bluuebunny/crossref_metadata_2025 huggingface dataset:
Franck–Condon factors in studies of dynamics of chemical reactions. II. Vibration–rotation distributions in atom–diatom reactions
Authors: Schatz George C. , Ross John
Published: February 2003

We study vibration–rotation distributions in two and three dimensional atom–diatom chemical reactions using the generalized Franck–Condon overlap theory of chemical dynamics presented in paper I. We begin by reducing the Franck–Condon overlap form of the transition amplitude to a partial wave series involving overlap expressions for the partial wave transition matrices. These quantities are then approximately separated into products of angular coordinate factors and vibration–translation factors. To evaluate the angular coordinate factor, we examine the limits of strong and weak potential and kinematic coupling, where simple dynamical approximations (such as sudden approximations) may be used, and we are thus able to do the integrations analytically. The vibration–translation factor is evaluated by reducing it to an overlap similar in form to one previously analyzed for collinear reactions (Ref. 1), with some differences arising from centrifugal forces. The resulting formulas for product internal state distributions are then applied to the 2D and 3D H+H2 reaction, and to the 3D exoergic reactions F+H2(D2) and H(D)+Cl2. For H+H2, we find that centrifugal effects may be ignored, and that the angular coordinate part of the Franck–Condon overlap provides an accurate qualitative description of the rotational distributions. In addition, we find that on reaction the projection quantum number distribution shows a marked propensity for the condition mj=mj′=0 for the z component of the rotational angular momentum. Centrifugal effects are important for the exoergic reactions considered, and the simple formulas developed for describing vibration–rotation distributions for such reactions give a qualitatively (and often quantitatively) accurate description of the shapes of these distributions and their dependence on dynamical and kinematic parameters such as mass ratios, potential anisotropy, and energy release behavior. In addition, a qualitative understanding of the Levine–Bernstein information theoretic procedure for relating 1D and 3D vibrational distributions is provided, and possible sources of errors in this method are identified.
Multiconfiguration wavefunctions for excited states. Selection of optimal configurations: The b 1Σ+ and d 1Σ+ states of NH
Authors: Banerjee Ajit , Grein F.
Published: February 2003

A systematic method is developed to select a set of optimal configurations for multiconfiguration (MC) wavefunctions describing low-lying and excited states of molecules at all internuclear separations. The method consists in a sequence of configuration selections combined with MC orbital optimizations. It is applied to an MC study of the b 1Σ+ and d 1Σ+ states of NH, at internuclear separations from 1 to 15 bohr, using a double-zeta plus polarization basis set with additional special orbitals. Wavefunctions and energies are given. Calculated spectroscopic constants for the b 1Σ+ state are Re=1.982 bohr (1.975), De=4.496 eV (4.55), and Te=2.69 eV (2.63), and for the d 1Σ+ state Re=2.106 bohr (2.11), De=4.491 eV (4.667), and Te=11.24 eV (11.06), with experimental values in parentheses.
Monolayer ledges on basal ice surfaces
Authors: Bartley David L.
Published: February 2003

Monolayer ledges on basal ice–vapor surfaces are analyzed. A large class of ledges is specified in terms of allowable kinks and jumps. Probabilities of jumps of each type considered are calculated for a variety of restricted ledges corresponding to the series of quasistable clusters available at fixed supersaturation. Mean ledge positions are estimated from the calculated probabilities.
Theory of the Krishnan effect: Application to dilute colloidal suspensions
Authors: Baram Asher , Gelbart William M.
Published: February 2003

In a 90°, depolarized light scattering experiment the initial and final polarizations are necessarily perpendicular but can be (1) both horizontal (H) with respect to the scattering plane or (2) one horizontal and the other vertical (V). Let the corresponding integrated (over frequency) intensities be denoted by IHH and IHV (=IVH), and the ’’Krishnan ratio’’ IVH/IHH by ρK. ρK=1, to within experimental error, for the isotropic (rotationally invariant) phases of virtually all liquids. In this paper we consider the general expressions for IHH and IVH which arise from ’’self’’ (i.e., one molecule at a time) and ’’pair’’ (cooperative) scattering processes, and study the possibility that ρK≠1. For rigid molecules whose dimensions are not too large compared to the wavelength (λ0) of light, the pair contributions can be written as a finite sum of averages (over the angle-dependent radial distribution function) of successively higher-order spherical harmonics and Bessel functions. The self terms assume a particularly simple form and their contribution to ρk−1 is shown to depend exclusively on the magnitude of kL: Here L is the molecular length (we consider rigid rods whose width d≪L) and k= (2)1/2 2π/λ0 is the wave vector transfer for the 90° scattering process. The pair terms are evaluated by using the radial distribution function for a dilute gas of hard rods, as is appropriate for several colloidal suspensions of physical interest. We calculate ρK in this way up to terms O (ρ3) and find deviations from unity as big as 15%. We suggest that similar model calculations can be carried out for bulk polymer samples and thermotropic liquid crystals, and that comparison between the resulting theoretical and experimental values of ρK can be used to infer qualitative features of the orientational order.
Quenching of the diffraction oscillations in the Ar–N2 total differential cross section
Authors: McGuire Paul
Published: February 2003

The quenching of diffraction oscillations in the Ar-N2 system is investigated computationally. (AIP).
Multichannel calculations in molecular physics: Study of coupled square wells—an exactly soluble model
Authors: Noyola-Isgleas Arturo , Fierros-Palacios Angel , Larsen-Roman Sigurd Yves , Franklin Jerrold
Published: February 2003

We consider two Schrödinger type differential equations subject to attractive square well potentials. Analytical and numerical results for the R matrix, the eigenphase shifts, and the bound states are presented and interpreted for various regimes. The effect of the closed channels on the phase shifts associated with the open channels, an effect of paramount importance in molecular physics, is analyzed.
The friction constant in a concentrated polymer–solvent system by Rayleigh scattering: Polystyrene in n-butyl acetate
Authors: Hecht A. M. , Geissler E.
Published: February 2003

Using an optical heterodyne technique (homodyne, in the definition used by the Malvern group), the intensity of the light scattered by concentration fluctuations and their correlation time were measured for a series of solutions of polystyrene in n-butyl acetate in the mass concentration range 0.18 to 0.57, between 0 and 70 °C. The elasticity deduced for the polymer matrix is found to be temperature independent, in contrast with polystyrene:cyclohexane and in conflict both with the Flory–Huggins theory and with recent scaling theories. This result is attributed to steric hindrances in the rotation of the solvent molecules. As in the case of polystyrene:cyclohexane, the diffusion constant D passes through a maximum as a function of concentration at a given temperature, indicating that the viscosity entering into the definition of D is a local viscosity and not that of the bulk solvent. On the assumption that the time constant Tr, which is central to de Gennes’ theory of the macroscopic viscosity, is proportional to the local viscosity, the dependence of Tr upon the coherence length ξ can be deduced without introducing explicitly the concentration dependence of these quantities. It is found that Tr∝1/ξ3.35±0.18, as compared with de Gennes’ theory, which predicts Tr∝1/ξ2 in the semidilute regime.
Rate constants for the reaction of the OH radical with CH3SH and CH3NH2 over the temperature range 299–426 °K
Authors: Atkinson R. , Perry R. A. , Pitts J. N.
Published: February 2003

Rate constants for reaction of the OH radical with CH3SH and CH3NH2 were determined over the temperature range 299–426 °K by a flash photolysis–resonance fluorescence technique. The Arrhenius expressions obtained were k (CH3SH) =8.89×10−12e(790±300)/RT cm3 molecule−1 sec−1 and k (CH3NH2) =1.02×10−11e(455±300)/RT cm3 molecule−1 sec−1, with rate constants at room temperature of (3.39±0.34) ×10−11 and (2.20±0.22) ×10−11 cm3 molecule−1 sec−1 for CH3SH and CH3NH2, respectively.
Proton longitudinal relaxation time measurements in dilute hydrogen chloride gas
Authors: Kalechstein Wallace , Lemaire Claude , Armstrong Robin L.
Published: February 2003

Experimental measurements of the proton longitudinal relaxation time in dilute hydrogen chloride gas are reported. The measurements were made at room temperature, at 61 MHz and for gas densities between 0.005 and 0.5 amagat. The scalar spin–rotation interaction provides the dominant relaxation mechanism. The data are well accounted for using existing relaxation theory and the single relaxation time approximation. The experiment provides a fundamental test of nuclear spin relaxation theory for a gas of heteronuclear diatomic molecules.
Electronic energy transfer in argon–xenon mixtures excited by electron bombardment
Authors: Gleason R. E. , Bonifield T. D. , Keto J. W. , Walters G. K.
Published: February 2003

The time dependences of the vacuum–ultraviolet emissions from xenon-doped argon are measured following pulsed excitation by a low intensity e-beam. The argon continuum at ∼1250 Å is observed to be quenched in electronic energy transfer collisions with xenon atoms, forming Xe* (1P1), with the rate constant (4.39±0.05) ×10−10 cm3/sec. The xenon continuum near 1470 Å is found to be due to collision induced radiation from Xe* (3P2) with the rate constant (3.2±0.7) ×10−16 cm3/sec, as well as radiation from high vibrational levels of Xe2* (0u+) which is formed from Xe* (3P1) and Xe(1S0) with the rate constant (2.1±0.2) ×10−31 cm6/sec with argon as the third body. Collisional de-excitation of Xe* (3P1) to Xe* (3P2) by argon is observed with the rate constant (1.5±0.3) ×10−14 cm3/sec. The xenon continuum at ∼1720 Å is formed from Xe* (3P2) and Xe(1S0) with the rate constant (2.15±0.25) ×10−32 cm6/sec with argon as the third body.
An NMR study of molecular motions in solid fluoropentamminecobalt (III) chloride
Authors: Reynhardt E. C. , Watton A. , Petch H. E.
Published: February 2003

Molecular motions have been studied in a powdered sample of fluoropentamminecobalt (III) chloride by using pulsed and continuous wave NMR techniques. Activation energies of 1.8 and 2.4 kcal/mole associated with threefold reorientations of ammine groups have been isolated. At higher temperatures half of the complex ions execute rapid anisotropic reorientations with an activation energy of 6 kcal/mole. The other ions remain stationary up to 360 K, where the complex changes structure or decomposes. Good agreement between relaxation and second moment measurements has been obtained. The motions are discussed in terms of structural features observed in other ammine complexes.
The non-Newtonian viscosity of dilute solutions of stiff polymers
Authors: Titulaer U. M.
Published: February 2003

The nonlinear stress response to an imposed oscillatory shear field is studied for a model polymer with constraints. The constraints are described by means of harmonic forces; their strength is taken to infinity in the final result. An explicit expression for the expectation value of the stress tensor to all orders in the applied shear field is given. In the stiff limit this expression becomes identical to one that can be derived on the basis of some earlier theories, where the constraints are treated in a different fashion.
Generalized theory of nucleation kinetics. III. Nucleation in dilute systems and/or in systems with a limited number of effective single elements
Authors: Ziabicki Andrzej
Published: February 2003

The kinetic theory of nucleation has been modified by dropping out the implicit assumption that the molar fraction of single structural elements (atoms, molecules) participating in cluster formation is close to unity. The concentration of effective single elements CH (controlled by dilution of the system or by introducing restrictions to the formation of clusters) strongly affects critical conditions of the phase transition, critical cluster size, and steady-state nucleation rates. The extended theory can be obtained from the classical equations if the free energy of a cluster composed of g single elements, ΔF (g) is replaced by Δ? (g) =ΔF (g) −kT ln(CH) g, the additional term being related to the change in the mixing entropy of effective single elements (CH) with (1−CH) ineffective structural units. The revised theory can be applied to transitions in multicomponent systems and systems composed of asymmetric orientable elements, like liquid crystals, polymers etc.
A class of approximate topological formulas for total π-electron energy
Authors: Gutman Ivan
Published: February 2003

A class of approximate topological formulas, Eqs. (12), (15), (25), and (27) for total π-electron energy E is derived. These formulas reflect the dependence of E on a number of structural details of the molecule: the number of atoms, bonds, rings, and nonbonding MO’s, the ring size, the algebraic structure count, and the branching. The formulas are obtained using analytical considerations and contain no fitting parameters.
Calculation of the chemical composition of iodine at high temperatures
Authors: Lewis D. B. , Woolsey G. A.
Published: February 2003

The chemical composition of iodine vapor has been calculated assuming local thermodynamic equilibrium and using the partition functions of the constituent particle species. Data are presented for three pressures: 0.36 torr, 46 torr, and 1 atm, over the temperature range 400 to 12 000 K.
Depolarized Rayleigh scattering and orientational motion of polyethylene glycol
Authors: Jones Dane R. , Wang C. H.
Published: February 2003

The depolarized Rayleigh light scattering spectra of pure polyethylene glycols and their solutions in an optically isotropic solvent were studied as a function of polymer molecular weight, concentration, and temperature. Reorientational relaxation times were obtained by fitting the observed spectra to Lorentzian functions convoluted with the experimental spectrum. The relaxation times were found to increase with increasing molecular weight and with dilution. The depolarized Rayleigh results for the polyethylene glycols were compared with results for other polymer systems and with results obtained using other types of relaxation studies. By studying the dependence of the relaxation times on viscosity, temperature, and concentration and comparing the results with theoretical predictions of molecular and end-group reorientations, we have associated the depolarized Rayleigh spectra of polyethylene glycol with rotation of the entire polymer molecule.
Thermomagnetic torque in N2O, OCS, and CHF3
Authors: Ball R. M.
Published: February 2003

The thermomagnetic torque (TMT) has been measured in the polar gases N2O, OCS, and CHF3 in the pressure range 0.01 to 1.0 Torr at room temperature. The TMT characteristics of these gases are similar to those of nonpolar gases. The parameters (τP/ΔT)∞m were found to be −39±2×10−6 dyn cm Torr/K for N2O, −20±1×10−6 dyn cm Torr/K for OCS, and −3.0±0.2×10−6 dyn cm Torr/K for CHF3. The parameters (H/p)∞m were found to be 14±1 kOe/Torr for N2O, 38±2 kOe/Torr for OCS, and 45±2 kOe/Torr for CHF3.
Single crystal ESR study of Cu2+ in diglycine barium chloride monohydrate
Authors: Krishnan V. G. , Sathyanarayan S. G. , Sastry G. Sivarama
Published: February 2003

ESR studies have been made on Cu2+ doped diglycine barium chloride monohydrate. Two Cu2+ lattice sites, Cu2+(I) and Cu2+(II), have been identified. Cu2+(I) exhibits only one set of four hyperfine lines in all directions and the g and A values show this complex to have a ‖3z2−r2〉 ground state. For the second lattice site, Cu2+ is found to enter into a substitutional Ba2+ site. The spectra for Cu2+(I) and Cu2+(II) lattice sites are found to have intensities in the ratio 10:1 suggesting a preferential occupation of the interstitial position by the Cu2+ ion. The spin Hamiltonian parameters and the molecular orbital coefficients are evaluated for the two sites.
Diatomic molecules as perturbed Morse oscillators. III. Perturbed eigenfunctions and Franck–Condon factors
Authors: Dwivedi Parmesh H. , Huffaker J. N.
Published: February 2003

Formulas are presented for eigenfunctions of rotating perturbed Morse oscillators as linear combinations of Morse eigenfunctions. Numerical techniques for computing these eigenfunctions are discussed and illustrated, and orthonormality properties are described in the attempt to determine the accuracy of the eigenfunctions. Franck–Condon factors are computed for the B 1Πu–X 1Σg + band system of Na2, the B 3Πg–X 1Σg + and C 3Πu–B 3Πg systems of N2, and the A 1Σu +–X 1Σg + system of Ca2 and compared with published results of RKR computations. Rotational effects are included for the latter band system.
Temperature dependence of the width of the Raman Q branch in liquids nitrogen and oxygen
Authors: Clouter M. J. , Kiefte H.
Published: February 2003

Interferometric measurements have been made of the width of the Q branch in the Raman spectra of saturated liquids N2 and O2 over the complete range from the respective triple points (63.15 K,54.35 K) to within 1 K of the critical point (126.26 K,154.58 K) in each case. The temperature dependence is similar for both liquids in the region above the normal boiling points (77.36 K,90.19 K): Near the latter temperatures the widths are 2 to 3 GHz, and the dominating feature is a sharp increase in width (to about 20 GHz) as the critical points are approached. A similar but less pronounced increase was also observed near the triple point in liquid O2. In the case of liquid N2, however, the linewidth remained approximately constant in the region below the normal boiling point.
A high pressure test of the theory of energy transfer by sensitized luminescence
Authors: Bieg K. W. , Drickamer H. G.
Published: February 2003

Measurements have been made of sensitized luminescence in the phospor KCl:Ag:Tl in the fcc phase to 18 kbar. The studies include both lifetimes and relative emission intensities of the two dopants. Both the lifetimes and relative intensities gave quantitative agreement with theory over the pressure range. The calculated efficiencies ranged from ∼0.08 to ∼0.28. Measurements from 100 to 300 °K at 8 and 18 bar also agreed with theory. Some more limited measurements on KCl:Ag:Pb in the same pressure range also exhibited satisfactory correspondence to the theory.
Infrared intensities: A model for the quantitative prediction of the vibrational strengths of SF6 and UF6
Authors: Person Willis B. , Overend John
Published: February 2003

Using an atomic polar tensor for the F atom obtained from an analysis of the experimental infrared intensities of CH3F and normal coordinate analyses of SF6 and UF6, the infrared spectra of SF6 and of UF6 have been predicted and compared with experiment. The absolute infrared intensities predicted for the infrared-active fundamental vibrations of SF6 agree quite well with experimental values; for UF6, the experimental values are less well-determined, but the agreement also appears to be satisfactory. The intensities of the binary combination bands have also been calculated, using anharmonic force constants introduced by the use of curvilinear coordinates supplemented by terms in the valence-bond stretching coordinates estimated from an assumed Morse potential, and using the calculated intensities for the fundamentals neglecting electrical anharmonicities. These predictions also appear to be in semiquantitative agreement with the experimental data.
Nonsteady hydrodynamics of biopolymer motions
Authors: McCammon J. A. , Wolynes Peter G.
Published: February 2003

Many biopolymers undergo conformational fluctuations in which large subunits move relative to each other under the influence of significant mechanical restoring forces. It is shown that nonsteady hydrodynamic effects may be important in the solvent response to such macromolecular motions. This result is in marked contrast to the familiar case of conformational fluctuations of random coil polymers, which move under the influence of weak entropic forces. The nonsteady solvent response is shown to affect the details of the biopolymer motion and to produce special qualitative features in the Raman light scattering spectrum of such macromolecules.
An atom–dipole interaction model study of the molecular second hyperpolarizability of selected haloalkanes
Authors: Sundberg Kenneth R.
Published: February 2003

The group dipole interaction model of the molecular second hyperpolarizability is specialized to describe a molecule composed of interacting isotropic atoms. The model is applied to a set of selected haloalkanes, and using established values for the atom polarizabilities, the atom second hyperpolarizabilities for carbon, hydrogen, fluorine, chlorine, bromine, and iodine are evaluated. The experimental and theoretical molecular second hyperpolarizabilities agree to within an average of 15%.
Theoretical transition dipole moments and lifetimes for the A 1Σ+u→X Σ+g system of Na2
Authors: Stevens W. J. , Hessel M. M. , Bertoncini P. J. , Wahl A. C.
Published: February 2003

Multiconfiguration self-consistent field (MCSCF) calculations have been carried out on the X 1Σ+g, A 1Σ+u, and B 1Πu states of Na2. The calculated potential energy curves are in good agreement with the experimental X and A RKR curves of Hessel and Kusch. Both the A→X and B→X transition moments have been calculated as a function of nuclear separation using MCSCF wavefunctions. These calculations are in excellent agreement with the recent experimental determinations of the B→X transition moment. A values and lifetimes of several A-state vibrational and rotational levels for the A→X transition have been calculated using the theoretical transition moment and the experimental potential curves of Hessel and Kusch. These again are in excellent agreement with the recently measured lifetimes.
Feshbach projection operator calculation of the potential energy surfaces and autoionization lifetimes for He(2 3S) –H and He(2 3S) –H2
Authors: Hickman Albert P. , Isaacson Alan D. , Miller William H.
Published: February 2003

A new technique has been developed for calculating potential energy surfaces and widths for autoionizing molecular systems. The method is based on the use of the Feshbach projection operators P and Q, which are defined within the space of configurations generated from a basis set of square-integrable orbitals. Autoionizing states are then obtained by diagonalizing a particular subblock of the finite Hamiltonian constructed by the CI procedure. These states decay because they are only approximate eigenfunctions of the full finite Hamiltonian. Their lifetimes are evaluated using continuum functions which are expanded in the basis orbitals. Such an expansion is shown to be sufficiently accurate. The present technique is compared with the stabilization method and is found to illuminate certain features of that procedure. Potential surfaces V* and widths Γ have been calculated for the Penning ionization transitions He(2 3S)+H→He+H++e− and He(2 3S)+H2→He+H2++e−.
Penning ionization of H2 by He(2 3S): Quantum mechanical scattering calculations within the rigid-rotor approximation
Authors: Hickman Albert P. , Isaacson Alan D. , Miller William H.
Published: February 2003

Elastic, rotationally inelastic, and total ionization cross sections have been calculated quantum mechanically for the scattering of metastable He(2 3S) atoms by H2 in the energy range 0.010–0.500 eV. The potential surface V* and autoionization width Γ previously calculated by the authors were used. The scattering calculation was based on the Arthurs and Dalgarno formalism for the scattering of an atom by a rigid rotator. The coupled channel equations were solved numerically for the complex, angularly dependent potential V*−(i/2) Γ. The results show that the total ionization cross section σi increases sharply with energy. Hence the ionization rate constant has a strong temperature dependence. The calculated rate constants are in good agreement with the experimental results of Lindinger et al.
Microwave optical double resonance spectrum of NH2. II. Collision-induced transitions in ? 2A1 and observations of a new vibronic state
Authors: Hills G. W. , Curl R. F.
Published: February 2003

Strong microwave optical double resonance (MODR) signals have been observed for NH2 around 6.4 GHz and 15.7 GHz. These transitions occur between a previously unobserved rovibronic level and, respectively, the J=1/2 and J=3/2 spin–rotational levels of 110, ? 2A1 π (0,10,0). This previously unknown level has been tentatively assigned as the 220, J=3/2, ? 2B1 (0,12,0). The N and J assignments are certain, the Ka is fairly certain, and the vibronic assignment is very tentative. In addition to MODR signals observed by direct laser excitation of the levels involved in the microwave transition, these microwave transitions are MODR active when a number of rotational levels of the ? 2A1 (0,10,0) band are excited. These collision-induced signals are interpreted qualitatively in terms of a simplified model. Observations of the collision-induced transition (tentatively assigned as) 220, J=3/2←322, J=5/2, ? 2B1 (0,12,0) are also reported.
Time-resolved fluorescence of the a 3Π0+ state of BrCl
Authors: Wright J. J. , Spates W. S. , Davis S. J.
Published: February 2003

The lifetime and several quenching rates have been measured for the a 3Π0+ state of BrCl by means of laser-induced time-resolved fluorescence. The vibrational levels were found to be thermalized so that about 92% of the decay was from the v′=0 and v′=1 levels with a combined observed lifetime of 18.5±3 μsec. Quenching rates for BrCl, Cl2, He, and air are 33×103 sec−1 Torr−1, 4×103 sec−1 Torr−1, 3.8×103 sec−1 Torr−1, and 53×103 sec−1 Torr−1, respectively.
Computational tests of angular momentum decoupling approximations for pressure broadening cross sections
Authors: Green Sheldon , Monchick Louis , Goldflam Rudolf , Kouri Donald J.
Published: February 2003

The utility of several approximate scattering methods for predicting collision induced spectral pressure broadening has been tested by comparison with accurate close coupling results. In particular, broadening of the pure rotational spectra of HD, HCl, CO, and HCN—all perturbed by low energy collisions with He atoms—has been computed using the effective potential formalism of Rabitz, the decoupled l-dominant approximation of DePristo and Alexander, and the jz-conserving coupled states method of McGuire and Kouri. For this last method, pressure broadening cross sections have been obtained with the new, correct expression recently derived by Goldflam and Kouri as well as with an earlier formalism based on an incorrect labeling of the scattering matrices. These methods were found to be generally disappointing for predicting pressure broadening with the exception of the new, correctly formulated jz-conserving coupled states method which was found to agree quantitatively (better than 5%) with close coupling values for all cases studied.
Photochemical kinetic equations in the absence of diffusion: An approximation for small conversion
Authors: Simmons E. L.
Published: February 2003

Equations are derived that describe the kinetics of a sample with slab geometry in which there is no diffusion of reactant or products. It is assumed that the photoproducts absorb radiation and that absorbing species not involved in the reaction are also present in the sample. The derivation is based on an approximation that greatly simplifies the kinetic equations. The approximation is valid only for short reaction times during which only a small amount of photoproducts are formed in the sample. Methods of employing the equations for the purpose of measuring quantum yield values are described.
Low frequency Raman modes in solid amorphous polystyrene and polymethyl methacrylate
Authors: Spells S. J. , Shepherd I. W.
Published: February 2003

Measurements of the low frequency Raman spectra of solid amorphous polystyrene and polymethyl methacrylate, below about 100 cm−1, are reported, showing two previously unresolved broad bands in each case. For polystyrene the mode frequencies at room temperature are about 60 and 10 cm−1; for polymethyl methacrylate about 60 and 20 cm−1. The spectra of strained samples, showing no changes in these low frequency modes, and of crystalline isotactic polystyrene are also reported. The temperature dependence of the mode intensities in polystyrene is found not to deviate significantly from a first-order variation in the temperature range from 125 to 250 °K and there is no evidence of frequency shifts in this range, within the accuracy of the determination of peak frequencies (±7 cm−1). However, frequency shifts are found in both bands with variation in molecular weight. The frequency shift for the 10 cm−1 band between samples with molecular weights of 105 and 340 is 8 cm−1, both measurements being at 100 °K. For the 60 cm−1 band the corresponding shift is 11 cm−1. Possible causes for these shifts are discussed, including changes in the density of states functions for skeletal normal modes, derived from a simple chain model. This model is, however, found to be inadequate in describing both the low frequency Raman spectrum and the anomalous values of low temperature specific heat of polystyrene.
A viscoelastic theory of the angular velocity correlation function
Authors: Montgomery John A. , Berne Bruce J.
Published: February 2003

Using a frequency-dependent rotational friction coefficient and viscosity, the angular velocity and linear velocity correlation functions are calculated for a spherical test particle in a viscoelastic fluid. The theory is compared with molecular dynamics data using values of the shear viscosity and viscoelastic relaxation time found from kinetic theory and from least squares fits to the data. While good fits are obtained in all cases, the viscosities obtained from fits of the angular velocity correlation function are much smaller than the Enskog values.
Symmetry considerations in the quantum treatment of collisions between two diatomic molecules
Authors: Alexander Millard H. , DePristo Andrew E.
Published: February 2003

To provide the necessary formal framework for future quantum studies of collisions between identical diatomic molecules, we give a detailed discussion of the correct treatment of parity and interchange symmetry within both the space-fixed and body-fixed formulation of the collision dynamics. The present development consolidates the various incomplete descriptions of this subject which have appeared in the literature as well as explicitly including the vibrational degrees of freedom. Additionally, the analysis is extended to include the effective potential, decoupled l-dominant and coupled states decoupling approximations. The degree of dimensionality reduction attainable is illustrated numerically.
Phonon assisted trap–trap triplet energy migration in the 0 °K limit in crystalline benzene
Authors: Colson Steven D. , Turner Robert E. , Vaida Veronica
Published: February 2003

The transfer of triplet excitation energy between isotopic traps in three component (host, trap, and supertrap) benzene isotopic mixed crystals has been studied. The trap–host energy difference was kept constant for the systems investigated while the trap–supertrap difference was varied. The transfer of triplet excitation energy was determined by measuring the trap-to-supertrap phosphorescence intensity ratio as a function of concentration. The results demonstrate the expected dependence of off-resonance trap–supertrap excitation energy transfer on the lattice (phonon) Franck–Condon factor. The efficiency is found to go through a maximum at a trap–supertrap energy separation of ∼50 cm−1, decreasing rapidly at higher and lower values. The observed inefficiency of trap–trap transfer (as compared to trap–supertrap transfer) is attributed to small differences in trap energies caused by crystal inhomogeneities.
Inelastic scattering calculations in polyatomic systems using an a
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o intermolecular potential energy surface: The CO2* (0,0,1,0)+H2(D2) →CO2(0,0,0,0)+H2(D2) systems
Authors: Sathyamurthy N. , Raff L. M.
Published: February 2003

An ab initio computation of the energy transfer dynamics in the (CO2,H2) and (CO2,D2) systems has been carried out. The intermolecular potential energy hypersurface has been obtained from the results of ab initio SCF computations using extended Gaussian basis sets. The potential energy has been computed for 1053 different geometries. Previously formulated cubic spline fitting procedures are employed to effect surface interpolation and to extract surface gradients. At small CO2–H2 center-of-mass separations, the potential energy is repulsive and nearly exponential in form. At larger separations, small attractive wells are found. At a given separation, the most stable conformation is a planar, parallel structure of C2v symmetry. Vibrational deexcitation probabilities, energy transfer mechanisms, and isotope ratios for the relaxation of the first excited state of the doubly degenerate bending mode of CO2 have been computed as a function of temperature by quasiclassical trajectories. In the temperature range below 600 K, the computed deexcitation probabilities are in fair to good agreement with the experimental shock tube data. At higher temperatures up to 1500 K, the computed results are too large by a factor of 3 or less. This error is interpreted to result at least partly from the assumption of classical motion. Computed isotope ratios are in fair accord with experiment over a 1000 K temperature range. The major relaxation mechanism is found to be V→R energy transfer for T?700 K and V→T transfer for 700 K<T?1500 K.
The status of discrete modes in multiphonon relaxation in solids
Authors: Englman Robert
Published: February 2003

The electron–phonon coupling linear in the lattice modes (l.m.) which accompanies the electronic excitation of impurities in solids has been treated as a multiphonon process in a few discrete modes and a residual one-phonon process in the l.m. (The latter process gives rise to relaxation which has been analyzed as electron–l.m. and discrete mode–l.m. energy exchanges.) The validity of this approach is contingent on the spectrum of the electron–phonon coupling being sufficiently narrow. Conversely, the generating function treatment of multiphonon relaxation, without introduction of discrete modes, depends (in principle), on the aforesaid spectrum not being exceedingly narrow. The results are applied to the relaxation processes in the excited states of an F center in KCl.
ENDOR Studies on x-irradiated Al(NO3)3⋅9D2O at 4.2°K
Authors: Rustgi Surendra N.
Published: February 2003

Paramagnetic radicals NO32− and NO2 have been identified in x-irradiated single crystals of Al(NO3)3⋅9D2O, with the help of ENDOR spectroscopy.(AIP)
The intrinsic reaction coordinate. An a
b i
n
i
t
i
o calculation for HNC→HCN and H−+CH4→CH4+H−
Authors: Ishida Kazuhiro , Morokuma Keiji , Komornicki Andrew
Published: February 2003

A practical method of calculating the intrinsic reaction coordinate starting at a saddle point is proposed. The method has been used in combination with the analytical evaluation of the energy gradient for the calculation of the reaction coordinate on an ab initio potential energy surface. The reaction coordinates are obtained for the HNC to HCN isomerization and the SN2 exchange reaction involving H−+CH4→CH4+H−.
On the mechanism for vibrational autoionization in H2
Authors: Dehmer P. M. , Chupka W. A.
Published: February 2003

The cross sections for the reactions of H2+ with He and Ne to form HeH+ and NeH+ are known to depend strongly on the H2+ vibrational level. When the H2+ reactant ions are produced by autoionization of Rydberg states, the measurement of these cross sections provides a means for studying the final vibrational state distribution of H2+ ions resulting from the autoionization process. Relative cross sections for hydride ion formation were determined for approximately 100 autoionizing Rydberg states in para-H2 between the H2+2𝒥g+,v=0 and v=6 convergence limits. The relative cross sections for formation of HeH+ and NeH+ (and thus the final vibrational state distributions of H2+) are nearly the same for all the autoionizing Rydberg states between any two H2+ vibrational convergence limits v and v+1 regardless of the initial vibrational level of the autoionizing state. Combining this observation with elementary considerations, we show that vibrational autoionization proceeds predominantly (≳ 95%) via a Δv=1 mechanism when this channel is energetically allowed. When autoionization via Δv=1 is forbidden, autoionization proceeds predominantly (≳ 75%) via the minimum change in Δv. In cases when autoionization proceeds via Δv≳1, the fraction of ions left in the highest available vibrational level decreases slightly with increasing initial vibrational level.
Theory of magnetic circularly polarized emission
Authors: Riehl J. P. , Richardson F. S.
Published: February 2003

A theory of magnetic circularly polarized emission (MCPE) based on the formalism and methods of quantum electrodynamics is developed for luminescent molecular systems. Differential transition probabilities for the (spontaneous) emission of left- and right-circularly polarized light are calculated and these quantities are related to the observables of MCPE spectroscopy. The emission intensity variables are further related to the emission analogs of the Faraday A, B, and C parameters. The influence of photoselection and reorientational relaxation (due to rotary Brownian motion) upon the MCPE observables is also considered. Results are obtained for two excitation–emission geometries—180° (head-on) and 90° (right angle). Explicit expressions are given for the A, B, and C Faraday parameters in both the orientationally relaxed limit (isotropic emitting sample) and the photoselected, frozen limit.
Analysis of the laser-excited fluorescence spectra of BO2
Authors: Russell D. K. , Kroll M. , Beaudet R. A.
Published: February 2003

The laser-excited fluorescence spectra of BO2 from several different vibronic states have been observed and measured under high resolution conditions. Analysis of the spectra yields molecular parameters for the ground electronic state, and provides an excellent basis for testing theories for the structure of 2π triatomic molecules.
Hartree-Fock (1s,2s) interorbital pair correlation energies and absolute overlap integrals
Authors: Light Stuart L. , Petersson G. A.
Published: February 2003

Interorbital pair correlation energies between 1s and 2s Hartree-Fock orbitals are shown to be proportional to the square of the absolute overlap integral ℱ‖φ1Sφ2S‖dτ. The proportionality constant is independent of the nuclear charge and the number of 1s and 2s electrons, but shows an exclusion effect of about 10% for each occupied 2p orbital. Using only four parameters (a proportionality constant and a 2p exclusion effect each, for singlet and triplet coupled pairs) we reproduce fifty-six ab initio pair energies (ranging from −0.00047 to −0.00891 hartree) with a root-mean-square deviation of 0.0001 hartree.
Optical absorption and laser excited fluorescence spectra of LaF3:Eu3+
Authors: Kumar U. Vignaneswara , Rao D. Ramachandra , Venkateswarlu Putcha
Published: February 2003

The optical absorption spectrum of LaF3:Eu3+ single crystal at liquid nitrogen temperature (LNT) showed two groups of lines around 5240 and 4640 Å which are attributed to the Stark levels associated with the 5D1 and 5D2 of Eu3+. The fluorescence spectrum of this system at LNT excited with the 4580 and 5145 Å lines of the Ar+ laser revealed 12 groups of lines originating in the Stark components of 5D0,1,2 and terminating in those of the 7F0,1,2,3,4,5 levels. Most of the Stark components of the energy levels of the LaF3:Eu3+ system lying below 20 000 cm−1 could be determined from these spectra. The polarizations of the lines indicate a C2v site symmetry, and all the observed Stark components are categorized under the symmetry species of this group.
Atom–diatom scattering: An extension of the field theoretic approach and a computational assessment
Authors: Varracchio E. Ficocelli
Published: February 2003

In a recent paper the problem of atom–diatom scattering has been approached with a field-theoretic formalism and an approximation scheme has been suggested for the optical and transition potentials, presiding elastic and inelastic processes, respectively. On the basis of experience gained from preliminary calculations, higher order approximations have been derived in this paper for the above mentioned effective potentials. New computations have been performed for a collinear collision model, with an exponential repulsive interaction potential and a harmonic oscillator representation for the molecule. The results obtained agree very well to the exact values of Secrest and Johnson, also for choices of the parameters where other approximate quantum mechanical treatments completely fail, suggesting that, in this extended scheme, the field theoretic scattering equations could be confidently applied to realistic three-dimensional computations.
Photoionization of ozone: Formation of O4+ and O5+
Authors: Weiss Morris J. , Berkowitz J. , Appelman E. H.
Published: February 2003

Ionization yields of O+, O2+, and O3+ have been studied as a function of wavelength in the photoionization of ozone from threshold to 600 Å with the primary process being the production of O3+. The ratio O3+ : O2+ : O+ at 600 Å is 1.00 : 0.36 : 0.05. The adiabatic ionization potential of O3+ is 990.32±0.3 Å (12.519±0.004 eV). Steplike structure occurs near threshold, at wavelengths corresponding to vertical ionization potentials reported in photoelectron spectroscopy studies. A weak onset for O2+ production appears at 13.082 eV (947.7 Å), followed by more intense O2+ production at 13.432 eV (932.2 Å). The onset for O+ formation appears weakly at 15.21 eV, rising continuously with decreasing wavelength. The formation of O5+ via the ion–molecule reaction O3++O3→O5++O was also observed and measured as a function of wavelength. The O4+ ion was also detected. Pressure dependences and enthalpy considerations make the reaction O3++O2→O4++O a highly probable mechanism for O4+ formation.
Comparison of models for H2–H2 and H2–He anisotropic intermolecular repulsion
Authors: Starr T. L. , Williams D. E.
Published: February 2003

Three empirical potential models (dumbbell, Kihara, and ellipsoidal overlap) are fitted to theoretical calculations of H2–H2 and H2–He repulsion. The dumbbell model gives a slightly better fit and has better transferability. This model is recommended because of its simplicity and physical meaningfulness.
Observation of deuterium quadrupole splittings of aromatic free radicals in liquid crystals by ENDOR and TRIPLE resonance
Authors: Biehl R. , Lubitz W. , Möbius K. , Plato M.
Published: February 2003

First measurements of deuterium quadrupole coupling constants of a polyatomic doublet state radical are reported. The quadrupole couplings were determined by ENDOR and electron–nuclear–nuclear TRIPLE resonance in liquid crystals. For the partially deuterated perinaphthenyl radical the out-of-plane component of the deuterium quadrupole coupling was measured to be e2q33Q/h =−(94±3) kHz for both molecular sites. Within experimental error these values are equal to those measured earlier for deuterated alternant hydrocarbons in the diamagnetic singlet ground state.
The dynamics of the reaction D2++N→ND++D
Authors: McClure Donald J. , Douglass Charles H. , Gentry W. Ronald
Published: February 2003

We report the results of a merged-beam study of the reaction D+2+N→ND++D over the range of relative kinetic energy from ∼0.005 to 10 eV. At low kinetic energies the reaction cross section is accurately proportional to the orbiting cross section calculated for the ion-induced dipole potential plus an experimentally estimated r−6 term. The reaction probability for the N(4Su) ground state is estimated to be at least 71% for low-energy orbiting collisions. The reaction mechanism is direct, with the ND+ product scattered preferentially forward with respect to the incident N atom velocity at initial kinetic energies as low as 0.031 eV. A deconvolution analysis of the measured product laboratory energy distributions shows a net conversion of internal to translational energy at initial kinetic energies less than 0.9 eV, and a net conversion of translational to internal energy at higher initial kinetic energies. We interpret the results in terms of the adiabatic electronic state correlations for NH+2 systems, taking into consideration previous work by Mahan and co-workers on the reaction N++H2→NH++H.
Pulsewidth dependence of ozone interference in the laser fluorescence measurement of OH in the atmosphere
Authors: Hanabusa M. , Wang Charles C. , Japar S. , Killinger D. K. , Fisher W.
Published: February 2003

By varying the pulsewidth of the output from two dye lasers, we have verified experimentally that the steady-state interference level of OH due to laser-induced dissociation of ozone decreases linearly with decreasing pulsewidth of the exciting radiation. At low pressures, further reduction in the interference level due to the transient nature of OH formation processes was also observed. These results should greatly facilitate measurements of OH concentrations in the atmosphere.
The effect of shape anisotropy on the neutron scattering from diatomiclike molecules
Authors: Evans Glenn T. , Fixman Marshall
Published: February 2003

The quasielastic neutron scattering cross section is calculated for a dense fluid consisting of homonuclear diatomiclike molecules having an axially symmetric translational diffusion tensor. To lowest order in the particle shape anisotropy, the main effect on the cross section is to augment the average translational diffusion constant by a term which involves the rank of the spherical harmonic for the orientational process and the anisotropy in the translational diffusion tensor.
Laser-induced fluorescence of CN(X2 Σ+) produced by photolysis of C2N2 at 160 nm
Authors: Cody R. J. , Sabety-Dzvonik Michael J. , Jackson W. M.
Published: February 2003

Laser-induced fluorescence has been used to investigate the energy partitioning into the electronic, vibrational, rotational, and translational degrees of freedom of CN radicals produced in the photolysis of cyanogen (C2N2) at λ=160 nm (7.75 eV, 62 500 cm−1). The CN radicals that were produced in the (X2 Σ+) state were found to be vibrationally and rotationally excited. The initial distributions of the rotational line intensities of the (0,0) and (1,1) bands in the violet (B2 Σ+↔X2 Σ+) band system are characterized by Boltzmann distributions in which Trot (0,0) ∼1400 K and Trot(1,1) ∼1100 K. The observed Boltzmann character of the rotational state distributions is explained by a type II predissociation, which produces statistically randomized energy distributions in the fragments. It is observed that approximately 95% of the vibrational energy in CN(X2 Σ+) appears in the v″=0 and v″=1 levels. Evidence is presented which shows that C2N2 predissociates to produce equal amounts of CN(A2 Πi) and CN(X2 Σ+).