is applied to two different molecular systems: the acetonitrile and the molecular nitrogen clusters. The MCGM is compared to two classical PES exploration methods: the Monte Carlo simulated annealing technique (MCSA) and the dynamic quenching method (DQM) used in Molecular Dynamics. The MCGM is much more effective than the MCSA and yields similar results to those of the DQM, without needing any initial cluster guess. Obtained isomers for clusters up to (CH3CN)6 and (N2)14 are described. In both systems, remarkably stable patterns are observed: the anti-parallel pair in the case of the acetonitrile, the pentagonal bipyramid and the icosahedron for nitrogen clusters. Lowest-energy isomers of (N2)n clusters are isomorphic to those of Arn.
Based on the experimental hyperfine structure data given in part I of this work and on the fine structure analysis of Wyart and Camus a parametric analysis of the hyperfine structure of the configuration 4f135d6s6p of neutral Yb has been performed leading to the following one-electron parameters for the magnetic dipole hyperfine structure of the isotope 173Yb a 4 01 = −375(11) MHz, a 5 01 = −68(17) MHz, a 6 10 = −3 090(80) MHz, and a 6 01 = −170(30)MHz and for the electric quadrupole hyperfine structure b 4 02 = 7 490(150) MHz, b 5 02 = 3 500(200) MHz, and b 6 02 = 4 700(200) MHz. Theoretical predictions are made for all unknown A and B constants of the configuration investigated.
The diffusion of NaH molecules in H2 gas and its reactive loss mechanism are described. The temporal behaviour of photochemically produced NaH in the reactive Na(3p)-H2(v″ = 2;3) collision system was observed by means of resonant coherent anti-Stokes Raman scattering (RECARS). A simple diffusion model, presented in this article, applies well in the lower pressure region (0 mbar). The binary diffusion constant for NaH molecules in H2, D0 = 0:7 cm2/s at 1 atm and 273 K, was determined. At higher pressures (up to 900 mbar), where the influence of diffusion decreased, an unexpectedly rapid decay of the NaH signal is observed. From this time behaviour we identify a new NaH loss mechanism and shown for the first time by means of time resolved CARS that this reaction can be described by a second-order rate equation in NaH.
Charge transfer collisions between C 60 3+ and C60 are studied for collision energies between 400 and 3600 eV. Single and double electron transfers are observed, both occuring under single collision conditions. Absolute charge transfer cross sections are determined as a function of collision energy. The cross section for single electron capture of approx. 300 Å2 is about two times larger than that for double electron transfer. For both processes the cross section increases slightly with increasing collision energy.
The interaction between C60 molecules with a graphite (0001) surface has been investigated by means of molecular dynamics simulations. The initial energies of the C60 molecules are 90 and 270 eV, respectively. An empirical model potential suggested by Takai et al. is used to describe the interaction between carbon atoms in the C60 molecule and between the atoms forming the graphite substrate. The interaction between the C60 atoms and the graphite atoms is modeled by a suitable Lennard-Jones potential. The resilience of scattered C60 molecules is observed and its energy distribution is in reasonable agreement with available experimental data, showing no significant dependence of the rebounding translational energy on the incident kinetic energy. The energy partition in the collision has been analyzed in detail and a two-step collision model speculated in the experiments has been discussed based on the simulation results.
The role of S = 1 excitations on the ground state of jellium clusters is analyzed within the randomphase approximation (RPA). The importance of the correlation-energy contribution to the residual interaction in this channel, in order to obtain a consistent description of the cluster electronic states, is stressed. The modification of the single-particle occupation numbers in the RPA ground state is obtained. This modification is found too large when considering only Coulomb exchange contributions to the functional, which clearly points the inadequacy of RPA for this approximate functional and the necessity to include correlation-energy contributions.
Excitation energies and transition probabilities for two electron one photon (K αα) transition 2s22p6: 1S → 1s2 2s2p5: 1P have been estimated for the inner shell ionised atoms Ne, Na, Mg, Al, Si, P, S, Cl and Ar. Similar transition data have also been evaluated for K αα s transition 2s2p: 1P → 1s2: 1S of the highly stripped He like ions Ne8+, Na9+, Mg10+, Al11+, Si12+, P13+, S14+, Cl15+ and Ar16+. The branching ratio between the X-ray hypersatellite line K α2 h (2s22p6: 1S → 1s2s22p5: 1P) and K αα line and that between K α2 h and K αα s transition has been estimated for all these ions. The method is based on variational principle with a correlated description of the initial and final states. Most of the data are new and agreement is very reasonable with the few available data.
When the cluster excitation energy is such that the hopping between different isomers becomes energetically allowed, the power spectrum corresponding to the very long-time dynamics is dominated by the 1/f-noise. The emergence of 1/f-noise is indicative of the absence of any characteristic time scales in the structure/conformation cluster dynamics. The 1/f-noise portion of the power spectrum covers all frequencies below the characteristic cluster modes and is essentially temperature independent once the cluster has “melted”.
The spin asymmetry and integrated spin-unresolved cross sections for the electron-impact ionization of atomic hydrogen are studied. The influence of individual final-state interactions on these quantities is elucidated by investigating the collision process within the plane-wave impulse approximation, the first Born approximation and within an independent Coulomb particle model. The predictions of the latter approximation for the shape and magnitude of the spin asymmetry are in good agreement with experimental data over the entire measured energy range. The effects of final-state electronic correlations are investigated by the inclusion of the electronic Coulomb density-of-states factor as well as by employing a three-body Coulomb wave-function for the description of the final state. For ionization of hydrogenlike ions the calculated asymmetry parameter has a positive slope near threshold indicating that such an energy dependence is due to the electron-nucleus interactions. At threshold the results are analyzed in light of the Wannier theory of threshold ionization.
Structural and optical properties of nonstoichiometric LinO and NanO (n = 3, 4) clusters containing one and two excess electrons are studied using ab-initio methods accounting for electron correlation. We show that calculated absorption patterns are excellent fingerprints of structural and bonding properties. The optical response of Li4O and Na4O clusters with the most stable tetrahedral type structures is characterized by a common feature, that is the appearance of a dominant intense transition in infrared regime although excess of electrons are not localized, as it is the case for small alkali-halide clusters, with cuboid corner vacancy (surface F-center in finite systems).
We have measured the UV-absorption spectrum (4f→5d transitions) of single europium atoms embedded in helium droplets using a beam depletion technique. The electronic levels involved in the transition lie spatially within the closed 6s2 shell leading to significant shielding from the surrounding helium environment. We observe the narrowest absorption lines ever reported in liquid helium and compare our data and experimental method, with experiments carried out in bulk liquid helium.
Structural and optical response properties of Li n Hn-m and NanFn-m (n = 2-6, m = 1, 2) clusters containing one- and two-excess electrons are studied using ab-initio methods accounting for electron correlation. The common feature of the optical response obtained for the most stable structures of NanFn-1 (n = 2-6) clusters is the appearance of a dominant intense transition in the infrared regime independently whether the single excess electron is localized at the cuboid corner vacancy (surface F-center) or at the external atom attached to the filled cuboid. In contrast, LinHn-1 (n = 2-6) clusters exhibit substantially different spectroscopic patterns with respect to halides also for the cases with the common structural properties. Optical response features of LinHn-2 (n = 3-6) clusters with two-excess electrons are characterized by dominant transitions in the visible regime reflecting segregation in “metallic” and ionic parts. In contrast, NanFn-2 = 3-6) can be divided according to their optical and structural properties into cuboid “lattice” defect species (Na4F n , Na n F4) and segregated metallic-ionic systems. For the former, the intense transitions occur in the infrared-visible, and for the latter only in the visible regime. It will be shown that the calculated absorption patterns are excellent fingerprints of structural and bonding properties.
Plasma modeling and diagnostics commonly involve solution of rate equations for the population densities of impurity ions in their excited and charge states. Construction of the rate equations requires a complete set of atomic transition rates for all the charge and excited states involved. However, the rates are themselves affected by the host plasma ions and electrons. The ionic and electronic effects in a two-component plasma are intimately interconnected, especially when the rate equations are simplified for computational purpose in the determination of ionization balance. We formulate a coherent approach to the problem of the plasma density effect, and apply it to carbon impurities in a hydrogen plasma. Both the plasma field distortion of atomic states and the corresponding rates by the plasma ions and stochastic plasma collisional transitions caused by the plasma electrons are included. The latter effect is estimated by constructing an effective collisional transition operator, and the electron-ion recombination processes are explicitly evaluated. It is shown that, these two effects of the ionic field distortions and electronic collisions tend to cancel each other, resulting in many cases in reducing the overall effect of the plasma density on the ionization.
We demonstrate coherent two-photon optical population transfer (STIRAP, J. Chem. Phys. 92, 5363 (1990)) between electronic states of atoms by use of two cw laser beams with high spectral resolution, which are modulated by an acoustooptical modulator (AOM) and provide a counterintuitive sequence of temporally delayed pulses of several hundred nanoseconds length. The efficiency of optical population transfer induced by this AOM-STIRAP method is shown in the lambda-type system of metastable Ne atoms, produced in a nozzle beam, where we compare directly spatially and phase separated pulses from cw laser beams (cw-STIRAP). As an application, efficient and selective transfer of population into a decaying electronically excited state of Na is demonstrated using the Doppler-free two-photon ladder-type transition 3S-3P-5S in a sodium cell. The experimental results are reproduced satisfactorily by use of a density matrix approach.
Sources of excited atoms in a metastable state produced by a discharge are important tools for several experiments in atomic physics. The axis defined by the electron current breaks the spherical symmetry and as a result, the magnetic sublevel cross section can be different and the atomic beam aligned. It is presented theoretical results for the energy dependence of the magnetic sub-level integral cross section for the 31S → 33P Mg transition and 41S → 43P on Ca, They suggest a strong energy dependent alignment at energies where the excitation cross section is also large. Analysis indicates that this may also be true for other atoms. The spin-orbit interaction, neglected during the excitation, is included after the collision. It produces a time dependent density matrix evaluated using state multipole. Collision experiments with these aligned atomic beams may provide information which can not be obtained when the atomic state description is an isotropic density matrix.
Accurate quantum-chemical ab initio calculations have been performed at the SCF and CEPA (coupled electron pair approximation) levels for the van der Waals interaction in the X 2 Σ + ground state of LiHe. An extended basis set has been used and the counterpoise correction for the basis set superposition error (BSSE) has been applied. The calculated potential energy curve has a very shallow minimum at 11.56 a 0 with a well depth of only 1.49 cm−1. This is too small to allow for a bound vibrational level. The analysis of the results shows that the interaction mainly consists of the Pauli repulsion between Li(1s 22s) and He (1s 2), which is decaying exponentially, and the attractive London dispersion energy. Van der Waals coefficients C6, C8, and C10 have been determined by a least squares fit to the long-range part of the calculated potential curve.
The nonlocal resonance model developed earlier for the description of the collision of low-energy electrons with HCl and HBr has been adapted to the electron-HI collision system. The parameters of the model have been determined by fitting experimental high-resolution data for the attachment cross section in HI in the energy range 0 - 170 meV. Moreover, ab initio electronic-structure data for the electronic ground-state potential-energy function of HI- have been taken into account. Within the resulting model, cross sections for vibrational excitation and dissociative attachment processes in HI and DI have been calculated over a reasonably wide energy range. No high-resolution experimental data are available for comparison.
The use of a generalized exponential function r v−1 exp(−ζr μ ) as a radial basis function in atomic calculations is studied with our special interest in the variationally optimum value of the parameter μ, since special cases of μ = 1 and μ = 2 correspond respectively to the radial parts of commonly-used Slater-type and Gaussian-type functions. Roothaan-Hartree-Fock calculations are performed for ground-state neutral atoms with atomic number Z = 2–54, singly-charged cations with Z = 3–55, and anions with Z = 1–53 within the single-zeta (or minimal basis) framework. For all the species examined, the optimtum μ values are found to be smaller than unity and increase towards unity as the atomic number increases. The present results support the use of Slater-type functions when μ is restricted to be an integer, but suggest from the variational point of view that even the exponential decay of Slater-type functions is too “strong” within the single-zeta approximation.
Electronic properties of silicon-fluorine and germanium-fluorine cluster anions (SinF m − n = 1–9, m = 1–3, GenF m − ; n =1–9, m = 1–3) were investigated by photoelectron spectroscopy using a magnetic-bottle type electron spectrometer. The binary cluster anions were generated by a laser vaporization of a silicon/germanium rod in an He carrier gas mixed with a small amount of SiF4 or F2 gas. Comparison between photoelectron spectra of SinF−/GenF− and Sin /Gen (n = 4–9) gives the insight that the doped F atom can remove one electron from the corresponding Sin n − /Ge n − cluster without any serious rearrangement of Sin/Gen framework, because only the first peak of Si n − /Ge n − , corresponding singly occupied molecular orbital (SOMO), disappears and other successive spectral features are unchanged with the F atom doping
Mass spectra of LaC n - were taken by using a laser-vaporization source (LVS) and pulsed arc cluster ion source (PACIS) applied to La-carbon composite rods (1:130 atomic ratio). The mass spectrum using the LVS with annealing procedure has shown several magic numbers for LaC n - (n = 44, 50, 60, and 70), whereas only small LaC n - (up to n = 14) have been observed in the mass spectrum using the PACIS. Photoelectron spectra of some of these have been measured using a magnetic-bottle type time-of-flight electron spectrometer. These results indicate the exsistence of a few conformational isomers for small La-containing carbon cluster negative ions LaC n - (n = 5–8).
