bluuebunny/crossref_metadata_2025 huggingface dataset:
Full configuration-interaction benchmark calculations for AlH
Authors: Bauschlicher Charles W. , Langhoff Stephen R.
Published: July 2002

The X 1Σ+, A 1Π, and C 1Σ+ states of AlH are studied using a [7s 5p 2d/4s 3p] basis set and incorporating valence correlation using the complete active space self-consistent-field (CASSCF)/second-order configuration-interaction (SOCI) and full configuration-interaction (FCI) methods. A principal focus of this work is to provide FCI calibration calculations for approximate methods of describing the energies, dipole moments, and transition moments when there is a variable degree of valence–Rydberg mixing. The state-averaged CASSCF/SOCI method is found to be capable of providing an accurate description of these states when the CASSCF active space is sufficiently flexible to describe both the important correlation effects and the Rydberg character. Having found a CASSCF/SOCI procedure that nearly reproduces the FCI results in a [7s 5p 2d/4s 3p] basis set, we take this procedure to near the basis set limit. The calculated dissociation energies (De in eV with experimental values in parentheses) are 3.16 (3.16) for the X 1Σ+ state, 0.24 (0.24) for the A 1Π state, and 0.80 (0.76) for the inner minimum of the C 1Σ+ state. Our calculated radiative lifetimes for the v=0, N=5 and v=1, N=7 levels of the A 1Π state are 64.3 and 96.6 ns, in quite good agreement with a previous CASSCF study, and with the experimental values of 66±4 and 83±6 ns. The radiative lifetimes for the v=0, N=0 and v=1, N=0 levels of the inner minimum of the C 1Σ+ state are 5.5 and 4.1 ns, respectively.
Quadrupole polarizabilities and hyperpolarizabilities of Kr and Xe from fourth-order many-body perturbation theory calculations
Authors: Maroulis George , Thakkar Ajit J.
Published: July 2002

Fourth-order Moeller–Plesset perturbation theory calculations on Kr and Xe atoms surrounded by strategically placed point charges are performed to predict the dipole (α1) and quadrupole (α2) polarizabilities, and the dipole–dipole–quadrupole (B) hyperpolarizabilities of Kr and Xe. A semiempirical value is thought to provide the best estimate for α1(Kr). Our recommended values for the other quantities are α1(Xe)=(27.7±0.3) e2 a20 E−1h, α2(Kr)=(99±2) e2 a40 E−1h, α2(Xe)=(210±6) e2 a40 E−1h, B(Kr)=−(343±10) e3 a40 E−2h, B(Xe)=−(812±24) e3 a40 E−2h.
Multiphoton optical and photoelectron spectroscopy of 4s–3d and 5s–4d Rydberg complexes of O2
Authors: Park Haiyoon , Miller Paul J. , Chupka William A. , Colson Steven D.
Published: July 2002

The 4s–3d and 5s–4d Rydberg complexes of O2 have been observed by (2+1) resonance-enhanced multiphoton ionization spectroscopy. All states show sharp rotational structure. Vibrational analysis (and rotational analyses of two bands) shows that all observed states are purely Rydberg in character with no detectable perturbations. Photoelectron spectra of the more intense peaks show the Δv=0 transition to be dominant, but large contributions from Δv≠0 transitions are observed and attributed to features in the ionization continuum. The Δv≠0 transitions are relatively weak for bands ascribed to excitation of a 3dδ Πg state and provide a method for producing nearly pure vibrational preparations (v+=0–3) of O+2 ions in the ground electronic state. Large amounts of O+ ions are observed and possible mechanisms of formation are discussed.
Vibrational hyperpolarizability of SF6
Authors: Shelton D. P. , Ulivi Lorenzo
Published: July 2002

Expressions are derived for the vibrational second hyperpolarizability γv of SF6, in the static limit and for the nonlinear-optical processes of the dc Kerr effect, electric-field-induced second harmonic generation (ESHG), and third harmonic generation (THG). These expressions give γv in terms of vibrational transition dipole, polarizability, and first hyperpolarizability matrix elements (μ,α,β). The matrix elements needed in the numerical evaluation of γv are obtained from the literature and from the Raman scattering measurements reported here. In the static limit it is found that γv≈20 γe, where γe is the electronic contribution to γ. At optical frequencies γv is still comparable to γe in magnitude, and it shows significant frequency dependence.
Rotational analysis of the Herman infrared bands of nitrogen
Authors: Huber K. P. , Vervloet M.
Published: July 2002

The Herman infrared bands of nitrogen have been recorded in rotationally cold emission from a supersonic jet discharge. The rotational analysis confirms their assignment to the transition C″ 5Πu→A′ 5Σ+g. Rotational constants have been derived for the lower state, as well as term values for the lowest rotational levels of the upper state.
On the electronic structure of the He+H2 system: Characterization of, and nonadiabatic interactions between, the 1 1A′ and 2 1A′ potential energy surfaces
Authors: Perry Jason K. , Yarkony David R.
Published: July 2002

The 1, 2 1A′ potential energy surfaces (PESs) of the He–H2 system, surfaces which correlate asymptotically with He(1S)+H2(X 1Σ+g, B 1Σ+u) system states, are characterized using MCSCF/CI wave functions. The existence of charge transfer structures of the form (HeH)+–H− on the two PESs is considered as are the electronic structure aspects of the nonadiabatic quenching process He+H2(B 1Σ+u )→He+H2(X 1Σ+g). While this work builds on previously reported theoretical treatments of these PESs, both qualitative and quantitative differences are found. In particular, our predicted entrance channel saddle point corresponds to a barrier of 1.5 kcal/mol on the 2 1A′ PES which is significantly lower than previous work. More significantly an extended region of large nonadiabatic effects characterized by the near degeneracy of the 1 1A′ and 2 1A′ PESs, E(2 1A′)−E(1 1A′)<0.5 kcal/mol, has been located. This region of the 2 1A′ PES, which is exothermic with respect to dissociation to He+H2(B 1Σ+u) and is characterized by general Cs, rather than C2v or C∞v geometries, was not uncovered in previous studies. Analyses based on the molecular dipole moment and the nonadiabatic coupling matrix elements 〈Ψ(2 1A′)‖(∂/∂Rα) Ψ(1 1A′)〉 are used to characterize this region.
Pressure broadening and line coupling in bending bands of CO2
Authors: Green Sheldon
Published: July 2002

The pressure broadening and line coupling cross sections in the Fano–Ben Reuven theory of line shapes are calculated for bending bands of CO2 in a bath of He atoms. Molecular collision dynamics are simplified by invoking the infinite order sudden (IOS) approximation for molecular rotational and vibrational angular momentum in a manner similar to but not identical with the method developed by Clary and shown to be accurate for CO2–He. Numerical values are obtained using a pairwise additive interaction potential developed by Clary. Predictions are in good accord with data for various infrared bands and pure rotational Raman spectra. It is found that all the pressure broadening and state-to-state cross sections depend on only a few dynamical factors (generalized IOS cross sections) and are therefore closely interrelated. Results are used to assess models developed previously to analyze line shapes in this and similar systems.
Classical, semiclassical, and quantum mechanics of a globally chaotic system: Integrability in the adiabatic approximation
Authors: Martens Craig C. , Waterland Robert L. , Reinhardt William P.
Published: July 2002

We examine the classical, semiclassical, and quantum mechanics of the Hamiltonian H= 1/2 (p2x+p2y+x2y2). The dynamics of this system are globally chaotic. However, the classical and quantum mechanical problems can be solved analytically by assuming an adiabatic separation of the x and y motion. We construct the canonical transformation to adiabatic action–angle variables and investigate the connection between this integrable approximation and the exact dynamics. In addition, we present a simple semiclassical formula that predicts energy levels in excellent agreement with the exact energy spectrum. The quantum adiabatic potential curves of this system have a very unusual structure—infinitely many curves cross at one point.
Stimulated emission pumping spectroscopy of jet-cooled C3
Authors: Rohlfing Eric A. , Goldsmith J. E. M.
Published: July 2002

We report a dispersed fluorescence spectrum obtained for excitation of a ∑+u–∑+g vibronic band of C3 at 33 588 cm−1, part of a newly discovered electronic system. Rotationally resolved stimulated-emission-pumping spectra of jet-cooled C3 using this ∑+u intermediate state are presented for dumping to the 0v121 (1≤v2≤13) and 6v121 (1≤v2≤5) levels in the 1Σ+g ground state. Vibrational term energies, rotational constants, and l-type doubling parameters are determined for each level.
Isotope effect in the photodissociation of HDO at 157.5 nm
Authors: Shafer Neil , Satyapal Sunita , Bersohn Richard
Published: July 2002

Experimental values for the absorption coefficient and the branching ratio of HDO at 157.5 nm are reported. The experimental evidence strongly supports the analysis of Zhang et al. (ref.10).(AIP)
Theoretical study of memory kernel and velocity correlation function for condensed phase isomerization. I. Memory kernel
Authors: Zhu S.-B. , Lee J. , Robinson G. W. , Lin S. H.
Published: July 2002

A unified memory kernel function is proposed on the basis of molecular dynamics simulations and comparison with experimental data. A frozen-solvent-like friction arises from the near-neighbor solvent cage effect, while the interaction between the system and the disordered heat bath or ‘‘broken cage’’ gives rise to a white-noise friction. On short time scales, the apparent friction is a ‘‘series addition’’ of these two limits, each solute molecule experiencing either an intact cage or a broken cage environment. For longer time scale behavior, the solvent is able to evolve, and some system molecules can experience both types of environments during the course of their own dynamics: the effective friction for these molecules then appears as a ‘‘parallel addition’’ of the same two contributions. This leads to a memory kernel having a simple exponential behavior. As the time scale for the system dynamics becomes still longer, the cage becomes indistinguishable from the heat bath, and a purely Markoffian relaxation with delta-function memory kernel ensues. This unified memory kernel model has been found to be applicable to ultrafast chemical reaction rates and diffusion properties both in molecular dynamics simulations and laboratory experiments.
Experimental determination of the Einstein coefficients for the N2(B–A) transition
Authors: Piper Lawrence G. , Holtzclaw Karl W. , Green B. David , Blumberg William A. M.
Published: July 2002

We have used a branching-ratio technique to measure the relative variation in the transition-dipole moment with internuclear separation for the N2(B–A) transition. Our spectral observations cover the range from 500 to 1800 nm, and use several different detectors and excitation sources. The data from different sets are consistent in the regions of spectral overlap. Using well established values for the radiative lifetimes of N2(B,v′≥5) allows the relative dipole-moment function to be placed on an absolute basis. From the dipole-moment function and a set of RKR-based Franck–Condon factors which we have computed, we derive Einstein coefficients covering the range v′=0–12 and v″=0–20. Our results indicate that currently accepted lifetimes for N2(B,v′=0–2) should be revised upwards by 20% to 40%.
Effect of carbon and hydrogen isotopic substitutions on the thermal diffusion of benzene
Authors: Rutherford W. M.
Published: July 2002

Measurements of the thermal diffusion factor of the benzene/carbon 13 substituted benzene pair and of the benzene/deuterated benzene pair are reported. The results show some interesting effects of mass distribution. (AIP)
Collision induced transitions between the A 2Πi(v=0) and X 2Σ+(v=10) states of CO+
Authors: Dentamaro Anthony V. , Katayama Daniel H.
Published: July 2002

We report on a laser double resonance study of collisional electronic transfer between the A 2∏i(v=0) and X 2Σ+(v=10) states of CO+. In particular, we have selectively populated individual rotational levels of the A 2∏i(v=0) manifold of CO+ with a pump laser and probed the adjacent X 2Σ+(v=10) manifold with a second pulsed laser. The only other molecules studied by this technique, N+2 and CN, are isoelectronic to CO+ and have efficient collisional transfer between their analogous A and X states even though perturbations are nonexistent or negligible. The pair of vibronic levels selected for CO+ has a zero energy gap and a much smaller Franck–Condon factor (FCF) than those studied for the A and X states of N+2 and CN. Previous works have indicated that the FCF is not important in these electronic transfer processes. We find, however, that unlike N+2 and CN, rapid collisional transfer between unperturbed rotational levels of the A 2∏i(v=0) and X 2∑+(v=10) electronic states of CO+ is not observed. Efficient electronic transfer occurred only through the perturbed levels of these two manifolds. In addition, we find that of the two spin components that make up the A 2∏i(v=0) rotational manifold, the propensity for collisional e↔f rotational transfer is more favorable for the 2∏3/2 member in agreement with theoretical considerations.
Two-photon spectroscopy of the 5p56p gerade states of Xe2
Authors: Lipson R. H. , Hoy A. R. , Chan E.
Published: July 2002

Two-photon resonant, three-photon ionization spectra of jet cooled Xe2, in the region of the Xe* 6p states between 70 000 and 80 000 cm−1, are reported. A new progression, near Xe* 6p[3/2]2, has been analyzed which is both vibrationally and isotopically resolved. Unambiguous assignment of upper state vibrational quantum numbers has resulted in precise molecular constants for the excited state. The transition has been assigned to 1g←0+g using ab initio potential curves. Implications concerning the often used ΔΩ=0 ‘‘propensity rule’’ are discussed.
Pressure dependence of zero-field splittings in organic triplets. I. Aromatics
Authors: Chan I. Y. , Qian X. Q.
Published: July 2002

Optically detected magnetic resonance experiments have been conducted at pressure up to 45 kbar for p-dichlorobenzene (DCB), s-tetrachlorobenzene (TCB) and quinoxaline (Q). The pressure dependence of their zero-field splitting parameters and, for DCB, the quadrupolar splitting are reported. For all three systems the D value decreases moderately with increasing pressure. The slope for TCB is approximately twice that for DCB. These results are interpreted in terms of pressure dependence of the mesomeric effect and the inductive effect of the substituents on the aromatic triplet state. For Q, the influence of pressure on spin-orbit interaction is also suggested.
Discussion of a ‘‘coherent artifact’’ in four-wave mixing experiments
Authors: Ferwerda Hedzer A. , Terpstra Jacob , Wiersma Douwe A.
Published: July 2002

In this paper, we discuss the nonlinear optical effects that arise when stochastic light waves, with different correlation times, interfere in an absorbing medium. It is shown that four-wave mixing signals are generated in several directions that spectrally track the incoming light fields. This effect is particularly relevant to transient hole-burning experiments, where one of these signals could easily be misinterpreted as a genuine hole-burning feature.
The production and characterization by resonance enhanced multiphoton ionization of H2(v=10–14) from photodissociation of H2S
Authors: Steadman Jhobe , Baer Tomas
Published: July 2002

The excitation of H2S at 66 000 cm−1 by two-photon absorption is shown to produce significant quantities of H2 molecules in very high vibrational, but low rotational levels. The vibrationally excited H2 has been identified by a 2+1+1 resonance enhanced multiphoton ionization (REMPI) process in whch the Ẽ,F̃ state is excited as an intermediate state. The major contribution of this double well state comes from the outer F̃ well because of the large H2 bond distance in the nascent H2(v) photoproduct. Excitation of the Ẽ,F̃ state by a 291 nm photon dissociates the molecule to the H+H*(n=3) products, and the excited atom is then ionized by a fourth photon. The analysis of the photoelectron spectra (PES) demonstrates that the intermediate H2S* molecule preferentially dissociates to vibrationally excited H2 molecules rather than ionizing by the absorption of an additional photon. It appears likely that the photodissociation of H2S is a practical method for producing highly vibrationally excited, but rotationally cool, H2 molecules for further dynamical studies.
Second virial coefficients for O2 and N2 mixtures with rare gases: A computational assessment
Authors: Gianturco F. A. , Venanzi M.
Published: July 2002

Second virial coefficients as function of temperature are computed for the title molecular systems interacting with He, Ne, and Ar. The relevant anisotropic forces are obtained via accurate potential functions tested earlier through the analysis of several, different properties of the various systems. The relevant quantum corrections are also computed, in addition to the classical results, and their effects analyzed vis à vis the available experimental data. The influence of such corrections on the very low-T behavior of the virial coefficients and on the determination of the Boyle temperatures is also shown and discussed. All examined potential functions are found to yield B(T) values in rather good accord with experiments, in spite of their marked differences in anisotropic behavior and in the shape of their potential well regions.
Theoretical line shapes for rotational spectra of HCl in Ar
Authors: Green Sheldon
Published: July 2002

An interaction potential for HCl–Ar recently derived from spectra of van der Waals complexes [J. M. Hutson, J. Chem. Phys. 89, 4550 (1988)] has been used to obtain converged close-coupling scattering S matrices from which pressure broadening (linewidth, line shift, and line coupling) cross sections have been computed within the Fano–Ben-Reuven formalism. Linewidths agree well with experimental data with the exception of the 0–1 line at low temperatures (an error of about 15% at 125 K). Line shifts, especially the largest, for the 0–1 line, are also in reasonable accord with experimental data. Line coupling is predicted to be quite small. The coupled-states molecular scattering approximation is found to be accurate except at very low collision energies. The infinite-order sudden approximations, on the other hand, is not reliable for this system. Inelastic scattering out of the spectroscopic levels accounts for only a fraction of the linewidth at thermal energies, especially in the lower rotational levels.
Effect of uniaxial stress on NaF:Cu+ electronic transitions
Authors: Weaver S. C. , McClure D. S.
Published: July 2002

We have performed uniaxial stress measurements on the 1Eg,1T2g, and 3Eg excited states of NaF:Cu+. Stress was applied along the [001] axis producing a tetragonal and a hydrostatic strain. From the tetragonal splitting, we have determined the Jahn–Teller energy in the 1Eg state to be 320 cm−1 thus supporting the recent reassignment of the states of Eg symmetry in this system [J. M. Berg and D. S. McClure, J. Chem. Phys. 90, 3915 (1989)].
Local and normal mode intramolecular vibrational relaxation in benzene
Authors: Gomez Llorente Jose M. , Hahn Ohyeon , Taylor Howard S.
Published: July 2002

This article addresses the importance of the structure of chaos in the phase space of planar benzene, especially around the local CH stretching mode. The structure imposes severe constraints on the ability of the classical mechanics to simulate the quantum mechanical flow of the energy out of the local mode, i.e., to simulate intramolecular vibrational relaxation (IVR). The phase space structure is inferred by computing ensemble averaged classical correlation functions and spectral densities. It is found that the region of phase space within a hyperradius of order h1/2 (which is the region corresponding to a quantum state) about the local mode is fairly well decoupled from the rest of the phase space and changes sharply from highly structured and quasiregular (although unstable) local mode character to chaotic normal mode character away from the CH bond. On one hand, the experimentally prepared quantum (packet) system must behave smoothly within the scale of h seeing only the dominant local mode character of this region. On the other hand, because some of the trajectories used to simulate the quantum flow from the local mode region are blocked and redirected back into the region, and the remainder (the majority of them) are outside the region and do not feel it, standard studies of the flow of ensembles of trajectories designed to mimic packet flow cannot be used to compute the local mode IVR rates of benzene. Instead, the scale of the phase space local mode structures, its isolated nature, and the constancy of the stability parameters everywhere in the local mode region point to the use of the stability parameters of chaotic trajectories as a measure of the IVR linewidth. These trajectories are run at the one-dimensional, local mode quantized energy, with no zero point energy in the other modes. This measure estimates the width at 10 000 cm−1 to be ∼20 cm−1, compared to the experimental width of 10 cm−1 for the local mode quantum number n=3, which is near this energy.
Modeling collisional energy transfer in highly excited molecules
Authors: Lim Kieran F. , Gilbert Robert G.
Published: July 2002

Data from classical trajectory simulations of the collision of a highly excited molecule with a monatomic bath gas are used to test the validity of the precepts used in the biased-random-walk (BRW) model for collisional energy transfer. This model assumes that energy migration during the collision is pseudorandom except for the constraint of microscopic reversibility, and leads to a simple displaced Gaussian form for the energy-transfer probability distribution. The BRW assumptions are shown to be of acceptable validity to exact classical trajectory simulations. A simple analytical approximation to the mean-square energy transfer per collision is obtained which reproduces the trajectory data to within an average of ±20%, and also gives acceptable accord with experimental data. The model shows that the magnitude of the average energy transferred per collision is governed by the time taken to traverse the overall interaction potential in and out from the appropriate collision diameter, by the internal energy, and by the average force exerted at the classical turning point of individual reactant-atom–bath-gas interactions.
Analysis of vibronic mode coupling in pentacene by fully resonant coherent four-wave mixing
Authors: Carlson Roger J. , Nguyen Dinh C. , Wright John C.
Published: July 2002

This paper develops the theoretical framework for the analysis of relative peak intensities in fully resonant, three laser, coherent four-wave mixing spectra of molecular vibrational and vibronic levels. For a Franck–Condon system, the relative vibronic peak intensities are shown to scale as the square of the absorption spectrum for all resonances not associated with the normal mode selected by the fixed vibrational resonance. A large enhancement of vibronic resonances involving the selected mode is predicted for typical potential well offsets. Higher order mode coupling causes deviations from these predictions by allowing other coupled resonances to be enhanced as well. Vibrational peak intensities are similarly related to the emission spectrum. Pentacene in benzoic acid mixed crystals at 2 K are used to experimentally study the mode mixing effects. It is shown that important coupling occurs between the 747.7 and 790.8 cm−1 vibronic modes of the pentacene S1 electronic state. Two possible mechanisms for this coupling are suggested. This work shows that the relative intensities of vibrational and vibronic resonances in fully resonant four-wave mixing spectra can serve as a useful probe of mode coupling.
Identification of the 3dπ 1Δg and 3Δ2g Rydberg state of O2 by multiphoton ionization and polarization studies
Authors: Park Haiyoon , Li Leping , Chupka William A.
Published: July 2002

The v′=0–4 levels of the 3dπ 1Δg state and v′=0–3 levels of the 3dπ 3Δ2g state of O2 have been identified and studied by multiphoton ionization and polarization techniques. The observed levels exhibit no detectable perturbation except for rather large Λ doubling probably due to L uncoupling, and heterogeneous predissociation probably by a Πg valence state. The observation of large amounts of O+ for the higher vibrational levels suggests large non-Franck–Condon effects in the ionization process.
Spin–orbit effects in the decomposition reaction N3H(X̃1A′)→N2(X1Σ+g)+NH(X3σ−, a1Δ)
Authors: Yarkony David R.
Published: July 2002

Using a flexible basis of better than double zeta-polarization quality and configuration interaction (CI) expansions of approximately 200 000 terms the electronic structure aspects of the spin-forbidden decomposition reaction N3H(X̃1A′)→NH(X3Σ−)+N2(1Σ+g) were studied. The spin–orbit interaction (Hso) was treated within the Breit–Pauli approximation including both the microscopic spin–orbit and spin–other–orbit contributions. Matrix elements of Hso between the lowest singlet state Ψ1a′(1A′)≡Ψ[1 1A′(0)] and the components of the lowest triplet state Ψ1a′(3A″) ≡iΨ[1 3A″(0)],  Ψ2a′(3A″) ≡i{Ψ[1 3A″(1)] −Ψ[1 3A″(−1)]}/(2)1/2 were determined in the asymptotic region corresponding to N2+NH, at the (experimental) equilibrium geometry of N3H(X̃ 1A′) and in the vicinity of the (approximate) minimum energy singlet–triplet crossing. At the approximate minimum energy crossing we find hsoz ≡〈Ψ1a′(1A′)‖Hso‖ Ψ1a′(3A″)〉 ≊39 cm−1 ≫hsoy ≡〈Ψ1a′(1A′)‖Hso‖ Ψ2a′(3A″)〉 ≊0.45 cm−1. The matrix elements hsoz,hsoy are interpreted in terms of a single configuration model and are compared with analogous quantities in the isolated NH molecule. A qualitative discussion of the decomposition reaction using a Landau–Zener approach is given.
Production of excited neutral and ionic photofragments following core-level excitation in molecules
Authors: Rosenberg R. A. , Wen C.-R. , Tan K. , Chen J.-M.
Published: July 2002

We have performed the first experiments to examine the neutral fragmentation paths following direct core-level excitation in a molecule. Using monochromatized synchrotron radiation in the range 100–140 eV, we have monitored the dispersed UV/optical fluorescence resulting from excitation of a Si 2p electron in SiF4. The main features in the fluorescence spectrum have been identified as emission from the SiF+4 D state and from excited SiF, Si, F, and Si+. Features in the fluorescence excitation spectra are assigned to excitation of a Si 2p electron to unoccupied valence orbitals, Rydberg orbitals, and shape resonances. There is a large enhancement in the yield of excited-state fragments following core-to-Rydberg excitation, which is due to the greater probability of the core-excited Rydberg state decaying, via a resonant Auger process, to highly excited, unbound states of SiF+4.
The prediction of spectroscopic properties from quartic correlated force fields: HCCF, HFCO, SiH+3
Authors: Green William H. , Jayatilaka Dylan , Willetts Andrew , Amos Roger D. , Handy Nicholas C.
Published: July 2002

Knowledge of a force field expanded through quartic displacements, together with a dipole field expanded through cubic displacements, yields all the harmonic and anharmonic molecular properties of interest to infrared spectroscopists. Such force fields may also explain much of the mechanism behind intramolecular vibrational energy redistribution. The ab initio quantum chemist can now calculate these fields, either at the self-consistent field level or with the inclusion of electron correlation effects. For accurate predictions, it is important to include electron correlations effects for at least the quadratic part of the force fields. Here we report studies using the second-order Mo/ller–Plesset method for the full quartic fields. We examine the effects of using large basis sets. The quadratic force constants are calculated analytically; cubic and quartic constants are calculated using central differences of second derivatives in reduced normal coordinates. Three molecules are studied. HCCF, for which a large quantity of experimental data has been recently analyzed by Holland, Newnham, and Mills. The calculations are sufficiently accurate that errors in the experimental assignments became apparent. HFCO, where the theoretical anharmonic constants are helpful in understanding the highly excited vibrational states probed by Moore and co-workers. SiH+3, whose high resolution absorption spectra has just recently been detected by Davies and co-workers. The conclusions are that this straightforward way of calculating spectroscopic properties is an extremely valuable tool for the understanding of spectroscopy.
On the noncrossing rule in polyatomic systems: Determination of a seam of actual surface crossings relevant to the quenching of H2(B 1Σ+u) by helium
Authors: Riad Manaa M. , Yarkony David R.
Published: July 2002

A recently developed method for determining avoided surface crossings using analytic gradient techniques is used to locate an actual crossing seam for the 1 1A′ and 2 1A′ potential energy surfaces of the He–H2 system. This seam is not related to any high symmetry nuclear configurations. The computational procedure, which is based on the minimization of ΔEIJ(R)2≡[EI(R)−EJ(R)]2, &(R)]2, uses different density matrices to simplify the construction of the energy difference gradient, the most costly step in the procedure. The actual crossing seam, R(r), is specified by the ordered triple R(r)≡[R(r), γ(r),r] for which ΔEIJ(R)=0. It is exoergic with respect to the He–H2(B 1Σ+u) asymptote for r≊[2.60, 5.70]. Here r≡R(H2), R=R(He–H2) and γ is the He–H2 angle. This seam defines a region nuclear coordinate space near which helium can efficiently quench H2(B 1Σ+u).
Reliability of one-electron approaches in chemisorption cluster model studies: Role of core-polarization and core–valence correlation effects
Authors: Illas F. , Rubio J. , Ricart J. M. , Daudey J. P.
Published: July 2002

Interaction of atomic oxygen with a Cu5 (Ag5) cluster model simulating the fourfold chemisorption of oxygen on Cu(100) and Ag(100) has been studied at the self-consistent-field (SCF) and configuration interaction (CI) levels using one-electron pseudopotentials to describe the inner shells of the cluster metal atoms. Core-polarization effects are introduced by means of a second-order perturbation method. Results show the method to be reliable and that there is no need for any molecular adjustment of the pseudopotential. Nondynamical and dynamical valence correlation effects are found to be very important. The separability of valence correlation effects is analyzed by comparison with previous calculations explicitly including the d electrons of the cluster metal atoms. Core–valence correlation effects decrease the interaction energy calculated at the valence CI level and are shown to be very important not only from the quantitative point of view but also from the qualitative one because different electronic states are differently affected, leading to a change in the ordering of the electronic states with respect to that found at the CI valence calculation level.
High resolution laser spectroscopy of the B 1Π−X 1∑+ transition of 23Na39K, and the perturbation between the B 1Π and c 3∑+ states
Authors: Katô Hajime , Sakano Mina , Yoshie Naoki , Baba Masaaki , Ishikawa Kiyoshi
Published: July 2002

The Doppler-free laser polarization spectrum of the B1Π −X1∑+ transition of 23Na39K was measured, and the molecular constants of the B1Π state of v=0∼16 were determined. The perturbation between the B1Π(v=8) and the c3∑+(v=22) levels at small J were studied in detail. By comparing the observed line intensities of the B1Π(v=16, J=14) −X1∑+(v″=0∼50, J=13 and 15) transition with the calculated Franck–Condon factors, the dependence of the transition dipole moment on the internuclear distance was determined. Absolute vibrational numbering of the c3∑+ state was done by comparing the calculated Franck–Condon factors with the observed line intensities of the c3∑+ −a3∑+ transition.
Theoretical study for the excited states of MoO4−nS2−n (n=0∼4) and MoSe2−4
Authors: Nakatsuji Hiroshi , Saito Shinji
Published: July 2002

Ground and excited states of six molybdenum complexes, MoO4−nS2−n (n=0∼4) and MoSe2−4, are systematically studied by symmetry adapted cluster (SAC) and SAC-CI theories. In the ground states, the ionicity of MoO2−4 is much larger than those of MoS2−4 and MoSe2−4. The calculated electronic spectra compare well with the observed spectra. New systematic assignments of the spectra are given. Most of the peaks are assigned to the electron-transfer type transitions from ligands to the metal. Many assignments are different from the previous ones. The observed relationships in the electronic spectra of these complexes are examined and corrected. The solvation effect is argued briefly.
Muonium addition reactions in the gas phase: Quantum tunneling in Mu+C2H4 and Mu+C2D4
Authors: Garner David M. , Fleming Donald G. , Arseneau Donald J. , Senba Masayoshi , Reid Ivan D. , Mikula Randall J.
Published: July 2002

The reaction kinetics for the addition of the muonium (Mu=μ+e−) atom to C2H4 and C2D4 have been measured over the temperature range 150–500 K at (N2) moderator pressures near 1 atm. A factor of about 8 variation in moderator pressure was carried out for C2H4, with no significant change seen in the apparent rate constant kapp, which is therefore taken to be at the high pressure limit, yielding the bimolecular rate constant kMu for the addition step. This is also expected from the nature of the μSR technique employed, which, in favorable cases, gives kapp=kMu at any pressure. Comparisons with the H atom data of Lightfoot and Pilling, and Sugawara et al. and the D atom data of Sugawara et al. reveal large isotope effects. Only at the highest temperatures, near 500 K, is kMu/kH given by its classical value of 2.9, from the mean velocity dependence of the collision rate but at the lowest temperatures kMu/kH≳30/1 is seen, reflecting the pronounced tunneling of the much lighter Mu atom (mμ=1/9 mp). The present Mu results should provide accurate tests of reaction theories on currently available ab initio surfaces.
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o study of the three lowest states X 2∑+, 2Π1/2, 2Π3/2, and B 2∑+ of the HeNe+ ion: Potential energy curves, Λ doubling, and predissociation rates of the rotational levels in the 2Π1/2 (v=0) state
Authors: Gemein Bernhard , de Vivie Regina , Peyerimhoff Sigrid D.
Published: July 2002

Multireference configuration interaction calculations are performed for the X 2∑+, 2Π, and B 2∑+ states in HeNe+ from 2.2 to 100 bohr. The various perturbations of the Born–Oppenheimer states are evaluated: the spin–orbit interaction is calculated up to second order by employing the Breit–Pauli Hamiltonian. The resulting potential curves for the A 2Π1/2 state are compared with the experimental data of Dabrowski and Herzberg (Ref. 1), the curve of the hereto undetected A 2Π3/2 is predicted. The Λ splitting of the A1 2Π3/2 and the A2 2Π1/2 states due to coupling of the electronic motion with the rotation of the nuclear framework is computed by employing a basis of vibronic eigenfunctions. The calculated parameters of p0=−1.407 cm−1 for A2 2Π1/2 agree well with the corresponding data of p0=−1.417 or −1.427 cm−1 derived from measurements (Refs. 1 and 2). The predissociation rates of the rotational levels in the A2 2Π1/2 (v=0) state are also calculated on the basis of the computed ab initio data. All results are compared with the experimental data of Carrington (Ref. 2). The mechanism for the predissociation of the A 2Π1/2 state (in the absence of a curve crossing) is elucidated.
Reaction cross section and product ion time-of-flight measurements for collisions of N+ and N+2 with CO2 at suprathermal energies
Authors: Gardner James A. , Dressler Rainer A. , Salter Richard H. , Murad Edmond
Published: July 2002

Charge exchange and dissociative ionization cross sections are reported for N+–CO2 and N+2–CO2 collisions at center-of-mass energies between 2 and 40 eV. Product ion kinetic energies are measured using time-of-flight techniques. Energy resonance considerations based on CO2 ionization Franck–Condon factors predict charge exchange to be slow for both of these reaction pairs. The N+–CO2 charge exchange, however, exhibits a large cross section and proceeds about one order of magnitude faster than the N+2–CO2 charge exchange. The formation of CO+ in N+–CO2 collisions is reported, while no dissociative ionization products are observed in the N+2–CO2 system.
One-color photolysis–ionization study of HN3: The N2 fragment internal energy distribution and μ-v-J correlations
Authors: Chu Jan-Jon , Marcus Peter , Dagdigian Paul J.
Published: July 2002

A one-color photolysis–ionization study of the photodissociation of hydrazoic acid near 283 nm has been carried out. The N2 and NH(a 1Δ) products have both been detected in a state-specific manner by resonance-enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The N2 fragments were observed in the v=0 vibrational mainfold and were found to be highly rotationally excited, with an average rotational excitation of 0.79 eV. The μ-J correlation parameter 𝒜(2)0 for N2 was determined to be positive and approximately equal to 0.5, indicating that the angular momentum vector J tends to be parallel with the transition dipole μ. Relatively little rotational energy was found in the NH product. From the observed polarization dependence of the NH REMPI mass peak profiles, we conclude that the recoil anisotropy parameter β is approximately −0.5 for low-J values and becomes positive by J=10. The observed width of these mass peaks provides information of the fragment recoil energy, and we estimate from the NH mass peaks that approximately 1.4±0.4 eV of the available energy appears as product relative translational energy. These experimental results are interpreted in terms of our understanding of the lowest excited singlet electronic state of this molecule.
Nonlinear dynamics of methyl rotation and intramolecular energy diffusion in p-fluorotoluene
Authors: Martens Craig C. , Reinhardt William P.
Published: July 2002

This paper examines the effect of large amplitude internal rotation on the rate and extent of intramolecular vibrational energy redistribution (IVR). We study a classical Hamiltonian modeling the vibrations of p-fluorotoluene in its first excited singlet (S1) electronic state. We find that the full many-dimensional vibrational phase space of this system can be approximately decomposed into two subsystems. The first consists of the methyl rotor and the lowest-frequency ring modes, which interest strongly and chaotically with the methyl rotor. Within this subsystem, energy is rapidly exchanged. The second subsystem consists of the remaining high-frequency modes, which do not strongly couple to the methyl rotor directly. The chaotic low-frequency ring–rotor dynamics generate an effectively random force on the remaining degrees of freedom. This intrinsically stochastic perturbation induces slower intramolecular energy diffusion and relaxation of nonequilibrium initial distributions in the higher-frequency ring modes.
Renormalization effects of bath-induced vibronic population transfer on stationary and time-resolved resonant light scattering spectra from molecules
Authors: Ohtsuki Y. , Hayashi M. , Fujimura Y.
Published: July 2002

Renormalization effects of the bath-induced vibronic population transfer on resonant light scattering (RLS) from molecules in condensed phases are theoretically studied based on the Markoffian master equation approach. By using the double space diagram technique, the self-energy originated from the bath-induced vibronic population transfer is analytically solved, and the analytic expressions for the intensities both of the stationary and of the time-resolved RLS spectra are derived. The renormalization effect is analyzed in terms of dimensionless molecular parameters, and model calculations are also performed to confirm theoretical results.
Absolute total cross sections for the charge transfer and dissociative charge transfer channels in the collisions of O+(4S )+H2
Authors: Flesch G. D. , Ng C. Y.
Published: July 2002

Absolute total cross sections for the reactions, O+(4S) + H2(X 1Σ+g)→O+H+2 [reaction (1)] and O+H+H+ [reaction (2)] have been measured in the center-of-mass collision energy (Ec.m.) range of 1.33–22.22 eV. The appearance energies for H+2 (1.70±0.10 eV) and H+ (4.50±0.10 eV) are in excellent agreement with the thermochemical thresholds for reactions (1) and (2), respectively. At Ec.m. higher than approximately 9 eV, the total cross sections for reactions (1) and (2) are greater than that for the exothermic channel forming OH++H.
Influence of monomer architecture on the shear properties of molecularly thin polymer melts
Authors: Homola A. M. , Nguyen H. V. , Hadziioannou G.
Published: July 2002

The effective viscosity and shear behavior of ultrathin films of two structurally different perfluoropolyether fluids have been investigated. The materials used were Fomblin Z, a linear random copolymer of fluoroethylene oxide and fluoromethylene oxide, and Fomblin Y, a branched random copolymer with fluoropropylene oxide and fluoromethylene oxide monomer units. The shearing experiments were conducted with the fluids confined between molecularly smooth surfaces at shear rates ranging from 200 to 4×103 s−1. It was found that when the thickness of the fluid films decreases from 10 to 2 nm, both perfluoropolyethers exhibited a sharp increase in viscosity, from bulk values to ‘‘surface viscosity’’ values that are many orders of magnitude larger. With increasing shear rates, the Z-type fluid showed a gradual decrease in the shear stress indicating an apparent ordering of the molecules due to the applied shear forces. On the other hand, with the Y-type copolymer, the shear stresses were significantly lower and were proportional to the shear rate resembling Newtonian fluids. The results are explained in terms of the differences in the molecular architecture of the fluids and suggest a close relation between the molecular structure of the polymer melt near a solid wall and its frictional and lubricating properties.
Liquid theory for band structure in a liquid. III. The mean spherical approximation for p bands and the numerical solution of the mean spherical approximation for both s and p bands
Authors: Chen Zhe , Stratt Richard M.
Published: July 2002

Formally, the problem of calculating the single-electron energy levels for a liquid requires that one diagonalize a 1023×1023 matrix, but previous work has made it clear that precisely the same information is available from the solution of a simple classical liquid problem. We extend our previous applications of this idea in several ways here: (1) the mean spherical approximation (MSA) for liquids is used to provide an explicit route to the density of states for a band resulting from a basis of p orbitals, (2) the previous MSA solution for s bands and the new MSA solution for p bands are both generalized to allow for nonorthogonality in the basis, and (3) numerical procedures are described for solving the integral equations resulting from these MSA theories. These developments mean that it is now computationally feasible to find the band structure of almost any simple liquid within a tight-binding model. We illustrate this point by computing the s and p bands expected from a hard-sphere liquid with a minimal basis of hydrogenic orbitals on each atom.
The H+para-H2 reaction: Influence of dynamical resonances on H2 (v′=1, j ′=1 and 3) integral cross sections
Authors: Kliner Dahv A. V. , Adelman David E. , Zare Richard N.
Published: July 2002

We have measured integral rate constants for the reaction H+para-H2→H2(v′=1, j′=1 and 3)+H at 11 center-of-mass collision energies (Erel) between 0.88 and 1.01 eV, a region in which dynamical scattering resonances are present. We have also measured the H2(v′ = 1, j′ = 3)/H2(v′ = 1, j′ = 1) population ratio at two additional values of Erel outside of this range. Tunable uv laser photolysis of HI was used to generate translationally hot H atoms of variable kinetic energy. Quantum-state-specific detection of the H2 reaction product was accomplished via (2+1) resonance-enhanced multiphoton ionization and time-of-flight mass spectrometry. The integral rate constants have a smooth dependence on Erel, in agreement with the recent quantum-mechanical (QM) calculations of Zhang and Miller and contrary to the experimental results of Nieh and Valentini. The QM results are in nearly perfect agreement with the present measurements for the dependence on Erel of both the integral rate constants and the H2(v′ = 1, j′ = 3)/H2(v′= 1, j′ = 1) population ratio. It is concluded that measurements of integral cross sections as a function of collision energy are not highly sensitive to the presence of dynamical resonances in this system.
The geometry of the ground state oxygen dimer ion (O2)+2 : T
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s-planar or rectangular?
Authors: Peel J. Barrie
Published: July 2002

A b initio calculations on the O+4 system, at the UMP3/6-31G* level of approximation show that the electronic ground state of O+4 favors a rectangular geometry (D2h), over a trans-planar geometry (C2h), with the latter more energetic by 0.54 eV (12.5 kcal mol−1). This result agrees with the conclusions of an ESR study on O+4, but is in disagreement with the conclusions of a complementary IR study.
Molecular dynamics of phenol at the liquid–vapor interface of water
Authors: Pohorille Andrew , Benjamin Ilan
Published: July 2002

Molecular dynamics results are presented for phenol at the water liquid–vapor interface at 300 K. The calculated excess free energy of phenol at the interface is −2.8±0.4 kcal/mol, in good agreement with the recent experimental results of Eisenthal and co-workers. The most probable orientation of the phenol molecule at the surface is such that the aromatic ring is perpendicular to the interface and the OH group is fully immersed in water. The hydroxyl substituent has a preferred orientation which is similar to the orientation of OH bonds of water at the pure water liquid–vapor interface. The transition between interfacial and bulk-like behavior of phenol is abrupt and occurs when the center of mass of the solute is located about 6 Å from the Gibbs surface of water. In this region the para carbon atom of the hydrophobic benzene ring can reach the interface and become partially dehydrated. This result suggests that the width of the interfacial region in which the behavior of a simple amphiphilic solute in water is influenced by the presence of the surface depends primarily on the size of its hydrophobic part. The role of the OH substituent was investigated by comparing phenol at the interface with two model systems: benzene with and without partial charges on carbon and hydrogen atoms. It is shown that in the absence of the hydrophilic substituent the solute is located further away from the liquid phase and is more likely to be oriented parallel to the interface. However, when the center of mass of the solute is moved into the interfacial region where the density of water approaches that of the bulk solvent, all three molecules become oriented perpendicularly to the surface. In this orientation the work of cavity formation needed to accommodate the hydrophobic ring in aqueous solvent is minimized.