bluuebunny/crossref_metadata_2025 huggingface dataset:
Longitudinal diffusion coefficients of Li+ and Na+ ions in He, Ne, and Ar: Experimental test of the generalized Einstein relation
Authors: Pai R. Y. , Ellis H. W. , Akridge G. R. , McDaniel E. W.
Published: February 2003

We have measured, with a drift tube mass spectrometer, the longitudinal diffusion coefficients of Li+ and Na+ ions in He, Ne, and Ar at a gas temperature of 300 °K. The measurements were made as a function of the energy parameter E/N, where E is the electrostatic drift field strength and N is the gas number density in the drift tube. The range of the steady-state, average ionic energy thereby covered extended from close to the thermal value up to 10.1 eV in the case of Li+ in He and a few eV less in the other ion–atom combinations. The experimental data are compared with the results of computations based on three different equations: the original Wannier equation derived for the polarization attraction model, our modification of this equation, and the generalized Einstein relation discussed in the paper immediately preceding this paper [Larry A. Viehland and E. A. Mason, J. Chem. Phys. 63, 2913 (1975)]. The latter relation is superior to the other two in the present application and gives excellent agreement with our experimental data. This is the same conclusion that was reached by Viehland and Mason in their analysis of our data on K+ ions in He, Ne, and Ar.
Laser-induced rate processes in gases: Reduction of generalized to ordinary master equation
Authors: Goodman Myron F. , Stone James , Thiele Everett
Published: February 2003

The generalized master equation (GME) describing the time evolution of energy level populations for a laser-pumped molecular gas, suffering phase-changing and thermalizing collisions, is analyzed in the weak radiation field limit for a two-level absorber. We show that the GME reduces to an ordinary master equation (OME) when the absorber response is perturbed by collisions, by phase interruptions in the light field, or by a combination of both. The GME to OME reduction is possible only in the low intensity and weak collision limits thereby placing a stringent limitation on the use of ordinary master equations to obtain accurate estimates of the evolution of energy level populations.
Phase coherence and collision models in radiatively driven oscillators
Authors: Stone James , Thiele Everett , Goodman Myron F.
Published: February 2003

We consider the effects on the energy absorbed and the distribution of energy resulting from a phased response of a molecule to a driving laser field. Contrasting a Landau–Teller collision model with a strong collision model we compare the dynamic and steady-state solutions to our generalized master equation with solutions to an ordinary master equation, for truncated (finite-level) and nontruncated (infinite-level) harmonic oscillators. We find significant coherent enhancement in upper energy level populations which is most pronounced when an intense but below saturation laser field is present; these coherent effects, predicted for truncated oscillator level populations, are contrasted with the infinite level cases. We show that at steady state an ordinary master equation predicts the average energy of the harmonic oscillator correctly but fails to predict the correct population distribution of energy states.
Hydrogen abstraction reactions by atomic fluorine. III. Temperature dependence of the intermolecular kinetic isotope effect for the thermal F+H2 reaction
Authors: Grant Edward R. , Root John W.
Published: February 2003

The intermolecular kinetic isotope effect for the title reaction has been investigated using a moderated nuclear recoil technique. The following Arrhenius temperature dependence was obtained: kH2/kD2 = (1.04±0.06)exp[(382±35)/RT]. This result is in quantitative agreement with an independent study based upon a discharge flow reactor method. The advantages and limitations of the nuclear recoil procedure are considered, and relevant literature results are critically reviewed.
CS(A 1Π→X 1Σ+) fluorescence from photodissociation of CS2 and OCS
Authors: Lee L. C. , Judge D. L.
Published: February 2003

The CS(A 1Π→X 1Σ+) fluorescence resulting from photodissociation of CS2 and OCS by vacuum ultraviolet emission lines extending from λλ686–1239 Å has been investigated. With the assumption that the electronic transition moment is constant, the Franck–Condon factors for the emission system and the populations of the vibrational levels of the CS(A 1Π) electronic state have been determined. The population data are approximately represented by a Poisson distribution, which is predicted from a theoretical model. A population inversion between the v=0 and 1 levels of the CS(A 1Π) state is found. The cross sections for the production of fluorescence are also presented.
Vibrational energy transfer in polyatomic molecule collisions. II. Role of multiquantum transitions during very energetic collisions
Authors: Jolicard G.
Published: February 2003

In a previous paper the processes of vibration energy transfer during a collision between two polyatomic molecules were studied using an operational calculation approach. This has enabled us to present analytical expressions for the probabilities of various events occuring during the collision of two diatomic molecules. A comparison with previous numerical calculations confirms the presence and the influence during these transfers of mechanisms which alter the characteristics of the nonperturbed oscillators and of multiquantum transitions.
The a3Σ+ → X1Σ+ and b3Π → X1Σ+ band systems of SiO and the a3Σ+ → X1Σ+ band system of GeO observed in chemiluminescence
Authors: Hager Gordon , Harris Richard , Hadley Steven G.
Published: February 2003

Chemiluminescence from SiO and GeO has been produced in a flowing gas system utilizing the reactions Si(3P) + N2O → SiO(b3Π,a3Σ+) + N2 ΔH = −150 kcal/mole Ge(3P) + N2O → GeO(a3Σ+) + N2 ΔH = −130 kcal/mole. An analysis of the chemiluminescence spectrum has shown that the emission originates in the b3Π and a3Σ+ states and terminates in the ground electronic state of SiO. The measured value of the spin–orbit coupling constant for the b3Π state was 75 cm−1. No vibrational constants for either the a3Σ+ or b3Π states could be determined. T0 for the b3Π state was found to be 33 853 cm−1 while that for the a3Σ+ state was found to be 33 409 cm−1. An analysis of the chemiluminescence of GeO was found to originate from the a3Σ+ state and terminate in the electronic ground state. T0 for the a1Σ+ state of GeO was found to be 27 552 cm−1. Good agreement between the experimentally observed vibronic band intensities and those calculated using the Rydberg–Klein method has been obtained.
Low-energy (≲3 eV) electron attachment to molecules in very-high pressure gases: C2H5Br
Authors: Goans R. E. , Christophorou L. G.
Published: February 2003

Significant changes have been observed in both the magnitude and the energy dependence of the rate of attachment of slow (≲3 eV) electrons to bromoethane (C2H5Br) in either N2 or Ar gases with increasing density of these media from 500 to 25 000 Torr for N2 and from 500 to 42 000 Torr for Ar. The observed density and energy dependences of the attachment rates are discussed on the basis of two plausible reaction schemes involving either one or two states of C2H5Br−. On the basis of the one-state reaction scheme, determinations were made of the absolute rate of formation of C2H5Br−* and of the relative magnitudes of the rates for dissociation and autoionization of the metastable C2H5Br−* ion as a function of the mean electron energy 〈ε〉. On the basis of the same reaction scheme, the lifetime of C2H5Br−* was estimated to be ?13 psec in the mean energy range 0.3–1 eV. Although the vertical attachment energy for the C2H5Br molecule is ∼0.75 eV, the present results imply that its adiabatic electron affinity is positive (≳0 eV).
Resolution corrections in total scattering cross section measurements
Authors: Gislason Eric A.
Published: February 2003

An expression is derived for the resolution correction based on the measured incident beam profile in the detector plane. The results are valid for any experimental configuration. Comparison is made with earlier treatments of this correction. The result given here will permit more accurate analysis of total scattering cross section measurements. Sample calculations are described and applied to recent determinations of the He–He potential. Finally, an expression is obtained for the small-angle elastic differential cross section which can be used to deconvolute experimental measurements.
Gas phase ion chemistry of HNO3
Authors: Fehsenfeld F. C. , Howard Carleton J. , Schmeltekopf A. L.
Published: February 2003

The ion chemistry of HNO3 is studied at 298 °K. The attachment of thermal electrons to HNO3 to produce NO−2 is found to have a rate constant of (5±3) ×10−8 cm3/sec. HNO3 reacts rapidly (k∼10−9 cm3/sec) with a large variety of negative ions including Cl−, NO2−, O2−, and CO−3. HNO3 is also found to bond strongly to NO3−. The proton affinity of HNO3 is determined to be 176±7 kcal/mole. The present results indicate a strong similarity between protonated HNO3 and hydrated NO2+, suggesting that these ion species are identical. The implication of the present results for atomospheric ion chemistry is discussed.
Self-consistent time-dependent Hartree–Fock calculations of dynamic polarizabilities, dispersion forces, and nuclear spin–spin coupling constants for the H2 and HF molecules
Authors: Oddershede Jens , Jo/rgensen Poul , Beebe Nelson H. F.
Published: February 2003

We present a self-consistent time-dependent Hartree–Fock scheme for calculating excitation frequencies, oscillator strengths, dynamic polarizabilities, dispersion forces, and indirect nuclear spin–spin coupling constants. Ab initio applications on the H2 and HF molecules have been performed. The effect of two-particle, two-hole excitation corrections are included. The results show that the method offers an accurate and economical alternative to configuration interaction for introducing correlation in second order properties. The spin–spin coupling constants, which depend on the triplet excited states, are most influenced by the self-consistency procedure.
Separation of isotropic and anisotropic hyperfine constants in disordered systems by analysis of electron paramagnetic resonance lines at two different microwave frequencies: Application to the molecular structure around excess electrons in γ-irradiated 10M sodium hydroxide alkaline ice glass
Authors: Bales Barney L. , Bowman Michael K. , Kevan Larry , Schwartz Robert N.
Published: February 2003

It is shown that the second moment of an unresolved electron paramagnetic resonance line in disordered systems can lead to separation of the isotropic and anisotropic hyperfine couplings if the spectrum is recorded at high enough microwave frequency to separate the main line from nuclear spin flip satellite lines. If resolution of main and satellite lines is not complete, separate determination of the isotropic and anisotropic couplings is still possible if measurement can be made at two different microwave frequencies of which one frequency is high enough to partially resolve the main and satellite lines. This analysis is applied to excess electrons trapped in 10M NaOH alkaline ice glass at 77 K and the isotropic and anisotropic coupling constants are determined as a function of n, the number of equivalent nearest neighbor protons. From electron spin echo modulation patterns and the intensity and separation of satellite lines from the main line, the best value of n is determined to be 6. The value n=6 corresponds to a trapped electron surrounded by three water molecules with their molecular dipoles oriented toward the electron (2 protons per water molecule) or six water molecules with one OH bond oriented toward the electron. In both cases the protons are 2.1 Å from the electron with an isotropic coupling constant of ∼1–2 G. This agrees with independent measurements from analysis of electron spin echo modulation patterns.
Definitions and properties of generalized collision cross sections
Authors: Coombe D. A. , Snider R. F. , Sanctuary B. C.
Published: February 2003

A unified classification of binary collision cross sections is presented. This is accomplished with the aid of a collision superoperator, in terms of which both differential and kinetic theory cross sections are defined. Connections and comparisons are then made with standard expressions for chemical reaction cross sections and with those ’’spectral’’ cross sections describing collisional widths, shifts and overlapping of spectral lines. A detailed treatment of rotational invariance is included. Finally, it is shown how this general framework encompasses the more complicated cross sections that arise in the study of the field dependence of gas transport coefficients.
A vibrational correction to coherent x-ray scattering intensities of diatomic molecules
Authors: Stewart Robert F.
Published: February 2003

Born’s theory for the vibrational averaging of coherent x-ray scattering has been implemented with the use of generalized x-ray scattering factors. Estimates at 300 and 2000 °K for H2, N2, and CO are reported, where the generalized x-ray scattering factors are truncated at the quadrupole level. The vibrational effect gives a scattered intensity which oscillates about the intensity of the nonvibrating molecule with increasing scattering angle. The estimated vibrational correction is no larger than 1% for N2 and CO over the range of the scattering vector (S?15.5 bohr−1 or sinϑ/λ?2.33 Å−1) used in this work. For H2, however, the correction is as large as 5%.
Ab initiostudy of secondary isotope effects on molecular structure
Authors: Bartell L. S. , Fitzwater Susan , Hehre Warren J.
Published: February 2003

An ab initio calculation of a secondary isotope effect on a bond length has been carried out for the first time. Single determinant molecular orbital computations indicate that the mean C–C bond length at 0 °K is longer in C2H6 than in C2D6 by 0.0015 Å. A comparison with model calculations supports a steric interpretation in which H atoms act as if they are bulkier than D atoms owing to their larger bending amplitudes of vibration.
Mass-reduced quantum numbers: Application to the isotopic mercury hydrides
Authors: Stwalley William C.
Published: February 2003

The concept of mass-reduced quantum numbers is introduced and discussed. For a diatomic molecule, the mass-reduced vibrational quantum number η= (v+1/2)/(μ)1/2 and mass-reduced rotational quantum number ξ=J (J+1)/μ are used and exemplified. Isotopically combined methods of potential determination from spectroscopic data follow from introduction of these quantum numbers, e.g., the isotopically combined Rydberg–Klein–Rees (RKR) method. These concepts are applied in detail to the isotopic mercury hydrides. It is shown that mass-reduced functions exist which accurately describe, for example, vibrational spacings [ΔG (η)(=dG/dη) = μ1/2ΔG (v+1/2)], rotational constants [B (η) =dE (η,ξ)/ dξ=μB (v)], and centrifugal distortion constants [D (η) =μ2D (v)]. The vibrational and rotational mass-reduced functions are used to construct an isotopically combined RKR potential. This potential, when joined to the long-range potential and extrapolated at short range reproduces the energy levels fairly well. In a companion paper, it is shown to yield the energy dependence of the total scattering cross section accurately. It should be noted that many other possible methods for potential determination are inadequate in this case. The interesting pseudocrossing region (∼3–5 Å), where an unusual flat shoulder in the potential occurs, is discussed, as are the two loci of barrier maxima (the extra locus arising from this shoulder). In Appendixes A, B, and C, new assignments of the 0–6 band of the A–X system of HgD are reported, the long-range C8 coefficient estimated, and the dissociation energies of HgH and HgD accurately determined to be D0=3020+1−7 cm−1 and 3207±1 cm−1, respectively.
The role of complex trajectories in elastic scattering
Authors: Gordon Robert J.
Published: February 2003

Undulations in differential elastic cross sections have previously been attributed to interference between the amplitudes of classical trajectories having the same observed scattering angle. From this point of view, oscillations resulting from a monotonically repulsive central potential are hard to understand, since in this case there is no interference from real trajectories. We have examined in detail the complex trajectories for the potential energy function 4r−12, and found that there are complex impact parameters with real positive and negative scattering angles. We showed that the semiclassical formulation of Ford and Wheeler is accurate, provided that the first complex stationary phase point for negative scattering angles is included.
Photodissociation of alkyl iodides
Authors: Donohue T. , Wiesenfeld J. R.
Published: February 2003

The branching ratios for formation of I (5 2P1/2) and I (5 2P3/2) following broad-band photolysis of a number of alkyl iodides and their fluorinated and deuterated analogs have been directly determined using time-resolved atomic absorption spectroscopy. The yields of ground and electronically excited atoms are strongly dependent on the structure of the alkyl group, those possessing secondary or tertiary α-carbon atoms displaying the lowest yields. The observed correlation between alkyl group structure and branching ratio is discussed using simple symmetry arguments to assess the role of spin–orbit coupling in the photodissociative excited state.
Pair approximation equations for interfaces and free surfaces in the Ising model
Authors: Weeks John D. , Gilmer George H.
Published: February 2003

We present a simple derivation of the pair approximation equations for spin distribution functions in the Ising model. In a uniform system, the pair equations are equivalent to those found by the Bethe or the quasichemical methods. Our derivation requires self-consistency between approximations for the singlet and pair distribution functions and is easily generalized to study inhomogeneous systems. A simple and efficient numerical method for solution of the equations is presented. We study in particular properties of the interface or free surface in an anisotropic Ising model with coupling between spins in adjacent planes given by J⊥, which may differ from the coupling between spins in the same plane. In the limit J⊥→∞, the model is equivalent to the ’’solid-on-solid’’ model used in theories of crystal growth. In this limit the mean field theory for the interface fails completely, while the pair approximation gives results at low temperatures in good agreement with previous calculations. Other possible applications for the pair equations are briefly discussed.
Quantum vibrational transition probabilities from real classical trajectories: Collinear atom–diatom collisions
Authors: Gentry W. Ronald , Giese Clayton F.
Published: February 2003

Quantum vibrational transition probabilities may be estimated from single, real classical trajectories by exploiting the approximate correspondence between the classical and quantal motions. In this paper we explore the range of validity of several such correspondence methods for collinear atom–diatom collisions by comparing the results with those from rigorous quantum mechanical calculations. The model systems considered here include: (a) a harmonic oscillator with (1) a repulsive exponential, (2) a Landau–Teller, and (3) a Lennard-Jones interaction potential, and (b) a Morse oscillator with a repulsive exponential interaction potential. The range of the dimensionless mass parameter M is 0.00628 to 3.737 and the total system energy ranges from 1.55 to 10.0 in units of h/ωe. For both Morse and harmonic oscillator excitation the semiclassical results are generally more accurate for small M and for small initial and final vibrational quantum numbers. Quantitatively, the results are consistently better for a harmonic oscillator than for a Morse oscillator. The threshold behavior for harmonic oscillator excitation can be predicted quantitatively using a classical trajectory in which the arithmetic mean of the initial and final speeds is equal to the arithmetic mean of the translational speeds for the corresponding quantum transition. By detailed balancing, the probabilities for vibrational de-excitation in slow collisions are also predicted accuretaly.
Triplet diffusion and energy transfer in molten naphthalene
Authors: Kellogg R. B. , Prock A.
Published: February 2003

The diffusion length of triplet excitation in molten naphthalene near the melting point has been determined to be 10±2×10−3 cm. From this length and the measured lifetime, 0.140±0.06 sec, in the molten state the diffusion coefficient has been calculated to lie between 0.3×10−3 and 1.8×10−3 cm2 sec−1. This value is greater by more than an order of magnitude when compared to ’’molecular’’ diffusion coefficients. It is therefore taken as evidence for the existence of triplet excitons in molten naphthalene.
Shock tube decomposition of dilute mixtures of nitrogen trifluoride in argon
Authors: Dorko Ernest A. , Grimm U. W. , Scheller K. , Mueller Gerhard W.
Published: February 2003

The decomposition of 0.25%, 0.50%, and 1.00% NF3 in argon was studied in a reflected shock wave. In these dilute mixtures the unimolecular decomposition pathway can be studied independently of other modes of decomposition. The temperature range of the study was 1500–2400 °K and the total pressure range was 0.8–60 atm. The total gas concentration behind the reflected shock ranged from 5×10−6 to 3.6×10−4 mol/cc. Kinetic measurements were made by following the decay of emission of the ν3+ν1 combination band of NF3 at 5.18 μ. The unimolecular rate constants of the Lindemann mechanism were obtained from the emission decay data and were converted to second order rate constants by dividing by the total gas concentration. The Arrhenius parameters calculated by a least squares analysis of the second order rate constants are given as k=1014.7 exp(−56 300/RT) cc/mol sec. The activation energy for the reaction is very close to the first N–F bond energy of 57 kcal/mol. Calculations using the Slater integral were in good agreement with experimental results. The results suggest that under the reported conditions vibrational equilibrium is not maintained during reaction.
The formation and breakup of NO+⋅N2 clusters in N2 at low temperatures
Authors: Johnsen Rainer , Huang Chou-Mou , Biondi Manfred A.
Published: February 2003

Drift tube–mass spectrometer apparatus have been used to determine the equilibrium constant K and the forward rate coefficient k+ for the clustering reaction NO++N2+N2?NO+⋅N2+N2. The values of K are 2×10−16, 1.5×10−18, and ?5×10−19 cm3 at 130, 180, and 220 K, respectively, with an estimated uncertainty of ∼50%. The values of k+ are 8×10−30 and ?10−30 cm6/sec at 130 and 220 K, respectively, with an estimated uncertainty of ∼30%. The energy of the NO+–N2 bond deduced from the equilibrium constant data is Eb=0.18±0.02 eV. These results are compared with other measurements, and implications for proposed D-region reaction paths linking NO+ and H3O+⋅ (H2O)n ions are discussed.
Structure determination by dipolar coupling measurements of dilute nuclear spins in a polycrystalline solid
Authors: Hester R. K. , Cross V. R. , Ackerman J. L. , Waugh J. S.
Published: February 2003

The nuclear double resonance technique used to observe the dipolar oscillations in solids—oscillatory exchange of order between rare and abundant spins—has been applied in the study of polycrystalline benzene at 200 K. Results indicate that this technique can provide accurate structural parameters from powdered samples.
Diffraction pattern and structure of aqueous hydrochloric acid solutions at 20 °C
Authors: Triolo R. , Narten A. H.
Published: February 2003

Hydrochloric acid solutions have been studied at concentrations from pure water to saturation with both x rays (HCl/H2O) and thermal neutrons (DCl/D2O). Such an extensive study reduces greatly the nonuniqueness of interpretation usually associated with diffraction data from liquids. The structure of D2O molecules in liquid water is very similar to that found in the vapor phase, the OD distance of 0.955(5) Å being identical to the equilibrium distance in the vapor molecules. The existence of D3O+ ions with an OD distance of 1.017(5) Å in concentrated DCl is firmly established. The D3O+(H3O+) ions are hydrogen bonded to four water molecules with an H3O+⋅⋅⋅OH2 distance of 2.52(2) Å and a D3O+⋅⋅⋅D distance of 1.61(2) Å. The Cl− ions appear to be four-coordinated by oxygen atoms at a mean distance of 3.13(1) Å, but no pronounced Cl⋅⋅⋅D distances indicating a well-oriented hydration shell about Cl− could be identified.
High pressure studies on spiropyrans
Authors: Wilson D. G. , Drickamer H. G.
Published: February 2003

High pressure optical measurements have been made on two spiropyrans dissolved in films of polystyrene (PS) and polymethyl methacrylate (PMMA). Three types of studies were made. (1) In the pressure region 30–140 kbars we observed pressure induced coloration (piezochromism) which was independent of time, rate of pressure application, or irradiation. The evidence favors a direct thermal transition between the uncolored and colored ground states. The volume change involved in the transition is of the order of 0.5–1.5 cm3/mol. (2) In the pressure range 8–40 kbars, it was demonstrated that pressure drastically reduced the efficienty of the photochromic process. This process proceeds via a triplet state of the uncolored molecule. With increasing pressure the energy of a triplet state increases with respect to the excited singlet state, which reduces the probability of intersystem crossing and therefore of formation of the colored species. (3) In the pressure range 0–10 kbars the rates of thermal bleaching were studied and discussed in terms of a current theory of the processes involved. Some measurements of rates of photocoloration and photobleaching are also described.
Reactions of ions in excited electronic states: (CO+⋅) *+CO→C2O+⋅+CO
Authors: Bowers Michael T. , Chau Marian , Kemper Paul R.
Published: February 2003

The formation of C2O+⋅ ions from electronically excited CO+⋅ ions has been studied in an ion cyclotron resonance (ICR) spectrometer. By analysis of the data it was possible to determine the rate constant for formation of C2O+⋅ to be k3⩽ (8.5±1.5) ×10−10 cm3 sec−1⋅molecule−1, the lifetime of the precursor CO+⋅ excited state to be k4−1⩾10−2 sec and the ratio of active CO+⋅ excited states to the remainder of the CO+⋅ formed in the electron beam to be k2/k1⩽3.8±0.4×10−3. A comparison is made between the CO results presented here and those for the isoelectronic N2 system reported earlier [J. Chem. Phys. 61, 4394 (1974)]. It is suggested that a barrier to N4+⋅ formation from electronically excited N2+⋅ quartet ions may exist in the nitrogen system from spin correlation rule considerations. Reasons for the existence of the barrier in the N2 system and the apparent absence of such a barrier in CO are suggested. The heat of formation of C2O+⋅ is approximately determined to be ΔHf(C2O+⋅) =392±10 kcal/mole.
Diffusion in two dimensions: Comparison between diffusional fluorescence quenching in phospholipid vesicles and in isotropic solution
Authors: Vanderkooi J. M. , Fischkoff S. , Andrich M. , Podo F. , Owen C. S.
Published: February 2003

The formalism developed in a previous paper is applied to the study of diffusion in an isotropic medium and in phospholipid vesicles as models for spherical and cylindrical diffusion theory, respectively. Pyrene was chosen as a fluorescence probe and diffusion was studied by observing quenching of monomer fluorescence by excimer formation at high pyrene concentration. The process of diffusion-dependent fluorescence quenching was better described by a theory which takes into account the finite time required to establish a diffusion gradient than by a time-independent theory. Furthermore, better fit to the data was obtained when the appropriate diffusion model (spherical or cylindrical) was used. The diffusion coefficient for pyrene in dimyristoyl lecithin at 30 °C was 3×10−8 cm2/sec. Proton magnetic resonance was used to determine the location of pyrene in the lipid.
Observation of 13C–14N dipolar couplings in single crystals of glycine
Authors: Griffin R. G. , Pines A. , Waugh J. S.
Published: February 2003

Dipolar splittings are normally not observable because the large number of nuclear couplings usually present produces extensive broadening of the NMR lines. In this note the 13C–14N splitting in glycine is observed by using rf fields of sufficient strength to decouple the protons from the magnetically dilute nuclei. (AIP)
Effect of temperature on the yield of solvated electrons in liquid ethanol
Authors: Dixon Robert S. , Lopata Vincent J.
Published: February 2003

The decay of the solvated electron and the relative yields of free and geminate solvated electrons have been measured in ethanol in the range 150–290 °K. The estimated absolute yields at low temperature agree well with sub-nanosecond values at room temperature, however the yield of solvated electrons appears to be substantially less than the total ionization yield. (AIP)
Pressure dependence of the isothermal compressibility and a modified form of the Tait equation
Authors: Wilhelm Emmerich
Published: February 2003

The pressure dependence at low pressures of the isothermal compressibility βT of liquids has been computed via a van der Waals type model of the liquid state for the temperature range 1 ? T/Tt ? 1.4 (where Tt denotes the triple point temperature). The results are in good agreement with the scarce experimental data: for most organic solvents, the quantity −(∂βT/∂P)T/βT2 ranges from 9–11. For water, the theory correctly predicts a much lower value. In the indicated range, the variation with temperature is small, in agreement with experiment and suggested by a modified Tait equation.
Dissociation processes of polyatomic molecules
Authors: Band Yehuda B. , Freed Karl F.
Published: February 2003

A general quantum mechanical theory is considered for the diverse dissociation processes of polyatomic molecules. The theory emphasizes the use of the proper normal vibrational coordinates for both the initial state of the polyatomic molecule and the final state of the dissociation fragments. The present theory provides a significant advance over all previous ones that invoke the quasidiatomic hypothesis, wherein the reaction coordinate is taken to be an initial normal mode, the remaining fragment normal modes are assumed to be identical to initial state normal modes, and the dissociation into vibrationally excited fragments is assumed to arise from the final state interactions occurring during the recoil of the fragments. Our more general theory of the vibrational and translational energy distributions in the fragments leads to a generalized Franck–Condon description of dissociation processes in terms of multidimensional bound-continuum matrix elements which can be reduced to one-dimensional ones that are amenable to analytic approximation. The Franck–Condon theory leads to simple criteria for the occurrence of fragment vibrational population inversions. These criteria are dependent on known spectroscopic constants for both the initial molecule and fragments and the repulsive potential between the receding fragments. The effects of final state interactions on the product vibrational distribution are incorporated by the use of a semiclassical model which demonstrates that the Franck–Condon rearrangement type process is dominant for a large number of systems. The theory is explicitly used to evaluate the vibrational distributions for photodissociation of HCN and ICN. The theory may also be applied to predissociation, unimolecular decomposition, and collisional induced dissociation.
Theory of propagation of discontinuities in kinetic systems with multiple time scales: Fronts, front multiplicity, and pulses
Authors: Ortoleva Peter , Ross John
Published: February 2003

We consider a reaction–diffusion system far from chemical equilibrium, with kinetics on multiple time scales and the capacity of attaining one or multiple homogeneous stable steady states. For such systems we present a theory of the structure (concentration profile) and velocity of fronts based on a matched asymptotic solution of the singular perturbation problem. We analyze single front (wave) propagation as a transition occurs from one stable stationary state to another; show the possibility of multiple mode front propagation; and discuss single pulse propagation in systems with one stable stationary state. For each process we confirm the theory by comparison with numerical solutions of the coupled differential equations for a model system. Finally, by application of the theory we discuss a reduction of the complexity of the stability analysis of these fronts.
19F magnetic resonance absorption in solid tetrafluorides
Authors: Knispel R. R. , Petch H. E. , Pintar M. M.
Published: February 2003

The field dependence of the second moment of the fluorine nuclear magnetic resonance absorption was studied in PbF4, ZrF4, CeF4⋅nH2O, VF4, and TiF4 over the 9–18 kG range. The chemical shift and the zero field value of the second moment are discussed in terms of the known general features of the respective crystal structures and comparisons are made with the results on other tetrafluoride compounds which have been previously reported.
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o and semiempirical study of multiple surfaces and their analytic continuation for collinear F(2P3/2, 2P1/2)+H2→FH+H
Authors: Jaffe Richard L. , Morokuma Keiji , George Thomas F.
Published: February 2003

The potential energy surfaces for the 2Π and 2Σ states in the collinear H2+F(2P3/2,1/2) →HF+H reaction have been calculated with ab initio restricted Hartree–Fock (GRHF) and GRHF–CI (configuration interaction) methods. The 2Π state in the CI calculations exhibits a well in the entrance valley. The effects of spin–orbit interaction have been examined in terms of a 2×2 secular equation in which the experimental spin–orbit coupling constant for the fluorine atom is used. GRHF calculations for complex values of nuclear coordinates are carried out to locate complex intersection points. Furthermore, two sets of semiempirical potential energy surfaces are constructed to simulate GRHF and GRHF–CI results. Dynamical implications of these surfaces are discussed in terms of complex intersections and nonadiabatic coupling constants.
Electronic energy level and intensity correlations in the spectra of the trivalent actinide aquo ions. II. Cm3+
Authors: Carnall W. T. , Rajnak K.
Published: February 2003

The free-ion energy level scheme of Cm3+ (aquo) to ∼40 000 cm−1 was determined. Energy level assignments were made based on the correlation between observed band intensities and those calculated using the Judd–Ofelt theory. Use of an expanded set of effective operators provided the basis for computation of free-ion energies in good agreement with the experimental assignments.
Configuration interaction studies on low-lying states of O2
Authors: Moss Barry J. , Goddard William A.
Published: February 2003

We report configuration interaction calculations as a function of internuclear distance for the nine states of O2 corresponding to the (3σg)2 (1πu)4 (1πg)2 and (3σg)2 (1πu)3 (1πg)3 configurations, that is, X 3Σ−g, a 1Δg, b 1Σ+g, c 1Σ+g, C 3Δu, A 3Σ+u, B 3Σ−u, 1Δu, 1Σ+u. By using the generalized valence bond (GVB) orbitals of the X 3Σ−g state, we obtain good quality GVB–CI wavefunctions with only a moderate number of configurations (72 to 98 spatial configurations) despite the use of a large basis set (double-zeta plus d polarization functions). The calculated De for the X 3Σ−g state is 4.88 eV, 93% of the experimental value. The calculated adiabatic excitation energies are on the average about 0.1 eV from the experimental values.
Rotational diffusion of linear molecules in fluids. Extended diffusion theory for perpendicular bands of linear molecules including vibrational relaxation
Authors: Lévi G. , Marsault J. P. , Marsault-Hérail F. , McClung R. E. D.
Published: February 2003

We present modifications of the classical and semiclassical extended diffusion theories, for the perpendicular bands of linear molecules, wherein the collisional events which change the angular momentum of the molecule simultaneously bring about changes in the orientation of the transition moment vector. The infrared band shapes for N2O and C2H2 in dense gas mixtures are found to be in excellent agreement with the theoretical band shapes.
An l-dominant simplification of the close-coupled equations for collisions between atoms and diatomic molecules
Authors: DePristo Andrew E. , Alexander Millard H.
Published: February 2003

A major obstacle to the efficient quantum-mechanical treatment of collisions between atoms and diatomic molecules is the rapid proliferation in the number of coupled equations due to the 2j+1 projection degeneracy associated with each rotational level. Recently, considerable effort has been devoted to the development of practical methods for reducing the dimensionality of these coupled equations while preserving the physically important aspects of the coupling between rotational levels. Reduction is often achieved by simplifying the role played by the centrifugal barrier. However, when collisions at large total angular momentum (large J) make a major contribution to the total cross section, the orbital angular momentum l exerts considerable importance. A model calculation on the Ar+N2 system indicated that at large J, the T matrix is dominated by transitions between states characterized by small centrifugal barriers. Consequently, we have developed an ’’l-dominant’’ (LD) approximation which consists in neglecting all channels with l≳J. Simple relations have been constructed to replace the flux corresponding to the eliminated channels. The LD method is expected to work best for inelastic processes, which are insensitive to the phase of the T matrix. Opacity functions for Ar–N2 collisions were calculated and compared with conventional close coupling (CC) results as well as the predictions of the effective potential (EP) and coupled states (CS) simplification schemes. The LD results reproduce, both qualitatively and quantitatively, the exact values and are considerably more accurate than those corresponding to the EP and CS methods, particularly in the important large-J region. An additional comparison study indicates that the degree of accuracy of the LD method is largely insensitive to the anisotropy of the potential. Computationally, the LD method represents a significant savings over a full CC calculation.
Coordinate invariance, the differential force law, and the divergence of the stress-energy tensor
Authors: Epstein Saul T.
Published: February 2003

Hermitian operators linear in momenta generate coordinate transformations. The associated hypervirial theorems are written in the form of moments of a differential force law, and a connection is made with the stress-energy tensor of the Schrödinger field in configuration space.
The magnetic moment of Se2
Authors: Büchler A. , Meschi D. J.
Published: February 2003

Atoms of the oxygen group form diatomic molecules with a multiplet ground state. Of these, O2 and S2 conform to Hund’s (b) coupling and have an appreciable magnetic moment, whereas Te2 is better described as having case (c) coupling with a very small effective magnetic moment. Although the Se2 molecule is intermediate between these two cases, it is closer to case (c) than to case (b). A method of calculating the magnetic moment and Zeeman effect of a molecule of this series with Hund’s case (c) coupling is described and applied to Se2. The calculations result in a value of 5.97×10−2 μB for the mean effective moment of the Se2 molecule at 1000 K in a field of 14 kG. This value was found to be consistent with the results obtained experimentally by the use of a molecular beam deflection technique similar to that of Stern and Gerlach.
Rate constants for the reaction of OH radicals with propylene and the butenes over the temperature range 297–425 °K
Authors: Atkinson R. , Pitts J. N.
Published: February 2003

Absolute rate constants for the reaction of OH radicals with propylene and the four butenes have been determined over the temperature range 297–425 °K using a flash photolysis–resonance fluorescence technique. The Arrhenius expressions obtained are: k1(propylene) =4.1×10−12 e(1080±300)/RT cm3 molecule−1⋅sec−1, k1(1-butene) =7.6×10−12 e(930±300)/RT cm3 molecule−1⋅sec−1, k1(isobutene) =9.2×10−12 e(1000±300/RT cm3 molecule−1⋅sec−1, k1(cis-2-butene) =1.04×10−11 e(970±300)/RT cm3 molecule−1⋅sec−1, and k1(trans-2-butene) =1.12±10−11 e(1090±300)/RT cm3 molecule−1⋅sec−1. The room temperature rate constants are compared and discussed with respect to the literature values.
CO2–N2 intermolecular V–T and V–V potentials via CNDO
Authors: Witriol N. M. , Stettler J. D. , Sabin J. R. , Trickey S. B.
Published: February 2003

V–T and V–V intermolecular potentials have been calculated within the CNDO approximation for the CO2 (asymmetric mode) –N2 system. The procedure followed was to compute the total energy of the CO2–N2 system for a variety of intermolecular separations, orientations, and normal mode displacements, and subtract the relevant isolated CO2 and isolated N2 total energies. The V–T and V–V intermolecular potentials were then obtained by fitting a second-order polynomial expansion in the normal mode displacements of both molecules to the total intermolecular energy for each intermolecular orientation and displacement. The resulting elastic potentials are much too deep. The inelastic potentials are highly orientation dependent, generally not well represented by a P2(ϑ) expansion in the relative angular orientation, and different for each specific V–T or V–V process.
Electronic state of iron in the oxygen and carbon monoxide adducts of heme proteins
Authors: Koster A. S.
Published: February 2003

The Fe Kβ fluorescence emission spectrum was used to study the coordination of iron in myoglobin adducts. The electronic structure of oxymyoglobin can be described as an equilibrium between a singlet state with a coordinated oxygen molecule and a triplet state with a coordinated superoxide anion. Carbon monoxide myoglobin contains divalent, zero spin iron.
Pure-state molecular beams: Production of rotationally, vibrationally, and translationally selected CsF beams
Authors: Bromberg E. E. A. , Proctor A. E. , Bernstein R. B.
Published: February 2003

Beams of CsF in selected vibrational (v=0,1,2,3,4) and rotational (J,M=1,0) states, with narrow velocity distributions, have been produced via an improved microwave molecular beam electric resonance (MBER) method utilizing ’’displacement focusing.’’ The apparatus consists of an effusive graphite oven source, a slotted-disk velocity selector, and two electrostatic quadrupole fields. The first field focuses the (J,M) = (2,0) state through an off-axis collimating orifice into a microwave cavity. Here microwave radiation stimulates the J=2→1 transition for a specified v state. Then a second (’’refocusing’’) quadrupole delivers the specified (v,J,M) = (v,1,0) state to an image point (a collimated surface ionization mass filter detector). The final focused and state-selected beam, say (0,1,0), has a purity ≳95% with respect to J,M. However, the best over-all (v,J,M) state purity achieved was 83% (0,1,0), with 17% of ’’background’’ [of which 4% is actually (0,1,0), 4% is (1,1,0), 4% (v,1,0) with v?2, and the remaining 5% unselected (scattered) CsF]. Using the present effusive oven source, the best intensity achieved for a state-selected (0,1,0) beam (of velocity 3.65×104 cm sec−1) was 1×105 molecule sec−1, corresponding to a flux density of 1.3×109 molecule cm−2 sec−1.
Statistical mechanical calculations of the surface tension of fluids
Authors: Salter S. J. , Davis H. T.
Published: February 2003

Surface tensions predicted from the Kirkwood–Buff formula for three different models of the interfacial doublet distribution function are compared. For liquid argon, the best agreement between theory and experiment is found for the model in which the interfacial pair correlation function is taken to have the value of a one-phase fluid at the density halfway between the molecular pair. This model, in fact, predicts surface tensions whose magnitudes and temperature dependence are in excellent agreement with experiment for argon, nitrogen and methane. Some preliminary calculations for carbon dioxide and methanol indicate that the nonspherical and polar character of the interactions of the molecules of these fluids must be accounted for in attempting to apply any of the models to them.
Intermolecular vibration in the low-frequency Raman spectra of l-alanine crystal
Authors: Loh Eugene
Published: February 2003

The low-frequency, ? 150 cm−1, polarized Raman spectra of l-alanine single crystals determined by Wang and Storms can be interpreted simply in terms of the intermolecular vibrations, i.e., librations and/or translations, of the four oriented alanine molecules in the unit cell of the orthorhombic crystal.
Variational time-dependent Hartree–Fock calculations. I. Applications to four-electron atomic and molecular systems
Authors: Stewart R. F. , Watson D. K. , Dalgarno A.
Published: February 2003

Solutions of the time-dependent Hartree–Fock (TDHF) equations are obtained by a variational treatment that employs Slater basis sets. Application is made to several four-electron atomic and molecular systems. For the beryllium isoelectronic sequence the variational approach is comparable in accuracy to numerical integrations of the TDHF equations. It provides also a description of the resonances that arise from excitation of the inner-shell electrons. Similar calculations for the molecule LiH show that the TDHF method is capable of producing accurate oscillator strengths for molecular systems. Results are given for LiH and for the isoelectronic BeH+ molecule.
Optical–optical double resonance laser spectroscopy of BaO
Authors: Field Robert W. , Capelle Gene A. , Revelli Michael A.
Published: February 2003

BaO molecules were sequentially excited by collinearly propagating radiation from two cw lasers, an argon ion laser (488.0 and 496.5-nm lines) and a tunable (560–630 nm) rhodamine 6G dye laser. This sequential excitation technique, called optical–optical double resonance (OODR), is a special case of two-photon spectroscopy in which two visible or uv wavelength photons of different frequencies resonantly excite a molecule from an initial level (v″,J″) to a final level (v*,J*) by way of a real intermediate level (v′,J′). Two types of optical double resonance experiments were performed on BaO. The first, excitation spectroscopy, revealed 19 vibronic levels of 1Σ+ electronic symmetry between 36 490 and 38 620 cm−1 above E (X 1Σ,re). These 19 1Σ+ levels belong to two or more perturbed electronic states. No 1Π levels were found, although levels belonging to the upper state of the BaO B (1Π) –X 1Σ Parkinson bands occur in this energy region. The second type of experiment, photoluminescence spectroscopy, enabled observation of extended rotationally resolved photoluminescence progressions out of various two-photon-excited levels (v*,J*) into BaO X 1Σ (v″=0–34). Vibrational energies and rotational constants obtained from these photoluminescence progressions were used to construct an RKR potential energy curve for BaO X 1Σ+ through v″=40, from which it was shown that the X 1Σ+ and a 3Π curves cross at r=0.276 nm and E=22 750 cm−1 [above E (X 1Σ,re)]. Laser frequency jitter of less than 10−3 cm−1 resulted in large intensity fluctuations of the OODR signal, implying that the double resonance linewidth was narrower than the Doppler width of the (v′,J′) → (v*,J*) transition. This sub-Doppler width effect resulted from excitation of (v′,J′) molecules that were velocity selected relative to the laser propagation direction.