bluuebunny/crossref_metadata_2025 huggingface dataset:
Observation of an anomalous increase in total cross sections with high vibrational excitation in the Li2 (v=0–21)+Na reactive system
Authors: Rubahn H.-G. , Slenczka A. , Toennies J. P.
Published: July 2002

The effect of initial vibrational excitation on Li2(v,J)+Na collisions was studied experimentally and theoretically. The relative integral cross sections σ(v,J) were measured in a crossed beam experiment for vibrational states v populated thermally or by optical pumping, and for final states detected by laser induced fluorescence. It is found that σ(v,J) increases by 35±7% between v=0 and v=20, while the dependence on the initial rotational state J is insignificant, within ±5%. The effect of vibrational enhancement is about twice as large as that found for the Li2–rare gas systems for the same amount of vibrational excitation. The effect is about an order of magnitude larger than predicted by our quasiclassical trajectory calculations on a LEPS potential energy surface, thus suggesting that additional long-range potential terms ought to be included. We also attempted to measure state-selectively the LiNa (v′,J′) products from the thermal reaction and found an upper limit of the reactive cross section of 1 Å2. This contrasts with the trajectory calculations that predict a 20 times larger cross section; we thus conclude that the reactive part of the potential also needs to be re-evaluated.
The finite basis representation as the primary space in multidimensional pseudospectral schemes
Authors: Lemoine Didier
Published: July 2002

We emphasize the merits and the superiority of the most complete nondirect product representation in non-Cartesian coordinates. Beyond the proper choice of basis set we show how to further optimize the spectral range in multidimensional calculations. The combined use of a fully pseudospectral scheme and the finite basis representation (FBR) as the primary space ensures the smallest prefactor in the semilinear scaling law of the Hamiltonian evaluation with respect to the FBR size. In the context of scattering simulations we present a simplified asymptotic treatment which increases the efficiency of the FBR-based pseudospectral approach. An optimal 6D pseudospectral scheme is proposed to treat the vibrational excitation and/or dissociation of a diatomic molecule scattering from a rigid, corrugated surface, and serves to illustrate our discussion. A 5D numerical demonstration is made for the rotationally inelastic scattering of N2 from a model LiF surface.
Bonding geometry and mechanism of NO adsorbed on Cu2O(111): NO activation by Cu+ cations
Authors: Fernández-García M. , Conesa J. C. , Bagus P. S. , Rubio J. , Illas F.
Published: July 2002

An ab initio molecular cluster model approach has been used to investigate the adsorption geometry and the nature of the interaction of the NO molecule with the Cu2O(111) surface. The two possible NO orientations, N- and O-down, have been studied for adsorption on both onefold and threefold surface positions. We show that, in all cases, the most important contribution to the bonding is the electrostatic interaction, with negligible or small (depending on the orientation and position) contributions from chemical effects. In the monocoordinated position it is found that the two NO orientations exhibit opposite vibrational frequency shift with respect to the free NO molecule. For the three-coordinated site the vibrational shift is found to be nearly zero. In the first case, when NO is bound through the O atom, its shift is negative, as observed experimentally on Cu2O surfaces. Therefore, the present model calculations suggest that the species which is experimentally observed is O-down chemisorbed NO on a onefold unsaturated site. Also, it is shown that this negative shift arises from electrostatic effects because the other significant contributions to the frequency, i.e., the Pauli repulsion and the NO polarization, cancel each other.
A density-functional study of cluster reactivity: Ammonia reacting with a free Ga5As5 cluster
Authors: Mackey J. , Lou L. , Nordlander P. , Smalley R. E.
Published: July 2002

It is well known that the chemical reactivity of small gallium–arsenide clusters can be sensitive to cluster size and geometrical structure. We have performed ab initio calculations probing the interaction of NH3 with different sites of a free Ga5As5 cluster. Our results show that bonding tends to be strongest at gallium sites. Adsorption of NH3 is found to induce strong relaxation of the cluster global geometry. The cluster sites are compared with sites on the (111) bulk surface of gallium arsenide, and the possibility for exploring the similarity between cluster reactivity and surface reactivity is discussed.
Kinetics of step-site filling for CO/Ni(9,1,1): A pulsed molecular beam-surface infrared study
Authors: Sinniah K. , Reutt-Robey J. E. , Robinson Brown A. , Doren D. J.
Published: July 2002

Pulsed molecular beam-surface infrared measurements of the kinetics of CO populating step sites on Ni(9,1,1) are reported and interpreted in terms of elementary surface rate processes. An analytic model is developed to describe the distribution of CO between step and terrace sites in the equilibrium limit, and refine our previous determination of the binding energy difference between these sites to ΔEs-t=0.6±0.2 kcal/mol. Time-resolved surface infrared measurements indicate that the equilibrium step coverage is reached within 100 ms of the chemisorption event. This rapid migration across the (100) terraces to step sites implies a barrier to surface hopping of <5.5 kcal/mol. On a longer time scale of minutes, the CO population at step sites increases further as the equilibrium point is shifted by the dissociative adsorption of residual hydrogen. These slower step filling rates are described with a kinetic model, in which hydrogen adsorption is the rate-limiting step.
The atomic softness matrix
Authors: Cioslowski Jerzy , Martinov Martin
Published: July 2002

The conventional approach to the electronegativity equalization principle suffers from a serious conceptual drawback that stems from discontinuities in the first derivative of the electronic energy with respect to the total charge of a molecule. A formalism that avoids the resulting ill-defined atomic hardness matrices employs a simple definition of the atomic softness matrix σ. The matrix σ, which is a generalization of the Hückel atom–atom polarizability matrix, can be easily and rigorously computed to arbitrary accuracy with any of the contemporary electronic structure methods. Properties of the atomic softness matrix are reviewed and illustrated with several numerical examples involving the first-row hydrides and a linear polyyne.
Emission spectroscopy of dissociating H2S: Influence of nonadiabatic coupling
Authors: Heumann Bernd , Schinke Reinhard
Published: July 2002

The emission spectroscopy of H2S excited in the first absorption band around 195 nm is investigated theoretically using ab initio potential energy surfaces (PES) and transition dipole moment functions. As shown in our previous studies, the photodissociation involves two excited electronic states, one which is binding and another one which is dissociative. The nonadiabatic coupling between these two states is very strong and described in a diabatic representation in which only the binding state is optically excited while the dissociative state is dark. As in the case of H2O excited in the 165 nm band, the emission spectrum shows a long progression of stretching states up to seven HS vibrational quanta. In contrast to water, however, some weak activity in the bending mode is also observed. Most remarkable is a prominent wavelength dependence which is attributed to the strong nonadiabatic coupling between the two excited electronic states. The agreement with experimental data is only fair; the essential features of the measurements are qualitatively reproduced, finer details such as the wavelength dependence are, however, not well described. It is concluded that more accurate ab initio input data are required in order to reproduce all details of the measured emission spectra.
Ab initio study of the phenol-water cation radical
Authors: Hobza Pavel , Burcl Rudolf , Špirko Vladimír , Dopfer Otto , Müller-Dethlefs Klaus , Schlag Edward W.
Published: July 2002

The phenol-water cation radical has been investigated by ab initio theory using the spin-restricted open-shell Hartree–Fock and spin-restricted open-shell second-order Mo/ller–Plesset theories with 3-21G*(O) and 6-31G* basis sets. The full geometrical optimization was performed for several hydrogen-bonded structures and one hemibonded structure. Clearly, the most stable structure has been found for Cs symmetry with the linear hydrogen bond between the proton of the OH group of the phenol cation radical and the oxygen of the water, and the water hydrogens pointing away from the phenyl ring. For this structure harmonic (and for some intermolecular modes anharmonic) vibrational frequencies have been computed for various isotopic complexes. The computed shifts of phenol-localized intramolecular modes on complexation and on deuteration as well as the calculated intermolecular frequencies of the different isotopic complexes allow for an assignment of vibrational frequencies observed in the experimental zero-kinetic-energy (ZEKE) photoelectron spectra. Five out of a possible six intermolecular vibrations and several intramolecular modes have been assigned, including the 18b vibration which shows a strong blue shift in frequency upon complexation. Structure and properties of the phenol-water cation radical are compared with those of the corresponding neutral complex.
A multiconfiguration self-consistent-field response study of one- and two-photon dipole transitions between the X 1Σ+ and A 1Π states of CO
Authors: Sundholm Dage , Olsen Jeppe , Jo/rgensen Poul
Published: July 2002

The one- and two-photon dipole transitions between the X 1Σ+ and the A 1Π states of CO have been studied by means of multiconfiguration self-consistent-field linear and quadratic response methods. The vibrationally averaged oscillator strength for the 0–0 one-photon dipole transition between the X 1Σ+ and A 1Π states obtained using the linear response method is 1.31×10−2 as compared to the experimental results of 0.96(14)×10−2, 1.08(7)×10−2, and 1.11×10−2. The two-photon transition probability rate constant, obtained using the quadratic response method, for the 0–1 vibrational band of the X–A transition of 7×10−59 cm4 s is more than six orders of magnitude smaller than the experimental result of 3.5×10−52 cm4 s. We suggest that the experiment should be reconsidered. The dipole moment of the A 1Π state obtained from quadratic response calculations on the ground state at R=2.332 a.u. is −0.0441 a.u. suggesting an anomalous polarity also for the A 1Π state. The experimental value is ±0.059(20) a.u.
Mode-selective infrared excitation of linear acetylene
Authors: Kaluža Matjaž , Muckerman James T.
Published: July 2002

Quantum-mechanical simulations of excitation of linear acetylene (HCCH) with a few synchronized, infrared, linearly polarized, transform-limited, subpicosecond laser pulses reveal optimal pathways for the selective laser-controlled excitation of the stretching modes in the molecule. Examples presented include a double-resonance excitation of a CH-stretching local mode state, a single-pulse excitation of a predominantly symmetrical CH-stretching state, an optimal two-pulse dissociation of the molecule into C2H + H, and a two-pulse sequence which induces stimulated emission and dumps the energy from a highly excited CH-stretching state into a CC-stretching state. The resulting optimal laser pulses fall within the capabilities of current powerful, subpicosecond, tunable light sources. The spectroscopy of the model that is relevant for finding selective excitation pathways is discussed. The wave function of the molecule is represented in a harmonic normal-mode basis, a discrete variable representation, and in an eigenstate basis. A real-time Lanczos propagator and an energy-shifted, imaginary-time Lanczos propagator are employed.
Electronic spectra of ND3 in the region of 64 500–73 500 cm−1 and their vibronic assignments
Authors: Li Xinghua , Vidal Carl Rudolf
Published: July 2002

Using a monochromatic VUV beam generated by two-photon resonant sum frequency mixing in a Mg–Kr mixture, we have investigated the electronic transitions of ND3 in the 64 500–73 500 cm−1 region by detecting the fluorescence from the predissociation fragments of ND3 or by detecting the ions produced via VUV+visible resonance enhanced multiphoton ionization (REMPI). Vibronic transitions from the ground state to the 2n, n=7–12 levels of the B̃ 1E″ state are rotationally resolved and the relevant molecular constants are deduced. Two parallel progressions are observed, of which one has been analyzed previously by Douglas [Faraday Discuss. Chem. Soc. 35, 158 (1963)]. We attribute the progression observed by Douglas as transitions to the 2n31 vibronic levels of the B̃ 1E″ state, the other one as transitions to the 2n3141 vibronic levels of the same electronic state. Transitions to the υ2′=0 vibronic level of the previously assigned D̃ 1E″ state have been rotationally resolved and the electronic symmetry of this electronic state is reassigned to E′.
Observation of surface enhanced multiphoton photoemission from metal surfaces in the short pulse limit
Authors: Aeschlimann M. , Schmuttenmaer C. A. , Elsayed-Ali H. E. , Miller R. J. D. , Cao J. , Gao Y. , Mantell D. A.
Published: July 2002

Photoelectrons with excess kinetic energy corresponding to several absorbed photons above the work function have been measured from atomically clean Cu(110) and Cu(100) surfaces under ultrahigh vacuum conditions. The power dependence of the photoemission yield does not follow a simple power law dependence corresponding to the number of photons absorbed. This behavior is reminiscent of other above threshold ionization (ATI) or tunnel ionization (TI) processes observed for atoms in the gas phase. The photoelectrons are generated with laser pulsewidths less than 100 fs in duration and peak powers as low as 100 MW/cm2. These intensities are on the order of 105 times lower than that required to observe similar phenomena in the gas phase. The relatively low intensities and correlation with surface roughness suggests a contribution from a surface enhancement mechanism. Thermal heating and space charge effects have been ruled out, and the possibility of electric field enhancement at the surface due to the coupling of photons into surface plasmons is discussed. The nonlinear yield and enhancement of the photoemission produced by short pulse excitation needs to be considered when discussing photoinduced hot electron reaction channels at metal surfaces.
The nuclear quadrupole coupling constants and the structure of the para–para ammonia dimer
Authors: Heineking N. , Stahl W. , Olthof E. H. T. , Wormer P. E. S. , van der Avoird A. , Havenith M.
Published: July 2002

Expressions are derived for the nuclear quadrupole splittings in the E3 and E4 (para–para) states of (NH3)2 and it is shown that these can be matched with the standard expressions for rigid rotors with two identical quadrupolar nuclei. The matching is exact only when the off-diagonal Coriolis coupling is neglected. However, the selection rules for rotational transitions are just opposite to those for the rigid rotor. Hyperfine splittings are measured for the J=2←1 transitions in the E3 and E4 states with ‖K‖=1; the quadrupole coupling constants χaa=0.1509(83) MHz and χbb−χcc=2.8365(83) MHz are extracted from these measurements by the use of the above mentioned correspondence with the rigid rotor expressions. The corresponding results are also calculated, with and without the Coriolis coupling, from the six-dimensional vibration–rotation–tunneling (VRT) wave functions of (NH3)2, which were previously obtained by Olthof et al. [E.H.T. Olthof, A. van der Avoird, and P.E.S. Wormer, J. Chem. Phys. 101, 8430 (1994)]. From the comparison of χaa with the measured value it follows that the semiempirical potential and the resulting VRT states of Olthof et al. are very accurate along the interchange (ϑA,ϑB) coordinate. From χbb−χcc it follows that this potential is probably too soft in the dihedral angle γ̄=γA−γB, which causes the torsional amplitude to be larger than derived from the experiment.
A statistical theory of mixtures: Application in the calculation of the vacancy formation parameters in rare gas solids
Authors: Soulayman S. Sh. , Zanchetta J. V. , Giuntini J. C.
Published: July 2002

A general statistical theory is proposed here to calculate the Gibbs energy of multicomponent mixtures. This theory is based on the assumptions of the quasichemical approximation and on the unsymmetrized self-consistent-field method. The effectiveness of the proposed theory is demonstrated through its application to determine vacancy formation parameters in rare gas solids (Ar, Kr, and Xe). In the calculations the pair potentials of Lennard-Jones and Barker et al. are used. The third-order triple-dipole forces of Axilrod–Teller–Muto are taken into consideration. The results are compared with the available theoretical calculations, Monte Carlo computations, and experimental estimations for some parameters of the vacancy formation.
Monte Carlo study of CO hydrogenation on cobalt model catalysts
Authors: Hovi J.-P. , Lahtinen J. , Liu Z. S. , Nieminen R. M.
Published: July 2002

Useful information on catalytic reactions can be achieved using Monte Carlo simulations combined with experimental data from model catalysts. We present a comprehensive analysis of the simulation studies of CO hydrogenation on a hexagonal surface using a discrete computer model for the irreversible reaction kinetics with no interactions between the surface species but their mutual reactions. The simulation results are compared to experimental data from a cobalt foil model catalyst at 101 kPa and 525 K. As a result, the following microscopic picture of the reaction on the catalyst surface is extracted: the rate-limiting reaction step is the termination of the carbon chains (α-hydrogenation), hydrogen atoms occupy different adsorption sites from other reactants, and the diffusion of hydrogen along the surface is fast. The model is also used to address the relevance of the ensemble effects for CO dissociation and the surface sensitivity of the CO hydrogenation reaction. Our simulation results imply that these aspects have little effect on the rates of hydrocarbon formation.
Instability in the dynamics of long DNA undergoing gel electrophoresis in a strong field
Authors: Semenov A. N. , Subbotin A. V.
Published: July 2002

We present a model describing reptation motion of a long DNA chain undergoing gel electrophoresis. Internal modes of the chain in an effective tube are associated with lateral loops (hernias), and the effect of the field on these lateral loops is analyzed. An equation describing reptation dynamics of the chain is derived. It is shown for fields exceeding some critical value that stationary reptation motion becomes unstable. Mobility of the chain in the weak fields is also calculated.
Dynamics of a molecule’s growth: Ultrasonic relaxation studies
Authors: Parthun M. G. , Johari G. P.
Published: July 2002

Longitudinal velocity and attenuation of ultrasonic waves traveling through a liquid at different temperatures have been measured as its molecules combine irreversibly to form large entities and thereby decrease the diffusivity and increase the configurational restrictions to their dynamics. In addition, the number of covalent bonds formed are measured by calorimetry. From these data, the longitudinal modulus and compliance are calculated, and the molecular relaxation time and related properties are deduced and interpreted in terms of the number of covalent bonds formed, by a formalism that connects the size of the molecules in the liquid with its elastic behavior. This relaxation time increases monotonically with increase in the molecule’s size, tending to infinity as the number of covalent bonds formed approaches the Avogadro number. The complex plane plots of the modulus and compliance have a shape that is described by a skewed arc function, with a temperature dependent exponent γ, that ranges in values from 0.33–0.31 for modulus and 0.39–0.45 for compliance. Departure from this shape suggest significant contributions from nonzero shear viscosity for relatively small size of molecules, and contributions from a faster or β-relaxation process when the molecular size is large. The study reveals the behavior of a liquid as it is altered by a permanent change in the size of the diffusing entities.
Potential curves and nonadiabatic matrix elements for collisions involving fragments of the HeN+ molecular ion
Authors: Gu Jian-ping , Buenker Robert J. , Hirsch Gerhard , Kimura Mineo
Published: July 2002

Ab initio multireference CI calculations have been carried out for the HeN+ molecular ion in order to describe collision processes between its constituent neutral and ionized atoms. The accuracy of these calculations is evaluated by means of a comparison of results obtained at large internuclear separations with the corresponding asymptotic energies deduced from atomic spectral data. Energy values are computed for the eleven lowest He++N and He+N+ atomic limits and average discrepancies relative to the experimental excitation energies up to 110 000 cm−1 are found to lie in the 1000–3000 cm−1 range, of which only 200 cm−1 appears to be the fault of the configuration interaction (CI) technique itself, with the main portion of the error stemming from the choice of atomic orbital (AO) basis instead. The HeN+ X 3Σ− ground state is calculated to have a De value of only 1414 cm−1, but the excited 2 3Π state has a much larger value of 22 133 cm−1 by virtue of an avoided crossing with the lower state of this symmetry. The corresponding radial nonadiabatic coupling is responsible for a large cross section for an excitation process between the N+(3Pg)+He and N+(3Du)+He channels which indirectly provides an efficient electron-capture mechanism leading to the N(4Su)+He+ exit channel. Additional nonadiabatic matrix elements for rotational and spin–orbit coupling have also been obtained and analyzed, as well as transition moments between the various HeN+ molecular states calculated.
A new definition of atomic charges based on a variational principle for the electrostatic potential energy
Authors: Lee Tai-Sung , York Darrin M. , Yang Weitao
Published: July 2002

A unique definition of atomic charges in molecules is presented based on a variational principle involving the electrostatic potential energy. The method requires only the electron density as input, and does not rely on an arbitrary set of fitting points as do conventional electrostatic potential fitting procedures. The dipole moments and electrostatic potentials calculated from atomic charges obtained from this method agree well with those from self-consistent-field calculations. The new method also provides a spherical-atom potential model that may be useful in future generation molecular simulation force fields.
Activation barriers for series of exothermic homologous reactions. IV. Comparison of measurements to theory for reactions of s2p1 atoms with N2O
Authors: Belyung David P. , Futerko Peter M. , Fontijn Arthur
Published: July 2002

The kinetics of the Al+N2O→AlO+N2 reaction has been studied in a high-temperature fast-flow reactor (HTFFR). The expression k(520–1030 K)=4.6×10−12 (T/K)0.5 exp(−778 K/T) cm3 molecule−1 s−1 has been derived from a nonlinear regression analysis of the measured rate coefficients. 2σ precision limits are about ±5% and accuracy limits are estimated to be about ±23%. Combining the data with a 296 K literature measurement yields k(296–1030 K)=5.8×10−15 (T/K)1.37 exp(−90.4 K/T) cm3 molecule−1 s−1, with 2σ estimated accuracy limits of about ±27%. The semiempirical approach used previously to predict activation barriers for s1 and s2 metal atom reactions with N2O has been modified to allow predictions of group 13 atom reactions with N2O. The activation barriers for the B, Ga, In, and Tl atom reactions are calculated from the experimental activation barrier of the Al+N2O reaction.
Imaging of wave functions and potentials from time-resolved and frequency-resolved fluorescence data
Authors: Shapiro Moshe
Published: July 2002

Imaging of the amplitude and phase of time-evolving wave functions and excited-state potentials, using fluorescence data, is performed. The method relies on the use of both frequency-resolved and time-resolved fluorescence to reduce the problem of wave function imaging to a solvable set of linear algebraic equations. The potential inversion is performed by a new formula which expresses an excited state potential in terms of the ground state potential, the transition frequencies, and the transition-dipole matrix elements, whose sign is shown to be derivable from the spectral line strengths.
A highly connected random master equation
Authors: Zwanzig Robert
Published: July 2002

In a highly connected master equation, each state is connected to a substantial fraction of all other states. A special case, in which the connections are made at random, is investigated here by means of an effective medium approximation. The eigenvalue spectrum of the resulting effective medium agrees well with the spectrum of the original master equation.
Overtone Raman spectrum and molecular polarizability surface of CO2
Authors: Tejeda G. , Maté B. , Montero S.
Published: July 2002

The Q branches of the Raman bands associated to the 2ν1:4ν02:ν1+2ν02 Fermi resonance of 12C 16O2 have been observed in the gas phase at 2543, 2671, and 2797 cm−1 and, in addition, at 2514 cm−1, one hot band from their first excited vibrational state is seen. From the analysis of their cross sections, jointly with those of the main Fermi diad, ν1:2ν02 and its hot bands, an improved description of the molecular polarizability surface has been achieved. The following derivatives of the mean polarizability have been obtained: ∂ᾱ/∂q1=12.43×10−42 CV−1 m2; ∂2ᾱ/∂q22σ=2.81×10−42 CV−1 m2 (σ=a,b); ∂2ᾱ/∂q21=0.45×10−42 CV−1 m2; ∂2ᾱ/∂q23=0.67 or 0.15×10−42 CV−1 m2; ∂3ᾱ/∂q1∂q22σ=−0.06×10−42 CV−1 m2, in terms of the dimensionless normal coordinates, and ∂ᾱ/∂S1=3.15×10−30 CV−1 m; ∂2ᾱ/∂S22σ=0.36×10−20 CV−1; ∂2ᾱ/∂S21=2.9×10−20 CV−1; ∂2ᾱ/∂S23=2.0 or 0.5×10−20 CV−1; ∂3ᾱ/∂S1∂S22σ=−1.7×10−10 CV−1 m−1, in terms of the symmetry coordinates. The thermal dependence of the mean polarizability is discussed in terms of these quantities.
Three-dimensional quantum wave packet study of the Ar–HCl photodissociation: A comparison between time-dependent self-consistent-field and exact treatments
Authors: García-Vela A. , Gerber R. B.
Published: July 2002

The time-dependent self-consistent-field (TDSCF) approximation is used to study the photodissociation of the Ar–HCl cluster in a three-dimensional framework. The results are compared with numerically exact quantum calculations, and the properties and accuracy of the TDSCF approach are evaluated on this basis. The TDSCF approximation is used in Jacobi coordinates, and the total wave function is factorized into a wave packet for two coordinates associated with the H atom, and a wave packet for a single coordinate that describes the relative motion of the heavy particles. Quantitative agreement between the TDSCF and the exact results is found for most quantities calculated. The calculations show that photodissociation, and in particular the departure of the H atom is predominantly a direct process, but an appreciable amount of wave packet amplitude moving in excited state resonances is also found. This amplitude seems significantly larger than obtained in recent calculations by Schröder et al. [J. Chem. Phys. 100, 7239 (1994); Chem. Phys. Lett. 235, 316 (1995)]. The validity and computational efficiency of the TDSCF approach for realistic systems of this type is discussed.
Radiative and radiationless decay of resonances resulting from electronically nonadiabatic interactions: A computational approach valid for both narrow and broad linewidths and large energy shifts
Authors: Han Seungsuk , Yarkony David R.
Published: July 2002

The description of resonances originating from several coupled electronic states in a diabatic or approximate diabatic basis can offer both conceptual insights and computational challenges. In a three-state problem, two bound electronic states strongly coupled to a single dissociative continuum, large resonance energy shifts (thousands of cm−1), and linewidths varying over 4 orders of magnitude can be encountered. In this work a nonperturbative computational approach is developed to treat this class of resonances. Expressions for both the radiative and radiationless decay rates are developed. Although the approach is nonperturbative, the linewidth is expressed in a Golden-Rule-type formula. The resonance energy is obtained from the iterative solution of an eigenvalue problem in the bound state space. These attributes enable efficient determination both narrow and broad linewidths and large resonance energy shifts. The approach is used to characterize both radiative and radiationless decay of the 2,3 3Πg states of Al2 using a rigorous three-state diabatic basis. Lifetimes ranging from tenths of picoseconds to nanoseconds are determined. The corresponding resonance energy shifts are on the order of 4000 cm−1.
Vibrational Stark effect of CO on NaCl(100)
Authors: Lambert David K. , Poppe G. P. M. , Wijers C. M. J.
Published: July 2002

The effect of local electric field at the NaCl(100) surface on the vibrational spectrum of adsorbed (vs free) CO is calculated and compared with previous experiments. We consider the change in both the vibrational frequency ν and the integrated IR absorption S. The calculation is based on the known molecular properties of free CO and uses S at F=0 as calculated earlier by Poppe, Wijers, and van Silfhout [Phys. Rev. B 44, 7917 (1991)]. We show how to account for the position dependence of F along the molecular axis in calculating ν and S. This is important for the NaCl(100) surface since F varies strongly in the empty site where the CO adsorbs. The ratio of calculated to observed Δν in the limit of low coverage is 0.98±0.17; the ratio of calculated to observed S with monolayer CO coverage is 1.10±0.12. The calculated effects agree with the data for CO on NaCl(100). We also discuss how the vibrational Stark effect may apply to CO on MgO(100) and at cation sites in zeolites.
Vibrational and rotational energy distribution of ClO produced in reactions of O(1D) atoms with HCl, CCl4, and chlorofluoromethanes
Authors: Matsumi Yutaka , Shamsuddin Sayed Mohammed
Published: July 2002

Nascent state-resolved ClO(X 2Π) radicals produced in reactions of O(1D) with HCl, CCl4, CFCl3, CF2Cl2, and CF3Cl have been measured, using vacuum-ultraviolet laser-induced fluorescence of the ClO(C 2Σ−–X 2Π) transition. Rotational temperatures of ClO (X 2Π3/2, v=0 and 1) are 900–1000 K for chlorofluoromethanes and 2050±60 K (v=0) and 1770±180 K (v=1) for HCl. The rotational distribution for the O(1D)+HCl system is interpreted by the orbital-to-rotational angular momentum transfer mechanism. Vibrational level populations of the ClO(v=0–3) products are inverted for CCl4 and chlorofluoromethanes, but not for HCl. Vibrational excitations in the v=0–3 levels are higher for the reactions with the larger available energies.
Theoretical modeling of photodissociation dynamics of CH3I on MgO(001)
Authors: Setzler J. V. , Huang Z.-H. , Guo H.
Published: July 2002

The 257 nm photodissociation dynamics of CH3I adsorbed on a MgO(001) surface is studied using classical molecular dynamics method. The substrate is modeled by a 6×6×3 slab of movable ions surrounded by a semi-infinite array of static ions. A single adsorbate molecule is aligned with the surface normal, the methyl end pointed either toward or away from the substrate. The system is equilibrated by using a Monte Carlo method to obtain the starting configuration. Fragment final state distributions are calculated for kinetic energy, angle of departure, and rovibrational states. Upon photodissociation of the adsorbate with the methyl end pointed toward the surface, the methyl fragments experienced vibrational cooling, in agreement with experimental results. Some rotational excitation is predicted for fragments produced from the methyl down orientation. The kinetic energy distributions of both the methyl and iodine fragments are qualitatively similar to those obtained by experiment. The results are compared with those obtained by the same model for CH3I adsorbed on LiF(001). Trapping of iodine atoms by the surface has also been investigated in this simulation.
Two isomers of CuO2: The Cu(O2) complex and the copper dioxide
Authors: Wu Hongbin , Desai Sunil R. , Wang Lai-Sheng
Published: July 2002

Photoelectron spectroscopy of CuO−2 has been studied at three detachment wavelengths: 532 nm, 355 nm, and 266 nm. Vibrationally resolved spectra for two distinct isomers are observed: the Cu(O2) complex and the copper dioxide molecule. The Cu(O)2 complex has an electron affinity (EA) of 1.503 (10) eV with a ground state vibrational frequency of 530 (50) cm−1. Its first electronic excited state is 7400 (300) cm−1 above the ground state. The Cu(O2)− anion is also observed to undergo photodissociation to Cu−+O2 at both 532 nm and 355 nm detachment wavelengths. The copper dioxide molecule is found to have a high EA of 3.46 (4) eV. Three low-lying excited states are observed within 1 eV above the ground state. The first two excited states of the copper dioxide molecule both possess a totally symmetric vibrational frequency of 640 (60) cm−1.
Infrared laser transient absorption spectroscopy of the ethyl radical
Authors: Sears Trevor J. , Johnson Philip M. , Jin Pei , Oatis Susan
Published: July 2002

The observation and analysis of the high resolution spectrum of the CH2 rocking fundamental of the ethyl radical, C2H5, at wavelengths close to 18.9 μm is reported. The band origin is found to be at 528.1 cm−1. The spectrum shows evidence for a very low barrier to internal rotation, or torsion, in this species. A simple model Hamiltonian, based on an assumed structure with G12 symmetry, qualitatively reproduces the observations and implies a torsional barrier of approximately 20 cm−1 in both the zero point and excited vibrational states. The Hamiltonian cannot reproduce the observations to the level of the experimental accuracy and the importance of several neglected terms is tested and discussed. The observed torsional splittings imply that, within the confines of the model, the a-inertial and internal rotation axes in the molecule are coincident to within 1°. In addition to their intrinsic interest, the spectra will be useful for future state resolved studies of the kinetics of chemical reactions involving the radical.
Determination of the K–Ar interaction potential in the XΣ and AΠ state from laser spectroscopic data
Authors: Bokelmann Frank , Zimmermann Dieter
Published: July 2002

The absorption spectrum of the van der Waals molecule KAr has been recorded between 12 870 and 13 040 cm−1 by means of high-resolution laser excitation spectroscopy in a supersonic jet expansion. About 1600 molecular absorption lines could successfully be assigned to the rovibrational structure of the electronic transition A 2Π←X 2Σ+ of 39KAr. In addition, a few absorption lines due to the isotopomer 41KAr have been detected. Spectroscopic parameters have been obtained for vibrational levels 0...3 of X 2Σ+ and 6...11 of A 2Π. Additional experimental information was provided by observing the spectral distribution of the fluorescence. The interatomic potentials of the X 2Σ+ and the A 2Π states have been derived from the experimental data by means of a fully quantum-mechanical approach using suitable analytical functions. Our final results for equilibrium distance Re and well-depth De are 5.404(5) Å and 40.1(6) cm−1 for X 2Σ+, 3.37(3) Å and 405(15) cm−1 for A 2Π1/2, and 3.34(3) Å and 427(15) cm−1 for A 2Π3/2. In addition, we could deduce qualitative information on the B 2Σ+ interaction potential from our experimental data yielding Re=7.10 Å and De=23 cm−1 as preliminary values.
Characteristic features of the electrostatic potential for negative atoms within the work formalism
Authors: Sen K. D. , Muthunayaki R. S.
Published: July 2002

The characteristic topological features of the electrostatic potential for the negatively charged atoms defined by the Sen–Politzer theorem [J. Chem. Phys. 90, 4370 (1989)] calculated using the local exchange-only work potential derived from Coulomb’s law are found to be in good agreement with the corresponding Hartree–Fock results. The perspective of the work formalism attaches new significance to the topological features exclusively in terms of the Fermi–Coulomb hole density.
The effect of charged impurities on a glass transition in a polar medium
Authors: Dakhnovskii Yuri , Lubchenko Vassiliy
Published: July 2002

A phase transition from a liquid to glass state is studied for polar solvents in the presence of ions. The entropy and free energy of the system above and below the transition temperature, Tc, is found on the basis of the theory of random dipole–dipole and charge–dipole interactions . The dependence of the transition temperature on the concentration of charged impurities, c, is found. It is shown that a relative charge concentration of only 10−5 is needed to increase Tc by 20 K. The transition temperature is also dependent on the value of the charge. For a Fe+3/Fe+2 reactive complex the transition temperature is changed by 10 K for c=10−6. For dissolved salts we describe the relationship between Tc and the separation distance of the ion pair. The glass transition is insensitive to the size of dissolved ions. The theory proposed has only one adjustable parameter, the elementary dipole moment of the solvent. The theory allows to explain a peak dependence in Tc measured as a function of the alcohol content Y in Li+Cl−⋅5H2O⋅Y alcohol(methanol, ethanol, 1-propanol) mixtures.
On the relaxation of a two-level system: Beyond the weak-coupling approximation
Authors: Reichman David R. , Silbey Robert J.
Published: July 2002

The model of two nondegenerate quantum levels coupled linearly and off-diagonally to a bath of quantum mechanical harmonic oscillators studied previously by Laird, Budimir, and Skinner is re-examined. Interpretations are made for both the fourth order population relaxation and dephasing processes. Some of the methods used are applied to the standard spin-boson problem. The question of experimental detection of predicted phenomena is discussed. An approximate method, based on a canonical transformation of the original Hamiltonian is proposed to study the problem.
Poling behavior of optical absorption spectra in carbazoles with one- and two-dimensional charge-transfer character
Authors: Isoshima Takashi , Wada Tatsuo , Zhang Ya-Dong , Brouyère Eddy , Brédas Jean-Luc , Sasabe Hiroyuki
Published: July 2002

Spectral change in optical absorption due to molecular alignment induced by electric poling has been studied experimentally and theoretically, in one- and two-dimensional charge-transfer carbazole molecules. After poling, an increase in absorbance at λmax was observed in 3,6-dinitrocarbazoles which present a two-dimensional charge-transfer character, while the absorbance at λmax decreased in 3-monosubstituted carbazoles which possess a one-dimensional charge-transfer character. From molecular-orbital calculations and evaluations of the molecular polarizability spectra for random and uniaxial orientations of the molecules, an explanation is provided for the poling behavior in terms of the vector directions of the transition and ground-state dipole moments of the molecules. Theoretical investigation of various conformers has been made, resulting in the suggestion that the poling behavior of absorption spectra should be strongly affected by the conformation of the acceptor groups in the case of disubstituted carbazoles.
Static and dynamic vibrational dephasing in a dense fluid
Authors: Schvaneveldt Stephen J. , Loring Roger F.
Published: July 2002

We present a theory of the statically broadened vibrational line shape of a molecule in liquid solution. In this limit of static broadening, the molecule vibrates in a static potential posed by fixed solvent molecules in a configuration chosen from the equilibrium distribution of fluid configurations. The line shape is calculated within the instantaneous normal mode approximation, in which the solute’s potential is approximated by a harmonic surface whose curvature agrees with that of the exact potential at the solute’s initial configuration. Within this approximation, the line shape is related to a configuration-averaged phonon Green’s function, which is calculated approximately with an analytical procedure. This theory represents a modification of our previous treatment of vibrational line shapes [J. Chem. Phys. 102, 2326 (1995)], in which the solvent dynamics were included. Comparison of the line shapes for static and dynamic solvents permits determination of the relative importance of static (inhomogeneous) and dynamic (homogeneous) contributions to line broadening. We carry out such comparisons for a harmonic diatomic in a Lennard-Jones solvent over a wide range of temperature and density.
Reactions of O, H, and Cl atoms with highly vibrationally excited HCN: Using product states to determine mechanisms
Authors: Pfeiffer Joann M. , Metz Ricardo B. , Thoemke John D. , Woods Ephraim , Crim F. Fleming
Published: July 2002

Oxygen, hydrogen, and chlorine atoms react with vibrationally excited HCN to produce CN and OH, H2, or HCl, respectively. The experiments presented here use direct vibrational overtone excitation to prepare states of HCN having four quanta of C–H stretching excitation [(004) state] or three quanta of C≡N stretching and two quanta of C–H stretching excitation [(302) state] and laser-induced fluorescence to determine the rotational and vibrational states of the CN product. We find that the reaction of HCN with O produces CN having little vibrational and rotational energy, with 85% of the CN in v=0, 12% in v=1, and 3% in v=2. The CN from the reaction of H with HCN is slightly more energetic, with 77% in v=0, 17% in v=1, and 6% in v=2. By contrast, the reaction of Cl with HCN produces CN with a considerable amount of excitation, about 30% is in v=1 and at least 10% is in v=2, depending on the initial vibrational state of the HCN reactant. The enhanced excitation of the CN product of the reaction with Cl reflects the contribution of a different mechanism. We conclude that the O-atom reaction forms CN exclusively by a direct abstraction reaction, the H-atom reaction produces CN primarily by direct reaction at the collisional energies of our experiment, and the Cl-atom reaction forms CN by the dissociation of an intermediate complex in addition to the direct abstraction reaction.
Neutron diffraction from shear ordered colloidal dispersions
Authors: Dux Christian , Versmold Heiner , Reus Valerie , Zemb Thomas , Lindner Peter
Published: July 2002

Recently, it has been pointed out that the structure of shear ordered colloidal dispersions can adequately be accounted for by the intensity distribution I(l) along Bragg rods [Phys. Rev. Lett. 75, 763 (1995)]. Information concerning packing of the shear induced layers and on their stacking order is contained in I(l). In this paper I(l) is determined by measuring the small angle neutron scattering distribution as a function of the sample orientation. For the investigated charge stabilized system with particle diameter σ=143 nm and a distance of nearest neighbors in the layers a=237 nm, a structure close to random close-packed hexagonal layers is found. As compared with close-packed systems the distance between the layers c is elongated so that c≊a. Further, it is shown that the application of shear leads to a more uniform distribution of I(l) along the rods, which demonstrates that the loss of correlation between the hexagonal layers under sheared conditions can also be determined by small angle neutron scattering.
Reaction-path dynamics in curvilinear internal coordinates including torsions
Authors: Nguyen Kiet A. , Jackels Charles F. , Truhlar Donald G.
Published: July 2002

We describe a procedure for calculating generalized normal mode vibrational frequencies along a reaction path in curvilinear coordinates for a polyatomic system involving torsions. We apply the method to calculate generalized normal mode frequencies and rate constants for the OH+H2→H2O+H reaction using variational transition state theory with a multidimensional semiclassical tunneling approximation for the transmission coefficient. The results are compared to rate calculations carried out in rectilinear Cartesian coordinates to test the sensitivity of the calculated thermal rate constants to the choice of coordinate.
Molecular dynamics study of solvation effects on acid dissociation in aprotic media
Authors: Laria Daniel , Kapral Raymond , Estrin Darío , Ciccotti Giovanni
Published: July 2002

Acid ionization in aprotic media is studied using molecular dynamics techniques. In particular, models for HCl ionization in acetonitrile and dimethylsulfoxide are investigated. The proton is treated quantum mechanically using Feynman path integral methods and the remaining molecules are treated classically. Quantum effects are shown to be essential for the proper treatment of the ionization. The potential of mean force is computed as a function of the ion pair separation and the local solvent structure is examined. The computed dissociation constants in both solvents differ by several orders of magnitude which are in reasonable agreement with experimental results. Solvent separated ion pairs are found to exist in dimethylsulfoxide but not in acetonitrile. Dissociation mechanisms in small clusters are also investigated. Solvent separated ion pairs persist even in aggregates composed of rather few molecules, for instance, as few as 30 molecules. For smaller clusters or for large ion pair separations cluster finite-size effects come into play in a significant fashion.
Infrared spectrum and theoretical study of the dinitrogen pentoxide molecule (N2O5) in solid argon
Authors: Bencivenni L. , Sanna N. , Schriver-Mazzuoli L. , Schriver A.
Published: July 2002

Fourier Transform Infrared (FTIR) spectra of the covalent dinitrogen pentoxide molecule isolated in a solid argon matrix at 10 K are reported. The measured frequencies in the 200–4000 cm−1 region are in close agreement with gas phase values previously reported. An accurate and sophisticated ab initio study showed that the stable equilibrium structure, C2 symmetry, was in agreement with the structure previously determined by electron diffraction data.
Density functional study of strong hydrogen-bonded systems: The hydrogen diformiate complex
Authors: Süle Péter , Nagy Á.
Published: July 2002

The structure and various properties of HCOO−.HOOCH (hydrogen diformate) anionic hydrogen-bonded system are studied. Single, double as well as triple hydrogen-bonded structures are obtained by geometry optimization up to the level of MP2/6-311++G(d,p) and by various density functional approaches. Optimized structures and calculated complexation energies are compared with experimental observation. At least 78% covalent character is found in the strong O–H–O-type hydrogen-bond. Multiple H-bonded structures provide calculated binding energies which are in the closest agreement with experiment. The experimental complexation energy is reproduced within almost chemical accuracy in the range of 35–37 kcal/mol, while the experimental observation is 36.8 kcal/mol. The secondary interaction like C–H.O contacts represent at least 3–4 kcal/mol energy lowering. The O–O bond distance is found in the range of 2.41–2.45 Å by various density functional methods, while the experimental value is 2.45 Å. The estimated counterpoise correction to basis set superposition error is also presented in the binding energies. The performance of density functional methods based on Gaussian and Slater-type basis is compared using the G92-DFT and ADF codes, respectively. For comparison the energetics of hydrogen diacetate is given as well.