bluuebunny/crossref_metadata_2025 huggingface dataset:
Shock compression data for liquids. III. Substituted methane compounds, ethylene glycol, glycerol, and ammonia
Authors: Dick R. D.
Published: February 2003

Hugoniot data are presented for the liquid forms of the substituted methanes—dichloromethane, dibromomethane, di-iodomethane, and chloroform—ethylene glycol, glycerol, and ammonia. High explosive techniques were used to cover the range of dynamic pressures of 0.7 to 82.0 GPa. Chloroform transforms to a new form at 25.0 GPa pressure. Di-iodomethane data indicate a low pressure transition at 2.3 GPa and a second transition at a pressure greater than 66.0 GPa. The shock velocity–particle velocity (Us–Up) data for these two liquids are best represented by a linear relationship over the various forms. The Us–Up data for dichloromethane, dibromomethane, ethylene glycol, and glycerol are best fit by a quadratic expression in Up. Ammonia Us–Up data fit a linear relationship. Only dibromonethane and ammonia Us–Up curves extrapolate to the known sound speed. The others intercept at values that are 13% to 33% higher than the measured sound speed, indicative of low pressure transitions.
Precision acoustic measurements with a spherical resonator: Ar and C2H4
Authors: Mehl James B. , Moldover Michael R.
Published: February 2003

The spherical acoustic resonator is a remarkably accurate and convenient tool for the measurement of thermophysical properties of gases at low and moderate densities. The speed of sound (c) in a gas of interest can be measured with an accuracy of 0.02% merely by measuring the frequencies of the radial resonances when the resonator is filled with the gas of interest and then repeating the frequency measurements with a reference gas such as argon. The resonance frequencies of the radial modes are easily measured because these modes have very high Q’s, typically 2000–10 000. In this work the precision and accuracy of speed of sound measurements have been substantially improved by including a detailed acoustic model of the resonator in the analysis. Many of the important parameters of the model can be determined from acoustic measurements: Painstaking mechanical measurements are not required. We have used a spherical resonator to measure the speed and attenuation of sound in C2H4 in the temperature range 0–100 °C and the pressure range 0.15–1.0 MPa. Our measured values of c in C2H4 have a precision of 0.003% and agree with those of Gammon within the scatter of Gammon’s data (±0.02%). This agreement is remarkable when one considers that our spherical resonator is operated in the frequency range 4–13 kHz while Gammon has used a more conventional, cylindrical, variable path, acoustic interferometer operating at 500–2500 kHz. To attain this agreement, we did not have to make any highly accurate measurements other than frequency. Our measured values of the bulk relaxation frequency of C2H4 are within the scatter of the more recent values of the literature. In the course of our ’’calibration’’ measurements with argon we have redetermined the leading acoustic virial coefficient of argon. Our values for the virial are in satisfactory agreement with those in the literature. We include several practical suggestions for increasing the accuracy and/or versatility of spherical resonators.
General theory of the Raman scattering close to a plane surface. Evanescent Raman spectra
Authors: D’Hooge L. , Vigoureux J. M. , Menu C.
Published: February 2003

We present a general theory of Raman scattering near the surface of a plane dielectric medium. All possible cases are considered but only the ones in which evanescent waves are present are discussed (evanescent excitation or/and evanescent scattering). In each case the intensity and angular distribution patterns of the scattered light are discussed. The problem of measurement of depolarization ratios of Raman bands is also considered.
Continuous wave intracavity dye laser spectroscopy. II. A parametric study
Authors: Harris Stephen J. , Weiner Anita M.
Published: February 2003

The sensitivity enhancement of cw intracavity dye laser spectra of I2 and of water vapor has been measured for four dye laser tuning systems, two cavity lengths, and a wide range of pumping powers. The longitudinal mode structure of the dye laser has also been examined. The sensitivity enhancement increases with the bandpass of the dye laser tuning element and is independent of the cavity length. It is only weakly dependent on pumping power except near threshold, where it climbs steeply. The experimental results are compared with predictions of several theories which seek to explain the basis for the enhancement. None of the predictions are in quantitative agreement with our results; and, except for the prediction that enhancement increases near threshold, the theories also give generally poor qualitative agreement with experiment.
Threshold resonances in the electron photodetachment spectrum of acetaldehyde enolate anion. Evidence for a low-lying, dipole-supported state
Authors: Jackson Robert L. , Hiberty P. C. , Brauman John I.
Published: February 2003

The electron photodetachment spectra for the enolate anions of acetaldehyde and acetaldehyde-d3 have been measured using an ion cyclotron resonance spectrometer in conjunction with a tunable dye laser. Sharp resonances at threshold are observed for both anions, and are attributed to a low-lying excited electronic state of the anion, which we relate to postulated ’’dipole-supported’’ states of electron–polar molecule systems. The electron affinity of the neutral enolate radicals CH2CHO and CD2CDO are found to be 1.824±0.005 and 1.819±0.005 eV, respectively.
The anharmonic force field of CClF3 and the determination of the Coriolis coefficients and anharmonic coefficients from the infrared spectrum
Authors: Scanlon Kerin , Suzuki Isao , Overend John
Published: February 2003

The spectroscopic parameters of CClF3 have been calculated from an assumed anharmonic Urey–Bradley force field. These calculated values have been used as an aid to the interpretation of the observed infrared spectrum. The Coriolis splitting of the K subbands of the parallel transitions to the combination states formed from two degenerate fundamentals, e.g., n4+n5, has been observed and used in the determination of the Coriolis coefficients.
IR photochemistry: A unified approach for single-channel reactions. II. Treatment of experimental data
Authors: Baldwin Alan C. , Barker John R.
Published: February 2003

In the preceding paper of this series [J. Chem. Phys. 74, XXXX (1981)], it was shown that multiphoton decomposition yields versus fluence or time can be expressed as cumulative log–normal distribution functions (CLNDF). In the present paper, this idea is exploited for the purpose of presenting and analyzing experimental data. Such data from several different laboratories are used to show the effects of convolution of the laser spatial profile. Also shown are examples of homogeneous behavior, ’’active /inactive’’ population subsets, and more general cases. This approach permits a unified description of the experimental system, regardless of the potential complexities.
Stochastic sensitivity analysis in chemical kinetics
Authors: Costanza Vicente , Seinfeld John H.
Published: February 2003

The stochastic sensitivity analysis problem in chemical kinetics is defined as determining the probability density function (pdf) of the concentrations given probability density functions for the parameters and initial conditions. The joint concentration parameter pdf is found to satisfy the equation (∂p/∂t)+div(Fp) = 0, where the system dynamics are given by ? = F(x). The properties of the solution of this equation are studied, and the approach is applied to analyze the sensitivity of the kinetics of the photolysis of a mixture of carbon monoxide, nitrogen dioxide, nitric oxide, and water in air to uncertainties in the initial concentrations of the nitrogen oxides and in the values of two photolysis rate constants. Comparisons to other sensitivity analysis approaches are discussed.
Mode coupling at low densities
Authors: Masters A. J. , Keyes T. , Madden P. A.
Published: February 2003

The low density limit of the general expression for the diffusion constant off a spherical particle is investigated. The particle is assumed to have an arbitrary size and interact with the fluid particles through a highly repulsive and central potential. (AIP)
A mean free path kinetic theory of void diffusion in a porous medium with surface diffusion. Asymptotic expansion in the Knudsen number
Authors: Ho Fun Gau , Strieder William
Published: February 2003

A mean free path kinetic theory of void transport with Fickian diffusion on the pore walls has been developed for diffusion in a porous medium. A variational upper bound expression for the effective dissusion coefficient for a bed of overlapping solid spheres is expanded asymptotically including terms to third order in the Knudsen number. The effects of tortuosity are rigorously considered by explicitly including the flux of diffusing material around obstructions in its path in the trial functions. Parallel addition of void and surface diffusivities appears to be a good approximation of the variational effective diffusivity equation for all porosities less than 0.70 and small Knudsen number. For small void fractions, the terms of the asymptotic expansion can be summed to give the simple parallel in surface, but series in Knudsen and bulk diffusivities combination in analytical form. (AIP)
Reactions of Ar+ with various neutrals
Authors: Lindinger W. , Howorka F. , Futrell J. H. , Dotan I.
Published: February 2003

Rakshit and Warneck’s1 recent results for the reactions of Ar+ in the 2P1/2 and 2P3/2 states with various neutrals are commented upon. It is shown that the different rate coefficients for the two argon states reflect the effects in the drift tube used in their research. (AIP)
Vibrational relaxation of HF(v = 3,4,5)
Authors: Jursich G. M. , Crim F. F.
Published: February 2003

Measurements of laser excited vibrational overtone fluorescence show that the self-relaxation rates for HF(v = 3,4,5) are (3.02±0.30)×10−11, (7.28±0.27)×10−11, and (15.1±0.8)×10−11 cm3 molecule−1 s−1, respectively, and that single vibrational quantum energy transfer is the dominant relaxation path. The relaxation rates scale as v2.7 and are independent of initial rotational quantum number up to J = 3. The large energy transfer probabilities are consistent with the significant contribution from orbiting collisions found in trajectory calculations, and comparing the measurements to several calculations demonstrates the importance of including vibration–rotation coupling in the theory.
Reactive modes in condensed phase reactions
Authors: Grote Richard F. , Hynes James T.
Published: February 2003

The rate constant k for condensed phase chemical reactions is found when a saddle point normal mode analysis holds and when the dynamic solvent forces are of the generalized Langevin type. If the reactive and nonreactive modes are uncoupled, the deviation of k from its transition state value ktst is governed by the nonadiabatic friction on the reactive mode. In the more typical case where the modes are coupled k/ktst is governed by an effective nonadiabatic reactive mode friction which completely accounts for intramode coupling. Some simple illustrations of mode coupling effects on k are given.
Diffusion-controlled reactions in the presence of strong electromagnetic fields
Authors: Monchick L.
Published: February 2003

A Fokker–Planck equation has been derived for a system of particles that can exist in several quantum states, that can react, and that can interact with time-dependent fields. A generalized Smoluchowski equation can be deduced in which the potential of mean force can be time dependent. The description of chemical reactivity in terms of optical potentials is replaced by a set of radiation boundary conditions if a suitable limiting procedure is used. As an example, the special case is considered of the diffusion-controlled reaction of two particles in the presence of an electromagnetic field strong enough to influence the course of the reaction.
Theory of high-spin⇄low-spin transitions in transition metal compounds induced by cooperative molecular distortions and lattice strains
Authors: Kambara Takeshi
Published: February 2003

In the previous paper [J. Chem. Phys. 70, 4199 (1979)] the high-spin ⇄ low-spin transitions were studied by using the model based on the static treatment of intramolecular distortion, in which the discontinuous spin transitions may be induced by the Jahn–Teller effect. In the present paper the intramolecular distortions are treated as dynamical variables and the intermolecular coupling between the intramolecular distortions is taken into account. It is shown that the results for the present treatment are equivalent to those for the previous treatment except that the meanings of the parameters are different between the two treatments. The effect of the coupling between transition metal ions and a lattice strain on the spin transition is also studied. The lattice strain may drive the spin transition accompanying the cooperative intramolecular distortion, even when the intramolecular coupling is not so strong that it can singly induce the spin transition. The discontinuous change of the lattice strain then takes place together with an occurrence of the cooperative intramolecular distortion. When there is no cooperative intramolecular distortion, the lattice strain varies continuously with temperature and the spin state also varies continuously, only that the discontinuous thermal change of the lattice strain may occur in a very narrow range of the coupling strength.
MCSCF–CI calculations of the ground state potential curves of LiH, Li2, and F2
Authors: Jönsson Bo , Roos Björn O. , Taylor Peter R. , Siegbahn Per E. M.
Published: February 2003

The potential curves for LiH(X 1Σ+), Li2(X 1Σ+g), and F2(X 1Σ+g) have been calculated using a wave function comprising all single and double replacements from an MCSCF reference state. It is shown that in order to obtain satisfactory results, the reference function should describe properly not only the dissociation of the molecule, but also the united atom limit. This implies that for Li2, 1πu, and for F2,2πu orbitals should be included in the reference state. Calculated dissociation energies are 2.48 (2.52) eV for LiH, and 1.00 (1.05) eV for Li2 (experimental values within parentheses). For F2 only 1.31 (1.66) eV is obtained with 2πu excluded from the reference orbital space. A calculation including this orbital yields 1.40 eV even at the MCSCF level.
Quantum corrections to the second virial coefficient for potentials with a hard core
Authors: Gibson W. G.
Published: February 2003

The high-temperature behavior of the quantum-mechanical second virial coefficient is investigated for the case where the intermolecular potential has a hard core and is differentiable outside the core. The method of hard-sphere basis functions is used to expand the direct part as B = Bcl+BI(λ/a)+BII(λ/a)2 +BIII(λ/a)3+⋅⋅⋅ (λ is the thermal wavelength, a is the hard-sphere diameter), and explicit closed-form expressions are given for BI, BII, and BIII. This extends and corrects the work of previous authors. For the case where the exterior potential joins smoothly onto the core, BI = 0, BIII = 0, and BII reduces to the usual Wigner–Kirkwood term for the exterior potential.
The dielectric constant of SF6 near the critical point
Authors: Thijsse Barend J.
Published: February 2003

Measurements are reported of the dielectric constant of SF6 at the critical density (±1%) over a temperature range of −10−3〈(T−Tc)/Tc〈+10−2. The experimental cell was especially designed to suppress spurious effects resulting from temperature gradients and gravity. Temperature stability was better than ±20 μK and capacitance stability better than 0.5 ppm over several days. The critical temperature was determined as 318.718±0.002 K and was completely free of drift. Capacitance data were reproduced within a few ppm over the duration of the experiment. No direct evidence of a (T−Tc)1−α anomaly was found; an upper bound of 5 ppm is established. Effects of fluid stratification and phase separation were clearly seen and are well understood. We find a value of 0.325±0.005 for the critical exponent β.
Electron stimulated oxidation of silicon surfaces
Authors: Muñoz M. C. , Sacedón J. L.
Published: February 2003

Experimental evidence of electron stimulated oxidation (ESO) has been given for Si(111) 7×7 surface. In a first stage, the oxide thickness as a function of time shows a linear relationship; in a second stage, the growth rate quickly decreases and a pressure dependent saturation oxide thickness is reached. During the oxidation process an electrical potential does exist across the oxide, as is required in the Cabrera–Mott theory. The linear kinetics and the electrical potential are shown to be explicable in terms of a modified coupled-current approach based on the Cabrera–Mott theory, provided a semiphenomenological pressure dependent parameter is included. This represents a contribution of the surface reaction to the transport equation. The saturation has been explained as due to the decrease of the negative surface charge (donor levels) which produces a decrease of the electron current.
Electronic structure of iron–dioxygen bond in oxy-Hb-A and its isolated oxy-α and oxy-β chains
Authors: Tsai T. E. , Groves J. L. , Wu C. S.
Published: February 2003

A comprehensive study of the temperature dependence of the Mössbauer spectra of oxygenated hemoglobin (oxy-Hb) and its isolated oxygenated α chains (oxy-α) and β chains (oxy-β) has been completed. A model of the iron–dioxygen bond in oxygenated hemoglobin (oxy-Hb) is proposed where the ground state of the iron ions in oxy-Hb is assumed to be a quantum mixture of ferrous and ferric states of the Mulliken’s electron-donor–acceptor type. The temperature dependence of the Mössbauer quadrupole splitting can be explained by anharmonic vibration of the oxygen molecule in the iron ion’s binding potential well. Various previously proposed models and experimental data on the electronic structure of the iron–dioxygen bond in oxy-Hb are then discussed in terms of this model.
Autoionization of OCS by threshold photoelectron spectroscopy
Authors: Delwiche Jacques , Hubin-Franskin Marie-Jeanne , Guyon Paul-Marie , Nenner Irène
Published: February 2003

Autoionization of carbonyl sulfide between 12 and 16 eV has been investigated by photoionization using the pulsed synchrotron radiation from ACO Orsay’s storage ring. The threshold photoelectron spectrum and the total photoionization spectrum of carbonyl sulfide have been recorded at high resolution in the wavelength range between 112.0 and 65.0 nm (11–19 eV). Threshold energy electrons are observed in specific wavelength regions: (i) at excitation energies where the X̃, Ã, B̃, and C̃ ionic states are formed by a direct process; (ii) in the Ã state region where resonant autoionization to Ã can be understood classically within the Franck–Condon approximation; (iii) in the Ã–X̃ Franck–Condon gap between 90 and 110 nm, where resonant autoionization leads to very sharp electron energy distribution strongly peaked at zero energy. Here the mechanism must be more complex.
Cooperative vibronic spectra involving rare earth ions and water molecules in hydrated salts and dilute aqueous solutions
Authors: Stavola Michael , Isganitis Louis , Sceats Mark G.
Published: February 2003

Several cooperative vibronic bands have been observed in the excitation and emission spectra of polycrystalline hydrates and dilute aqueous solutions of TbCl3. A radiative transition involving the simultaneous electronic transition of a Tb3+ ion and vibrational transition of a nearby water molecule gives rise to these bands. Theoretical calculations, based on an electrostatic coupling model, explain the intensities of the cooperative vibronic transitions. Selection rules are derived and quantitative estimates of intensities are made.
Dynamic scaling of dilute polymer solutions
Authors: Marqusee J. A. , Deutch J. M.
Published: February 2003

Dynamic scaling relations are presented for the diffusion coefficient and intrinsic viscosity for dilute polymer solutions in d dimensions. The functional integration description of Adler and Freed is used with the correct d-dimensional hydrodynamic interaction and assumptions of power law dependence to obtain the scaling relations. Recursion relations for the exponents are determined in the asymptotic region of large N by an interdimensional scaling argument.
Matrix reactions of copper atoms and ozone molecules. Infrared spectrum of CuO
Authors: Tevault D. E. , Mowery R. L. , De Marco R. A. , Smardzewski R. R.
Published: February 2003

The matrix reactions of copper atoms with ozone have been studied by infrared spectroscopy. Two products were found in the initial deposit: CuO3, which has a strong absorption at 802.3 cm−1, and CuO, which has its main feature, corresponding to the 63Cu16O isotopic species, at 628.0 cm−1 in solid argon. Copper atom reactions with oxygen-18-enriched ozone samples were used to obtain species identifications. The CuO3 absorption was very similar in frequency and isotopic splitting behavior to alkali and alkaline earth metal ozonide species previously studied in matrices. The CuO frequency is in good agreement with gas phase measurements which put the ground state CuO vibrational fundamental at 631.3 cm−1. Temperature cycling of the Cu–O3 matrices leads to CuO4 formation by the secondary reaction of CuO with unreacted ozone.
A tunneling model for the decay of luminescence in inorganic phosphors: The case of Zn2SiO4:Mn
Authors: Avouris Phaedon , Morgan T. N.
Published: February 2003

We have investigated phosphorescence, thermoluminescence, and photostimulated luminescence in manganese activated zinc silicate phosphors, both with and without arsenic doping. Both phosphorescence and photostimulated luminescence intensities are found to decay as the reciprocal of the time, a result which requires a different interpretation from that given by the usual model of electron release from a distribution of trap levels. To account for these results we propose a new model based on the radiative recombination of electrons and holes through tunneling from shallow traps or from excited states of the deeper traps. A simple expression is derived which describes the decay of both types of luminescence in phosphors under very general conditions for which tunneling is the dominant recombination mechanism. Excellent agreement between theory and experiment is found.
Time resolved photodissociation spectroscopy of 1,3,5-hexatriene radical cation
Authors: Morgenthaler Lee N. , Eyler John R.
Published: February 2003

The photodissociation spectra of the parent ions arising from both cis and trans isomers of 1,3,5-hexatriene are identical within 10 ms of formation. It appears that both ions have a common structure soon after electron impact ionization. The rate of collisional quenching of vibrationally excited ions has been measured using a pulsed ion cyclotron resonance (icr)-pulsed dye laser technique. Quenching occurs with nearly unit efficiency in collisions with the parent neutral. Approximately fifty collisions with argon are required to relax the vibrational excitation in the excited ions.
Electron beam excited mixtures of O2 in argon. III. Energy transfer to O2 and O3
Authors: Keto J. W. , Hart C. F. , Kuo Chien-Yu
Published: February 2003

We report here measurements of the reaction rates for quenching of Ar*2 (3Σ+u) and charge transfer from Ar+2 to O2 and O3. These rates have been measured in electron beam excited mixtures of argon and oxygen as a function of argon pressures from 500 to 11 000 Torr with partial pressures of oxygen varying from 50 to 172 mTorr. Interpreting our measured loss rates as resulting from energy transfer to both O2 and O3 we extract bimolecular rate coefficients for excitation transfer from Ar*2 of 4.6±2.6×10<u−11 cm3 sec−1 and 4.6±0.6×10−9 cm3 sec−1 to O2 and O3, respectively. For charge transfer from Ar+2 to O2 we extract a bimolecular rate and a termolecular rate of 8±6×10−11 cm3 sec−1 and 9±2×10 −31 cm6 sec−1;and for Ar+2 to O3 we obtain rates of 5±3×10−9 cm3 sec−1 and 4.0±0.8×10−29 cm6 sec−1. Because we extract the contribution to the reaction rates by O3 we obtain significantly smaller energy transfer rates to O2 than earlier workers.
Structural and magnetic studies of hexagonal β-242Pu2O3
Authors: McCart Bruce , Lander G. H. , Aldred A. T.
Published: February 2003

Neutron diffraction and magnetic susceptibility measurements have been used to characterize the structural and magnetic properties of β-Pu2O3 with the hexagonal La2O3 structure. High-resolution neutron powder patterns combined with profile analysis have given the cell constants, atomic positional parameters, and the stoichiometry with high precision. The latter is in agreement with the value determined from thermodynamic methods. The antiferromagnetic structure in the temperature range 4 K<T<TN∼19 K consists of Pu moments in the basal plane, but they vary along the c direction in a pseudospiral way. Two components are required to describe this modulation. The value of the maximum Pu moment at 10 K is ∼0.85 μB, each component having an amplitude of ∼0.6 μB/Pu. At 4 K a spin rotation takes place so that μ∥c. Susceptibility measurements reflect the existence of TN but show very little response near the spin rotation temperature.
Appropriate input for the charged-sphere GMSA
Authors: Stell George , Hafskjold Bjo/rn
Published: February 2003

We give a new means of determining the parameters that appear in our generalized mean spherical approximation (GMSA). It is slightly different from our earlier SCOZA recipe and yields radial distribution functions that are somewhat better in an overall way throughout the molten-salt regime, where the results represent the most accurate analytic theory available for charged hard spheres.
Observation of critical dynamics in ND4Br by high-resolution neutron spectroscopy
Authors: Rowe J. M. , Rush J. J. , Pynn R. , Heidemann A. , Michel K. H.
Published: February 2003

Neutron inelastic scattering measurements are reported for ND4Br. Measurements were performed at 213 K (below Tc) in order to determine the instrumental resolution and at 225 °K (Tc+7 K). (AIP)
On the interpretation of solute induced solvent structure
Authors: Hirata Fumio , Rossky Peter J.
Published: February 2003

Computer simulation results in which a water molecule is considered, formally, as a solute in liquid water are examined in order to clarify the role played by solution geometry in the interpretation of solvation structure. An analysis of spatial correlations and intermolecular hydrogen bonding in the solvation ’’shell’’ is used to show that solvent molecular packing around the solute can substantially distort the quantitative measures of structure and that this must be considered carefully in their interpretation. (AIP)
Laser photofragment spectroscopy of the 1,3-butadiene cation at 5145 Å
Authors: Preuninger F. N. , Farrar J. M.
Published: February 2003

We present preliminary results of a laser beam–ion beam study of photodissociation of the 1,3-butadiene cation at 5145 Å. The kinetic energy distribution for C3H3+ production shows forward–backward symmetry, indicative of a transition dipole parallel to the fragmenting carbon–carbon bond. The most probable kinetic energy release is 75 meV, substantially larger than that observed in recent photoion–photoelectron coincidence measurements. (AIP)
A Monte Carlo evaluation of thermal desorption rates
Authors: Adams John E. , Doll Jimmie D.
Published: February 2003

The recently reported method for computing thermal desorption rates via a Monte Carlo evaluation of the appropriate transition state theory expression [J. E. Adams and J. D. Doll, J. Chem. Phys. 74, 1467 (1980)] is extended, by the use of importance sampling, so as to generate the complete temperature dependence in a single calculation. We also describe a straightforward means of calculating the activation energy for the desorption process within the same Monte Carlo framework. The result obtained in this way represents, for the case of a simple desorptive event, an upper bound to the true value.
X-ray diffraction study and models of liquid methane at 92 K
Authors: Habenschuss Anton , Johnson Elijah , Narten A. H.
Published: February 2003

New x-ray diffraction data for liquid methane near the triple point are presented. The data are analyzed using molecular scattering factors for methane which permit extraction of the carbon–carbon structure and correlation functions. Several site–site pair potentials are used with the reference interaction site model (RISM) to predict the structure and correlation functions for a dense methane-like fluid, and these are compared to the structure and correlation functions derived from the diffraction study. In addition, the accuracy of RISM is assessed by comparison with published computer simulation calculations on methane. Our results indicate that although the RISM distribution functions for ’’tetrahedral’’ molecules such as methane hae some physically unrealistic features, they compare well with molecular dynamics results. Site–site interaction potentials give an adequate description of liquid methane, provided that attractive forces are included in the potential. Although methane molecules are ’’seen’’ by x rays as spherically symmetric, orientational correlations between pairs of neighboring molecules are clearly present in the liquid near the triple point.
Infrared photodissociation of van der Waals molecules containing ethylene
Authors: Casassa Michael P. , Bomse David S. , Janda Kenneth C.
Published: February 2003

Vibrational predissociation line shapes in the ν7 region of the ethylene spectrum are measured for van der Waals molecules of ethylene bound to Ne, Ar, Kr, C2H4, C2F4, and larger ethylene clusters. The predissociative rate is very fast for this group of molecules. The vibrationally excited state lifetimes are 0.44, 0.59 and 0.89×10−12 sec for (C2H4)2, Ar⋅C2H4, and C2H4⋅C2F4 respectively. That the observed line shapes are homogeneous is demonstrated by the fact that a low-power, narrow frequency bandwidth laser can dissociate a large fraction of the initial ensemble of ethylene clusters. The observed transition probability is proportional to the number of ethylene subunits for clusters containing three or fewer ethylene subunits. These observations are interpreted in terms of intramolecular energy flow directly from ethylene ν7 to the weak van der Waals modes of motion.
Electron impact ionization cross sections of F2 and Cl2
Authors: Stevie F. A. , Vasile M. J.
Published: February 2003

Total ionization cross sections for the production of positive ions by electron impact ionization of F2 and Cl2 have been measured over the energy range 12 to 100 eV. The technique involves the ionization of a modulated molecular beam with cross sections determined by measurement of a calibrant gas. Mass spectrometric measurements insured that impurities did not contribute to the measured ion currents. The 70 eV ionization cross section of F2 was found to be 1.1×10−16 cm2, and the 70 eV ionization cross section of Cl2 was 7.1×10−16 cm2.
Chemical pumping of pure rotational HF lasers
Authors: Smith John H. , Robinson Dean W.
Published: February 2003

Pure rotational laser oscillation has been observed in HF following flash photolysis of various mixtures of trifluoromethyl halide, acetylenic compound, and argon in the ration of 1:1:100. Specifically, mixtures of CF3I+C2H2, CF3I+CH3C2H, and CF3Br+C2H2 exhibit different patterns of laser intensity, and, further, are markedly different from patterns generated by the known photoelimination system CH2CF2. The above systems show laser intensity at lower rotational levels than could be explained by simple V→R collision induced mechanisms (as in the case of CH2CF2). Further, the intensities in these anomalously low rotational levels appear to be a function of the identity of the electronically excited halide (I or Br) produced photolytically. A computer simulation model has been constructed which relaxes nascent HF through V→R transfer (formed vibrationally and rotationally excited by elimination) but incorporates an additional pumping mechanism, that of quenching of I or Br by these relaxing HF molecules. Most of the experimentally observed laser intensities are explained on the basis of this new pumping mechanism.
Collision dynamics and the thermal rate coefficient for the reaction H+O2→OH+O
Authors: Miller James A.
Published: February 2003

Classical trajectory methods have been used to predict the rate of the reaction H+O2→OH+O. Both quasiclassical (QCT) and quasiclassical quantum mechanical threshold (QCT–QMT) methods were employed in the analysis. The QCT–QMT calculations are in excellent agreement with experimental results over the entire temperature range considered (250<T<2500 K). In particular, the modified Arrhenius expression from the QCT–QMT analysis k = 5.13×1016 T−0.816 exp(−16 507/RT) cm3/mol is in excellent agreement with Schott’s high temperature shock tube result k = 1.22×1017 T−0.907 exp(−16 620/RT) cm3/mol and is less than a factor of 2 too high compared with experimental results in the 250–300 K temperature regime. The negative Arrhenius curvature is found to be due to a dynamic effect associated with the L+HH mass combination of the reactants. Simple formulations of statistical models for the reaction (transition-state theory, the statisitical model for complex reactions, and the unified statistical model) are found to given incorrect dependence of the reaction cross section on relative translational energy. This incorrect dependence of σR on ET leads to positive Arrhenius curvature rather than the negative curvature obtained both experimentally and from the trajectory calculations.
The electronic structure of hydroxyl molecules trapped in small neon clusters
Authors: Gamache Robert R. , Cade Paul E.
Published: February 2003

The electronic structure of small NenOH clusters, with n = 6 or 12, is examined within the approximate Restricted Hartree–Fock framework. The purpose of these calculations is to examine the integrity and modifications of the trapped molecule OH within the host atom cluster, Nen, and to contribute to understanding molecules trapped in matrices. We have limited this study to small planar neon clusters and hence do not present a full three-dimensional picture of a molecule trapped in a microcrystal. The Ne6OH system is stable with respect to Ne6 and OH by ∼10 kcal/M and trapping of a hydroxyl into the cluster is exothermic. In a rough comparison between electronic factors and van der Waals contributions, it is claimed that the van der Waals contributions do not account for the most important aspects of the trapping in this small cluster. The ionization potentials of OH are all lowered when OH is trapped in this small cluster. An interpretative study based on electronic charge densities is presented. The integrity and modifications of the hydroxyl are examined by consideration of orbital, total, and certain difference densities for OH when free and trapped in the cluster. We find that there is apparently a small transfer of charge from the neon cluster to the hydroxyl molecule and the density maps show many features characteristic of normal bonding density maps, although at a much lower scale. The barrier to rotation in the plane is obtained and found to be very small consistent with finding for OH trapped in a neon matrix. We find there are no significant changes in the potential energy curve, E(R), for the hydroxyl molecule in the matrix as compared to the free molecule.
Undersaturation dependence of the evaporation rate of potassium chloride
Authors: Nordine Paul C. , Gilles Paul W.
Published: February 2003

Theories of ledge dynamics for crystals evaporating by the stepwise mechanism predict a change from crystal edge control to dislocation control of the evaporation kinetics as the gas phase saturation is reduced. This effect was studied by measuring the rate of evaporation from the (100) face of high purity KCl crystals vs gas phase saturation with a specially designed spherical effusion cell of variable orifice area. The experiments minimized or eliminated variation in sample temperature, area, dislocation density, and purity and gave results of sufficient precision to confirm the theoretical predictions. Ledges arising from dislocations do not influence the evaporation rate of KCl if the incident to equilibrium flux ratio (Vi/Veq) exceeds 0.93±0.02, at 460 ° C. This conclusion is based on an observed change in slope of evaporation rate versus incident flux at a value of Vi/Veq consistent with the theory of stepwise evaporation and independent estimates or measurements of surface energies, surface reaction activation energies, and ledge spacings on evaporated crystals.
Photophysics of CF3NO
Authors: Spears Kenneth G. , Hoffland Lynn D.
Published: February 2003

We use laser spectroscopic methods to measure fluorescence lifetimes of CF3NO. These lifetimes correspond to radiationless decay from specific torsional and other vibrational levels of the S1 state of CF3NO. The lifetime data and collisional quenching studies are used to support a model of S1 internal conversion to S0, followed by dissociation to CF3+NO. Relative changes in radiationless rates as a function of optical excitation are successfully calculated by a radiationless rate theory. The torsional mode is proved to be a promoting mode for the radiationless transition, which is the first example of this concept. The predicted vibrational and torsional populations in S0 are used to make specific predictions of trends in the rotational and vibrational energy content of the NO photofragment.
Simple amorphous salts: Spectra and glass transition temperatures
Authors: Consani Keith , Devlin J. Paul , Ray Alex , Farrar Henry , Wilson Edmond W.
Published: February 2003

Vapor deposits of alkali metal nitrate salts prepared at liquid nitrogen temperatures have been shown to be structurally amorphous. The present study of the amorphous nitrates has produced infrared lattice mode spectra for NaNO3 and KNO3 and, through observation of the amorphous to crystal phase change using a light scattering technique, has extended the estimated Tg values to include RbNO3 and CsNO3. The complete range of nitrate Tc values extends from 171 K for CsNO3 to 253 K for LiNO3, reflecting a cation dependence similar to that for Tg of bulk metaphosphate glasses. The amorphous NaNO3 and KNO3 infrared lattice bands are extremely broad and, although some band structure is apparent, the subbands that are largely responsible for the great breadth are not as prominent as for the corresponding molten salts. Since band narrowing should occur at low temperatures, leading to more pronounced subbands, the spectra suggest that the amorphous deposits may be relatively more disordered than the melts, i.e., the melt is not randomly structured. Tg values have also been estimated for a series of alkali halide deposits, and lattice mode infrared spectra have been measured for NaCl and KBr films. Despite earlier reports to the contrary, anhydrous alkali halide vapors apparently form pure amorphous deposits at 90 K. In particular, KBr and NaCl deposits have relatively broad lattice absorptions that sharpen markedly at 180 and 185 K, near the temperatures at which light scattering measurements also indicate formation of microcrystals. In general, the new data indicate that amorphous alkali halide salts have crystallization temperatures in the 150–185 K range, well below the corresponding nitrate values. However, a lack of obvious ion size effects, so marked for nitrates and metaphosphates, was not expected and suggests a need for additional data.
Demonstration of efficient nonlinear optical crystals with vanishing molecular dipole moment: Second-harmonic generation in 3-methyl-4-nitropyridine-1-oxide
Authors: Zyss J. , Chemla D. S. , Nicoud J. F.
Published: February 2003

Previous research toward more efficient nonlinear optical organic crystals has aimed at increasing the first-order hyperpolarizabilitiy of individual molecular units. We show that the general features derived from these studies (high conjugation, charge transfer, and molecular asymmetry) leave room for the cancellation of the molecular dipole moment. This can be usefully exploited in two directions: improving the crystal growth conditions and a possibility of acting on the crystal point group structure through otherwise hidden parameters. Pyridine-1-oxide, owing to the electronic ’’push–pull’’ property of the N–oxide bond, allows for substitution in the 4 position of donor as well as acceptor groups, among which the nitro group ensures dipole cancellation. The possibility of charge transfer from or toward the N–oxide bond according to the nature of the radical in the 4 position is investigated and confirmed by means of second-harmonic generation experiments in powder or solution (DCFISH) on a number of compounds. 3-methyl-4-nitropyridine-1-oxide is shown to be the best candidate, among related compounds, for crystal growth. A detailed study of its linear and first-order nonlinear properties is presented. Crystalline samples grown in solution are seen to be of excellent optical quality. Its efficiency compares to that of the best available phase- matchable organic crystals [d = (23±3)×10−9 esu]. Besides, the optical broadband phase-matching potential of the crystal could be useful in a number of applications like frequency doubling of ultrashort pulses in the near infrared.
Interaction energy for open-shell systems
Authors: Neumann D. B. , Krauss M.
Published: February 2003

General expressions necessary for direct calculation of damped multipolar atomic interaction energies are presented. The ab initio method requires the computation of the zeroth and first order wave functions of each atom and can be easily applied to the interaction of open-shell atoms. Applications of this technique are given here for the case of the dipole–dipole interaction of O(1S) with O(1S, 1D, and 3P) and, using effective core potentials, Hg(1S) with Hg(1S and 3P).
Orientational pair correlations in dilute solutions of rodlike molecules
Authors: Evans G. T. , Andersen G. R. , Mazo R. M.
Published: February 2003

The orientational correlation times for the first and second rank spherical harmonics of single particle and collective orientational variables are calculated assuming that the rotorlike molecules undergo correlated rotational and translational motion. Rotation–translation coupling arises through the Oseen tensor operating between the rotors and introduces a concentration dependence in the single particle and collective times. A linear concentration dependence stems from the averaged Liouville operator, as found previously, and through the memory function, whose direct calculation has been hitherto ignored. Even in the presence of the memory function, the Keyes–Kivelson ratio of the collective and single particle correlation times is unaffected to linear order in the concentration of the tagged molecules. However, the memory function and the associated torque correlation function incorporate a linearly increasing concentration dependence in the single particle times.
Extended Browian dynamics approach to diffusion-controlled processes
Authors: Lamm Gene , Schulten Klaus
Published: February 2003

The method of Browian dynamics to simulate trajectories of particles diffusing under the influence of external forces is extended to the approximation of local linear forces and to reflective boundaries possibly confining the diffusion space. A simple algorithm for the generation of random diffusive displacements is developed and its validity demonstrated by comparison with analytical and numerical descriptions of sample one-dimensional diffusion distribution.
Surface stress anisotropy of treated glass and liquid crystal alignment
Authors: Mada Hitoshi
Published: February 2003

We measured the surface energy and its anisotropy both for a liquid crystal (7CB) and for some treated glasses which make the liquid crystal align. The treated glasses were prepared in the following five ways: (1) rubbing the glass surface with a cloth, (2) coating with PVA and rubbing, (3) coating with an inorganic surfactant of SiO and rubbing, (4) coating with carbon and rubbing, and (5) 60 ° oblique evaporation of SiO. The surface energy was obtained by measuring the contact angle of the liquids whose surface tensions are known. The interfacial energy and its anisotropy were calculated from the experimental results. The magnitude of the anisotropy of the interfacial energy is in good qualitative agreement with the orientational order parameter of the liquid crystal near the surface. Therefore, the alignment of the liquid crystal on the treated substrate is dependent on the anisotropy of the interfacial energy.
The classical Boltzmann equation of a gas of diatomic molecules
Authors: Curtiss C. F.
Published: February 2003

A classical Boltzmann equation and collision integral of a gas of nonvibrating diatomic molecules is developed. It is shown that the generalization of the usual equation for structureless particles involves additional integrations over the angular momentum vector of the second molecule and two angles associated with the phases of the rotational motion of the two molecules. These latter angles are constants of the free motion of the molecules defined in a manner analogous to that of the usual impact parameter, i.e., they are determined by the free molecule trajectory as extrapolated to the point of closest approach.
Molecular calculation of the dielectric constant of N2 and CO2
Authors: Chen C. C. , Steele William A.
Published: February 2003

The formal equation which relates the deviation from the Clausius–Mossotti equation for the dielectric constant to the dipoles induced in a nonpolar molecular fluid by the external field-induced dipoles and the permanent multipole moments is taken as a starting point. The molecular integrals appearing in this equation are explicitly written in terms of angle-averaged and angle-dependent pair correlation functions for the fluid. The intermolecular interactions for N2 and CO2 are modeled as the sums of diatomic potential functions plus quadrupolar energies. Simulated pair correlation functions are used in a calculation of the two- and three-molecule contributions to the deviation from Clausius–Mossotti behavior for liquid N2. Pair correlation functions calculated in the limit of zero density were used to evaluate second dielectric virial coefficients for N2 at 300 and 124 °K and for CO2 at 323 °K. A comparison with experiment is moderately satisfactory, considering the uncertainties in the parameters of the calculation. It is concluded that the contributions due to orientational correlations and to quadrupole-induced terms play an important role in determining the deviations from Clausius–Mossotti behavior for the systems considered.
A new representation for thermodynamic properties of a fluid
Authors: Kohler Friedrich , Haar Lester
Published: February 2003

It is proposed to consider the contribution of the nonisolated molecular pairs to the configurational Helmholtz energy, i.e., the quantity f*/RT−Bρ, where the product of the second virial coefficient B times molar density ρ covers the contribution of the isolated pairs. Especially illuminating is the difference function Δ = (f*/RT−Bρ)real  fluid −(f*/RT−Bρ)hard  body  fluid. From the few examples given it seems that this difference function is of a general nature provided that the anisotropy of the hard bodies corrsponds to the anisotropy of the molecules of the real fluid. The difference function can be correlated empirically in a simple way and can be used for estimating thermodynamic properties at intermediate and low densities from high density results.