bluuebunny/crossref_metadata_2025 huggingface dataset:
Integral equation theory of flexible polyelectrolytes. I. Debye–Hückel approach
Authors: Dymitrowska Magdalena , Belloni Luc
Published: July 2002

The polymer reference interaction site model (PRISM) with standard Percus–Yevick (PY) or hypernetted chain (HNC) closure equations is solved numerically for solutions of flexible polyelectrolytes. The monomers interact via a screened Coulomb potential (Debye–Hückel approach). The wormlike form factor characterized by a persistence length is used for the intrachain correlations. The equilibrium and structural results obtained for a large variety of parameters are interpreted in terms of the monomer–monomer interchain pair distribution function and of the structure factor. Issues like the scattered intensity peak position and the osmotic pressure are addressed. The domain within the phase diagram without HNC solutions is also presented.
Phase coexistence and interface structure of a Lennard-Jones fluid in porous media. Application of Born–Green–Yvon equation
Authors: Trokhymchuk Andrij , Sokołowski Stefan
Published: July 2002

The Born–Green–Yvon (BGY) equation with Fischer–Methfessel (FM) approximation is used to calculate the density profiles of the planar liquid–vapor interface of Lennard-Jones fluid in a hard-sphere disordered rigid matrix. The density profiles are compared with the results of canonical ensemble Monte Carlo simulations. Both theory and simulation show that the thickness of the interface increases in the presence of disordered media. The phase diagrams for two different matrix densities have been evaluated and compared with the predictions of mean field and optimized cluster theories. Comparison with Monte Carlo data for low matrix densities indicates that the BGY-FM theory works reasonably well. Possible extensions of the theory are also discussed.
A spectroscopic study of CaOCH3 using the pump/probe microwave and the molecular beam/optical Stark techniques
Authors: Namiki Kei-ichi C. , Robinson J. Scott , Steimle Timothy C.
Published: July 2002

The Stark effect on the R22q(0,0.5) (ν=17 682.9251 cm−1) and P11q(0,1.5) (ν=17 682.1966 cm−1) branch features of the (0,0) B 2A1–X 2A1 band system of calcium methoxide, CaOCH3, was measured and analyzed to give the magnitude of the permanent electronic dipole moments, |μ|, of 1.58(8) D and 1.21(5) D for the X 2A1 and B 2A1 states, respectively. The dipole moments are compared with other monovalent calcium compounds and those predicted from a simple electrostatic model. Pure rotational transitions in the X 2A1 state were recorded using the pump/probe microwave-optical double resonance technique. The proton magnetic hyperfine splitting pattern confirms a C3v symmetry of the ground electronic state. The determined Fermi contact and dipolar parameters are: aF=−0.421(27) MHz; Taa=1.070(45) MHz, and |Tbb−Tcc|=0.292(47) MHz. The latter parameter is associated with the lifting of the K-degeneracy of this symmetric top molecule and mixing the I0=1/2 and I0=3/2 levels.
Structural evolution of phase-separating model colloidal liquids by Brownian dynamics computer simulation
Authors: Lodge J. F. M. , Heyes D. M.
Published: July 2002

Brownian dynamics, (BD), simulation has been used to follow the phase separation of Lennard-Jones-type particles quenched from a supercritical state point into the vapor–liquid or vapor–solid co-existence parts of their phase diagrams. Calculations were performed with spherical particles interacting via 12:6, 24:12, and 36:18 interaction laws at subcritical temperatures and low-volume fractions (φ⩽0.2). Structural properties were followed as the systems evolved using pictures of the configurations, radial distribution function, and the low-angle scattering peak of the structure factor. The time dependence of the interaction energy was also followed. The scaling behavior of these quantities as a function of time was found to be similar to that observed in light scattering experiments during the phase separation of real colloidal systems. The aggregate structure that developed with time was sensitive to the range of the attractive part of the potential and its underlying phase diagram (the 36:18 system does not have a liquid phase). The 12:6 systems soon formed compact structures, whereas the systems generated using the shorter-ranged potentials persisted in a more diffuse, tenuous network for the duration of the simulations. Apart from at very short times for all potential laws, the only convincing evidence for a long-lived fractal structure was for the 36:18 systems at the lowest quench temperatures (kT/ε=0.3, where ε is the depth of the potential). The local structure in the dense regions of the network was sensitive to the range of the potential, exhibiting in the vapor–solid co-existence part of the phase diagram glassylike features for the 12:6 systems and crystalline local order for the 24:12 and 38:18 states. The 12:6 systems close to the metastable region of the vapor–liquid two-phase part of the phase diagram exhibited latency in the appearance and growth of the small angle scattering peak. The 24:12 and 36:18 systems also displayed latency at the higher temperatures both in the growth of the peak height and its movement to lower scattering vectors.
An ab initio study of specific solvent effects on the electronic coupling element in electron transfer reactions
Authors: Henderson Thomas M. , Cave Robert J.
Published: July 2002

Specific solvent effects on the electronic coupling element for electron transfer are examined using two model donor–acceptor systems (Zn2+ and Li2+) and several model “solvent” species (He, Ne, H2O, and NH3). The effects are evaluated relative to the given donor–acceptor pair without solvent present. The electronic coupling element (Hab) is found to depend strongly on the identity of the intervening solvent, with He atoms decreasing Hab, whereas H2O and NH3 significantly increase Hab. The distance dependence (essentially exponential decay) is weakly affected by a single intervening solvent atom–molecule. However, when the donor–acceptor distance increases in concert with addition of successively greater numbers of solvent species, the decay with distance of Hab is altered appreciably. Effects due to varying the orientation of molecular solvent are found, somewhat surprisingly, to be quite modest.
Spectroscopic evidence for perturbed NO dimers on oxidized Mo(110)
Authors: Queeney K. T. , Pang S. , Friend C. M.
Published: July 2002

We report herein the first direct experimental evidence of a nitric oxide dimer (NO)2 which is significantly perturbed from gas- and condensed-phase (NO)2 by bonding of one of the nitrosyls to a metal surface. Infrared reflectance absorbance spectroscopy of isotopically mixed overlayers is used to identify the formation of this species from NO adsorption on oxidized Mo(110) and to characterize it as a second-layer NO[ν(14NO)=1871 cm−1] bound to a surface nitrosyl [ν(14NO)=1728 cm−1], The spectroscopic signature of this species is a small (∼8 cm−1) splitting of ν(NO) of each of the nitrosyls upon isotopic mixing, rather than the appearance of three different frequencies for νs(NO) and νa(NO). The formation of such a dimer on oxidized Mo(110) does not result in N–N bond formation, in contrast to the evolution of both N3 and N2O via a dinitrosyl intermediate on the same surface. This result suggests that, on surfaces which interact strongly enough with NO to form chemisorbed nitrosyls stable above room temperature, N–N bond formation is favored by surface-bonding of both nitrosyls, rather than coupling via the weak N–N bond of a condensed-phase type (NO)2 dimer.
Effect of temperature on electron attachment to and negative ion states of CCl2F2
Authors: Wang Yicheng , Christophorou Loucas G. , Verbrugge Joel K.
Published: July 2002

The effect of temperature on electron attachment to dichlorodifluoromethane (CCl2F2) has been investigated for temperatures up to 500 K and for mean-electron energies from thermal to 1.0 eV using an electron swarm method. The measurements were made in mixtures of CCl2F2 with nitrogen. The electron attachment rate constant increases with temperature over the entire temperature and mean-electron energy range investigated. The variation of the thermal value of the electron attachment rate constant with temperature compares well with earlier measurements of this quantity and shows an increase by a factor of 10 when the temperature is raised from 300 to 500 K. From a comparison of published data on the electron affinity, electron attachment using the swarm method, electron attachment using the electron beam method, electron scattering, electron transmission, indirect electron scattering, and related calculations, the lowest negative ion states of CCl2F2 have been identified with average positions as follows: a1(C–Clσ*) at +0.4 eV and −0.9 eV, b2(C–Clσ*) at −2.5 eV, a1(C–Fσ*) at −3.5 eV, and b1(C–Fσ*) at −6.2 eV; an electron-excited Feshbach resonance is also indicated at −8.9 eV.
Electronic spectroscopy of the Al–H2 complex: Excited state dynamics and orbital alignment of the AlH(A 1Π) product
Authors: Yang Xin , Dagdigian Paul J.
Published: July 2002

The weakly bound Al⋯H2 complex was prepared in a pulsed supersonic beam and studied with laser fluorescence excitation spectroscopy. Transitions to bound bend–stretch levels in the electronic state correlating with the Al(5s)+H2 asymptote have been observed. Resonance fluorescence from the excited levels could be detected only very weakly. These excited levels decay mainly nonradiatively, and the excitation spectrum was obtained by monitoring emission from the lower Al atomic levels, 4s and 3d, and AlH A→X chemiluminescence due to AlH(A 1Π) formed in the reactive decay of the excited Al(5s)⋯H2 complex. The action spectra show a progression of bands which were assigned as a progression in the excited state Al–H2 stretch vibrational mode. The AlH chemiluminescence was recorded with 0.8 nm spectral resolution. Emission in the A–X (0,0) and (1,1) bands, from the v′=0 and 1 vibrational levels of the A 1Π state, was observed. The relative emission intensity of the Q branches was found to be depressed from that expected if the e and f levels of the excited state were equally populated. The experimental spectra were compared with simulations in order to estimate the rovibrational Λ-doublet state distribution in the A 1Π state. A strong propensity to form e levels, with considerable rotational excitation, was found. The Λ-doublet propensity is believed to be a consequence of the A′ symmetry of the Al(5s)+H2 potential energy surface on which the reagents initially approach one another.
Simultaneous observation of dipole-bound and valence electron states in pyridine tetramer anion
Authors: Han Sang Yun , Kim Jeong Hyun , Song Jae Kyu , Kim Seong Keun
Published: July 2002

Coexistence of dipole-bound and valence electron states was observed by photoelectron spectroscopy in pyridine tetramer anion despite the fact that pyridine itself does not bind electrons by either mechanism. The photoelectron yield for the two states showed distinctly different laser polarization dependence. The fact that these two states have rather disparate thermodynamic stabilities suggests that there may exist an energy barrier separating the two forms in configuration space of the cluster.
Molecular-dynamics simulations of solvent effects on the C–H stretching Raman bands of cyclohexane-d11 in supercritical CO2 and liquid solvents
Authors: Frankland S. J. V. , Maroncelli M.
Published: July 2002

Molecular-dynamics simulations are used to elucidate the molecular basis for the solvent effects on the isolated C–H stretching bands observed in the Raman spectrum of cyclohexane-d11. The main focus is on modeling the density dependence of the spectrum in supercritical CO2 recently reported by Pan, McDonald, and MacPhail [J. Chem. Phys. 110, 1677 (1999)], but several liquid solvents (CCl4, CS2, and CH3CN) have also been examined. The frequency shifts and line shapes of the Raman spectrum are simulated using a rigid solute and standard line shape theory in the limit of pure dephasing. Three models for the vibration–solvent coupling are considered. The simplest model, which is based on ground-state forces alone, provides a surprisingly good representation of the density dependence of the linewidths–line shapes but predicts the wrong sign for the gas-to-solution frequency shifts. This failure is due to the neglect of changes in bond polarizability upon vibrational excitation. Allowing for this polarizability difference via a semiempirical approach provides an accurate description of both the linewidths and frequency shifts with a physically reasonable vibrational difference potential. Interpretation of the instantaneous frequency shifts simulated with this model leads to the following general conclusions concerning the solvent effect on these spectra: (i) The relatively small gas-to-solution frequency shifts observed in experiment are the result of the near cancellation of much larger positive and negative contributions from repulsive and attractive interactions. (ii) Fluctuations in the instantaneous frequency are sufficiently fast (correlation times ∼100 fs) that the spectra are homogeneously broadened in all solvents examined. (iii) The dynamics of the solvent–solute interactions that determine the Raman line shapes are quite well described by an isolated binary collision (“IBC”) type picture. (iv) The simplicity of the dynamics, and the success of this IBC description, is due at least in part to the special, localized character of these isolated C–H stretching modes. (v) The linear density dependence of the linewidths observed in supercritical CO2 reflects the modest extent of local density augmentation in the cyclohexane–CO2 system.
Ab initio calculation of the frequency-dependent interaction induced hyperpolarizability of Ar2
Authors: Fernández Berta , Hättig Christof , Koch Henrik , Rizzo Antonio
Published: July 2002

The frequency-dependent interaction induced polarizability and second hyperpolarizability of the argon dimer are computed for a range of internuclear distances employing the coupled cluster singles and doubles response approach. The frequency dependence of the interaction-induced properties is treated through second order in the frequency arguments using expansions in Cauchy moments and hyperpolarizability dispersion coefficients. The dielectric, the refractivity, the Kerr and the hyperpolarizability second virial coefficients are computed for a range of temperatures employing a recent accurate ab initio potential for the ground state of the argon dimer. For most of the computed virial coefficients good agreement is obtained between the present ab initio results and the available experimental data.
Effect of uniform electric field on homogeneous vapor–liquid nucleation and phase equilibria. II. Extended simple point charge model water
Authors: Gao G. T. , Oh K. J. , Zeng X. C.
Published: July 2002

The effects of a uniform electric field on homogeneous vapor–liquid nucleation of the extended simple point charge (SPC/E) model water have been simulated. A grand canonical Monte Carlo simulation approach [I. Kusaka et al., J. Chem. Phys. 108, 3416 (1998)], which directly gives the equilibrium distribution of physical clusters, is employed to calculate the formation free energy of the SPC/E water cluster. The results show that the formation free energy is lowered in a uniform field if the chemical potential of the supersaturated vapor is fixed; in this case, the field enhances the rate of nucleation. However, if the vapor supersaturation or pressure is fixed, the formation free energy increases with the field, that is, the field reduces the rate of nucleation. This conclusion is consistent with an earlier study using the i/v cluster theory for weakly dipolar Stockmayer fluid in a uniform field (Part I). A byproduct of this work is the vapor–liquid coexistence (binodal) of the SPC/E water in the presence of the electric field.
Electronically excited states in size-selected solvated alkali metal atoms. III. Depletion spectroscopy of Na(NH3)n-clusters
Authors: Brockhaus Peter , Hertel Ingolf V. , Schulz Claus Peter
Published: July 2002

The first electronically excited state of small Na(NH3)n clusters up to n=22 is studied by means of depletion spectroscopy. A drastic decrease of the excitation energy from the 3s→3p transition of the Na atom (16 950 cm−1) down to 6000 cm−1 for the Na(NH3)4 cluster, the closing of the first solvation shell, is observed. For larger clusters the excitation energy increases slightly toward the bulk value (6300 cm−1) which represents the absorption of the “solvated” electron. For all Na(NH3)n clusters with n⩾3 a strong absorption peak is observed near 6600 cm−1. By comparison with deuterated sodium–ammonia clusters this absorption can be assigned to an intramolecular vibrational overtone of the ammonia molecule. This indicates a strong coupling between electronical and vibrational excitation in the Na(NH3)n clusters.
Surface-aligned photochemistry: Photolysis of HCl adsorbed on LiF(001) studied by Rydberg-atom time-of-flight spectroscopy
Authors: Giorgi Javier B. , Kühnemuth Ralf , Polanyi John C.
Published: July 2002

Photolysis of HCl adsorbed on LiF(001) was studied by means of hydrogen Rydberg-atom time-of-flight (HRTOF) spectroscopy. Experiments were performed using 193.3 nm excimer laser radiation or 121.6 nm tripled dye-laser radiation for photolysis. The H-atom translational energy distributions using 193.3 nm (6.41 eV) radiation showed three main features: a high-energy channel corresponding to elastically scattered H atoms (EL) peaked at 1.85 and 1.65 eV (leaving behind Cl and Cl*, respectively); a second channel ∼1.2 eV lower in energy resulting from surface-aligned inelastic collisions (INEL, peaking at 0.6 eV); and a thermalized channel thought to be the product of multiple collisions and trapping of the scattered H. For 121.6 nm (10.2 eV) photolysis, the H-atom translational energy distributions showed four features: EL, with a maximum at 5.5 eV (the Cl and Cl* channels could not be separated at this high recoil energy); INEL(1), peaking at 3.2 eV; a more complex inelastic pathway, INEL(2), with a maximum at 0.5 eV; and, finally, a thermalized channel. Angular distributions for the elastically scattered H atoms indicated that they scattered from F− in the underlying LiF(001) at 40 and 48° off-normal for photodissociation of HCl(ad) at 193.3 and 121.6 nm, respectively. These two angles are far from 71°, the angle that would be observed from specular scattering of the H atom from the surface plane; this is strong evidence for the localized atomic scattering (LAS) identified in earlier studies performed in this laboratory. It appears that the higher the energy of the H projectile, the deeper it penetrates into the impacted surface atom, giving rise to the larger scattering angle. As in previous work, an exchange reaction was invoked to explain the ∼1.2 eV energy loss in the INEL(1) channel, together with observed retention of direction in H following this strongly inelastic encounter.
Vibrational energy relaxation of oxygen in liquid mixtures with argon
Authors: Everitt K. F. , Skinner J. L.
Published: July 2002

We calculate the vibrational energy relaxation rate constants for oxygen molecules in oxygen/argon liquid mixtures of various mole fractions at 77 K. Our results are uniformly about a factor of 5 larger than experimental values, which are in the range of 100 s−1. When the mole fraction-dependent results are scaled by their values for neat oxygen, theory and experiment are in excellent agreement.
Shape distribution and correlation between size and shape of star-branched tetrahedral lattice chains in athermal and theta systems
Authors: Zifferer Gerhard
Published: July 2002

By means of Monte Carlo simulation, linear (F=2) and star-branched tetrahedral lattice chains with F=3–12 arms of length n=480 connected to a hard core consisting of MIN(5,F+1) segments (the total number of segments thus ranging from 963 to 5765) were produced for athermal and theta conditions. Nonreversal random walks (random walks without backfolding bonds) were generated as a reference. Several quantities (asphericity factor δ*, prolateness factor S*, and shape factors sfi*) characteristic of the instantaneous shape of molecules—being based on the orthogonal components of the squared radius of gyration taken along the principal axes of inertia—were computed. The probability distributions of these quantities were calculated and their interdependence as well as their correlation with quantities characteristic of the size of configurations was analyzed. Shape and size of star-branched chains for athermal as well as for theta conditions are highly correlated as earlier found for linear chains and random walk stars. The broadness of distribution functions decreases with increasing number of arms, but remains appreciably for F=12, the system with the largest functionality evaluated.
Photoionization cross sections and asymmetry parameters for silane
Authors: Machado L. E. , Lee M.-T. , Brescansin L. M.
Published: July 2002

We present calculated results of photoionization cross sections and photoelectron angular distributions for ionization out of the two outermost 2t2 and 3a1 valence orbitals of silane for photon energies ranging from near threshold to 50 eV. The iterative Schwinger variational method in the exact static-exchange level is applied to obtain the continuum photoelectron orbitals. A generally good agreement is observed between our calculated cross sections and the experimental data available in the literature. In addition, a striking similarity has been observed between the photoionization properties of the 1t2 orbital of methane and the 2t2 orbital of silane.
Vibrationally mediated photolysis dynamics of H2O in the vOH=3 manifold: Far off resonance photodissociation cross sections and OH product state distributions
Authors: Votava Ondrej , Plusquellic David F. , Nesbitt David J.
Published: July 2002

Vibrationally mediated photodissociation dynamics of water on the first excited electronic state surface (Ã) has been studied with slit jet-cooled H2O prepared in the complete polyad of vOH=3 overtone stretch levels (|03+〉, |12+〉, |12−〉, and |03−〉). (Notationally, |n1n2±〉 refers to symmetric/antisymmetric combinations of local mode OH stretch excitation, roughly corresponding to n1 and n2 quanta in the spectator and photolyzed OH bond, respectively.) At 248 nm photolysis wavelength the Condon point for bond cleavage occurs in the classically forbidden region, primarily sampling highly asymmetric H+OH exit valley geometries of the potential energy surface. Rotational, vibrational, spin orbit, and lambda doublet distributions resulting from this “far off resonance” photodissociation process are probed via laser induced fluorescence, exploiting the high efficiency laser excitation and light collection properties of the slit jet expansion geometry. Only vibrationally unexcited OH products are observed for both |12±〉 and |03±〉 initial excitation of H2O, despite different levels of vibration in the spectator OH bond. This is in contrast with “near-resonance” vibrationally mediated photolysis studies by Crim and co-workers in the |04−〉 and |13−〉 manifold, but entirely consistent with theoretical predictions from a simple two-dimensional quantum model. Photolysis out of the rotational ground H2O state (i.e., JKaKc=000) yields OH product state distributions that demonstrate remarkable insensitivity to the initial choice of H2O vibrational stretch state, in good agreement with rotational Franck–Condon models. However, this simple trend is not followed uniformly for rotationally excited H2O precursors, which indicates that these Franck–Condon models are insufficient and suggests that exit channel interactions do play a significant role in photodissociation dynamics of H2O at the fully state-to-state level.
Selective generation and control of excited vibrational wave packets in the electronic ground state of K2
Authors: Pausch R. , Heid M. , Chen T. , Kiefer W. , Schwoerer H.
Published: July 2002

We investigate the generation and real-time monitoring of coherent vibrational wave packets in the electronic ground state of supersonic jet-cooled potassium dimers. Vibrationally excited wave packets with mean quantum numbers v̄=6 and v̄=11 are generated by a stimulated Raman process which is enhanced by an electronic resonance. Two ultrashort laser pulses of different wavelengths induce the pump and the dump process. The population of the final hot ground-state wave packets is successfully controlled by a variable time delay between the pump and the dump process, which enables us to wait with the dumping for the optimal Franck–Condon overlap between the intermediate and the predicted final vibrational wave packet in the electronic ground state.
Exact high-density limit of correlation potential for two-electron density
Authors: Ivanov Stanislav , Burke Kieron , Levy Mel
Published: July 2002

Present approximations to the correlation energy, Ec[n], in density functional theory yield poor results for the corresponding correlation potential, vc([n];r)=δEc[n]δ/n(r). Improvements in vc([n];r), are especially needed for high-quality Kohn–Sham calculations. For a two-electron density, the exact form of vc([n];r) in its high-density limit is derived in terms of the density of the system and the first-order wave function from the adiabatic perturbation theory. Our expression leads to a formula for the difference 2Ec[n]−∫vc([n];r)n(r)dr, valid for any two-electron density in the high-density limit, thus generalizes previous results. Numerical results (both exact and approximate) are presented for both Ec[n] and ∫vc([n];r)n(r)dr in this limit for two electrons in a harmonic oscillator external potential (Hooke’s atom).
Theory of laser cooling of polyatomic molecules in an electronically excited state
Authors: Wadi Hicham , Pollak Eli
Published: July 2002

A detailed theoretical study is presented for the vibrational population distribution of polyatomic molecules which results from electronic excitation from a thermal ground state. If the vibrational frequencies of the excited state are lower than the ground-state frequencies and if position shifts are not too large, then there exist excitation frequencies for which the excited-state vibrational distribution will be cooled in comparison to the ground state. An analytic theory for the vibrational distribution in the excited state is obtained by noting that the fast dephasing of a polyatomic molecule after excitation allows for the development of a Gaussian approximation for the excitation process. We show that the equilibrium energy distribution of a polyatomic molecule as well as the nascent distribution after excitation are well approximated as Gaussian. The average energy in the excited state is then found to be a quadratic function of the excitation frequency. If cooling takes place, it will usually be maximal for an excitation frequency which is to the red of the ground electronic state to ground electronic state excitation frequency. Cooling is not necessarily a quantum effect, it may also be found in the classical limit, in which one ignores quantization of the vibrational levels. The generality of the Gaussian approximation opens the way for theoretical treatment of anharmonic polyatomic molecules, using quantum Monte Carlo techniques.
Classical-quantum correspondence in multiphoton dissociation of diatomic molecules by chirped laser pulses
Authors: Kim Jung-Hoon , Liu Wing-Ki , Yuan Jian-Min
Published: July 2002

The classical and quantum dynamics of diatomic molecules driven by chirped laser pulses are investigated, with particular attention given to the dependence of the classical-quantum correspondence on the microscopic parameters of the systems. For this purpose, several molecules with different effective Planck’s constants are employed and their respective results are compared. Based on the bucket dynamics which has been successfully applied to explain the dissociation mechanism, we propose a criterion that determines whether a particular molecule will show a good correspondence between classical and quantum calculations in a given parameter region. It is found that, when the size of the bucket is bigger than the effective Planck’s constant, the classical predictions of dissociation probabilities agree well with the quantum mechanical results.
Quantum scattering study of electronic Coriolis and nonadiabatic coupling effects in O(1D)+H2→OH+H
Authors: Drukker Karen , Schatz George C.
Published: July 2002

In this paper we develop an approximate quantum scattering method capable of determining cross sections for reactive A+BC collisions, with A being an open shell atom and BC being a closed shell diatomic molecule. This method is based on time-independent coupled channel calculations, and absorbing potentials are used to describe reaction. The coupled channel expansion includes all electronic states of the atom that correlate to a selected atomic term, and a converged set of rotational states of the diatomic. Diatomic vibration is approximated as an adiabatic degree of freedom. The method is used to study the title reaction, including all five of the electronic surfaces that correlate to O(1D)+H2 as well as terms in the Hamiltonian that couple these surfaces. These couplings include: electronic and rotational Coriolis coupling, and electrostatic nonadiabatic coupling. Coriolis coupling causes all five states to interact and is most important at long range, while electrostatic coupling produces strong interactions between the 11Σ and 11Π states at short range (where these states have a conical intersection) and weak but non-negligible interactions between these states at long range. The most important three of the five surfaces (11Σ and 11Π, or 11A′, 11A″ and 21A′) and the electrostatic nonadiabatic coupling between them are taken from the recent ab initio calculations of Dobbyn and Knowles [A. J. Dobbyn and P. J. Knowles, Mol. Phys. 91, 1107 (1997); Faraday Discuss. 110, 247 (1998)], while the other surfaces (11Δ or 21A″ and 31A′) are based on a diatomics-in-molecules potential. Our results for the fully coupled problem indicate that Coriolis coupling is significant between the electronic fine structure levels so that electronic alignment is not strongly preserved as the reactants approach. However, the fine structure averaged reaction probability is relatively insensitive to the electronic Coriolis mixing. Averaged reaction probabilities from a centrifugal decoupled calculation where both electronic and rotational Coriolis interactions are neglected are in good agreement (10% or better) with the results of the fully coupled calculations. We find that electrostatic nonadiabatic coupling between the lowest Σ and Π states is significant, even at energies below the Π barrier where only the long-range nonadiabatic coupling between these states is important. As a result, the low energy cross section summed over electronic states receives a ≈10% contribution from the Π state. We find that the total cross section decreases with energy for energies below ≈3.5 kcal/mol and increases slightly at higher energies, with the increase due to reaction over the Π barrier. We find that the Π barrier contribution to the cross section is about twice that obtained by treating the reaction adiabatically, with the difference due to nonadiabatic dynamics on the 21A′ state.
Microwave spectrum and molecular structure of PNO
Authors: Okabayashi Toshiaki , Yamazaki Emi , Tanimoto Mitsutoshi
Published: July 2002

The microwave spectra of P14N16O and its isotopomers P15N16O and P14N18O were observed in a dc glow discharge plasma of a mixture of nitric oxide and hydrogen gases over solid red phosphorus placed on the stainless steel electrode. Rotational transitions of the parent P14N16O species were measured in the ground state as well as in the vibrationally excited ν1 (PN str.), ν2 (bend), and 2ν2 states. The l=0 substate of the 2ν2 state interacts with the ν1 state through a Fermi resonance. The rotational constants determined for the ground states of the three isotopomers yield the substitution structure, rs(PN)=151.6516(87) pm and rs(NO)=119.5025(80) pm.
Reaction rates for fluctuating barriers with asymmetric noise
Authors: Ankerhold Joachim , Pechukas Philip
Published: July 2002

Thermally activated escape over a high barrier fluctuating randomly in time is investigated for asymmetric flipping rates. By means of the Fokker–Planck approach approximate methods developed for symmetric noise are applied to calculate the relaxation eigenvector and ultimate relaxation rate for piecewise linear potentials in the Smoluchowski limit. The ranges of validity of these approaches are discussed. Furthermore, the exact result for weak thermal noise but arbitrary barrier flipping rates is derived and its dependence on the asymmetry studied in detail.
The cavity ringdown spectrum of the visible electronic system of thiophosgene: An estimation of the lifetime of the T1(ã  3A2) triplet state
Authors: Moule D. C. , Burling I. R. , Liu H. , Lim E. C.
Published: July 2002

To obtain insights into the photophysical properties of collision-free T1(ã 3A2) thiophosgene, Cl2CS, the cavity ringdown (CRD) spectrum of the T1←S0 absorption system was recorded under supersonic jet conditions and compared with the corresponding excitation spectrum of the total emission. It was found that none of the T1←S0 bands in the CRD spectrum appears in the excitation spectrum, indicating that the T1 thiophosgene decays almost exclusively by the nonradiative T1→S0 intersystem crossing (ISC). An estimation of the T1 nonradiative lifetime was made using the T1–S0 spin-orbit coupling and the Franck–Condon factors for the T1→S0 ISC based on the ab initio equilibrium structures and vibrational frequencies for the T1 and S0 states, computed at the MP2/6-31G(d,p) and MP4/6-31G(d,p) level of theory. The nonradiative life, calculated as the 1/e of the survival probability, is approximately 20 ps for barrier heights of 770–845 cm−1 and out-of-plane angles of 32.07°–32.69°. (The thiophosgene adopts a pyramidal conformation with the C=S bond bent from the ClCCl plane by about 32°.) The computed lifetime is comparable to the lower-limit lifetime of ∼50 ps obtained from the quantum yields ΦP⩽10−3ΦF with ΦF≈1.0 and the measured S1 fluorescence lifetime of 4 μs. The short T1 nonradiative lifetime of thiophosgene can be attributed to the large matrix elements of the T1–S0 spin-orbit coupling (V=150 cm−1) and the strong out-of-plane deformation of the T1 state relative to the planar S0 state, that provides the large Franck–Condon factors for the T1→S0 ISC. It is proposed that a similar pyramidal deformation is also responsible for the absence of phosphorescence from T1 thiocyclobutanone. Consistent with this supposition, T1 cyclopentanone, which is nearly planar at the thiocarbonyl carbon center, exhibits strong phosphorescence under similar experimental conditions.
Interpolated potential energy surface and reaction dynamics for O(3P)+H3+(1A1′) and OH+(3Σ−)+H2(1Σg+)
Authors: Bettens Ryan P. A. , Hansen Trevor A. , Collins Michael A.
Published: July 2002

An ab initio potential energy surface for the triplet state of OH3+ has been constructed at the MP2/6-311G(2d,p) level of theory. Classical simulations of the title collisions have been carried out to evaluate the rate coefficients for three reactions, including H3++O→H2O++H. Examination of the singlet-triplet energy gap across the triplet surface has shown no evidence for significant surface crossing effects on the dynamics.
Dynamics of collisional alignment in supersonic expansions: Trajectory studies of He+CO, O2, and CO2
Authors: Fair Joanna R. , Nesbitt David J.
Published: July 2002

Classical trajectory calculations have been performed on experimentally determined intermolecular potentials for He–O2, He–CO, and He–CO2 in order to simulate the collisional formation of rotationally aligned molecular distributions in a supersonic expansion. These calculations verify that multiple collisions between the light “diluent” gas and heavier “seed” rotor molecules result in a distribution of rotor molecules with negative alignment (a2<0), i.e., a preference for j perpendicular to the expansion axis. These rotational alignment effects are found to be robustly insensitive to collision energy and qualitatively similar for all three collision systems, thereby providing a useful basis for comparison with experimental studies. The asymptotic alignment is observed to depend strongly on the angular momentum, increasing monotonically with j. When analyzed on a collision-by-collision basis, this j dependence can be traced to gyroscopic stability, i.e., higher j states are classically more resistant to the collisional loss of alignment. In addition, collisional formation of the alignment is found to reflect comparable contributions from both elastic (mj-changing) and inelastic (j-changing) collisions. Finally, the calculations indicate that molecules with j aligned parallel to the expansion axis are correlated with faster average velocities than molecules with j perpendicular to the axis, which is consistent with the He+CO experimental studies of Harich and Wodtke [J. Chem. Phys. 107, 5983 (1997)], as well as the He+N2+ drift tube studies of Anthony et al. [J. Chem. Phys. 106, 5413 (1997)].
Theoretical studies of rotation induced Fermi resonances in HOCl
Authors: Chen Rongqing , Guo Hua , Skokov Sergei , Bowman Joel M.
Published: July 2002

Theoretical investigations of rotation induced Fermi resonances in HOCl are carried out using several different quantum mechanical methods. Due to shape differences of the eigenfunctions, nearby vibrational levels may be energetically tuned to form Fermi (or anharmonic) resonances by varying rotational quantum numbers. Such rotation induced Fermi resonances have been observed experimentally in HOCl, for example, for bright states (3,2,0) and (4,0,0) by Abel et al. [J. Chem. Phys. 104, 3189 (1996) and ibid. 106, 3103 (1997)]. Using an ab initio potential, this work shows that the (3,2,0) state is significantly mixed with the (2,3,3) state near J=28 and K=4, and J=14 and K=3, while the (4,0,0) state forms a Fermi pair with (3,2,1) near J=43 and K=8. The wave functions of the Fermi pairs display significant deformation due to the mixing. Both the rotation induced degeneracy and coupling strength are found to be important.
Infrared spectrum and energy levels of the CO dimer: Evidence for two almost isoenergetic isomers
Authors: Brookes Matthew D. , McKellar A. R. W.
Published: July 2002

The CO dimer has been studied in the region of the CO stretching vibration (2139–2152 cm−1) with a pulsed supersonic jet and a tuneable diode laser probe. By using both pinhole and slit jet configurations, and varying the distance downstream of the laser probe, the dimer spectrum was obtained over a range of effective rotational temperatures from about 1 to 12 K. Using this temperature dependence and the technique of combination differences, over 120 transitions belonging to 13 subbands were assigned in terms of 24 ground state (vCO=0) and 36 excited state (vCO=1) rotational energy levels of (CO)2. The levels fall into two groups, corresponding to isomers with effective intermolecular separations of either about 4.4 or 4.0 Å. The 4.4 Å isomer is the ground state, while the 4.0 Å isomer is a low-lying (0.88 cm−1) excited state. This energy ordering is inverted when vCO=1. Previous calculations suggest that both forms are planar and roughly T-shaped, with the 4.4 Å isomer in a C-bonded configuration and the 4.0 Å isomer in an O-bonded configuration. Measurements on an isotopically mixed dimer, C1613O–12O16O, indicate that C–O vibrational coupling between the monomer units in a dimer is very weak.
State-to-state unimolecular reaction dynamics of HOCl near the dissociation threshold: The role of vibrations, rotations, and IVR probed by time- and eigenstate-resolved spectroscopy
Authors: Callegari A. , Rebstein J. , Jost R. , Rizzo T. R.
Published: July 2002

We use infrared-visible double resonance overtone excitation to prepare HOCl molecules in single, well-characterized rotational levels of high OH stretching states just above the dissociation threshold on the ground potential energy surface. Combined with time-resolved laser induced fluorescence (LIF) detection of the OH product, this approach allows us to monitor the dependence of unimolecular dissociation rate on the angular momentum (J,Ka,Kc), total energy, and vibrational character of the state of the reactant molecule as well as on number of dissociation channels available to the OH product. Dissociation rates from single states of the parent molecule are distributed over more than two orders-of-magnitude in a fashion that appears largely independent of the excess energy and the total angular momentum. In several instances we observe a one-order-of-magnitude difference in dissociation rate between states that are nearby in rotational quantum number and/or energy. Superimposed on these state-to-state rate fluctuations is a general trend toward decreasing unimolecular dissociation rate with increasing Ka quantum number. Moreover, the measured rates, which range from 1 to 300 μs−1, are much slower than the predictions of statistical theories. We present a simple model calculation to explain the observed phenomena.
Dual transformation for wave packet dynamics: Application to Coulomb systems
Authors: Kawata Isao , Kono Hirohiko
Published: July 2002

A dual transformation technique that can deal with awkward Coulomb potentials is developed for electronic wave packet dynamics. The technique consists of the variable transformation of the Hamiltonian and the transformation of the wave function with a normalization constraint. The time evolution is carried out by the alternating-direction implicit method. The operation of the transformed Hamiltonian on the wave function is implemented by using three- and five-point finite difference formulas. We apply it to the H atom and a realistic three-dimensional (3D) model of H2+. The cylindrical coordinates ρ and z are transformed as ρ=f(ξ) and z=g(ζ), where ξ and ζ are scaled cylindrical coordinates. Efficient time evolution schemes are provided by imposing the variable transformations on the following requirements: The transformed wave function is zero and analytic at the nuclei; the equal spacings in the scaled coordinates correspond to grid spacings in the cylindrical coordinates that are small near the nuclei (to cope with relatively high momentum components near the nuclei) and are large at larger distances thereafter. No modifications of the Coulomb potentials are introduced. We propose the form f(ξ)=ξ[ξn/(ξn+αn)]ν. The parameter α designates the ρ-range where the Coulomb potentials are steep. The n=1 and ν=12 transformation provides most accurate results when the grid spacing Δξ is sufficiently small or the number of grid points, Nξ, is large enough. For small Nξ, the n=12 and ν=1 transformation is superior to the n=1 and ν=12 one. The two transformations are also applied to the dissociation dynamics in the 3D model of H2+. For the n=12 and ν=1 transformation, the main features of the dynamics are well simulated even with moderate numbers of grid points. The validity of the two transformations is also enforced by the fact that the missing volume in phase space decreases with decreasing Δξ.
Charge transfer to solvent (CTTS) energies of small X−(H2O)n=1–4 (X=F, Cl, Br, I) clusters: Ab initio study
Authors: Majumdar D. , Kim Jongseob , Kim Kwang S.
Published: July 2002

Charge transfer to solvent (CTTS) energies of small halide–water clusters, X−(H2O)n=1–4 (X=F, Cl, Br, I) have been studied using first-order configuration interaction as well as time dependent discrete Fourier transform density functional methods. The only available experimental data are the recently reported CTTS energies for I−(H2O)n=1–4 clusters by Johnson and co-workers [D. Serxner, C. E. H. Dessent, and M. A. Johnson, J. Chem. Phys. 105, 7231 (1996)]. These results are in good agreement with our predicted values. The calculated CTTS energies indicate that there is regularity in the change of CTTS energies with respect to the change of halide anion as well as the cluster size. Our investigations have shown that this observed trend of CTTS energies of X−(H2O)n clusters could be quantitatively explained by the ionization potential of the halide anions and the binding energies of the respective clusters.
Phase transitions in alloy clusters physically adsorbed on model surfaces
Authors: Ocasio Mayra , López Gustavo E.
Published: July 2002

The lowest energy equilibrium structure for Ne19, Ar19, Ne9Ar10, and Ne12Ar7 clusters adsorbed on model surfaces was determined using simulated annealing techniques. In all cases, two-dimensional structures were obtained and in the case of the alloy clusters, a core structure was observed. Phase transitions were identified by calculating anomalies in the constant volume heat capacity as a function of temperature. In all cases, the usual melting and evaporation transitions were identified. In the case of alloy systems, various low temperature transitions were observed. These transitions are associated with order–disorder transitions which are similar to what is observed in alloy solids in bulk. The effect that surface corrugation has on these transitions was also analyzed.
Two-dimensional vibrational spectroscopy. IV. Relationship between through-space vibrational coupling and intermolecular distance
Authors: Hahn Sangjoon , Kwak Keyngwon , Cho Minhaeng
Published: July 2002

By means of the ab initio calculation, the second-order nonlinear derivatives of the dipole moment and polarizability of a dimer, CO⋯HCl, with respect to the two, CO and HCl, stretching modes are found to be inversely proportional to r3, where r is the intermolecular distance. The observed r-dependencies are interpreted by taking into account the dipole–induced-dipole interaction. Thus, it is shown that the intermolecular distance between two vibrational chromophores can be estimated by quantitatively examining the cross-peak intensity in a given 2D (two dimensional) vibrational spectrum.
Ab initio calculation of energies and lifetimes of metastable dianions: The C22− resonance
Authors: Sommerfeld T. , Tarantelli F. , Meyer H.-D. , Cederbaum L. S.
Published: July 2002

Most small dianions known in the solid state and solutions cannot exist as isolated entities and decay in the gas phase by electron autodetachment. These dianions show rare-gas-like closed-shell electronic ground states and represent a new type of metastable system. Here we study the prototype closed-shell resonance C22− in the framework of the complex absorbing potential method. We investigate in detail a number of unsettled methodological issues. In particular, there is no “natural” choice of orbital set for closed-shell metastable states and we study several orbital sets as well as other basis set and correlation effects on resonance energy and width. Closed-shell resonances typically show several open decay channels and we compute partial widths for the three open channels of C22−. Finally, we study the complex potential energy curve and compare our bond lengths and vibrational frequencies with geometrical parameters which have been obtained ignoring the metastable character of C22−.
Including dispersion in configuration interaction-singles calculations for the spectroscopy of chromophores in solution
Authors: Canuto Sylvio , Coutinho Kaline , Zerner Michael C.
Published: July 2002

In this paper we prove that a configuration interaction electronic structure calculation on a supermolecule that contains only single excitations includes dispersion interactions between the two subsystems when energy differences are taken between the Hartree–Fock (molecular orbital) ground state and low energy excited states in which single excitations dominate. This theorem is proven up to second order in perturbation theory.