bluuebunny/crossref_metadata_2025 huggingface dataset:
Shifts of the vibration–rotation absorption lines of diatomic molecules perturbed by diatomic polar molecules. A theoretical analysis
Authors: Boulet C. , Robert D. , Galatry L.
Published: February 2003

In the frame of the second order time-dependent theory of the molecular line shape, the line shifts for diatomic molecules perturbed by diatomic ones are carefully analyzed. Particular emphasis is put on two types of physical situations depending upon the nature of the perturber with respect to that of the active molecule. The available literature presents data which may be classified in two categories. The first one (here called ’’normal shifts’’) is illustrated by the vibration–rotation line shifts of HCl–HCl, HF–HF, and HCl–DCl and exhibits an aperiodic behavior of the line shifts vs the initial rotational quantum number. The second one (called ’’oscillating shifts’’) is illustrated by the vibration–rotation line shifts of DCl–HCl and DF–HF and is characterized by an oscillating behavior with a semiperiod of one rotational quantum number. These behaviors are particularly well reproduced by the present calculations in a quantitative way. A detailed analysis of the physical mechanisms responsible for these two types of behavior is presented and permits a precise determination of the nature of the collision induced transitions included in these mechanisms. Moreover, an extension of this study is proposed in order to predict changes of the phase of the oscillations. This is the case, for instance, for HCl perturbed by HF in the O–2 band.
Dielectric models for solvent effects on electronic spectra
Authors: Sullivan Donald E. , Deutch J. M.
Published: February 2003

The effect of solvent dielectric constant on the frequency shifts and intensity of the electronic absorption spectrum of a dissolved molecule is investigated using Wertheim’s dielectric model. The results show significant differences from those based on use of the Lorentz–Lorenz and Onsager–Böttcher continuum theories, as well as from the results of a lattice model recently considered by Fulton.
Rare gas pressure broadening and shifting of HF vibration–rotation absorption lines—explanation of anomalous line shifts
Authors: Jarecki J.
Published: February 2003

We remove the discrepancy of a previous calculation between experimental and calculated rare gas pressure-induced HF vibration–rotation line shifts by choosing new values for two intermolecular force constants and including anisotropic potential phase shifts to all orders. Two previously unexplained features of krypton and xenon line shifts are explained, and the agreement between previously calculated linewidths and experimental values is improved.
Problems in the measurement of the Stark effect on mixed crystals: Space charge, local field tensor, and sign of the dipole moment parameter
Authors: Udagawa Y. , Hanson D. M.
Published: February 2003

The electronic Stark effect on the S1←S0 and T1→S0 transitions of quinoxaline in durene and naphthalene host crystals is used to demonstrate how the problem of space charge can be eliminated, to reveal the inadequacy of the Lorentz local field approximation, and to demonstrate a technique that provides critical information about the sign of the dipole moment change in centrosymmetric crystals.
Chemiluminescence from the gas phase reaction of atomic boron with the alkali metal fluorides
Authors: Sridharan U. C. , McFadden D. L. , Davidovits P.
Published: February 2003

Gas phase reactions of boron with CsF, RbF, KF, and NaF were studied in a merged flow. Atomic boron was produced in a microwave discharge of a B2H6–helium mixture. The densities of ground state boron and alkali atoms produced in the reaction were determined by absorption measurements. Light from excited alkali atoms in the reaction region was recorded and the excited state population densities were calculated. Our evidence indicates that alkali atoms (M) in the first excited P states are produced primarily in the direct reaction B+MF→BF+M. Several additional processes contribute to the production of higher excited states. Population inversion is observed between a number of excited states.
Collisional laser-induced vibrational excitation of CO2
Authors: Burak Itamar , Gamss Leonard A.
Published: February 2003

A mixture of low pressure CO2 and a buffer gas is subjected to P (18) 10.6 μ CO2 laser pulses. The transient behavior of the laser-induced 4.3 μ fluorescence is observed. The peak of the fluorescence signal is used as a monitor for the excitation density of the (001) excited vibrational level. The excitation density is measured for various pressures of the buffer gas and the effect is considered theoretically. At low pressures only two vibrational–rotational levels are coupled to the radiation field. The amount of excitation is determined by the power broadening of the inhomogeneous Doppler transition. When the pressure of the buffer gas is increased, the noninteracting rotational levels are coupled to the excitation process through collisions. T2 collisional processes interfering with the coherent excitation are also considered. The molecules are specified according to their orientation with respect to the field polarization (M quantum number) and their velocity, v, along the direction of propagation of radiation. The specific field-molecular interaction for a molecule in the (Mv) state is coupled to a master equation taking into account the multitude of rotational, orientational, and velocity energy transfer processes. A solution is presented for the total excitation density. The pressure effect is simulated by a linear change of the relaxation parameters. A comparison between the theoretical results and the experimental data is made.
Product magnetic defection slotted disk velocity analysis molecular beam kinetics: Li+CH3NO2, CCl4, and CH3I
Authors: Sholeen Charlotte M. , Herm Ronald R.
Published: February 2003

Recoil velocity spectra of LiNO2, LiCl, and LiI reactively scattered from crossed thermal beams of Li and CH3NO2, CCl4, or CH3I have been measured by positioning a combination of an inhomogeneous electromagnet (to deflect aside nonreactively scattered Li atoms) and a slotted disk velocity analyzer between the beam collision zone and surface ionization detector. Transformation of these data into the center-of-mass (CM) coordinate system provides contour maps of the differential reaction cross sections. The CM product angular distributions from the three reactions are quite different: Li+CH3I favors backward scattering (i.e., a reversal in Li atom direction during the reaction) although the CM distribution is poorly resolved due to unfavorable kinematics; Li+CCl4 favors sideways scattering; and Li+CH3NO2 favors forward scattering. The product recoil energy distributions are quite similar for all three reactions and indicate potential energy surfaces with substantial repulsion between the products. These results are discussed in relation to previous work on related reactions and interpreted in terms of the electronic structure of ion-pair intermediates.
The uniform phase space representation of product state distributions of elementary chemical reactions
Authors: Faist Michael B.
Published: February 2003

The uniform phase space (UPS) representation of translational or internal degrees of freedom of the products of a chemical reaction is presented. Procedures are developed to deal with cases in which the final state distribution is discrete or approximately continuous; extensions cover multidimensional distributions. Explicit realistic models are used to demonstrate these situations.

In the UPS representation, the statistical or phase space distribution over product states is constant, yielding equal volumes of phase space for equal intervals along the coordinate in the UPS representation. As a consequence, an actual distribution over product states, cast into the UPS representation, becomes a direct measure of its deviance from a statistical distribution. The information content (in the information theoretic sense) is invariant under transformation to this representation, and its value can be readily approximated by a simple histogrammic procedure. Calculations for realistic model problems illustrate the superiority of the UPS representation of the energy disposal in elementary chemical reactions.
Rotationally resolved resonance scattering from a predissociating molecule: Ammonia-d3 excited at 2139 Å and 2144 Å
Authors: Hackett P. A. , Back R. A. , Koda S.
Published: February 2003

Rotationally resolved resonance scattering spectra of ND3 excited within the 0000←00+00 and 0100←01−00 bands of the ?←? transition have been photographed. The quantum yield of this emission has been measured (1×0.5×10−5) and is in agreement with that predicted for simple ’’fluorescence.’’ However, the spectra show evidence for excitation of rovibronic levels up to 20 cm−1 off of resonance with the exciting line, and rotational lines within a single vibrational band obey ’’Raman’’ selection rules. Thus the position of rotational lines gives information about ground state energy levels only. Information about excited state energy levels is contained in the relative intensity of vibrational progressions in the emission spectra.
HFClF: Structure and bonding
Authors: Novick Stewart E. , Janda Kenneth C. , Klemperer William
Published: February 2003

The structure of the complex formed between HF and ClF has been determined by molecular beam electric resonance spectroscopy. The molecule HFClF is a slightly asymmetric prolate top. The spectroscopic constants determined from K=0 spectra of several isotopic species are as follows: The atomic arrangement in the complex is HFClF with the three heavy atoms collinear. The proton is off axis by 55°. The FCl van der Waals bond length is 2.76 Å. Comparison of the structure of HFClF with that of (HF)2 shows striking similarities. It appears that the bonding in both complexes is quite similar.
Semiclassical calculations of cross sections for fine-structure transitions in Rb (62P) and Cs (82P) colliding with rare gas atoms
Authors: Pascale J. , Stone P. M.
Published: February 2003

Calculations of cross sections for fine-structure transitions in the second and third excited 2P state of rubidium and cesium, respectively, have been carried out using a semiclassical theory for collisions with ground state He, Ar, and Xe atoms. The primary difference from previous theories for the first 2P state of an alkali is the full formulation of the semiclassical theory in terms of molecular states of the alkali–rare gas atom complex. The molecular states are consistent with the interaction potential and both radial and rotational coupling are explicitly included in the calculations. In the collision energy range investigated (from threshold to 0.3 eV) the cross sections are found to have an undulatory structure, especially prominent for the Rb–Ar system. Except for the Cs–He system, the thermally averaged cross sections agree with experiment within a factor of 1.4–2.8. For cesium, these results reduce considerably the large discrepancy between previous calculations of Nikitin and Reznikov (1971) and experiment.
Determination of T2/T1 for inversion transitions in NH3 by analysis of power-broadened line shapes
Authors: Amano T. , Schwendeman R. H.
Published: February 2003

A computer-coupled microwave spectrometer has been used to record the line shapes of 14 inversion transitions in 15NH3 at several pressures under conditions of low and moderate microwave powers. From the low-power line shapes, values of Δν/p= (2πpT2)−1 have been determined and used as input data for the analysis of the power-broadened line shapes from which values of T2/T1 have been obtained. The T2/T1 results generally increase with J and K and range from 1.11±0.06 for the (J,K) = (1,1) transition to 1.58±0.09 for the (5,5) transition. The data appear to imply that the ratio of the rate constant for adiabatic collisions to that for diabatic collisions is about 0.35 in NH3.
Vibrational relaxation and energy gap law breakdown in the X 3Σ− state of NH and ND in solid argon
Authors: Bondybey V. E.
Published: February 2003

The lifetimes of the v=1 levels in the ground electronic states of NH and ND were measured. It is shown that the rate of vibrational relaxation in NH is faster by at least a factor of 150, contrary to the predictions of the energy gap law. This is due to the localized rotational mode being the energy acceptor and to the larger amplitude of center of mass motion in the hydride. The absolute rates are considerably slower than the corresponding rates in the excited A 3Π state owing to the lower polarizability and lower barrier to free rotation in the ground electronic state.
Electron correlation in small metal clusters. Application of a theory of self-consistent electron pairs to the Be4 system
Authors: Dykstra Clifford E. , Schaefer Henry F. , Meyer Wilfried
Published: February 2003

A knowledge of the properties of small metal particles is essential to the understanding of catalysis on a molecular level. In this regard, one particularly important property is the rate at which the dissociation energy of a small metal cluster approaches the bulk cohesive energy. The present research concerns the effect of electron correlation on the dissociation energy of a particularly stable beryllium cluster, the tetrahedral Be4 system. A contracted Gaussian basis set of size Be(9s 4p/5s 2p) was adopted in conjunction with the recently developed theory of self-consistent electron pairs (SCEP). Several new theoretical and computational wrinkles are discussed, including the incorporation of the SCEP/coupled electron pair approximation (SCEP/CEPA). The Be4 results provide strong evidence for the reliability of the Hartree–Fock approximation for alkaline earth cohesive energies. As suggested earlier the Be4 dissociation energy appears to be ∼40 kcal/mole. Analogous studies of the Be2 molecule are reported.
Viscosity of multicomponent mixtures of four complex gases
Authors: Kestin J. , Khalifa H. E. , Wakeham W. A.
Published: February 2003

In a series of earlier papers it has been established that the viscosities of the pure gases CH4, CO2, CF4, and SF6 as well as those of their binary mixtures conform to the extended law of corresponding states formulated by Kestin, Ro, and Wakeham. In this paper, the kinetic-theory expressions for multicomponent gas mixtures are used in conjunction with the law of corresponding states and the appropriate binary scaling parameters σij and εij to generate the viscosities of several ternary and quaternary mixtures of these gases. The calculated viscosities are compared with direct measurements (accuracy ±0.2%) performed in an oscillating disk instrument in the temperature range 25–200°C. The comparison reveals that the generated data depart from the measured viscosities by a maximum of 0.5% and that the standard deviation is only 0.25% which is comparable with the experimental uncertainty. The validity of the calculation scheme presented for predictive purposes is thereby confirmed. It is emphasized that the calculation is based exclusively on previously published information and that it does not in any way utilize the data on multicomponent mixtures. Thus the calculation can be described as a true ’’prediction.’’
Measurement of the rate coefficients for the bimolecular and termolecular de-excitation reactions of He(23S) with Ne, Ar, N2, CO, CO2, and CH4
Authors: Lee F. W. , Collins C. B.
Published: February 2003

This work reports the measurement of second and third order excitation transfer reactions of metastable 23S helium atoms with Ne, Ar, N2, CO, CO2, and CH4. In this study He  (23S) destruction frequencies have been experimentally determined from measurements of the optical absorption of the 3889 Å, 23S→43P, helium transition in high pressure afterglows of mixed gases excited by intense electron beam discharge. Data have been obtained as functions of helium pressure over the range from 800 to 2000 torr and as functions of the partial pressure of reactant from 14 to 200 mtorr. From this data pressure dependent rate coefficients have been extracted and subsequently resolved into contributions from second order, bimolecular, and third order, termolecular, components. The bimolecular components have been found to agree with tabulated values appearing in the literature. The sensitivity of the method has been sufficient to detect termolecular components as small as 0.3×10−30 cm6/sec and values were found to range from this threshold value for Ne to 8.3×10−30 cm6/sec for CO2. The size of these termolecular rates suggests the general importance of such third order reactions in higher pressure plasmas and afterglows such as those found in e-beam lasers.
A classical trajectory study of inelastic scattering in the system He+H2
Authors: Pattengill M. D.
Published: February 2003

The classical trajectory study method has been used to investigate inelastic scattering in the system He+H2 using previously developed collision models. The calculations spanned the total energy range E=0.403 eV to E=1.900 eV. For all cases studied, H2 was initially in the ground vibrational–rotational state. Despite the fact that over half the energies employed were above the threshold (Eth?0.8 eV) for vibrational excitation of H2, the scattering was found to populate only the lowest two allowed rotational levels of the vibrational ground state. The classical cross sections obtained are compared with those from previous exact quantal (close-coupled), effective potential, and coupled states calculations.
The effect of porous barriers on the molecular composition and total flux of a reactive gas mixture
Authors: Mohazzabi Pirooz , Searcy Alan W.
Published: February 2003

When a gas mixture that is governed by a pressure dependent equilibrium is passed through a porous barrier, the exit molecular fluxes are functions of the equilibrium constant and the atomic concentrations at the exit face of the barrier rather than of the molecular weights and impingement pressures as has been tacitly assumed for Knudsen flow conditions. It is shown that whether or not molecules of the gas react with each other, the ratio of the total atomic escape flux of each elemental constituent to the total atomic impingement flux of that element for Knudsen flow should be a/l, where l is the barrier thickness and a is the same constant, of the order of the average diameter of barrier pores for each element in the gas mixture. The equilibrium prediction is confirmed for the sodium chloride monomer/dimer equilibrium mixture passed through alumina barriers with pores of the order of 1–10 μ diameter and various thicknesses. Measured dimer fluxes are reduced by the barriers to less than 10−3 times the dimer fluxes in conventional effusion; yet the measured fluxes agree to within about 30% with predictions from the measured monomer fluxes and the monomer/dimer equilibrium constant. The prediction that total atomic flux reductions for different vapors have the same dependence on a/l is confirmed by measurement with sodium chloride vapor and zinc vapor. The influence of porous barriers on the apparent heats of vaporization of monomers and dimers and the possibility of significant influence of surface diffusion on the experimental measurements are discussed.
Some aspects of electron–phonon interaction in the thermal modulation spectra of molecular crystals
Authors: Loo B. H. , Francis A. H. , Hipps K. W.
Published: February 2003

The details of the temperature dependent intensity and linewidth of zero-phonon bands in molecular crystal absorption and emission spectra are considered in relation to the line shape functions observed in thermal modulation spectra. Theory suggests and experiment confirms that in the limit of the Condon and adiabatic approximations for linear electron–phonon coupling, the extraordinary temperature dependence of zero-phonon bands may be utilized to separate them from diffuse background absorption or emission. Qualitative consideration of line broadening from quadratic electron–phonon coupling utilizing a configuration coordinate model suggests that even when these terms dominate the linear interaction, increased resolution may be obtained in a thermal modulation spectrum. The theoretical considerations are illustrated with several experimental examples and some applications of the technique are discussed.
Rotational relaxation in HD–inert gas mixtures
Authors: Agrawal P. M. , Saksena M. P.
Published: February 2003

An expression for the inelastic collision cross section σ (lm→l′m′) has been derived for the rotational transition in the HD molecule caused by a collision with an inert gas atom. The interaction potentials for the HD systems have been derived from the theoretical potentials for the H2 systems by shifting the center of mass of the molecule. The rotational relaxation number values have been calculated for HD–He, HD–Ne, and HD–Ar systems in the temperature range from 10 to 100 °K. A good agreement has been obtained between theory and experiment. Temperature dependence and the effect of replacement of an isotope in a homonuclear molecule have been discussed in detail.
A
b i
n
i
t
i
o vertical spectra and linear bent correlation diagrams for the valence states of CO2 and its singly charged ions
Authors: England W. B. , Rosenberg B. J. , Fortune P. J. , Wahl A. C.
Published: February 2003

Correlated and uncorrelated ab initio vertical spectra are reported for the valence states of CO2, CO2+, and CO2−. Calculations with polarized and unpolarized basis sets are compared at each level. Ground state quadrupole moments are computed. Linear molecule to bent molecule SCF correlation diagrams are also reported.
Calculations of g tensors for electron spin resonance in the energy weighted maximum overlap model
Authors: Dalgaard Esper , Linderberg Jan
Published: February 2003

Anisotropic g tensors for electron spin resonance (ESR) are calculated in the energy weighted maximum overlap model (EWMO) for a number of inorganic radicals. The g tensor for the hitherto unobserved radical HBF is predicted. Calculated principal components and directions of the g tensors for the HSO2 and FSO2 radicals are used to suggest a choice between different solutions for the g tensor and hyperfine interaction tensor recently obtained by McDowell et al. in a study of hydrogen and fluorine atom addition to SO2. The magnetic field is included to all orders in the calculations and London’s field-dependent atomic orbitals are used in order to obtain a translationally invariant formulation of the method. The g tensor is obtained to first order in spin–orbit coupling, which is included in the model via atomic orbital energy parameters.
On combined statistical and j
z-conserving coupled states approximations
Authors: Kouri Donald J. , Shimoni Yair , Heil Timothy G.
Published: February 2003

In this paper we consider the collision of an atom and diatomic molecule within the framework of the jz-conserving coupled states approximation. Particular attention is focused on the dependence of jz CCS cross sections upon the rotor magnetic quantum number. It is found computationally that for the He+H2 and He+CO2 systems, the jz CCS partial cross sections σJλ0j0→j are approximately independent of the particular value of λ0, so that the full cross section can be estimated quite well by σj0→j?[π/k2j0(2j0+1) ΣJ(2J+1)(2 min(j0,j,J)+1) ‖TJλ0(j0‖j) ‖2, where ‖λ0(j0, j, J) ‖? min (j0, j, J). This is equivalent to a statistical approximation so far as the magnetic states of the rotor is concerned. By utilizing this fact, one need carry out jz CCS calculations only for λ0=0 and then employ the above expression for σj0→j to obtain all possible integral cross sections. This leads to an approximation scheme requiring the same computational effort as Rabitz’s effective potential method. However, the accuracy of the present approximation is expected to be better than the E.P. for the strongly interacting He+CO2 system. Furthermore, if only transitions involving larger j0, j are required, one may choose λ0=J for J< min (j0, j) and λ0=min (j0, j) for J? min (j0, j). This results in even fewer equations to solve than the effective potential method. It enables one to systematically truncate the number of states coupled in order to calculate a given j0→j cross section since j′<λ0 do not enter the equations. Thus, the truncated statistical jz approximation is the first fully quantal approach to inelastic molecular collisions which permits one to (a) select a transition ji→jf in which one is interested and (b) within the jzCCS approximation, rigorously eliminate all states j′< min (ji, jf, J) for each J. Yet another level of approximtation can be obtained by combining the results of the statistical jz and truncated statistical jz approximations. The averaged results are found to be much more accurate than either approximation alone and in fact are within a very few percent of the full jzCCS results for the He+H2 and He+CO2 systems. The computational effort is simply the sum of that for the statistical jz and truncated statistical jz approximations. This is still very much less than a full jzCCS calculation and only slightly more work than the E.P. method. The results for the j0, j transitions with j0, j<λ0 will be the same as the statistical jz method. Numerical results are presented for the He+H2 and He+CO2 systems to illustrate these new statistical jz approximations.
Electron spin resonance study of the radical (C6H5)2As produced in a single crystal matrix
Authors: Geoffroy M. , Ginet L. , Lucken E. A. C.
Published: February 2003

ESR spectra of x-irradiated single crystals of (C6H5)3AsO were investigated. One of the two radical species trapped in the crystal was identified as being the diphenylarsino radical (C6H5)2Ȧs. The g and 75As magnetic coupling tensors were determined together with the 75As quadrupole interaction tensor. These ESR parameters are shown to be consistent with a bond angle CAsC of 98°±2°. The spectra parameters of (C6H5)2Ȧs are compared with those of (C6H5)2Ṗ and ṄH2.
Measurements of rotational and vibrational quantum transition probabilities in the scattering of Li+ from N2 and CO at center of mass energies of 4.23 and 7.07 eV
Authors: Böttner Rudolf , Ross Ulrich , Toennies J. Peter
Published: February 2003

Angular distributions and time of flight spectra for the scattering of Li+ ions from ground state N2 and CO molecules have been measured over a range of scattering angles (0°⩽ϑc.m.⩽60°) and for Ec.m. =4.23 and 7.07 eV. The time of flight spectra show a number of maxima which have been attributed to unresolved rotational excitation superimposed upon vibrational quantum transitions corresponding to n=0→0, 0→1, and 0→2. The laboratory spectra have been transformed into the center of mass system to obtain relative inelastic differential cross sections. Significant differences between the vibrational inelastic cross sections for N2 and CO are observed at large angles at the highest energy studied. The results are discussed in terms of the SCF potential hypersurfaces. The differences in the vibrational cross sections cannot be explained in terms of the infinite order sudden approximation and the vibrational matrix elements of the SCF hypersurfaces. This suggests that the inclusion of rotational–vibrational coupling % in the dynamics and/or an improved hypersurface are called for.
Off-lattice Monte Carlo study of ring polymers
Authors: Bruns W. , Naghizadeh J.
Published: February 2003

Off-lattice nonintersecting rings of up to 99 bonds were generated by Monte Carlo methods. It was found that the mean square radius of gyration varies as N1.24, N being the number of bonds. The distribution function from the center of mass which is Gaussian for unperturbed rings is distorted to a non-Gaussian form as a result of interactions. The pair distribution function, however, retains its unperturbed functional form to a high degree of accuracy, after perturbation by repulsive forces.
O2(1Σg+) relaxation in collisions. I. The influence of long range forces in the quenching by diatomic molecules
Authors: Braithwaite M. , Davidson J. A. , Ogryzlo E. A.
Published: February 2003

Rate constants for the quenching of O2(1Σg+) to O2(1Δg) by nine diatomic molecules have been calculated on the assumption that the relaxation results from long-range interactions between the transition quadrupole of O2(1Σg+) and the transition dipole and quadrupole of the quenchers. The results suggest that in most cases this may be the dominant mode by which the relaxation of O2(1Σg+) is induced. With this assumption, the major channels for the quenching processes have been identified for most quenchers.
An analysis of the optical spectra of single crystals of Cs2NaPrCl6
Authors: Cheng Chris , Dorain Paul B.
Published: February 2003

The optical spectra of single crystals of Cs2NaPrCl6 are reported. A complete f2 analysis gives the parameters: F2=302.0 cm−1, F4/F2=0.1600, F6/F2=0.01596, ξ4f=754.8 cm−1, α=19. cm−1, A4〈r4〉=288.6 cm−1 and A6〈r6〉=39.6 cm−1. A comparison of these parameters with those of the free Pr+3 ion indicates a covalency of 3%. The magnetic susceptibility is calculated and compared to that measured previously.
Microwave optical double resonance spectrum of NH2. I. Ground state, 14N hyperfine coupling constants, and spin rotation constants
Authors: Hills G. W. , Cook J. M. , Curl R. F. , Tittel F. K.
Published: February 2003

Magnetic dipole allowed transitions between the two J components of the same rotational state in the ground vibronic state of NH2 have been detected using microwave optical double resonance. The hyperfine structure of some of these transitions have been analyzed to yield values for the 14N magnetic coupling constants (in MHz) of (O)I=27.9 (0.6), (aa)I=−43.3 (1.5), and (cc)I=87.4 (0.9), where the quoted error in parentheses is three standard deviations. In addition, a preliminary analysis of the fine structure splittings determines the spin rotation constants (in MHz) to be As=−9290.0 (160), Bs=−1331.9 (38), and Cs=3.9 (57). A poor fit of the fine structure arises from an inadequate Hamiltonian, and the statistical uncertainty quoted for these parameters is probably meaningless. The derived constants for the gas phase are compared with the available solid state ESR data as well as some theoretical studies.
Negative ion formation from surfaces under bombardment of a superthermal potassium neutral beam
Authors: Helbing R. K. B. , Hsieh Y. F. , Pol V.
Published: February 2003

A polycrystalline tungsten surface was bombarded wiuth superthermal neutral potassium beam and the ion species released from the surface were studied by means of a 90° magnetic mas spectrometer. (AIP).
Negative ions from phosphorus halides due to cesium charge exchange
Authors: Mathur B. P. , Rothe Erhard W. , Tang S. Y. , Reck Gene P.
Published: February 2003

An experiment has been conducted in which cesium atoms in the kinetic energy range 2–350 eV collide with phosphorus halides. Parent anions and fragments are formed. Molecular energies are obtained from threshold measurements. The electron affinites for PCl3, POCl3, PBr3, PCl2Br, PBr2Cl, and POCl2 are found to be 0.8, 1.4, 1.6, 1.5, 1.6, and 3.8 eV, respectively. The P–X bond energies for PCl3, POCl3, and PBr3 are 3.3, 3.5, and 2.6 eV, respectively.
Liquid carbon tetrachloride: Atom pair correlation functions from neutron and x-ray diffraction
Authors: Narten A. H.
Published: February 2003

Neutron and x-ray diffraction data for liquid CCl4 at 20 °C are presented and analyzed to yield structure and correlation functions for C–Cl and Cl–Cl atom pairs in different molecules. In the liquid, a central molecule is interlocked with four of its nearest neighbors and this arrangement explains the strong orientational correlations that exist over short radial distances. The data are sufficiently accurate to provide a sensitive test for molecular theories of the liquid state and for intermolecular force models in computer experiments.
Tunneling rotation of the methyl radical in solids
Authors: Lee J. Y. , Rogers Max T.
Published: February 2003

Hyperfine coupling elements, energy levels, and ESR transition frequencies have been computed for the methyl radical taking into account the large anisotropy of the proton hyperfine couplings. The interpretation of low-temperature ESR and ENDOR spectra of methyl radical is discussed with particular attention to the tunneling rotation about the threefold axis.
An SCF method for hole states
Authors: Hsu Hsiang-lin , Davidson Ernest R. , Pitzer Russell M.
Published: February 2003

An SCF method is derived for doublet states with one vacancy in an orbital within the occupied manifold (hole states). This method gives an upper bound to an excited state energy. Hence it is a stable procedure which is bounded from below and cannot collapse to a lower energy SCF state. This new procedure is compared with several other open-shell SCF procedures which have been advocated for the ground doublet state.
Thermochemical studies of the gaseous lower-valent fluorides of molybdenum
Authors: Hildenbrand D. L.
Published: February 2003

Equilibria among the gaseous molybdenum fluorides were studied by high temperature mass spectrometry, and the results were used to derive thermochemical heats of formation and bond dissociation energies for the gaseous species MoF, MoF2, MoF3, MoF4, and MoF5. Effusion beams containing the gaseous fluorides were generated by the reaction of SF6 or MoF6 with elemental Mo at temperatures of 1100–2200 K, and the thermochemical results were derived from a second-law analysis of the equilibrium data. Values of the standard heats of formation, ΔHf °298, obtained are as follows: for MoF, 65.0±2.2 kcal/mol; MoF2, −40.2±2.9 kcal/mol; MoF3, −141.5±3.5 kcal/mol; MoF4, −228.0±3.9 kcal/mol; and MoF5, −296.7±8.6 kcal/mol. The value for MoF yields the dissociation energy D00(MoF) =110.3±2.2 kcal/mol (4.78±0.10 eV). Estimated spectroscopic and molecular constants are consistent with the gaseous equilibrium data, indicating that assumptions about the derivation of equilibrium constants from ion currents are reasonable. ]
Vibrationally inelastic scattering at high energies: H++H2
Authors: Collins Francis S. , Cross R. James
Published: February 2003

The sudden approximation is used to compute integral and differential cross sections for H++H2(v=0) →H++H2(v′=1–4) using an ab initio DIMZO (diatomics-in-molecules, zero overlap) potential-energy surface. Good agreement was obtained above 100 eV with calculations of Gentry and Giese using the DECENT approximation. Agreement with the experiments of Hererro and Doering seems fair if their data are properly analyzed. Several useful numerical approximations are developed and tested. These result in savings of several orders of magnitude in computation time.
Low-energy elastic and electronic-energy exchange scattering of He* by Kr
Authors: Bentley John , Fraites James L. , Winicur Daniel H.
Published: February 2003

Differential cross sections for He* scattered by Kr are obtained at three relative energies, 63.6–82.3 meV. Location of rainbow maxima and ’’rapid’’ quantum oscillations, observed at all energies, are used to determine parameters for a double Lennard-Jones potential. The parameters, ε=8.0±0.6 meV and rm=4.8±0.25 Å, are close to those for Li+Kr and are substantially different from those of He+Kr. The effects of direct electronic-energy exchange scattering are observed as excess scattered intensities between 10° and 30° lab. These effects are compared to those arising from chemi-ionization.
Reduced free-particle Green’s functions in quantum-mechanical perturbation calculations
Authors: McDowell H. Keith , Porter Richard N.
Published: February 2003

Several new methods are given by which contributions to terms involving single excitations in many-body perturbation series expansions of electronic properties of atoms and molecules can be calculated accurately. All are based upon solution of a Fredholm identity connecting the reduced free-particle Green’s function with a reduced Green’s function appearing in the Goldstone diagram that is to be evaluated. The most generally applicable approach appears to be one based upon the method of moments and the expansion of an appropriately defined first-order wavefunction in a set of polynomials that are orthogonal with respect to a weight function appearing naturally in the configuration-space representation of the diagram. This ’’polynomial moment’’ method is compared with a ’’power moment’’ method and also with iterative solutions of the Fredholm identity. Accuracy of the numerical values of the diagram so obtained varies from exact (for the dipole polarizability for the hydrogen atom), through 10 significant figures (for the single-particle excitation term in the second-order energy for a two-electron atom in a 1/Z expansion) to 5 significant figures (for the first-order term in a 1/Z expansion of the electron density at the nucleus of a two-electron atom). These methods obviate explicit use of hydrogenic Green’s functions and require at most the inversion of a matrix of order ∼15.
Energy transfer and relaxation phenomena in CaF2:Er3+
Authors: Tallant David R. , Miller Marvin P. , Wright John C.
Published: February 2003

A selective laser excitation technique has been used to study energy transfer between the individual ions of specific crystallographic sites in charge compensated CaF2:Er3+. Unique energy transfer rates have been obtained directly from the fluorescent transients of the selectively excited sites. No energy transfer processes were observed between sites corresponding to single Er3+ ions. For those sites composed of Er3+ clusters, very efficient and complex intracluster energy transfer was observed despite large energy defects requiring phonon assistance. The ion pair relaxation processes compete efficiently with multiphonon relaxation because smaller contributions from lattice phonons are required. Analogous energy transfer interactions were found to occur between Yb3+ and Er3+ ions in mixed cluster sites in double-doped crystals. Detailed relaxation and energy transfer mechanisms are presented. Kinetic expressions describing excited state relaxation are derived from these mechanisms and are used to provide theoretical fits to experimental fluorescence transients confirming the relaxation mechanisms proposed.
Unified rotational dynamics of molecular crystals with orientational phase transition
Authors: Michel K. H. , De Raedt H.
Published: February 2003

A unified theory for the rotational dynamics of molecular crystals with orientational phase transitions is given. As basic secular variables one takes symmetry adapted functions, which describe the molecular orientations, and the angular momenta of the molecules. Using Mori’s projection operator technique, one obtains a coupled set of two dynamic matrix equations for the corresponding relaxation functions. The coupling is proportional to the order parameter and accounts for the reactive coefficients. The corresponding collective excitations are librons. The damping of librons is described by two transport coefficients that account for orientational relaxation and angular momentum relaxation, respectively. By approaching the phase transition, both matrix equations decouple, the orientational relaxation describes the critical dynamics. In the disordered phase, this equation describes collective hindered rotations. In the same framework one describes phases of partial order as CD4 II. Comparison is made with neutron scattering experiments.
Primary oxidation and reduction products in x-irradiated aspartic acid
Authors: Adams Stephen M. , Budzinski Edwin E. , Box Harold C.
Published: February 2003

The primary reduction products identified by ESR–ENDOR spectroscopy in single crystals of DL-aspartic acid hydrochloride irradiated at 4.2°K are anions formed by addition of an electron to the carbonyl oxygen atoms of the carboxylic acid groups. The main consequence of the oxidation process is to produce a hole centered mainly on atomic chlorine.
Spin-dependent deuterium isotope effect on the radiationless decay of a porphyrin triplet state
Authors: Goncalves A. M. Ponte , Burgner R. P.
Published: February 2003

The effect of deuteration on the radiationless decay of the lowest triplet state of a porphyrin free base is found to be spin dependent. The EPR spectra of deuterated tetrafluorophenyl porphin (TFPP) are reported. (AIP).
Semiclassical calculation for collision induced dissociation
Authors: Rusinek Isak , Roberts R. E.
Published: February 2003

The classical S-matrix theory of Miller and Marcus has been used to compute collision induced dissociation probabilities (Pdiss) for particle–oscillator one dimensional collisions. The continuum of dissociation states was discretized in a straightforward way by confining the oscillator to a suitably large box. From this simple discretization an expression for Pdiss in the limit of an infinite box is derived. Numerical results for a truncated harmonic oscillator with an exponential repulsive interaction with the colliding particle are presented for several masses and potential parameters. The behavior of Pdiss as a function of collision energy, well depth and initial vibrational excitation is studied. The energy profile of Pdiss shows a prominent structure which is directly related to the initial vibrational state of the oscillator. The relative dissociation ’’efficiencies’’ of the different vibrational levels of the oscillator depend drastically on the collision energy, with highly excited vibrational states being more ’’efficient’’ dissociation candidates at low (near threshold) energies but with the opposite behavior at higher kinetic energies. The dependence of the dissociation probability on potential parameters and masses is analogous to the trends for a comparable energy transfer process.
Theoretical Compton profile anisotropies in molecules and solids. I. Formulation of the MSC procedure and application to lithium fluoride
Authors: Ramirez B. I. , McIntire W. R. , Matcha R. L.
Published: February 2003

A theoretical calculation of directional Compton profiles and corresponding anisotropies in a diatomic molecule is presented. The different occupied molecular orbitals (MO) give interesting Compton profiles and anisotropies which are related to their roles in the binding of the molecule. The results are extended to approximate a face-centered-cubic (fcc) crystal, using a molecular simulated crystal procedure (MSC) in which the MO are modified to match the symmetry of the crystalline environment. The formalism is applied to LiF at an equilibrium spacing of 3.55 a.u., approximately the crystalline spacing. The results compare favorably with previous anisotropy calculations and with experimental measurements of Compton profile anisotropies in LiF.
Coriolis coupling and the Raman spectra of SF6 and CF4 in the liquid and plastic phases
Authors: Gilbert M. , Drifford M.
Published: February 2003

It is shown how to correct the J model to take account of first order Coriolis coupling on triply degenerate lines of spherical tops. The Debye limit of the J model is examined. Corrected J spectra are then compared to the experimental Raman lines of SF6 and CF4 in their condensed phases.
Characterization of γ-ray induced paramagnetic centers in bayerite by means of EPR and ENDOR spectroscopies
Authors: Derouane Eric G. , Vedrine Jacques C.
Published: February 2003

Four different paramagnetic centers, induced in bayerite samples by irradiation, have been characterized by EPR and ENDOR spectroscopies. Two of them, respectively, of V- and F-types, have been found to be mobile at room temperature and frozen at lower temperature. The first one, with g1= 2.021±0.002, g2=2.010±0.005, and g3=2.005±0.002, is identified with an O− species stemming from the ionization of OH− groups present in the structural cavity walls of the bayerite. The second one, with g∥=1.9970±0.0005 and g⊥=2.0020±0.0005, corresponds to solvated electrons. A third center, with g∥=2.035±0.002 and g⊥=2.0025±0.0005, with strongly hindered motion even at room temperature, is assigned to an O−2 species located at the center of the structural cavity, U. The last center is a doublet with a 502.5±0.5 Oe hyperfine splitting and giso= 2.0022±0.0001; it is due to H atoms located in U. Particular attention has been paid to the ’’matrix’’ ENDOR line at the free proton nuclear frequency and to the nature of the nuclear and electronic relaxation mechanisms that govern its shape and intensity. ENDOR spectra were simulated considering various possibilities for the proton distribution and the relaxation mechanisms. The best agreement is obtained with: (i) a nuclear relaxation time that is angular independent, (ii) a discrete lattice proton distribution in agreement with literature data; (iii) a lower limit for the distance between the paramagnetic center and the different lattice protons that results in the matrix ENDOR line (R≳3.4 Å=0.34 nm). This lower limit excludes all nearby protons that experience direct contact hyperfine interaction; (iv) an upper limit for the distance R. It corresponds to the most distant protons that are involved in the ENDOR mechanism via electron–nuclear dipolar interaction. This limit is shown to increase when microwave power is increased; (v) spin–packet linewidths that are shown to be dependent on the electron spin–lattice relaxation time.
Many-body perturbation theory applied to Be+ ion
Authors: Hata Jiro , Sakai Masako
Published: February 2003

The Bruecner–Goldstone many-body perturbation theory has been applied to the study of the electron correlation problem of atomic beryllium ion in its ground-state 2S. An orthonormal complete set is generated from VN–1 potential. In the present calculation, the final correlation energy is −0.0510 a.u., as compared with −0.0477 a.u. from the estimate made by Weiss. The l dependence of the factors, a and b, appearing in the geometric sum approximation is also investigated, and is found to be independent of it in present calculation.
A dimer model with angular forces: Equation of state
Authors: Freasier B. C.
Published: February 2003

A dimer model with angular forces on the square lattice is analyzed. On the basis of low and high density expansions with their Padé approximants it is concluded that it is likely the system exhibits a first order phase transition if the angular forces are fundamentally attractive and do not counteract each other.
The ligand hyperfine interaction with rare earth ions. II. Calculation of the paramagnetic NMR shift
Authors: McGarvey Bruce R.
Published: February 2003

The isotropic and anisotropic 19F NMR shifts for Yb3+ doped into alkaline earth fluorides is calculated using a covalent model for the spin transfer. The good agreement between theory and experiment is strong evidence for the covalent model being the correct model in the case of Yb3+. The calculations reveal that the main contribution to the isotropic shift does not come from the Fermi contact interaction, as has been generally assumed, but rather from a second order Zeeman term due to a spin–orbit interaction between the nuclear spin and the orbital motion of unpaired electrons in the ligand p orbitals. It has been found that the isotropic shift of Yb3+ can be reasonably estimated by calculating the high temperature limit of the shift (kT≫ crystal field splittings) even though kT is about one-third the crystal field in the sytem studied. A general method of calculating the NMR shift in the high temperature limit is developed which is readily applicable to multispin ions and is applied to the Er3+ system.