bluuebunny/crossref_metadata_2025 huggingface dataset:
Bulk properties from finite-cluster calculations. V. Pseudo-Wannier orbitals from molecular orbital calculations on finite clusters
Authors: Cioslowski J.
Published: July 2002

A unitary transformation of the canonical Hartree–Fock orbitals which yields pseudo-Wannier orbitals in periodic finite systems is introduced. These orbitals are localized within individual unit cells and therefore can serve as a suitable one-electron basis for calculating the electron correlation energy. They also provide a natural partitioning scheme which allows for studying the finite-size effects on the total energy and the first-order electronic properties. The similarity sum which is maximized by the above transformation can serve as a measure of deviation of the electronic distribution in the system under consideration from that of an infinite periodic system.
Free energy and stability of macromolecules studied by the double scanning simulation procedure
Authors: Meirovitch Hagai , Vásquez M. , Scheraga Harold A.
Published: July 2002

The double scanning method (DSM) is a computer simulation technique suggested recently by Meirovitch [J. Chem. Phys. 89, 2514 (1988)]. This method is a variant of the usual or ‘‘single’’ scanning method (SSM) of the same author, which was extended by us to polypeptides [Biopolymers 27, 1189 (1988); this paper is designated here as paper II]. The two methods are step-by-step construction procedures from which the entropy and the free energy can be estimated. The transition probabilities are obtained by scanning the so-called ‘‘future’’ chains, which are continuations of the chain in future steps up to a maximum of b steps. With the SSM, the process is carried out by exact enumeration of the future chains; this is time consuming, and therefore b is limited to small values. With the DSM, on the other hand, only a relatively small sample of the future chains is generated by applying an additional scanning procedure. This enables one to increase b at the expense of approximating the transition probabilities. Increasing of b, however, is important in order to treat medium- and long-range interactions more properly. In this paper (as in our paper II), we apply the DSM to a model of decaglycine without solvent, described by the potential energy function ECEPP at 100 and 300 K. Using the SSM with the maximal value, b=4, we found in paper II that, at 100 K, the α helix rather than the statistical coil is the most stable state. The present DSM simulation at T=100 K (based on b=5) is more efficient than the SSM, and a structure with significantly lower energy than that of the α helix is found. It is argued that b can be increased further to 7 at this temperature. At 300 K the DSM, like the SSM, shows that the statistical coil is the most stable state of decaglycine. However, the DSM is found to be less efficient than the SSM. It is argued, however, that the DSM is expected to be advantageous (even at 300 K) to simulate more complex polypeptides that are stable in small regions of phase space (such as the α-helical state). Finally, it should be pointed out that the present method can be employed to treat a wide range of macromolecular models, such as those for synthetic polymers and nucleic acids.
Vibrational relaxation of highly excited SiF4 and C6F5H by Ar
Authors: Beck Kenneth M. , Gordon Robert J.
Published: July 2002

The vibrational relaxation of SiF4 and C6F5H in an Ar bath was measured as a function of initial energy using the method of time-resolved optoacoustics. SiF4 was found to relax nonexponentially, with a rate constant which varied approximately as the vibrational energy raised to the 3/2 power. This behavior is similar to that observed earlier for SF6+Ar below the bottleneck. In contrast, C6F5H was observed to decay exponentially, as expected for a large molecule in the quasicontinuum. These two types of behavior are discussed in the context of previously proposed propensity rules for the vibrational relaxation of highly excited molecules.
Comment on ‘‘A perturbation–trajectory method for the study of gas–surface collision dynamics’’
Authors: Riley Merle E. , Coltrin Michael E. , Diestler Dennis J. , Jezercak Michael , Agrawal Paras M. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

The perturbation−trajectory method (PTM) of Jezercak, Agrawal, Thompso and Raff for application to gas−surface collision processes is commented upon. The authors contend that the PTM does not function as a true thermal reservoir or heat bath. (AIP)
Quadratic crystal field contributions to the ground S-state splitting of Gd3+
Authors: Siu G. G.
Published: July 2002

The linearity of contributions to the S-ground-state splitting (zero-field splitting, ZFS) of Gd3+ is confirmed by studies on the quadratic effects of the ZFS mechanisms, which are proved to be negligible in most cases. However, the quadratic crystal field (QCF), though small, may have marginal influence on the superposition model analyses of the experimental spin-Hamiltonian parameters. General expressions are thus obtained for QCF contributions to ZFS in ions with the open-shell f 7 configuration. These are used to calculate the quadratic contributions for Gd3+ in garnet host crystals. They show that these are small and yet should be allowed for when making superpositon model analyses of phenomenological parameters.
Simple model of coherent energy transfer by Fermi resonance
Authors: Clarke David L. , Collins Michael A.
Published: July 2002

A simple model is presented to describe a mechanism by which large quanta of vibrational energy may be transferred coherently through a hydrogen bonding network. Exact results and numerical simulations of both the quantum and classical dynamics are presented. The physical context of the model is briefly described and order of magnitude estimates of the energy transfer time scale are discussed.
Calculation of the potential of mean force using molecular dynamics with linear constraints: An application to a conformational transition in a solvated dipeptide
Authors: Elber Ron
Published: July 2002

An algorithm is proposed for the calculation of mean force potentials using molecular dynamics with linear constraints. The new algorithm is based on free energy perturbation and is especially useful when the analytical form of the reaction coordinate is not known (e.g., for a numerically evaluated steepest descent path). It employs only Cartesian coordinates which makes it easy to use for a variety of systems. In particular, it is useful when the reaction coordinate cannot be represented by a single local coordinate or by a combination of a small number of local (internal) coordinates. The linear constraints are solved explicitly and the computational effort required is similar to that of dynamics with no constraints. The potential of mean force for a conformational transition of a solvated valine dipeptide is calculated. In aqueous solution the barriers for the conformational transitions are reduced with respect to the barriers in vacuo, the helix configuration becomes stabilized.
Quantum corrections for rate constants of diabatic and adiabatic reactions in solutions
Authors: Warshel A. , Chu Z. T.
Published: July 2002

A method for the calculation of quantized rate constants of chemical reactions in condensed phases is presented. The method focuses on the evaluation of nuclear tunneling corrections for classical activation free energies of diabatic and adiabatic reactions. The diabatic problem is treated by the quasiharmonic dispersed polaron model, using both a second order quantum mechanical rate constant, which is exact for quasiharmonic systems, and a semiclassical approximation based on an analytical density matrix formulation. The adiabatic free energy functionals are obtained by using the corresponding diabatic system as a reference state. A path integral formulation is also used both as a guide for the derivation of the adiabatic correction and as an alternative method for problems of limited dimensionality. The close relationship between the free energy functionals of the present approach and those developed in our earlier studies of electron transfer and proton transfer reactions is pointed out. The method is illustrated by studying several simple one and two dimensional problems and by stimulating deuterium isotope effects in proton transfer reactions in solutions. The calculations of the isotope effects reproduce the corresponding experimental trend.
Br*+CO2 revisited: An interrogation of E–V energy transfer with time-resolved diode laser spectroscopy
Authors: Sedlacek Arthur J. , Weston Ralph E. , Flynn George W.
Published: July 2002

High resolution (0.0003 cm−1) time-resolved diode laser absorption spectroscopy has been used to reinvestigate the electronic–vibrational energy transfer from spin–orbit excited bromine, Br(2P1/2), to carbon dioxide. The experiments are carried out by generating Br* atoms with pulsed 193 nm laser photolysis of CF3Br and monitoring the subsequent energy transfer by following directly the temporal evolution of selected vibrational states of CO2 (1001, 0201, 0221 and 0001). By comparing the temporal profile of the 1001 state with that predicted by various kinetic models it has been established that quenching of Br* by CO2 occurs via the nearly resonant 1001 state. The E–V rate measured in the present study, (4.8±0.6)×105 Torr−1 s−1, agrees well with that reported previously using infrared fluorescence probes. It has also been determined that the efficiency of this energy transfer is 0.87±0.15; that is, on average, 87% of the spin–orbit excited energy of the Br* atoms will show up as CO2 vibrational energy. The rate constant for the vibrational relaxation of the 1001 state is found to be (4.1±0.5)×106 Torr−1 s−1, also in good agreement with previous low resolution fluorescence measurements.
Thioformaldehyde single rotational level photophysics: Longer than radiative lifetimes and reduced fluorescence yields in the isolated molecule
Authors: Dunlop James R. , Clouthier Dennis J.
Published: July 2002

Single rotational level fluorescence lifetimes and relative fluorescence quantum yields have been measured under collision-free conditions for a variety of rotational levels in the 41 vibrational state of S1 H2CS. About half of the rotational levels studied decay with a radiative lifetime of ≊170 μs. Other levels decay with lifetimes longer than the radiative lifetime and these states correlate with perturbations observed in the spectrum, as detected by Doppler-limited laser-induced fluorescence and sub-Doppler intermodulated fluorescence techniques. The levels coupled to T1 and most of the levels coupled to S0 have relative fluorescence yields near unity. A few levels in S1 show reduced quantum yields of fluorescence which persist at pressures as low as 5×10−7 Torr. A sequential coupling model is proposed to qualitatively account for the observed excited state dynamics of these levels.
Triple-resonance spectroscopy of the higher excited states of NO2 : Rovibronic interactions, autoionization, and ℓ-uncoupling in the (100) manifold
Authors: Campos Francis X. , Jiang Yanan , Grant Edward R.
Published: July 2002

Ionization-detected absorption spectra of the (100) vibrationally autoionizing states of NO2 have been recorded from double-resonantly prepared N′=1 and N′=7 rotational levels of the 3pσ 2Σ+u (100) Rydberg intermediate state. Photoselection associated with three-color triple-resonant absorption spectroscopy resolves single rotational lines in discrete electronic states that lie above the adiabatic ionization threshold at total energies as high as 78 800 cm−1. Most features observed can be assigned to sσ, dσ, and dπ series converging to the (100) vertical threshold. Identified transitions extend over an interval of principal quantum numbers ranging from 9 to more than 40. A fourth short progression of sharp transitions is assigned as a segment of a g series. The complete spectrum of (100) transitions is modulated in intensity by a sequence of dips that form a series of states, recognized as pσ and pπ, converging to the (110) threshold. Evidence is cited for a mirror-image effect in autoionizing rates, where less-dissociative symmetric-stretch excited states autoionize much more efficiently than above-threshold bending excited states. The spectra span a range from the Hund’s case-b limit to that of case-d coupling. Though extensively obscured by broadening and shifting due to coupling with the ionization continuum and the discrete states of the (110) manifold, resonances in the (100) series show rotational structure consistent with the spectrum of accessible transitions and the variation expected in rotational spacing as Rydberg angular momentum systematically uncouples from the molecular axis. Autoionizing resonances differ significantly in width and line shape, evidencing a systematic variation in the strength with which discrete states are coupled with underlying continua. Fano line shapes indicate a component of direct, virbrationally off-diagonal photoexcitation to the (000) continuum, which interferes with discrete state absorption.
Accurate solution of a highly asymmetric electrolyte: Molecular dynamics simulation and integral equation
Authors: Linse Per
Published: July 2002

An asymmetric electrolyte has been investigated by means of molecular dynamics simulations and integral equation techniques. The charge asymmetry of the electrolyte is 1:20 which corresponds to small micelles formed by ionic amphiphiles. In the simulations, the long ranged Coulombic force was replaced by a spherical Ewald truncated (SET) potential. The effect of the long ranged part was taken into account by applying the reference hypernetted chain (RHNC) perturbation method on the simulation data. The result of this procedure converges to the result of the Coulombic potential as the force gets more long ranged in the simulation. An alternative way to treat the long ranged interaction in simulations, the minimum image convention, breaks down for the present system. The almost exact thermodynamic and structural results for the Coulombic system are used to examine the validity of the hypernetted chain (HNC) approximation. Although the HNC equation gives accurate energy and pressure, it overestimates the micelle–ion and ion–ion correlation which results in a too weak effective micelle–micelle repulsion. Moreover, the HNC approximation results in a too high critical temperature and the present results suggest that mixed integral equations may be useful to obtain more reliable phase boundaries.
Seven isomers of protonated nitrosyl fluoride
Authors: Meredith Cynthia , Davy Randall D. , Schaefer Henry F.
Published: July 2002

A b initio molecular electronic structure methods have been used to examine protonation of the species nitrosyl fluoride (FNO) and nitrogen hypofluorite (FON). Protonated isomers are postulated for both FNO and FON, and each is investigated to determine whether a corresponding stable potential minimum exists. Equilibrium geometries have been optimized and harmonic vibrational frequencies obtained for each isomer at the SCF double zeta (DZ) and double zeta plus polarization (DZ+P) levels of theory. Molecular structures have also been optimized with the DZ+P single and double excitation configuration interaction method (CISD). Our findings suggest that, for both FNO and FON, protonation at the nitrogen is energetically favored over protonation at the oxygen. In addition, it is predicted that protonation at the fluorine atom of either FNO or FON results in a loosely bound complex of HF and NO+, a complex which is in fact the global minimum for FHNO+. Among the six conventionally bonded isomers (each incorporating three reasonably normal chemical bonds), the nitrogen- and oxygen-protonated nitrosyl fluoride species lie lowest energetically. An interesting structural prediction is that nitrogen protonation of FON significantly decreases the F–O bond distance.
Rotationally specific mode–to–mode vibrational energy transfer in D2CO/D2CO collisions. II. Kinetics and modeling
Authors: Bewick C. P. , Martins J. F. , Orr B. J.
Published: July 2002

Time-resolved infrared-ultraviolet double resonance (IRUVDR) spectroscopy is used to study the kinetics of collision-induced rovibrational energy transfer between the ν6 and ν4 modes of D2CO in the vapor phase. As in paper I [J. Chem. Phys. 93, 8634 (1990)] of the series, attention rests on the existence of V–V transfer channels which are rotationally specific with respect to both J and Ka. Infrared excitation by the 10R(32) CO2 -laser line prepares D2CO in two discrete rovibrational states, (J,Ka,Kc)=(11,4,7) and (7,2,6), of the v6=1 vibrational manifold. D2CO/D2CO collisions then disperse this selected population to various states of the (ν4,ν6) rovibrational manifold, through a combination of rotational energy transfer (RET) and ν6→ν4 transfer. This yields an extensive range of (J,Ka) -resolved IRUVDR kinetic curves, demonstrating the collision-induced evolution of rovibrational population and enabling that evolution to be modeled by means of a master-equation approach.

The features of the model of best fit are as follows: the dominant Ka -resolved channel of ν6→ν4 transfer is that with Ka=4→6; accompanying J-resolved ν6→ν4 transfer channels favor ΔJ=0, with state–to–state rate constants scaling as J3.4; additional (J,Ka) -resolved ν6→ν4 channels allow a spread of J- and Ka -changing V–V transfer. These features are consistent with the accepted mechanism of ν6→ν4 transfer in D2CO, involving enhancement by a combination of Coriolis coupling and rotor asymmetry perturbations. In addition to ν6→ν4 transfer, RET provides the predominant channels of collision-induced relaxation: J-changing RET is described by a conventional fitting law based on the energy gap ‖ΔE‖ for the state-selected molecule; Ka -changing RET favors even values of ΔKa and, contrary to previous expectations, is J selective with a propensity for ΔJ=0. The physical implications of these results are discussed.
Energy partitioning in the 266 nm dissociation of HN3 and relative transition probabilities in the NH (c 1Π↔a 1Δ) system
Authors: Nelson H. H. , McDonald J. R.
Published: July 2002

The emission spectra of NH(c 1Π) and laser induced fluorescence excitation spectra of the NH (a 1Δ) fragment produced from the 266 nm photolysis of HN3 have been measured and several new vibronic bands in the c 1Π↔a 1Δ system have been characterized. The dispersed emission measurements are used to determine the transition probabilities connecting the v=0, 1 levels of the (c 1Π) state and the v=0, 1, 2, and 3 levels of the (a 1Δ) state and, ultimately, the nascent vibrational distribution in the NH product. Measurement of the collision-free Doppler broadened line shapes of single rotational features allows the NH translational energy to be determined in several (a 1Δ) vibronic levels. These studies complete the characterization of the distribution of excess dissociation energy among all the internal and external degrees of freedom of the fragments in this dissociation.
State-to-state reactive scattering amplitudes from single-arrangement propagation with absorbing potentials
Authors: Neuhauser Daniel
Published: July 2002

A new paradigm is presented for calculation of reactive state-to-state transition amplitudes. The wave function is propagated in one arrangement (either reagents or the sought products, the choice being at one’s convenience); other arrangements are blocked with an absorbing potential. Reactive information is then obtained from the integral expression for the T matrix (〈ψ‖H−H0‖Ψ〉). The approach is exemplified on a collinear system, yielding accurate transition probabilities that are insensitive to the parameters of the absorbing potential. Expressions for the complete T matrix in the new reactive IOS are then derived, based solely on an IOS assumption in one of the arrangements, without a need to invoke matching procedures between different arrangements.
Iodoethane photolysis: Which C–H bond leads to H-atom formation?
Authors: Brum Jeffrey L. , Deshmukh Subhash , Koplitz Brent
Published: July 2002

Results are reported on the 193 and 248 nm photolysis of iodoethane, specifically with respect to H-atom production. Experiments using selectively deuterated iodoethanes, ICD2CH3 and ICH2CD3, reveal that at 193 nm the carbon–hydrogen bond cleavage is not carbon-atom specific. However, following photolysis at 248 nm, it is clear that C–H (or C–D) bond dissociation occurs preferentially at the β carbon atom.
Theoretical study of the A 3Σ−–X 3Π transition in SiC
Authors: Langhoff Stephen R. , Bauschlicher Charles W.
Published: July 2002

A theoretical study of the A 3Σ−–X 3Π infrared transition in SiC is presented using atomic natural orbital Gaussian basis sets in conjunction with multireference configuration interaction calculations. Dipole moment functions have been computed for both states as well as the A–X electronic transition moment function. The calculations predict that T00 is 3700±200 cm−1, so that the transition recently observed in emission near 4500 cm−1 and assigned to the 0–0 band of the A 3Σ−–X 3Π system of SiC is actually the 1–0 band.
The permanent electric dipole moment of CaOH
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Steimle Timothy C. , Shirley Jeffrey E.
Published: July 2002

Theoretical calculations yield electric dipole moments ( μ) of 0.98, 0.49, and 0.11 Debye at the computed equilibrium geometries of the X 2Σ+, A 2Π, and B 2Σ+ states of CaOH. Thus the pure rotational spectra in these electronic states are expected to be weaker than the corresponding alkaline monohalides. This is consistent with our failure to observe the pure rotational spectrum of the X 2Σ+ state by microwave-optical double resonance spectroscopy, and is in agreement with our experimental determination of μ<1 D for the A 2Π3/2 state. However, the vibrational–rotational spectrum is quite strong, since dμ/dr is quite large in these ionic molecules. Using the free-ion polarizabilities, the modified Rittner model predicts a μ value of about −8 D for the X 2Σ+ state, which is in error by about 9 D. The theoretical values for total dipole moment, as well as contributions from Ca+ and OH−, can be reproduced using effective polarizabilities. However, this analysis shows that the effective polarizability of OH− is a much smaller fraction of the free-ion value than for Ca+, because Pauli repulsion restricts the OH− polarization toward the Ca+. Some account of this effect seems to be a prerequisite for the successful application of either Rittner or ligand field models to ionic systems.
Reply to the Comment on: Electron scattering in dense atomic and molecular gases
Authors: Rupnik K. , Asaf U. , McGlynn S. P.
Published: July 2002

The Freeman comments (vide supra) are dissected into two parts, criticism of the choices of the polarizability α for H2, N2, and CH4, and urgings for a redirection of scattering theory, particularly for molecules. These parts are discussed separately.
Coherence effects in half collisions: Collision induced fluorescence in Hg(2537 Å)–Ar
Authors: Segal D. , Harris I. D.
Published: July 2002

We have measured the polarization of collisionally redistributed light as a function of detuning from line center for the Hg3P1–1S0 transition in the presence of Ar. We present a polarization profile for the red wing between 5 and 18 cm−1 and polarizations for two detunings in the blue wing. We compare this type of experiment to the complementary method of photodissociation and discuss the issue of coherence between excitation path-ways. The data are analyzed using a simple molecular reorientation model suggested by others which includes coherent excitation. The model is shown to be unable to completely account for the high polarizations measured in this experiment. We suggest reasons for the failure of this model.
Size consistent multireference single and double excitation configuration interaction calculations. The multireference coupled electron-pair approximation
Authors: Ruttink P. J. A. , van Lenthe J. H. , Zwaans R. , Groenenboom G. C.
Published: July 2002

A new size consistent extension to the multi reference configuration interaction method is described. The method termed multireference coupled electron pair approximation (MRCEPA) is akin to a multireference CEPA(0) approach, though nonlinear terms do receive separate attention. We show the performance of the approach in some model systems as well as in an application to calculation of ground and excited ππ* states of ethylene.
A theoretical investigation of the low-lying electronic states of the molecule BeH+
Authors: Machado Francisco B. C. , Ornellas Fernando R.
Published: July 2002

A theoretical study at the multireference singles and doubles configuration-interaction level is reported for the six lowest electronic states of BeH+. Of the four new states described, the 1Π and 3Π are bound states supporting at least 8 and 12 vibrational levels, respectively. Two 3Σ+ states are repulsive. For the two known 1Σ+ states improved results are also presented. For all these states, dipole moments and transition moments have been computed. For the bound states, besides the vibrational levels, transition probabilities, radiative lifetimes, vibrationally averaged dipole moments, and a set of spectroscopic constants are also reported.
Analytical energy gradients for multiconfiguration self-consistent field wave functions with frozen core orbitals
Authors: Busch Thilo , Esposti Alessandra Degli , Werner Hans-Joachim
Published: July 2002

A method to calculate analytical energy gradients for multiconfiguration self-consistent field (MCSCF) wave functions with frozen core orbitals is presented. Since the core orbitals, which are taken from a closed shell SCF calculation, are not variationally optimized in the MCSCF procedure, it is necessary to determine their derivatives by solving a set of coupled perturbed Hartree–Fock (CPHF) equations. The technique is similar to the calculation of energy gradients for CI wave functions, but is complicated by the fact that the SCF and MCSCF orbitals are different. This makes it necessary to perform a transformation between the two orbital basis sets at an intermediate stage. The CPHF equations are solved by an iterative method, in which optionally part of the Hessian matrix can be constructed and inverted explicitly. Some applications of the method are presented. For the molecule P2S, optimized geometries for two isomers and a saddle point are compared for MCSCF wave functions with frozen and fully optimized core orbitals. It is demonstrated that in both cases virtually identical results are obtained and that the frozen-core approximation leads to significant savings in computer time. Some preliminary results are also reported for tetrasilabicyclo[1.1.0]butane, Si4H6.
Virial sum rules for nuclear electric shieldings and geometrical derivatives of dipole and quadrupole molecular moments
Authors: Lazzeretti Paolo , Malagoli Massimo , Zanasi Riccardo
Published: July 2002

The virial theorem has been used to derive sum rules for dipole- and mixed-dipole–quadrupole nuclear electric shieldings and corresponding geometrical derivatives of dipole and quadrupole moments in a molecule. Test calculations have been carried out on a series of first- and second-row hydrides. The virial sum rules can be effective tools to prove the accuracy of theoretical nuclear shieldings and analytic geometrical derivatives. As the latter are related to ir intensities, the virial sum rules can give important indications on the reliability of theoretical predictions for this spectroscopical parameter.
Energies of CH2OH, CH3O, and related compounds
Authors: Curtiss Larry A. , Kock L. David , Pople John A.
Published: July 2002

The Gaussian-2 (G2) theoretical procedure, based on ab initio molecular orbital theory, is used to calculate the energies of CH2OH, CH3O, and their cations. The resulting G2 adiabatic ionization potential for CH3O of 10.78 eV supports the new value of 10.726±0.008 eV reported by Ruscic and Berkowitz [J. Chem. Phys. 95, xxxx (1991)] from a photoionization study. A previous photoelectron result of 7.37±0.03 eV is probably an incorrect assignment. The G2 ionization potential for CH2OH is 7.45 eV, consistent with the experimental value of 7.55 eV. The calculated O–H and C–H bond dissociation energies of CH3OH are 105.0 and 96.2 kcal/mol, respectively. The results suggest that the D0(H–CH2OH) from kinetics measurements may be too low.
Electron scattering in dense CO2 gas: Photoionization spectra of CH3I perturbed by CO2
Authors: Asaf U. , Steinberger I. T. , Meyer J. , Reininger R.
Published: July 2002

A photoionization study of CH3I in the presence of dense CO2 (density range: 1.6–10.0×1019 cm−3 ) revealed the prethreshold 2E3/2 Rydberg transitions of methyl iodide perturbed by CO2. From the measured pressure shift of the nd Rydbergs (n=11,12,13,14) the scattering length of CO2 was deduced as A=−0.224 nm, in accord with cross-section data. The vibrational structure of CO2 above 9.54 eV was found to be superimposed on the CH3I absorber photoionization spectrum, yielding a spectrum more detailed than absorption spectra of pure CO2.
Effect of ion vibration for proton-transfer reaction of ammonia cation
Authors: Tachibana A. , Suzuki T. , Teramoto Y. , Yoshida N. , Sato T. , Yamabe T.
Published: July 2002

Ion vibration gives rise to a new pulsing dipole moment which affects the long-range interaction with the target molecule. This pulsing long-range interaction mechanism has been applied to the proton-transfer reaction NH+3(ν)+NH3→NH2+NH+4 in which the vibrational mode-specific depression of the reaction rate is observed. Significance of the vibrationally induced dipole moment of NH+3(ν) has been clarified in such a way that the origin of the depression is ascribed to the interaction of this pulsing dipole moment with the permanent dipole moment of NH3. The results of calculation agree qualitatively well with the experimental result [Chupka and Russell, J. Chem. Phys. 48, 1527 (1968)].
Photodissociation of K2 (X 1Σ+g–B 1Πu)
Authors: Kleiber P. D. , Wang J.-X. , Sando K. M. , Zafiropulos V. , Stwalley W. C.
Published: July 2002

A semiclassical model based on an orbital following picture to describe the dissociation dynamics of K*2 (B 1Πu) is outlined. The results of this simple model are in complete quantitative agreement with an exact quantum calculation by Dubs and Julienne based on a half-collision multichannel quantum defect analysis (accompanying paper). These theoretical works give a complete description of the process and indicate that dissociation occurs adiabatically from the Hund’s case (a) to the Hund’s case (c) basis, followed by a sudden recoupling at long range to the Hund’s case (e) basis. The experimentally observed bound–free excitation spectrum and the polarization of the atomic product fluorescence are in excellent agreement with the result of a full thermally averaged calculation based on this theoretical dynamical model.
A variational Monte Carlo study of argon, neon, and helium clusters
Authors: Rick Steven W. , Lynch D. L. , Doll J. D.
Published: July 2002

Clusters of rare gas atoms provide an interesting setting for the study of the issue of quantum mechanical localization. The properties of these clusters of 2–7 atoms are calculated using variational Monte Carlo methods. To our knowledge, this is the first variational Monte Carlo study of localized clusters and new solidlike wave function forms, including shadow, multiple bond length, and Boltzmann-like wave functions, are reported. Diffusion Monte Carlo methods provide an independent, exact value of the ground state energy, useful as a check of the variational results. The properties of the variational wave functions, when analyzed in terms of probability distribution functions, quench studies, and visual examination of the wave functions, indicate delocalized helium and localized argon and neon clusters.
About the convergence of reactive infinite order sudden calculations on parallel computers
Authors: Laganà Antonio , Aguilar Antonio , Gimenez Xavier , Lucas Jose’ M.
Published: July 2002

A problem of reactive infinite order sudden approximation calculations is the correct evaluation of the convergence of fixed angle contributions to the cross section with the reactant orbital quantum number l. Parallel computing, in addition to speeding up the calculation, offers the advantage of extending the convergence check to higher l terms with no additional consumption of computing time. An application to the Cl+HCl reaction is given.
Coupled wave packets study of the dynamics of dissociative ion–molecule charge exchange
Authors: Aguillon F. , Sidis V. , Gauyacq J. P.
Published: July 2002

A coupled wave packet method is presented which allows us to treat exactly the vibrational and dissociative motions in nonadiabatic atom–molecule collisions, whose relative motion is described by a classical trajectory. It consists of a time propagation of the coupled vibrational wave functions defined over a grid. It is applied to the dissociative charge exchange (DCE) process in atom–molecule collisions. Model cases are investigated that put forward the basic characteristics of the DCE process. Analysis of the time evolution of the wave packets leads to a direct view of the collision dynamics and to an interpretation of the dissociation mechanisms. Striking phenomena are revealed, that were not predictable by the previously existing approximate methods.
Theoretical study of methyl hypoflourite (CH3OF) and related compounds
Authors: Curtiss Larry A. , Pople John A.
Published: July 2002

The Gaussian-2 (G2) theoretical procedure, based on ab initio molecular orbital theory, is used to calculate the energies of CH3OF, CH3OF+, and related compounds. In this study we have found methyl hypofluorite to have a trans Cs structure and to be stable with respect to loss of fluorine by 45.9 kcal/mol. The energies of fragmentation processes of methyl hypofluorite calculated from G2 theory are in agreement with those measured by Ruscic, Appelman, and Berkowitz [J. Chem. Phys. 95, XXX (1991)] and support their interpretation of the photoionization data. The theoretical enthalpy of formation ΔH0f0(CH3OF) of −21.0 kcal/mol is in agreement with the experimental value (≥−23.0±0.7 kcal/mol) derived from the photoionization data. The ordering of the O–F bond strengths in the series of molecules OF, HOF, and CH3OF is OF>HOF>CH3OF and the C–O bond strength is 6–8 kcal/mol weaker in methyl hypofluorite than in methanol.
Hard dumbbells in contact with a hard wall: An application of the density functional theory
Authors: Sokol/owski S.
Published: July 2002

A modified version of the density functional theory recently used by Poniewierski and Ho/yst [Phys. Rev. A 41, 6871 (1990)] for studies of liquid crystals is applied to investigate orientational ordering of hard dumbbells in contact with a hard wall and between two hard walls. A comparison with the results of grand ensemble Monte Carlo simulations indicates that the considered approach may be used with advantage in the study of the structure of hard dumbbells adjacent to a wall.
Possible rotationally resolved detection of NO2 in the C̃ 2A2 state by optical–optical double resonance spectroscopy
Authors: Shibuya Kazuhiko , Kusumoto Tadashi , Nagai Hidekazu , Obi Kinichi
Published: July 2002

This Note describes the first rotationally resolved detection of NO2 in the C̃ 2A2 state by optical–optical double resonance (OODR) spectroscopy. The C̃ 2A2–X̃ 2A1 transition is symmetry forbidden. Direct experimental evidence of the C̃ 2A2 state had not been provided until the recent vibrationally resolved detection using photoelectron spectroscopy of NO−2 by Weaver et al. [J. Chem. Phys. 90, 2027 (1989)]. We have measured hundreds of the OODR spectra in the 500 ∼600 nm range and found that 11 rovibronic levels in the spectral region of 513.8∼516.2 nm belong to the Ka = 1 stack of a vibrational level of C̃ 2A2.
Crystal structures of N2O to 12 GPa by x-ray diffraction
Authors: Mills R. L. , Olinger Bart , Cromer D. T. , LeSar R.
Published: July 2002

The structures and phase transitions of N2O were studied by powder x-ray diffraction in a tungsten–carbide anvil device from about 100 to 300 K and 2 to 12 GPa. Two solid phases, α-N2O and β-N2O, were observed. The α pattern is consistent with the known low-pressure low-temperature ordered cubic form, space group Pa3, up to 4.8 GPa where transition to a new β solid occurs. From refinements using photographic x-ray intensities, the β-N2O structure was determined to be orthorhombic Cmca. There are four molecules in a unit cell with a=4.954 Å, b=4.497 Å, and c=6.201 Å at 5.81 GPa and 298 K. The molecular axes lie parallel to the bc plane and are tipped at an angle of 37.2° to the b axis. Random head-to-tail orientation is probable in both solids. The phase diagram and values of the molar volume for N2O are compared with published data for the isoelectronic molecule CO2. Recent theoretical calculations correctly predict the β-N2O structure.
Nested interaction representations in time dependent quantum mechanics
Authors: Tannor David J. , Besprozvannaya Anna , Williams Carl J.
Published: July 2002

Two significant developments in the formulation of the equations of motion in the interaction representation (IR) are introduced in this paper. These developments have direct bearing on the efficient propagation in the interaction representation. The first of these developments involves the concept of sequential, or ‘‘nested’’ interaction transformations on the Hamiltonian and the wave function. Two such nested IRs are described. The first is a momentum- or ‘‘P-adapted’’ IR which, in addition to removing wave packet spreading, systematically maintains the average momentum 〈P〉 at zero. This facilitates a grid which not only has a minimal range, but whose points are maximally sparse. Evaluating the Hamiltonian operator in the P-adapted interaction representation involves only one simple algorithmic modification to the ordinary IR: replacing eiH0t/ℏV(R)e−iH0t /ℏ→eiH0t/ℏV(R+〈P〉t/m) eiH0t/ℏ, i.e., using the potential energy function which is downstream from the wave function by the classical propagation distance. A PR-adapted IR is also defined, which systematically maintains both the average position 〈R〉 and momentum 〈P〉 at zero.

The PR-adapted IR also involves only one modification to the ordinary IR: replacing eiH0t/ℏV(R)e−iH0t /ℏ→eiH0t/ℏV(R+〈R〉 +〈P〉t/m)eiH0t/ℏ. The second formal development consists of the application of the operator equation eiH0t/ℏV(R)e−iH0t /ℏ=V(R+Pt/m) in conjunction with the nested IRs. This formal identity, which is essentially the Heisenberg representation for V(R), provides a convenient route to propagation in the interaction representation without reconstructing the Schrödinger wave function as an intermediate. The time propagation in all representations is performed using an iterative Lanczos reduction scheme combined with a second- order Magnus expansion. Numerical results for the exponential and the 1/R potential are presented, and illustrate that the new representations can easily lead to a savings of an order of magnitude in the size of the grid required for the propagation.
A supersonic molecular beam optical Stark study of CaOH and SrOH
Authors: Steimle T. C. , Fletcher D. A. , Jung K. Y. , Scurlock C. T.
Published: July 2002

The permanent electric dipole moments of CaOH and SrOH in their X 2Σ+, A 2Π3/2, A 2Π1/2, and B 2Σ+ states have been measured using the technique of supersonic molecular beam optical Stark spectroscopy. For CaOH the values obtained were μ(X 2Σ+)=1.465(61)D, μ(A 2Π1/2)=0.836(32)D, μ(A 2Π3/2)=0.766(24)D, and μ(B 2Σ+)=0.744(84)D, while for SrOH the values were μ(X 2Σ+)=1.900(14)D, μ(A 2Π1/2)=0.590(45)D, μ(A 2Π3/2)=0.424(5)D, and μ(B 2Σ+)=0.396(61)D. The results are compared with values from a recent ab initio calculation for CaOH and with the predictions of a semiempirical electrostatic polarization model.
Two-color (2+1′) multiphoton ionization threshold photoelectron study of the Ar–NO van der Waals complex: Observation of intermolecular vibrational progressions of the Ar–NO+ cation
Authors: Takahashi Masahiko
Published: July 2002

Two-color (2+1′) multiphoton ionization high-resolution threshold photoelectron spectroscopy has been applied to the Ar–NO van der Waals complex in a supersonic free jet. The resonant ionization process studied may be expressed by Ar–NO(X 2Π1/2,v″=0) +2hν1→Ar–NO*(C 2Π,v′)+hν2→Ar –NO+(X 1Σ+,v+). Two strong anharmonic vibrational progressions have been observed in the threshold photoelectron spectra having frequencies of 79 and 94 cm−1. These vibrations have been assigned to the intermolecular bending and stretching motions of the (Ar–NO)+ complex cation. From the threshold photoelectron spectra, the adiabatic ionization potential of Ar–NO has been determined as 73 869±6 cm−1 (86 cm−1 higher than previously reported), and the dissociation energy of the (Ar–NO)+ cation has been calculated as 951 cm−1. The structural change of Ar–NO that occurs on photoionization has been calculated with the aid of simple Franck–Condon calculations, which suggest that the intermolecular bond distance of Ar–NO decreases by 1.03 Å, while the O–N–Ar angle decreases by 11.3°.
Quantum calculation of thermal rate constants for the H+D2 reaction
Authors: Park Tae Jun , Light J. C.
Published: July 2002

Thermal rate constants for the H+D2 reaction on the LSTH potential-energy surface are determined quantum mechanically over T=300–1500 K using the quantum flux–flux autocorrelation function of Miller [J. Chem. Phys. 61, 1823 (1974)]. Following earlier works [T. J. Park and J. C. Light, J. Chem. Phys. 91, 974 (1989); T. J. Park and J. C. Light, ibid. 94, 2946 (1991)], we use the adiabatically adjusted principal axis hyperspherical coordinates of Pack [Chem. Phys. Lett. 108, 333 (1984)] and a direct product C2v symmetry-adapted discrete variable representation to evaluate the Hamiltonian and flux. The initial representation of the J=0 Hamiltonian in the ℒ2 basis of ∼14 000 functions is sequentially diagonalized and truncated to yield ∼600 accurate eigenvalues and eigenvectors for each symmetry species block. The J>0 Hamiltonian is evaluated in the direct product basis of truncated J=0 eigenvectors and parity decoupled Wigner rotation functions. Diagonalization of the J>0 Hamiltonian is performed separately for each KJ block by neglecting Coriolis coupling and approximating K coupling by perturbation. Both eigenvalues and eigenvectors are corrected by the perturbation. Thermal rate constants for each J, kJ(T), are then determined by the flux–flux autocorrelation function considering nuclear spins. Due to the eigenvector corrections, both parity calculations are required to determine kJ(T). Overall thermal rate constants k(T) are obtained by summing kJ(T) over J with the weight of 2J+1 up to J=30. The results show good agreement with experiments.
Infrared multiphoton excitation dynamics of CF3I. I. Populations and dissociation rates of highly excited rovibrational states
Authors: Abel B. , Hippler H. , Troe J.
Published: July 2002

Transient detection of I(2P3/2) by resonantly enhanced multiphoton ionization (REMPI) was used to monitor in-pulse and after-pulse dissociation of CF3I excited by infrared (IR) multiphoton absorption. After-pulse reaction is characterized by time-dependent dissociation rate coefficients. The apparent reaction rates increase with increasing laser intensity. These observations are attributed to strong rotational dependencies of the specific dissociation rate constants k(E,J) and to CO2 laser-intensity-dependent rotational distributions of the excited CF3I. The corresponding rotational distributions are reconstructed from the observed experimental time profiles of the reaction rates. In addition to the detection of the I(2P3/2) dissociation product from infrared multiphoton excitation, populations of vibrationally highly excited CF3I* were identified via electronic excitation in the visible, subsequent fast dissociation and REMPI detection of the resulting I(2P1/2). At weak IR laser intensities these vibrational distributions were found to be strongly bimodal. However, the bimodal character weakens with increasing laser intensity, tending towards a single broad distribution at very high laser intensities.
Motion of megabase deoxyribonucleic acid during field-inversion gel electrophoresis: Investigation by nonlocal Monte Carlo
Authors: Duke T. A. J. , Viovy J. L.
Published: July 2002

We give a detailed description of a new Monte Carlo method for the simulation of the forced dynamics of long chain polymers in a constrictive environment. The model is based on the reptation theory but admits, in addition, the possibility that loops of the chain (‘‘hernias’’) may escape laterally out of the tube. A discrete representation of the molecule, in which individual chain segments are either taut or slack, permits the extensional mode of the molecule within the tube to be taken into consideration. The dynamics is modeled by the nonlocal hopping of ‘‘defects’’ (regions of slack) along the chain, with Monte Carlo rules based on the stochastic equations of motion of the taut portions of the molecule. We use the technique to investigate the motion of long deoxyribonucleic acid (DNA) molecules, containing millions of base pairs, during field-inversion gel electrophoresis. For the pulse ratios most commonly used in practice, we find that the separation patterns display two regions of band-inversion. This anomalous behavior is linked to the strong transient response of the molecules when the field is reversed; sudden field inversion induces the formation of a chain configuration shaped like an extended V after an interval of time that increases linearly with the chain length. The DNA molecules that have the minimum and the maximum migration speeds are those whose transient response times are approximately equal to the forward and the reverse pulse time, respectively.
The decay of triplet pyrazine and methylpyrazine in supersonic jets. Substitution effects
Authors: Sneh Ofer , Cheshnovsky Ori
Published: July 2002

We have measured the decay rates of optically excited triplet states of pyrazine and methylpyrazine in a supersonic jet. The excess vibrational energy dependence of the radiationless rate constants in the energy range between the T1 and the S1 electronic origins of the isolated molecules was explored. Decay rates between 5×102 and 105 s−1 were found in the 1800 cm−1 range of excess vibrational energy above the origin of the T1 state in pyrazine. In methylpyrazine the decay rates increase from 8×102 to 3.3×104 in the first 600 cm−1 excess energy range above the T1 origin. The decay rates are free of mode specificity and rotational effects. The wide dynamic range of the T1→S0 radiationless rates of pyrazine is substantially enhanced by methyl substitution. The results support a model which suggests that certain vibrational modes, those which undergo large frequency decreases in the excited state, control the strong vibrational energy dependence of the T1→S0 intersystem crossing of pyrazine. These large frequency changes result from the interaction of the near lying 3nπ* and 3ππ* states (the ‘‘proximity effect’’). The enhancement of the excess energy dependence of the decay rates by methyl substitution indicates a further decrease in the frequency of these modes. This effect of the methyl group is consistent with the ‘‘proximity effect.’’ The frequency reduction of these modes in the S1 manifold of methylpyrazine also support this mechanism.