bluuebunny/crossref_metadata_2025 huggingface dataset:
Collisional energy transfer in polyatomic molecules: A study of anharmonicity effects in Kr+CO2
Authors: Schatz George C. , Mulloney Thomas
Published: February 2003

The importance of anharmonicity in determining the rates of collisional energy transfer in Kr+CO2 is studied by combining the usual classical trajectory method with an accurate characterization of the semiclassical stationary ’’states’’ of the CO2 molecules (i.e., the ’’good’’ action variables) before and after each collision. A linear model of the collision dynamics is assumed, which means that only energy transfer involving the symmetric and asymmetric stretch modes of CO2 is described. Most of our studies focus upon analyzing the average changes in symmetric and asymmetric stretch good actions, and the average T→V energy transfer. For Kr+CO2, comparison of these collisional energy transfer moments using an anharmonic force field for CO2 with those using the corresponding harmonic force field indicates that anharmonicity effects are very important in the collisional energy transfer process, with errors in moments by factors of 10 or more incurred when the harmonic approximation is used. At high collision energies, where the primary energy transfer process is T→V, the anharmonic molecule behaves as a ’’stiffer’’ oscillator when compressed than harmonic with the result that a factor of 10 less energy is absorbed. At low collision energy where the energy transfer is primarily V→V and is dominated by the low frequency components of the collisional interaction force, energy transfer is much larger for the anharmonic molecule since the frequencies associated with the harmonic molecule (i.e., the fundamental frequencies) are much higher than one particular combination frequency (2ω1−ω3) which exists in the anharmonic molecule. This single frequency then dominates all energy transfer properties, causing all energy transfer moments to have the same dependence on collision energy. One can summarize these results by the statement that diagonal anharmonicity effects dominate in the high collision energy impulsive limit while off diagonal anharmonicities control energy transfer at low collision energies. We also examine the dependence of collisional energy transfer in Kr+CO2 on the level of sophistication of the anharmonic force field, with the conclusion that (at least for the low vibrational states of CO2), a quartic force field is necessary but apparently also sufficient to quantitatively describe the collision dynamics. In addition, the effect of initially exciting CO2 to the states (N100) with N1=0,1,2,3,8, and 16 is investigated with the conclusion that anharmonic coupling becomes increasingly important as the amount of vibrational excitation is increased, with its effect on T→V energy transfer generally much smaller than that on V→V transfer.
No linear dependence or many-center integral problems in momentum space quantum chemistry
Authors: Monkhorst Hendrik J. , Jeriorski Bogumil/
Published: February 2003

It is shown that the position space LCAO-type wave functions for a many-center one electron system can be easily obtained via a suitable momentum space approach. These wave functions and corresponding variational approximations to the eigenvalues are calculated via diagonalizations of simple overlap matrices. In the proposed approach the problems of many-center integrals and instabilities due to overcompleteness of basis sets do not appear at all.
The b 1Σ+↔X 3Σ− transitions of NI, NBr, and NCl in solid argon at 12 K
Authors: Miller John C. , Andrews Lester
Published: February 2003

The NI, NBr, and NCl radicals have been synthesized from nitrogen and halogen atoms produced in a microwave discharge and subsequently trapped in solid argon at 12 K. The b 1Σ+↔X 3Σ− transition has been observed in both absorption and laser emission experiments, and the spectroscopic parameters are compared to gas phase values for NBr and NCl. Constants for NI are reported for the first time. Vibrationally unrelaxed emission is observed and discussed in terms of current models.
Specific heats of the pyrochlore compounds Cd2Re2O7 and Cd2Ru2O7
Authors: Blacklock K. , White H. W.
Published: February 2003

The specific heats of the pyrochlore compounds Cd2Re2O7 and Cd2Ru2O7 were measured below 20 K. The data was found to be well described by the expression C=γT+αT3+βT5 below 6 K for both compounds. The Debye temperatures were calculated from the T3-term coefficients. The values for γ were found to be 26.5 mJ/mol-K2 for Cd2Re2O7 and 12.3 mJ/mol-K2 for Cd2Ru2O7. The ϑD values were 285 and 310 K respectively.
Pole approximation for unimolecular decay dynamics
Authors: Numrich Robert W. , Kay Kenneth G.
Published: February 2003

Time-dependent transition probabilities and final product distributions for unimolecular decay are approximated from the corresponding integral expressions in terms of poles and residues reflecting contributions from bound and resonance states. Poles in the integrands at complex resonance energies are located by an efficient iterative search for the poles of low-dimensional resolvent matrices. These matrices and the residues needed for the calculation are computed using Wigner’s R-matrix method. Calculations are performed for two collinear triatomic systems previously treated by an accurate technique. By comparing the approximate and accurate results, the magnitude of branch cut contributions to time-dependent transition probabilities is determined. The pole approximation is found to yield time-dependent probabilities that are qualitatively (and, sometimes, quantitatively) accurate once direct dissociation has ceased. This approximation, however, is found to be less satisfactory for calculation of final product distributions unless direct molecular dissociation is negligible.
Repulsive intermolecular potential between two H2 molecules
Authors: Ree Francis H. , Bender Charles F.
Published: February 2003

A b initio calculations of the intermolecular potential Φ between two H2 molecules have been carried out for intermolecular separations corresponding to the repulsive region of Φ. In most of the calculations a contracted Gaussian basis set consisting of two s and one p functions per H was used to construct a self-consistent field (SCF) configuration and SCF plus all singly and doubly excited configurations at fixed H–H bond length. To ascertain the accuracy of the computed results, three types of more elaborate calculations were carried out. These are based on a larger Gaussian basis set (five s and two p functions per H), full configuration interaction, and variation of the H–H bond length. We found that Φ depends weakly on these factors at r≲4.5 bohr. Analytic expressions are presented to represent Φ in terms of intermolecular distance and three angles defining orientations of two H2 molecules. The minimum-energy orientation of two H2 as they approach each other is predicted to have a simple shape, two H2 forming a parallelogram. The results obtained in this work are compared with those for other Φ’s obtained from theoretical and experimental considerations. Since Φ’s based on the condensed-state data are less repulsive than the ab initio Φ, we conclude that the many-body effects in condensed states are significant over a nearly entire range of repulsion.
Spontaneous emission lifetimes in the ground electronic states of HD+ and H2+
Authors: Peek James M. , Hashemi-Attar Ali-Reza , Beckel Charles L.
Published: February 2003

Excited state lifetimes are reported for spontaneous emission from 22 excited vibrations of HD+ and 19 excited vibrations of H2+, in their ground electronic states. (AIP)
The variational method and the stochastic–Liouville equation. III. Infinite elements for CIDN(E)P
Authors: Zientara Gary P. , Freed Jack H.
Published: February 2003

The variational finite-element method introduced by Zientara and Freed for the solution of the stochastic-Liouville equation is modified to utilize the advantages of an infinite outer element. Within this infinite element, the correct asymptotic forms of the solutions may be used, and they may be matched to those of the inner finite elements. Large reductions in the computational effort are realized by this scheme while maintaining high accuracy as illustrated in the example of high-field chemically induced dynamic spin polarization. Applicability of this method extends to solutions of partial differential equations in chemical physics characterized by a large spatial region with simple interactions and a restricted region in which more complex behavior occurs, such as is found in treatments of chemical reactions modulated by liquid state diffusive processes and in scattering theory in quantum mechanics.
Convergency studies of second and approximate second order multiconfigurational Hartree−Fock procedures
Authors: Yeager Danny L. , Jo/rgensen Poul
Published: February 2003

The convergency of second order and approximate second order multiconfigurational Hartree−Fock procedures has been examined. Preliminary calculations on Be and O2 show that second order procedures straightforwardly can be applied and converged in a few iterations both for ground and excited states. Approximate second order schemes such as the ’’super CI’’ approach converged much slower. The present calculations preserve orbital orthogonality explicitly in each iteration without using a Schmidt, Löwdin, or other orthogonalization procedure.
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o calculation of spin densities in hydrocarbon radicals
Authors: Chipman Daniel M.
Published: February 2003

A b initio calculations are reported for the isotropic hyperfine coupling constants arising from contact spin density distributions in hydrocarbon radicals. Representative sigma radicals (vinyl and ethynyl), nonconjugated pi radicals (methyl and ethyl), and conjugated pi radicals (allyl and cyclohexadienyl) are examined in the unrestricted Hartree-Fock approximation with a wide variety of Gaussian basis sets, ranging from crude minimal sets to a large uncontracted set. The unrestricted Hartree–Fock method itself gives poor results for this property, but is found to yield results of semiquantitative accuracy after a spin projection is performed. Some significant differences are found between results after full quartet annihilation and after complete spin projection. Due to a systematic cancellation of errors, the crude minimal STO-3G basis set gives generally better results than more sophisticated minimal basis sets. Extended basis sets give only a slight overall improvement on the STO-3G results. The standard contraction schemes for large Gaussian basis sets are found to be quite appropriate for spin density calculations. It is concluded that the projected unrestricted Hartree–Fock method can be a useful tool for study of spin density distributions in hydrocarbon radicals even when deployed with highly contracted Gaussian basis sets.
+
2 (A 2Πμ→X 2Π= (g−italic)=) and O+2 (b 4Σ−g→a 4Πu) emissions by photoionization of O2
Authors: Wu C. Y. Robert , Phillips E. , Lee L. C. , Judge D. L.
Published: February 2003

Cross sections for the production of the O2+ (A 2Πu→X 2Πg) fluorescence in the 2000–5000 Å region and the O2+ (b 4Σ−g→a 4Πu) fluorescence in the 3000–8000 Å region have been measured at selected atomic emission lines and throughout the synchrotron radiation continuum from 175 to 740 Å. The electronic transition moments and vibrational populations of the resulting ions have also been determined. The partial cross section for producing the O2+ (b→a) system shows structure closely related to the total absorption spectrum in the 500–700 Å region. In contrast with this, the O2+ (A→X) system shows only weak structure and its spectrum is quite different from that of the absorption spectrum. The various channels and processes for evolution of the O2 molecules excited into these Rydberg states are discussed.
Crystal structure and absolute piezoelectric d14 coefficient in laevorotatory Bi12SiO20
Authors: Abrahams S. C. , Bernstein J. L. , Svensson C.
Published: February 2003

Laevorotatory Bi12 SiO20 crystallizes in the cubic system with space group 123 and a=10.10433±0.00005 Å at 298 K. The integrated intensities of 5488 reflections within a hemisphere of reciprocal space with (sinϑ/γ?1.15 Å−L were measured with monochromatic MoKα radiation. A total of 977 independent and significantly nonzero Fmeas were used to determine the crystal structure by the method of least squares. The final agreement factor R=0.0316. The Si–O distance within the geometrically regular SiO4 tetrahedron is 1.647±0.005 Å. The Bi–O bond distances, ranging from 2.064±0.009 to 2.647±0.003 Å, with two additional contacts of 3.06 and 3.161 Å formed by the Bi inert 6s2 electron pair are all within 0.025 Å of the corresponding distances Bi12 GeO2. The absolute sense of the piezoelectric d14 coefficient is such that tensile stress along [111] generates positive polarity on (111), the face toward which the oxygen apex of the SiO4 tetrahedra points,i.e.,d1450. The magnitude of d14 was determined in a static experiment to be 37.8±2.0x10−12CN−1.
Calculations of atomic level structure of alkali atoms in rare-gas solids
Authors: Dawson J. F. , Balling L. C.
Published: February 2003

The pseodopotential method of Baylis has been used to calculate potential energies of alkali atoms in rare-gas crystals. The purpose of these calculations is to see if the optical spectra of matrix-isolated alkali atoms can be understood on the basis of the alkali–rare-gas interaction. We have investigated substitutional and interstitial sites as well as larger vacancies in which two or more rare-gas atoms are replaced by an alkali atom. Some of these sites show characteristics suggested by recent experimental results.
Quantum number and energy scaling for nonreactive collisions
Authors: DePristo Andrew E. , Augustin Stuart D. , Ramaswamy Ramakrishna , Rabitz Herschel
Published: February 2003

Two new theoretical developments are presented in this article. First an energy corrected sudden (ECS) approximation is derived by explicitly incorporating both the internal energy level spacing and the finite collision duration into the sudden S-matrix. An application of this ECS approximation to the calculation of rotationally inelastic cross sections is shown to yield accurate results for the H+–CN system. Second, a quantum number and energy scaling relationship for nonreactive S-matrix elements is derived based on the ECS method. A few detailed illustrations are presented and scaling predictions are compared to exact results for R–T, V–T, and V–R, T processes in various atom–molecule systems. The agreement is uniformly very good — even when the sudden approximation is inaccurate. An important result occurs in the analysis of V–T processes: the effects of anharmonic wave functions (coupling) and decreasing vibrational energy gaps (energetics) are separated. Each factor makes significant contributions to the deviation of the anharmonic from the harmonic scaling relationship.
Talmi transformation and the multicenter integrals of harmonic oscillator functions
Authors: Maretis Dimitris K.
Published: February 2003

The multicenter integrals over harmonic oscillator functions expressed in terms of spherical polar coordinates are evaluated analytically using generalizations of the Talmi transformation. The results thus obtained are closed formulas containing only finite summations and known quantities. Numerical integration is no longer needed.
Further theoretical studies of the spin–spin coupling constant of HD. Coupled Hartree–Fock calculations of the paramagnetic orbital term and new calculations of the Fermi-contact term
Authors: Schulman Jerome M. , Lee Wellington S.
Published: February 2003

This paper presents calculations of the paramagnetic orbital contribution J1b to the spin–spin coupling constant of JHD. Values obtained in the uncoupled and coupled Hartree–Fock approximations were found to be 0.559 and 0.776 Hz, respectively. An analysis of the various contributions in terms of group theory is presented. The paper also contains a coupled Hartree–Fock calculation of the Fermi-contact term as well as completion of the previously reported diagrammatic perturbation study of JHD in the Hartree–Fock potential.
Fluorescence quenching due to the electron transfer. Indole–chloromethanes in rigid ethanol glass
Authors: Namiki Akira , Nakashima Nobuaki , Yoshihara Keitaro
Published: February 2003

Fluorescence quenching of indole by chlorinated methanes in rigid matrices at low temperature have been studied with picosound-laser excitation and steady-state illumination. Observed decay curves were significantly deviated from a single exponential in the presence of highly concentrated chloromethanes, and analyzed by the generalized decay function for the exchange interaction. The fluorescence quantum yields decreased exponentially with the concentration of added quenchers. These results were explained with the electron transfer mechanism from the excited indole to chloromethanes which were caused by the short range electron exchange interaction. The detailed mechanism of the electron transfer from the excited states are discussed and the similarity between the electron transfer and the triplet–triplet excitation transfer is emphasized.
Viscoelastic properties of rigid and semiflexible particles in solution. I
Authors: Iwata Kazuyoshi
Published: February 2003

A theory of Erpenbeck and Kirkwood, which describes viscoelastic behavior of macromolecules in a general way, is transformed so that it can be applied practically to ’’semiflexible particles,’’ which are partly deformable but have definite shape in solution. A basic idea of the present work is that, by introducing proper auxiliary coordinates and an imaginary rigid core which is made zero at the final stage of calculation, derivatives ∂?i/∂xi and ∂xj/∂?i and Jacobian J=∂{?i}/∂{xj} associated with a coordinate transformation from a lab system {?i} to a body system {xj} can be computed explicitly, and contravariant components of a diffusion-coefficient tensor in a ’’diffusional space’’ are derived easily. The particles are assumed to be composed of many points, which are arranged arbitrarily and move within the particles under the influences of intramolecular potentials, hydrodynamic resistance of solvent and ’’internal friction’’ caused by other points in the same particles. As the base for treating the semiflexible particles.
Analysis of multiple pulse NMR in solids. III
Authors: Burum D. P. , Rhim W. K.
Published: February 2003

A 52-pulse cycle recently introduced in a communication, which has a substantially increased resolving power compared to previously available techniques, is analyzed in detail. Also, a new 24-pulse cycle which is essentially equivalent to the 52-pulse cycle in resolving power is introduced. These pulse cycles achieve their enhanced resolution compared to the REV-8 cycle by removing the effects of the homonuclear dipolar interaction in solids to higher orders of magnitude without reintroducing any dipolar terms eliminated by REV-8. Principles of pulse cycle decoupling are introduced which greatly simplify the systematic development of compound multiple pulse cycles. Beginning with the 2-pulse solid echo sequence, we show how these principles could have been used in designing the WAHUHA and REV-8 cycles and the 14-pulse cycle recently introduced by Haeberlen. We further demonstrate the application of this method by developing several new pulse cycles such as a 12-pulse cycle equivalent to the 14-pulse cycle, the 24-pulse cycle, and the 52-pulse cycle. Finally, proton chemical shift tensor components for several organic solids which were measured using the 52-pulse cycle are reported without detailed discussion.
The cyclic ozone isomer
Authors: Burton Peter G.
Published: February 2003

Analysis of basis set effects in large correlated wave function calculations is undertaken in relation to the energy separation of the cyclic (D3h) and open (C2v) forms of ozone. The present calculations attain some 60% of the valence correlation energy of ozone with the largest bases, superseding a number of recent investigations of the D3h−C2v separation. The conclusion is reached that these species are isomers corresponding to different local minima on the ground state surface, below the ozone dissociation limit. The energy separation of these species is narrowed down to 40–88 kJ/mol, with the likeliest value, 50±kJ/mol, emerging from the present calculations (’’doulbe zeta plus polarization’’ is in error ∼25 kJ/mol). The formation and implications of the secondary D3h ozone isomer are briefly discussed.
Theoretical Compton profile anisotropies in molecules and solids. IV. Parallel–perpendicular anisotropies in alkali fluoride molecules
Authors: Matcha Robert L. , Pettitt Bernard M. , Ramirez B. I. , McIntire William R.
Published: February 2003

Calculations of Compton profiles and parallel–perpendicular anisotropies in alkali fluorides are presented and analyzed in terms of molecular charge distributions and wave function character. It is found that the parallel profile associated with the valence pi orbital is the principal factor determining the relative shapes of the total profile anisotropies in the low momentum region.
Chemical potential and electronegativity in terms of electron density
Authors: March N. H.
Published: February 2003

The chemical potential, and hence according to the recent work of Parr et al. the absolute magnitude of electronegativity, is expressed explicitly in terms of quadratures on the electron density by employing gradient expansions.
Molecular theory and the shape of the adsorption isotherm
Authors: Cortes G. Joaquin
Published: February 2003

The adsorption isotherm of a gas in the presence of a solid surface is discussed in terms of the interaction energy of gas molecules with one another and with the solid surface. (AIP)
Markov chains in the 2-d Ising model: Exact solutions for special cases
Authors: Poland Douglas
Published: February 2003

Verhagen’s conditions for Markovian behavior in the 2-d plane-square Ising lattice gas model with both nearest and next nearest neighbor interactions are cast in the form of a single nonlinear equation that is solved numerically. If ε1 and ε2 are the nearest and next nearest neighbor interaction energies, respectively, then there are no solutions for both ε1 and ε2 negative (representing attractive interactions). For ε2≳0, there is a locus in the ε1-density plane for both ε1≳0 and ε1<0 where the model is Markovian and where one can calculate the activity exactly as a function of density and the interaction parameters; the special point ε1=+∞ corresponds to the analytic solution found by Verhagen.
A new interpretation of Petty and Moran’s ion-impact Ne–H+3 experiment
Authors: Carney G. D.
Published: February 2003

The calculated energy intervals for breathing and bending vibrations of H+3 given by Carney and Porter (2) agree with the observed ion-impact spectroscopy experiments of petty and Moran (1). The vibrational anhormonicity in H+3 and rotational excitation processes are discussed.(AIP)
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o SCF MO calculations of the potential surfaces of thiocarbonyls. III. Ground state and first excited triplet state of thiourea, (NH2)2CS
Authors: Kapur A. , Steer R. P. , Mezey P. G.
Published: February 2003

Equilibrium geometries for the ground and first excited triplet states of thiourea have been calculated using ab initio SCF MO methods. Apart from the pyramidality of the NH2 groups, the calculations for thiourea indicate a planar ground state analogous to simple tetraatomic thiocarbonyls. The first triplet state out-of-plane bending mode frequencies have been calculated by a perturbational method. The rotation of the NH2 moiety about the C–N bond has been studied for the ground and first excited triplet states and the barriers to these internal rotations have been calculated.
Vibrational spectra of nitric oxide chemisorbed on Pt(100)
Authors: Pirug G. , Bonzel H. P. , Hopster H. , Ibach H.
Published: February 2003

The chemisorption of NO on a Pt(100) single crystal surface at 140 and 300 K was investigated by high resolution electron energy loss spectroscopy, combined with Auger electron spectroscopy and LEED. Chemisorption of NO was predominantly molecular. The unreconstructed Pt(100)−1×1 surface was characterized by a single NO stretch frequency at 1615 cm−1 and two low frequency vibrations at 230 and 380 cm−1. These latter modes are likely to represent the Pt–NO stretch and Pt–N–O bending vibrations, respectively. The adsorbed molecule has therefore Cs symmetry in this case and is bent with respect to the surface normal. Low coverage NO adsorption at 140 K on the reconstructed Pt(100)−5×20 surface gave rise to three frequencies at 1690, 465, and 310 cm−1. Thus NO is also adsorbed in a bent configuration on this surface. At higher coverage an additional NO stretch frequency at 1790 cm−1 indicated a second adsorption site. Low frequency modes could not be clearly resoled for this species. NO adsorption on the same surface at 300 K was characterized by frequencies at 1805, 1630, and 380 cm−1, independent of NO coverage. At this temperature the reconstruction of the surface is lifted locally where NO adsorption took place. In analogy to the 1×1 surface it was concluded that the major NO state corresponds to a bent NO molecule while the 1805 cm−1 frequency represents NO adsorbed on a defect site which is created by the structure transformation.
Proton affinities of hydrogen halides determined by the molecular beam photoionization method
Authors: Tiedemann P. W. , Anderson S. L. , Ceyer S. T. , Hirooka T. , Ng C. Y. , Mahan B. H. , Lee Y. T.
Published: February 2003

Gas-phase proton affinities of HF (95.5±1.4 kcal/mol), HCl (135±1 kcal/mol), and HBr (141±1 kcal/mol) have been measured along with the HX+–HX binding energies for the HCl and HBr systems (20±2 and 23±2 kcal/mol, respectively) by photoionization of van der Waals molecules produced in a supersonic expansion. A detailed discussion of the potentialities and limitations of the method is presented.
Molecular beam scattering studies of orbiting resonances and the determination of van der Waals potentials for H–Ne, Ar, Kr, and Xe and for H2–Ar, Kr, and Xe
Authors: Toennies J. Peter , Welz Wolfgang , Wolf Günther
Published: February 2003

The velocity dependence of the integral cross sections for the scattering of H atoms and normal-H2 molecules from the rare gases has been measured for primary beam velocities from 300 m/sec (Ec.m.=0.5 meV) to about 3000 m/sec (Ec.m.=100 meV). Three distinct resonance maxima were observed in each of the systems H–Xe, H–Kr, H2–Kr, and H2–Ar. At least one resonance maximum was found in H2–Xe and H–Ar, while H–Ne showed no maxima. The H2–Ar cross sections were also measured with a cold beam of pure para-H2 consisting entirely of spherically symmetric j=0 molecules. No noticeable difference was found when compared to the n-H2 cross sections indicating that the observed resonances depend only on the spherical symmetric potential. Extensive comparisons with spherical symmetric model potentials showed that all the observed resonances are due to temporary capture in the well of the effective potential. This phenomenon is usually referred to as ’’orbiting.’’ The energetic location of the observed orbiting resonances and glory maxima were found to be very sensitive to the potential shapes and parameters. Only models which included at least two terms in the long range dispersion potential C6/R6 and C8/C8 could describe the data in a satisfactory way. For H–Ar the data are in very good agreement with the experimental potential of Bassi et al. and in reasonable agreement with the ab initio MCSF potential of Wagner et al. For H2–Ar, –Kr, –Xe good agreement is found with the potentials recently determined from infrared spectra by Le Roy and co-workers. For all the other systems new best fit potentials have been determined. The best fit values of ε and Rm are for H–Ne: 1.90 meV and 3.15 Å; H–Ar: 4.16 meV and 3.62 Å; H–Kr: 5.90 meV and 3.57 Å; H–Xe: 7.08 meV and 3.82 Å; H2–Ar: 6.30 meV and 3.57 Å; H2–Kr: 7.19 meV and 3.72 Å, and H2–Xe: 8.10 meV and 3.92 Å.
Vibrationally stimulated addition reactions between hydrogen halides and unsaturated hydrocarbons: A negative result
Authors: Herman Irving P. , Marling Jack B.
Published: February 2003

No evidence for enhancement of gas phase addition reactions between hydrogen halides and unsaturated hydrocarbons was observed in cases where one reactant was prepared in a high vibrational state by laser excitation of an overtone transition. The specific reaction systems studied were HCl(ν=5 or 6)+ various unsaturated hydrocarbons, and also C2H2(00500)+HBr or HCl, at room temperature. The reactant states were prepared by one photon-overtone absorption inside a cw dye laser, to an energy level above the activation energy of the corresponding thermal reaction. It is estimated that the rate constant for addition of these vibrationally excited molecules is at least 10–1000 times slower than the rate corresponding to the pre-exponential factor of the associated thermal reaction, depending on the employed rate of collisional quenching. Also, the reaction of C2H2(00500)+HBr did not appear to accelerate even when heated to 300° C.
Crossed beam (E–V
R
T) energy transfer experiment
Authors: Hertel I. V. , Hofmann H. , Rost K. A.
Published: February 2003

A molecular crossed beam apparatus which has been developed to perform electronic-to-vibrational, rotational, translational (E–V,R,T) energy transfer studies is described. Its capabilities are illustrated on the basis of a number of energy transfer spectra obtained for collision systems of the type Na*+Mol(ν,j) →Na+Mol (ν′,j′) where Na* represents a laser excited sodium atom and Mol a diatomic or polyatomic molecule. Because of the lack of reliable dynamic theories on quenching processes, statistical approaches such as the ’’linearly forced harmonic oscillator’’ and ’’prior distributions’’ have been used to model the experimental spectra. The agreement is found to be satisfactory, so even such simple statistics may be useful to describe (E–V,R,T) energy transfer processes in collision systems with small molecules.
Energy distribution in the photodissociation products of van der Waals molecules: Iodine–helium complexes
Authors: Sharfin Wayne , Johnson Kenneth E. , Wharton Lennard , Levy Donald H.
Published: February 2003

The process of photodissociation of the van der Waals molecules I2He, I2He2, and I2He3 has been studied. Vibronic state distributions of the electronically excited product I2* fragment produced upon photodissociation have been measured by observing the dispersed fluorescence spectra of these fragments. It was found that for each complex I2Hen, the first observed dissociation channel involved the loss of n vibrational quanta from the I2 stretch even though channels involving the loss of fewer quanta were energetically open. The n quantum channel was by far the dominant channel although some weaker n+1 processes were observed. Branching ratios are reported. It was found that the various complexes can be identified by a band shift rule which accurately predicts the spectral shift between the absorption features due to the complex and the absorption band of uncomplexed iodine. This rule says that the spectrum shifts a constant amount for each additional helium atom in the complex. The cross section for collisional vibrational relaxation of I2 by helium in a supersonic expansion was measured as a function of position from the supersonic nozzle. It was shown that the effective vibrational relaxation cross section increases as the translational temperature and relative kinetic energy decreases.
Mössbauer spectroscopic studies of ferric myoglobin single crystals in a small applied magnetic field
Authors: Harami T.
Published: February 2003

Mössbauer experiments on single crystals of some ferric myoglobin (Mb) compounds which include high spin compounds (S=5/2), metMb and MbF, and a low spin compound (S=1/2), MbN3, have been done in the presence of a small external magnetic field perpendicular to the γ rays at liquid He temperature. The Mössbauer spectra of these compounds were analysed by a spin Hamiltonian formalism. For metMb and MbF, saturation fields of 480±5 and 510±5 kOe and quadrupole interactions eOVzz/2 of 1.10±0.05 and 0.70±0.05 mm sec−1 were obtained, respectively. The value of a hyperfine coupling constant Az was determined from spectra obtained with the applied field nearly along the a axis. The difference between Az and A⊥ was less than 5% and 4% for metMb and MbF, respectively. For MbN3 the following Mössbauer parameters were obtained: an asymmetry parameter η=0.4±0.1, the quadrupole interaction eQVzz/2 =−2.10±0.05 mm sec−1 and an isomer shift=0.30±0.05 mm sec−1. The axis of the largest component of an electric field gradient is rather close to the heme plane and deviates by 32° from one of four Fe–N directions. Taking the results of EPR into account, it was found that the environment around the heme iron is lower than orthorhombic symmetry, being almost monoclinic symmetry.
Generalized electron gas–Drude model theory of intermolecular forces
Authors: Waldman Marvin , Gordon Roy G.
Published: February 2003

An electron gas theory of intermolecular forces which unifies the treatment of long- and short-range effects is improved and extended in several ways. (1) The shell polarization model takes full account of the forces from both cores and shells. (2) The amount of polarization is calculated with full self-consistency in the forces on the shells. (3) Exact agreement with the known long-range forces is achieved by an effective mass description of the shell dynamics. (4) Anisotropic polarizations appropriate to a linear molecule interacting with an atom are included. The theory is applied to the systems H2–He, H2–Ar and HCl–Ar. Fairly good agreement is found with the available but incomplete experimental data including the angle dependence of the potentials. However, some evidence suggest that the predicted repulsion is somewhat too large when a rare gas atom approaches a hydrogen atom along the molecular axis.
Fourier transform method for the classical trajectory problem
Authors: Nesbet R. K. , Clary D. C.
Published: February 2003

By introducing an impluse potential at an external boundary, or by reflecting the scattering potential at such a boundary, collision dynamics is converted into an equivalent bound-state problem. Recent theoretical developments in classical bound-state dynamics. which make use of the geometrical structure in phase space known as an invariant toroid, can then be applied to collision problems. Fast Fourier transform techniques are used to construct the invariant toroid, which is the locus of an indefinitely extended nonperiodic classical trajectory. This approach to collision dynamics is applied to the collinear atom–oscillator problem modelling He–H2 vibrational excitation. Results are compared to previous classical and quantum dynamical studies. The proposed method obtains in a single calculation the complete ensemble of trajectories that correspond to a semiclassical wave function. At given total energy and for given global action integral values all trajectories are parallel straight lines when expressed in the coordinate space of true angle variables obtained by the Fourier transform calculation. These aspects of the formalism reduce the computation of inelastic transition probabilities and action integrals to simple geometric exercises.
Rotational laser emission by HF in the ClF–H2 chemical laser
Authors: Cuellar Enrique , Pimentel George C.
Published: February 2003

Rotational laser emission from high J states of HF in the range 800–1100 cm−1 (v=0, J=22→21 to J=33→32; v=1, J=23→22 to J=29→28; and v=2, J=24→23 and J=25→24) has been initiated through flash photolysis of ClF: H2: Ar=2:1:23 mixtures at 63 Torr. The short threshold time (2–4 μsec) indicate that V→V up-pumping in HF‡–HF‡ collisions followed by V→R energy transfer cannot satisfactorily explain the observed emissions. The reaction H+ClF→HF+Cl must be involved in the pumping process, even though the branching ratio favors formation of HCl instead of HF. Two mechanisms are proposed and both could be operative. The rotationally excited HF that emits may be formed directly, as has been proposed by Polanyi and co-workers for this and other H+XY reactions on the basis of chemiluminescence results. It is also possible that multiquantum V→R energy transfer contributes population to the high J states. While HF and argon collision partners may participate in such pumping, the ClF molecule may be particularly effective because its closely spaced rotational levels allow near resonant energy transfer paths.
Orientational order and phase transitions in a two-dimensional triangular octopolar array
Authors: Maki Kazuo , Nosé Shūichi
Published: February 2003

As a prototypical model for a system of methane molecules adsorbed on a surface of graphite, we have carried out a classical molecular field theoretical investigation of a two-dimensional triangular octopolar array. Two successive phase transitions are found. The upper one is of second order, while the lower one is of first order. The changes in site symmetries are D6h→D3h→Cs in the order of decreasing temperature.
Effects of pressure on the dielectric properties and phase transitions of the 2-D antiferroelectric squaric acid: (H2C4O4 and D2C4O4)
Authors: Samara G. A. , Semmingsen Dag
Published: February 2003

The pressure dependences of the dielectric properties and phase transitions of squaric acid, H2C4O4, and its deuterated analog, D2C4O4, were investigated. The transition temperatures at 1 bar are 375.5±1.0 K and 527.5±2.0 K, respectively, and decrease with pressure at initial rates of 105.6±2.0 K/GPa and 102.5±2.0K/GPa. It is argued that the large hydrogen isotope effect on Tc can be accounted for on the basis of the elongation of the O–H⋅⋅⋅O bond resulting from deuteration. This elongation causes a modification of the double-well potential along this bond. It is not necessary to invoke proton tunneling. The lack of an appreciable isotope effect on dTc/dP supports the absence of tunneling. An interesting aspect of the results is the appearance of a dome-shaped feature in the real part of the dielectric constant just above Tc in both crystals. This feature, which is reminiscent of the behavior of the magnetic susceptibility of antiferromagnets above the Néel temperature, is discussed. The results on D2C4O4 suggest the possible existence of a pressure-induced phase transition at ∼0.8 GPa. The dielectric loss of this crystal is large both below and above Tc. This is attributed to the high conductivity of D2C4O4 on approaching the melting (or dissociation) temperature. The activation energies for conduction and their pressure dependences both above and below Tc are determined and discussed.
Counterpoise-corrected SCF and Gordon–Kim electron gas interaction potentials for NH3-He
Authors: Davis Stephen L. , Boggs James E. , Mehrotra Suresh C.
Published: February 2003

An SCF interaction potential surface has been calculated for the NH3–He system, using the ’’functional counterpoise’’ or ’’ghost orbital’’ method, to minimize superposition error. The resulting corrected SCF surface is in good agreement with near Hartree–Fock limit interaction energies. For comparison, electron gas surfaces for the system were computed with and without the Rae self-exchange correction, using both of the basis sets employed in the SCF calculations. The electron gas surface with inclusion of the Rae correction is in qualitative agreement with the SCF surface. The agreement deteriorates as the intermolecular separation R increases. Also, as R increases, the electron gas interaction energies become quite dependent on the basis set used to describe the molecular charge density.
Theoretical study of the four lowest doublet electronic states of the hydroperoxyl radical: Application to photodissociation
Authors: Langhoff S. R. , Jaffe R. L.
Published: February 2003

Potential-energy curves have been calculated for the four lowest doublet states of the hydroperoxyl radical as a function of the H–O–O bond angle and the oxygen–oxygen and oxygen–hydrogen bond lengths. A polarized double-zeta basis of Cartesian–Gaussian functions was used. An extensive configuration–interaction treatment and subsequent extrapolation procedure was used to obtain a uniform description of the four electronic states over a wide range of nuclear geometries. The calculated equilibrium oxygen–oxygen bond lenths for the ground state, (1)  2A″, and the low-lying (1)  2A′ excited state are in excellent agreement with the values determined from absorption spectra in the near infrared. An analysis of the shapes of the potential curves for the (2)  2A″ and (2)  2A′ states, as a function of the oxygen–oxygen and oxygen–hydrogen bond lengths, indicates that OH and atomic oxygen should be the predominant photodissociation products. Calculated dipole moments are reported for each state. These results indicate that the (2)  2A″ and (2)  2A′ states are quite ionic. The (1)  2A′← (1)  2A″ transition in the near infrared is calculated to be quite weak, giving a radiative lifetime for the (1)  2A″ state of about 1 msec. The most intense transition is (2)  2A″← (1)  2A″ and it accounts for the broad continuum feature observed in the UV absorption spectrum with a maximum near 210 nm. Absorption cross sections for HO2 as a function of wavelength are calculated form the theoretical data and are found to be in excellent agreement with the measurements of Paukert and Johnston. The theoretical results support the HO2 photodissociation data that are at present used at Ames Research Center in model studies of ozone depletion.
Investigations of Soret excited resonance Raman excitation profiles in cytochrome c
Authors: Champion P. M. , Albrecht A. C.
Published: February 2003

Resonance Raman excitation profiles of the ’’oxidation state marker bands’’ in ferric- and ferrocytochrome c are obtained using tunable laser excitation in the Soret region (350–450 nm). The excitation profiles are asymmetric and peak distinctively to the blue of the Soret absorption maximum. The data are explained using a simple model that analytically includes the complete subspace of Franck–Condon active vibrations associated with each intermediate electronic state. In this simple multimode model the damping factor is a ’’soft’’ fitting parameter taken to be wavelength independent. Nevertheless, an excellent fit of the Soret absorption band and Raman excitation profile is achieved. Our theoretical approach places upper limits on the Soret damping factors that are on the order of 50 cm−1 (HWHM) for ferrocytochrome c and on the order of 300 cm−1 (HWHM) for ferricytochrome c.
Laser magnetic resonance spectrum of the ν2 band of PH2
Authors: Hills G. W. , McKellar A. R. W.
Published: February 2003

The ν2 bending fundamental band (ν0=1101.908 cm−1) of PH2 in the ? 2B1 ground electronic state has been studied using CO2 laser magnetic resonance in the 9.0 to 9.6 μm region. The short-lived PH2 radicals were produced in a flow system by the reaction of H atoms with PH3 molecules. A total of 25 molecular parameters (3 rotational, 5 distortion, and 4 spin-rotation constants in each of the (000) and (010) vibrational states, plus the ν2 band origin) have been determined from a least-squares fit of 122 observed resonances. The (000) state parameters are more accurate than, and in satisfactory agreement with, previous spectroscopic determinations, and the changes in the parameters upon excitation of the ν2 bending mode are discussed.
Ratio analysis of electron spin echo modulation envelopes in disordered matrices and application to the structure of solvated electrons in 2-methyltetrahydrofuran glass
Authors: Ichikawa Tsuneki , Kevan Larry , Bowman Michael K. , Dikanov S. A. , Tsvetkov Yu. D.
Published: February 2003

A new analysis for two pulse and three pulse electron spin echo modulation data is presented which is independent of the echo decay function. The experimental ratios of maxima to minima of the echo modulation curve are compared with simulated ones to obtain the number of interacting nuclei, their distances, and their isotropic coupling by a least squares fit. From the best simulation, the empirical echo decay function can be determined and the original experimental data can be simulated. This new method is applied to new three pulse and old two pulse [L. Kevan et al., J. Chem. Phys. 63, 409 (1975)] electron spin echo data on solvated electrons in 2-methyltetrahydrofuran (MTHF) glass at 77 K. This analysis together with previous second moment data gives a revised model for electron solvation in this matrix based on a statistical orientation of the MTHF ring carbon planes toward the electron. In this statistical configuration model three equivalent MTHF molecules form the first solvation shell with their ring carbon atom planes perpendicular to the electron and oriented statistically. The closest protons are 3.4 Å from the electron with a proton hyperfine constant of 0.4 MHz which is equivalent to a deuteron hyperfine constant of 0.06 MHz.
Barrier to internal rotation in ethane from the microwave spectrum of CH3CHD2
Authors: Hirota Eizi , Saito Shuji , Endo Yasuki
Published: February 2003

Rotational spectra of CH3CHD2 were observed by using a source-modulation microwave spectrometer. Of 26 observed transitions, 16 showed splittings due to internal rotation. An analysis of the observed spectra, which utilized an internal axis method modified to include centrifugal distortion effects, gave the following results: A=60 809.592 (59) MHz, B=17 770.87 (16) MHz, C=17 084.84 (16) MHz, F=257.70 (35) GHz, and V3=2.8783 (28) kcal/mole, with 2.5 times standard errors in parentheses. In the analysis V6=0 was assumed, and D, a constant associated with a cross product of inertia, was fixed to a calculated value. When the uncertainty in D is taken into account, the error of V3 increases to 0.0029 kcal/mole. Effects of the molecular vibrations on the barrier are discussed briefly.
Infinite order sudden approximation for reactive scattering. I. Basic l-labeled formulation
Authors: Khare V. , Kouri D. J. , Baer M.
Published: February 2003

An infinite order sudden (IOS) treatment of reactive scattering is developed taking into account recent results of nonreactive collision studies on the importance of l-labeling, nonconservation of helicity, and transformation properties of sudden approximation wave functions. The present IOS method should be sufficiently simple to apply to a number of chemically interesting atom–diatom reactions. Such applications are currently in progress.
Multiple-photon excitation of difluoroamino sulfur pentafluoride: A study of absorption and dissociation
Authors: Lyman John L. , Danen Wayne C. , Nilsson Alan C. , Nowak Andrew V.
Published: February 2003

Multiple-photon absorption and dissociation experiments with difluoroamino sulfur pentafluoride (SF5NF2) along with results of previous experiments with SF6 and S2F10 have led us to conclude that while the density of vibrational states of a polyatomic molecule influences the absorption process, the specific structure of an absorbing species is probably a more important factor. Absorption of intense infrared radiation by SF5NF2 occurs with virtually no decrease in the absorption cross section, up to a fluence of 0.03 J/cm2. This fluence level is sufficient for isotopically selective cleavage of the weakest molecular bond (S–N). The reaction probability increases with about the third power of the laser energy fluence. The infrared absorption spectrum of SF5NF2 has three strong absorption bands between 850 and 1000 cm−1. Reaction probability is much greater for irradiation at frequencies within the center band than within the high frequency band.