bluuebunny/crossref_metadata_2025 huggingface dataset:
Canonical sampling of classical phase space: Application to molecular vibration-rotation dynamics
Authors: Fukui Kazuhiko , Cline Joseph I. , Frederick John H.
Published: July 2002

We analyze a recently introduced method for canonically sampling classical phase space using Nosé-Hoover chain constant temperature molecular dynamics. An advantage of this method is that a thermal ensemble can be generated without the use of a Monte–Carlo rejection procedure. The method is shown to give thermal distributions for two realistic molecular model systems, representing H2O and CF3NO, including rotational degrees of freedom. We demonstrate how the method can be extended to generate a “bithermal” sampling in which the rotational and vibrational degrees of freedom have different effective temperatures and how to adapt the results to include excitation of an initial thermal ensemble by a monochromatic light source. We also suggest a method for approximating the corresponding quantum distribution through a simple scaling of the classical distribution.
Laser-induced fluorescence spectroscopy of Ar2HF at vHF=3: An examination of three-body forces
Authors: Chuang Cheng-Chi , Tsang Susy N. , Hanson Jason G. , Klemperer William , Chang Huan-Cheng
Published: July 2002

The vibrational spectrum of Ar2HF in the 11 320–11 430 cm−1 region is recorded by intracavity laser-induced fluorescence. The intramolecular vibrational state, Σ0, in combination with the intermolecular vibrations, assigned as Πin-plane, Πout-of-plane and Σ1, of the complex have been observed. The Σ0 state correlates adiabatically with j=0 of HF (v=3); the Πin-plane, Πout-of-plane, and Σ1 states correlate adiabatically with j=1 of HF (v=3), respectively. We have determined the vibrational band origins (and rotational constants) of ν0=11 323.784 cm−1 (A=0.120 15, B=0.058 30, C=0.038 94 cm−1), ν0=11 387.730 cm−1 (A=0.122 68, B=0.057 05, C=0.038 42 cm−1), ν0=11 426.815 cm−1 (A=0.120 27, B=0.058 15, C=0.038 71 cm−1) and ν0=11 427.400 cm−1 (A=0.120 26, B=0.058 15, C=0.038 71 cm−1) for Σ0, Πin-plane, Πout-of-plane, and Σ1 states, respectively. The vibrational red shift for the pure HF stretch from vHF=0–3 is 49.023 cm−1. The in-plane and out-of-plane bending frequencies are 63.947 and 103.031 cm−1. The Σ1 state, which may be viewed as the Ar2FH structure is located 103.616 cm−1 above the Ar2HF Σ0 state. The spectral line shapes appear to be well fitted by a Doppler profile with FWHM≈120 MHz, indicating that the predissociation linewidths have a Lorentzian component of less than 10 MHz. These results are compared with those of Farrell and Nesbitt [J. Chem. Phys. 105, 9421 (1996)] for vHF=1. The present experimental data set is also compared with the quantitative predictions by Ernesti and Hutson [Phys. Rev. A 51, 239 (1995)] and therefore serves as a rigorous test for modeling nonadditivity of intermolecular interactions and their vibrational dependence. These comparisons show that the vibrational dependence of three-body terms is accurate in the region of potential minimum. For configurations far from the energy minimum, appreciable discrepancies appear to exist. The vibrational variation of the Πin-plane bending frequency is relatively poorly predicted, which strongly suggests the inadequacy in the present modeling of the intriguing nonadditive forces for this prototypical system.
Photodissociation of ozone in the Chappuis band. I. Electronic structure calculations
Authors: Woywod Clemens , Stengle Martin , Domcke Wolfgang , Flöthmann Heiner , Schinke Reinhard
Published: July 2002

Potential-energy surfaces of the 1 1A′, 1 1A′′, and 2 1A′′ states of ozone and corresponding transition-dipole-moment surfaces have been computed as a function of the two bond distances and the bond angle. The calculations are based on the complete-active-space self-consistent field (CASSCF) and multiconfigurational second-order perturbation theory (CASPT2) electronic-structure models. For the calculations of the A″1 surfaces, which exhibit a conical intersection, a diabatic representation has been constructed, employing a direct diabatization method implemented at the CASSCF level. The slow variation of the diabatic potentials and transition dipole moments with nuclear geometry allows us to perform the ab initio calculations on a widely spaced grid. The complete potential-energy and transition-dipole-moment surfaces are then efficiently obtained by interpolation. This procedure leads to very significant savings of computing time compared to the mapping of the rapidly varying potentials and derivative couplings in the conventional adiabatic representation. Diabatic potentials at the CASPT2 level have been obtained by applying the adiabatic-to-diabatic transformation constructed at the CASSCF level to the adiabatic CASPT2 potentials. The properties of the resulting adiabatic and diabatic A″1 potential-energy surfaces are discussed, with emphasis on the 1 1A′′–2 1A′′ conical intersection, which is of relevance for the photodissociation dynamics of ozone in the Chappuis band. The computation of the photoabsorption cross section and the comparison between theory and experiment are discussed in the accompanying paper.
Interelectronic moments of atoms in position and momentum spaces
Authors: Koga Toshikatsu , Matsuyama Hisashi
Published: July 2002

Two methods are presented for the calculation of interelectronic moments 〈un〉 in position space and 〈vn〉 in momentum space of atomic systems within the Hartree–Fock framework, where u and v are, respectively, the position and momentum distances of any pair of electrons. The methods are applied to the neutral atoms from He to Xe in their ground state, and the moments 〈un〉 with n=−2 to +4 and 〈vn〉 with n=−2 to +3 are reported based on the numerical Hartree–Fock calculations. Some trends of these interelectronic moments are also discussed as a function of atomic number.
State-to-state reactive scattering of F+H2 in supersonic jets: Nascent rovibrational HF(v,J) distributions via direct IR laser absorption
Authors: Chapman William B. , Blackmon Bradley W. , Nesbitt David J.
Published: July 2002

Pulsed discharge sources of supersonically cooled F radicals have been crossed with supersonically cooled H2 to study the F(2P3/2,2P1/2)+H2→HF(v,J)+H reaction under single collision conditions with a collision energy of 1.8(2) kcal/mol. The HF(v,J) product states are probed via direct absorption of a single mode, tunable near IR laser perpendicular to the plane of intersection of the two jet axes. The high spectral resolution (Δν≈0.0001 cm−1)permits the quantum state HF(v,J)distribution to be determined with complete resolution of final rovibrational levels. The J-dependent integral cross sections for HF(v=3,J) are compared with exact quantum scattering calculations by Castillo and Manolopoulous on the ground adiabatic potential energy surface of Stark and Werner. Agreement between theory and experiment is quite good for low J (<3) states. However, theory substantially underpredicts the experimental distributions for high J(⩾3) states near the energetic cutoff for ground spin orbit state F(2P3/2)atoms, which may indicate the presence of non-adiabatic reaction channels involving spin orbit excited F*(2P1/2) atoms.
Neutral and cation spectroscopy of fluorene–Arn clusters
Authors: Zhang Xu , Pitts Jonathan D. , Nadarajah Ravindrakumar , Knee J. L.
Published: July 2002

Fluorene–Arn complexes formed in a pulsed supersonic jet have been studied in their S1 state using two color REMPI spectroscopy with mass resolved detection. The appearance and shifts of the S1 origins relative to the fluorene monomer are measured for cluster sizes up to n = 30. The shifts and appearance of these bands are used to identify multiple conformations at low n and have indicated a shift from two sided clustering by Ar at low n to primarily one sided clustering at large n. The ionic ground state of the smaller clusters (n⩽6) are studied using mass analyzed threshold ionization (MATI) spectroscopy. The change of the ionization potentials as a function of cluster size has been determined. In the case of the fluorene–Ar4 cluster, the MATI spectrum of two separate cluster conformations was measured, revealing significantly different ionization potentials. Vibrational dynamics has been studied in several smaller clusters (n⩽3) by measuring MATI and ZEKE spectra when pumping vibronic transitions in the fluorene chromophore. Significantly enhanced coupling of the chromophore to van der Waals modes is observed in going from n = 1 to n = 3.
Quantum molecular dynamics of large systems beyond separable approximation: The configuration interaction classical separable potential method
Authors: Jungwirth Pavel , Fredj Erick , Gerber R. Benny
Published: July 2002

A nonseparable method for time-dependent quantum simulations of large polyatomic systems is presented and applied to the dynamics of the I2Ar17 cluster, following electronic excitation of the iodine molecule. The new method is an extension of the classical separable potential (CSP) approximation, in which the evolution of each mode is governed by a time-dependent mean potential due to the other modes and the total wave packet is a product of single mode wave functions. The computational effectiveness of the CSP approach stems from the use of classical molecular dynamics (MD) trajectories, carried out at the outset of the procedure, for obtaining the effective single-mode potentials. The present method generalizes the CSP scheme by a configuration interaction (CI) treatment, in which the total wave packet is represented as a linear combination of separable terms, with coefficients determined from the time-dependent Schrodinger equation. The single mode wave functions for each configuration are propagated along effective potentials that are generated using individual classical trajectories. The classical MD simulation is also used for simplifying the dynamical equations for the CI coefficients. Thus, the selection of correlations that are included quantum mechanically is guided by classical mechanics, which is the basis for the computational efficiency of this approach. The CI wave packet for the I2Ar17 system with 51 vibrational degrees of freedom was propagated for 500 fs following I2 (B←X) excitation. About 1500 configurations proved sufficient for convergence of the CI series. The separable approximation to the wave function holds for 60 fs and begins to break down upon the first collision of the iodine atoms with argons. After the second iodine-argon collision this breakdown is almost complete, and at t=500 fs the CSP term represents less than 5% of the correlated wave packet. Both absorption and resonance Raman spectra are, however, well described by the separable CSP method, since they are determined within the first 60 fs. The CI-CSP method offers very good accuracy due to inclusion of important correlation effects between different modes, while remaining computationally feasible for systems up to 100 degrees of freedom and more.
A pairwise and two many-body models for water: Influence of nonpairwise effects upon the stability and geometry of (H2O)n cyclic (n=3–6) and cagelike (n=6–20) clusters
Authors: Masella Michel , Flament Jean Pierre
Published: July 2002

Three flexible models (PW, CMP and TCPE) are presented to study interactions occurring in water clusters. The total interaction energy is decomposed into five terms; repulsive, charge–charge, intramolecular relaxation, polarization, and hydrogen bonding. The parameters of the first three terms are the same for all of the three models. The polarization term is taken as a classical molecular many-body polarization potential for the CMP and TCPE models, while nonpairwise effects are omitted in the PW model. As nonpairwise effects occurring in water clusters greater than the dimer have a topological origin, such effects are introduced in the hydrogen bonding term of the TCPE model. Parameters of the three models are derived from ab initio calculations at the MP2/6-311+G(2df,2p) level on three water dimer structures and the cyclic water trimer. Application of the three models to water clusters (H2O)n from n=3 to 20 exhibits that the nonpairwise effects in such clusters represent about 20% of their total binding energy and that the two nonpairwise effects considered suffice to describe most of the many-body effects. In particular, the many-body polarization term is mostly responsible for interoxygen distance contractions (relative to the dimer) in clusters greater than n=3. In cyclic clusters, this term is responsible for 40%–70% of the total nonpairwise effect energy, while in cagelike clusters, for about 50%.
Symmetry-adapted perturbation theory of nonadditive three-body interactions in van der Waals molecules. II. Application to the Ar2–HF interaction
Authors: Moszynski Robert , Wormer Paul E. S. , Heijmen Tino G. A. , van der Avoird Ad
Published: July 2002

Symmetry-adapted perturbation theory (SAPT) of three-body forces is applied to characterize the nonadditive interactions in the Ar2–HF trimer. The origins of the anisotropy of the nonadditive Ar2–HF potential are discussed, and the results are compared with the existing ab initio data. The multipole-expanded expressions for the induction, induction–dispersion, and dispersion nonadditivities in terms of the multipole moments and (hyper)polarizabilities are derived for the special case of atom–atom–diatom complexes, and the validity of the multipole approximation is investigated by comparison of the expanded and nonexpanded energies computed at the same level of the theory and in the same basis sets. Finally, recent (semi)empirical models of nonadditive interactions in Ar2–HF based on the exchange quadrupole electrostatic interaction are analyzed in terms of contributions as defined by SAPT. It is shown that the present level of the SAPT theory correctly accounts for the terms included in the exchange quadrupole electrostatic model.
The Cotton–Mouton effect of liquid water. Part II: The semi-continuum model
Authors: Ruud Kenneth , Ågren Hans , Dahle Pål , Helgaker Trygve , Rizzo Antonio , Coriani Sonia , Koch Henrik , Sylvester-Hvid Kristian O. , Mikkelsen Kurt V.
Published: July 2002

We present gauge-origin independent calculations of the Cotton–Mouton effect of liquid water. The liquid is represented by a semi-continuum model such that the central molecule is surrounded by its first solvation shell, which explicitly accounts for the strong interaction between the water molecule of interest and its closest neighbors. The long-range interactions with the solvent are modeled by a dielectric continuum surrounding the water molecule and the first solvation shell. We employ large basis sets, using London atomic orbitals in order to obtain gauge-origin independent results close to the Hartree–Fock limit. It is demonstrated that the direct interaction between neighboring molecules leads to a large effect on the calculated Cotton–Mouton constant, which undergoes a sign change from the gas to liquid phase, as observed previously for the linear electro-optical effect [K. V. Mikkelsen et al., J. Chem. Phys 102, 9362 (1995)]. Our best estimate for the molar Cotton–Mouton constant, −38.1⋅10−20 G−2 cm3 mol−1 (corrected for local field effects), is in reasonable agreement with the experimental value of −118(15)⋅10−20 G−2 cm3 mol−1. We expect that the remaining discrepancy is mainly due to an inadequate treatment of electron correlation.
An ab initio study of the dissociation of HNCO in the S1 electronic state
Authors: Stevens Jonathan E. , Cui Qiang , Morokuma Keiji
Published: July 2002

Regions of the S1 potential energy surface of HNCO relevant to N–H and C–N bond photodissociation have been investigated with ab initio calculations. Geometries of minima and transition states on S1 as well as those of the product photofragments and the HNCO ground state have been optimized with the CASSCF method, and their energies calculated with MRSDCI and CASPT2 methods. Deep planar trans and cis minima exist on the S1 surface, and are connected by transition states for isomerization. The S0→S1 electronic transition is brighter for trans configurations than for cis, and the initial excitation and dynamics are most likely to proceed through trans configurations. The N–H fission on S1 has a substantial barrier; it occurs more easily through the planar cis transition state, which is about 20 kcal/mol above the dissociation threshold, than through the trans transition state. The C–N fission on S1 can take place both via the planar trans and the planar cis transition state with a low barrier over the dissociation threshold; the reverse barrier is estimated to be a few kcal/mol.
Organic gas/surface interaction: Fraction (γ value) of the kinetic energy used for ionization
Authors: Kishi Hiroshi , Fujii Toshihiro
Published: July 2002

When hyperthermal beams of organic compounds strike a heated Re surface, their positive ions are scattered from that surface. Within the energy range studied (several eV), the efficiency of ion production is a function of incident kinetic energy (Ek) and surface temperature (Ts). This indicates that the ionization probability (I) correlates well with both Ek and 1/Ts. The conversion efficiency of translational to ionization energy in collisions of superthermal molecules with a surface was studied from the resulting ionization behavior. The γ value, which is defined as a fraction of the kinetic energy used for ionization, is estimated from two unique Ek and Ts correlations. The gases used in this study were acetone, benzene, naphthalene, and α-pinene. The γ value was molecule dependent. For example, at one extreme is the α-pinene molecule whose γ value when derived from the I vs Ek relationship is 0.13 [the I(Ts) dependence gives γ=0.10 under the condition that Ek=4.5 eV]. Thus the γ values estimated from the two independent methods agree fairly well.
Theoretical exploration of femtosecond multi-state nuclear dynamics of small clusters
Authors: Hartmann Michael , Pittner Jiřı́ , Bonačić-Koutecký Vlasta , Heidenreich Andreas , Jortner Joshua
Published: July 2002

We investigate ultrafast multi-state nuclear dynamics in a triatomic cluster. In particular, we explore how the intracluster nuclear dynamics of the Ag3−/Ag3/Ag3+ system is reflected in the femtosecond pump-probe negative ion-to neutral-to positive ion (NENEPO) signals. The nuclear dynamics is based on classical trajectories on the ground electronic adiabatic state potential hypersurfaces obtained from accurate ab initio quantum chemistry calculations. The nuclear dynamics of Ag3 initiated from the linear transition state involves distinct sequential processes of configurational relaxation to the triangular configuration, intracluster collisions, and the onset of IVR, resonant, and dissipative IVR, and vibrational equilibration. We determined the timescales for these processes and discussed their dependence on the initial cluster temperature. The Wigner representation of the density matrix was utilized to simulate the NENEPO-zero kinetic energy (NENEPO-ZEKE) signal and the total (integrated over the photoelectron energy) NENEPO signal. We show how geometrical change, completion of IVR and vibrational coherence effects can be identified in the NENEPO signals. A comparison of the calculated NENEPO signals with the available experimental data is presented.
Fractal kinetics and surface reactions
Authors: Moiny F. , Dumont Martine , Dagonnier R.
Published: July 2002

Simple models of bimolecular surface reactions occurring on a regular lattice are investigated by means of Monte-Carlo simulations. Fractal kinetics, observed in various steady-state regimes, appear to be due to the fractal nature of the adsorbate clusters generated by the microscopic mechanisms of the kinetic scheme. Two-site mechanism rates involve anomalous orders which are tentatively assigned to cluster properties defined in terms of 2d-percolation specifics.
Rate coefficients for the reaction and relaxation of vibrationally excited H2O(|04〉−) with H atoms and H2O
Authors: Hawthorne Giles , Sharkey Paul , Smith Ian W. M.
Published: July 2002

Rate coefficients are reported for the removal of H2O in its |04〉− vibrational state in collisions with H atoms and with H2O. Pulses of tunable infrared radiation at 719 nm excite H2O molecules in gas mixtures in which the concentrations of both H2O and H atoms are known. OH radicals formed in the reaction between H and H2O(|04〉−) are observed by laser-induced fluorescence and the time delay between pulses from the infrared “pump” and ultraviolet “probe” lasers is varied to provide kinetic data. Rate coefficients have been determined for (i) the self-relaxation of H2O(|04〉−) by other water molecules, k2=(2.2±0.2)×10−10 cm3 molecule−1 s−1, and (ii) the removal of H2O(|04〉−) by H atoms, k1=(4.95±0.6)×10−10 cm3 molecule−1 s−1.
Hydrogen bonding of water with aromatic hydrocarbons at high temperature and pressure
Authors: Furutaka Seiya , Ikawa Shun-ichi
Published: July 2002

Infrared spectra of HDO dissolved in benzene, toluene, ethylbenzene, o-xylene, m-xylene, and mesitylene at 373–473 K and 100 bar have been measured. It is found that the peak positions of the OH bands assigned to monomeric HDO shift to lower wave numbers as ionization energies of the aromatic compounds decrease. It is concluded that the interaction of water and aromatic compounds in the high temperature–pressure mixtures can be described as π-hydrogen bonding as for that in their complexes observed in the low temperature matrices and jet-cooled clusters.
An order–disorder phase transition in monolayer CO/LiF(001)
Authors: Vu N.-T. , Jack D. B.
Published: July 2002

Monte Carlo simulations of CO physisorbed on a LiF(001) surface show that a monolayer of CO molecules forms an ordered p(2√×√)R45 herringbone structure which undergoes an order–disorder phase transition around 30 K. The CO molecules sit near the Li+ sites (C atom down) with a tilt of ∼40° from the surface normal.
Discrete variational quantum reactive scattering method with optimal distorted waves. I. Theory
Authors: Groenenboom Gerrit C.
Published: July 2002

The size of the interaction region grid in a discrete Kohn variational reactive scattering calculation may be minimized by using distorted waves (DWs) in the trial wave function. Fully converged state-to-state results may be obtained with a small grid if (1) closed channels are included in the coupled channels expansion of the DWs and (2) asymptotically vanishing DWs are included in the trial wave function. This may be done without spoiling the sparsity of the interaction region Hamiltonian, which allows the use of an iterative method for solving the linear equations. We define boundary conditions for the regular, irregular, and asymptotically closed DWs, that minimize the number of DWs needed for convergence. The application to the reaction H+O2→ OH+O, is given in part II.
Using tethered triblock copolymers to mediate the interaction between substrates
Authors: Chern Shyh-Shi , Zhulina Ekaterina B. , Pickett Galen T. , Balazs Anna C.
Published: July 2002

Using scaling analysis and a self-consistent field (SCF) theory, we compress two copolymer-coated surfaces and isolate conditions that yield multiple, distinct minima in the interaction profile. We focus on planar surfaces that are coated with ABC triblock copolymers. Tethered to the surface by the last monomer in the C block, the copolymers are grafted at relatively low densities. The surrounding solution is a poor solvent for both the A and C blocks, and is a good solvent for the B blocks. Through scaling theory, we pinpoint the parameters that yield two minima in the interaction profile. The SCF calculations reveal the changes in the morphology of the polymers as the layers are compressed. Through both studies, we determine how the morphological changes give rise to the observed surface interactions. The results provide guidelines for creating polymer-coated colloidal systems that can form two stable crystal structures. Such systems could be used for bistable, optical switches. The findings also yield a prescription for creating systems that exhibit additional minima in the free energy of interaction.
One-electron resonances and computed cross sections in electron scattering from the benzene molecule
Authors: Gianturco F. A. , Lucchese Robert R.
Published: July 2002

One-electron resonances arising in electron scattering processes from a nonlinear polyatomic target, the benzene molecule, have been examined using various related methods. First, we have carried out calculations over a broad range of collision energies (from about 0.001 eV up to about 30 eV) by solving the scattering equations which use a parameter-free exact-static-exchangeplus-correlation-polarization potential to treat the electron–molecule interaction in all scattering symmetries. The entire range of features produced by the calculations was then related to specific structural properties of the scattering functions which give rise to the resonances. This analysis was done by using wave functions obtained with a local model potential approximation to the full potential used in the scattering calculations. These scattering functions were also related to the virtual orbitals obtained from a minimum basis set self-consistent field calculation. Additionally, for each resonant state found in this study we related the energy and width of the resonance to the partial wave components of the nearly adiabatic potential energy curve of the appropriate symmetry generated from the local model potential.
Interaction of a magnetic field with the a′3Σ+–A1Π complex in CO
Authors: Sykora Thomas , Vidal C. R.
Published: July 2002

A theory describing the effects of a DC magnetic field on a 3Σ+−1Π complex includes the influence on the energy positions of the 3Σ+ rovibrational levels, the change of their lifetimes as well as the strength of the 3Σ+←1Σ+ intercombination transitions. This is measured on the a′3Σ+(v=14)←X1Σ+(v=0) transition in CO where strong zero field mixing of the a′3Σ+(v=14) state with the A1Π(v=4) state occurs through spin-orbit coupling. a′←X excitation spectra are taken under fields of up to 1 Tesla. Lifetimes of various a′(v=14),N,Fi rotational levels are measured as a function of a magnetic field, where changes as large as 30% reveal a strong influence of the field on the mixing with the A1Π(v=4) state. A change in the singlet character of the a′3Σ+(v=14),N,Fi rovibrational levels alters the line intensities of their forbidden a′←X transition which arises due to intensity borrowing from the allowed A←X transition. From the measured lifetime values at 1 Tesla the changes in line intensities as well as absolute oscillator strengths for several a′(v=14)←X(v=0) rotational lines are derived. An increase in population transfer from X1Σ+(v=0),J to a′3Σ+(v=14),N,Fi of up to 51% is deduced. Finally, the energy levels and the lifetimes of the a′(v=14),N,Fi rotational levels in the high field regime beyond 4 Tesla are discussed.
Modeling weak interaction elements affecting the structures and vibrational red-shifts of ArnHF clusters (n=1 to ∞)
Authors: Dykstra Clifford E.
Published: July 2002

It is shown that a concise model potential can well account for the ground vibrational state rotational constants of Ar1−4HF clusters, the HF vibrational frequency shift of Ar1−4HF clusters, and the vibrational frequency shift of HF in an argon matrix. The model potential explicitly incorporates direct intermolecular polarization, which is a significant contributor to the stabilities of the clusters. Direct polarization produces an Ar–HF interaction that is different for HF in its ground vibrational state and its first excited vibrational state. That difference in interaction accounts for a sizable share of the HF red-shift for Ar clusters of all sizes. The calculations include full treatment of vibrational dynamics via quantum Monte Carlo for the ArnHF clusters with n=1–12, and these show small effects of weak mode vibrations on the HF red-shift. Calculations of the fully optimized equilibrium structures for n=1–176 clusters plus calculations with optimized lattice structures of four, five, six, and seven solvation shells (through n>2000) were used for extrapolation to an infinite cluster limit value for the HF red-shift. The second solvation shell is found to be noticeably important in the matrix red-shift. Three-body dispersion in the model potential has a direct effect on the red-shifts in large clusters, but the indirect effect via the influence on Ar–Ar separation is small.
Vibrational autoionization and predissociation in high Rydberg states of nitric oxide
Authors: Pratt S. T.
Published: July 2002

New results on the competition between autoionization and predissociation in the high Rydberg states of nitric oxide are presented. These results provide insight into the earlier work of Park et al. [Phys. Rev. Lett. 76, 1591 (1996)] that shows evidence for substantial mixing between Rydberg series and ionization continua with even and odd orbital angular momenta (l). New data based on fluorescence-dip spectroscopy, detection of neutral predissociation products, and photoelectron spectroscopy suggest that the A′ 2Σ+ and I 2Σ+ dissociative valence states play an important role in this l mixing. Zeeman splittings observed in a magnetic bottle electron spectrometer also result in an improvement in the assignment of these high Rydberg states.
A mode-selective differential scattering study of the C2H2++methanol reaction: Influence of collision intermediates, collision times, and transition states
Authors: Qian Jun , Green Richard J. , Anderson Scott L.
Published: July 2002

We report the vibrational and collision energy dependence of cross sections and product branching in the reaction of C2H2+ with CD3OD, CD3OH, and CH3OD. We also report axial recoil velocity distributions, along with modeling. At low collision energies, reaction is mediated by a picosecond lifetime complex of the [C2H2:methanol]+ form. The bottleneck that controls overall reaction efficiency appears to be formation of the complex, and reactivity is influenced by collision energy and C2H2+ CC stretch excitation, but not by bending vibration. The most energetically favorable exit channel from the complex is isomerization to covalently bound C3H6O+ complexes, but this does not occur. Instead the [C2H2:methanol]+ decays by breakup to C2H2+CH4O+, C2H3+CH2OH+, and C2H+CH3OH2+ channels. Changes in the branching with available energy provide some insight into the nature of the transition states that control decay of the complex. As collision energy is raised above ∼1 eV, the reaction gradually becomes direct, i.e., the collision time drops to well below the rotational period of the collision complex (<∼0.5 ps). In this regime, the dominant charge transfer and hydride abstraction products mostly form in large impact parameter collisions. At high energies there is little dependence of either reaction efficiency or product branching on collision energy or reactant vibrational state, suggesting that both are probably controlled largely by collision geometry.
Internal rotation effects in the rotationally resolved S1(1Lb) ←S origin bands of 3-methylindole and 5-methylindole
Authors: Remmers Karen , Jalviste Erko , Mistrı́k Ivan , Berden Giel , Meerts W. Leo
Published: July 2002

The rotationally resolved ultraviolet (UV) excitation spectra of the S1(1Lb)←S0 origin bands of 3-methylindole and 5-methylindole have been measured and analyzed. As a result of an internal rotation of the methyl group, each spectrum consists of rotational lines of overlapping 0a1←0a1 and 0e←0e torsional transitions. Like indole, 3-methylindole and 5-methylindole undergo axis reorientation upon electronic excitation. The Hamiltonian used to describe all observed spectral features includes a pure rotational part, a pure torsional part, and terms describing the interaction between the internal rotation and the overall rotation. It also accounts for the axis reorientation effect. Values for the barrier heights of the methyl torsion, the angle of the methyl top axis with the inertial axes, and the rotational constants are obtained for both the S0 and the S1 state. From an analysis of the intensities of the rotational transitions, the direction of the transition moment and the axis reorientation angle are obtained. Due to quantum interference effects in the 5-methylindole spectrum the sign of these angles could be determined.
The vibrational relaxation of I2 (X 1Σg+) in mesitylene
Authors: Liu H. J. , Pullen Stuart H. , Walker Larry A. , Sension Roseanne J.
Published: July 2002

Transient absorption measurements between 400 nm and 570 nm are used to extract information on the vibrational relaxation of iodine in the complexing solvent mesitylene. The well characterized nature of the I2-arene complex makes it an excellent prototype for the study of relaxation processes in the presence of weak interactions. The data and analysis presented here demonstrate the rapid nonexponential vibrational relaxation of I2 in the interacting solvent mesitylene. The peak of the population distribution has dropped below n=10 by 11 ps and n=7 by 15.5 ps. The energy relaxation is characterized by a biexponential decay with time constants of 4.41±0.08 ps and 20.3±0.7 ps. Quantitative comparisons of relaxation in a variety solvents are made by using a simple time-delay to peak absorption characterization of the relaxation. The initial 4.4 ps decay in mesitylene is significantly faster than the time scales for relaxation in noninteracting hydrocarbon solvents. The difference in the relaxation rate cannot be attributed to a change in vibrational frequency as the vibrational frequency of I2 has only a small dependence on the solvent. It is suggested that the vibrational relaxation of I2 in mesitylene through the high-lying levels is better characterized as an “intramolecular” vibrational energy redistribution process than relaxation to a solvent bath. The ultrafast vibrational relaxation occurs via the anharmonic coupling of the I–I stretching coordinate and the I-MST stretching coordinate of an I2-MST complex.
Charge localization and fragmentation dynamics of ionized helium clusters
Authors: Ovchinnikov M. , Grigorenko B. L. , Janda K. C. , Apkarian V. A.
Published: July 2002

The dynamics of Hen+, n=3–13, clusters formed by electron impact ionization of the neutral is studied theoretically using mixed quantum/classical dynamics by both mean-field and surface hopping methods. Potential energy surfaces and nonadiabatic couplings among them are determined from a semiempirical, minimal basis DIM Hamiltonian. The dynamics of hole hopping, hole localization, and cluster fragmentation are described through trajectory data. He3+ clusters, with initial conditions given by the zero-energy quantum distribution of nuclear coordinates, dissociate through two-channels, He+He+He+ and He+He2+ with relative yields of 20% and 80%. The motif of hole localization on a pair of atoms, and subsequent dissociation of the initial pair with hole hop to a new pair is observed in trimers, and repeats in larger clusters. In the larger clusters, hole hopping among He2 pairs provides an additional, less important mechanism of charge migration. The coupled electronic-nuclear dynamics of triatomic units describes the mechanism of energy loss, by transfer of vibrational to translational energy. This leads to ejection of energetic neutral atoms as well as the ejection of He2+ prior to evaporative cooling of the cluster. He2+ is the exclusive charged unit produced in the fragmentation of He13+ clusters. In bulk He the same dynamics should lead to fast vibrational relaxation t<10 ps and formation of He3+ as the positive ion core.
Experimental and ab initio study of the infrared spectra of ionic species derived from SF6 and SF4 and trapped in solid neon
Authors: Lugez Catherine L. , Jacox Marilyn E. , King Rollin A. , Schaefer H. F.
Published: July 2002

When a Ne:SF6 mixture is subjected to Penning ionization and/or photoionization by neon atoms in their first excited states, between 16.6 eV and 16.85 eV, and the products are rapidly frozen at approximately 5 K, the infrared spectrum of the resulting deposit includes prominent absorptions of SF4 and SF5. Other prominent product absorptions are assigned to SF5+ and SF5−, and one absorption is tentatively assigned to SF6−. Similar experiments on a Ne:SF4 sample yield SF4+, SF3+, and SF4−, as well as some F2SO+ and F2⋯SO, derived from F2SO impurity. The spectroscopic identifications of the SFn cations and anions are aided by the results of ab initio calculations. Because the vertical electron detachment energy of SF5− is exceptionally high, this species is stable on unfiltered mercury-arc irradiation of the deposit.
Molecular-dynamics simulation of collisional energy transfer from vibrationally highly excited azulene in compressed CO2
Authors: Heidelbach C. , Fedchenia I. I. , Schwarzer D. , Schroeder J.
Published: July 2002

Results from nonequilibrium molecular-dynamics simulations of collisional energy transfer from vibrationally highly excited azulene in compressed CO2 are compared with experimental results from our laboratory obtained under comparable physical conditions. As observed in the experiment, the cooling rates show a purely monoexponential decay of the excess energy. The influence of the microscopic solvent shell structure on these processes is investigated using the full three-dimensional anisotropic CO2 structure around azulene obtained from the simulation. The analysis shows that local heating effects of any kind do not play a role in our model system. Predictions of the pressure dependence of the energy transfer rates by the isolated binary collision model are compared with results from the simulations using two different definitions of the collision frequency in dense fluids.
Dynamics of Rydberg states of nitric oxide probed by two-color resonant four-wave mixing spectroscopy
Authors: McCormack E. F. , Di Teodoro F. , Grochocinski J. M. , Pratt S. T.
Published: July 2002

Two-color resonant four-wave mixing (TC-RFWM) spectroscopy has been used to probe highly excited v=0 and v=1 Rydberg states of nitric oxide. Transitions to n=16–30, v=0, Rydberg states, and the 8p, 9p, 7f, 8f, 8s, and 9s, v=1 Rydberg states from the A 2Σ+, v′=0 and 1 states have been recorded. The decay rate of the 8p and 9p, v=1 states has been extracted from the observed line profiles by using a recently developed model for the excitation of quasibound resonances in TC-RFWM spectroscopy. Transitions from the A 2Σ+, v′=1 state to the X 2Π3/2, v″=10 state have also been observed, allowing an absolute calibration of the TC-RFWM signal intensity. This calibration is used to determine an excited-state absorption cross section for the 9p, v=1 Rydberg state.
Manipulation of nuclear spin Hamiltonians by rf-field modulations and its applications to observation of powder patterns under magic-angle spinning
Authors: Ishii Yoshitaka , Terao Takehiko
Published: July 2002

A theoretical approach to design frequency-, phase-, and/or amplitude-modulated radio frequency (rf) fields for realizing Hamiltonian manipulations in nuclear magnetic resonance (NMR) is presented. In this approach, the evolution operator for a spin system under a general modulated rf field is represented without any a priori assumptions about the modulation wave form, using a time-dependent Euler transformation in the spin space. Then, the average internal Hamiltonian for a general periodic modulation can be expressed using the Euler angles. By equating it to the target Hamiltonian into which an internal Hamiltonian is desired to be modified, the time-dependent Euler angles are determined. General solutions of necessary modulations are obtained from the Euler angles with an infinite number of freely adjustable parameters. The great degree of freedom allows us to further improve the performance of the Hamiltonian manipulation. As useful examples of this approach, two amplitude-modulated fields and one phase-modulated rf field for restoring C13 chemical-shift anisotropies or C–13N15 dipolar interactions under magic-angle spinning (MAS) were developed and experimentally demonstrated. In an experiment, the C13 chemical-shift powder pattern of the methylen carbon in glycine was successfully observed under MAS with a spinning speed of 10 kHz by applying an amplitude-modulated rf field. It was found that the rotor-synchronous π-pulse cycle developed by Tycko et al. [J. Magn. Reson. 85, 265 (1989)] for observing chemical-shift powder patterns under MAS does not satisfactorily work under the fast spinning speed because of the inevitably high duty ratio of π-pulses. The presented method has the advantage of applicability to high-speed spinning. Moreover, Hamiltonian manipulations by the rf modulations require no critical adjustments of the experimental parameters, and are insensitive to rf inhomogeneity.
Effective Hamiltonian for near-degenerate states in relativistic direct perturbation theory. II. H2+-like systems
Authors: Rutkowski A. , Schwarz W. H. E. , Kozłowski R. , Bȩczek J. , Franke R.
Published: July 2002

The recently developed effective Schrödinger-like Hamiltonian equation (EHA) in a model space of near-degenerate nonrelativistic two-component spinors is applied to the relativistic energy corrections at first, second, and third order of c−2 within the framework of relativistic direct perturbation theory (DPT). The dominant singular part of the total relativistic correction is already recovered by the lowest-order effective Hamiltonian in the spirit of degenerate perturbation theory, while the perturbative expansion needs to account for only the small remaining part. Numerical results for groups of excited potential curves of the one-electron H2+-like quasimolecule Sn299+ are presented and discussed. In general the most efficient approach is first-order EHA-DPT for the set of states, followed by single-state DPT of higher orders.
Adsorption of a hard-sphere fluid in a slitlike pore with molecularly rough walls
Authors: Bryk Pawel , Pizio Orest , Sokolowski Stefan
Published: July 2002

We have investigated adsorption of a hard-sphere fluid in a slitlike pore with composite walls that permit penetration of fluid particles into the wall body for a given depth. Each of the walls is prepared by quenching of a layer of an ideal gas on a hard wall substrate. Our interest is in the adsorption isotherms and in partitioning of the fluid between random microporous layers of a given thickness and an empty space between these layers. The inhomogeneous replica Ornstein–Zernike equations and the Percus–Yevick approximation in conjunction with the Born–Green–Yvon equation for the density profile are used to study the system. The theoretical predictions are compared with the grand canonical ensemble Monte Carlo simulations. The adsorption isotherms and in-plane correlation functions are evaluated and compared with the reference system. We have shown that the presence of random matrix layers deposited on the pore walls significantly influences the structure of the confined fluid. The adsorption isotherms reflect decreasing penetrability of the fluid into a pore with microporous walls. In-plane correlations for two particles located at a rough wall are stronger than the correlations at a hard wall.
Ab initio potential energy surface by modified Shepard interpolation: Application to the CH3+H2→CH4+H reaction
Authors: Takata Takeyuki , Taketsugu Tetsuya , Hirao Kimihiko , Gordon Mark S.
Published: July 2002

An ab initio potential energy surface for the six-atom reaction CH3+H2→CH4+H was constructed, within C3v symmetry, by a modified Shepard interpolation method proposed recently by Collins et al. Selection of data points for the description of the potential energy surface was performed using both the Collins method and the dynamic reaction path (DRP) method. Although the DRP method is computationally more expensive, additional data points can be determined by just one simulation. Analyses of distributions of the data points, reaction probability, and errors in energy and energy gradients determined by the two different methods suggest a slight advantage for the DRP sampling in comparison with the iterative sampling.
Adsorption of ortho and para H2 on NaCl(001)
Authors: Briquez S. , Picaud S. , Girardet C. , Hoang P. N. M. , Heidberg J. , Voßberg A.
Published: July 2002

Fourier transform infrared spectroscopy (FTIR) experiments together with interaction potential calculations are performed to interpret the adsorption–desorption properties of para and ortho H2 molecules physisorbed on the (001) faces of NaCl crystallites. Isotherms of the adsorption process show preferential adsorption of para-H2 from gaseous normal-H2 at surface temperature around 10 K. Isobars of the desorption process show preferential desorption of para-H2 from a mixed ortho-H2 and para-H2 adsorbate. A quantum description of the molecular motions corroborates preferential adsorption of p- and (M=±1) o-H2 on the cation sites, as proposed in previous classical studies. It is shown that the adsorption energy is smaller for the para than for the (M=±1) ortho species in agreement with experimental measurement of desorption energy, while (M=0) o-H2 is only slightly bound to the substrate. This indicates that the ionic surface can discriminate all the H2 species. The determination of the bar spectrum for a set of independently adsorbed H2 species can interpret most of the infrared signals assigned to adsorption on the terrace sites. More specially the experimental behavior, with temperature and coverage, of the frequencies and relative intensities of the pure vibrational peaks and of the translation–vibration combination bands is very well explained in terms of hindered external motions of the admolecule.
Dirac–Fock–Roothaan calculations using a relativistic reduced frozen-core approximation
Authors: Watanabe Yoshihiro , Matsuoka Osamu
Published: July 2002

The reduced frozen-core approximation (RFCA) that has been previously used for nonrelativistic calculations is extended to relativistic calculations. The RFCA adopts a new method for the orthogonalization of valence basis functions to core orbitals. Orthogonalization is performed using corelike basis functions consisting of fewer primitive basis functions than core orbitals. Dirac–Fock–Roothaan calculations on HI and ThO show that the relativistic RFCA can reduce computing time and closely reproduce the total and valence orbital energies and spectroscopic constants obtained by all-electron calculations.
A multipole acceptability criterion for electronic structure theory
Authors: Schwegler Eric , Challacombe Matt , Head-Gordon Martin
Published: July 2002

Accurate and computationally inexpensive estimates of multipole expansion errors are crucial to the success of several fast electronic structure methods. In this paper, a new nonempirical multipole acceptability criterion is described that is directly applicable to expansions of high order moments. Several model calculations typical of electronic structure theory are presented to demonstrate its performance. For cases involving small translation distances, accuracies are increased by up to five orders of magnitude over an empirical criterion. The new multipole acceptance criterion is on average within an order of magnitude of the exact expansion error. Use of the multipole acceptance criterion in hierarchical multipole based methods as well as in traditional electronic structure methods is discussed.
Weak-field optimal control of Na2 photodissociation
Authors: Paci Jeffrey , Shapiro Moshe , Brumer Paul
Published: July 2002

The weak-field version of optimal control theory is applied to the design of laser pulses to achieve control over product formation. Computations on the optimal control of the two-photon dissociation of Na2, yielding the products Na(3s)+Na(3p) and Na(3s)+Na(4s), are described. The optimization is performed by maximizing either the difference between, or the ratio of, the probabilities of producing a product in these two channels. The dependence of the quality of control on the potential surfaces involved is also investigated as is the strategy through which optimized control is achieved.