bluuebunny/crossref_metadata_2025 huggingface dataset:
The microwave spectrum of CH2DOH
Authors: Quade C. Richard , Suenram R. D.
Published: February 2003

The torsional–rotational spectrum of CH2DOH has been investigated in order to gain fuller understanding of the internal rotation parameters as well as to provide molecular radio astronomers with a set of measured transition frequencies which can be used for an interstellar search for the molecule. a- and b-dipole transitions have been assigned within the three substates of the torsional ground state and c-dipole transitions have been assigned from the e1 to the o1 substate. The potential energy coefficient V1=12±1 cm−1 has been determined with V2 constrained to zero. Although the torsional wave functions are by no means sharp, the molecule does localize to symmetric and asymmetric forms.
A study of vibrational relaxation of H2 in He
Authors: Lin C. S.
Published: February 2003

The vibrational relaxation rate constant kl→0 (T) over T?500 K was computed for para-H2 in He from fully converged CC cross sections. While the calculated rate constant is much too large, it displays a hump around T=100 K, presenting, for the first time, a plausible elucidation of the humps in the experimental results of Audibert et al. Analyses of the present results and that of Raczkowski et al. seem to indicate that the overestimation by the present work is perhaps mainly due to the nature of the intermolecular potential and that a close-coupling scheme and an accurate potential surface should reproduce the experimental results.
Collision dynamics of three interacting atoms: Energy transfer and dissociation in collinear motions
Authors: Beard L. H. , Micha David A.
Published: February 2003

The general theory of collisions in three-body systems is applied to vibrational excitation and dissociation in atom–diatom collisions. A multiple collision expansion is introduced for high relative energies, concentrating on single- and double-collision contributions. The general formalism is illustrated with calculations in collinear models for the H+D2 system. They include results for hard-core and exponentially repulsive interactions, and for harmonic oscillator and Morse diatomic potentials. Excitation probabilities are given within the single collision approximation with and without a peaking factorization. Comparison with exact results show that these assumptions are generally valid above threshold energies. For the case of collisional dissociation it is found that double collision events, describing the final channel distortion in the diatomic breakup, are essential to properly cover the energy transfer region between breakup and excitation. Results for dissociation of vibrational excited diatomics show the important role played by initial diatomic momentum distributions and more specifically, by the dynamical form factors of the theory.
Dynamics of gas–surface interactions: 3D generalized Langevin model applied to fcc and bcc surfaces
Authors: Tully John C.
Published: February 2003

A three dimensional generalized Langevin formalism is presented and applied to Ar and Xe interactions with Pt (111). Approximations for the random force and friction terms are proposed which permit realistic description of the motion and response of the surface atoms, including proper correlations among neighboring atoms. A ’’ghost atom’’ technique is developed which provides convenient numerical solution of the generalized Langevin equations in such a way that the fluctuation–dissipation theorem relating the random force and friction is satisfied rigorously. A simple prescription for determining parameters in the random force and friction is outlined. The prescription is applied to a four-atom active zone to obtain explicit relationships for all parameters, depending only on the bulk and surface Debye frequencies, for fcc (100), (110), and (111), and bcc (100) and (110) surfaces. Calculations of energy accommodation, sticking probabilities, and thermal desorption rates are reported for Ar and Xe on Pt(111). The sensitivity of these properties on the parameters of the random force and friction, i.e., on the phonon spectrum, is examined. The dependence is found to be sufficiently weak to demonstrate convincingly that the generalized Langevin approach with relatively simple parameterization is capable of describing gas–surface dynamics with high accuracy.
Charged hard sphere results for the one-component plasma
Authors: Palmer R. G.
Published: February 2003

The relationship of the charged hard sphere fluid to the classical one-component plasma is examined in general and within the mean spherical model. Predictions for th one-component plasma are derived and compared with the Monte Carlo data. (AIP)
The influence of the molecule continuum on inelastic vibrational energy transfer in atom–molecule collisions
Authors: Scherzinger Ann L. , Secrest Don
Published: February 2003

A study is made of the vibrational energy transfer in atom–diatomic molecule collisions for molecules with a dissociative continuum. The collision energy is kept below the dissociation limit. Comparisons are made of calculations performed using only the discrete molecular basis set, not including continuum functions, with exact calculations. A collinear model is used in these calculations in which the molecule is taken to be a Morse oscillator. It is found that for molecules with a shallow well, such as van der Waals molecules, the discrete molecular basis set never leads to converged scattering cross sections. For strongly bound molecules at low collision energies, the transitions among the lower states of the molecule are accurately described by only the discrete oscillator states, but at high collision energies or for states higher up in the well, the transition probabilities are often in error by more than a factor of 2. This is seen even when the calculation has appeared to converge with the discrete basis set. Errors due to the neglect of the continuum are not limited to transitions among states near to the continuum.
Three-body association reactions of He+, Ne+, and Ar+ ions in their parent gases from 78 to 300 K
Authors: Johnsen Rainer , Chen Aikwo , Biondi Manfred A.
Published: February 2003

Three-body association coefficients for the conversion of atomic He+, Ne+, and Ar+ ions to molecular ions have been determined using drift-tube mass-spectrometer techniques at gas temperatures from 78 to 300 K. The 300 K results for the conversion of He+ to He2+ agree well with those obtained by other investigators using various techniques; however at 78 K, our results disagree strongly with one of the previous determinations. In the cases of conversion of Ne+ to Ne2+ and Ar+ to Ar2+, the rate coefficients differ substantially for ground state ions in their 2P3/2 and 2P1/2 fine structure states; also, the temperature dependence of this effect is distinctly different for the two gases. It does not appear that available theories give an adequately quantitative description of these association processes.
Localized atomic orbitals for atoms in molecules. I. Methodology
Authors: Aufderheide Keith H.
Published: February 2003

A method has been devised for the construction of localized atomic orbitals (LAOs) for atoms in molecules. The LAOs reflect the atomic hybridizations suggested by the localized molecular orbital (LMO) analyses of Edmiston and Ruedenberg. The procedure involves definition of a new function, L(Aa‖A?), a measure of a particular type of exchange energy associated with the distribution between AOs a and ? on atom A. The terms L(Aa)=L(Aa‖Aa) constitute an atomic localization sum, L(A), which is required to be a maximum in the LAO basis on A. The properties of the components of L(A) are explored so as to achieve maximal computational efficiency. The maximization process is contingent upon an iterative sequence of 2×2 rotations. In this paper it is shown that the method will yield satisfactory results for all closed shell free atoms plus for the F atom in the HF molecule. The minimal basis of LAOs on F in HF are partitioned as (1) a core orbital with a population of about two electrons, (2) three trigonally equivalent lone pair orbitals each with a population of about two electrons, and (3) a bonding orbital pointing toward H with a population of somewhat less than one electron. The results match both classical and LMO predictions. In general, the LAO and LMO methods, although independent, yield complementary pictures of localized valence and bonding, as is shown further in the second publication of this series. There are, however, several features of the LAO method for molecules which may make it an attractive alternative to the LMO process, including computational time requirements for larger systems and a much preferable description of the valence in C2.
A
b i
n
i
t
i
o study on the isopropyl radical
Authors: Pacansky J. , Dupuis M.
Published: February 2003

A b initio UHF calculations were performed on seven conformations of the isopropyl radical which were generated by rotations of the two methyl groups, respectively. For each conformation, all of the geometric parameters were optimized using the gradient method. The calculations show that all seven conformations are separated by very small differences in energy. Furthermore, the planarity and the potential energy required for a pyramidal distortion of the radical center is quite different for each conformation.
Excited states of CH+: Potential curves and transition moments
Authors: Saxon R. P. , Kirby K. , Liu B.
Published: February 2003

Wave functions and potential curves for the ground X 1Σ+ state and eight excited states of 1Σ+ and 1Π symmetry of CH+ have been obtained using ab initio configuration interaction (CI) methods. In order to take proper account of valence–Rydberg mixing, Rydberg functions were included in the basis set. The orbitals used in the set of reference configurations for the CI wave functions included both valence and Rydberg orbitals, determined from a multiconfiguration self-consistent field and a natural orbital calculation, respectively. Transition moments between the ground electronic state and the 2 1Σ+, 3 1Σ+, A 1Π, and 2 1Π states have been computed, and the importance of these states in photodissociation of CH+ is discussed.
Cavity model for localized electrons in nonpolar media
Authors: Ichikawa Tsuneki , Yoshida Hiroshi
Published: February 2003

A cavity model has been developed for the localization of excess electrons in nonpolar media. The assumption of a continuum gives the analytical expression for the potential depth of a cavity, which is mainly determined by the short-range repulsive interaction between an excess electron and medium molecules. The shape of the cavity potential can be approximated adequately by a finite spherical well deep enough for the electron localization in alkane glassy matrices. Based on the present model, the broad optical absorption spectra due to the localized electrons in 3-methylpentane glassy matrix are attributed to bound-free transitions. The shape and the intensity of the electron spectra observed at 77 and 4.2 K can be reproduced by the theoretical spectra obtained by solving rigorously the spherical well problem. The energy of quasifree electrons, the total energy for the electron localization, and the cavity radius estimated from the simulation of the absorption spectra are in reasonable agreement with the available experimental results.
Angular dependence of light scattering structure coefficients for large anisotropic particles
Authors: Aragón S. Sergio R.
Published: February 2003

The effects of particle and experimental geometries on the scattered electric field correlation function are studied in detail for a dilute solution of cylindrically symmetric anisotropic particles, in the Rayleigh–Debye approximation. Previous results for this system are simplified considerably through the use of the spherical harmonic coupling rule and the scattering angle and polarization dependence of the structure factors is computed explicitly. Expressions containing the particle form factor contributions to the first two dynamical term weighting coefficients are given and the interpretation of experiments which show the complexity of the anisotropy is discussed in detail.
1
P
1→3P1 excitation transfer for Ca and Sr with the noble gases
Authors: Wright J. J. , Balling L. C.
Published: February 2003

Cross sections for 1P1→3P1 excitation transfer in Ca and Sr have been measured for the noble gases, using simultaneous measurements of the integrated 1P1 and 3P1 fluorescence intensities and the observed 1P1 decay rate. The cross sections range from 10−18 to 10−16 cm2.
Infrared multiphoton excitation and inverse electronic relaxation in SO2
Authors: Wolk Gary L. , Weston Ralph E. , Flynn George W.
Published: February 2003

Visible luminescence (λ=270–470 nm) has been observed from S16O2 and S18O2 at pressures of 0.2 to 20 Torr following irradiation by an intense infrared laser (λ=9.3 μm). Our experiments show that the luminescence is not due to dielectric breakdown or recombination of dissociation fragments, but rather is fluorescence from the first excited singlet states of SO2 following inverse electronic relaxation from highly excited vibrational levels of the ground electronic state. Crossover from the ground to excited electronic states may also be collisionally assisted. Spectroscopic and kinetic measurements are consistent with previous studies on 1B1 emission from SO2. The pressure dependence of the fluorescence yield exhibits two distinct pressure regimes, while the dependence of visible emission on laser pump wavelength follows the small signal infrared absorption spectrum. The threshold for detection of fluorescence is 17–20 J/cm2 with 9.3 μm radiation as the excitation source. These observations are discussed in terms of recently proposed theories which describe the photophysics of vibrationally excited states coupled to a radiative continuum through higher electronic states.
Raman scattering from NH4+ and ND4+ in beta-alumina
Authors: Bates J. B. , Kaneda T. , Wang J. C. , Engstrom Herbert
Published: February 2003

Polarized Raman spectra in the internal and external mode region of NH4+ and ND4+ ions in beta-alumina have been measured. The forms of the scattering tensors for the internal modes are calculated taking into account the disordered distribution of ions on the crystallographic sites and treating each ion as an independent scattering center. The results of the analysis suggest that the ammonium ions occupy three nonequivalent sites in beta-alumina. Model calculations indicate that the ions prefer the Beevers–Ross and midoxygen sites with the C2 axes of the ions located in the conducting plane. The librational modes of NH4+ and ND4+ evidently occur in the region from 200 to 450 cm−1, but overlap with phonons of the host lattice precludes positive identification. The in-plane and out-of-plane translational modes are identified in the region between 100 and 200 cm−1.
Point defect formation under the influence of NH3 and H2O atmospheres in the surface of NH4Cl
Authors: Kessler A. , Betz E.
Published: February 2003

An increase of the current passing through crystals of NH4Cl under the influence of NH3 and H2O atmospheres is investigated. From an analysis of the data it is inferred that, as a consequence of a proton transfer reaction, the same kind of defects which cause the extrinsic and intrinsic conductivity in NH3-saturated and in undoped NH4Cl, respectively, are formed in the surface.
X-ray diffraction study of concentrated chromium (III) chloride solutions. I. Complex formation analysis in equilibrium conditions
Authors: Magini Mauro
Published: February 2003

Concentrated chromium (III) chloride solutions, both acidic and neutral, have been examined by x-ray diffraction in equilibrium conditions; i.e., a long period of time after their preparation. Complex formation between Cr3+ and Cl− ions have been quantitatively analyzed through direct comparison of experimental and calculated structure functions. The average number of chloride ions bonded to chromium in each solution has been determined. The values obtained are consistent with the indications given by the distribution curves and in good agreement with those expected on the grounds of equilibrium constants. The average aggregation numbers were determined by assuming the presence, in neutral solutions, of the charged species Cr(H2O)3+6, Cr(H2O)5Cl2+, and Cr(H2O)4Cl+2. In addition experimental evidence of the neutral species Cr(H2O)3Cl3 in the acidic solution has been obtained. Octahedral water coordination for chloride ions has been confirmed. Well defined second coordination shell around Cr3+ ions is strongly supported by the structural parameters obtained for water molecules belonging to two subsequent hydration shells of the cations.
On the 3d24s2, 3d34s1, and 3d4 states of Ti
Authors: Bauschlicher Charles W.
Published: February 2003

The 3F, 5F, and 5D(3d4) states of Ti are investigated using CI and MCSCF calculations.(AIP)
Multiple quantum NMR study of correlation of two methyl groups
Authors: Tang J. , Pines A.
Published: February 2003

Multiple quantum NMR was used to study a system of two adjacent methyl groups in an oriented molecule to see if there was any degree of correlated motion. The 4-quantum spectrum which is sensitive to two-body correlations can distinguish simply between the cases of uncorrelated and correlated motion. The experimental multiple quantum spectra of partially oriented 2,3-dimethylmaleic anhydride in a liquid crystal show that the two methyl groups rotate quite independently.
Influence of deuteration on the order–disorder phase transition of anthracene–tetracyanobenzene
Authors: Dalal N. S. , Haley L. V. , Northcott D. J. , Park J. M. , Reddoch A. H. , Ripmeester J. A. , Williams D. F. , Charlton J. L.
Published: February 2003

Raman measurements of intermolecular vibrational frequencies, differential scanning calorimetry, and ESR measurements of triplet excitons are reported for crystalline anthracene–tracyanobenzene and various deuterated analogs. Deuteration of all the α- or all β-anthracene positions lowers the temperature of the order–disorder transition by 10–12 K. Perdeuteration lowers it by 17 K. The effect may result from decreased repulsive interactions when the CH bond is replaced by the shorter CD bond. High excitation light intensities can change the apparent transition temperatures and expand the measured coexistence range of the high- and low-temperature phases. (AIP)
Mobilities of NO2−, NO3−, and CO3− in N2 over the temperature range 217–675 K
Authors: Eisele F. L. , Perkins M. D. , McDaniel E. W.
Published: February 2003

Zero-field mobilities of NO2−, NO3−, CO3−, in N2 are reported over the temperature range 271–675 °K. The ions were formed by electron bombardment of mixtures of the trace gases N2O, NO2, and CO2 in N2. (AIP)
The far-infrared spectrum and barrier to planarity of 2-cyclohexene-1-one
Authors: Smithson T. L. , Wieser H.
Published: February 2003

The far-infrared spectra of 2-cyclohexene-1-one around 100 cm−1 is discissed and frequency assignments are considered, that are different from those of ref 2. (AIP)
On the relation between random coupling and sequential intramolecular dynamics
Authors: Druger S. D.
Published: February 2003

The relation between various recently studied random coupling models for intramolecular dynamics is examined. It is shown that the frequently made assumption of factorable coupling is in fact highly restrictive, that it implies strong correlations among the values of specific coupling matrix elements, and that it can sometimes lead to time-dependent behavior inconsistent with that expected for real systems. A specific random-coupling limit inconsistent with factorable coupling is rigorously shown equivalent to regarding each intermediate state to be coupled independently to its own quasicontinuum, and this limit therefore implies the expected sequential behavior. A general theory of the relation between uncorrelated matrix elements and sequential molecular dynamics (without the factorable coupling assumption) is then presented and shown to yield the expected sequential behavior under conditions of random coupling. The theory is extended by developing, within the same framework, a scheme for treating the correlated couplings as a perturbation on the uncorrelated couplings, and additional uncorrelated-coupling conditions sufficient to assure sequential behavior are obtained.
A systematic theoretical study of harmonic vibrational frequencies: The ammonium ion NH4+ and other simple molecules
Authors: Yamaguchi Yukio , Schaefer Henry F.
Published: February 2003

Analytic gradient techniques have been used to predict the harmonic vibrational frequencies of HCN, H2CO, H2O, CH4 and NH4+ at several levels of molecular electronic structure theory. Basis sets of double zeta, double zeta plus polarization, and extended plus polarization quality have been used in conjunction with self-consistent-field and configuration interaction methods. For the four spectroscopically characterized molecules, comparison with theory is particularly appropriate because experimental harmonic frequencies are available. For the 16 vibrational frequencies thus considered, the DZ SCF level of theory yields average errors of 166 cm−1 or 8.0%. The DZ+P SCF results are of comparable accuracy, differing on the average from experiment by 176 cm−1 or 8.3%. With the extended basis set, the comparable SCF frequency errors are only slightly less. The explicit incorporation of correlation effects qualitatively improves the agreement between theoretical and experimental harmonic vibrational frequencies. The DZ CI frequencies differ on the average by only 44 cm−1 or 2.0%. Perhaps surprisingly, the use of larger basis sets in conjunction with CI including all singly and doubly excited configurations leads to larger average errors in the vibrational frequencies. For example, the DZ+P CI frequencies have average errors of 80 cm−1 or 3.5%. Thus it seems clear that higher excitations (probably unlinked clusters especially) have a significant effect (order of 50 cm−1) on the theoretical prediction of polyatomic vibrational frequencies. The apparent discrepancy between the theoretical and experimental equilibrium geometry of CH4 is resolved here, and shown to have been a simple consequence of basis set incompleteness. Finally, the gas phase NH4+ equilibrium bond distance is predicted to be 1.022 Å, or 0.01–0.02 Å shorter than found by Ibers and Stevenson for NH4+ in crystalline NH4Cl and NH4F.
Correlation diagrams for rigid and nonrigid three-body systems
Authors: Kellman Michael E. , Amar François , Berry R. Stephen
Published: February 2003

A correlation diagram is constructed for the energy levels of a three-body system. The principle limiting cases for which the diagram is constructed are the equilateral triangle of three identical particles and the completely nonrigid cluster with pairwise harmonic forces. The artificial symmetries of these limits can be broken by conventional methods to make the diagram suitable for real molecules. The degree of nonrigidity can be expressed with a single parameter, a ratio of two excitation energies, one of which becomes the energy of the first rotational excitation and the other, the energy of the first vibrational excitation, in the idealized rigid limit. For real molecules, the parameter is a ratio of averages of the excitation energies to those states that correlate with the corresponding ideal limiting cases. The method is related to, and gives some insights regarding the Elliott model of nuclei.
Molecular dynamics simulations extended to various ensembles. I. Equilibrium properties in the isoenthalpic–isobaric ensemble
Authors: Haile J. M. , Graben H. W.
Published: February 2003

Equilibrium molecular dynamics simulations of the Lennard-Jones fluid have been performed to serve as tests of a method recently suggested by Andersen for obtaining results in the isoenthalpic–isobaric ensemble. The method involves the introduction of adiabatic volume fluctuations into the simulation by treating the volume as an additional dynamic variable having its own equation of motion. Our findings are that (a) it is a simple task to modify an existing molecular dynamics program to the new method and (b) results for the static properties of the fluid are the same as those obtained by other Monte Carlo and molecular dynamics techniques. At this stage the method still needs refinement in order to produce the correct dynamic properties of the fluid.
The Fokker–Planck–Langevin model for rotational Brownian motion. II. Comparison with the extended rotational diffusion model and with observed infrared and Raman band shapes of linear and spherical molecules in fluids
Authors: Lévi G. , Marsault J. P. , Marsault-Hérail F. , McClung R. E. D.
Published: February 2003

A detailed comparison of two extensions of the Debye model for molecular reorientation in liquids, Gordon’s extended J-diffusion model and the rotational Fokker–Planck–Langevin model, is presented. It is shown that the two models, although they represent very different physical pictures of rotational dynamics in fluids, lead to remarkably similar reorientational correlation functions, memory functions, correlation times and spectral densities. Only for values of the angular momentum correlation time appropriate to low density fluids do the two models give significantly different results, but the validity of the FPL model is questionable in this region. The results of an infrared study of the ν3 band of N2O in N2O/O2 along the coexistence curve from 105–153 K, and in N2O/N2 along the 80 bar isobaric line from 100–300 K are presented. The reorientational correlation functions from these measurements, and from earlier infrared studies of CO/N2 and Raman studies of CF4 and N2O liquids are compared with correlation functions computed with the EDJ and FPL models. Neither the EDJ nor the FPL model gave a substantially better description of the spectral band shape data.
Thermal diffusion in the gas mixture CH4–Ar
Authors: Shahin F. , Abdel-Latif B. T. , Farag N.
Published: February 2003

The temperature dependence of the thermal diffusion factor αT for the CH4–Ar system has been studied by a two-bulb method in the temperature range 300–1100 °K. The plots of αT vs the temperature show a normal behavior with a gradual increase of αT with temperature. The composition dependence of αT has also been studied for this system, the measurements covering the composition range 10%–90%CH4. The αT vs composition of CH4 curves show a minimum at all temperatures, which are more clearly defined at high temperatures. Analysis of the gas mixtures was carried out by means of the well-known Katharometer technique.
Four particle cluster approximation for the Maier–Saupe model of the isotropic–nematic phase transition
Authors: Van der Haegen R. , Debruyne J. , Luyckx R. , Lekkerkerker H. N. W.
Published: February 2003

The cluster variation theory for the Maier–Saupe model of the isotropic–nematic phase transition is extended to the four-particle level. As in the case of the Heisenberg ferromagnet, the irregularities of the three-particle cluster approximation applied to cubic lattices, where there are no triangles of nearest neighbors, disappear. The extension from three- to four-particle clusters yields improved values of all quantities, characteristic for the phase transition.
Triplet CH+(CD+) emission from chemiluminescent ion–molecule reaction C+ (4P)+H2 (D2)
Authors: Kusunoki I. , Ottinger Ch.
Published: February 2003

Luminescence induced by collisions of low-energy metastable C+( 4P) ions in a beam with H2 and D2 gas was studied spectroscopically. Nine bands of the CH+(b 3Σ−−a 3Π) transition were observed, of which only three had previously been known. Of the corresponding CD+ transition twelve bands were observed for the first time. Vibrational constants were derived which are in good agreement with independent recent ab initio results. Preliminary results on the vibrational distribution of the CH+ (b) reaction product were also obtained.
Contributions of the interaction of internal rotation with other vibrations to the effective potential energy for internal rotation in molecules with symmetric internal rotors
Authors: Quade C. Richard
Published: February 2003

The theory of vibration–large amplitude internal motion interaction in molecules is used to calculate these contributions to the effective Hamiltonian for internal rotation when the internal rotor has a threefold axis of symmetry. Calculations are made for four deuterium isotopic species of methyl alcohol, two isotopic species of acetaldehyde and trifluoroacetaldehyde, and perfluoroacetyl fluoride. The dynamic contribution to the potential energy coefficient V6 ranges from −3 to −6 cm−1. The potential energy coefficient V3 shows a dependence of a few tens of cm−1 upon the state of the perpendicular vibrations. A cos6τ dependence of the reduced torsional coefficient is of the order of 0.05% of 𝒢ττ. The mixing of internal rotation with other vibrations cannot account for the decrease in barrier for methyl alcohol with deuterium substitution; actually the calculation gives an increase in barrier of comparable magnitude to the decrease observed for CD3OH compared to CH3OH.
Magnetization and NMR in pure and mixed crystal Fe[S2CN(C2H5)2] 2ClxBr1−x: 3D-Ising ferromagnetism and evidence for a strain induced ground state
Authors: DeFotis Gary C. , Cowen J. A.
Published: February 2003

The magnetization along easy and hard magnetic axes has been measured in three single crystal samples of Fe[S2CN(C2H5)2]2Clx Br1−x (x=1.000, 0.712, and 0.204), between 1.2 and 4.2 K and in applied fields up to 13 000 Oe. In the pure chloride an effective saturation moment of 3.28±0.16 μB per iron ion is observed along the [101] easy axis. The effective saturation magnetization observed in mixed crystals is slightly smaller than for the pure chloride and exhibits a weak dependence on the chloride–bromide ratio. This implies that the ground state of the iron ion in the bromide species is a weakly mixed component of ‖±3/2≳, which is not the case for the pure bromide. Along the hard axes, [010] and (1̄01), a linear dependence of magnetization on applied field is observed, to fields in excess of 12 000 Oe. These data are in essential agreement with previously measured zero-field susceptibilities. Numerous proton resonances appear in the NMR spectra of pure and mixed crystal samples. Four lines with nearly identical frequencies in pure and mixed crystals are observed, providing further evidence that similar ground states characterize all the iron ions in these crystals. The modulation and splitting properties of the observed resonances constitute direct evidence for a canted sublattice structure. The temperature dependence of a proton resonance in the pure chloride implies that the system is of 3D-Ising character, though an unusual value for the critical exponent is observed, β=0.265±0.01. The results are discussed in light of current theories of critical behavior in 3D-Ising systems.
Solid state studies. XIX. Heteroionic coupling in the vibrational spectra of phase IV of ammonium nitrate in the 1400 cm−1 region
Authors: Kearley Gordon J. , Kettle Sidney F. A. , Ingman John S.
Published: February 2003

A detailed Raman study of phase IV (room temperature) NH4NO3, ND4NO3, NHnD4−nNO3(0<n<4), and the corresponding species with 15N substitutes in the 1400 cm−1 region reveals that not only do δ(H–N–H) modes couple together, as do ν (N–O) vibrators, but these two sets may couple with each other when they subtend a common factor-group species. This heteroionic vibrational coupling is particularly important in NH4NO3. Infrared data are not readily accessible but are not inconsistent with this interpretation. The occurrence of heteroionic vibrational coupling is correlated with the crystal structure of phase IV.
Collisional influences on vibration–rotation spectral line shapes: A scaling theoretical analysis and simplification
Authors: DePristo Andrew E.
Published: February 2003

The recently developed energy corrected sudden (ECS) scaling theory for nonreactive molecular collisions is combined with the impact theory of spectral line shapes. Through the use of angular momentum algebra, we show that only a restricted fundamental set of collisional transitions enter into the linewidth and line shift determination. This development significantly reduces the complexity and difficulty in dynamical calculations of the line shape parameters. A further simplification in the linewidth and line shift equations is derived based upon the physical properties of the ECS theory; the result is a relationship between linewidths (or line shifts) and a small set of dynamical quantities: rotationally inelastic rates, modified vibrationally inelastic rates, impact parameter correlation functions and vibration-to-vibration inelastic rates. The theory is sufficiently general to treat pure rotation, fundamental, overtone and hot bands, and vibrationally hot perturbers. A brief consideration of the physical implications of such a relationship is presented here; future articles will contain more detailed discussions with special emphasis on the direct extraction of the dynamical quantities from experimental linewidth and line shift data.
Dynamics of the quenching of Na(3 2P
J) by CO(X 1Σ+, v=0)
Authors: Hsu David S. Y. , Lin M. C.
Published: February 2003

The vibrational energy distribution of the CO formed in the E→V energy transfer reaction: Na(3 2PJ) + CO (X 1Σ+, v=0)→Na(3 2S1/2) + CO (X 1Σ+, v?9) was determined at five temperatures between 486 and 672 K. The electronically excited Na atom was optically pumped by a dye laser operating at 590 nm and the CO vibrational energy distribution measured by means of a cw CO laser operating at 5 μm. The CO was found to have a near Poisson distribution, peaking at v=2, with little temperature effect. The efficiency of the E→V transfer was determined to be ∼35%. These findings are consistent with those predicted by a curve-crossing model involving an ionic Na+(CO)− complex.
Properties of clusters in the gas phase: V. Complexes of neutral molecules onto negative ions
Authors: Keesee Robert G. , Lee Naisin , Castleman A. W.
Published: February 2003

Ion–molecules association reactions of the form A−(B)n−1+B=A−(B)n where studied over a range of temperatures in the gas phase using high pressure mass spectrometry. Enthalpy and entropy changes were determined for the stepwise clustering reactions of (1) sulfur dioxide onto Cl−, I−, and NO2− with n ranging from one to three or four, and onto SO2− and SO3− with n equal to one; and (2) carbon dioxide onto Cl−, I−, NO2−, CO3−, and SO3− with n equal to one. From these data and earlier hydration results, the order of the magnitude of the enthalpy changes on the association of the first neutral for a series of negative ions was found to parallel the gas-phase basicity of those anions. For any given ion, the relative order of the addition enthalpies among the neutrals was found to be dependent on the polarizabilities of the neutrals and on the covalency in the ion-neutral bond. Dispersion of charge via covalent bonding was found to significantly affect the succeeding clustering steps.
Energy disposal by F atom abstraction reactions: HF vibrational–rotational distributions from F+HBr and HI
Authors: Tamagake K. , Setser D. W. , Sung J. P.
Published: February 2003

HF infrared chemiluminescence from the reactions of F atoms with HCl, HBr, and HI was used to assign vibrational–rotational populations of the HF product. Experiments were done in both a cold-wall, arrested vibrational–rotational relaxation apparatus and in a fast-flow, arrested vibrational relaxation apparatus. Since the total HF formation rate constants are known for these reactions, absolute 300 K rate constants for formation of HFvJ are established. The mean vibrational energy disposal to HF including estimates for HF (v=0) is <fV≳HCl=0.51, <fV≳HBr=0.59 and <fV≳HI=0.59. The mean HF rotational energy decreased from 0.18 to 0.12 in the HCl–HI series. The <fV+fR≳ sum is virtually constant for the three reactions, but <fV≳ does increase slightly as the reactions become more exoergic. The HBr reaction yields ∼10% Br(2P1/2), the upper spin–orbit state; but I(2P1/2) is not formed from HI. Independent work by Nip and Clyne on the HCl reaction suggest that Cl(2P1/2) also is formed in ∼10% yield. The dynamics of these reactions are considered with the aid of information-theoretic analysis. The possibility of two components, direct and complex, for product formation is considered. The HF vibrational distribution from the HBr and HI experiments in the cold-wall apparatus showed an unusual dependence upon reagent flow and the ’’best’’ distributions were not found for the lowest flow. The selection of the ’’best’’ initial vibrational populations were assisted by the data from the fast-flow apparatus, which included experiments with DCl and DBr as well as HCl, HBr, and HI.
A new approach to molecular collisions: Statistical quasiclassical method
Authors: Lee Hai-Woong , Scully Marlan O.
Published: February 2003

A new approach is presented in which classical mechanics is combined with quantum statistics to describe molecular collisions. In this approach, the dynamics of collisions is described by classical trajectories as in the widely used quasiclassical method. However, initial and final internal states are represented in phase space in a quantum statistical way, using the Wigner distribution function. Results of calculations performed on a collinear He–H2 collision indicate that this new method is more accurate than the quasiclassical method, especially when the initial vibrational energy is low. Moreover, the new method is capable of describing classically forbidden processes that cannot be accounted for by the quasiclassical method.
Influence of the heterogeneities of the solid on the surface diffusion of adsorbed gases
Authors: Horas Jorge A. , Marchese Jose , Rivarola Juan B. P.
Published: February 2003

A model for the surface diffusion of adsorbed gases on porous solids is proposed. In it the heterogeneities of the solid and its influence on the diffusion coefficient are taken into account. The results obtained with the model are compared with new experimental data. A satisfactory agreement in covering ranges which run from very low values to the monolayer is achieved. The values of the parameters involved are found to be very reasonable.
Importance of angular momentum constraints in the product energy partitioning of model H–C–C–→H+C=C dissociation
Authors: Wolf Ralph J. , Hase William L.
Published: February 2003

Using classical trajectories to study the dynamics of H–C–C→H+C=C it is found that orbital angular momentum is conserved on the average in passing from the potential energy barrier to products. This result yields a very accurate dynamical model for predicting the average product energy partitioning.
Isotopically selective, infrared multiphoton dissociation of CF3Br
Authors: Nève de Mévergnies M. , del Marmol P.
Published: February 2003

A weak isotope effect based on the ratio 79Br/81Br is observed in the photodissociation of CF3Br by radiation from a TEA CO2 laser.(AIP)
Calculation of single orbital relaxations accompanying core ionization in the isoelectronic ten electrons hydrides by the Xα–SW method
Authors: Tse John S.
Published: February 2003

SCF-Xα-SW calculations are performed on the ground state and core 1s hole state for CH4, NH3, H2O and HF using the overlapping sphere model, Table 1 contains the molecular charge reorganization after the renormal of an 1s electron in these molecules. The screeming of the core is further discussed. (AIP)
On the thermal decomposition of SO2 diluted in Ar behind reflected shock waves
Authors: Saito Ko , Yokubo Teiji , Murakami Ichiro
Published: February 2003

The primary reaction rate constants of SO2 pyrolysis were measured in mixtures of 20 and 100 ppm in Ar by monitoring the UV emission. The results are summarized as k1=1014.55 exp(−439 kJ mol−1/RT) cm3 mol−1 sec−1 in the temperature range of 4300–6200 K.
Comment on ‘‘Photoselection in uniaxial liquid crystals: The effect of rotational Brownian motion on measurements of orientational distribution functions’’
Authors: DuPré Donald B. , Chapoy L. Lawrence
Published: February 2003

The effect of rotational Brownian motion on measurements of orientational distribution functions in uniaxial liquid crystals is discussed. The comments of Naqvi1 on the authors previously published2,3 papers are answered.(AIP)
The structure of the liquid lead at 670 K through molecular dynamics
Authors: Giró A. , González J. M. , Padró J. A. , Torra V.
Published: February 2003

By using the molecular dynamics method the radial distribution function, g(r), for liquid lead has been calculated. The temporal evolution has been generated by means of an oscillatory potential, obtained by joining together the semiempirical results given by Jacobs and Andersen for short range to those given by Johnson, Hutchinson, and March for long range.
Temperature dependence of the quenching of Mg(3P
J) by H2: Evidence for a change from physical to chemical exit-channel control
Authors: Breckenridge W. H. , Nikolai W. L.
Published: February 2003

The rate constant for the quenching of Mg(3s3p 3PJ) by H2 increases sharply with temperature in the 612–841 °K range. A simple equation is shown to fit this data as well as a rate-constant value reported by Taieb and Broida at lower temperature: This two-term expression results from the assumption that inefficient E to V, R energy transfer occurs in competition with a chemical exit channel in which there is no activation barrier beyond the 11.5 kcal/mole endothermicity of the reaction: Mg(3PJ)+H2→MgH+H. A transfer therefore occurs from physical to chemical exit-channel control in this unusual case as the temperature is increased.
A reference trajectory approach to Langevin equations in gas phase collision dynamics
Authors: Schatz George C. , Moser Mark D.
Published: February 2003

In this paper, a new approach to the development of Langevin-like equations for studying gas phase collisional energy tranfer and other dynamical problems is introduced based on the use of reference trajectories to describe memory effects and nonlinear interactions. In this development, the exact equations of motion are first expressed in terms of the deviations of the coordinates and momenta from some reference trajectory values and then linearized about those values. A partitioning between fast and slow variables is then assumed, and those members of the above mentioned linearized equations which refer to the fast variables are re-expressed as integral equations. A ’’local Brownian-like’’ approximation is then made in the memory kernel appearing in the integral equations to reduce them to algebraic equations, and upon substitution of these into the slow variable equations of motion, we obtain Langevin-like equations for the slow variables. In these equations the interaction between slow and fast variables appears as frictionlike and random forcelike terms, and in these terms, information about nonlinear interactions and correlated motions (including recurrences) is evaluated using the reference trajectory. In order to keep the deviations from the reference trajectory small during each collision, this trajectory is best chosen as the ensemble averaged trajectory, and we find that a good approximation to this for many problems is provided by a trajectory in which all initial vibrational energies are set equal to zero. Applications of this Langevin-like approach to several models of gas phase VT collisional energy transfer show that it is capable of quantitative predictions (errors typically<20%) of the first and second (classical) moments of the final translational distributions, provided that the initial translational energy is low enough to make the collision duration long compared with typical vibrational periods, and that the initial vibrational energy is low enough to make the deviations about the reference trajectory small. Often these restrictions are not particularly severe. For example, in a collinear Kr+CO2(000) model, the average energy transfer is accurate to 5% for initial translational energies as high as 10 eV, while for a collinear He+H2 model, energy transfers accurate to 30% or better are obtained with five quanta of initial vibrational excitation in the H2. In addition, accurate results are obtained even when the average energy transfer is of different sign than that of the reference, and in spite of the fact that the width of the translational distribution is a factor of 10 or more larger than its first moment. We also demonstrate that the Langevin equation works well when the average energy transfer becomes comparable to a quantum of vibrational energy (i.e., in the nonperturbative regime) provided that the necessary time scale separations for invoking the Langevin treatment exist.
Theoretical electronic plus vibrational investigation of some hydrogen-bonded complexes. I. Stretching cubic and quartic force constants of FH⋅⋅⋅OH2
Authors: Bouteiller Y. , Allavena M. , Leclercq J. M.
Published: February 2003

A b initio SCF–MO calculations have been carried out on the FH⋅⋅⋅OH2 dimer to obtain the energy surface, the equilibrium geometry, the electric dipole moment and its derivatives, with different basis sets including, in the most elaborated case, polarization functions on all the atoms. Stretching force constants related to the νFH and νF⋅⋅⋅O modes have been computed up to the fourth order, by fitting a two-dimensional energy surface. As previously reported, the nonrigid structure of the FH⋅⋅⋅OH2 complex is confirmed. Finally, a vibrational method, including all the stretching force constants computed for the dimer, is proposed to reconstruct the νFH and νF⋅⋅⋅O modes.