bluuebunny/crossref_metadata_2025 huggingface dataset:
Pseudospectral Hartree–Fock gradient calculations
Authors: Won Youngdo , Lee Jung-Goo , Ringnalda Murco N. , Friesner Richard A.
Published: July 2002

Techniques for the calculation of analytic first derivatives of the Hartree–Fock energy are reported, within the context of the pseudospectral ab initio method. Using these gradients, geometry optimization is carried out on several molecules at the 6-31 G** level. The resultant geometries are compared to those from conventional ab initio molecular-orbital calculations, and it is shown that bond lengths agree to within 0.003 Å, while bond angles are within 1°.
Mapping of transition-state wave functions: I. Rotational state distributions following the decay of long-lived resonances in the photodissociation of HONO(S1)
Authors: Schinke R. , Untch A. , Suter H. U. , Huber J. R.
Published: July 2002

We investigate final rotational state distributions following the decay of long-lived resonance states with k*=0, 1, and 2 quanta of internal bending excitation. The calculations are related to the photodissociation of HONO on the S1 electronic state surface, truncated to two degrees of freedom namely the HO–NO dissociation bond and the ONO bending angle. The decay of the k*=0 resonance yields a smooth Gaussian-type distribution, in very good agreement with recent measurements. The distributions following the decay of the excited bending states show a bimodal behavior with the main maxima at high rotational states. The final angular momentum distributions reflect the coordinate-dependence of the dissociation wave function in the region of the transition state, mediated by the dynamics in the exit channel when the wave packet slides down the steep potential slope. A qualitative interpretation of the rotational state distributions is provided by a simple classical model which applies the transition-state wave function as a weighting for trajectories starting on a line that separates the intermediate complex from the product channel.
Time resolved stimulated light scattering from a solvated chromophore: A molecular dynamics study
Authors: Walsh Alan M. , Loring Roger F.
Published: July 2002

We calculate the signal in a time-resolved, coherent light scattering experiment on a dilute solution of a nonpolar solute dissolved in a nonpolar solvent. We model the solute and solvent molecules as electronic two level systems, and assume that only the solute interacts with radiation. The electronic degree of freedom is treated perturbatively within the Born–Oppenheimer approximation. The nuclear degrees of freedom are treated classically, and their effects are modeled by a molecular dynamics simulation. We assume the electronic absorption spectrum is dominated by inhomogeneous broadening. The validity of this assumption for our model is verified through the calculation of the absorption line shape with and without the inhomogeneous broadening assumption. A quantitative measure of the homogeneous dephasing time is obtained from a calculation of the photon echo signal for our model. The time dependence of the light scattering signal is shown to reflect both dynamics of the fluid in its electronic ground state and in its electronic excited state. We analyze the relative importance of contributions to the signal from these two types of dynamics as a function of experimental time scale and laser frequency.
Structure of the benzene–Ar2 cluster from rotationally resolved ultraviolet spectroscopy
Authors: Weber Th. , Neusser H. J.
Published: July 2002

Rotationally resolved spectra of the two vibronic bands 610 and 1620 and a vibronic van der Waals band of the benzene–Ar2 cluster are presented, whose vibronic assignments are based on the analysis of their rotational structures. A fit to the rotational line positions in the symmetric top spectra yields an accurate set of rotational constants in the ground and the excited electronic state and the exact values for the band origins of the bands. From these values the spectral shift between corresponding cluster and monomer bands as well as the frequency of the van der Waals symmetric stretching vibration in the excited electronic state are precisely determined. The structure of the cluster is identified to be symmetric with one Ar atom located on the C6 axis on each side of the benzene ring at a distance of 3.58 Å in the S0 state and 3.52 Å in the S1 state. These bond lengths exactly agree with our recent values for benzene–Ar. From the result that the bond lengths are equal for the dimer and the trimer we conclude that there is no Ar–Ar interaction through the intermediate benzene ring plane.
Molecular modeling of solvation. Cl−(D2O)
Authors: Zhao Xin Gui , Gonzalez-Lafont Angels , Truhlar Donald G. , Steckler Rozeanne
Published: July 2002

The isotopic fractionation factor in the monohydrated gas-phase cluster Cl−(H2O), i.e., the equilibrium constant for Cl−(H2O)(g)+D2O(g)⇄Cl−(D2O)(g) +H2O(g), is used to test models used for solution-phase simulations and to test semiempirical and ab initio molecular orbital theory for their detailed structural and vibrational predictions for both the solute–solvent bond properties and the interaction-induced intramolecular changes in water itself. The isotope effect is studied at a consistent level of vibration–rotation theory, i.e., the harmonic-oscillator-rigid-rotator approximation, using four different kinds of approach, namely extended-basis-set ab initio electronic structure calculations, both (i) with and (ii) without electron correlation, (iii) semiempirical molecular orbital theory at the level of neglect of diatomic differential overlap, and (iv) analytic force fields based on site–site interactions. The calculations of type (i) show good convergence and are compared both to experiment, which presumably tests the harmonic approximation, and to the results of efforts (ii), (iii), and (iv), which presumably tests the structural and vibrational properties predicted by these more approximate approaches. We find significant effects of anharmonicity and electron correlation; semiempirical molecular orbital theory does remarkably well; and there is a wide variation in predictions among the 25 analytic force fields tested. Finally we combine the well converged ab initio results for the properties of the well, the vibrational red and blue shifts, and the isotope effect on the equilibrium constant with the previous ab initio calculations of Dacre for the repulsive interactions to obtain a new interaction potential for chloride ion with nonrigid water that also predicts a more accurate enthalpy of complexation and binding energy than do the chloride–water potentials available in the literature.
Trapping constant, thermal conductivity, and the microstructure of suspensions of oriented spheroids
Authors: Torquato S. , Lado F.
Published: July 2002

The n-point probability function Sn(rn)  is fundamental to the study of the macroscopic properties of two-phase random heterogeneous media. This quantity gives the probability of finding n points with positions rn ≡{r1,...,rn} all in one of the phases, say phase 1. For media composed of distributions of oriented, possibly overlapping, spheriods of one material with aspect ratio ε in a ‘‘matrix’’ of another material, it is shown that there is a scaling relation that maps results for the Sn for sphere systems (ε=1) into equivalent results for spheriod systems with arbitrary aspect ratio ε. Using this scaling relation it is then demonstrated that certain transport and microstructural properties of spheriodal systems generally depend upon purely shape-dependent functions and lower-order spatial moments of S2 (minus its long-range value) of the equivalent spherical system. Specifically, the following three distinct calculations are carried out for both hard, oriented spheroids and overlapping (i.e., spatially uncorrelated), oriented spheroids: (1) bounds on the diffusion-controlled trapping constant; (2) bounds on the effective conductivity tensor; and (3) fluctuations in the local volume fraction as measured by the ‘‘coarseness.’’ These computations enable us to investigate the effects of statistical anisotropy (i.e., particle asymmetry) and particle exclusion volume on the aforementioned quantities.
Electronic spectral shifts of aromatic molecule–rare-gas heteroclusters
Authors: Shalev Eli , Ben-Horin Narda , Even Uzi , Jortner Joshua
Published: July 2002

In this paper, a semiempirical theory for the spectral shifts of the electronic origin of the S0→S1 transition of (aromatic molecule)⋅(rare-gas)n heteroclusters is advanced and applied. Neglecting the modifications of intermolecular overlap and exchange interactions upon electronic excitation, the dispersive contributions to the spectral shift are evaluated to second order, accounting for finite-size structural features of the large molecule by the utilization of the multicenter monopole representation of the intermolecular interactions. The spectral shifts for nonpolar aromatic hydrocarbons in or on rare-gas heteroclusters are represented in terms of differences between electrostatic interactions involving an electrostatic field (due to the molecular transition monopoles charge distribution) and an induced dipole (originating from the rare-gas polarizability). The transition monopoles incorporated all the one- and two-electron ππ* excitations of the aromatic molecule, which were represented by Hückel or self-consistent molecular orbitals (MO). The dispersive spectral shifts were semiempirically scaled to correct the systematic overestimate of the transition monopoles within these simple MO schemes. The red spectral shift was recast in terms of a sum of two-atom (carbon atom–rare-gas) and three-atom (carbon atom–rare-gas–carbon atom) contributions, which are subsequently summed over the contributions of all the rare-gas atoms, with each term involving products of an electronic factor and a geometric factor.

The electronic factors exhibit a linear dependence of the spectral shift on the rare-gas polarizability, while the geometric factors incorporate the structural effects of the contributions of the individual rare-gas atoms to the spectral shift, predicting the nonadditivity of the spectral shift per added rare-gas atom and the exhibition of distinct spectral shifts for different structural isomers. The semiempirical theory in conjunction with structural information emerging from (pairwise) potential optimization for small and medium-sized (n=1–8) heteroclusters and from molecular dynamics simulation for medium-sized and large heteroclusters (n=5–34) was applied. The theory accounts for the spectral shifts of small and medium-sized rare-gas heteroclusters of pentacene and tetracene as well as for small, medium-sized, and large heteroclusters of 9,10 dichloroanthracene, but not for the general pattern of the spectral shifts for anthracene⋅An (A=Ar, Kr; n=1–8) and for perylene⋅Arn (n=2). The confrontation between theory and experiment for the spectral shifts of pentacene⋅Arn (n=1–8), tetracene⋅Arn (n=1–8), tetracene⋅Krn (n=1–8), 9,10 dichloroanthracene⋅Arn (n=1–34) and 9,10 dichloroanthracene⋅Krn (n=1–20), allows for the elucidation of the size dependence of spectral shifts, for the identification of structural isomers in small and medium-sized heteroclusters, and for the exploration of spectral inhomogeneous broadening in large heteroclusters.
Onset and evolution of ‘‘magic numbers’’ in mass spectra of molecular clusters
Authors: Casero R. , Soler J. M.
Published: July 2002

We have calculated the evaporation rate for every cluster size of Ar+n and Xe+n in the microcanonical ensemble (as a function of its internal energy) up to n=37. This allows us to perform numerical experiments simulating the evolution of the size and energy distributions in a cluster beam up to arbitrary long times. We find that there is a time lag for the onset of magic numbers (especially abundant cluster sizes) of molecular clusters. This time is required for the clusters to cool down by evaporation and become solid-like and it may differ by several orders of magnitude for different species thus explaining apparently contradictory experiments. For longer times we find, in agreement with theoretical predictions but contrary to simple intuition, that the relative abundance of magic/nonmagic clusters decreases with time.
A dynamical definition of atomic clusters
Authors: Xie J. , Northby J. A. , Freeman David L.
Published: July 2002

The assignment of an atom to a particular cluster of atoms is defined dynamically in terms of the power flow to the atom. The definition avoids some of the ambiguities present in structural definitions of cluster membership and enables a precise determination of the time at which evaporation and capture events occur in clusters. The definition is illustrated with molecular dynamics studies of evaporation events in Lennard-Jones clusters, and comparisons are made with structural definitions.
Rotational and vibrational effects in the E 1Σ+g–X 1Σ+g two-photon transitions of H2, HD, and D2
Authors: Huo Winifred M. , Rinnen Klaus-Dieter , Zare Richard N.
Published: July 2002

A theoretical study is reported for the dependence of the E, F 1Σ+g–X 1Σ+g (vE=0, vX) Q-branch two-photon transitions on the initial vibrational and rotational quantum numbers vX and J for H2, HD, and D2. The magnitude of the two-photon transition moment ‖Mfo‖ increases with vX between 0 and 2, then decreases rapidly at higher vX. This behavior is ascribed to (constructive and destructive) interference effects in the two-photon transition, which vary with the nodal behavior of the initial-state vibrational wave function. The J dependence of ‖Mfo‖ is most strongly affected by the changes in vibrational wave functions caused by centrifugal distortion of the vibrational potentials. At low to intermediate J, the Franck–Condon overlap improves, resulting in a slow, monotonic increase of ‖Mfo‖ with J. At higher J, resonant tunneling between the inner and outer wells of the E, F state introduces sharp variations in the ‖Mfo‖ structure. Near the top of the barrier, nonresonant tunneling in the E-state vibrational wave function causes a general decrease in ‖Mfo‖ with J.
Electronic shell closings in metal cluster plus adsorbate systems: Cu+7CO and Cu+17CO
Authors: Nygren Martin A. , Siegbahn Per E. M. , Jin Changming , Guo Ting , Smalley Richard E.
Published: July 2002

The stability of CO-chemisorbed small clusters of copper have been studied both by first principles calculations and by experiment. Evidence is found that the shell model (which predicts that clusters of 8, 18, and 20 electrons are particularly stable) is useful both for the bare metal clusters, and for these clusters with a chemisorbed CO−provided the CO is considered to have contributed two electrons. Experiments supporting this view are reported for gold clusters as well.
Subpicosecond transient infrared spectroscopy of adsorbates. Vibrational dynamics of CO/Pt(111)
Authors: Beckerle J. D. , Cavanagh R. R. , Casassa M. P. , Heilweil E. J. , Stephenson J. C.
Published: July 2002

The vibrational dynamics of excited CO layers on Pt(111) were studied using infrared (IR) pump–probe methods. Resonant IR pulses of 0.7 ps duration strongly pumped the absorption line (ν≊2106 cm−1 ) of top-site CO. Weak probe pulses delayed a time tD after the pump were reflected from the CO-covered Pt(111) surface, and dispersed in a monochromator to determine the absorption spectrum of the vibrationally excited CO band, with time resolution <1 ps and monochromator resolution <1 cm−1. Transient spectra were obtained as a function of CO coverage, surface temperature, and laser fluence. Complex spectra for tD<0 show features characteristic of a perturbed free induction decay, which are expected based on multiple-level density-matrix models. For tD≥0, the CO/Pt absorption exhibits a shift to lower frequency and an asymmetric broadening which are strongly dependent on fluence (1.3–15 mJ/cm2 ). Spectra return to equilibrium (unexcited) values within a few picoseconds. These transient spectral shifts and the time scale for relaxation do not depend (within experimental error) on coverage for 0.1≤ΘCO≤0.5 ML or on temperature for 150≤Ts≤300 K. A model for coupled anharmonic oscillators qualitatively explains the tD>0 spectra in terms of a population-dependent decrease in frequency of the one-phonon band, as opposed to a transition involving a true CO(v=2) two-phonon bound state. The rapid relaxation time and its insensitivity to Ts and ΘCO are consistent with electron–hole pair generation as the dominant decay mechanism.
A theoretical study of Na(H2O)+n (n=1–4)
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Partridge Harry , Rice Julia E. , Komornicki Andrew
Published: July 2002

The successive H2O binding energies of Na(H2O)+n are in excellent agreement with experiment and are rather insensitive to electron correlation since the bonding is predominantly electrostatic. A point-charge model shows that changes in the successive binding energies are due primarily to ligand–ligand repulsion. Vibrational frequencies and infrared intensities are determined for Na(H2O)+n (n=1–4) at the self-consistent-field and second-order Mo/ller–Plesset levels of theory to facilitate experimental study of these ions.
Radiative properties of solvated molecules in dielectric clusters and small particles
Authors: Gersten Joel , Nitzan Abraham
Published: July 2002

The radiative lifetime of molecules solvated in finite size clusters and particles is studied as a function of size. Four regimes of behavior are indicated by our present and previous theoretical results and by the available experimental data: The microscopic regime (up to a few tens of solvent molecules), where the lifetime is sensitive to microscopic structural details of the cluster; the electrostatic regime (up to sizes ∼0.1λ, where λ is the radiation wavelength in the cluster), where the lifetime follows the predictions of classical electrostatics of dielectric environments; the electromagnetic regime (sizes of the order of λ), where the behavior is dominated by electromagnetic resonances in the particles; and the bulk regime (sizes much larger than λ). In the last three regimes the radiative lifetime may be approximated as a product of a cavity factor and a solvent factor. The first depends on the shape of the microscopic cavity surrounding the molecule and the second depends on the shape and size of the solvent particle. For spherical particles and for spherical or mildly spheroidal cavities, the lifetime changes from being longer than that of the free molecule in the electrostatic regime to being shorter in the bulk regime, in agreement with recent experimental results. The transition region occurs in the electrodynamic size regime. In the ‘‘bulk regime’’ (very large particles) molecules near the particle surface (within ∼ one wavelength) are strongly affected by electromagnetic Mie resonances and show strong size-dependent deviation from the bulk behavior which characterizes molecules in the interior. The size dependence of the radiative lifetime stands in marked qualitative contrast to the size dependence of the solvent induced frequency shift, which approaches its bulk limit much earlier—when the cluster size becomes much larger than the microscopic cavity size. Finally, the ratio between the integrated absorption profile and the radiative decay rate does not depend on the cluster size.
Low-energy electron-impact excitation of the ã 3B1u(π→π*) state of ethylene
Authors: Sun Qiyan , Winstead Carl , McKoy Vincent , Lima Marco A. P.
Published: July 2002

A two-state close-coupling calculation of electron-impact excitation of ethylene to the ã 3B1u state (π→π*) is carried out using a version of the Schwinger multichannel method developed for distributed-memory parallel computers. The calculated integral cross section shows a sharp rise at threshold, in agreement with available experimental data. The integral and differential cross sections are useful in understanding the mechanism of this excitation process and indicate the possible presence of a core-excited shape resonance near threshold.
Ultraviolet photoelectron spectroscopy and photofragmentation studies of excess electrons in potassium iodide cluster anions
Authors: Yang Y. A. , Bloomfield L. A. , Jin C. , Wang L. S. , Smalley R. E.
Published: July 2002

Ultraviolet photoelectron spectroscopy (UPS) and photofragmentation data for (KI)−n, K(KI)−n , and K2(KI)−n (n up to 13) are presented. The excess electrons in these clusters are all loosely bound with vertical detachment energies below 1.6 eV, and the photofragmentation data correlated well with the UPS results. Our results for the stoichiometric clusters are in good agreement with theoretical predictions, and the ground state geometry of these clusters are discussed. For the excess potassium cluster series, we suggest the possible existence of electron spin pairs in some of these cluster anions.
Thirteen-atom clusters: Equilibrium geometries, structural transformations, and trends in Na, Mg, Al, and Si
Authors: Röthlisberger Ursula , Andreoni Wanda , Giannozzi Paolo
Published: July 2002

We report the results of an extensive structural study of Na13, Mg13, Al13, and Si13 carried out with the Car–Parrinello method. Several and mostly unforeseen noncrystalline structures are discovered to characterize the low portion of the potential energy surface. Crystalline structures are shown either to correspond to high-energy local minima or to be highly unstable. The low-energy structural pattern appears to change significantly from one element to the other. Specific characteristics as well as trends are discussed.
Hylleraas wave functions revisited
Authors: Koga Toshikatsu
Published: July 2002

Optimality of the combination of N terms used in the Hylleraas N-term wave function is examined for several two-electron atoms. It is pointed out that the N-term wave function given in the literature is not always the best N-term function for a fixed N. For example, the known Hylleraas six-term function Ψ6=exp(−ζs) (1+c1u+c2t2+c3s+c4s2+c5u2) yields the energy of −2.903 329 354 a.u. for the helium atom, whereas a new six-term function Ψ6,opt=exp(−ζs) (1+c1u+c2t2+c3u2+c4s2u+c5s3u) gives a much lower energy of −2.903 452 763 a.u. It is also shown that the best combination of N terms depends largely on the nuclear charge.
Laser fluorescence study of the Pb+F2, Cl2 reactions: Internal state distribution of the PbCl product and radiative lifetimes of PbF(A,B) and PbCl(A)
Authors: Chen Jing , Dagdigian Paul J.
Published: July 2002

The reactions of Pb atoms with molecular reagents F2 and Cl2 have been studied in a molecular beam apparatus by laser fluorescence detection of PbF and PbCl products. The experiments were performed in a beam-gas configuration under single-collision conditions. The PbCl product from Pb+Cl2 was observed in the vibrational levels v≤17 of the ground X1 electronic state. The PbCl product vibrational state distribution was derived; the average vibrational excitation was found to be 21% of the total available energy. For Pb+F2, PbF(X1) product was detected in only the v=0 and 1 vibrational levels, and the rovibrational state distribution could be characterized by a 300 K Boltzmann distribution. By modulating the Pb beam, it was concluded that this observed product was an artifact and did not arise from bimolecular gas-phase collisions. The radiative lifetimes of the PbF(A,B) and PbCl(A) states were also measured and were found to equal 4.9±0.3 μs, <10 ns, and 1.14±0.06 μs, respectively, averaged over the observed vibrational levels.
Structure of symmetric polyolefin block copolymer thin films
Authors: Foster Mark D. , Sikka Mohan , Singh Navjot , Bates Frank S. , Satija Sushil K. , Majkrzak Charles F.
Published: July 2002

The microstructure of thin films of nearly symmetric poly(ethylene–propylene)–poly(ethylethylene) (PEP-PEE) diblock copolymers (f=0.55, where f is the volume fraction of PEP) was characterized by neutron reflectometry (NR). A symmetric film structure in which the PEE block segregates preferentially to both interfaces is observed above and below the bulk order–disorder transition (ODT). Measurements at room temperature for several chain lengths, N, provide a real-space picture of the change in interdomain interfacial profiles associated with the crossover between the strong and weak segregation limits. The polymer/air and substrate/polymer interfaces are observed to induce an ordered microstructure even when the center of the film is disordered. The characteristic dimension of this near surface microstructure is larger than the corresponding bulk value for values of χN lying between those of the bulk Gaussian-to-stretched-coil and order–disorder transitions, where χ is the segment–segment interaction parameter. This behavior is attributed to the correlation of large amplitude composition fluctuations in the film with the interfaces. A mean-field behavior prevails for χN<(χN)GST,bulk, where some preferential segregation occurs at the interfaces, but the characteristic dimension once again matches that in the bulk.
A systematic analysis of the spectra of trivalent actinide chlorides in D3h site symmetry
Authors: Carnall William T.
Published: July 2002

The optical spectra of actinide ions in the compound AnCl3 and doped into single-crystal LaCl3 were interpreted in terms of transitions within 5fN configurations. Energy-level calculations were carried out using an effective-operator Hamiltonian, the parameters of which were determined by fitting experimental data. Atomic and crystal-field matrices were diagonalized simultaneously assuming an approximate D3h site symmetry. Spectroscopic data were taken from the literature but in most cases supplemented by unpublished measurements in absorption and in fluorescence. Data for each ion were analyzed independently, then the model parameters were intercompared and in many cases adjusted such that in the final fitting process the principal interactions showed uniform trends in parameter values with increasing atomic number. Consistent with analyses of the spectra of lanthanide ions in both LaCl3 and LaF3, abrupt changes in magnitude of certain crystal-field parameters were found near the center of the 5fN series. This resulted in two groups of parameter values, but with consistent trends for both halves of the series, and generally very good agreement between observed and computed energies. A new energy-level chart based on computed crystal-field level energies for each trivalent actinide ion has been prepared.
Spectroscopy of mass-selected tantalum dimers in argon matrices
Authors: Hu Zhendong , Shen Bo , Lombardi J. R. , Lindsay D. M.
Published: July 2002

The absorption (‘‘scattering depletion’’) and Raman spectra of tantalum dimers in an argon matrix have been measured for the first time. The principal absorption band of the dimer (maximum at 480 nm) shows a vibrational progression with ω′0 =260 (15) cm−1 and T0≤21,330 (20) cm−1. Weaker absorptions are observed between 200 and 350 nm and in the region 550–700 nm. Raman spectra (obtained by exciting into the 480 nm band) give ω″e =300.2 (12) cm−1 with ωex″e =1.2(2) cm−1.
Electronic polarizabilities, potential functions, and spectroscopic constants for diatomic molecules of alkali halides and alkali hydrides
Authors: Kumar M. , Shanker J.
Published: July 2002

The two different approaches for the evaluation of molecular state electronic polarizabilities of ions based on Seitz–Ruffa (SR) energy level analysis and Wilson–Curtis–Coker model are critically analyzed by calculating the values of dipole moments within the framework of the original Rittner and the modified T-Rittner models. It is found that the polarizabilities based on SR energy level analysis along with the T-Rittner model are distinctly superior. These polarizabilities are used for determining spectroscopic constants for diatomic molecules of alkali halides and alkali hydrides. Various potential functions for the overlap repulsive energy proposed so far are used to estimate the binding energy and its various order derivatives. A comparison of calculated values with the experimental data shows that the modified Varshni–Shukla potential, among seven traditional potentials, and the Narayan–Ramaseshan (NR) potential, among three ion-dependent potentials, yield the best agreement with experimental data. Using these potentials we have calculated the values of spectroscopic constants viz. vibrational anharmonicity constant, rotational–vibrational coupling constant, and higher order spectroscopic constants. It is concluded that the results obtained by NR potential are distinctly superior to those obtained by the modified Varshni–Shukla potential function.
State-selected Rydberg electron attachment to sulfur hexafluoride clusters at different collision energies
Authors: Desfrançois C. , Khelifa N. , Lisfi A. , Schermann J. P.
Published: July 2002

Rate constants for negative-ion formation have been measured for the electron transfer reactions between state-selected Rydberg atoms and sulfur hexafluoride clusters at various collision energies. At low values of the principal quantum number of the Rydberg atoms, post-attachment interactions not only lead to Coulombic complex formation and internal to translational energy exchange, but also to a new effect which corresponds to an evaporative process due to the influence of the positive atomic core upon the negative clusters.
Optical absorption probes of LixNa4−x (x=1,2): Geometry, substitution isomers, and composition-dependent electron delocalization
Authors: Pollack Stuart , Wang C. R. Chris , Dahlseid Tina A. , Kappes Manfred M.
Published: July 2002

Photodepletion spectra (415–763 nm, 10 Å effective resolution) were obtained for LiNa3 and Li2Na2 and compared to previous Na4 and Li4 measurements as well as to model calculations ranging from classical electrostatic to ab initio molecular orbital. Findings include (i) the assignment of planar rhombic singlet ground states for both molecules; (ii) in Li2Na2, the possibleobservation of predominantly one of three energetically close-lying planar rhombic or near rhombic substitution isomers; and (iii) in the sodium/lithium tetramer series, evidence for increasing localization of itinerant valence electrons with increasing cluster lithium content.
The 3Πu ← X 3Δ1g band system of jet-cooled Ti2
Authors: Doverstål Mats , Lindgren Bo , Sassenberg Ulf , Arrington Caleb A. , Morse Michael D.
Published: July 2002

A band of jet-cooled 48Ti2 has been located in the near infrared by resonant two-photon ionization spectroscopy. Rotational analysis has shown the band to be an Ω′=0±←Ω″=1 transition, which is consistent with the 3Δg ground state proposed by Bauschlicher et al. [J. Chem. Phys. 95, 1057 (1991)]. The band is assigned as a 3Π0u ← X 3Δ1g transition, and lower and upper state bond lengths have been determined as r0(X 3Δg)=1.9422±0.0008 Å and r0(3Πu)=1.997±0.009 Å (1σ error limits, corrected for spin–uncoupling effects). Comparisons are made to the TiV and V2 molecules, a rationale for the unusual filling order of the 3d-based molecular orbitals is provided, and molecular orbital assignments are considered for the excited 3Πu state.
Use of spin-labeling techniques to study the dynamics of surface Penning ionization
Authors: Oró D. M. , Lin Q. , Zhang X. , Dunning F. B. , Walters G. K.
Published: July 2002

Spin labeling techniques, specifically the use of electron-spin-polarized He(2 3S) metastable atoms coupled with energy-resolved spin analysis of the ejected electrons, are used to investigate the dynamics of metastable atom deexcitation by molecules in a film several monolayers thick condensed on a cooled Cu(100) surface. Data for CO2, H2O, Cl2, and O2 films show that metastable deexcitation is analogous to gas-phase Penning ionization, i.e., the condensed molecules can be considered as a ‘‘solid gas.’’ Detailed differences are, however, evident that can be attributed to the presence of neighboring molecules. Data for C60 films are also presented which suggest the possibility of plasmon excitation as is observed in photoionization and electron energy loss data.
Gauge-origin independent multiconfigurational self-consistent-field theory for vibrational circular dichroism
Authors: Bak Keld L. , Jo/rgensen Poul , Helgaker Trygve , Ruud Kenneth , Jensen Hans Jo/rgen Aa.
Published: July 2002

Multiconfigurational self-consistent-field (MCSCF) theory is presented for the gauge-origin independent calculation of vibrational circular dichroism. Origin independence is attained by the use of London atomic orbitals (LAO). MCSCF calculations on ammonia and its isotopomer NHDT demonstrate that atomic axial tensors and vibrational rotational strengths converge fast with the size of the basis set when LAOs are used. The correlation effects are significant both for the atomic tensors and the vibrational rotational strengths even for the single configuration dominated NHDT molecule.
Photoelectron spectroscopy of rare gas dimers revisited: Vibrationally resolved photoelectron spectrum of argon dimer
Authors: Pradeep T. , Niu B. , Shirley D. A.
Published: July 2002

Photoelectron spectra of rare gas dimers Ar2, Kr2, and Xe2 have been measured using the He i radiation at 584 A with a resolution of 13 meV. All the six ionic states in the He i region have been resolved for the first time, including some of the purely repulsive states. Ionization energies are presented to an accuracy ≤0.003 eV. Dissociation energies (D0) of the ionic states are calculated using the best estimate adiabatic ionization energies. The D 2Σ1/2g+ states of all the dimers are weakly bound, contrary to an earlier theoretical prediction. Part of the vibrational structure of two of the electronic states of Ar2 has been resolved. For the A 2Σ1/2u+ state, as many as 14 vibrational excitations are observed. These excitations are assigned to transitions to the higher vibrational levels of v=32–45. For the B 2Π3/2g state, three vibrational structures are resolved and are assigned to v=2–4 of the ionic state. Accurate values of the spectroscopic constants of these states are presented. The present values are compared with the values available in the literature. The De values for the A 2Σ1/2u+ and the B 2Π3/2g states are estimated to be 1.361 and 0.104 eV, respectively.
Rydberg electron transfer to hydrogen iodide: Dissociative and nondissociative electron capture
Authors: Carman Howard S. , Klots Cornelius E. , Compton Robert N.
Published: July 2002

Rydberg electron transfer (RET) to jet-cooled hydrogen iodide (HI) molecules has been studied for alkali atoms excited to ns and nd Rydberg levels (9<n<40). I− ions produced by dissociative electron capture are observed for all Rydberg levels studied, even though dissociative RET becomes endothermic for n<52. For n≳23, rate constants for I− formation are in good agreement with previous results for low energy free electron attachment. For lower values of n, the RET results deviate from the free electron results due to electrostatic interactions between the nascent positive (Rydberg core) and negative ions. A simple calculation based upon the ‘‘free electron model’’ agrees quantitatively with the rate constant data for n≳13. For Rydberg levels with n<13, long-lived HI− (and DI−) ions are observed which appear to be stable with respect to both dissociation and autodetachment. These observations indicate that a bound and stable anion state exists, correlating with the lowest H(D)+I− limit, with a potential well which supports at least one vibrational level for both HI− and DI−.
Electron flow and electronegativity equalization in the process of bond formation
Authors: Cioslowski Jerzy , Stefanov Boris B.
Published: July 2002

Charge-constrained calculations make it possible to rigorously analyze electron flow and electronegativity equalization in the process of bond formation. Such an analysis is performed for the prototypical H2, HF, and LiH molecules. As the bonds are stretched, the dependence of the electronegativity difference on the extent of charge transfer undergoes a transition from approximate linearity to a steplike discontinuous character. With the help of the second-order perturbation theory, the bond hardness is related to the matrix elements of the fragment-electron-count operator and is shown to increase exponentially with the bond length R at the dissociation limit. For polar bonds, the magnitude of the in situ electronegativity difference ΔχAB decreases quickly with R due to the decreasing polarization of the fragments. However, ΔχAB levels off for large distances, and most of the reduction in charge transfer that accompanies bond dissociation can be attributed to the dramatic increase in the bond hardness. The charge-constrained calculations provide both the evidence and explanation for the energy derivative discontinuities that are observed in isolated atoms and molecules.
Magnetism and spin dynamics in MnPS3 and pyridine intercalated MnPS3: An electron paramagnetic resonance study
Authors: Joy P. A. , Vasudevan S.
Published: July 2002

A detailed investigation of the magnetic properties of single crystals of the two-dimensional layered Heisenberg antiferromagnet MnPS3 and pyridine intercalated MnPS3 by static magnetic susceptibility and electron paramagnetic resonance (EPR) is reported. A detailed analysis of the angular and temperature variation of the EPR linewidths and line shapes has been carried out. While MnPS3 orders antiferromagnetically at TN=78 K, the pyridine intercalated compound exhibits isotropic weak ferromagnetism below 70 K. It is known that in the intercalated compound there is a loss of center of inversion so that a canting of spins could arise from either Dzyaloshinsky–Moriya (DM) dijSi×Sj interaction or single ion anisotropy, DSz2. The angular dependence of EPR linewidth and line shape of MnPS3 even at 300 K shows none of the features expected for a low-dimensional magnetic system at high temperatures; a (3 cos2 θ−1)2 angular dependence of linewidth and Lorentzian line shape at θ=55°. Instead, over the entire temperature range studied, the linewidth showed a 1+cos2 θ dependence and the line shapes predominantly Gaussian. The linewidth as well as the Gaussian character increased with decreasing temperature. The results have been interpreted to show that there is substantial intralayer spin–spin correlations and that critical fluctuations manifest at temperatures much above TN. The pyridine intercalated compound, however, shows the expected W-shaped angular dependence of linewidth with a minimum at θ=55° and 135°. The angular variation of linewidth could be fitted to a A+B cos2 θ+C(3 cos2 θ−1)2 dependence. However, the near-Lorentzian line shape at all angles implies that other relaxation mechanisms are operative which is also evidenced from the cos2 θ contribution to the angular variation of linewidth. Evidence for both DM as well as single ion anisotropy in the intercalated compound is presented. The presence of both of the above terms would explain the isotropic weak ferromagnetism observed in the static susceptibility of pyridine intercalated MnPS3.
Ionization induced fragmentation of size selected neutral sodium clusters
Authors: Bewig L. , Buck U. , Mehlmann Ch. , Winter M.
Published: July 2002

The fragmentation behavior for different ionization methods of size selected neutral sodium clusters n≤5 after ionization in a time scale from picoseconds up to tens of microseconds is the subject of this work. Size selection by scattering the cluster beam from a He target beam in combination with a quadrupole mass filter and time resolved measurements at specific laboratory angles enables us to determine the neutral precursor masses of the detected ions. The fragmentation of the neutral clusters depends on the different types of ionization and moreover on the excess energy that is deposited into the cluster during the ionization process. The fragmentation behavior of the sodium trimer follows clearly the energetics. No fragmentation with energetically well-defined 290 nm photons from a laser (0.38 eV excess energy), slight fragmentation with 265 nm laser photons (0.78 eV excess energy), severe fragmentation caused by the photons of the broad band spectrum from the Xe arc lamp with λ≥220 nm (up to 1.75 eV excess energy), and finally total fragmentation under electron impact ionization with 25 eV electrons (2–4 eV excess energy). Similar results have been obtained for the other cluster sizes, mostly in agreement with the energetics that result from calculated and experimental ionic and calculated neutral dissociation energies combined with experimental ionization potentials. An exception is the tetramer fragmentation using the Xe arc lamp with the preferential product ion Na+2. In the measured angular distributions, the fragmentation processes Nan→Na+2 (7≤n≤9) and Na8→Na+5 are observed.
On the preparation and measurement of superpositions of chiral amplitudes
Authors: Cina Jeffrey A. , Harris Robert A.
Published: July 2002

We examine the preparation and detection of superpositions of chiral amplitudes of a handed molecule, showing that specific sequences of phase-controlled ultrashort light pulses enable the preparation and measurement of chiral coherences. It is found that certain choices of relative optical phase between the pulses of the preparation sequence set up left–right superpositions that would be inaccessible by tunneling dynamics alone.
Observation of three resonances in the third harmonic generation spectrum of conjugated polymers: Evidence for the four-level essential states model
Authors: Mazumdar S. , Guo F.
Published: July 2002

Much of the theoretical effort to understand the mechanism of optical nonlinearity in π-conjugated polymers has been focused towards explaining the early observation of only two resonances in the third harmonic generation spectrum of trans-polyacetylene. A recent theory of third order optical nonlinearity predicts three, instead of two, resonances, with their intensities decreasing with increasing frequencies. This is a fundamental difference between the band and the exciton pictures of π-conjugated polymers. Very recent experimental results obtained with cis-polyacetylene, as well as earlier experiments on two blue forms of polydiacetylenes, are in complete agreement with this prediction. An isolated single strand of cis-polyacetylene should exhibit exciton behavior. We briefly discuss the latest experimental results for trans-polyacetylene.
Solvation forces and colloidal stability: A combined Monte Carlo and density functional theory approach
Authors: Frink Laura J. D. , van Swol Frank
Published: July 2002

Osmotic stress (OS) techniques have found wide application in the crystallization of proteins, the study of intermolecular forces between biocolloids, and in studies of the role of solvent in molecular recognition events. In these cases, the role of the solvent is not well understood. We present here a combined Monte Carlo simulation and smoothed density functional theory approach with which model colloid–solvent mixtures may be studied in a potential of mean force approximation. In particular, our simulations focus on parallel cube colloids in Lennard-Jones solvents. We have applied this approach to simulate some OS experiments in which the dependence of the intermolecular separation (h) of DNA strands in aqueous solutions on an applied osmotic pressure (Π) have been measured. Our simulations demonstrate that an underlying oscillatory solvation potential qualitatively produces the features of Π–h relationships observed in the experiments. In particular, increasing Π produces a transition to a more dense colloidal crystal. More importantly, the transition Π decreases with increasing temperature as observed in experiments. In addition, hydrostatic pressure pb and Π are shown to operate conversely in isothermal solvent controlled OS experiments. In addition to characterizing some key features of solvent controlled processes, these simulations reconcile OS experiments with surface forces apparatus (SFA) experiments, and suggest new experimental routes to studying intermolecular forces.
Vibrational spectroscopy of NO+(H2O)n: Evidence for the intracluster reaction NO+(H2O)n→H3O+(H2O)n−2 (HONO) at n
≥
4
Authors: Choi Jong-Ho , Kuwata Keith T. , Haas Bernd-Michael , Cao Yibin , Johnson Matthew S. , Okumura Mitchio
Published: July 2002

Infrared spectra of mass-selected clusters NO+(H2O)n for n=1 to 5 were recorded from 2700 to 3800 cm−1 by vibrational predissociation spectroscopy. Vibrational frequencies and intensities were also calculated for n=1 and 2 at the second-order Mo/ller–Plesset (MP2) level, to aid in the interpretation of the spectra, and at the singles and doubles coupled cluster (CCSD) level energies of n=1 isomers were computed at the MP2 geometries. The smaller clusters (n=1 to 3) were complexes of H2O ligands bound to a nitrosonium ion NO+ core. They possessed perturbed H2O stretch bands and dissociated by loss of H2O. The H2O antisymmetric stretch was absent in n=1 and gradually increased in intensity with n. In the n=4 clusters, we found evidence for the beginning of a second solvation shell as well as the onset of an intracluster reaction that formed HONO. These clusters exhibited additional weak, broad bands between 3200 and 3400 cm−1 and two new minor photodissociation channels, loss of HONO and loss of two H2O molecules. The reaction appeared to go to completion within the n=5 clusters. The primary dissociation channel was loss of HONO, and seven vibrational bands were observed. From an analysis of the spectrum, we concluded that the n=5 cluster rearranged to form H3O+(H2O)3(HONO), i.e., an adduct of the reaction products.
Electronic absorption spectra of large benzene ⋅Arn clusters
Authors: Knochenmuss Richard , Ray Douglas , Hess Wayne P.
Published: July 2002

We report the mass resolved resonant two-photon ionization spectra of C6D6⋅Arn clusters up to n=70. Shifts of the benzene S1←S0 610 vibronic band were studied as a function of both cluster size and expansion conditions. We find that clusters in different size ranges exhibit remarkably different spectra, which also depend on expansion conditions and, hence, cluster internal energy. Below n=16, spectral features trend toward the blue with increasing size. Above n=16, all features exhibit weak size dependence over wide size ranges. In the n=20–40 range, four distinct bands were found, which we suggest could be due to fully enclosed fluxional (−55 cm−1 vs free C6D6), partially enclosed rigid (−43 cm−1), and one-sided rigid (−32 cm−1) or one-sided fluxional (also −32 cm−1, broader) structural types. Above n=40, only one band was definitely identified (at −26 cm−1), which may be due to a one-sided rigid structural type. The trends in spectral shift vs size give no indication of approaching bulk-like solvation of the benzene. Only one subpopulation between n=20 and 40 may show similarity to macroscopic benzene–argon solutions.
An analytical analysis of the square-well fluid behaviors
Authors: Tang Yiping , Lu Benjamin C.-Y.
Published: July 2002

Analytical expressions of the Helmholtz free energy, internal energy, and compressibility for the square-well fluid are developed from the first-order solution of the mean spherical approximation with further application of the Hilbert transform proposed previously. The developed expressions, including that of the first-order radial distribution function, are satisfactorily compared with the Monte Carlo simulation data available in the literature.