bluuebunny/crossref_metadata_2025 huggingface dataset:
Classical formulation of the spectroscopy of nonadiabatic excited-state dynamics
Authors: Stock Gerhard , Miller William H.
Published: July 2002

A general classical formulation of the spectroscopy of nonadiabatically coupled electronic states is developed within the theoretical framework of the classical electron analog model due to Meyer and Miller. Adopting the Heisenberg representation for the calculation of the electric polarization, several approaches are presented to establish a quantum-classical correspondence. It is shown that a consistent classical formulation is obtained by replacing the quantum-mechanical commutators, which appear in quantum-mechanical perturbation theory, by the corresponding classical Poisson brackets which come from classical perturbation theory. The more usual practice of replacing the Heisenberg dipole operators directly by the corresponding classical dipole functions is shown to be an approximation to classical perturbation theory. For a coupled electronic three-state system explicit expressions are derived for the nonlinear polarization. Furthermore the practical application to femtosecond pump-probe spectroscopy is discussed in some detail.
Radius of gyration of uniform H-comb polymers in two and three dimensions
Authors: Bishop Marvin , Saltiel Craig J.
Published: July 2002

Brownian dynamics simulations are made for continuum, excluded volume, uniform H-comb polymers in two and three dimensions. It is found that the radius of gyration power law exponent is 1.50±0.01 and 1.22±0.01 in two and three dimensions, respectively. The g ratio of continuum H-combs is in excellent agreement with lattice Monte Carlo, experimental data, and renormalization group predictions in three dimensions; in two dimensions the leading order renormalization group predictions are unreliable.
Many-body methods for excited state potential energy surfaces. I. General theory of energy gradients for the equation-of-motion coupled-cluster method
Authors: Stanton John F.
Published: July 2002

The formal theory is presented for calculating the analytic first derivative of the energy with respect to arbitrary perturbations within the equation-of-motion coupled-cluster (EOM-CC) approximation. Through use of the Dalgarno–Stewart interchange theorem (Z-vector method), terms involving derivatives of the ground state cluster amplitudes are eliminated, leading to the definition of a new quasiparticle de-excitation operator which simplifies the theory and significantly reduces the expected cost associated with studying potential energy surfaces for excited electronic states. For both illustrative and pragmatic reasons, the final equations are cast in a form similar to that developed for ground state CC energy derivatives, involving contraction of effective one- and two-particle density matrices with matrix elements of the differentiated Hamiltonian. Some aspects regarding calculation of the gradient are discussed with particular attention devoted to similarities between the structure of the present formulas and those which have been previously implemented for the ground state problem.
Molecular dynamic simulations for crystallization of metallic liquids under different pressures
Authors: Qi D. W. , Moore R. A.
Published: July 2002

Molecular-dynamics simulations, using 500 particles, have been performed in order to study the crystallization of supercooled liquid Na under 1 and 2000 atm of pressure. The pseudopotential method is used in order to explicitly include conduction electron contributions, which are found to require careful evaluation by the method of a two-dimensional interpolation. The liquid and crystal structures are analyzed using a pair analysis technique. Under the above increase in pressure and with the same cooling rate, the liquid-bcc phase transition point shifts upward by ∼25 K and the transition zone narrows.
Charge transfer and collision-induced dissociation reactions of CF2+ and CF2+2 with the rare gases at a laboratory collision energy of 49 eV
Authors: Manning Michelle , Price Stephen D. , Leone Stephen R.
Published: July 2002

Multiple product channels are observed for the reactions of CF2+ and CF2+2 with the rare gases at a laboratory collision energy of 49±1 eV. A dication beam is produced in an electron impact ion source and mass selected using a quadrupole mass spectrometer. The ion beam is focused into a collision region and a time-of-flight mass spectrometer is used to monitor the reaction products. Reactions of CF2+ produce CF+, C+, and F+ ions and reactions of CF2+2 result in CF+2, CF+, C+, and F+ ion formation accompanied by the corresponding rare gas ions when charge transfer occurs. The relative yields of these products are measured directly. For reactions of both dications, there is a substantial increase in the total reaction cross section as the rare gas collision partner changes from He to Xe. Collision induced dissociation is the primary reaction between CF2+ and He, while charge transfer dominates the reactions involving Ne through Xe. Stable CF+ states are populated during charge transfer between CF2+ and Ar. Dissociative charge transfer to form C+ ions and F atoms is favored for collisions of CF2+ with Ar, Kr, and Xe. Both He and Ne undergo almost exclusively collision induced dissociation reactions with CF2+2. Nondissociative charge transfer to populate stable states of CF+2 is the most important reaction pathway in collisions of Ar with CF2+2, and dissociative charge transfer to form CF+ ions and F atoms is the principal reaction of Kr and Xe with CF2+2. The trends in charge transfer reactivity are successfully modeled using Landau–Zener theory.
Adiabatic ionization energy of CH3SSCH3
Authors: Li Wai-Kee , Chiu See-Wing , Ma Z.-X. , Liao C.-L. , Ng C. Y.
Published: July 2002

The ionization energy (IE) for CH3SSCH3 has been measured by the pulsed molecular beam photoionization mass spectrometric method. The experimental IE of 8.18±0.03 eV is in excellent agreement with the theoretical prediction of 8.15 eV calculated using the ab initio Gaussian-2 procedure, indicating that the experimental ionization onset can be assigned as the adiabatic IE for CH3SSCH3. The observation of the adiabatic IE(CH3SSCH3) is attributed to the low potential energy barrier for rotation about the S–S bond, which allows CH3SSCH3 to exist dynamically in a wide range of CSSC dihedral angles.
Radical addition to HNO. Ab initio reaction path and variational transition state theory calculations for H+HNO→H2NO and H+HNO→HNOH
Authors: Page Michael , Soto Maribel R.
Published: July 2002

Features of the potential energy surface for hydrogen atom addition to both the nitrogen atom and the oxygen atom of HNO have been investigated at high levels of ab initio theory. For both reactions, vibrational frequencies, moments of inertia, and energies were determined along steepest descent paths at the multiconfigurational self-consistent field level of theory, correlating all valence electrons except the 2s electrons on oxygen (complete active space multiconfigurational self-consistent field distributing 11 electrons among 9 active orbitals). Energies were refined along these reaction pathways using large-scale multireference configuration interaction calculations (all single and double excitations from all configurations generated by a five-electron-in-four-orbital reference space) and large basis sets (correlation-consistent polarized valence triple zeta). The calculated barriers for H-atom addition to the nitrogen atom and the oxygen atom of HNO are 2.68 and 8.99 kcal/mol, respectively. Because of the relatively large barrier for addition to the oxygen atom of HNO, this channel cannot compete with addition to the nitrogen to form the H2NO radical. However, the HNOH radical can likely be formed indirectly by a 1,2 hydrogen migration. Variational transition state theory rate calculations performed over a wide range of temperatures using the ab initio potential energy surface information as input show a substantial variational effect for the calculated association rate constant. The rate of H+HNO association to form a radical complex is slower than the rate of hydrogen atom abstraction at all temperatures; by a factor of more than 20 at room temperature to about a factor of 3 at 2000 K.
Vibrational predissociation in S1 1-methylindole van der Waals clusters
Authors: Outhouse E. Allison , Demmer David R. , Leach Gary W. , Wallace Stephen C.
Published: July 2002

The excited state dynamics of the 1-methylindole(Ar)1, 1-methyl(d3)indole(Ar)1, 1-methylindole(CH4)1, and 1-methyl(d3)indole(CH4)1 van der Waals clusters have been investigated in a free jet expansion. Time and energy resolved emission spectroscopic techniques were used to determine intramolecular vibrational redistribution rates and vibrational predissociation rates. All of the clusters were found to predissociate when excited with sufficient vibrational energy in the S1 state. The predissociation dynamics were found to be consistent with a serial model in which energy transfer from the 1-methylindole skeletal modes to the van der Waals modes precedes the dissociation step. These results are compared with earlier results on indole clusters. The density of vibrational states in the van der Waals clusters was found to be the most important factor in determining the vibrational dynamics. The role of the chromophore skeletal vibrations is also discussed.
Binding of radical species to surfaces: Cluster models for OH on Cu(111)
Authors: Hermann K. , Witko M. , Pettersson L. G. M. , Siegbahn P.
Published: July 2002

The chemisorptive interaction of the hydroxy radical (OH) with the Cu(111) surface and its consequences for the surface electronic structure are studied by CunOH (n ranging from 1 to 25) cluster models using ab initio Hartree–Fock and correlated wave functions. A comparison of the adsorbate binding at the Cu(111) on top, bridge, and the two threefold hollow [face-centered-cubic and hexagonal-close-packed (fcc and hcp)] sites based on extended geometry optimizations yields the fcc site energetically favored with a binding energy EB=3.1 eV at the configuration interaction (CI) level. Correlation contributions account for about 0.9 eV of the binding and are almost independent of the binding site and cluster size. The OH–metal interaction is characterized by considerable Cu to OH charge transfer filling the partially occupied O 2p lone pair orbitals (OH 1π), while OH to Cu charge transfer involving the OH 3σ orbital is small. As a result, the adsorbate is negatively charged in the presence of the surface and ionic binding contributions become important. This makes the OH–metal binding very similar to that of other radicals such as, e.g., OCH3. The adsorbate binding character is also reflected in the results of cluster core and valence hole states corresponding to OH adsorbate ionization. These results may be compared with experimental photoionization data as they become available.
Subpicosecond resolution studies of the H+CO2→CO+OH reaction photoinitiated in CO2–HI complexes
Authors: Ionov S. I. , Brucker G. A. , Jaques C. , Valachovic L. , Wittig C.
Published: July 2002

Time resolved studies of the title reaction have been carried out by photodissociating the HI moiety within weakly bound CO2–HI complexes. The HOCO° intermediate decomposes via a unimolecular decomposition mechanism, and the emerging hydroxyl radicals are monitored with subpicosecond temporal resolution by using laser-induced fluorescence. The measured rates are in good agreement with several theoretical predictions: Rice, Ramsperger, Kassel, and Marcus (RRKM calculations; classical trajectory simulations on the best available potential energy surface; and recent quantum scattering calculations.
Dissociation energy of neutral and ionic benzene-noble gas dimers by pulsed field threshold ionization spectroscopy
Authors: Krause H. , Neusser H. J.
Published: July 2002

Delayed pulsed field threshold ionization of clusters excited to high long-lived Rydberg states is used to study their dissociation behavior. Benzene–Ar and benzene–Kr dimers are excited by resonance enhanced two-photon ionization to Rydberg levels close to various ionization thresholds. The field ionized threshold ions are monitored and separated from the non-energy-selected ions in a reflecting field mass spectrometer with high mass resolution. The appearance of threshold ions at the daughter ion mass indicates the onset of a dissociation process. Daughter ions are first observed for the 16161(3/2) level of the two investigated dimers. This leads to an upper limit of the dissociation energy of benzene–Ar of 340 cm−1 which is probably higher than the true dissociation energy. For the first time threshold ions are observed for large internal energies of some 5 eV in the core indicating that high Rydberg states maintain their long lifetime even if the core is electronically or vibrationally excited by several eV.
An electronic structure study of H2 and CH4 interactions with MgO and Li-doped MgO clusters
Authors: Anchell James L. , Morokuma Keiji , Hess Anthony C.
Published: July 2002

An ab initio study is presented concerning the chemisorption of hydrogen on a model of the (100) surface of MgO and Li-doped MgO. The local surface environment was modeled employing cubic and tetragonal clusters composed of 8 and 12 atoms, respectively. The lattice constant for the clusters was fixed at the experimentally determined value for bulk MgO and the geometry of the adsorbate was optimized at the unrestricted Hartree–Fock (UHF) level of theory. Correlation energy was treated at the second-order unrestricted Mo/ller–Plesset (UMP2) level at the UHF optimized geometry. It was found that H2 undergoes heterolytic dissociation at neighboring three-coordinated Mg and O sites (denoted Mg3c and O3c) in MgO with activation energies of 4.2 and 2.4 kcal/mol at the UHF and UMP2 levels, respectively. Li-doped MgO did not support heterolytic dissociation at neighboring Mg and O sites. Instead H2 was found to dissociate homolytically without barrier at two O3c sites and to undergo hydrogen atom abstraction at O3c and O4c sites. At the UHF/UMP2 level, it was found that at O3c sites, abstraction occurs with a 17.9/3.0 kcal/mol barrier, and at O4c sites, abstraction occurs with a 15.7/0.6 kcal/mol activation energy.
Observation of the A 2Πu←X 2Πg dissociative transition in isolated O2− using mass-selected photofragmentation spectroscopy
Authors: Lavrich David J. , Buntine Mark A. , Serxner David , Johnson Mark A.
Published: July 2002

We report the observation of the A 2Πu electronically excited state of the isolated, gas phase O2− superoxide anion by photofragmentation in the range of 4.5 eV, well above the onset of the electron detachment continuum at 0.45 eV. The action spectrum of O− photoproducts displays an abrupt onset in the vicinity of the known O2− dissociation energy. Quantum yield studies indicate that the dissociative pathway is about 3% relative to the direct electron photodetachment channel. The angular distribution of the ejected O− ions reveals a primarily parallel electronic transition (β=+1.2±0.1), allowing the 2Πu identification of the excited electronic state. This state has been observed previously in fluorescence from O2− in alkali crystals at low temperature, and appears to be the same excited state invoked to account for dissociative attachment of electrons onto neutral O2..
Thermal disorder in adsorbed Cl on Si(100)
Authors: Cheng C. C. , Gao Q. , Dohnalek Z. , Choyke W. J. , Yates J. T.
Published: July 2002

Thermal broadening of Cl+ electron stimulated desorption ion angular distribution (ESDIAD) patterns, related to the vibrational motion of the adsorbate, has been investigated by subtraction of patterns measured at different temperatures. In the case of Cl on Si(100), the observed difference pattern between 130 and 305 K indicates that azimuthally isotropic motion of Cl occurs about the most probable Si–Cl bond direction. Equal distributions from the in-plane and out-of-plane bending motions are observed, correlating with the ∼200 cm−1 frequencies associated with these motions.
A mean field approach to the structure of polyelectrolytes
Authors: Bratko D. , Dawson K. A.
Published: July 2002

We describe a variational mean field study of polyelectrolyte expansion based on the application of the Gibbs–Bogoliubov inequality and a generalized Gaussian trial Hamiltonian. The screened electrostatic interactions among the charged beads on the polyion are approximated by a pairwise additive Yukawa potential while we treat the excluded volume effects in terms of the Dirac δ function in the way usual in studies of neutral polymers. Expressing the Hamiltonian in terms of Fourier components, the variational procedure yields a set of Euler equations that are analyzed by the method of dominant balance to study the scaling regimes in various limiting situations. The method predicts correct scaling laws for weakly screened polyelectrolytes, dominated by long-ranged Coulombic repulsions. At strong screening or low degrees of ionization, when the polymer resembles a self-avoiding walk, the calculations overestimate the scaling exponent, the value of ∼4/3 replacing the Flory value, a deficiency known from earlier applications of the theory to nonionic macromolecules. The numerical solution to the Euler equations is used to calculate the mean square distances between monomer pairs in cyclic polyions as functions of the relative distance along the polymer backbone. Effects of the degree of polymerization and electrolyte screening are studied and the difficulties in providing a general numerical solution to the variational problem are discussed.
A molecular dynamics study of small acetonitrile clusters
Authors: Del Mistro G. , Stace A. J.
Published: July 2002

A molecular dynamic study has been undertaken on small acetonitrile clusters containing between two and nine molecules, with each molecule being treated as a rigid rotor. As a function of kinetic temperature, calculated dynamic quantities, such as the root mean square bond length fluctuation, show that the clusters exhibit distinct solidlike and liquidlike forms. Starting from ordered structures, very marked size differences are observed in the melting temperatures of the clusters, with odd-sized species being found to adopt a liquidlike state at temperatures ∼80 K below neighboring even-sized clusters. This phenomenon is attributed to a strong propensity for polar acetonitrile molecules to form stable antiparallel pairs. Thus, clusters of an odd size contain an unpaired molecule which becomes mobile at comparatively low temperatures. Power spectra calculated from velocity (translational and rotational) autocorrelation functions, show that rotational motion about the symmetry (C3) axis begins at temperatures significantly below the recognized melting points. In contrast, motion about each of the axes perpendicular to the symmetry axis remains hindered even in clusters that are liquidlike. Structure observed in the calculated power spectra for acetonitrile clusters at low temperatures, compares favorably with that seen in the bulk material from several different types of experiment. Such observations are assisted by the fact that the spectra are dominated by features attributable to stable molecular dimers. Qualitative support for some of the results reported here is to be found in recent experiments by Buck and co-workers [Ber. Bunsenges. Phys. Chem. 96, 1275 (1992)] on the photodissociation of small acetonitrile clusters. In particular, the latter show strong evidence of differences in behavior between odd- and even-sized clusters; it is suggested that the experiments could be sampling from a mixture of solidlike and liquidlike forms.
Preparation of ions in selected rotational states by delayed pulsed field ionization
Authors: Merkt F. , Mackenzie S. R. , Softley T. P.
Published: July 2002

A novel method of preparing ions in selected rovibronic states is presented. It is based on pulsed field ionization of long-lived high-n Rydberg states lying just below the different rovibronic energy levels of the ion. The potential of the method is illustrated with the example of hydrogen: A clean population of H2+ ions is prepared in the X 2∑g+ (v+=2, N+=0, 1, 2, 3) states. The method ought to be applicable to the preparation of a wide range of small molecular ions in selected rotational states and opens new possibilities in the study of state-to-state ion–molecule reactions.
On the nature of the interaction energy in the Ar–ClF complex
Authors: Sadlej Joanna , Chal/asiński G. , Szczȩśniak M. M.
Published: July 2002

The analysis of the potential energy surface of the Ar–ClF complex is performed using the perturbation theory of intermolecular forces. The three minima on the potential energy surface correspond to the linear Ar—Cl–F configuration (global minimum De=233 cm−1), the linear Ar—F–Cl configuration (De=133 cm−1), and the T structure in which the Ar atom is nearly perpendicular to the molecular axis of Cl–F (De=146 cm−1). The calculated parameters of the minima are in full accord with the recent ab initio study by Tao and Klemperer [J. Chem. Phys. 97, 440 (1992)]. The absolute minimum results from the attractive dispersion and polarization energies which help overcome a considerable exchange repulsion. The secondary linear minimum Ar—F–Cl, is due, in large measure, to the dispersion energy accompanied by a weaker exchange repulsion. The T configuration is characterized by the weakest repulsion and the dispersion energy roughly equal to that in Ar—F–Cl. The analysis of the angular behavior of the Heitler–London interaction energy leads us to believe that the charge distribution of the Cl–F molecule possesses a concave shape along the molecular axis at the Cl end of the molecule. This indentation in the charge cloud allows subsystems to approach close to one another in the linear Ar—Cl–F arrangement, and also causes an appreciable stiffness of the Ar–Cl–F bending mode.
Comment on ‘‘State-specific unimolecular reaction of NO2 just above the dissociation threshold’’ [J. Chem. Phys. 99, 254 (1993)]
Authors: Wittig C. , Ionov S. I.
Published: July 2002

Spectroscopic studies of NO2 photoinitiated unimolecular decomposition by Miyawaki et al. indicate a very loose transition state just above threshold. If extrapolated to higher energies, this contradicts our time resolved studies of NO2 photodissociation that demonstrate a tighter transition state. We point out that both sets of data are consistent with variational RRKM theory, which predicts tightening of the transition state with increasing energy.
Photofragment spectroscopy and dissociation dynamics of N2+2: A 1Πu–X 1Σ+g
Authors: Martin P. A. , Bennett F. R. , Maier J. P.
Published: July 2002

The 9–0 and 9–1 transitions of the A 1Πu–X 1Σ+g system of N2+2 have been observed by fast ion beam photofragment spectroscopy. Multireference configuration interaction ab initio calculations predict v′=8 to be the highest bound level which seemed to be supported by experimental evidence. The dominant predissociation mechanism for the v′=9 level appears to be through tunneling, unlike the v′=6, 7, and 8 levels, which show an e/f symmetry-dependent electronic predissociation. Calculated tunneling lifetimes from our own modified A 1Πu state potential gives good agreement with the experimental lifetimes. In addition to the 9–1 and 9–0 bands, we have also measured the previously unobserved 7–0 and 8–0 rovibronic transitions thus extending Cosby, Müller, and Helm’s study to the blue [Phys. Rev. A 28, 766 (1983)]. Molecular constants have been determined for the v=0 of the X 1Σ+g state and v=7, 8, and 9 of the A 1Πu state.
Potential energy surface for the hydrogen–iodine reaction
Authors: Anderson James B.
Published: July 2002

The potential energy surface for the H2–I2 system was determined in analytic variational electronic structure calculations with single- and double-substitution configuration interactions for the 16 valence electrons and with effective potentials for the iodine core electrons. The favored pathway for the overall reaction H2+I2→HI+HI was found to pass through the region of the collinear configuration I–H–H–I. The pathway is accessible to bound and unbound iodine atom pairs and it allows the bimolecular and termolecular reactions, H2+I2→HI+HI and H2+I+I→HI+HI, both proposed by Bodenstein 100 years ago.
Approximate calculations of transport properties for the He–CH4 system
Authors: Gianturco F. A. , Serna S.
Published: July 2002

The infinite order sudden (IOS) treatment of the dynamics that yields the necessary collision integrals for the calculation of transport coefficients is tested for a gaseous mixture that includes a nonlinear polyatomic partner, the methane molecule, interacting with He. The use of a fully anisotropic potential obtained before from crossed beam experiments is shown to provide rather good agreement with the available experiments for diffusion and viscosity coefficients over a broad range of temperatures. The comparison with more accurate calculations already carried out by us also shows extremely good agreement between the IOS calculations and the earlier, much more costly, evaluation through a quantum coupled states dynamics. The present calculations therefore indicate that, even for a more complicated nonlinear molecular partner in the mixture, IOS results for diffusion and viscosity coefficients can provide a way of further improving the chosen form for the full potential energy surface of the system.
Pt3Au and PtAu clusters: Electronic states and potential energy surfaces
Authors: Dai Dingguo , Balasubramanian K.
Published: July 2002

We carried out complete active space multiconfiguration self-consistent-field calculations followed by multireference singles+doubles configuration interaction with the Davidson correction which included up to 3.55 million configurations employing relativistic effective core potentials on Pt3+Au and PtAu clusters. Four low-lying electronic states were identified for Pt3+Au. The 2A2 electronic state (C3v) was found to be the ground state of Pt3Au. Spin–orbit effects were found to be significant. We also computed six low-lying electronic states of PtAu and four low-lying electronic states of PtAu+. The 5/2 (2Δ) and 0+(1Σ+) states were found to be the ground states of PtAu and PtAu+, respectively.
Proper integral equations for interaction-site fluids: Exact free-energy expressions
Authors: Lue Leo , Blankschtein Daniel
Published: July 2002

We have developed expressions for the residual Helmholtz free energy and the residual chemical potential in terms of the correlation functions and bridge functions in the context of the interaction-site formalism for the Chandler–Silbey–Ladanyi equations. Unlike the corresponding expressions for the extended-RISM equation, these expressions are formally exact for systems described by interaction-site models. The new expressions are similar in form to those for multicomponent simple fluid mixtures and are found to reduce to them in the extended-atom limit, where the bond lengths approach infinity. We have also found that the residual Helmholtz free energy satisfies a variational principle for a certain class of closure relations. This finding could facilitate the development of more efficient methods for solving the Chandler–Silbey–Ladanyi equations. We have also derived explicit expressions for the residual Helmholtz free energy, residual chemical potential, residual pressure, and residual internal energy in the hypernetted-chain approximation of the Chandler–Silbey–Ladanyi equations. It is noteworthy that the derived expressions depend solely on the correlation functions of the system at full coupling, thus making the computation of the various fluid properties simpler and more efficient by eliminating the need to perform a numerical integration over a coupling constant. We have also found that the residual Helmholtz free energy associated with the hypernetted-chain approximation of the Chandler–Silbey–Ladanyi equations satisfies a variational principle. Furthermore, in the extended-atom limit, all the derived expressions associated with the hypernetted-chain approximation of the Chandler–Silbey–Ladanyi equations reduce to those corresponding to the multicomponent simple fluid mixtures.
The role of electron–electron interactions in liquids
Authors: Kavanaugh Theresa C. , Stratt Richard M.
Published: July 2002

Electron–electron interactions appear to play qualitatively vital roles in the behavior of expanded liquid metals; these systems display phenomena that simply do not occur in a single-electron picture. Motivated by a desire to understand such liquids, and to model electron interaction effects in liquids more generally, we show in this paper how one can formulate and solve a Hartree–Fock theory within a liquid by using liquid theory methods. The work generalizes the previous efforts of Logan and co-workers by removing the restrictions to model band shapes and Hubbard Hamiltonians. The Pariser–Parr–Pople Hamiltonian used here has the added feature of an interatomic Coulombic interaction and therefore allows us to assess the role of interaction-induced fluctuations in the local field at each atom. The model also requires a calculation of a quantity with the significance of a bond order, a concept of some possible utility in a wide variety of electronic-structure-in-liquids problems.
Monomer–excimer kinetics in solution. II. Statistical nonequilibrium thermodynamic approach
Authors: Molski Andrzej , Naumann Wolfgang
Published: July 2002

The statistical nonequilibrium thermodynamic theory of diffusion-influenced reactions is employed to study reversible excimer formation in solution. Three types of rate coefficients for bimolecular excimer formation are discussed: (i) molecular rate coefficients defined by one-way reactive fluxes, (ii) phenomenological rate constants based on form of the macroscopic rate equations, and (iii) modified, time-dependent phenomenological rate coefficients. Relations are derived linking: (i) the molecular rate coefficients for reversible and irreversible excimer formation rates, and (ii) the steady-state molecular rate constant and the Laplace transform of the time resolved irreversible rate coefficient. The relationship between the present approach and the microscopic–stochastic theory of excimer formation is discussed.
The low-lying states of AlCu and AlAg
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Partridge Harry
Published: July 2002

The singlet and triplet states of AlCu and AlAg below about 32 000 cm−1 are studied using the internally contracted multireference configuration-interaction method. A more elaborate study of the X 1Σ+ ground state of AlCu is undertaken using extended Gaussian basis sets, including the effect of inner-shell correlation and including a perturbational estimate of relativistic effects. Our best estimate of the spectroscopic constants (r0, ΔG1/2, and D0) for the X 1Σ+ state with the experimental values in parentheses are: 4.416(4.420) a0, 295(294) cm−1, and 2.318(2.315) eV. The calculations definitively assign the upper state in the observed transition at 14 892 cm−1 to the lowest 1Π state. The calculated spectroscopic constants and radiative lifetime for the 1Π state are in good agreement with experiment. The calculations support the tentative assignments of Behm et al. for three band systems observed in the visible region between 25 000 and 28 000 cm−1. However, the computed spectroscopic constants are in very poor agreement with those deduced from an analysis of the spectra. Analogous theoretical results for AlAg suggest that the (2)3Π, (3)3Π, and (3)1Σ+ states account for the bands observed, but not assigned, by Duncan and co-workers.
New integral equation theory for primitive model ionic liquids: From electrolytes to molten salts
Authors: Babu C. Satheesan , Ichiye Toshiko
Published: July 2002

A new closure to the Ornstein–Zernike (OZ) equation is proposed for ionic liquids and is investigated for primitive models of high valency (2:2) aqueous electrolyte solutions and molten salts. The new closure, which is related to an earlier closure for the soft-sphere case proposed by Ichiye and Haymet, may be viewed as a prescription for the so-called ‘‘bridge functions.’’ These functions are approximated by zero in the hypernetted-chain (HNC) closure which is generally used for ionic systems. In both the new closure and the soft-sphere closure, the recognition that the unlike bridge function is opposite in sign from the like bridge function leads to an approximation for these missing graphs by adding (for the unlike case) or subtracting (for the like case) a set of graphs similar to those used in Percus–Yevick theory to the HNC equation. Compared to the HNC closure, the pair correlation functions predicted for primitive models by the new closure are generally in much better agreement with Monte Carlo (MC) simulations of molten salts and aqueous 2:2 electrolytes. The fundamental improvement of this paper over the Ichiye–Haymet work is that the separation of long- and short-range part of c(r) for the hard-sphere case is clearly defined, whereas it was done numerically for the soft-sphere case. Moreover, the present theory is in better agreement with MC simulations both in the molten salt as well as in the dilute solution regimes than the soft-sphere case. Finally, a study was made of the transition of the like charge pair correlation functions from monotonic behavior at low densities to a nonmonotonic behavior at high densities. The new closure clearly predicts such a transition region at concentrations near 0.02 M and temperatures near 314 K. There is also a region below 0.02 M and 314 K where the new closure fails to converge. Compared to MC simulations, the critical region predicted by the new closure appears to be a lower estimate. However, for the HNC closure there is only a remote possibility of such a transition region since the correlation functions are nonmonotonic even at lower concentrations, a feature which is corrected in the new theory.
The Ã 1A″ state of isocyanogen (CNCN)
Authors: Sherrill C. David , Schaefer Henry F.
Published: July 2002

Ab initio electronic structure theory has been used to characterize the lowest 1A″ excited state of isocyanogen. The geometries, vibrational frequencies, and infrared intensities of the X̃ 1Σ+ and Ã 1A″ states have been determined using the single and double excitation configuration-interaction (CISD) method in conjunction with a triple-ζ plus double polarization (TZ2P) basis set. Adding the Davidson correction and including a set of f polarization functions in the basis set (CISD+Q TZ2Pf), we estimate the energy difference between the ground and first excited singlet state of isocyanogen to be Te = 41 700 cm−1. With a C–N–C bond angle of 117°, this bent 1A″ excited state may be useful in stimulated emission pumping (SEP) experiments on the NCCN■CNCN isomerization.
Ab initio study of the molecules BC and B2C
Authors: Martin Jan M. L. , Taylor Peter R.
Published: July 2002

The potential energy surface for the B2C molecule and the potential energy curve for the ground state of BC have been investigated using full-valence complete active space SCF (CASSCF), augmented coupled cluster [CCSD(T)] and multireference treatments. The ground state of B2C is an extraordinarily stable ring (∑ De=261.6±1 kcal/mol) with two 2-electron π systems. The first excited state is linear BCB (1Σ+g), which is essentially biconfigurational due to a (4σg)–(3σu) near degeneracy. Anharmonic spectroscopic constants were obtained from quartic force fields at the CCSD(T) level with a correlation-consistent basis set of [4s3p2d1f] quality. A severe Fermi resonance exists between the bending and symmetric stretching modes. All computed intensities are fairly weak. Spectroscopic constants for BC using elaborate multireference techniques were very well reproduced using the CCSD(T) method with a spin-restricted Hartree–Fock reference configuration, but not with an unrestricted Hartree–Fock reference. This suggests that even moderate levels of spin contamination that do not significantly affect relative energies may have a detrimental effect on computed spectroscopic constants.
Extension of the quasistatic far-wing line shape theory to multicomponent anisotropic potentials
Authors: Ma Q. , Tipping R. H.
Published: July 2002

The formalism developed previously for the calculation of the far-wing line shape function and the corresponding absorption coefficient using a single-component anisotropic interaction term and the binary collision and quasistatic approximations is generalized to multicomponent anisotropic potential functions. Explicit expressions are presented for several common cases, including the long-range dipole–dipole plus dipole–quadrupole interaction and a linear molecule interacting with a perturber atom. After determining the multicomponent functional representation for the interaction between CO2 and Ar from previous published data, we calculate the theoretical line shape function and the corresponding absorption due to the ν3 band of CO2 in the frequency region 2400–2580 cm−1 and compare our results with previous calculations carried out using a single-component anisotropic interaction, and with the results obtained assuming Lorentzian line shapes. The principal uncertainties in the present results, possible refinements of the theoretical formalism, and the applicability to other systems are discussed briefly.
Rigorous formula for the mean lifetime of diffusion-controlled second-order reactions in solution
Authors: Sumi Hitoshi
Published: July 2002

In general cases of second-order reactions in solution, a species R of molecules with natural lifetime λ0 reacts with another species S with an intrinsic rate constant k(r) that varies with the mutual distance r, following mutual diffusion in the field of an intermolecular potential U(r). An average rate constant k2 is defined from the mean lifetime λ (given by the mean first-passage time) of R as the proportionality coefficient of λ−1−λ−10 to the concentration of S when concentrations of R and S are both sufficiently low. It is proved rigorously that k2 has a general formula of 1/(k−1TST+k−1D), where kTST represents the transition-state-theory rate constant determined by assuming the thermal-equilibrium distribution of r in the potential U(r), and is independent of the mutual diffusion constant D, while the D dependence of k2 is isolated in kD(≳0) which decreases as D decreases. Since k2 nearly equals kTST for a large D but approaches kD as D decreases, kD is called the diffusion limited rate constant. So far it has only been known that this formula is applicable when molecules react coming into contact, but this formula is shown to hold in the general cases. Traditionally, the mean lifetime has been calculated by numerically solving Kolmogorov’s backward equation, which is a second-order differential equation. A new method is given to calculate k2 and kD for arbitrary λ0, D, k(r), and U(r) by solving Fredholm’s integral equation of the second kind. This equation can numerically be solved with much higher precision than the backward equation, since in the digitization approximation of a continuous variable, a double differential in the latter is approximated by a double difference among slightly different small terms, while an integral in the former is approximated by a sum of them.
A density functional study of FeCO, FeCO−, and FeCO+
Authors: Castro Miguel , Salahub Dennis R. , Fournier René
Published: July 2002

The binding energies, structural parameters, and vibrational frequencies of FeCO, FeCO−, and FeCO+ were studied with a linear combination of Gaussian-type orbitals-density functional (LCGTO-DF) method. The ground state of FeCO is found to be 3Σ− and the calculated dissociation energy, with respect to ground state Fe(5D,3d64s2) and CO (1Σ+), is 30 kcal/mol; after correcting for the atomic states separation of the iron atom this value becomes 17 kcal/mol, which is relatively close to the most recent experimental values 8.1±3.5–10.5±3.7 kcal/mol. Quartet ground states were found for both FeCO+ and FeCO− and the calculated dissociation energies (with respect to ground state Fe+, Fe−, and CO) are 50 and 31 kcal/mol, respectively. There is agreement between theory and experiment in that D(FeCO+)≳D(FeCO−)≳D(FeCO). The ωe’s we calculate for FeCO are, in cm−1, 658 (Fe–C stretch), 1982 (C–O stretch), and 368 (bend). These values are reasonably close to their experimental counterparts, 530±10, 1950±10, and 330±50. For FeCO− we have found 566 and 272 cm−1 for the Fe–C stretch and bend modes while the experimental values are 465±10 and 230±40 cm−1. A frequency of 1831 cm−1 is predicted for the C–O stretch of FeCO−. The σ-donation (CO→Fe) and π-back-donation (Fe→CO) charge transfer mechanism is operative in these species.
Absorption and fluorescence of Tl atoms in rare gas matrices
Authors: Wurfel Brent E. , Thoma Anton , Schallmoser Günter , Lammers Andreas , Bondybey Vladimir E.
Published: July 2002

Tl containing Ar, Kr, and Xe matrices were produced by electrode sputtering in a self-igniting pulsed-discharge unit. The absorption and laser-induced fluorescence spectra of the thallium atoms have been investigated. The results are compared with our recent observations of gas phase diatomic thallium–rare gas (Tl–Rg) clusters. The observed linewidths, shifts, and relaxation dynamics are qualitatively interpreted on the basis of the pairwise Tl–Rg interaction potentials.
Finite-temperature characterization of ferrocene from first-principles molecular dynamics simulations
Authors: Margl Peter , Schwarz Karlheinz , Blöchl Peter E.
Published: July 2002

A first-principles molecular dynamics calculation of ferrocene [Fe(C5H5)2] at 300 K, based on the local density approximation, is presented. Kohn–Sham equations and forces on the atoms are obtained using the projector augmented wave technique. Frequencies and eigenmodes are derived by fitting a system of harmonic oscillators to the molecular dynamics trajectory. Frequency shifts for deuterated ferrocene [Fe(C5D5)2, ferrocene-d10] and the corresponding eigenvectors are obtained by diagonalizing the dynamical matrix after renormalizing it according to the changed masses. Most experimental and theoretical frequencies agree to about 2.5% (or 20 wave numbers), with the exception of six modes for which a new assignment is proposed.
Charge induced H2 spectrum in γ-ray irradiated para-H2 crystals
Authors: Momose Takamasa , Kerr Karen E. , Weliky David P. , Gabrys Charles M. , Dickson Robert M. , Oka Takeshi
Published: July 2002

A sharp spectral line has been observed in γ-ray irradiated para-H2 crystals and assigned to the pure vibrational Q1(0)(v=1←0, J=0←0) transition induced via the Condon effect by charges distributed in the crystal. The remarkable sharpness and stability of the signal is reported and discussed.
Transient nucleation in binary vapor of water and sulfuric acid
Authors: Nishioka Kazumi , Fujita Kenshi
Published: July 2002

Transient nucleation is studied for a binary vapor of water and sulfuric acid, in which the acid is assumed to form a hydrate H2SO4⋅2H2O. The rate equation is solved numerically under the condition that only monomers of water and the hydrate exist initially, and it is found that (1) At the first stage the number densities of clusters with single acid molecule but containing more and more water molecules increase very rapidly. (2) Soon the rates of increase in the densities of those clusters slow down, then the densities of clusters with two acid molecules develop from the region around the valley in the contours of the reversible work of cluster formation. These observations seem to support the idea that in the analysis of steady state nucleation the population of clusters may be treated as in equilibrium with respect to exchange with water molecules in the vapor. (3) As for the effect of the difference in the formulas for the reversible work of cluster formation, the conventionally used formula and the one due to Nishioka and Mori, on the transient nucleation behavior, the conventionally used formula may cause significant errors when the critical nucleus obtained from it contains erroneous number of acid molecules. (4) The time lag to reach the steady state is found to be much longer than 1 ms, and it is not possible to continue the computer simulation up to the steady state in the present system.
Surface diffusion of In on Ge(111) studied by optical second harmonic microscopy
Authors: Suni I. I. , Seebauer E. G.
Published: July 2002

Surface diffusion of In on Ge(111) has been measured by optical second harmonic microscopy. This technique employs surface second harmonic generation to directly image submonolayer surface concentration profiles. The coverage dependence of the diffusivity D can then be obtained from a Boltzmann–Matano analysis. In the coverage range 0.1<θ<0.48, the activation energy Ediff decreased with increasing coverage, ranging from 31 kcal/mol at θ=0.1 to 23 kcal/mol at θ=0.48. Over the same coverage range, the pre-exponential factor D0 decreased from 5×102 to 1×10−1 cm2/s. This gradual change reflects a change in diffusion mechanism arising from the disordered nature of the Ge(111) surface. At low coverages, In adatoms sink into the top layer of Ge, and diffusion is dominated by thermal formation of adatom-vacancy pairs. At high coverages, diffusion occurs by normal site-to-site hopping. The gradual change in diffusion parameters with coverage was interrupted by an apparent phase transition at θ=0.16. At this point, both Ediff and D0 peaked sharply at 41 kcal/mol and 6×105 cm2/s, respectively. The desorption energy Edes was measured by temperature programmed desorption. Edes decreased from 60 kcal/mol at submonolayer coverages to 55 kcal/mol at multilayer coverages.
Molecular transition state, resonances, and periodic-orbit theory
Authors: Burghardt I. , Gaspard P.
Published: July 2002

The dynamics of the molecular transition state, in a reaction or photodissociation process, may be analyzed by semiclassical methods. We investigate the classical dynamics of the transition state in the dissociation HgI2 (X 1Σ+g)→hνHgI(X 2Σ+)+I, and apply the semiclassical quantization methods based on periodic-orbit theory. A series of resonances is characterized in a low-energy regime, where the classical dynamics is regular, and at high energies, after a transition to chaos has occurred. In a complementary fashion, we analyze the quantum-mechanical propagation of wave packets. The spectrum which is derived from the quantum-mechanical autocorrelation function is compared with the semiclassical results.
Far-wing excitation study on the fine-structure transition of Hg(3P1–3P) in collisions with N2
Authors: Ohmori Kenji , Kurosawa Takahiro , Chiba Hisashi , Okunishi Misaki , Sato Yukinori
Published: July 2002

Measurements are made of the far-wing excitation spectra of HgN2 collisional quasimolecules for the production of Hg*(3P0) and Hg*(3P1) in the vicinity of the Hg resonance line. It is found that the A–X excitation is by far more effective in producing Hg*(3P0) than the B–X excitation. This is contrary to the case of bound–bound excitation of the HgN2 van der Waals complex [C. Jouvet and B. Soep, J. Chem. Phys. 81, 2229 (1984); Fuke et al., ibid. 86, 4745 (1987); Yamanouchi et al., ibid. 89, 2975 (1988)].