bluuebunny/crossref_metadata_2025 huggingface dataset:
Dissociation of methane and ethane on Pt(110): Evidence for a direct mechanism under thermal conditions
Authors: Luntz A. C. , Winters Harold F.
Published: July 2002

The dissociative sticking S0 for CH4, C2H6, and their deuterated analogs has been measured on Pt(110) as a function of surface temperature Ts under thermal ‘‘bulb’’ conditions at low gas pressures (Tg=300 K). Large increases in S0 with Ts are observed for all species, in partial qualitative agreement with previous measurements. S0 for CH4 has also been measured at a high Ar background pressure (Tg≊Ts) and shows a much faster increase with Ts than the low pressure case. This is interpreted as evidence that the dissociation mechanism for CH4 under these bulb conditions is direct rather than precursor mediated, a conclusion which is in conflict with previous interpretations. In addition, the increase in S0 with Ts for C2H6 and C2D6 exhibits concave curvature at low Ts, i.e., is non-Arrhenius, and this is difficult to reconcile with standard precursor models. Both experimental results, however, are shown to be in qualitative accord with a previously developed dynamical model for direct dissociation, as long as the dissociation limit is endothermic.
Electronic energy-level structure and correlation crystal-field effects of Er3+ in Cs3Lu2Br9
Authors: Hehlen Markus P. , Güdel Hans U. , Quagliano John R.
Published: July 2002

Single crystals of Cs3Lu2Br9:1% Er3+ were grown using the Bridgman technique. From highly resolved polarized absorption and luminescence measurements at 15 and 4.2 K, respectively, 101 crystal-field levels from 27 different 2S+1LJ(4f11) multiplets of Er3+ in C3v symmetry were assigned. A Hamiltonian including 16 atomic and 6 crystal-field parameters was fitted to a set of 87 crystal-field levels and gave a rms standard deviation of 23.84 cm−1. Inclusion of one freely varying correlation crystal-field (CCF) parameter lowered the overall rms standard deviation to 19.25 cm−1 and provided a dramatic improvement of the calculated crystal-field splittings of specific CCF-sensitive J multiplets.
New trends in the state-to-state photodissociation dynamics of H2O(Ã)
Authors: Brouard M. , Langford S. R. , Manolopoulos D. E.
Published: July 2002

The water molecule, rotationally state selected in the third and fourth OH stretching overtone (‖04〉−, ‖05〉−) and stretch–bend combination (‖04−2〉) levels, has been photodissociated via the Ã state at λ≂282 nm. The photofragment rotational state distributions, determined by OH(A–X) laser induced fluorescence (LIF), differ from those reported previously by Andresen and co-workers, in which water was initially prepared in the ‖01〉− level and photodissociated at 193 nm, and from those by Crim and co-workers, in which H2O was photodissociated via the ‖04〉− level at wavelengths shorter than 282 nm. These differences become more pronounced with increasing angular momentum in the parent water molecule and with an increasing number of quanta in the intermediate OH stretching overtone state. The Franck–Condon theory of Balint-Kurti, previously employed successfully to account for the product state distributions arising from the 193 nm photodissociation of H2O‖01〉−, qualitatively reproduces the trends observed in the present study if it is assumed that dissociation occurs preferentially from extended RH-OH configurations of the ‖04〉− and ‖05〉− overtones. The product OH state distributions are thus shown to be sensitive indicators of the bending and rotational motions of H2O(X̃) in the wide amplitude stretching region of the ground state surface.
An improved potential energy surface of Ar–C2H2
Authors: Yang Moonbong , Watts Robert O.
Published: July 2002

The intermolecular interaction of the Ar–C2H2 system was investigated using the coupled-states approximation to analyze the scattering process at a collision energy of 649.5 cm−1 (80.5 meV) and a close-coupling method to predict bound state energy levels. In both cases, the acetylene molecule is treated as a rigid rotor. A primary rainbow peak observed in the total differential scattering experiment provides useful information on the well depth while dimer rotational frequencies give accurate information on the shape around the minimum of the ground state potential. Four existing potentials for the system were tested against the total differential scattering cross section and rotational spectroscopic measurements. No one potential showed good agreement with both measurements. An improved potential energy surface was obtained by fitting simultaneously both scattering and spectroscopic data.
Activated rate processes: The reactive flux method for one-dimensional surface diffusion
Authors: Bader Joel S. , Berne B. J. , Pollak Eli
Published: July 2002

We have implemented a semiclassical dynamics simulation method to investigate the effects of finite barrier heights and nonlinear potentials on the rate of diffusion of a particle which is coupled to a frictional bath and is traveling on a one-dimensional potential energy surface. The classical reactive flux method has been modified to account for semiclassical tunneling and above-barrier reflection. A novel perturbation theory treatment of the semiclassical dynamics is developed to simulate the motion of the particle when the coupling to the frictional bath is small and the particle’s motion is nearly conservative. Our simulation results support the theoretical prediction that the diffusion constant increases as friction decreases. We also find supporting evidence for an inverse isotope effect, as the diffusion constant for a classical particle can be larger than that of a corresponding quantum mechanical particle. The escape rate and the average energy of escaping particles are also found to be in good agreement with theoretical predictions.
Electronic fine structure transitions and rotational energy transfer of NO(X 2Π) in collisions with He: A counterpropagating beam study
Authors: Meyer Henning
Published: July 2002

The collision dynamics of NO(X 2Π) with He is investigated at a collision energy of 147 meV using the method of counterpropagating pulsed molecular beam scattering. One-dimensional product velocity distributions are determined through ion time-of-flight analysis. State specific detection of NO is achieved through (2+1) resonance enhanced multiphoton ionization detection via the E 2Σ+ intermediate state. Calculated frequencies and line strength factors for the two-photon transition enable the extraction of state resolved degeneracy averaged integral and differential cross sections. The overall behavior of the integral cross sections for fine structure conserving and changing transitions, weighted by the degeneracy of the final state, is well described by two different exponential energy gap laws. Differential cross sections for both types of transitions exhibit very different rotational rainbow structures. For the multiplet changing transition, the scaling law suggests an increased energy transfer efficiency while the rotational rainbow structure indicates a larger effective anisotropy. Therefore, the dynamics at the probed collision energy is clearly dominated by fine structure changing collisions when magnetic sublevel specific cross sections for a specified energy transfer are compared.
The conservation of quantum zero-point energies in classical trajectory simulations
Authors: Lim Kieran F. , McCormack Drew A.
Published: July 2002

We propose a computational technique to constrain the vibrational modes of a classical molecule to have energy greater than the quantum zero-point energy (ZPE). The trajectory of any mode with energy less than ZPE is projected to a neighboring point in phase space where the energy is equal to the ZPE and the phase angle of the mode is unchanged. All other modes are then perturbed in such a way as to conserve the total energy of the system. This technique is similar in principle to the method of holonomic constraints. We apply this ‘‘semiholonomic’’ TRAPZ (trajectory projection onto ZPE orbit) scheme to the two mode Hénon–Heiles system and find that it results in a decrease of ergodicity. Periodic limit cycle internal vibrational energy redistribution is observed. Implications of this method for the conservation of ZPE in quasiclassical trajectory simulations are discussed.
Production of OH by dissociating ozone–water complexes at 266 and 355 nm and by reacting O(1D) with water dimers
Authors: Hurwitz Y. , Naaman R.
Published: July 2002

In the present study the production of OH was monitored when ozone water complexes were dissociated at 266 and 355 nm and when O(1D) atoms were reacted with water dimers. The results indicate that the absorption of ozone at 355 nm is enhanced by two orders of magnitude when the ozone is complexed with water. In addition it is shown that the rotational energy distribution of the OH product is very similar when it is produced in an intracluster process, or by reaction of O(1D) with water dimers. The results are rationalized by complex induced potential energy surfaces shift. The shifts may strongly depend on the relative conformation of the water and the ozone in the complex.
The density and polarization of an ion-dipole-electrolyte near a charged wall
Authors: Díaz-Herrera Enrique , Forstmann F.
Published: July 2002

The structure of the hard sphere dipolar liquid and the electrolyte with added hard sphere ions near a charged hard planar electrode has been investigated with the reference hypernetted chain integral equation (RHNC). We find a decrease of the dielectric function ε near the wall, a decrease of ε in the fluid due to saturation, a field dependent change in the dipole density near the wall, and a decrease of the ion density near the electrode due to solvation. Related to the demixing instability of the ion-dipole mixture, the ions suddenly concentrate near the surface at higher fields and lead to a sharp increase in the differential capacitance. Also electrostriction in the pure dipolar liquid with a field in the bulk is considered.
Velocity and internal state distributions of photodesorbed species from N2O/Pt(111) by 193 nm light
Authors: Masson Denis P. , Lanzendorf Eric J. , Kummel Andrew C.
Published: July 2002

Polarized ultraviolet light from an excimer laser (193 nm) was used to photodesorb and photodissociate N2O adsorbed on a cold (80 K) Pt(111) surface. The photodesorbed species and their time of flight (TOF) were monitored by resonantly enhanced multiphoton ionization (REMPI) spectroscopy. We have identified three major channels. The photodesorption of molecular N2 is observed only in the slowest channel where N2 produced by fragmenting the N2O is thermalized on the surface before desorbing. Evidence for this behavior includes both low (∼90 K) rotational and translational temperatures of the N2 fragments as well as a lack of correlation between rotational and translational energy. In the next fastest channel, hyperthermal N2O with a kinetic energy of 0.4±0.1 eV is seen to photodesorb. The photodesorbed hyperthermal N2O also has a substantial degree of internal vibrational excitation. The angular distribution of the N2O channel is peaked toward the surface normal. In the fastest channel, the release of ballistic oxygen atoms, a prompt axial recoil with no collisions with neighboring adsorbates, is seen along the tilted N2O molecular bond axis. The ballistic oxygen atoms leave the surface either in the ground state O(3P) or in the first electronically excited state O(1D). The kinetic energy of the O(3P) and of the O(1D) photoproducts is similar (0.5 eV) suggesting a common dissociative intermediate. In all of the channels observed, the dependence of the photoproducts yield on the polarization of the photodesorption laser indicates a hot carrier mediated mechanism at the surface. We propose a dissociative electron attachment model to explain the photochemistry of N2O/Pt(111) with 193 nm light.
Vibrational mode selectivity in the unimolecular decomposition of CH2NNO2
Authors: Rice Betsy M. , Grosh John , Thompson Donald L.
Published: July 2002

Vibrational mode selectivity has been investigated for the unimolecular decomposition of methylene nitramine (MN), CH2NNO2, by using classical trajectories computed on a (previously reported) potential-energy surface (PES) that is based on ab initio results. The PES allows for the two known primary decomposition pathways: (I) N–N bond scission to form H2CN and NO2 and (II) concerted molecular elimination giving HONO and HCN. Of particular interest in this study is the influence of selective vibrational mode excitations on the rates and branching of the decomposition via the competing reaction channels, which have barrier heights that differ by only 2 kcal/mol. Rates were computed for 57.7 kcal/mol above the zero-point energy for initial conditions corresponding to overtone excitations of each of the 15 normal modes. In each case, zero-point energy was assigned to each of the normal modes, and then one of the modes was assigned 57.7 kcal/mol excitation energy giving a total of 85.0 kcal/mol. One calculation was done at this energy with initial conditions corresponding to a microcanonical (statistical) distribution in which the energy was randomly distributed among all of the vibrational modes. Comparisons of the mode selective results with this statistical rate show that there is substantial enhancement of the decomposition rates for the two reactions for excitation of three of the normal modes (those with frequencies of 275, 442, and 753 cm−1).

Excitation of the other 12 modes yielded rates in accord with the statistical rates for the 2 reactions. The eigenvectors of the normal modes of the molecule that give mode selectivity in the reaction rates project onto the reaction coordinates for the two pathways. One other mode (with frequency 606 cm−1) also projects strongly onto the reaction path for II), but there is no enhancement of the rate when it is excited. However, we found that energy transfers out of this mode to other modes not coupled to either reaction coordinate. The remaining 11 modes do not project to any significant extent onto either reaction coordinate. The results show that projection of a vibrational mode onto the reaction coordinate is necessary for mode selective reaction, but is not sufficient since energy transfer into the reaction coordinate must compete with energy flow to other modes that do not project onto (i.e., are uncoupled from) the reaction coordinates.
Negative ion chemistry of SF4
Authors: Stevens Miller Amy E. , Miller Thomas M. , Viggiano A. A. , Morris Robert A. , Van Doren Jane M. , Arnold Susan T. , Paulson John F.
Published: July 2002

A selected ion flow tube was used to conduct an extensive study of negative ion–molecule reactions of SF4 and SF−4. Rate constants and product ion branching fractions were measured for 56 reactions. The reactions bracket both the electron affinity of SF4 (1.5±0.2 eV or 34.6±4.6 kcal mol−1) and the fluoride affinity of SF3 (1.84±0.16 eV or 42.4±3.2 kcal mol−1). These results may be combined to give the neutral bond energy D(SF3–F)=3.74±0.34 eV or 86.2±7.8 kcal mol−1, independent of other thermochemical data except for the accurately known electron affinity of F. The heat of formation of SF−4 is derived from the electron affinity of SF4: ΔfH(SF−4)=−9.2±0.3 eV or −212.9±7.5 kcal mol−1. Lower limits to EA(SF2) and EA(SF3) are deduced from observation of SF−2(35%) and SF−3(65%) ion products of the reaction S−+SF4. Rapid fluoride transfer from both SF−2 and SF−3 to SF4 places upper limits on the electron affinities of SF2 and SF3. The combined results are 0.2 eV≤EA(SF2)≤1.6 eV and 2.0 eV≤EA(SF3)≤3.0 eV. We review the status of measurements of EA(SFn), n=1–7.
Photodissociation of CH2. V. Three-dimensional adiabatic potential energy surfaces and transition dipole moments
Authors: Beärda Robert A. , van Hemert Marc C. , van Dishoeck Ewine F.
Published: July 2002

Full three-dimensional adiabatic potential energy surfaces are presented for the lowest five 3A″ and five 3A′ states of CH2. Both the 1 3A′ and 2 3A″ states are dissociative with respect to the C–H coordinates, consistent with our earlier two-dimensional results. All higher lying states are found to be bound for this coordinate, although the barrier toward dissociation is small for some states. In terms of angle dependence, the 1 3A′ state shows a flat behavior, but tends towards larger angles as dissociation proceeds. Most excited 3A′ states are somewhat bent with only a small barrier to linearity. Transition dipole moments connecting the ground state with the excited triplet states are presented as well. The 1 3A′ state is the only state of that symmetry with a large transition dipole moment in the Franck–Condon region. Other 3A′ states exhibit large values only if one bond is stretched compared with the ground state equilibrium geometry. The 1 3A″, 3 3A″, and 4 3A″ states are also slightly bent with a small barrier to linearity. However, the 2 3A″ state has an absolute minimum at very small angles (less than 60°), and shows a considerable local minimum (∼1.5 eV) for the linear configuration. The 5 3A″ state prefers the linear shape. The 3 3A″ state has the largest transition dipole moment function in the Franck–Condon region, but the transition moments to other 3A″ states can exhibit large values outside this region. The 2 3A″ and 3 3A″ states undergo an avoided crossing in the Franck–Condon region, so that a coupled states treatment is necessary for a correct description of the photodissociation dynamics. In order to provide the corresponding transition dipole moments in an appropriate form, a transformation to the principal axes of inertia was performed. The adopted transformations are discussed in detail.
Size consistency and size extensivity of linear response properties using the perturbed electron propagator
Authors: Deleuze Michael , Pickup Barry T.
Published: July 2002

The size-consistency and the size-extensivity aspects of linear response properties obtained using the coupled electron propagator approach are examined in the limit of a complex system dissociating into noninteracting fragments, and in the thermodynamic limit of a periodic inhomogeneous system with finite densities. This analysis is carried out considering forms of the perturbed electron propagator exact up to first-order and second-order in the correlation potential. Conclusions obtained can be extrapolated to higher orders on the basis of the topology of self-energy and perturbed one-electron density diagrams.
Structure of H+3 in high electric fields with implications for the structure of adsorbed H3
Authors: Reckzügel M. C. , Drachsel W. , Block J. H.
Published: July 2002

Total energy surfaces of linear and triangular H+3 in an external electric field are calculated using density functional theory in the nonlocal spin density approximation. For fields between 2 and 3 V/Å energy surfaces indicate that linear H+3 is unstable while triangular H+3 is stable. A new Born–Haber cycle is proposed which predicts from experimental appearance energy data that H+3 has a triangular structure upon arriving at the retarder. The field desorption of linear and triangular H3 is discussed in light of this prediction.
Interpretation of the polarization infrared spectrum of CO2 monolayers adsorbed on ionic substrates
Authors: Picaud S. , Briquez S. , Lakhlifi A. , Girardet C.
Published: July 2002

We interpret the polarization infrared spectrum of linear molecules adsorbed on ionic substrates on the basis of semi-classical interaction potentials. The frequency shifts and splittings and the intensities of the infrared signals are analyzed in terms of strong electrostatic surface fields with the additional influence of the lateral coupling between admolecules. Application to the CO2 monolayer adsorbed on clean NaCl and MgO(100) surfaces is discussed. A consistent quantitative comparison between calculated and measured peak intensities of the polarization spectrum requires the presence of two types of domains for the layer, oriented along the perpendicular step directions 〈100〉 and 〈010〉 with a relative weight 1:4. This corroborates the experimental assignment of the parallel and perpendicular polarized peaks associated with the bending and asymmetric stretching vibrational modes of the admolecule.
Temperature dependent quenching of A 2Σ+ NO between 215 and 300 K
Authors: Zhang Rong , Crosley David R.
Published: July 2002

Collisional quenching of the v′=0 vibrational level of the A 2Σ+ state of nitric oxide has been studied over the temperature range 215 to 300 K for the collision partners CO2, O2, H2O, NH3, H2S, and NO itself. The pressure dependence of the time decay of laser-induced fluorescence furnished the quenching cross sections σQ. The temperature is measured in situ. For all colliders, σQ is large and increases as the temperature decreases; this includes NO and O2 which do not show a temperature dependence above 300 K. The temperature dependence of σQ for each collider can be adequately described by the simple attractive force model of Parmenter and co-workers.
Supercooling of H2 clusters produced in free-jet expansions from supercritical states
Authors: Knuth E. L. , Schünemann F. , Toennies J. P.
Published: July 2002

A combined theoretical and experimental investigation has been undertaken to determine optimum conditions for achieving rapid cooling of H2 clusters in nozzle-beam expansions with the goal of producing superfluid H2 clusters. Theory predicts that a temperature less than 6.6 K, well below the 13.8 K triple-point temperature of p-H2, is required. Terminal specific enthalpies of clusters are determined experimentally from terminal velocities of clusters measured using the time-of-flight technique. The results are interpreted in the context of isothermal and adiabatic spinodals for p-H2 constructed using thermodynamic methods and a van der Waals equation-of-state model. The lowest terminal enthalpies are achieved in expansions starting from supercritical source conditions and crossing the binodal curve of the phase diagram with densities far to the liquid side of the critical point. In this case the clusters are formed via relatively late fragmentation of metastable liquid H2. These clusters are expected to be liquid and, at the point in the expansion at which collisions cease, to have temperatures of about 9 K. Before arriving at the detector they are cooled further by evaporation to temperatures between 4 and 5 K. Further experiments are needed to determine if these clusters are superfluid.
Photodissociation dynamics of (NO)2 on LiF(001): Characterization of vibrationally excited NO fragments
Authors: Jackson R. C. , Polanyi J. C. , Sjövall P.
Published: July 2002

The photodissociation of adsorbed NO dimers on LiF(001) was studied in the monolayer regime at 248 nm using resonantly enhanced multiphoton ionization (REMPI) and Fourier transform infrared (FTIR) absorption spectroscopy. Vibrationally excited NO photofragments were observed in v′=0–9. The vibrational energy distribution was found to have a maximum at v′=0 and a second region of substantial population between v′=2 and 9. The rotational and translational energy distributions of the photofragments showed no major change with vibrational excitation. By contrast, the translational energy displayed a systematic increase with increasing rotational excitation. Photodissociation at 1 ML (NO)2 coverage yielded average vibrational, rotational, and translational energies of 0.48, 0.05, and 0.13 eV, respectively, in the NO fragments. The vibrational and rotational energy distributions of the fragments were unchanged for 0.06 ML, while the translational energy increased by approximately 30% in going to this lower coverage. The angular distribution was peaked in the normal direction at both coverages. The results are interpreted in terms of an excitation/deexcitation mechanism, for which the observed energy distributions can be rationalized by assuming differing equilibrium geometries between the ground and excited states of the adsorbed dimer.
Mass-resolved two-photon spectra of Kr2
Authors: Lipson R. H. , Dimov S. S. , Cai J. Y. , Wang P. , Bascal H. A.
Published: July 2002

(2+1) resonantly enhanced multiphoton ionization spectra of jet-cooled Kr2 between ≊91 800 and 94 100 cm−1 have been recorded. Single isotopomer excitation spectra were obtained using time-of-flight mass detection. Vibronic structure for several Rydberg ← ground state transitions involving excited gerade states which dissociate to Kr+Kr* (4p55p) have been unambiguously analyzed for the first time, and molecular constants derived. Bond lengths were estimated from Franck–Condon calculations. These results compare favorably with recent ab initio calculations. Excited state predissociation effects were inferred from excitation spectra obtained by monitoring atomic daughter fragment ions in TOF.
The spectroscopy of phase transitions in clusters. A molecular dynamics study of infrared frequency shifts and line shapes in small acetonitrile clusters
Authors: Stace A. J. , Del Mistro G.
Published: July 2002

A combination of molecular dynamics and perturbation theory has been used to investigate the changes in infrared peak shape and frequency shift molecular clusters may exhibit as they undergo a phase transition from a solidlike to a liquidlike state. Four A1 vibrational modes of the acetonitrile molecule in (CH3CN)n clusters with n in the range 2–9, have been studied as a function of kinetic temperature. The calculated response of each vibrational mode to a phase transition is quite different: in some cases neither a frequency shift nor a change in linewidth are observed and, at the other extreme, the linewidths can double and the vibrational frequencies either red- or blueshift by 2–3 cm−1 as clusters move from a solidlike to a liquidlike-state. However, sharp changes in linewidth and frequency shift at the transition temperature are only seen in the larger clusters. Certain of the calculated spectral features are found to be very sensitive to the presence of molecular rotational motion in ‘‘warm’’ solidlike clusters; an observation which could have important implications in other areas of cluster spectroscopy.
How can (semi)local density functional theory account for the ground-state total energy of highly ionized atoms of the first three periods in the periodic table?
Authors: Kristyán Sándor
Published: July 2002

We investigate three generally used exchange-correlation functions (good for most other properties) in order to discern their ability to reproduce ground-state total energies of highly ionized atoms as well as the sum of their first two ionization energies. Total ground-state energies of closed shell atoms with N electrons and Z atomic numbers are considered for 2≤N≤Z≤18, and N=2,4,6,8,10. The sum of the first two ionization energies, I1+I2, is calculated for closed shell atoms with Z=2,4,6,8,10. The density functional theory (DFT) methods investigated are remarkably successful in accounting for the ground-state total energy of the ionized states of atoms, although their accuracy significantly varies with the positive charge of the ionized atom. Interestingly, the conventional Hartree–Fock self-consistent field (HF-SCF) method is more ‘‘rigid’’ with respect to this type of variance in accuracy. The Becke gradient corrected exchange function gives good results, but the Becke exchange with the Lee–Yang–Parr correlation function is better. However, there are some ionized states of atoms for which even the best density functional methods do not exceed the accuracy of the conventional Hartree–Fock SCF method. The simple Dirac–Slater functional gives poor results. The comparison of these methods to accurate ab initio calculations and experimental data are reported in detail. Interestingly, the accuracy of these methods (as a function of the degree of ionization) may reflect the shell structure of the atom.
Gauge-invariant basis sets for magnetic property calculations
Authors: Sugimoto Manabu , Nakatsuji Hiroshi
Published: July 2002

The use of augmented basis sets of the form, {χ, rtχ, rtruχ, ...} (rt, ru=x, y, z), is proposed for calculating magnetic properties which are almost gauge-origin independent. It is derived from Epstein’s theorem which states the sufficient condition for unitary invariance. Test calculations using the coupled-Hartree–Fock/finite perturbation method show that the augmented sets correctly reduce the origin dependence of magnetic shielding constants, and that the results agree well with the experiment. Through systematic modifications of the basis set, a practical procedure in choosing basis functions to be added is suggested.
Nonlinear relaxation and solvation dynamics in a Coulomb lattice gas
Authors: Knödler D. , Dieterich W. , Lonsky C. , Nitzan A.
Published: July 2002

The transient response of ions upon sudden creation or removal of a local, external charge in a one-component Coulomb lattice gas is investigated by Monte Carlo simulations. Our model can be regarded as a simplified description of solvation dynamics processes in electrolyte systems. Effects of the nonlinearity of the relaxation are pointed out and are contrasted with predictions from linear (Debye–Hückel and Debye–Falkenhagen) theories.
Isotope effect in electron stimulated desorption: The role of internal degrees of freedom in CO desorption from Pt(111)
Authors: Szabó András , Yates John T.
Published: July 2002

Electron stimulated desorption (ESD) of CO+, O+, metastable neutral CO*, and ground state neutral CO from the CO/Pt(111) was studied, using isotopic substitution of CO. Four isotopic versions of CO were compared in their desorption behavior. Contrary to the prediction of theoretical models that suggest a decrease of the ESD yield with increasing mass of a given desorption product, the CO+ and CO* ESD yields were found to be larger from the (heavier) 12C18O than from the (lighter) 13C16O adsorbate. The O+ ESD yields followed the expected trend Y(16O+)≳Y(18O+). No isotope effect was observed for ground state neutral CO desorption. Qualitative arguments explaining the anomalous isotope effect, and emphasizing the importance of internal dynamics, in particular, the rotation of diatomic desorption products in the ESD process, are presented. Here it is shown that the velocity of departure of the carbon end of the rotating CO molecule may control neutralization or quenching effects for CO+ or CO* produced by electron stimulated desorption. Thus, neutralization or quenching of the excited CO species occurs via carbon-centered orbitals of the departing species.
Theory of nonequilibrium effects on the conformation of polymers
Authors: Eu Byung Chan , Gan Hin Hark
Published: July 2002

Two sets of integral equations are presented in this paper for dynamic pair correlation functions for polymeric liquids subjected to shearing. One set of the integral equations is a dynamic and polymeric extension of the Percus–Yevick integral equation for the equilibrium pair correlation function of a simple fluid, whereas the other is that of the hypernetted chain equation. These integral equations are coupled, in the case of uniform temperature, to the momentum balance equation and the constitutive equations for the stress tensor and the diffusion fluxes in the system. When this coupled system of evolution equations (generalized hydrodynamic equations) is solved subject to appropriate initial and boundary conditions, the dynamic structure factors and correlation functions can be computed for the polymeric liquid of interest. The first-order iterative solution of the integral equations for correlation functions is computed for the case of equilibrium polymers. It yields a qualitatively correct chain-length dependence of the maximum position of the end-to-end distance distribution function and thus the mean square end-to-end distance of the polymer. The shear-induced effect on the end-to-end distance distribution function is numerically computed based on the first-order perturbation solution, which indicates that the distribution becomes anisotropic, having two twofold symmetry axes, when the polymer is sheared.
Momentum-space ionization spectroscopy of iso-dichloroethylene: A comparative (e,2e) study of electron momentum distributions of the frontier-orbital and innervalence many-body states in iso-dichloroethylene and its cis and trans isomers
Authors: Chuaqui M. H. , Mei L. , Mathers C. P. , Allison M. L. , Ying J. F. , Leung K. T.
Published: July 2002

The ionization energy spectra of iso-dichloroethylene (iso-C2H2Cl2) at electron momentum of ∼0 and 0.7 a.u. have been obtained by molecular (e,2e) spectroscopy in the symmetric noncoplanar geometry at 1200 eV impact energy. Momentum distributions (MDs) of selected ionic states, including the outervalence states X(3b1)−1 (frontier-orbital state) and G(6b2)−1, and the innervalence states H(9a1)−1, S1(8a1 )−1, S2(5b2)−1, and S3(7a1)−1 as well as an unassigned many-body feature, have also been determined and compared with ab initio calculations using self-consistent-field wave functions of 4-31G, 6-31G, and 6-31++G** basis sets. Combined with our earlier (e,2e) results on the cis- and trans-C2H2Cl2, these MD measurements can be used to investigate the effects of isomeric changes on the valence-shell electronic structure of these disubstituted ethylene derivatives. The sensitivity of the experimental MDs to chemical bonding details can be demonstrated. In addition, ‘‘new’’ many-body features at 19–22 eV and above ∼26 eV have been observed in our ionization energy spectra, which support the breakdown of Koopmans’ theorem as predicted by an earlier Green’s function calculation. Furthermore, our MD measurements can be used to attribute the new many-body features observed at 19–22 eV to (9a1)−1 satellite states and to characterize the many-body states above ∼26 eV predominantly as (7a1)−1 satellite states.
High resolution rotational analysis of the B 3Π–X 3Δ (1,0) band of titanium monoxide
Authors: Amiot Claude , Azaroual El Mehdi , Luc Paul , Vetter Raymond
Published: July 2002

The B 3Π–X 3Δ (1,0) band of titanium monoxide has been studied at sub-Doppler resolution (0.002 cm−1) by crossing a beam of TiO molecules with a cw tunable laser beam and by collecting the laser-induced fluorescence. The rotational structure of 42 branches belonging to the 3Π–3Δ transition has been analyzed up to rotational quantum numbers equal to 94. Spectroscopic data have been reduced to a set of 24 molecular constants, using a case (a) effective Hamiltonian. The rotational, spin–orbit and Λ-doubling constants are discussed in terms of the leading configurations which give rise to the X 3Δ and B 3Π electronic states. It is shown that for the B state, existing ab initio calculations are not able to reproduce the second order spin–orbit effect and the Λ doubling effect.
Argon in condensed phase: Quantitative calculations of structural, thermodynamic, and transport properties from pure theory
Authors: Ermakova Elena , Solca Jan , Huber Hanspeter , Welker Marc
Published: July 2002

Molecular dynamics simulations are performed with a new ab initio argon–argon pair potential from the literature for pressures up to 1000 MPa and temperatures up to 600 K in the supercritical phase as well as for points in the liquid phase of argon. Many properties, such as the compression factor z, the pair distribution function g, the enthalpy H, the internal energy U, the molar heat capacities CV,m and CP,m, the speed of sound c, the adiabatic and isothermal compressibilities βS and βT, the thermal pressure coefficient γV, the self-diffusion coefficient D, the thermal conductivity λ, and the shear viscosity η are calculated and compared to experimental values. These comparisons together with comparisons from calculations of similar quality for condensed neon give hints of the influence of many-body interactions on different properties. Structural and transport properties, but also thermodynamical properties like heat capacities, compressibilities and sound velocities, are quite accurately described within this model even at high pressures and in the liquid phase. Therefore, the calculated values are expected to give reliable predictions in ranges where experimental data are not available and it may be concluded, that many-body interactions have little influence on these properties. However, pressure, internal energy and enthalpy, show increasing absolute deviations with higher density and are not well reproduced without the inclusion of many-body interactions.
Quadrupole coupling constants in linear (HCN)n clusters: Theoretical and experimental evidence for cooperativity effects in C–H⋅⋅⋅N hydrogen bonding
Authors: King B. F. , Farrar T. C. , Weinhold F.
Published: July 2002

Ab initio quadrupole coupling constants χ(D), χ(14N) for deuterium and nitrogen nuclei are obtained for the sequence of linear (HCN)n clusters (up to n=6) at the RHF/6-31+G* level, complementing previous studies of n-dependent ‘‘cooperative’’ effects in these C–H...N hydrogen-bonded species. For the dimer and trimer, the theoretical values are compared with experimental equilibrium values of Gutowsky and co-workers and found to successfully reproduce both the magnitudes and patterns of measured shifts. For larger (HCN)n clusters, the n-dependent trends in χ(D), χ(14N) appear to be correlated with cooperativity effects found previously (binding energies, geometrical parameters, dipole moments, ir frequencies and intensities). This suggests that quadrupole coupling measurements can provide a useful probe of cooperative H-bonding phenomena in gaseous and condensed media.
Dynamic structure factor in a bidisperse polymer melt
Authors: Wilson Jeffrey D. , Loring Roger F.
Published: July 2002

We present calculations of the single-chain dynamic structure factor for a polymer melt composed of linear molecules of the same chemical identity but of two different chain lengths. The fluid is treated within a dynamical mean-field approach, in which each molecule is represented as a freely jointed chain moving among stochastic obstacles. The obstacles are of two types, each representing the obstruction of local conformational changes by one of the species present. The obstacle dynamics are determined self-consistently by equating the relaxation rate of an obstacle of a given type to the smallest conformational relaxation rate of the species that it represents. Calculation of the dynamic structure factor is mapped onto the solution of a random walk with dynamical disorder, in which a walker moves on a one-dimensional lattice with hopping rates that randomly fluctuate among three states. The relevant random walk problem is solved within the effective medium approximation, and the results are employed to examine the dependence of the dynamic structure factor on time, wave vector, chain lengths, and fluid composition.
The visible excitation spectrum of jet cooled NO2: The chaotic behavior of a set of 2B2 vibronic levels
Authors: Georges Robert , Delon Antoine , Jost Rémy
Published: July 2002

We have observed a set of 350 2B2 vibronic levels of NO2 in the 16 000–19 360 cm−1 energy range by the laser induced fluorescence (LIF) technique combined with a supersonic jet. This work extends (i.e., a larger energy range) and improves (i.e., a better detection threshold) our previous study [J. Chem. Phys. 95, 5701 (1991)]. 42 new 2B2 vibronic levels have been detected in this range where 159 vibronic levels were previously observed. In the 16 580–19 360 cm−1 energy range we estimate that the 315 observed levels represent 96% of the existing 2B2 levels. The correlation properties of this large and almost complete set of 315 2B2 vibronic levels have been analyzed. We present the next neighbor distribution, the Σ2(L), and Δ3(L) statistics, the Fourier transform (FT) of the stick spectrum with constant intensities (‖FT‖2), and the intensity distribution. The results of these analyses confirm the chaotic behavior of the 2B2 vibronic levels in this energy range: there are strong level repulsion, long range correlations and a Porter–Thomas intensity distribution. The correlation ‘‘hole’’ observed in the ‖FT‖2 of the stick vibronic spectrum is close to the one of the Gaussian orthogonal ensemble (GOE). However we have found a significant deviation from completely chaotic behavior (GOE type). Two peaks in the FT indicate recurrences (periods of 50 and 150 fs) i.e., periodic motions. We conclude that chaos is established within the 2B2 vibronic levels of NO2, after few hundred femtoseconds.
Binary nucleation kinetics. II. Numerical solution of the birth–death equations
Authors: Wyslouzil Barbara E. , Wilemski Gerald
Published: July 2002

We numerically solve the complete set of coupled differential equations describing transient binary nucleation kinetics for vapor-to-liquid phase transitions. We investigate binary systems displaying both positive and negative deviations from ideality in the liquid phase and obtain numerical solutions over a wide range of relative rates of monomer impingement. We emphasize systems and conditions that either have been or can be investigated experimentally. In almost every case, we find behavior consistent with Stauffer’s idea that the major particle flux passes through the saddle point with an orientation angle that depends on the rates of monomer impingement. When this is true, the exact numerical steady state nucleation rates are within 10%–20% of the predictions of Stauffer’s analytical theory. The predictions of Reiss’ saddle point theory also agree with the numerical results over a wide range of relative monomer impingement rates as long as the equilibrium vapor pressures of the two pure components are similar, but Stauffer’s theory is more generally valid. For systems with strong positive deviations from ideality, we find that the saddle point approximation can occasionally fail for vapor compositions that put the system on the verge of partial liquid phase miscibility. When this situation occurs for comparable monomer impingement rates, we show that the saddle point approximation can be rescued by evaluating an appropriately modified nucleation rate expression. When the two impingement rates differ significantly, however, the major particle flux may bypass the saddle point and cross a low ridge on the free energy surface. Even in these rare cases, the analytical saddle point result underpredicts the numerical result by less than a factor of 10. Finally, we study the transition from binary to unary nucleation by progressively lowering the vapor concentration of one component. Both Reiss’ and Stauffer’s rate expressions fail under these conditions, but our modified rate prescription remains within 10%–20% of the exact numerical rate.
On the density of volume states in the isobaric ensemble
Authors: Attard Phil
Published: July 2002

The conventional implementation of the isobaric (fluctuating volume) ensemble neglects the density of volume states. Here it is shown that the latter is proportional to the inverse of the volume. It is argued that this expression should always be used, and that it has measurable consequences in simulations of small systems, in simulations that seek an accurate equation of state, and in simulations of the critical, spinodal, and coexistence regions.
Comparisons between statistics, dynamics, and experiment for the H+O2→OH+O reaction
Authors: Yang Chao-Yie , Klippenstein Stephen J.
Published: July 2002

The accuracy of the variable reaction coordinate (VRC) implementation of transition state theory (TST) is investigated for the bimolecular reaction of H with O2 via direct comparisons with quantum scattering theory for J=0, classical trajectory simulations for a wide range of J, and experimental canonical rate constants. The DMBE IV potential energy surface of Varandas and co-workers is employed in each of the theoretical calculations. The first two comparisons indicate that the VRC-TST approach overestimates the cumulative reaction probability (CRP) for this reaction by a factor of 2.3, roughly independent of E and J for moderate energies. The trajectory simulations further indicate that this failure of TST is primarily the result of the rapid redissociation of a large fraction of the initially formed HO2. An estimate for the quantum CRP on the basis of the combined dynamical and statistical results is seen to provide a useful alternative to the more standard quasiclassical trajectory estimates. A thermal averaging over the E and J-dependence of the TST estimates for the CRP provides canonical rate constants, k(T), which, when corrected for the above-mentioned overestimate, are still a factor of 1.7–2.0 times greater than the experimental data. This discrepancy is most likely the result of either (i) inaccuracies in the DMBE IV surface and/or (ii) an overestimate of the contribution to the reactive flux from the nearly degenerate first excited state in the exit channel region.
Selected ion flow drift tube studies of the reactions of Si+(2P) with HCl, H2O, H2S, and NH3: Reactions which produce atomic hydrogen
Authors: Glosík J. , Zakouřil P. , Lindinger W.
Published: July 2002

The reaction rate coefficients, k, for the reactions of ground-state Si+(2P) with HCl, H2O, H2S, and NH3, have been measured as a function of reactant ion/reactant neutral center-of-mass kinetic energy, KECM, in a selected ion flow drift tube (SIFDT) apparatus, operated with helium at a temperature 298±2 K. The values k of the studied reactions have very pronounced, negative energy dependencies; the rate coefficients decrease by about 1 order of magnitude as KECM increase from near thermal values to ∼2 eV. The results are interpreted in terms of a simple model assuming the reactions to proceed via the formation of long-lived complexes. These intermediate complexes decompose back to reactants or forward to products, the unimolecular decomposition rate coefficients for these reactions being k1 and k2, respectively. It is found that a power law of the form k−1/k2=const(KECM)m closely describes each reaction.
Seminumerical contact transformations: From internal coordinate rovibrational Hamiltonian to effective rotational Hamiltonians. Framework of the method
Authors: Lukka Tuomas J. , Kauppi Esa
Published: July 2002

This paper presents a formulation of seminumerical contact transformations for rovibrational spectroscopy. Effective rotational Hamiltonians are obtained starting from a rovibrational Hamiltonian with an exact kinetic energy operator in curvilinear internal valence coordinates. Like the accuracy of the variational methods, the accuracy of this method can be increased by using more computational power. Error estimates are also calculated. Main motivations for using seminumerical contact transformations in rovibrational spectroscopy are considered. As an example, a calculation is carried out for H2S. No remarkable deviations between the calculated and the observed effective constants were observed for the states considered (ground states, ν2, ν1, ν3, 2ν2, 2ν1+ν2, ν1+ν2+ν3, ν2+2ν3). New ideas for further research of this subject are suggested.