bluuebunny/crossref_metadata_2025 huggingface dataset:
Studies on free energy calculations. II. A theoretical approach to molecular solvation
Authors: Resat Haluk , Mezei Mihaly
Published: July 2002

Using the concepts of scaled particle theory, an analytical theory is developed to investigate the limiting behavior of solvation free energies at the particle creation limit. The new theory directly incorporates the weakly attractive, dispersion interaction terms into the analytical calculations. For neutral molecular systems, the effects of longer ranged electrostatic interactions are also incorporated, albeit in an ad hoc way, and the validity of the utilized assumptions are then demonstrated with numerical examples. It is shown that it is possible to blend the numerical and analytical methods to increase the reliability of quantitative results, and, at the same time, to achieve savings on computational expenditure for certain types of calculations. Different methods of performing the thermodynamic integration in solvation free energy calculations are also compared. Studied examples clearly show the importance of proper treatment of the divergence at the particle creation limit in obtaining quantitatively reliable results for the solvation free energies.
On the choice of inertial axes for interpreting spectroscopic properties of van der Waals complexes
Authors: Ernesti Andreas , Hutson Jeremy M.
Published: July 2002

Properties such as the dipole moments and nuclear quadrupole coupling constants of van der Waals complexes are important in the determination of intermolecular potential energy surfaces from high-resolution spectra. The properties are often interpreted in terms of angular expectation values. It is shown that, when calculating such properties, it is important to use an inertial axis system that satisfies the Eckart conditions. Projections onto other axes, such as the intermolecular vector or the instantaneous principal axes, can lead to substantial errors when the individual monomers have large moments of inertia.
Infrared absorption of cis–cis peroxynitrous acid (HOONO) in solid argon
Authors: Lo Wen-Jui , Lee Yuan Pern
Published: July 2002

Nitric acid (HONO2) isolated in solid argon at 12 K was irradiated with 193 nm emission from an ArF excimer laser. Recombination of the photofragments led to formation of peroxynitrous acid (HOONO) in various conformers. In addition to previously reported lines due to trans–perp HOONO, lines at 3285.4, 1600.3, 1395.0, 927.2, 794.3, and 629.1 cm−1 were observed; they are assigned to cis–cis HOONO having a five-membered hydrogen-bonded ring. The observed vibrational frequencies and the corresponding isotopic shifts of both conformers are in agreement with recent theoretical calculations. Cis–cis HOONO was photolyzed much more rapidly than trans–perp HOONO upon irradiation at 308 nm with a XeCl laser. Lines due to a HO2...NO complex were also observed; they disappeared upon irradiation with the globar source of the infrared spectrometer. The mechanism of formation of various conformers of HOONO is discussed.
Solvation of electronically excited I2−
Authors: Maslen P. E. , Papanikolas J. M. , Faeder J. , Parson R. , ONeil S. V.
Published: July 2002

The interaction potentials between the six lowest electronic states of I−2 and an arbitrary discrete charge distribution are calculated approximately using a one-electron model. The model potentials are much easier to calculate than ab initio potentials, with the cost of a single energy point scaling linearly with the number of solvent molecules, enabling relatively large systems to be studied. Application of the model to simulation of electronically excited I−2 in liquids and CO2 clusters is discussed. In a preliminary application, solvent effects are approximated by a uniform electric field. If electronically excited (2Πg,1/2) I−2 undergoes dissociation in the presence of a strong electric field, the negative charge localizes so as to minimize the total potential energy. However, in a weak field the negative charge localizes in the opposite direction, maximizing the potential energy. Based on a study of the field-dependent potential surfaces, a solvent-transfer mechanism is proposed for the electronic relaxation of 2Πg,1/2I−2, in contrast to the conventional view of relaxation via electron transfer.
Monte Carlo simulations of hydrophobic interactions: A test particle approach
Authors: Forsman Jan , Jönsson Bo
Published: July 2002

By use of the Widom particle insertion method we have evaluated the hydrophobic force and free energy of solvation for both two Lennard-Jones as well as two hard sphere particles in water as a function of their separation. The entropy, energy, and free energy of a single hydrophobic particle were also calculated. By simulating water confined between two hard walls, we were able to obtain the free energy of interaction between a small nonpolar particle and a nonpolar wall. The results show ‘‘contact separation’’ to be the most stable configuration in all cases studied, but the free energy barrier of separation for two nonpolar particles is rather small, of the order of kT. For a nonpolar solute in contact with a hydrophobic wall the barrier is significantly larger, of the order of 4 kT. The effect of system size and geometry on the free energy was investigated. The insertion method produces satisfactory accuracy, provided that the inserted particles are not too large, that a sufficient number of uncorrelated configurations are used, and that some effort is spent on obtaining a fully equilibrated system. The perturbation calculations can be accelerated by a simple prescreening of possible insertion sites.
Molecular dynamics simulation of a nematic liquid crystal
Authors: Komolkin Andrei V. , Laaksonen Aatto , Maliniak Arnold
Published: July 2002

The article describes molecular dynamics simulations of 4-n-pentyl-4′-cyanobiphenyl (5CB) in the nematic phase at 300 K using two interaction models. The first model comprises united atoms, while in the second, shorter simulation, the hydrogen atoms are explicitly included. Liquid crystalline order parameters were calculated using various definitions of molecular frames and were found to be in reasonable agreement with experiments. Distributions of dihedral angles and relative populations of various conformations in the alkyl chain have been determined. Translational and rotational diffusion processes were investigated using time correlation functions, and were compared with experimental results. Local order parameters, relevant for deuterium nuclear magnetic resonance (NMR) spectra, were determined for the segments in the alkyl chain. Proton NMR line shapes were calculated from the trajectory using an approximate method for determination of the dipole–dipole Hamiltonian matrix. These line shapes were found to be very sensitive to conformational distributions and therefore to the force field used in the simulation.
Endohedral adsorption in graphitic nanotubules
Authors: Breton J. , Gonzalez-Platas J. , Girardet C.
Published: July 2002

Calculations based on simple interaction potentials are performed to define the adsorption characteristics of molecules encapsulated in carbon nanotubules. The continuum approximation used to describe the cylindrical sheets is shown to work fairly well within a large range of tubule diameters. Criteria for molecule confinement are given which include the influence of the number of graphitic shells describing the whole tubule and they are compared to similar results obtained for carbon Buckyballs. Application to the most common encaged species, i.e., rare gas atoms and alkali-metal ions, confirms these general rules.
Vibrational photon echoes in a liquid and glass: Room temperature to 10 K
Authors: Tokmakoff A. , Zimdars D. , Sauter B. , Francis R. S. , Kwok A. S. , Fayer M. D.
Published: July 2002

Picosecond infrared vibrational photon echo experiments were performed on the asymmetric CO stretching mode (1983 cm−1) of tungsten hexacarbonyl in 2-methylpentane from room temperature to 10 K using a free electron laser. This is the first report of a room temperature infrared vibrational photon echo in a liquid.
Remarkably small critical exponent for the viscosity of a polymer solution
Authors: Berg Robert F. , Gruner Karen
Published: July 2002

We have measured the apparent critical exponent y characterizing the divergence of the viscosity η∝(T−Tc)−y near the liquid–liquid critical point of the mixture polystyrene in diethyl malonate. The data span the range in reduced temperature of 10−4<(T−Tc)/Tc<10−1. The sample was prepared from the same materials used by Gruner et al. in their capillary viscometer [Macromolecules 23, 510 (1990)]; however our torsion oscillator viscometer had a shear rate 80 times lower. This increased the range of reduced temperatures where shear effects could be neglected. In spite of the large reduction in shear rate and the different viscometry technique, the parameters fitted to our data and those of Gruner et al. are in agreement. For this polymer solution, y is in the range 0.028±0.003, close to recent results for two other polymer solutions measured in capillary viscometers. However, it is significantly smaller than the exponent for pure fluids (0.041± 0.001) and simple binary mixtures (0.042±0.002). It appears that polymer solutions are in a dynamic universality class different from that of simpler fluids.
Fourier representation method for accurate evaluation of the Coulomb and exchange lattice sums in extended chains
Authors: Delhalle J. , Cizek J. , Flamant I. , Calais J. L. , Fripiat J. G.
Published: July 2002

By considering chains of hydrogen and helium atoms with one Gaussian function centered on each atomic sites, i.e., the simplest possible metallic and insulating model systems retaining all the characteristics of the restricted Hartree–Fock–Roothaan method for extended chains, a scheme is proposed to carry out accurately and efficiently both Coulomb and exchange lattice sums which occur in a Gaussian basis set environment.
The extension of wave operator-Floquet formalism to molecular photodissociation processes with short laser pulses
Authors: Jolicard Georges , Billing Gert Due
Published: July 2002

Photodissociations induced by intense short laser pulses are investigated using a generalized Floquet formulation. This formulation replaces the continuous electric field shape function by a discrete step function. An initial active space is defined by using the Bloch’s wave operator theory and the corresponding target spaces, in which the dynamics is confined, are built for each new discrete value of the electric field magnitude. A powerful computational method of calculation of dissociation probabilities is elaborated on by calculating and storing some basic data at each step of the discretized shape function—the eigenvalues of the target spaces, the overlap matrix between two successive groups of Floquet eigenvectors, and finally the projection of the Floquet eigenvectors onto the bound states. The calculation is achieved by exploiting these data in a fast integration of the dynamic projected into the active space. An illustrative example, concerning the photodissociation of H+2, reveals that the procedure is particularly suitable for studying the influence of the pulse shape.
Quasiregular spectral features of the strongly chaotic Fermi resonant system
Authors: Zembekov Alexander A.
Published: July 2002

Numerical evidence is reported of the quasiregular power spectra in the strongly chaotic system of coupled Morse and harmonic oscillators with the 1:2 frequency ratio. The spectra are shown to consist of a ‘‘regular’’ part, characterized by sharp peaks and a chaotic one which resembles a smooth chaotic background. The regular part does not seem to be related to the visible islands of stable motion as is the case of the standard map. The observed regularity of the spectra is shown to be associated with the repeated trapping of the chaotic trajectory by the localized regions of marginal stability. The appearance of such regions is the direct consequence of the global tangent bifurcations near the borderline of the system. The analysis of the spectra of local Lyapunov exponents and periodic orbits analysis seem to account for the most important qualitative features of the underlying chaotic dynamics. In particular we show that a relatively small number of low-period saddle-center pairs born in tangent bifurcations is sufficient to characterize marginally stable sets and to determine their recurrence properties as well.
Ab initio molecular dynamics study on the thermal stability of Na8 microcluster
Authors: Tse John S. , Klug Dennis D.
Published: July 2002

The thermal stability of a Na8 microcluster has been modeled with the first principles quantum molecular dynamics technique. Three distinct transitions were observed and characterized with the calculation of their phonon spectra. The results show that Na8 cluster can exist in rigid, disordered and liquidlike phases. These observations substantiate a previous classical molecular dynamics calculation using an empirical potential.
A model of reversible reaction with slow intramolecular relaxation
Authors: Kurzyński Michal/
Published: July 2002

The model studied describes intramolecular stochastic dynamics in terms of diffusion in a parabolic potential along a coordinate of various possible physical interpretations. The chemical transformation is considered as a transition perpendicular to this coordinate. Equations of dynamics are formally equivalent to a Schrödinger equation with a spin that can take two values. In the slow diffusion limit the reaction can be either an ordinary activated process, studied already in the literature, or can be controlled by a nonchemical variable—the mean value of the diffusion coordinate. For the latter case an asymptotically exact nonlinear kinetic equation is derived and discussed. Importance of the initial stage of the reaction, when a local equilibrium is being reached between species for each value of the diffusion coordinate, is indicated. Results of the approximation proposed are compared with the actual course of reaction found by direct numerical solution of the starting equations. A distinct case of ‘‘gated’’ reversible reaction, when fast perpendicular transitions are sharply localized at a certain point on the diffusion coordinate, is considered separately.
Radiative lifetimes of HBr+ and DBr+ (2Π1/2, v=0, 1)
Authors: Jullien S. , Lemaire J. , Fenistein S. , Heninger M. , Mauclaire G. , Marx R. , Chambaud G. , Rosmus P.
Published: July 2002

Radiative lifetimes of HBr+ and DBr+(2Π1/2, v=0, 1) have been determined using the monitor ion technique in a triple cell ICR spectrometer with Fourier transform detection. The experimental lifetimes corresponding to the vibrational transition v=1→0 are 10.2±1 ms and 56+12−11 ms for HBr+ and DBr+, respectively. The lifetimes calculated by a coupled electron pair approximation (CEPA) method similar to the method used previously for HF+ and HCl+ are 8.8 ms for HBr+ and 35.8 ms for DBr+, in rather good agreement with the experimental results. A comparison with previous experimental and theoretical results on hydrogen halides shows a shortening of the lifetime between the neutrals and the corresponding ions and a lengthening with isotopic substitution (H by D) as well as with substitution of F by Cl or by Br. The lifetimes corresponding to spin–orbit relaxation are very long 852+621−282 ms for HBr+ and 965+546−323 ms for DBr+. They can be considered as equal within the quite large experimental uncertainty on the contribution of collisional deactivation.
Semiclassical calculation and analysis of dynamical systems with mixed phase space
Authors: Sepúlveda Miguel Angel , Heller Eric J.
Published: July 2002

A semiclassical method for the propagation of arbitrary wave packets in a multidimensional Hamiltonian is presented. The method is shown to be valid for treating Hamiltonian systems whose classical phase space is a combination of chaotic and quasiperiodic motion (mixed dynamics). The propagation can be carried out long enough for the nonlinearities of the system to be important. The nonlinear dynamics is reflected in spectra and correlation functions. We suggest this new semiclassical method can be a tool for analyzing the nonlinear aspects of the vibrational spectra.
Pressure dependence of magnetic coupling in ionic solids from abinitio cluster model calculations
Authors: Casanovas Jordi , Illas Francesc
Published: July 2002

The dependence of the magnetic coupling constant, J, with the pressure has been studied by an ab initio cluster model approach in a typical ionic solid such as KNiF3. By computing J at different values of the lattice parameter R, we predict a power law of the form ‖J‖≊R−n with 10.5<n<13.3 in good agreement with the suggested experimental value. This behavior is found even for the simplest Ni2F cluster model described by a complete active space configuration interaction wave function in which the active orbitals correspond to the open shell of each Ni2+ cation.
Crossed beam rovibrational energy transfer from S1 glyoxal. III. Quantitative H2 and He cross sections for (0, K′=0) and (72, K′=0) and comparison with theory
Authors: Gilbert Brian D. , Parmenter Charles S. , Krajnovich Douglas J.
Published: July 2002

Results of crossed-molecular beam inelastic scattering experiments from two levels of 1Au (S1) trans-glyoxal in collisions with H2 (Ec.m.=80 meV, 650 cm−1) and He (Ec.m.=95 meV, 770 cm−1) are reported. Relative inelastic scattering cross sections with quantitative error bars are obtained. S1←S0 laser excitation was used to prepare either the 00 level or the 72 level (εvib=466 cm−1) with the high rotational selection of K′=0 and J′=0–10. The final high levels populated by rotationally and rovibrationally inelastic scattering were monitored by dispersed fluorescence with K′ state resolution. Fluorescence from only those molecules involved in inelastic scattering was obtained from the difference signal of spectra with the target gas beam (H2 or He) ON and OFF. Those dispersed fluorescence spectra were analyzed with a computer fitting procedure to extract 52 relative state-to-state cross sections for scattering from the (00, K′=0) level and 84 for the (72, K′=0) level. The cross sections have been compared quantitatively with the results of the three-dimensional quantal scattering calculations of Clary, Dateo, Kroes, and Rettschnick. The agreement between the experimental and theoretical cross sections is nearly quantitative for both target gases and both initial states. Disagreements between experiment and theory occur only in the details of K′ distributions within the rovibrational channels. The vibrationally inelastic scattering is extremely selective among the many accessible channels. The cross sections for Δυ7=±1 changes in the lowest frequency mode ν7′ = 233 cm−1, a CHO–CHO torsion, exceed those involving the other 11 modes by at least an order of magnitude. As judged by the relative magnitudes of rovibrational and rotational cross sections, rovibrational scattering is surprisingly efficient. In fact, when comparing cross sections for transferring energy by (T→R) vs (T→R,V) with similar ΔE, rovibrational cross sections are the greater in numerous examples.
A simplified approach to molecular dynamics simulations of liquid crystals with atom–atom potentials
Authors: Cross C. W. , Fung B. M.
Published: July 2002

Molecular dynamics simulations have been performed for the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl (5CB) using a realistic atom–atom potential as well as a pseudorealistic atom–atom potential. The latter treats the phenyl rings in the 5CB molecule as simple spheres. This method allows a considerable decrease in computational time while still retaining important aspects of the structural anisotropy. Molecular ordering has been studied using both approaches by calculating order parameters for the principle molecular axis as well as for individual molecular segments. Molecular conformations and dynamics have been examined using dihedral angle distribution functions for the biphenyl core and for the alkyl chain. Analysis of the data allows the inter and intra molecular contributions to the conformational equilibria of the chain to be separated. This information can be used to determine the conformational equilibria of the alkyl chain under the influence of the nematic field created by surrounding molecules. Transport properties have been examined by calculating diffusion coefficients in directions both parallel and perpendicular to the nematic director. All properties calculated using the pseudomolecular potential are compared with those obtained from the more complete atom–atom potential to assess the validity of the simplified approach.
Tunneling splittings in the S1 electronic states of symmetrically substituted 3,7-dichlorotropolone, 3,5,7-trichlorotropolone, and 3,7-dibromotropolone
Authors: Sekiya Hiroshi , Tsuji Takeshi , Ito Sayaka , Mori Akira , Takeshita Hitoshi , Nishimura Yukio
Published: July 2002

The S1–S0 electronic spectra of 3,7-dichlorotropolone, 3,5,7-trichlorotropolone, and 3,7-dibromotropolone have been measured in a supersonic free jet to investigate the effect on proton tunneling of symmetrical substitution of Cl or Br atoms for H atoms of tropolone. The 000 tunneling splittings in S1 of 3,7-dichlorotropolone, 3,5,7-trichlorotropolone, and 3,7-dibromotropolone are 45, 31, and ≲1 cm−1, respectively. The increase of the tunneling splitting for 3,7-dichlorotropolone compared to that for tropolone (19 cm−1) is attributed to a decrease in the O–H...O heavy atom separation, leading to smaller potential barrier height. The decrease of the 000 tunneling splitting for 3,5,7-trichlorotropolone compared to 3,7-dichlorotropolone and remarkable decrease of the 000 tunneling splitting for 3,7-dibromotropolone are ascribed to significant changes of the normal coordinates compared to those of tropolone.
Sub-Doppler infrared spectroscopy of HCCCN–BF3 (v1) and HCN–BF3 (v1 and 2v1)
Authors: Kerstel E. R. Th. , Pate B. H. , Mentel T. F. , Yang X. , Scoles G.
Published: July 2002

Using a molecular beam opthothermal spectrometer, we have recorded the sub-Doppler ir rovibrational spectra of the CH stretching vibrations of the C3v symmetric-top van der Waals molecules HCCCN–BF3 (v1 fundamental) and HCN–BF3 (v1 fundamental and first overtone). At first sight, all spectra appear regular, showing no signs of extensive (local) perturbations as might have been expected from the similarities in structure between HCN–BF3 and the stable molecule HCCCF3, of which the 2v1 spectrum in particular presents a clear example of intermediate case intramolecular vibrational energy relaxation. However, closer inspection reveals that the HCCCN–BF3 spectrum shows evidence of an isotope dependent vibrational predissociation rate, the 11B complex dissociating at least twice as fast as its 10B counterpart. The complexation induced red shift equals 2.70 cm−1, and is in the same range as observed for the majority of the HCCCN and HCN complexes studied to date. In contrast, the HCN–BF3 spectra show instrument limited linewidths, therewith providing only a lower limit to the lifetime, but the complexation induced frequency shifts are clearly anomalous: the fundamental spectrum is red shifted by only 0.15 cm−1, while the overtone spectrum is blue shifted by 0.66 cm−1. These observations are rationalized by invoking long-range (anharmonic) vibrational interactions in both the HCCCN–BF3 and the HCN–BF3 molecule.
Optical dephasing on femtosecond time scales: Direct measurement and calculation from solvent spectral densities
Authors: Vöhringer P. , Arnett D. C. , Westervelt R. A. , Feldstein M. J. , Scherer N. F.
Published: July 2002

The connection between dephasing of optical coherence and the measured spectral density of the pure solvent is made through measurements and calculations of photon echo signals. 2-pulse photon echo measurements of a cyanine dye in polar solvents are presented. Signals are recorded for both phase matched directions enabling accurate determination of the echo signal time shift. Echo signals are calculated by two approaches that employ the response function description of nonlinear spectroscopy; (i) a single Brownian oscillator line shape model, and (ii) the line shape obtained using the solvent spectral density. The strongly overdamped Brownian oscillator model incorporates only a single adjustable parameter while the experimental data present two fitting constraints. The second model incorporates the measured solvent spectral density. Both give very good agreement with the experimental results. The significance of the second method lies in this being a new approach to calculate nonlinear spectroscopic signals, for comparison with experimental data, that uses directly the measured spectrum of equilibrium fluctuations of the solvent. This approach also provides a better conceptual perspective for deriving insight into the nature of the solute–solvent coupling mechanism. Comparing the parameters for the strength of interaction in a variety of polar solvents it is found that the coupling involves the solvent polarizability and not the solvent polarity. The interaction mechanism cannot be deduced from the Brownian oscillator calculations.
Electronic excitation dynamics of Li(H2)2: Dissociation mechanisms, lifetimes, and the validity of a hybrid quantum/classical approach
Authors: Li Zhiming , Gerber R. Benny
Published: July 2002

The dissociation dynamics of the cluster Li(H2)2, following the 2s→2p excitation of the Li atom, is studied in the framework of a collinear model. The process was investigated by exact quantum wave packet calculations, and the results were used to test a hybrid quantum/classical method, in which the highly quantum mechanical initial state of the cluster is described by a wave function, and the latter is used to sample initial positions and momenta for a classical treatment of the excited state dynamics. We found that the dynamics was dominated by two predissociation processes, Li*(H2)2→Li*–H2+H2 and Li*(H2)2→Li*+(H2)2, with the latter process having a higher yield. A relatively long dissociation lifetime, ∼10 ps, was found for the excited cluster. The slow vibrational predissociation rate was interpreted as due to the very low density of state involved. The hybrid quantum/classical approach was found to give product vibrational energy and velocity distributions in good accord with the distribution from exact calculation. However, the lifetimes from the hybrid approach were found to be much shorter than those from the exact quantum calculations. The hybrid approach is thus applicable even to photoexcitation of quantum clusters for studying certain selected properties.
Photodissociation of HOD (νOD=3): Demonstration of preferential O–D bond breaking
Authors: Cohen Y. , Bar I. , Rosenwaks S.
Published: July 2002

It has been predicted that photodissociation of vibrationally excited HOD may preferentially yield either OD+H or D+OH, depending on the vibrational mode and the dissociation wavelength. To date, only the former preference has experimentally been demonstrated. In the present work preferential O–D bond breaking has been achieved from the photolysis of HOD (νOD=3) at 193 nm. HOD was prepared in a specific rovibrational level of the second overtone of the O–D stretch via infrared excitation. The subsequent photolysis led to enhancement of both OH and OD production, the OH/OD branching ratio being 2.6±0.5. The results agree with the predictions of Imre and co-workers [J. Phys. Chem. 93, 1840 (1989)] on the enhancement of νOD≥3 photodissociation but differ in the branching ratio obtained at the specific photolysis wavelength.
Investigation of the role of triplet states in the Wulf bands of ozone
Authors: Braunstein M. , Martin R. L. , Hay P. J.
Published: July 2002

Results of spin–orbit interaction calculations on the three lowest triplet states of ozone, 3B2, 3A2, and 3B1, are presented. These calculations use a one-electron, one-center approximation of the spin–orbit operator and a modest number of configurations within a 3s2p1d basis to estimate the amount of singlet character in these predominantly triplet states and triplet character in the predominantly singlet ground state. The results are given in the context of recent studies which predict that one of these triplet states gives rise to the Wulf optical absorption bands of ozone, a series of weak diffuse peaks in the near infrared, just above the O2+O dissociation threshold. Without spin–orbit coupling, these triplet states are spin forbidden from the singlet ozone ground state, 1A1. The present results point to the 3A2 and 3B1 states as playing a possible role in the Wulf bands, but more complete calculations will be needed to confirm this.
Microwave spectroscopy and interaction potential of the long-range He...Ar+ ion
Authors: Carrington Alan , Leach Christine A. , Marr Andrew J. , Shaw Andrew M. , Viant Mark R. , Hutson Jeremy M. , Law Mark M.
Published: July 2002

We have measured and interpreted a microwave spectrum of the HeAr+ ion in which all of the observed energy levels lie within 8 cm−1 of the lowest dissociation limit, He(1S)+Ar+(2P3/2). We use an ion beam technique in which the HeAr+ ions are formed by electron impact, accelerated to kilovolt potentials, and mass-analyzed. After passage through an appropriate section of waveguide, the ions enter an electric field lens in which state-selective fragmentation occurs; the Ar+ ions produced in the lens are separated from all other ions by means of an electrostatic analyser and detected with an electron multiplier. Microwave transitions induced in the waveguide section result in population transfer which produces detected changes in the electric field-induced Ar+ fragment current. Many transitions have also been observed by a microwave–microwave double resonance technique. We have observed 68 lines spanning the frequency range 6–170 GHz; no immediately recognizable pattern is apparent. We have measured the Zeeman splitting produced by a small axial magnetic field for almost every line, which enables us to determine the values of the total angular momentum J involved in each transition, and also effective g factors for the two levels involved. We are therefore able to construct a purely experimental pattern of 37 levels lying within 8 cm−1 of the dissociation limit. The data are treated first by means of a conventional effective Hamiltonian in a case (c) basis, which allows electronic and vibrational quantum numbers to be assigned to most of the levels; the assignments are approximate, however, because very strong rotational-electronic coupling undermines the Born–Oppenheimer approximation.

A more complete theoretical treatment is then presented, using the coupled-channel method in a case (e) representation to calculate the energy levels without making the Born–Oppenheimer approximation. The microwave transition frequencies and g-factors are fitted, together with earlier ultraviolet spectra, to provide a new interaction potential (designated MAL1) for He interacting with Ar+(2P3/2 and 2P1/2). The MAL1 potential is substantially more accurate than previous potentials, especially in the long-range region and for the A1 2Π3/2 state, which had not been observed before. An important new feature of the MAL1 potential is that the long-range C6 coefficient is strongly anisotropic, so that the different electronic curves have substantially different C6 coefficients.
The adsorption of acetylene on Ni(110): An experimental and theoretical study
Authors: Weinelt M. , Huber W. , Zebisch P. , Steinrück H.-P. , Ulbricht P. , Birkenheuer U. , Boettger J. C. , Rösch N.
Published: July 2002

The adsorption of acetylene on Ni(110) was investigated by temperature programmed desorption (TPD), low energy electron diffraction (LEED), angle resolved ultraviolet-photoelectron spectroscopy (ARUPS), and near edge x-ray absorption fine structure (NEXAFS) measurements, as well as by detailed model cluster calculations and slab model band structure calculations. By combining the experimental results and those of the cluster studies an orientation of the molecules is deduced with the C–C axis parallel to the surface and preferentially aligned along the substrate troughs ([11̄0] azimuth) as well as with a highly coordinated adsorption site in the substrate troughs. A detailed analysis of the photoemission spectrum is given. The proposed adsorption geometry is corroborated by band structure calculations for various alternative sites and orientations that are shown to be very sensitive to the azimuthal orientation of the adsorbed molecules.
Asymmetric adiabatic correction to the rotation–vibration levels of H2D+ and D2H+
Authors: Polyansky Oleg L. , Dinelli Bianca M. , Le Sueur C. Ruth , Tennyson Jonathan
Published: July 2002

Calculations on H2D+ and D2H+ have shown that the energy levels of these asymmetric isotopomers of H3+ cannot be reproduced using effective potential energy surfaces with D3h symmetry. It is shown that for these ions the adiabatic correction to the Born–Oppenheimer approximation has an asymmetric component which can be expressed as a mass-independent surface multiplied by a mass factor. An expression for this function is obtained from ab initio calculations. Use of this adiabatic correction is found to resolve the discrepancy with the levels of H2D+ and D2H+. The ab initio calculations reported reproduce the observed H2D+ transitions with an average error (obs−calc) of −8 MHz for the rotational transitions, −0.06 cm−1 for the ν1 band, −0.13 cm−1 for ν2, and −0.19 cm−1 for ν3. These errors are nearly constant for all transitions within a vibrational band. This gives a very accurate ab initio framework for predicting unobserved transition frequencies.
CH3 state distributions form the reactions of O(1D) with saturated and chlorinated hydrocarbons
Authors: Schott R. , Schlütter J. , Olzmann M. , Kleinermanns K.
Published: July 2002

CH3 product state distributions arising from the reaction of O(1D) with CH4, C2H6, C3H8, and i-C4H10 were characterized using resonantly enhanced multiphoton ionization (REMPI). The vibrational distributions in the ν1 symmetric stretch and in the ν2 ‘‘umbrella’’ mode are noninverted in all cases. The results are compared to statistical adiabatic channel-phase space theory (SACM-PST) calculations. The ν2 excitation is much less excited than statistically expected for the reactions with CH4 and C2H6 but nearly statistical in the case of the heavier hydrocarbons. For all reactions but the one with C2H6, the ν1 excitation is similar to the distribution statistically expected. The CH3 rotational distribution from the reaction of O(1D) with CH4 is much hotter than room temperature as comparison with simulated rotational band contours showed. From the integrated CH3 band intensities, we obtained approximate reaction cross sections for the CH3 channel with a decrease in the order CH3≳C2H6≳C3H8≳i-C4H10. In the reactions of O(1D) with CH3Cl, CH3CH2Cl, and CH3CH2CH2Cl, we observed CH3 as a primary product. CH3–ν1 excitation is small in all cases but ν2 excitation is considerable with even an inverted v=1/v=0 distribution in the case of the reaction with C2H5Cl. The relative reaction cross sections for the CH3 channel decrease in the order CH4≫CH3Cl≳CH3CH2Cl≳CH3CH2CH2Cl.
Vibrational state controlled bond cleavage in the photodissociation of isocyanic acid (HNCO)
Authors: Brown Steven S. , Berghout H. Laine , Crim F. Fleming
Published: July 2002

We report the bond selected photodissociation of isocyanic acid (HNCO). This molecule dissociates from its first excited singlet state, breaking either the N–H bond to form H+NCO (X 2Π) or the C–N bond to form NH (a 1Δ)+CO (1∑+). The threshold for production of NH lies about 3900 cm−1 above that of NCO, and we detect both of these channels by laser induced fluorescence on either the NH or the NCO fragment. Dissociating the molecule out of a vibrationally excited state on its ground electronic surface containing four quanta of N–H stretch (4ν1) enhances the efficiency of the NCO channel over the NH channel by a factor of at least 20. We reach this conclusion by comparing the results of such a vibrationally mediated photodissociation experiment to those from a conventional single photon dissociation at the same total energy (about 1000 cm−1 above the threshold for the NH channel). Our estimate of the branching ratio in the one photon dissociation at this energy is roughly ΦNCO/ΦNH≊20, and it grows to ΦNCO/ΦNH≥400 in the vibrationally mediated photodissociation.
Diffusion-controlled reaction rate to asymmetric reactants under Coulomb interaction
Authors: Traytak S. D. , Tachiya M.
Published: July 2002

The rate constant for diffusion-controlled reactions between asymmetric reactants described by the simple model of Solc and Stockmayer under the influence of Coulomb-type interaction is considered. Using the method of dual series relations, we calculate the rate constant with a high accuracy and obtain some approximate analytical formulas. We compare our results with an approximate analytical formula derived before by the constant-flux approximation and with numerical calculations based on the Brownian dynamics simulation. It is shown that in the case of strong attractive potential the rate constant only slightly depends on the size of the active site and tends to the classical Debye result for isotropically reactive particles with a further increase in the Onsager length. Moreover, it is shown that for small-sized active sites the effect of the interaction potential is to scale the rate constant for neutral reactants by a Boltzmann factor, which was first conjectured by Zhou [Biophys. J. 64, 1711 (1993)].
Lifetimes of Rydberg states in zero-electron-kinetic-energy experiments. II. Electric field induced and collisional enhancement of Xe autoionization lifetimes
Authors: Vrakking Marc J. J. , Lee Yuan T.
Published: July 2002

Lifetime measurements are reported for high principal quantum number (n=43–87) autoionizing Rydberg states of the Xe atom, prepared using state-selective excitation with a narrow bandwidth laser. It is demonstrated that the lifetimes can be increased by l-mixing in dc electric fields, as well as by l,m-mixing through the presence of surrounding Xe+ ions. The influence of l,m-mixing interactions on the peak intensities in zero-electron-kinetic-energy (ZEKE) photoelectron spectroscopy experiments is discussed, and a possible method for increasing the applicability of ZEKE experiments is suggested. In addition, experimental evidence is given for spin–orbit state changing Xe*(n)–Xe collisions.
Solvation of lithium chloride in water–hydroxylamine mixtures: A theoretical investigation by means of Monte Carlo simulations
Authors: Suwannachot Yuttana , Hannongbua Supot , Rode Bernd M.
Published: July 2002

Monte Carlo simulations have been performed for LiCl containing mixtures of water and hydroxylamine. The structure of solvate species formed by the ions depending on the solvent composition has been analyzed through radial distribution functions and coordination number distributions and is discussed in relation to solvation energy contributions. A number of factors, including preferential ligand binding, spatial conditions, and H-bonding possibilities, determine the formation of solvated ion species, whose average composition is, in most cases, arising from a wide variety of coordinations.
Relation between the orientational ordering and the tricritical behavior for smectic-A to smectic-C phase transition
Authors: Bräuniger Thomas , Fung B. M.
Published: July 2002

The smectic-A to smectic-C transition for two binary mixtures of liquid crystals has been studied. These systems are mixtures of a 4′-(2-chloroalkoxy)-4-heptyloxybiphenyl, designated B7 or C7, with 4-butyloxyphenyl-4′-decyloxybenzoate (10O4̄). Pure B7 and C7, in their racemic forms, display characteristics of a first-order smectic-A to smectic-C transition. With increasing mole fraction of 10O4̄, the behavior of the phase transition changes from first order to second order. The order parameter of the mesogenic core of B7 and C7 in these systems was determined by the use of 13C nuclear magnetic resonance. It was found that the order parameter of the liquid crystal systems at the transition temperature is discontinuous for a first-order smectic-A to smectic-C transition, and continuous for a second-order transition. The composition of the tricritical mixture (XTCP, where X is the mole fraction of 10O4̄) was determined to be XTCP=0.12±0.01 for the B 7/10O4̄ system and XTCP=0.13±0.01 for the C7/10O4̄ system. The former has not been reported before, and the latter agrees reasonably well with previous studies.
Comment on ‘‘The protonated water dimer: Extensive theoretical studies of H5O+2’’ [J. Chem. Phys. 101, 4878 (1994)]
Authors: Pudzianowski Andrew T.
Published: July 2002

An inappropriate comparison in the title article between a calculated D0 and one group of experimental dissociation enthalpies is pointed out. The quoted D0 is used to derive a ΔH of 35.0 kcal/mol, in good agreement with other high-level ab initio results and a different group of experimental values.
A density-functional study of cluster reactivity. II. Two ammonia molecules reacting with a free Ga5As5 cluster
Authors: Mackey J. , Lou L. , Nordlander P.
Published: July 2002

We present an investigation of the interaction of two ammonia molecules with the Ga5As5 cluster. The (NH3)2Ga5As5 complex is found to be stable, in agreement with experiments. We find a significant decrease in the binding energy for the second ammonia, compared to single ammonia adsorption in agreement with experimental results. We investigate the site dependence of the ammonia–ammonia interaction and demonstrate that the adsorption of a single ammonia on certain sites prevents further adsorption of ammonia on certain other sites.
Mode-selective photoisomerization in 5-hydroxytropolone. I. Experiment
Authors: Ensminger Frederick A. , Plassard Jason , Zwier Timothy S. , Hardinger Steven
Published: July 2002

Laser-induced fluorescence excitation, dispersed fluorescence, and population labeling spectra of the S0–S1 transition of 5-hydroxytropolone (5-HOTrOH) have been recorded in a supersonic free jet. In the ground state, the two in-plane orientations of the 5-OH substituent produce two isomers, syn and anti relative to the 2-OH. Population labeling is used to identify transitions in the spectrum originating from the syn and anti zero-point levels. In the excited electronic state where 2-OH H-atom tunneling is expected to be more facile, the single vibronic level dispersed fluorescence spectra identify excited state levels with widely varying syn–anti character. Many of the levels in the low-energy region of the spectrum are nearly pure syn or anti in character, thereby showing little coupling to the isomerization reaction coordinate. Other levels are strongly syn–anti mixed via Fermi resonance interactions. A few levels exhibit strong syn–anti mixing despite being relatively isolated from other levels. These levels have strong syn–anti coupling matrix elements which reflect a strong coupling to the reaction coordinate. Analysis of the dispersed emission identifies a ‘‘promoter’’ vibrational mode W, with syn (anti) ground-state frequency 336 (337) cm−1 which appears to play a key role in coupling syn and anti levels. The accessibility of excited state levels of mixed character makes it possible to efficiently and reversibly isomerize 5-hydroxytropolone in a highly mode-specific fashion.
Interference control without laser coherence: Molecular photodissociation
Authors: Chen Zhidang , Shapiro Moshe , Brumer Paul
Published: July 2002

Control over channel-specific line shapes and branching ratios in photodissociation is shown to be achievable by irradiating a molecule with two intense cw lasers whose relative phase need not be well defined. Control results from quantum interference between nonlinear pathways induced by the intense fields, within which the relative laser phase cancels. The interference, and hence the product yields, can be manipulated by changing the relative frequencies and intensities of the two lasers. In this paper this theory of high field control is developed, and computations on the photodissociation of Na2 are presented. Control over product yields is shown to be extensive, even with inclusion of rotational states. For example, the branching ratio between the Na(3s)+Na(3p) and Na(3s)+Na(4s) products can change by as much as a factor of 10 as the frequencies are tuned.
Sticky spheres in adhesive pores
Authors: Jamnik A.
Published: July 2002

The structure of Baxter’s adhesive fluid [R. J. Baxter, J. Chem. Phys. 49, 2770 (1968)] in planar pores with adsorbing walls maintaining the equilibrium with the bulk fluid is considered. The wall–fluid correlations are studied on the basis of the solution to the Percus–Yevick/Ornstein–Zernike equation, obtained by using the analytic results for the direct correlation function of the bulk fluid. In the case of strongly adsorbing walls, the density profiles of hard sphere fluid resemble those in the pore with nonadsorbing walls but with the width decreased by the thickness of the adsorbed monolayers. The profiles of strongly adhesive fluid show maxima at the distances from the wall–fluid contact plane which correspond to the most probable configurations of strongly adhered successive molecular layers. A characteristic feature observed in all cases is the discontinuity in the slope of the profile at the distance one molecular diameter below the wall–fluid contact plane as the consequence of the exclusion volume of adsorbed monolayer. The distribution of the fluid between the bulk phase and the interior of the pore is oppositely affected by the strength of the wall–fluid and the fluid–fluid adhesion. Its dependence on the degree of confinement is more pronounced in narrow pores than at bigger separation between the walls.
On the selection of magnetic states in angle-resolved photodissociation
Authors: Seideman Tamar
Published: July 2002

The information contained in magnetic-state-selected photodissociation differential cross sections is examined by means of a quantum mechanical time-independent theory. Motivated by recent experimental demonstration of the possibility to select molecules with respect to their magnetic-rovibronic state and measure the energy-resolved angular distribution following their photofragmentation, we examine analytically and numerically the sensitivity of such angular distributions to the structure and the dynamics of the studied system. It is found that magnetic-state-selected cross sections contain significant information with respect to both the electronic structure (the potential energy surfaces as well as the transition dipole vector) and the reaction dynamics of photoinitiated reactions, which cannot be obtained from more averaged observable. In particular we find that such cross sections provide a mapping of the transition dipole vector which couples the ground state with the excited manifold. This feature is traced to the coherent excitation of a small subset of helicity states in the absorption process. It is suggested that the information contained in these angular distributions can be appreciated and extracted by preceding the experimental measurements with theoretical analysis.
Ab initio study of the acetylene and vinylidene dications fragmentation
Authors: Duflot D. , Robbe J-M. , Flament J-P.
Published: July 2002

The C2H++2 fragmentation processes have been studied using the complete active space self-consistent field method followed by a multireference perturbative configuration interaction, in order to interpret recent charge separation spectroscopy experiments. For two-body processes, the calculated appearance thresholds of the C2H+/H+ and CH+/CH+ fragment pairs are in good agreement with the experimental data. It is shown that the C2H++2→CH++CH+ dissociation occurs with an important rotation of the CH+ ions. The presence of the CH+2 ion is explained by a preliminary isomerization of acetylene to vinylidene dication. This reaction has been studied for the lowest lying states of C2H++2 (3Σ−g and 1Δg) and compared with other acetylenic ions isomerizations (C2H2, C2H+2, C2H−2). For three-body processes, the calculations are consistent with the mechanisms proposed by the experimentalists.