bluuebunny/crossref_metadata_2025 huggingface dataset:
ΔM
j transitions in homonuclear molecule scattering off corrugated surfaces. Square and rectangular lattice symmetry and purely repulsive interaction
Authors: Proctor T. R. , Kouri D. J. , Gerber R. B.
Published: February 2003

In this paper, we present the first formal and computational studies of Δmj transitions occurring in homonuclear molecule-corrugated surface collisions. The model potential is a pairwise additive one which correctly incorporates the fact that Δmj transitions occur only for corrugated surfaces (provided the quantization axis is chosen to be the average surface normal). The principal results are: (a) Δmj transitions are extremely sensitive to lattice symmetry; (b) strong selection rules obtain for specular scattering; (c) the magnitude of Δmj -transition probabilities are strongly sensitive to surface corrugation; (d) the Δmj transitions depend strongly on diffraction peak; (e) the ratio of molecular length to lattice dimension (r/a) has a strong influence on the magnitude of Δmj -transition probabilities [with the probabilities increasing as (r/a) increases]; (f) Δmj rainbows are predicted to occur as a function of the (r/a) ratio increases; (g) Δmj transitions and the Δmj rainbow are expected to accompany Δj-rotational rainbows; (h) such magnetic transition rainbows accompanying Δj rainbows are suggested as an explanation of recent experimental observations of quenching of NO polarization for larger Δj transitions in NO/Ag(111) scattering.
Linear response theory of long range solvent effects on nπ* circular dichroism of helical polypeptides
Authors: Rabenold David A. , Rhodes W.
Published: February 2003

A linear response theory is presented for the long-range solvent effect on the nπ* circular dichroism spectra of long helical polypeptides. A time dependent Hartree approximation that includes intrapolymer electrostatic field effects is employed. Periodic boundary conditions are used to describe the polymer while the solvent is considered to be a polarizable isotropic continuum. For an α-helical system in water the solvent is predicted to cause a 23% decrease in the nπ* rotational strengths.
Calculation of partial widths and isotope effects for reactive resonances by a reaction-path Hamiltonian model: Test against accurate quantal results for a twin-saddle point system
Authors: Skodje Rex T. , Schwenke David W. , Truhlar Donald G. , Garrett Bruce C.
Published: February 2003

We calculate the partial widths of three collisional resonances in a collinear system with mass combinations HFH and DFD on a low-barrier model potential energy surface. We compare accurate quantal results to results obtained with a reaction-path Hamiltonian model in which the resonances are interpreted as quasibound states trapped in wells of adiabatic potential curves and their decay probabilities are calculated by semiclassical tunneling calculations and a Feshbach golden-rule formula with the decay mediated by an internal centrifugal interaction proportional to the curvature of the reaction path. The model successfully predicts when vibrationally nonadiabatic decay dominates over the adiabatic mechanism for decomposition of the resonances and it predicts the nonadiabatic partial widths with an average error of 25%.
Electron energy loss spectroscopy of molecular fluorine
Authors: Wang R.-G. , Wang Z.-W. , Dillon M. A. , Spence David
Published: February 2003

We have obtained electron energy loss spectra in molecular fluorine in the energy-loss range 6–24 eV at scattering angles between 3 ° and 9 ° for an incident electron energy of 200 eV. For energy losses above 11.5 eV, our spectra are in excellent agreement with earlier electron scattering and photoabsorption measurements and we resolve for the first time the vibrational levels of the F 1Πg parity forbidden state, which is the lowest Rydberg state of F2 and serves as the lower level of the orange emission band system. We are able to unambiguously reassign vibrational quantum numbers to the F 1Πg state, which have been in doubt for several decades. In the region 5–11.5 eV, of particular interest to F2 laser modeling, we are unable to find any electronic transition which cannot be attributed to minor, identifiable, impurities known to exist in F2.
Non-Markovian theory of activated rate processes. IV. The double well model
Authors: Carmeli Benny , Nitzan Abraham
Published: February 2003

The transition rates associated with a particle moving in a double potential well under the influence of thermal noise and friction is considered as a generalization of Kramers’ theory of activated rate processes. We obtain expressions for these transition rates which are valid for all friction and for a general (non-Markovian) interaction between the particle and its thermal environment. Nonthermal equilibrium effects in the steady state distribution in the well as well as effects of trajectories returning unrelaxed from the far wall are explicitly taken into account. The results reduce to all the previously obtained results of the single well model. We use the theory to analyze the experimental results of Hasha, Eguchi, and Jonas.
Finite duration of collisions and vibrational dephasing effects on the Ar broadened HF infrared line shapes: Asymmetric profiles
Authors: Marteau Ph. , Boulet Christian , Robert D.
Published: February 2003

The existence of an asymmetry for the HF rovibrational absorption isolated lines induced by Ar pressure is clearly observed. This confirms the expected behavior based on a previous theoretical analysis of the finite duration effect on the molecular line shapes in the core region at moderate densities. The observed profile is well represented by the addition to the usual Lorentzian line shape of a dispersion component. This component is proportional to a characteristic parameter, linear with respect to the density of perturber and explicitly connected to the intermolecular potential. The a priori calculation of this parameter from an empirical potential leads to a super-Lorentzian shape in the low frequency side in agreement with the experiment. Moreover, the expected increase of the asymmetry when going from the fundamental to the harmonic band is well observed. Such an increase results from the modification of the vibrational dephasing process due to molecular anharmonicities.
A crossed molecular beam study of NO+O3→NO*2 +O2: The effect of ozone rotational energy
Authors: Kahler Carol C. , Ansell Ellen , Upshur Carol Merten , Green William H.
Published: February 2003

The chemiluminescent branch of the reaction between NO and O3 to produce NO*2 and O2 was studied by crossing a supersonic NO beam with a variable temperature effusive O3 beam and monitoring visible light emission. Signal was measured for two ozone beam source temperatures, 300 and 181 K, and a ratio of S300/S181=3.0±0.4 was obtained. This ratio represents the effect of changing translational energy as well as internal O3 energy. The translational energy contribution to the signal ratio was calculated using a collision energy dependence of σ ∝ (E/2.3−1)2.05, which was derived from experiments in which the temperature of the supersonic NO source was varied. The effect of vibrational energy was accounted for using published rate constant ratios. The remaining signal ratio of 2.3±0.3 is attributed to the effect of ozone rotational energy, which results in a dependence of (Erot)n where n=1.3–1.8.
Tests of using large valence spaces in quasidegenerate many-body perturbation theory: Calculations of O2 potential curves
Authors: Takada Toshikazu , Freed Karl F.
Published: February 2003

Potential curves are calculated for the oxygen molecule using the effective valence shell Hamiltonian ℋ v method based on quasidegenerate many-body perturbation theory (QDMBPT). Spectroscopic constants of the 12 bound valence states are compared with those obtained from experiment and those calculated by Saxon and Liu from extensive MCSCF and CI calculations. The excellent agreement indicates that the quasidegenerate many-body perturbation scheme is a reliable ab initio method even with larger valence spaces in which the quasidegenerate condition is strongly violated. Approximations to the correlated effective valence shell Hamiltonian full valence shell CI matrix are tested, and new simplifications are found to give results very similar to the complete calculations, thereby providing useful approximation schemes for larger systems. A remarkable property of the QDMBPT-ℋ v method is the fact that all the states at a given internuclear distance are calculated using a common set of core and valence orbitals; the perturbation formulation accounts for the valence orbital readjustments normally required in traditional configuration interaction calculations.
Energy variational principle and charge conservation for a many-particle system in a magnetic field
Authors: Kobe Donald H. , Kennedy Paul K.
Published: February 2003

When the energy variational principle is applied to a many-particle system in a magnetic field, the trial wave function is generally chosen without regard to the gauge used for the vector potential. This procedure leads to gauge-dependent energy expectation values and charge nonconservation. By mutiplying the trial wave function by a phase factor with a spatially dependent phase function and varying the energy expectation value with respect to the phase function, the charge conservation condition is obtained and a gauge-invariant energy expectation value results.
An energetically low-lying silacyclopropyne isomer of SiC2
Authors: Grev Roger S. , Schaefer Henry F.
Published: February 2003

We have discovered a low-lying cyclic isomer of Si–C–C which is best described as a three-membered ring with a weak carbon–carbon triple bond. In these theoretical studies a double zeta plus polarization basis set was used initially. At the self-consistent-field (SCF) and two-configuration (TC) SCF levels of theory the ring structure is a transition state leading to linear Si–C–C. A configuration interaction (CI) treatment at the SCF optimized cyclic geometry, however, show it to lie 1.1 kcal/mol below the linear structure. Extension of the basis set to include a second set of d functions on each atom gives a final prediction that the ring structure lies ∼5 kcal below linear SiCC.
The quantum parameter and critical binding of helium dimers
Authors: Ghassib H. B.
Published: February 2003

A recent paper by Uang and Stwalley is taken as a starting point for relating the quantum parameter to the critical binding of diatomic helium molecules in a simple and direct manner. It is conjectured that the relations examined here may also prove to be useful in investigating the general problem of dimers and n-mers in helium fluids within a unified picture.
Homogeneous nucleation in vinyl acetate vapor
Authors: Chowdhury M. A.
Published: February 2003

Critical supersaturations for the the homogeneous vapor phase nucleation of vinyl acetate have been measured as a function of temperature, in an upward thermal diffusion cloud chamber. The experimental data are found to be in reasonable agreement with the classical (Becker–Doering–Zeldovich) theory of nucleation.
A new analysis of charge transfer and polarization for ligand–metal bonding: Model studies of Al4CO and Al4NH3
Authors: Bagus Paul S. , Hermann K. , Bauschlicher Charles W.
Published: February 2003

A new analysis of the intra-unit charge polarization and inter-unit donation for the interaction of ligands with metals is presented. The analysis is based on the calculation of SCF wave functions with constraints imposed on the variational space used for the molecular orbitals of metal–ligand clusters. With this approach, various kinds of ligand and metal charge rearrangement can be studied separately and their energetic contribution to the bonding can be determined. For the first time, this contribution for metal to ligand and ligand to metal donation is established. The constrained space orbital variation CSOV approach also clearly shows the similarities and the differences between the bonding of CO and NH3. The results are obtained for Al4CO and Al4NH3 clusters chosen to simulate the adsorption of the ligands at an on-top site of the Al(111) surface. However, the features found for these model systems have significance for the analysis of ligand–metal bonding in general. The analysis gives new estimates for the importance of charge donations which are different from those commonly believed. It also shows new important features of the interaction. One unexpected feature of the metal–NH3 interaction is that the electrostatic attraction of the effective dipole moments of the metal and ligand units makes an important contribution to the bond. Inter-unit donation of charge does not seem to be important for this system.
Conformatonal analysis of n-cyclophanes by molecular mechanics. II. 5-, 6-, and 7-metacyclophane
Authors: Carballeira L. , Pereiras A. J. , Rios M. A.
Published: February 2003

The optimal geometries, gas phase thermodynamic functions, heats of formation, and strain energies of 5-, 6-, and 7-metacyclophane have been calculated using a molecular mechanical method developed by Boyd et al., and the results compared with available experimental data and with similar results previously obtained for the n-paracyclophane isomers. In general, strain energies in the meta series are some 12 kcal/mol less than in the para series, the strain energy of an n-metacyclophane being roughly that of an (n+1)-paracyclophane. The meta compounds may thus be said to be one degree more stable than the para compounds.
The electrostatic field and the molecular dipole moment in the polymorphs of ice
Authors: Johari G. P.
Published: February 2003

The magnitude and the direction of the electrostatic field at the site of a water molecule in hexagonal and cubic ice and in ice III, IV, V, and VI have been calculated. The calculations take into account neighbors up to the third shell and the probability of each orientation of these neighbors in a dendritic manner. The electrostatic field has the direction of the dipole moment of the central molecule in all the polymorphs, but its magnitude decreases with increase in the density of the polymorphs. The field at a molecular site is lower the lower the symmetry of the molecular site. The increase in the dipole moment of water molecule resulting from this field has been calculated for all molecules in the unit cell, assuming that the increase is directly proportional to the field strength. The weighted average dipole moment in the ices is 30%–45% higher than the dipole moment of a water molecule in the vapor phase. The largest increase is for cubic ice and the smallest for ice VI. All calculated values of the dipole moments substantially differ from values anticipated from the model of point dipole at the center of an isotropically polarizable spherical cavity. The contribution to relative permittivity due to orientation polarization has been calculated using these values of the dipole moment and the known dipolar correlation factor. The relative permittivity agree reasonably well with the measured values for all polymorphs of ice.
The structure of liquid water at an extended hydrophobic surface
Authors: Lee Chyuan-Yih , McCammon J. Andrew , Rossky P. J.
Published: February 2003

Molecular dynamics simulations have been carried out for liquid water between flat hydrophobic surfaces. The surfaces produce density oscillations that extend at least 10 Å into the liquid, and significant molecular orientational preferences that extend at least 7 Å into the liquid. The liquid structure nearest the surface is characterized by ‘‘dangling’’ hydrogen bonds; i.e., a typical water molecule at the surface has one potentially hydrogen-bonding group oriented toward the hydrophobic surface. This surface arrangement represents a balance between the tendencies of the liquid to maximize the number of hydrogen bonds on the one hand, and to maximize the packing density of the molecules on the other. A detailed analysis shows that the structural properties of the liquid farther from the surface can be understood as effects imposed by this surface structure. These results show that the hydration structure of large hydrophobic surfaces can be very different from that of small hydrophobic molecules.
Linear excitations and the stability of the hard sphere glass
Authors: Stoessel J. P. , Wolynes P. G.
Published: February 2003

We extend Fixman’s self-consistent phonon theory of the thermodynamic properties and fluctuations of the hard sphere crystal to an amorphous lattice. In doing so we develop a theory of lattice vibrations in amorphous materials via a perturbation theory around the Einstein oscillator approximation. This development is analogous to the mean spherical model description of electronic excitations in liquids. The theory exhibits a mechanical instability rather close to computer simulated glass transitions, suggesting the possibility that the simulations probe only linear stability because of the short simulation times.
A charge transfer model of Lewis base intercalation compounds of FeOCl
Authors: Eckert H. , Herber R. H.
Published: February 2003

A number of FeOCl Lewis base intercalation compounds have been prepared and characterized by chemical analysis, x-ray powder diffraction, and Mössbauer spectroscopy. Detailed temperature dependent Mössbauer studies show these materials to be mixed valence compounds with electron exchange occurring on the time scale of the Mössbauer effect at temperatures above 100 K. In the magnetically ordered state the relative orientation of the EFG principal axis with respect to the magnetic field axis is a function of the nature of the intercalated material. These results indicate that a charge transfer model (as contrasted to a local interaction model) can provide an appropriate description of the guest-host interaction in these compounds.
Bistability, mushrooms, and isolas
Authors: Ganapathisubramanian N. , Showalter Kenneth
Published: February 2003

The iodate oxidation of arsenous acid exhibits single-hysteresis bistability in a continuous flow stirred tank reactor. Other patterns of multiple stationary states including mushrooms and isolas are exhibited by this system when a constant flow of solvent is introduced to the CSTR in addition to the usual flow of reactants. A simple empirical rate law model provides a near quantitative description of the behavior. This model is analyzed and compared to other model systems.
Hydrogen atom migration in the oxidation of aldehydes: O(3P)+H2CO
Authors: Dupuis Michel , Lester William A.
Published: February 2003

We report an ab initio study of hydrogen atom migration in methylenebis(oxy)H2CO2(3B2) to form triplet formic acid HCOOH (3A1). From HF, MCHF, and CI calculated energy barriers, we estimate the activation energy to be no less than 30 kcal/mol. We conclude that the hydrogen migration channel is not accessible in recent room temperature experiments on the O(3P)+H2CO reaction
Excitation decay due to incoherent energy transfer: A comparative study by means of an exact density expansion
Authors: Knoester J. , Van Himbergen J. E.
Published: February 2003

In this paper we consider a system of identical, randomly distributed donors, between which incoherent energy transfer takes place, described by coupled rate equations. It is proved, that the well-known diagrammatic series expansion of Gochanour, Andersen, and Fayer for the self-energy, while not an expansion in terms of the density of donors, does in fact imply an exact density expansion for the configuration averaged probability ρ(t), that an initially excited donor will still be excited at time t. Although this diagrammatic expansion has derived its greatest appeal from the search for approximate self-consistent solutions to the generalized diffusion coefficient, the exact density expansion for ρ(t) can also serve very useful purposes. This is demonstrated by a comparative study of model results for ρ(t), whose absolute accuracy or validity can be assessed.
Collision complex formation in the reactions of formyl radicals with nitric oxide and oxygen
Authors: Langford Andrew O. , Moore C. Bradley
Published: February 2003

The laser photolysis/resonance absorption technique has been used to measure absolute rate constants for the reactions of HCO and DCO with NO and O2 at 295 K. The values of (1.26±0.2)×10−11 and (4.65±0.6)×10−12 cm3 molecule−1 s−1 for the reactions of HCO with NO and O2, respectively, are in good agreement with previously reported values. Deuterium substitution was found to increase the rate constants by 25% and 10% in reactions with NO and O2, respectively. These results are not explained within the framework of a direct hydrogen transfer mechanism and suggest that collision complexes are formed in both reactions. Simple RRKM calculations for a complex with a lifetime long compared to vibrational energy redistribution times semiquantitatively reproduce the experimental isotope effects, as well as the experimentally observed decreases in rate constant which result from vibrational excitation of the HCO or from an increase in temperature.
Collision dynamics of three interacting atoms: Model calculations of H+H2 resonances
Authors: Kuruoglu Zeki C. , Micha D. A.
Published: February 2003

Collisions of H and H2 at thermal energies are studied within a three-body theory of atom–diatom rearrangement collisions. A previously developed general formalism based on a diabatic electronic representation is shown to be equivalent, for this system, to a treatment in terms of atomic spins. It further provides a novel approach to nuclear exchange symmetry. The interaction potential is parametrized by introducing a minimal valence-bond basis, and the collision dynamics is described with the Faddeev equations. These equations are reduced to two-body form, and are analyzed in terms of angular momentum components. A detailed description is given of the numerical procedure applied to the coupled integral equations that result from expanding in diatomic square integrable basis functions. Those equations are solved in momentum variables using quadrature techniques, and provide K-matrix elements. Converged calculations have been done for an s-wave model and for energies up to around the v=3 threshold of H2. Resonances have been located from the singularities of the K-matrix elements and are found at 0.203 eV above the v=1 threshold of H2, with a width of 0.167 eV, and at 0.202 eV above the v=2 threshold with a width of 0.274 eV. The position of the first resonance is in very good agreement with previous results of three-dimensional calculations. The second resonance is above the energy range of previously published work.
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o multiple root optimization MCSCF study of the C∞v/C
s excitation spectra and potential energy surfaces of N2O
Authors: Hopper Darrel G.
Published: February 2003

This paper presents the results from extensive theoretical studies of the electronic structure for nitrous oxide. These studies have included the development of correlated ab initio MCSCF/CI wave functions for each of some 30 states, including MCSCF excited root/same symmetry calculations where necessary. Also, in the case of seven lower states, the potential energy hypersurfaces and dipole moment functions have been scanned using both MCSCF/CI and SCF wave functions. Vertical spectrum calculations are the most accurate and have been carried out at several levels of sophistication, including SCF, MCSCF, and MCSCF/CI correlation for DZ, DZ+diffuse, DZ+d, and DZ+d+diffuse one-electron bases. From all of these calculations the adiabatic excitation spectrum is established and the behavior of a number of states upon dissociation under C∞v and Cs symmetry is examined. Adiabatic correlation diagrams are then constructed to summarize many of the results and to relate them to the asymptotic spectra O(k sL) +N2 ( j sΛ) and NO( j sΛ) +N(k sL). In the light of the present work, optical and electron impact data are interpreted and assigned. It is found that each feature observed in the 4–8 eV region is supported by an electronic transition (s) allowed only under Cs symmetry. These features derive such intensity as they have in consequence of vibrational motion of the molecule in its normal, linear ground state. In distinct contrast, the features observed above 8 eV are shown to involve vertical excitations allowed under C∞v as well as Cs symmetry. Several of these transitions have not been assigned previously. For others, such as the all important D1 Σ+ state, the present results provide the necessary hard evidence for making an assignment. Also reported are correlated ab initio, DZ+d treatments for the C2v isomer 1 1A1 (1 1A′) of normal nitrous oxide, and for the O–N2 van der Waals interaction. The 1 1A1, or ‘‘ring,’’ state is shown to be bound relative to O(1D)+N2 by some 1.2 eV. The O(3P)–N2 long range 1 3Π ab initio interaction potential is reasonably well fit by an exp-6 functional form V(r)=37.757 exp(−r/0.566 569)−47.139 r−6 in a.u.
Bounds on the atomic electronic density at the nucleus
Authors: King Frederick W.
Published: February 2003

A lower bound estimate for the electron density at the nucleus ρ(0) for S state atoms and ions is derived. A refinement is made to the best upper bound estimate for ρ(0) for atomic systems available in the literature. A numerical test of the quality of the bounds is presented for members of the helium isoelectronic sequence.
An adiabatic study of the X 1Σ+ and A 1Σ+ states of BeH+
Authors: Bishop David M. , Cheung Lap M.
Published: February 2003

Using a 330-term configurational-interaction wave function with an elliptical orbital basis, the potential curves for the electronic ground state (X 1Σ+), and first excited state (A 1Σ+) of BeH+ have been determined. For the X 1Σ+ state the curve is close to the best hitherto published and for the A 1Σ+ the curve is better than any previous results. With the corresponding wave functions, the adiabatic corrections to these curves have been found for the first time.
Vinylidene: Potential energy surface and unimolecular reaction dynamics
Authors: Carrington Tucker , Hubbard Lynn M. , Schaefer Henry F. , Miller William H.
Published: February 2003

New quantum chemistry calculations (with a triple zeta plus polarization basis set, and a single and double configuration interaction) have been carried out to determine the equilibrium points and the transition state for the vinylidene (H2C=C:)→acetylene (HC≡CH) isomerization. A classical barrier height (i.e., with no zero point energy effects) of 6.3 kcal/mol is obtained, and application of the Davidson correction for unlinked clusters reduces this to 5.4 kcal/mol. Our best estimate is that the true classical barrier lies in the range 2–4 kcal/mol. The dynamics of the vinylidene/acetylene isomerization is described with the framework of the reaction path Hamiltonian. The lifetime of vinylidene (in its ground vibrational state) with respect to this process is calculated to be 0.24 to 4.6 ps for a classical barrier of 2 to 4 kcal/mol. This lifetime decreases by a factor of ∼2 if one quantum of the CH2 scissors mode of vinylidene is excited, but is predicted to increase somewhat if a quantum of the C–C stretch is excited. These results are all consistent with the recent experimental observation of vinylidene via photodetachment of C2H−2.
Existence of cooperative absorption lines for Yb–(OH,OD) pairs: Absolute oscillator strengths
Authors: Dexpert-Ghys J. , Auzel F.
Published: February 2003

Due to its energy scheme simplicity, Yb3+ has been chosen in order to investigate the (Ln3+OH−) cooperative pair lines predicted by Dexter. For the first time by direct absorption such pair lines have been shown to exist in Yb(OH3),Yb(OOH) microcrystals and in [Yb(C2H5SO4),9H2O],[YbCl3,6H2O] crystals and in their aqueous solutions, either with light or heavy water. Absolute oscillator strengths of pairs at sum frequencies of the Yb electronic line (2F7/2−2F5/2) and of the ν1 stretching (OH)−, (OD)− vibrations have been found to be respectively (3.5±0.5)×10−9 and (2.6±0.3)×10−9. Results are discussed in terms of the cooperative model of Stavola–Dexter and of the vibronic sideband description, showing what is common to both models and what is different.
Electron diffraction studies of hot molecules. III. Stretching and bending anharmonicity in CF3Cl
Authors: Bartell Lawrence S. , Vance William , Goates Steven R.
Published: February 2003

Bond lengths, amplitudes of vibration, and asymmetries in distribution of nonbonded atom pairs were determined for CF3Cl at temperatures ranging from 300 to 1220 K. The CF but not the CCl bond length followed the commonly invoked approximation Δr=(3a/2)Δl2+ΔK+Δδr. The coefficients of skewness of nonbonded radial distribution peaks revealed substantial bending anharmonicity comparable to that detected in a previous study of binary fluorides and accounted for by the valence-shell-electron-pair repulsion force field in its points-on-a-sphere representation.
Resonance Raman, fluorescence, and phosphorescence of palladium phthalocyanine in Shpol’skii matrices
Authors: Huang Tzer-Hsiang , Chen Wen-Hsiung , Rieckhoff K. E. , Voigt E. M.
Published: February 2003

The degenerate states of the first excited singlet and triplet of palladium phthalocyanine (PdPc) in Shpol’skii matrices at 4.2 K are lifted 67±2 cm−1 by the crystal field stabilized Jahn–Teller effect. Strengths of emission from the split triplets follow the Boltzmann relation, whereas those from the split singlets do not. The singlet and triplet emission yields and lifetimes indicate a reduction of nonradiative rates in Shpol’skii matrices as compared to those in alpha-chloronaphthalene (αCIN) observed previously. The concurrence of resonance Raman and relaxed fluorescence from the singlets in this system was verified from the study of the emission spectra. In deexcitation, it was found that the incoherent resonance fluorescences was insignificant as compared with the coherent resonance Ramans and, for some fundamental vibrations, the intersystem crossing (ISC) processes were not negligible. The vibronic spectra in emission and excitation are characterized by very weak intra- and interstate vibronic interaction and electron–phonon interaction as usual for metal Pc’s in Shpol’skii matrices. The absence of mirror symmetry between absorption and fluorescence for the totally symmetric vibrations, except the 1536 cm−1 fundamental, was found and could be attributed to the non-Condon effect induced by the vibronic interaction. The enhancement in the intensity of phosphorescence excitation over the fluorescence excitation and the appearance of extra vibronic bands in phosphorescence were observed and successfully explained.
Rate coefficients for the reaction HS+NO+M→HSNO+M (M=He, Ar, and N2) over the temperature range 250–445 K
Authors: Black G. , Patrick R. , Jusinski L. E. , Slanger T. G.
Published: February 2003

HS radicals were generated by photodissociation of H2S at 193 nm and their disappearance monitored by LIF. The three-body recombination of HS with NO has been studied over the temperature range 250–445 K using He, Ar, and N2 as diluent gases. The temperature dependences of the low-pressure, three-body rate coefficients are given by 10−(22.56±0.60)T−(3.28±0.27), 10−(23.27±0.50)T−(2.98±0.20), and 10−(24.43±0.94)T−(2.48±0.36) cm6 molecule−2 s−1 for He, Ar, and N2, respectively. A tentative value of (2.7±0.5)×10−11 cm3 molecule−1 s−1 for the high pressure limiting rate coefficient over the range 250–300 K is suggested on the basis of extrapolation. These kinetic data are evaluated in terms of unimolecular rate theory.
Angular distributions of electrons from resonant two-photon ionization of molecules
Authors: Hansen John C. , Berry R. Stephen
Published: February 2003

A phenomenological theory is developed for the angular distribution of photoelectrons produced by resonant two-photon ionization of diatomic molecules. The physical situation described by the theory is optical excitation of the target molecule to a real electronically excited intermediate state followed by ionization induced by absorption of a second photon of a frequency different from the first. If experiments include all possible polarizations, up to ten independent phenomenological parameters can be obtained. The phenomenological parameters are given in terms of microscopic physical parameters appropriate for diatomic molecules, transition amplitudes to the various final state channels for the photoelectron expressed in a molecule-fixed coordinate system. The form of this reduction depends upon which Hund’s coupling case is used to describe the molecular states and upon whether fine structure is resolved in the photoelectron spectrum. The theory is illustrated for a few specific examples.
UV laser photodissociation of molecular ions
Authors: Kutina R. E. , Edwards A. K. , Pandolfi R. S. , Berkowitz J.
Published: February 2003

An apparatus is described for performing molecular ion photofragment spectroscopy with an ultraviolet laser. Preliminary results are given for the photodissociation of H+2 , HD+, D+2 , D2O+, OD+, ND+3 , and ND+2 , using a laser energy of 6.42 eV. Where energetics permit alternative modes of dissociation, selectivity is observed and attributed to selection rules and correlation rules. With polyatomic ions, the distribution of recoil energies provides insight into the partition of excess energy between internal and kinetic energies of products. In the decomposition of ND+3 , results are obtained which call into question some recently reported thermochemical quantities.
Generalized reorientation cross sections. II. Scattering frame transformations and propensity rules
Authors: Monchick Louis
Published: February 2003

The generalized molecular reorientation cross sections derived in the first paper of this series are reformulated using an unsophisticated version of the internal angular momentum coupling scheme introduced by the Wisconsin school. In particular, we consider the transformation from quantization axes that diagonalize the wave amplitude in the magnetic rotational angular momentum indices to space-fixed quantization axes in either the center of mass or laboratory frames. The reformulated coupling scheme makes it apparent that these transformations bias the cross sections toward orientation conserving propensity rules. This is advanced as the reason why the self-same, close-coupled calculations have been used to support orientation and jz conserving propensity rules.
The cyclic, two-hydrogen bond form of the HO2 dimer
Authors: Fitzgerald George , Schaefer Henry F.
Published: February 2003

Among possible forms of the hydroperoxyl radical dimer H2O4, one of the most appealing and plausible is the six-membered ring 4. This cyclic isomer of H2O4 has been investigated using nonempirical molecular electronic structure theory. For the lowest triplet state of H2O4, the single configuration self-consistent field (SCF) method was used in conjunction with double zeta (DZ) and double zeta plus polarization (DZ+P) basis sets. At both levels of theory, the six-membered ring is predicted to be a minimum on the H2O4 potential energy hypersurface. The DZ SCF and DZ+P SCF dimerization energies are 4.5 and 4.9 kcal, respectively. Vibrational frequencies are predicted within the harmonic approximation and compared with the analogous monomer predictions and with the recent experimental findings of Diem, Tso, and Lee. It is concluded that the cyclic HO2 dimer is composed of two weak hydrogen bonds, each about one-half the strength of that observed for the water dimer.
Two-dimensional electron spin echo spectroscopy and slow motions
Authors: Millhauser Glenn L. , Freed Jack H.
Published: February 2003

A technique is described for the study of slow molecular motions that employs two-dimensional electron spin echo (2D-ESE) spectroscopy. An ‘‘echo-induced ESR spectrum’’ is obtained with a two pulse 90° (τ)/() 180° sequence by recording the echo height as the dc field is swept. A series of sweeps with different τ are Fourier transformed to yield a two-dimensional spectrum comprised of an echo-induced spectrum along the x axis and the homogeneous widths along the y axis. A convenient theoretical analysis appropriate for slow motions is given and it is shown that the 2D spectrum may be regarded as a graph of the widths vs the resonance positions of the individual ‘‘dynamics spin packets’’ that constitute the spectrum. An experimental study of the spin probe Tempone in 85% glycerol/H2O solvent for slow motions (i.e., rotational correlational times of order 10−5–10−6 s) shows variations in homogeneous widths (i.e., T−12) over the spectrum. This is analyzed in terms of canonical models of rotational reorientation—Brownian, free, and jump diffusion, and it is found that the variations of T−12 across the spectrum show features similar to a Brownian model. A special normalized contour plot of the 2D spectra is introduced which clearly emphasizes the variation of T−12 across the spectrum. The enhanced information content of these 2D spectra is discussed, and it is shown how 2D-ESE spectroscopy is both an improvement on and a complement to cw techniques for studying motional dynamics. Experimental extensions and limitations are also discussed.
Noninvariance groups for many-particle systems: Coupled harmonic oscillators
Authors: Kellman Michael E.
Published: February 2003

Noninvariance groups for many-particle systems are investigated in the context of the model problem of the coupling of a pair of harmonic oscillators to give normal modes. First, a recent paper analyzing normal modes in terms of breaking of the SU(2) invariance symmetry of the uncoupled system is reviewed. Next, the noninvariance group description of the one-dimensional oscillator spectrum in terms of infinite-dimensional unitary representations of SU(1,1) is summarized. Then, the analysis of normal modes in terms of a broken noninvariance SU(2,1) group for the two-dimensional problem is carried out. First, the T, U, and V SU(2) subgroup classifications of SU(3) are reviewed in the context of representations for the three-dimensional oscillator. Second, the analogous SU(2) and SU(1,1) subgroup classification of the infinite two-dimensional spectrum is presented. The SU(1,1) groups classify infinite sequences of excitation of the symmetric and antisymmetric stretch, respectively. Then, in an alternate approach, SU(1,1) representations for the spectra of the individual oscillators are coupled, analogous to vector coupling of angular momentum. Normal modes can be obtained in this manner, but only in the limit in which an arbitrary parameter labeling the group representations takes the value infinity. The relation of these results to the theory of group contractions and their implications for the description of truncated spectra (such as coupled Morse oscillators or π-electron spectra of linear polyenes) are briefly discussed.
Henry’s Law vaporization studies and thermodynamics of einsteinium-253 metal dissolved in ytterbium
Authors: Kleinschmidt Phillip D. , Ward John W. , Matlack George M. , Haire Richard G.
Published: February 2003

The cohesive energy of metallic einsteinium determines whether einsteinium is a trivalent or divalent metal. The enthalpy of sublimation, a measure of the cohesive energy, is calculated from the partial pressures of einsteinium over an alloy. The partial pressure of 253Es has been measured over the range 470–870 K, using combined target and mass spectrometric Knudsen effusion techniques. An alloy was prepared with einsteinium dissolved in a ytterbium solvent to produce a very dilute solution. Partial pressure measurements on the alloy were amenable to the experimental technique and a data analysis using a Henry’s law treatment of the data. Vapor pressure data are combined with an estimated crystal entropy S0298 and ΔC0p for ytterbium, to produce enthalpy, entropy, and free energy functions from 298 to 1300 K. The vapor pressure of einsteinium in a dilute einsteinium–ytterbium alloy is described by the equation log P(atm)=−(6815±216)/T+2.576±0.337, from which we calculate for the enthalpy of sublimation of pure einsteinium ΔH0298 (second law)=31.76 kcal/mol. The value of the enthalpy of sublimation is consistent with the conclusion that Es is a divalent metal.
Dynamic Monte Carlo simulations of dense polymer systems on the tetrahedral lattice: A liquid-glass-type transition
Authors: de la Batie Remi Dejean , Viovy Jean-Louis , Monnerie Lucien
Published: February 2003

The mean square displacement of chain beads, 〈R2i (t)〉 has been studied for systems having total bead densities in the range 0.875 to 0.980. On increasing the density, the bead diffusion presents a sharp step from a liquid-like behavior to a glass-like one. Such a transition is observed for systems containing either chains only or chains and free beads, but in this latter case it occurs at a higher total density. Furthermore, this transition has some geometrical features of a dynamic percolation and shows similarities to the equilibrium liquid-glass transition theory proposed by Cohen and Grest.
An average self-avoiding random walk on the square lattice lasts 71 steps
Authors: Hemmer S. , Hemmer P. C.
Published: February 2003

Attention is drawn to the fact that a self-avoiding walker is remarkably localized because of self-trapping. For the square lattice, where a walker may get trapped after any number n≥7 steps, we determine the distribution t(n) of walk lengths by a Monte Carlo calculation using 60 000 walks. The average walk length is found to be 70.7±0.2 steps. The average displacement of trapped walkers is merely 11.9 lattice units.
High resolution spectrum of the HCl dimer
Authors: Ohashi N. , Pine A. S.
Published: February 2003

Rotational structure in the spectrum of the H–Cl stretching bands of the HCl dimer has been fully resolved using a tunable difference-frequency laser. The spectrum of a natural isotopic sample was recorded under thermal equilibrium conditions near the condensation point (T∼130 K) of HCl at low pressures (3–4 Torr) and long path lengths (64–80 m). Rotational assignments have been made for K″=0, 1, and 2 subbands of a perpendicular-type band centered near 2880 cm−1 and a parallel-type band near 2857 cm−1 sharing a common vibrational ground state. For the ground state of the H 35Cl–H 35Cl dimer, the K″=0 rotational constant B̄=1944.29(7) MHz and the K″=1 asymmetry splitting B−C=19.5(3) MHz yield a zero-point center-of-mass separation of the two monomer constituents of 3.797(1) Å. Another difference band for the 35Cl isotope has also been observed as well as a similar set of bands for the mixed H 35Cl–H 37Cl species. The spectra suggest an extremely rapid tunneling motion between equivalent forms of nearly orthogonally oriented monomer units.
Radiative and collisional properties of CsH (A 1Σ+). II. Calculation of transition probabilities and lifetimes, comparison with experimental data
Authors: Telle H.
Published: February 2003

Radiative transition probabilities for the A 1Σ+→X 1Σ+ emission of CsH between all vibrational levels of the excited state (v′≤36) and all vibrational levels of the ground state (v″≤25) have been calculated. These transition probabilities are used to deduce the radiative lifetimes of the various vibrational levels (0≤v′≤36) of the CsH A 1Σ+ state; the bound–free emission probability, which comprises the major contribution for the high vibrational levels, is included for their calculation. A comparison with experimental intensity data and lifetime measurements is made; in general, the calculations are in accord with the experimental findings, and some detailed differences are discussed.
Photolytic cage effect and atom recombination of iodine in compressed gases and liquids: Experiments and simple models
Authors: Otto B. , Schroeder J. , Troe J.
Published: February 2003

Quantum yields for photolysis of molecular iodine and recombination rate coefficients of iodine atoms were measured by nanosecond laser flash photolysis in the liquid solvents CO2, C2H6, C3H8, and n-heptane at pressures up to 5 kbar. The experiments continue earlier studies with these solvents performed in the gas phase at high pressures. The results are represented in terms of simple models which involve cage breakout, cage energy relaxation, stabilization in clusters, and diffusion control. Experimental studies over wide density ranges allow for a separation of the various components of the mechanism.
Electron and thermal dissociation of BF3(g)
Authors: Farber M. , Srivastava R. D.
Published: February 2003

Electron impact fragmentation studies were performed on the BF3(g) molecule. Appearance potentials for BF+2, BF+, and F+ ions produced in the fragmentation process were 16, 24, and 30 eV, respectively. The appearance potential of BF+2 was found to be only 0.2 eV greater than the I.P. of BF3(g). The fragmentation ratio of the ionic concentrations at 70 eV for BF+3, BF+2, BF+, and F+ is 10/100/8/3. The primary mechanism for the thermal dissociation of BF3(g) is to the lower borofluorides in nearly equal concentrations. Dissociation to the dimeric borofluorides and F atoms is negligible, with F atom concentrations less than 1% of the three borofluorides. Ionization potentials for BF3(g), BF2(g), and BF(g) were 16, 9, and 11 eV (all ±1), respectively. The partial pressures for the isomolecular reaction BF3(g)+BF(g)=2BF2(g) in the temperature range 1780 to 1900 K yielded a ΔH298 of 17.1±0.5 kcal/mol, resulting in a ΔHf298 of BF2(g) of −141.0±0.5 kcal/mol, in good agreement with the photoionization and electron impact values.
The microwave spectrum of the methoxy radical CH3O
Authors: Endo Yasuki , Saito Shuji , Hirota Eizi
Published: February 2003

The pure rotational spectrum of the methoxy radical in the 2E ground electronic state was observed in the mm-wave region using a source-frequency modulation microwave spectrometer with a 1 m long free space absorption cell. The methoxy radical was produced in the cell by the reaction of methanol with 2450 MHz discharge products of CF4. Each rotational transition was found to split into fine and hyperfine components. A Hamiltonian was derived which includes the spin-orbit interaction, the vibronic Coriolis interaction, the (2,−1) and (2,2) rovibronic interactions, and the hyperfine interaction. It was successfully applied to explain the detailed feature of the observed spectrum and to derive molecular constants therefrom. The hyperfine coupling constants of the proton nuclei were determined for the first time. When the C–H bond length was assumed to be 1.0937 Å, the observed A and B rotational constants led to the C–O bond length of 1.376 Å, considerably shorter than the values hitherto accepted.
Doppler free polarization spectroscopy of SrCl: The B 2Σ+–X 2Σ+ system
Authors: Ernst W. E. , Schröder J. O.
Published: February 2003

A rotation-vibration analysis of the SrCl B 2Σ+–X 2Σ+ system is reported. Large parts of the (0,0), (1,1), and (2,2) bands were recorded using Doppler free polarization spectroscopy and microwave labeling was employed in order to simplify the complex spectra. The optical data were combined with the results of independent microwave measurements of the X 2Σ+ state in a weighted least squares fit. The heads of four Δv=+1 bands were measured for the vibrational analysis. The main Dunham parameters (2σ error in parentheses) for the Sr35Cl B 2Σ+ state are Y10=0.103 110 64(78) cm−1, Y11=−0.473 94(70)×10−3 cm−1, Y10=307.056(19) cm−1, and Y20=−1.0156(34) cm−1; Te=15 722.8176(14) cm−1.