bluuebunny/crossref_metadata_2025 huggingface dataset:
Identification of the lowest energy nuclear fluctuation modes promoting the photochemical H-transfer tunneling reaction in doped fluorene single crystals
Authors: Prass B. , Colpa J. P. , Stehlik D.
Published: July 2002

Data with improved accuracy are presented for the low temperature reaction rates k(T) of the H-tunneling reaction between one specific host neighbor molecule in fluorene single crystals and a photoexcited acridine guest. The constant low temperature reaction rate k0 cannot be separated from the precursor intramolecular decay rate. This does not affect the analysis of the temperature dependent reaction rate, which renders the identification of two distinct thermally excited nuclear fluctuation modes promoting the H-transfer reaction. The energies are 125(15) and 440(40) cm−1. They agree well with corresponding lines in the Raman spectra of fluorene crystals. For the first time this result offers the possibility to test quantum mechanical reaction mechanisms without the need of averaging over unspecified distributions of nuclear fluctuation modes.
Photodissociation dynamics of H2O and D2O in the first absorption band: A complete a
b i
n
i
t
i
o treatment
Authors: Engel V. , Schinke R. , Staemmler V.
Published: July 2002

We report a detailed theortical study of the photodissociation of H2O and D2O in the first absorption band (λ∼165 nm). The calculations are three dimensional and purely quantum mechanical. They include an ab initio potential energy surface for the Ã state and a calculated SCF dipole moment function for the X̃→Ã transition. The dynamical calculations are performed within the infinite-order-sudden approximation for the rotational degree of freedom of OH and the LHL approximation for the masses. The resulting vibrational–translational motion is then treated exactly in two dimensions using hyperspherical coordinates. This study does not include any adjustable parameters. The thermally averaged total absorption spectra for H2O and D2O agree perfectly with the experimental spectra. Even finer details such as the progression of ‘‘vibrational’’ structures are well reproduced. They are not induced by any selective absorption but can be explained on the basis of the Ã state potential energy surface and details of the dissociation dynamics. Vibrational excitation of the OH and OD products is significantly wavelength dependent. The distribution of the three lowest vibrational states at 157 nm is in good accord with recent LIF measurements. Particular attention is paid to the sensitivity of the final results with respect to the coordinate dependence of the transition dipole function, the parent nuclear wave function and the excited state potential energy surface.
A supertensor formalism for solute–continuum solvent interactions with an arbitrarily shaped cavity. I. Theory and implementation
Authors: Drummond Malcolm L. J.
Published: July 2002

A polarizable molecule is allowed to interact with its surroundings, represented by a dielectric continuum. The molecule lies in a cavity which is its van der Waals envelope. The supertensor formulation of many-body electrostatics is invoked to enable a consistent solution to be found. A computational implementation is tested on a dipole in a cavity. The following paper in this Journal reports results on molecular systems.
Model for bulk phases in mixtures of amphiphilic molecules and water
Authors: Halley J. W. , Kolan A. J.
Published: July 2002

We present a one parameter spin model for mixtures of amphiphilic molecules and water which is distinct from earlier models for the phase behavior of these systems. We show that the model on a two-dimensional square lattice supports ‘‘lamellar,’’ ‘‘cubic,’’ and ‘‘micellar’’phases somewhat like those observed experimentally. We present Monte Carlo data on the lamellar-isotropic line in the phase diagram in the temperature-chemical potential plane. We summarize the features of the phase diagrams which we have found for the two cases in which the directionality of the amphiphilic molecules in the model are represented by two component (xy-like) and three component (Heisenberg-like) spins, respectively.
One-dimensional reactive systems: The effect of diffusion on rapid bimolecular processes
Authors: Peacock-López Enrique , Keizer Joel
Published: July 2002

Bimolecular reactions in one dimension are studied using the fluctuation–dissipation theory. In particular, we calculate the rate constants for three reactive systems. First, for an infinite straight line, we consider the reaction A+A→A+P, and the isomerization 2A→A2. In the first case, we obtain a linear dependence between rate constant and the reactant concentration. In the second, we obtain the same linear dependence only when the characteristic dimerization time τd is much greater than the characteristic pumping time τp. On the other hand if τp≫τd, we find a rate constant which is independent of the reactant concentration. Our results are in qualitative agreement with recent computer simulations of these reactions. Second, we consider trapping, A+S→S, by static sinks on a ring. For the cases of interest, we find negligible corrections, due to curvature, to the functional form of the infinite straight line rate constant.
The dichroism and birefringence of a hard-sphere suspension under shear
Authors: Wagner Norman J. , Fuller Gerald G. , Russel William B.
Published: July 2002

Optical measurements were used to detect structural anisotropy in concentrated dispersions over a range of Peclet numbers. Silica spheres of 49 and 130 nm radii with grafted octadecyl chains were dispersed in cyclohexane at volume fractions from 0.1 to 0.4. The apparatus consisted of a Couette cell with the dispersion in the annulus probed by a HeNe laser beam parallel to the axis of rotation. The dichroism and birefringence of the transmitted beam varied linearly with shear rate at low Peclet numbers with an orientation coincident with the principle direction of shear. Increasing the Peclet number, by increasing the particle size, produced a nonlinear response with the orientation tending to align in the direction of flow. A theory coupling the nonequilibrium microstructure under shear to the optical properties of the suspension enables direct interpretation of the dichroism. Comparison of sample dichroism calculations for two different forms of the theory demonstrates that the optical technique can be used to discriminate between theories which predict the microstructure.
Lifetime of the A 2Σ+, v′=0 level of HS measured using the Hanle effect
Authors: Loge Gary W. , Tiee J. J.
Published: July 2002

H2S was photodissociated using an ArF excimer laser at 193 nm to form HS photofragments. LIF spectra of HS in the region of 324 nm were obtained using a pressure tuned dye laser with improved resolution of the R1 and RQ21 branches in the A 2Σ+←X 2Π3/2 transition. The zero field linewidths and the Zeeman splitting of the RQ21 (1.5) line in a magnetic field were obtained. The latter was used to verify the g values expected for a Hund’s case (b) 2Σ+ upper state and Hund’s case (a) 2Π lower state. Depolarization of fluorescence in a magnetic field using the RQ21 (3.5) line with σ-polarized LIF excitation was used to determine an estimated lifetime of 0.15–0.3 ns for the lowest vibrational level of the 2Σ+ state. A lower limit on the lifetime of 0.17 ns was determined from the measured adsorption linewidths. No rotational dependence on the linewidth was observed.
Dynamics of fluorescence of a dye molecule in solution
Authors: Kinoshita Shuichi , Nishi Nobuhiko
Published: July 2002

Transient and stationary fluorescence properties of rhodamine 6G in liquid ethanol have been investigated in a wide temperature range between 110 and 300 K. Various phenomena concerning the interaction between the dye molecule and the surroundings such as dynamic Stokes shift, stepwise temperature dependence of the fluorescence peak energy, and excitation-wavelength dependent fluorescence peak shift, have been observed. To explain these phenomena in a unified manner, a stochastic theory based on the Onsager cavity model has been developed. It is shown that the fluorescence as well as the absorption characteristics are explained well by this treatment. Various parameters characterizing the optical properties of the molecule, i.e., the dipole moments in the ground and excited states, transition energy in vacuum, temperature-dependent relaxation time of the dynamic Stokes shift and so on, can be derived from the comparison between the theory and experiment. Finally, the origin of the relaxation processes found in the present work is discussed in connection with the dielectric properties of the solvent and also with the resonance Raman scattering process.
Dynamics with the effective adiabatic theory: The Bloch equations
Authors: Carmeli Benny , Chandler David
Published: July 2002

This paper extends our earlier work on the effective adiabatic theory [J. Chem. Phys. 82, 3400 (1985)] to study relaxation of a two-level system coupled to a Gaussian dissipative bath—the spin–boson problem. Bloch equations are derived which, under the limited circumstances described herein, treat the role of bath fluctuations omitted in the equilibrium effective adiabatic reference system. Applications to the Lorentzian dissipative bath show that the theory agrees closely with numerical simulation results. Application to an Ohmic bath shows that the theory is in agreement with currently accepted results concerned with the problem of macroscopic quantum coherence.
Reconstruction of intermolecular potentials at fixed energy: Functional sensitivity analysis approach
Authors: Ho Taksan , Rabitz Herschel
Published: July 2002

The iterative, perturbative inversion procedure of Mackintosh et al. is modified for the inversion of (noise free) elastic molecular scattering data at fixed energy, in conjunction with singular system analysis and the Tihkonov–Miller-type regularization method. The singular system consisting of the first order functional relation between infinitesimal changes in the phase shifts and the intermolecular potential provides a natural basis for solving the corresponding linear Fredholm integral equation of the first kind. The regularization procedure stabilizes the ill-conditioned inverse problem arising from the above linear integral equation. Furthermore, in regions where the wavelength is small compared to the length scale of the potential, the first order functional derivatives or sensitivity densities of the phase shifts with respect to the potential δηl/δV(R) are approximated by their mean values to facilitate the regularization procedure and, therefore, the convergence of the problem. This semiclassical modification and the adoption of the singular system analysis are necessary for intermolecular potentials possessing a rapidly varying region that cannot be appropriately represented by a small number of spline functions. For illustration, a model of the He–Ne system has been adopted and the applicability of the proposed method at diffrent energies has been examined in detail.
The chemical potential of simple fluids in a common class of integral equation closures
Authors: Kjellander Roland , Sarman Sten
Published: July 2002

An explicit formula for the chemical potential (μ) of simple fluids is derived for a whole class of integral equation theories, including the Percus–Yevick approximation and some other, more recently proposed closures. This formula only requires the pair correlation functions for one single state of the system, and applies to both homogeneous and inhomogeneous fluids. The coupling parameter integration method to calculate the chemical potential—where one particle is gradually coupled to the rest—is also investigated. It is shown that the μ value obtained in the approximate theories is not unique, but depends on the integration path. This behavior is due to inconsistencies of the approximation, which are discussed in some detail. For a certain choice of integration path the μ values obtained using the latter method agree with those from the explicit formula. Numerical results for the different closures are presented for a hard sphere fluid. The accuracy of μ depends strongly on the quality of the bridge function inside the hard core diameter.
Picosecond fluorescence depletion spectroscopy. II. Intramolecular vibrational relaxation in the excited electronic state of fluorene
Authors: Kauffman J. F. , Côté M. J. , Smith P. G. , McDonald J. D.
Published: July 2002

This paper presents the results of the time-resolved study of intramolecular vibrational relaxation in the molecule fluorene. The results represent the first extensive study of a molecule using the technique which we have developed known as the time-resolved fluorescence depletion technique. Fluorescence depletion decays and dispersed fluorescence spectra of 19 vibronic features of fluorene are presented. The decays show a progression of dynamic behavior including stationary behavior at low densities of states, quantum beating at intermediate densities, and fast decay of the initially prepared state at high state densities. The data allow us to assign IVR lifetimes to several vibronic levels of fluorene from 27 ps at 1425 cm−1 of excess vibrational energy to ≤10 ps at ∼2000 cm−1. The degree of spectral congestion in the associated dispersed fluorescence spectra is shown to be related to the dynamic behavior of the vibronic features. In addition, the effects of intermolecular rotational coherences on the time-resolved fluorescence depletion decays of fluorene are explored. With these results we are able to confirm the assignments of the excited state rotational constants which we have made using rotational band contour simulations.
Escape probability of geminate electron–ion recombination in the limit of large electron mean free path
Authors: Tachiya M. , Schmidt W. F.
Published: July 2002

We investigated theoretically the geminate electron–ion recombination in the limit of large electron mean free path by using the concept of diffusion in energy space. The energy diffusion equation is derived and the energy diffusion coefficient is evaluated. An analytical expression for the escape probability of the electron–ion pair is derived. It reproduces the numerical results very well which were obtained previously by using a Monte Carlo method. Experimental implications of the present theoretical findings are discussed.
Spatially resolved surface enhanced second harmonic generation: Theoretical and experimental evidence for electromagnetic enhancement in the near infrared on a laser microfabricated Pt surface
Authors: Haller Kurt L. , Bumm Lloyd A. , Altkorn Robert I. , Zeman Ellen J. , Schatz George C. , Van Duyne Richard P.
Published: July 2002

Surface enhanced second harmonic generation (SESHG) has been observed for the first time from a Pt surface in a <10−3 Torr vacuum environment. Both ‘‘smooth,’’ mechanically polished Pt surfaces and rippled Pt microstructures prepared by laser microchemical etching in Cl2(g) were studied with the newly developed technique of SESHG imaging. The etching procedure and the behavior of the Pt surface under the SESHG imaging conditions is reported in detail. The rippled/smooth enhancement factor for SHG excited with 80 ps, 1064 nm pulses from a cw, mode-locked Nd-YAG laser focused to a 3.1 μm 1/e2 radius ranges from 4 to 17, in qualitative agreement with theoretical calculations for isolated Pt spheroids.
Inelastic He scattering from a xenon overlayer: Dynamical diffraction effects
Authors: Kosloff R. , Cerjan C.
Published: July 2002

A model study is presented which describes the interaction of a gas with a crystalline surface. Both the surface and gas are treated quantum mechanically using a type of mean field approximation for the coupling of the translating particles to the surface. By varying the angle of incidence of the incoming particle, a large diffraction effect is observed for a narrow range of angles. Significant differences between the static and thermal features of the gas–surface scattering are observed.
A theoretical analysis of Raman scattering from predissociating molecules
Authors: Lee Sungyul , Freed Karl F.
Published: July 2002

Raman scattering from predissociating molecules is studied by computations for model Morse potential systems using a quantum–mechanical time-independent multichannel scattering algorithm. An analysis is presented of the role of bound–continuum interference and of the weak emission from the dissociative surface.
Vacuum ultraviolet absorption spectra of thiirane and thietane
Authors: Tokue Ikuo , Hiraya Atsunari , Shobatake Kosuke
Published: July 2002

Photoabsorption cross sections of thiirane and thietane vapors have been studied in the 110–240 nm region using synchrotron radiation. A number of peaks from thiirane and thietane are arranged into four and five Rydberg series, respectively, converging to the first ionization potential. Many Rydberg peaks from both molecules are observed to possess vibronic structures. The vibrational progressions from thiirane are assigned to the CH2 wagging (ν4) and CS symmetry stretching (ν5) modes. For thietane, the CH2 wagging (ν5) and CS symmetry stretching (ν7) modes of the excited states are found to be active. Molecular geometries and vibrational frequencies for the excited states are discussed on the basis of ab initio calculations.
Surface damage caused by bombardment with low-energy (10–30 eV) argon
Authors: Metiu Horia , DePristo Andrew E.
Published: July 2002

We apply a recently developed combined molecular dynamics–local Langevin equation method to the simulation of the scattering of Ar by the (100) face of a face-centered cubic solid. The kinetic energies of the Ar are chosen to be low compared to the typical energies used in sputtering. We find that even at low energies, a significant amount of surface damage is inflicted by the Ar, leading to ejection of metal atoms into the gas phase, the formation of dislocations, and the production of isolated atoms trapped on the surface. We study both the probability that such events occur and individual trajectories which display the dynamic processes through which sputtering takes place or defects are created.
Laser excitation spectroscopy of the A and B states of jet-cooled copper dimer: Evidence for large electronic isotope shifts
Authors: McCaffrey John G. , Bennett Robert R. , Morse Michael D. , Breckenridge W. H.
Published: July 2002

Fluorescence excitation spectra recorded for the A–X system of jet-cooled Cu2 show conclusive evidence of a ΔΛ=0 transition, and the A state is thereby definitively assigned as 1Σ+u. A previous assignment of the B state as 1Σ+u is confirmed, but the vibrational levels of this state are complicated by the presence of a perturbation at v′=0. The perturbing state does not, however, appear to be either of the two optically accessible electronic states in this spectral region. Anomalously large electronic isotope shifts are observed for the A and B states, and this behavior is discussed in terms of the correspondingly large ‘‘specific mass shifts’’ observed in the optical spectra of atomic copper for transitions that couple states differing in the number of d electrons. Due to the large spin-orbit coupling constants in the ‘‘d-hole’’ configurations, it is proposed that the low-energy-excited molecular states of Cu2 derived from these configurations should be described by Hund’s case (c) coupling. Dynamical effects observed in the gas phase and in solid matrices are briefly discussed in terms of this bonding scheme.
Calculation of spin densities in diatomic first-row hydrides
Authors: Chipman Daniel M.
Published: July 2002

Fermi contact spin densities have been theoretically determined for the ground-state diatomic first-row hydrides CH, OH, and NH having open shell π electrons. Multiconfiguration self-consistent-field wave functions include the dominant configuration and single excitations from it describing the most important spin and orbital polarization effects. Optimization of the orbitals by precise numerical grid methods shows that this simple wave function model is capable of providing spin densities in satisfactory agreement with experiment. Gaussian basis sets suitable for use with this wave function model are determined by comparing to the precise numerical spin density results. Huzinaga’s popular (9s5p‖4s) primitive Gaussian basis provides a good starting point if augmented with diffuse and polarization functions and with a tight (high exponent) s function at hydrogen. Only the innermost few primitive functions may be contracted. Contraction coefficients may be determined on the basis of free atom Hartree–Fock calculations. These studies lead to economical contracted Gaussian basis sets that should be useful for spin density calculations on larger polyatomic radicals.
Direct measurement of solvent cage dynamics following photodissociation of iodine using picosecond Raman spectroscopy
Authors: Lingle Robert , Xu Xiaobing , Yu Soo-Chang , Chang Y. J. , Hopkins J. B.
Published: July 2002

Photodissociation of iodine in CCl4 at 532 nm deposits 6300 cm−1 of energy into the solvent cage immediately surrounding the solute. Picosecond Raman spectroscopy has been used to investigate the dissipation of this energy in the solvent coordinate by monitoring time-dependent shifts in the Raman lineshapes of the local solvent cage.
Two-level systems and low-temperature glass dynamics: Spectral diffusion and thermal reversibility of hole-burning linewidths
Authors: Littau K. A. , Bai Y. S. , Fayer M. D.
Published: July 2002

Intermediate time-scale time-dependent hole-burning measurements are reported on three glassy organic systems which undergo spectral diffusion: cresyl violet in ethanol at 1.30 and 2.13 K, resorufin in ethanol at 2.13 K, and resorufin in glycerol at 2.13 K. The hole width is observed to broaden on a log time scale from 0.1 to 5000 s for each ethanol system while no broadening is observed in the system of resorufin in glycerol. A detailed theoretical treatment is introduced which allows the raw data to be converted to the fluctuation rate distribution of the underlying modes responsible for dephasing. Using this theory, the broadening in ethanol is found to be the result of a distribution of glassy modes which is Gaussian on a log R scale with a center rate at ∼0.02 s−1. In addition, temperature cycling hole-burning results are reported on the system cresyl violet in ethanol. A hole is burned at 1.30 K and detected before, during, and after a temperature cycle to 2.13 K and back. The hole width is observed to broaden at the high temperature and then narrow again in a completely reversible manner when the temperature is again lowered. Theoretical calculations show this behavior to be entirely consistent with the tunneling two-level-system (TLS) model of glass dynamics but incompatible with other models such as particle or defect diffusion. The cycling data is shown to fall exactly on the theoretical curve calculated from the TLS model using no adjustable parameters.
A photoelectron spectroscopic study of the electrochemical processes in polyaniline
Authors: Snauwaert P. , Lazzaroni R. , Riga J. , Verbist J. J. , Gonbeau D.
Published: July 2002

The electronic structure of the base forms of electrochemically prepared polyaniline is studied by means of x-ray photoelectron spectroscopy (XPS). Model compounds for the amine, imine and crosslinked nitrogen sites provide the references for the analysis of the core level spectra in the polymer. In particular, the analysis of the nitrogen 1s spectra represents a useful method for accurate determination of the amine to imine ratio. The evolution in the concentration of the different nitrogen species as a function of the electrochemical potential is shown to be in good agreement with the results of cyclic voltammetry measurements. Varying the pH leads to large modifications in the stability domain of the different oxidation states.
Reduced-dimensionality quantum calculations of the thermal rate coefficient for the Cl+HCl→ClH+Cl reaction: Comparison with centrifugal-sudden distorted wave, coupled channel hyperspherical, and experimental results
Authors: Sun Qiyan , Bowman Joel M. , Schatz George C. , Sharp J. R. , Connor J. N. L.
Published: July 2002

Reduced dimensionality (RD) cumulative reaction probabilities (CRPs) are reported for the Cl+HCl→ClH+Cl reaction using two semiempirical extended London–Eyring–Polanyi–Sato potential energy surfaces. Comparison is made with CRPs from centrifugal-sudden distorted wave (CSDW) calculations, and with accurate coupled channel hyperspherical (CCH) CRPs for total angular momentum quantum number J=0. Rotational and bending energy-shift approximations are applied to the CCH CRPs for J=0 to obtain approximate CRPs for J>0. A test of this approximation is made using CSDW partial wave CRPs. New expressions for the thermal rate coefficient are derived using these approximations. The rate coefficients calculated from RD and energy-shifted CCH CRPs are in excellent agreement with each other. They also agree well with the CSDW and experimental rate coefficients.
The stability of N2(A′ 5Σ+g)
Authors: Marinelli W. J. , Kessler W. J. , Woodward A. M. , Rawlins W. T.
Published: July 2002

The energy and stability of the ‘‘high spin’’ 7Σ and 5Σ states of N2 have been the subject of many theoretical efforts. These states play an important role in N-atom recombination. The collisional coupling of these states to other excited electronic states strongly influences the electronic emission spectrum of N2 observed in atom recombination. Since these states are not dipole coupled to lower excited electronic states of N2 , they are considered metastable and are possible candidates for high energy density storage media. We have examined the stability of N2(5Σ) state in a discharge flow reactor. The 5Σ state is produced from the N2(A 3Σ) energy pooling reaction and probed via laser-induced fluorescence on the C″ 5Π–A′ 5Σ transition. While no laser-induced fluorescence from excitation of the A′ state is observed, comparison of signal levels with laser-induced fluorescence on the B 3Πg–A 3Σ−u transition enables us to establish an upper limit on the gas phase lifetime of this state at pressures of a few Torr. Experimental evidence is presented which suggests that the primary mode of A′-state quenching is through collisional coupling to the B 3Πg state. However, other matrix isolation experiments suggest that the A′ state may be stabilized in cryogenic rare gas matrices.
Photodissociation dynamics of H2S(D2S) following excitation within its first absorption continuum
Authors: Xie Xiaoxiang , Schnieder L. , Wallmeier H. , Boettner R. , Welge K. H. , Ashfold M. N. R.
Published: July 2002

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of H2S(D2S) following excitation at a number of wavelengths in its first absorption continuum. The respective bond dissociation energies were determined as D0(H–SH)=3.90±0.03 eV and D0(D–SD)=4.00±0.02 eV. The measured vibrational energy disposal in the partner ground state SH(SD) fragments and the product angular distributions support and extend the earlier results of van Veen et al. [Chem. Phys. 74, 261 (1983)], while the deduced rotational and spin–orbit state population distributions in these fragments appear to be generally consistent with the recent measurements of Weiner et al. [J. Chem. Phys. 90, 1403 (1989)]. We present a schematic adiabatic potential energy surface for the dissociative 1A″ excited state via which it is possible to rationalize virtually all existing dynamical measurements relating to this fragmentation process.
Dynamic percolation theory for diffusion of interacting particles
Authors: Granek Rony , Nitzan Abraham
Published: July 2002

The recently developed dynamic percolation theory is used to solve the problem of diffusion of interacting particles in lattice-gas models within an effective medium approximation. The approach is based on the observation that the motion of a tracer particle in a system of (similar or different) particles can be viewed as particle motion in a changing random environment. This makes it possible to use effective medium theory (EMT) solutions to the latter problem. The main conceptual problem of this approach is to relate the characteristic microscopic times for the evolution of the disordered background to the macroscopic diffusion. We discuss and compare several possible ansatzs for this relation and conclude that relating these times to the chemical diffusion rate is the most reasonable simple choice. Using this ansatz, we obtain EMT approximations for the tracer diffusion coefficient in the noninteracting lattice-gas (NILG, blocking interactions only) model and an approximate EMT relation between the chemical and the tracer diffusion coefficients in a lattice gas with nearest-neighbor interactions. Agreement with available simulation results is good whenever single bond EMT is expected to be reliable.
Three-dimensional analytic quantum theory for triatomic photodissociations. II. Angle dependent dissociative surfaces and rotational infinite order sudden approximation for bent triatomics
Authors: Grinberg Horacio , Freed Karl F. , Williams Carl J.
Published: July 2002

We extend to triatomic molecules with bent initial bound states our analytical quantum theory of triatomic photodissociations. The theory uses basis functions for the initial bound state wave function that are product functions in the natural normal (or local) modes appropriate to that state and a continuum wave function that is a product function in the natural scattering coordinates appropriate to the dissociative surface. This choice of wave functions produces three-dimensional nonseparable transition amplitudes which are reduced to analytical forms by introduction of the infinite order sudden and Airy approximations for the continuum wave function and a quadrature formula for the integral over bending motions. The present theory also lifts some assumptions that had been introduced previously to simplify the theory for isotropic repulsive potentials. Thus, we use the exact nonlinear relation between the bound state bending angle and the scattering angle to remove the previous small angle approximation that is shown to lead to inaccuracies in some cases. We also study the ramifications of the fact that the bound state bending vibration is generally a linear combination of pure bend and both bond stretches in the transformation between normal modes and scattering coordinates. Inclusion of this bend–stretch coupling, ignored in our previous analytical work and in many theories of triatomic photodissociation that employ expansions of the bound state wave function in a basis set expressed in dissociative surface scattering coordinates, is shown to have profound influence on computed fragment rotational distributions. The bend–stretch couplings serve often to wash out the additional high frequency oscillations that appear in prior approximate treatments in which these couplings are ignored. This then leads to fragment rotational distributions that contain only those oscillations resulting from the map of the initial bound state bending vibration. Comparison is made with three-dimensional close coupled and DVR calculations for the photodissociation of HCN from the bent C̃1A′ state. Calculations are also presented for initial states of high rotational angular momenta.
Cu7 cluster: Pentagonal bipyramid
Authors: Van Zee R. J. , Weltner W.
Published: July 2002

Electron-spin resonance evidence is presented supporting a pentagonal bipyramid structure (D5h symmetry) and a 2A″2 ground state for the 63Cu7 metal cluster, as previously found for Ag7 and alkali–metal septamers.
Nuclear magnetic shieldings and spin rotation constants of HF and N2
Authors: Oddershede Jens , Geertsen Jan
Published: July 2002

The paramagnetic contribution to the nuclear magnetic shieldings and the spin rotation constants are calculated using polarization propagator theory. Results are reported both in the first order approximation [equivalent to the coupled Hartree–Fock (CHF) method] and in the second order polarization propagator approximation (SOPPA). It is demonstrated how the constant Cgauge which gives the gauge origin dependence of the total nuclear magnetic shielding in a finite basis set [σ(Rc+d)=σ(Rc)+d⋅Cgauge] can be calculated from the polarization propagator. The magnetic shieldings of H and F in HF are nearly the same in CHF and SOPPA and results from both levels of theory agree well with experiment. For N2, σ(SOPPA)=−72.2 ppm, also in good agreement with the most recent measurement. However, for N2 there is a large correlation effect since σ(CHF)=−106.5 ppm. The computed spin rotation constant for N2 is M(15N)=20 kHz, i.e., about 2 kHz lower than its measured value. This value of M(15N) corresponds to σ(N)=−72.2 ppm.
Monte Carlo recursion study of cluster formation from vapor
Authors: Li Zhenqin , Scheraga Harold A.
Published: July 2002

A general formalism of cluster formation from monatomic vapor is developed, based on classical statistical mechanics in the isothermal–isobaric ensemble. The Gibbs free energy of clusters in a constrained equilibrium with vapor can be computed efficiently using a Monte Carlo recursion procedure. The application of our theory and method to the study of homogeneous nucleation of argon cluster, shows that small clusters under supersaturation conditions tend to have lower effective density compared with the corresponding bulk liquid, and more negative Gibbs free energy of formation than that predicted from the classical nucleation theory.
Investigation of CO photolysis at 193 nm using oxygen-atom two-photon laser-induced fluorescence near 225.6 nm
Authors: Merrow Clifton N. , Forch Brad E.
Published: July 2002

The photolysis of carbon monoxide (CO) with the ArF excimer laser (193 nm) has been investigated using a pump and probe laser experiment. Ground state oxygen 2p4 (3P2,1,0) atomic fragments which are produced in the ArF laser photolysis of CO are detected by monitoring the 844.7 nm laser-induced fluorescence following the two photon excitation of the oxygen 2p4 (3P2,1,0) →2p33p(3P2,1,0) transitions near 225.6 nm. The slope (n) of an Ln–Ln plot of the fluorescence signal intensity vs laser photolysis energy yields n=1.90±0.09, which indicates a quadratic dependence on the 193 nm photolysis of CO. A mechanism is proposed where the initial absorption of one ArF photon produces CO (a 3Π,v′=2), which subsequently absorbs one additional ArF laser photon into a dissociative state with suspected triplet character.
Vibrational distributions of the Ã 2Πu state of CO+2 and CS+2 produced by electron impact on jet-cooled CO2 and CS2
Authors: Tokue Ikuo , Shimada Hisanori , Masuda Akemi , Ito Yoshio , Kume Hiroshi
Published: July 2002

Fluorescence spectra of the Ã2Πu–X̃2Πg system of CO+2 and CS+2 ions, following electron impact on supersonic jet targets of CO2 and CS2, have been analyzed to determine vibrational populations. The vibrational distributions of the Ã2Πu state of CO+2 and CS+2 were found to be independent of the impact energy in the 30–300 eV range. The distribution of the CO+2(Ã) state is very similar to the result obtained by He i photoelectron spectroscopy (PES), which is a Franck–Condon-type vertical ionization, whereas that of the CS+2(Ã) state is significantly different from the PES data. This non-Franck–Condon behavior for formation of CS+2(Ã) in the electron impact is discussed.
Polarized emission spectroscopy of photodissociating nitromethane at 200 and 218 nm
Authors: Lao K. Q. , Jensen E. , Kash P. W. , Butler L. J.
Published: July 2002

We report the polarized emission spectra from photodissociating nitromethane excited at 200 and 218 nm. At both excitation wavelengths, the emission spectra show a strong progression in the NO2 symmetric stretch; at 200 nm a weak progression in the NO2 symmetric stretch in combination with one quantum in the C–N stretch also contributes to the spectra. We measure the angular distribution of emitted photons in the strong emission features from the relative intensity ratio between photons detected perpendicular to versus along the direction of the electric vector of the excitation laser. We find the anisotropy is substantially reduced from the 2:1 ratio expected for the pure CH3NO2 X(1A1)→1B2(ππ*)→X(1A1) transition with no rotation of the molecular frame. The intensity ratios for the features in the NO2 symmetric stretching progression lie near 1.5 to 1.6 for 200 nm excitation and 1.7 for 218 nm excitation. The analysis of the photon angular distribution measurements and consideration of the absorption spectrum indicate that the timescale of the dissociation is too fast for molecular rotation to contribute significantly to the observed reduction in anisotropy. The detailed analysis of our results in conjunction with electron correlation arguments and previous work on the absorption spectroscopy and final products’ velocities results in a model which includes two dissociation pathways for nitromethane, an electronic predissociation pathway and a vibrational predissociation pathway along the 1B2(ππ*) surface. Our analysis suggests a reassignment of the minor dissociation channel, first evidenced in photofragment velocity analysis experiments which detected a pathway producing slow CH3 fragments, to the near threshold dissociation channel CH3 + NO2(2 2B2).
Absolute infrared vibrational band intensities of molecular ions determined by direct laser absorption spectroscopy in fast ion beams
Authors: Keim E. R. , Polak M. L. , Owrutsky J. C. , Coe J. V. , Saykally R. J.
Published: July 2002

The technique of direct laser absorption spectroscopy in fast ion beams has been employed for the determination of absolute integrated band intensities (S0v) for the ν3 fundamental bands of H3O+ and NH+4. In addition, the absolute band intensities for the ν1 fundamental bands of HN+2 and HCO+ have been remeasured. The values obtained in units of cm−2 atm−1 at STP are 1880(290) and 580(90) for the ν1 fundamentals of HN+2 and HCO+, respectively; and 4000(800) and 1220(190) for the ν3 fundamentals of H3O+ and NH+4, respectively. Comparisons with ab initio results are presented.
A
b i
n
i
t
i
o (HeH2)+ energy surfaces and nonadiabatic couplings between them
Authors: Furlan Richard J. , Bent Gary , Russek Arnold
Published: July 2002

The energy surfaces of the three lowest adiabatic states of the (HeH2)+ triatomic molecular system have been calculated ab initio as functions of all three variables describing the triatomic geometry, using the BRLJHU set of quantum chemistry programs. The procedure is described by the acronym SA-MCSCF/CI, for state-averaged multiconfiguration self-consistent-field calculation, followed by a full configuration interaction calculation. In addition the nonadiabatic matrix elements which couple these adiabatic states have been calculated. Results have been obtained on a sufficiently fine mesh for interpolation by a spline-fit program to produce energy differences and nonadiabatic coupling matrix elements over the full mesh required for collisional excitation problems of He+ on H2 and H+2 on He involving these states.
Relation between Born–Green–Yvon solutions and chemical potential for a fluid inside a pore
Authors: Sokolowski S. , Fischer J.
Published: July 2002

Limiting values of the one-particle background correlation function for inhomogeneous fluids are considered and used as boundary values for the integration of the Born–Green–Yvon equation. For a system of hard spheres in contact with a hard wall different approximations of the pair correlation function are tested with respect to (a) the consistency of the limiting values, (b) the behavior of the background correlation function inside the wall, and (c) the form of the local density in front of the wall. Then the study concentrates on using the Fischer–Methfessel version of the Born–Green–Yvon equation together with the limiting value of the one-particle background correlation function inside the solid for describing density profiles of a fluid inside a pore. Comparison of the results against computer simulations shows quite reasonable agreement.
Negative-ion mass spectrometric study of ion-pair formation in the vacuum ultraviolet. II. OCS→S−+CO+, O−+CS+, and CO2→O−+CO+
Authors: Mitsuke Koichiro , Suzuki Shinzo , Imamura Takashi , Koyano Inosuke
Published: July 2002

Ion-pair formation from photoexcitation of OCS and CO2 has been studied by negative-ion mass spectrometry using synchrotron radiation in the 15–35 eV photon energy range. Negative ions S− and O− from OCS and O− from CO2 have been observed. The lowest onset energy in the photodissociation efficiency curve for each ion is in good agreement with the thermochemical threshold for the formation of the negative ion in the ground 2Pu state and its counterpart positive ion in the ground 2Σ+ state. There exist series of peaks with medium intensities in the efficiency curves of S− from OCS and O− from CO2; they are identified as resulting from predissociation of the Rydberg states converging to OCS+(B̃ 2Σ+) and CO+2(C̃ 2Σ+g), respectively. Broad peaks are observed at 18.4 eV (∼675 Å) in the efficiency curves for both S− and O− produced from OCS. Predissociation of the excited valence state formed by the intravalence 9σ→10σ transition is considered to mainly contribute to these features. In addition, a broadband feature is present in the wavelength range of 400–620 Å in the O− efficiency curve. The most likely candidate for the corresponding doorway state is the two-electron excited state involving simultaneous 9σ→10σ and 3π→4π transitions. In the case of the O− efficiency curve from CO2, two maxima observed at 21.4 (580 Å) and 23.0 eV (538 Å) are explained as resulting from the 3σu→5σg transition forming an excited valence state which effectively couples to the ion-pair continuum.
Monte Carlo reptation model of gel electrophoresis: Steady state behavior
Authors: Duke T. A. J.
Published: July 2002

We study the effect of tube length fluctuations on the mobility and orientation of DNA fragments during continuous field gel electrophoresis, using a Monte Carlo lattice simulation. The variation of mobility with molecular weight is found to be similar to that derived from models that assume a fixed tube length; in particular, the prediction of band inversion still applies. The dependence on field strength is substantially different, however. The mobility initially rises more rapidly with increasing field, but reaches a plateau at a much lower value. In contradiction to models that neglect fluctuations, but in agreement with experimental observations, the steady state orientation of the tube continues to increase after the mobility has saturated and the degree of alignment has a marked dependence on chain length. We discuss how these differences arise from alterations in the tube configuration caused by length fluctuations.
Visible multiphoton dissociation of Fe(CO)5 for production of iron atoms
Authors: Mitchell S. A. , Hackett P. A.
Published: July 2002

Ground state (a 5D) and metastable excited state (a 5F and a 3F) iron atoms have been produced by visible multiphoton dissociation of Fe(CO)5 at 552 nm in a static pressure gas cell at room temperature. The distribution of iron atoms among these states has been measured by using a pump and probe arrangement in which the probe laser pulse excites resonance fluorescence from iron atoms at variable time delay following the photolysis pulse. Collisional relaxation processes of metastable a 5F and a 3F iron atoms have been investigated by using a simple model to describe the main features of the overall relaxation process. Results for a variety of quenching gases including N2O, C2H4O, and O2 indicate that relaxation occurs mainly by transitions between adjacent multiplets, with little intermediate intramultiplet relaxation and no detectable removal by chemical reaction. An interpretation of these results is given in terms of schematic potential energy curves which represent the bonding capabilities of specific electronic configurations of iron atoms. These curves are discussed in an accompanying paper on studies of chemical reactions of ground state iron atoms.
Electronic energy shifts of a sodium atom in argon clusters by simulated annealing
Authors: Tsoo Chiachin , Estrin Dario A. , Singer Sherwin J.
Published: July 2002

Born–Oppenheimer electronic surfaces for NaArn clusters are calculated using simulated annealing with a floating Gaussian basis set. The electronic energy levels are obtained during a molecular dynamics simulation of nuclear motion. Classical dynamics is used to search for local minima of the cluster potential energy surface. Results are presented for NaArn, n=1, 2, 3, 4, and for two large clusters, NaAr20 and NaAr24. Predictions of vertical transition energies of the cluster solvated sodium atom are given. The blueshifted transition frequencies for NaAr20 and NaAr24, clusters in which the metal atom is densely surrounded by rare gas solvent, are comparable to the largest sodium blueshifts observed in argon matrix spectra.
Effect of interconversion between reactant configurational states on enzyme kinetics controlled by rototranslational diffusion motions
Authors: Baldo Marcello , Grassi Antonio , Raudino Antonio
Published: July 2002

In this paper we extend a previous model [M. Baldo, A. Grassi, and A. Raudino, J. Chem. Phys. 91, 4658 (1989)] describing the orientational effects in diffusion-controlled enzyme (or membrane surface) reactions. The present generalization takes into account the reactants internal motions involving the interconversion between configurational states, one of them being much more reactive than the others. The problem leads to a system of rotational–translational diffusion equations (RT-DEs) coupled through the interconversion reactions between the conformers. For sake of simplicity, we have restricted the analysis to the case of only two conformational states. The steady-state RT-DE with the proper boundary conditions has been solved by an exact analytical procedure, leading to a set of linear algebraic equations which have been numerically solved. The model allows one to calculate the kinetic constants of the enzyme reactions as a function of available experimental parameters, such as the rotational and translational diffusion coefficients, the reactant’s orientational constraints and the rates of interconversion between its different conformations. The numerical results show a monotonous but very nonlinear increasing of the enzyme kinetic constant on raising either the rotational diffusion constant or the interconversion rate between the P+ and P− reactant’s conformations. Well-defined regions where the influence of the above parameters on the enzyme kinetics reaches a maximum have been identified.
A centrifugal-sudden distorted-wave study of isotope effects for the reactions Cl+HCl→ClH+Cl and Cl+DCl→ClD+Cl
Authors: Schatz George C. , Amaee B. , Connor J. N. L.
Published: July 2002

A quantum calculation has been performed using the centrifugal-sudden distorted-wave (CSDW) method for the three-dimensional Cl+DCl→ClD+Cl reaction. Three potential energy surfaces have been employed: two extended London–Eyring–Polanyi–Sato surfaces [denoted Bondi–Connor–Manz–Römelt (BCMR) and Persky–Kornweitz 3 (PK3)] and a scaled and fitted ab initio one (denoted sf-POLCI). Quantities calculated include: cumulative reaction probabilities, integral cross sections, rotational product distributions, and rate coefficients. Differential cross reactions are also reported for the PK3 surface, which are compared with the results from a simple semiclassical optical model (close agreement is found). We also compare the Cl+DCl results with earlier CSDW calculations for Cl+HCl→ClH+Cl. The rotational distributions are strongly perturbed by isotope substitution and are sensitive to variations in the potential surface. In contrast, the H and D rate coefficients for all three surfaces agree with experimental values, except for Cl+DCl on PK3.