bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational energy transfer studies in CO2–hydrocarbon mixtures
Authors: Rao I. V. , Babu S. V. , Rao Y. V. C. , Rao V. Subba , Lalita K.
Published: February 2003

Rate constants for the transfer of energy from CO2(00°1) to ethane and n-butane were measured using the laser induced fluorescence technique. The large magnitudes and the negative temperature dependence of the energy transfer rates indicate that near-resonant energy transfer processes caused by long range forces are responsible for the deactivation of the asymmetric stretching mode of CO2. It is shown that the combination bands in the alkanes are likely to receive the energy from CO2(00°1). Using Sharma–Brau theory with Tam’s modification and our experimental results, the square of the transition dipole matrix elements of ν3+ν6 and ν9+ν11 bands of ethane are estimated. At any given temperature in the range 300–730 °K, the energy transfer cross section was found to increase linearly with n of CnH2n+2. It is suggested that the absorption intensity of the combination bands of the alkanes also increases linearly with n in the spectral range 2300–2400 cm−1.
Vibrations of potassium and cesium thiocyanates crystals. III. Long-wave lattice modes
Authors: Ra O/. , Ti S. S. , Kettle S. F. A.
Published: February 2003

In this paper, a dynamical calculation is reported for the orthorhombic crystals of potassium and cesium thiocyanates. A general description of the model used and theoretical results are presented. Although the model covers arbitrary wave vectors, and is applicable to both internal and external modes, numerical work is presently restricted to external long-wave vibrations. The vibrational frequencies theoretically calculated and experimentally observed show reasonable agreement. Rotatory and translatory modes belonging to the same symmetry species are found to be strongly coupled. Some erroneous assignments made previously are rectified.
Absorption and circular dichroism spectra of the c
i
s- and t
r
a
n
s-butadiene chromophores α- and β-phellandrene
Authors: Gross K. P. , Schnepp O.
Published: February 2003

The absorption and circular dichroism spectra of gas-phase (−) -α-phellandrene and (−) -β-phellandrene have been measured in the spectral region 300–135 nm. Solution spectra of these compounds were measured in perfluoro-n-hexane to 160 nm. The spectra were interpreted in terms of the excited electronic states of the cis- and trans-butadiene chromophores. The overlapping valence and Rydberg states were unambiguously differentiated by comparison of the vapor and solution phase spectra. Comparison between absorption and CD spectra made possible the symmetry assignments of magnetic dipole allowed (as opposed to electric dipole allowed) excited states of butadiene. For both molecular systems the first observed excited singlet state corresponds to 1Bu for the trans chromophore and to 1B2 for the cis chromophore. No evidence was found for a ’’lowest-lying’’ singlet state of Ag or A1 symmetry.
Theory of the spin Hamiltonian of CIO2 in single crystals of silver chlorate
Authors: Peixoto L. T. , Foglio M. E.
Published: February 2003

The EPR of ClO2 in gamma irradiated single crystals of AgClO3 was recently observed and the g and A tensors appeared rotated with respect to each other by approximately 6° about one of the molecular axis of ClO2. This effect was studied by considering a crystal field that breaks the C2v symmetry of ClO2. The crystal field was estimated from a point charge model that includes the Ag+ and ClO−3 ions that are the nearest to the ClO2 fragment. This model gives a good qualitative agreement with the experimental results: It predicts the same axis and direction of rotation that were observed and a value of the angle of rotation between 0.8° and 4.4° (depending on the position of the ClO2 in the crystal lattice). In this model the g tensor is practically unaffected by the crystal fields and the orientation of the ClO2 fragment is consequently determined by the principal axis of this tensor.
A potential surface for the two low-frequency out-of-plane modes of s-tetrazine
Authors: Malloy Thomas B. , Carreira L. A.
Published: February 2003

Previously reported data for s-tetrazine-d0 and -d2 have been used to determine a potential energy surface for the low-frequency out-of-plane ring modes. For s-tetrazine-d0 this potential function in reduced coordinates is V (qα,qτ) = (251.8/2) q2α+(337.5/2) q2τ+1.62q4α −5.18q2αq2τ, where V is in cm−1 and qα and qτ are dimensionless ring-puckering and ring-twisting coordinates. Based on the limited frequency data available for the d2 compound the following potential function was determined and used to predict the Q-branch structure for the low-frequency mode: V (qα,qτ) = (225.6/2) q2α+(337.5/2) q2τ +1.14q4α−4.30q2αq2τ.
Rotational selection rules for nonradiative processes
Authors: Howard W. E. , Schlag E. W.
Published: February 2003

Recent experiments indicate that nonradiative transitions not only differ for various single vibronic levels but are also strongly dependent on the rotational states involved. Thus the rotational selection rules should be considered for these rotational transitions. Rotational matrix elements and selection rules are presented for internal conversion between two triplet or two singlet states and for intersystem crossing between a singlet and a triplet for polyatomic molecules in Hund’s case b. The selection rules are, in general, different for each process and depend on the nature of the angular momentum coupling as well as the interaction operator.
Comparison of semiclassical treatments for evaluating Franck–Condon transition amplitudes for molecular dissociation
Authors: Band Yehuda B. , Morse Michael D. , Freed Karl F.
Published: February 2003

The full Franck–Condon amplitude for molecular dissociation processes can be written as a sum of products of half collision amplitudes, for vibrational–rotational transitions between single channel states of the repulsive electronic surface, and Franck–Condon (e.g., vertical) amplitudes, for transitions from the bound electronic state to the single channel states of the repulsive surface. The single channel Franck–Condon amplitudes, involving bound-continuum overlap integrals, are evaluated using various semiclassical approximations. A comparison is made between the semiclassical and exact quantum evaluations of the Franck–Condon single channel amplitudes by performing calculations for the photodissociation of HCN, ICN, and CO2, thereby testing the range of validity of the semiclassical methods. Also, the wavelength dependence of the isotope effect for HCN dissociation is described.
Mass diffusion measurements in liquid crystals by a novel optical method
Authors: Hervet H. , Urbach W. , Rondelez F.
Published: February 2003

Binary mass diffusion measurements have been performed in fluid media doped with photochromic dye molecules. A periodic concentration of tagged molecules is created by illuminating the sample with a fringe intensity pattern. The photo induced change of refractive index and/or absorption of the dye molecules creates an optical grating which is observed through Bragg diffraction of an auxiliary laser beam. When the flash excitation is switched off, this grating will relax since photoexcited molecules will diffuse into nonphotoexcited zones and vice versa. The main advantages of this method are that: (1) the duration of the experiment is strongly reduced compared to classical tracer methods since diffusion lengths, defined by the fringe spacing, are small (1–100 μm); (2) the possibility of studying anisotropic diffusion is readily available; and (3) small sample volumes are required. Results are reported on the diffusion of methyl red in a homogeneously aligned sample of MBBA. In the nematic phase, the diffusion is faster along the local optical axis than perpendicular, the anisotropic ratio D∥/D⊥ being 1.6±0.1 at 22 °C, in good agreement with previous tracer data. The temperature dependence yields an activation energy of 5.8±0.7 kcal/mole for D∥ and 6.0±0.8 kcal/mole for D⊥. In the isotropic phase, the activation energy is found to be 10±1.5 kcal/mole.
The coupled states approximation for scattering of two diatoms
Authors: Heil Timothy G. , Green Sheldon , Kouri Donald J.
Published: February 2003

The coupled states (CS) approximation is developed in detail for the general case of two colliding diatomic molecules. The high energy limit of the exact Lippmann–Schwinger equation is used to obtain the CS equations so that the sufficiency conditions of Kouri, Heil, and Shimoni apply. In addition, care is taken to ensure correct treatment of parity in the CS, as well as correct labeling of the CS by an effective orbital angular momentum. The analysis follows that given by Shimoni and Kouri for atom–diatom collisions where the coupled rotor angular momentum j12 and projection λ12 replace the single diatom angular momentum j and projection λ. The result is an expression for the differential scattering amplitude which is a generalization of the highly successful McGuire–Kouri differential scattering amplitude for atom–diatom collisions. Also, the opacity function is found to be a generalization of the Clebsch–Gordon weight atom–diatom expression of Shimoni and Kouri. The diatom–diatom CS body frame T matrix TJ(j1′j2′j12′λ12′‖ j1j2 j12λ12) is also found to be nondiagonal in λ′12, λ12, just as in the atom–diatom case. The parity and identical molecule interchange symmetries are also considered in detail in both the exact close coupling and CS approximations. Symmetrized expressions for all relevant quantities are obtained, along with the symmetrized coupled equations one must solve. The properly labeled and symmetrized CS equations have not been derived before this present work. The present correctly labeled CS theory is tested computationally by applications to three different diatom–diatom potentials. First we carry out calculations for para–para, ortho–ortho, and ortho–para H2–H2 collisions using the experimental potential of Farrar and Lee (as modified by Zarur and Rabitz). Our results are compared with CC results due to Green. In addition, we have derived and tested the statistical and truncated coupled states approximations for this system. Next, we carried out calculations for ortho–para H2–H2 scattering using the configuration interaction surface of Ostlund. Calculations at seven energies from 2000 to 14 000 cm−1 are reported; the CS results are compared to CC results. The third system studied is H2–HCl, using a potential adapted from an HCl–He electron gas surface by Green. The results are again compared with CC studies due to Green. The results are quite encouraging and indicate that for systems (and specific transitions) satisfying the validity conditions of Kouri, Heil, and Shimoni, the CS is quantiative just as in the atom–diatom case. Furthermore, we expect the accuracy to improve as the energy of the collision increases.
An analysis of the B 1Πu–X 1∑+g band system of Na2
Authors: Kusch P. , Hessel M. M.
Published: February 2003

A critical analysis of the B 1Πu–X 1Σ+g band system of the Na2 molecule has been made. The body of data on which the present analysis is based is much more extensive than that used in any previous analysis. The data for the analysis were obtained by a laser-induced fluorescence technique in which many collisionally induced satellite fluorescence series were observed in addition to the directly excited series. We have found the coefficients in a Dunham expansion that replicates the frequencies of 12 591 lines that originate in 1297 different V′, J′ levels with an rms deviation between the observed and calculated frequencies of 0.011 cm−1. The frequencies of the lines that appear in the present analysis are poorly replicated by use of the Dunham coefficients found by earlier workers. The RKR potential curves, for both the B and X states and the Franck–Condon factors for transitions between the two states, have been found. In contrast to previous analyses the theoretical centrifugal distortion coefficients, calculated from the rotationless RKR potential, agree closely with those directly found in the present analysis. The dissociation energies have been found to be 5988 cm−1 for the X state and 3116 cm−1 for the B state. The maximum of the potential barrier of the B state lies about 474 cm−1 above the 2P3/2+2S1/2 levels to which the B state dissociates.
Calculations of ESR line shapes of triplet molecular aggregates in solid matrices. I. Fine and hyperfine structures of molecular pairs and molecular linear chains in ordered matrices
Authors: Lemaistre Jean-Pierre , Kottis Philémon
Published: February 2003

We introduce a description by quasistationary collective electronic excitation states (recently called also miniexcitons) for simulations of temperature dependent ESR line shapes of triplet molecular aggregates embedded in ordered matrices. We put emphasis on the fact that in organic molecules, where the triplet spins are strongly bound to the orbital motion, the sites’ spin oscillate in phase; this electronic coherence, being caused by the electronic part of the resonance exchange quantum interaction, cannot be termed a short memory process. It is emphasized on the fact that the time dependent stochastic models are based essentially on the random phase approximation. The latter is shown to suppress the collective response of the sites and, therefore, to be of little use for simulations at low temperatures. We use a total effective hamiltonian which allows us to calculate a spin hamiltonian for each collective electronic excitation. With the help of this spin hamiltonian, we perform actual ESR line shapes simulations for triplet pairs and finite chains of molecules obtained for system Nh8/Nd8 and DBN. We discuss the variation of the linewidth with the size of the aggregate and show that extreme narrowing at very low temperatures is not due to exciton motion, but to the renormalization of the electronic spin density on the size of the aggregate. Then we discuss the temperature effect as a consequence of thermal transitions among the exciton states of the aggregate and derive from comparison with experiments a high temperature limit relaxation rate U° (T). Our simulations allow insight into the spin perturbation mechanism associated with the temperature.
The effect of spin delocalization on the proton magnetic relaxation in transition metal hexaaquo ions
Authors: Waysbort Daniel , Navon Gil
Published: February 2003

The calculation of the effect of covalency on the electron–nuclear dipolar relaxation in low spin d5 complexes was extended to include all octahedral transition metal complexes. In particular, an expression was given for hexaaquo complexes and numerical calculations were performed for the hexaaquomanganese (II) ion. Taking into account spin delocalization, the calculated dipolar relaxation is larger than the value obtained using the point dipole appproximation. This effect was found to be relatively small for hexaaquo complexes owing to cancellations in the contributions from spin densities in the various water molecular orbitals. The result of the proton–manganese distance obtained from NMR relaxation times is compared with the available crystallographic data.
Coupling of translational and reactive dynamics for a Fokker–Planck model
Authors: Northrup Scott H. , Hynes James T.
Published: February 2003

The coupling of translational and reactive dynamics is investigated for a Fokker–Planck description of relative particle motion in a solvent. A division into inner and outer relative coordinate spatial regions separated by a boundary layer is made. The inner region is characterized by a deep potential well associated with bound, or reacted, particles and a potential barrier. The outer zone is characterized by less rapidly varying forces which include solvent structural effects. With this division, a Fokker–Planck source equation for unreacted particles is derived. The source in this equation incorporates the complete inner region dynamics involving both reactive and nonreactive trajectories. This equation is then reduced via projection operator techniques to a spatial Smoluchowski source equation for unreacted particles in the outer region for the case of slow reaction. Validity conditions involving velocity relaxation are given. The sources here include both inner region short range repulsive effects and a reversible chemical reaction. The outer Smoluchowski description includes caging effects. The equivalent boundary condition formulation is given; thus, the radiative boundary condition relating diffusive and reactive fluxes is derived. New expressions for the reaction rate constants are found and examined in terms of more fundamental rate kernels and Fokker–Planck barrier crossing dynamics. The rate constants are evaluated for a Kramers-type treatment of the inner region. These rate constants include effects of velocity nonequilibrium. They reduce to transition state results under intermediate inner region friction conditions.
Observation of second-order Stark effects in silane, SiH4
Authors: Kreiner W. A. , Oka T. , Robiette A. G.
Published: February 2003

The Stark effect on rovibrational transitions of the tetrahedral molecule silane has been studied by laser Stark spectroscopy, i.e., by applying dc fields of up to 65 kV/cm and monitoring the absorption of fixed frequency ir laser lines. The coincidences observed occur between the ν4 fundamental of SiH4 and N2O laser lines. Using the inverse Lamb dip technique single M components of three high J transitions could be resolved, and two of these have been identified by rf–ir double resonance. In both cases the Stark shift is due to the second-order Stark mixing of A1 with A2 rotational levels. The analysis of the Stark effects is consistent with the ground state centrifugal distortion moment ϑzxy=3.34×10−5 D measured previously by Kagann, Ozier, and Gerry. Experiments to measure the frequency offset Δν=νM−νL of the molecular absorption lines from the nearest laser line have also been performed, using the beat between two N2O lasers. Direct evidence of the second-order Stark shift has been obtained by observing Stark Lamb dips with a continuously tuned laser.
Photofragmentation of CH3Cl+ at 366 nm
Authors: Orth Robert G. , Dunbar Robert C.
Published: February 2003

The photofragmentation of methyl chloride cation was examined at a wavelength of 366 nm, using the anisotropy associated with the trapping efficiency of an ion cyclotron resonance spectrometer. The direction of the transition dipole moment as determined from the angular distribution of the CH+3 photofragments is parallel to the carbon–chlorine bond. This was interpreted as supporting the contention that the optical transition involved in the dissociation process is 2E←2E. The average kinetic energy of the CH+3 fragment was estimated to be 0.58 eV, indicating that some of the excess energy remaining after dissociation is partitioned into the internal modes of the methyl cation fragment.
Photoionization of pyrene in alkane liquids
Authors: Piciulo P. L. , Thomas J. K.
Published: February 2003

The two photon laser photolysis of pyrene in alkanes, benzene and methanol have been studied. Initial excitation of pyrene leads to the first excited singlet state, which subsequently absorbs a second photon leading to photoionization. Detection of the photoionization level at 10−9 sec time intervals indicates a very small photoionization yield in alkanes compared to alcohols. It is shown that this is due to a rapid initial neutralization of the photoproduced ions in the alkane, and that the initial yields of two photon photoionization are comparable in alkanes and alcohols. The lifetimes of the ion neutralization are 6.7×10−10 sec in methanol, 2×10−12 sec in iso-octane, 2×10−11 sec in hexane and 6.7×10−12 sec in benzene.
The dissociation energies of gaseous BaPd and BaRh and calculated dissociation energies of selected diatomic group IIA–platinum metal intermetallic molecules
Authors: Gingerich K. A. , Choudary U. V.
Published: February 2003

The high-temperature Knudsen effusion mass spectrometric technique has been used to measure the second law and third law enthalpies of the reactions BaPd(g) =Ba(g)+Pd(g) and BaAu(g)+Pd(g) =BaPd(g)+Au(g). The third law enthalpies ΔH°298 (in kJ) have been obtained as 224.3±2.4 and 35.8±1.8. The corresponding second law values are 216.8±8.9 and 30.9±9.8 kJ, respectively. From these reaction enthalpies the following thermodynamic properties in kJ mol−1 (kcal mol−1) were derived: D°298[BaPd(g)]=221.8±5.0 (53.0±1.2), D°0[BaPd(g)]=220.0±5.0(52.6±1.2), ΔHf,298[BaPd(g)]=336.8±10 (80.5±2.5), and °Hf,0[BaPd(g)]=338.5±10 (80.9±2.5). The dissociation energy of BaRh in kJ mol−1 (kcal mol−1) has been determined as D°0[BaRh(g) =257.4±25 (61.5±6.0) or D°298[BaRh(g)]=259.4±25 (62.0±6.0). The dissociation energies of BaPd and BaRh have been interpreted in terms of an empirical valence bond approach assuming double bond formation and utilizing a 5d orbital of barium in BaPd. Values for the bond energies of the diatomic molecules of barium with the platinum metals and of SrPt calculated by the model are also given.
M
1V,V
N
N Auger spectra of tellurium and some of its compounds
Authors: Bahl M. K. , Watson R. L. , Irgolic K. J.
Published: February 2003

The energies and relative intensities of the MNN Auger peaks have been measured for tellurium metal and 22 of its compounds. The relative intensities are compared with theoretical values for LS and intermediate coupling of the final hole states. By comparing the photopeak and Auger chemical shifts, the changes in the total (static) relaxation energies were obtained. It was found that the relaxation energy change provides a useful parameter for characterizing nearest neighbor groups.
Semiclassical description of electron spin motion in radicals including the effect of electron hopping
Authors: Schulten Klaus , Wolynes Peter G.
Published: February 2003

The coherent electron spin motion in radicals induced by the hyperfine coupling to nuclear spins is described semiclassically. The nuclear spins are treated as constant classical vectors around which the electron spin precesses. The ensemble average over all nuclear spin configurations is taken yielding the electron spin correlation tensor <S(0)S(t) ≳. Borrowing from the theory of rotational diffusion the effect of electron hopping between molecules on the spin correlation tensor is described. The treatment is applied to the time evolution of the electron spin state of a radical pair initially prepared in a singlet state.
Low-frequency vibrational spectra and conformations of bicyclo [3.2.0] hept-6-ene and 2-oxabicyclo [3.2.0] hept-6-ene
Authors: Villarreal J. R. , Laane J.
Published: February 2003

The low-frequency (100–500 cm−1) infrared and Raman spectra of bicyclo [3.2.0] hept-6-ene and its 2-oxa analog have been analyzed. The large collection of transitions involving the four out-of-plane ring modes were assigned and complex interaction was found. For the oxa compound a detailed energy level diagram was determined from the 30 transitions observed. Analysis of the ring-puckering spectra demonstrated that the asymmetric potential energy functions had only one energy minimum within 4 kcal/mole of the ground state, which corresponds to a boat species. Whether a second minimum is present at higher energies could not be determined.
Comparison of planar trajectory and classical centrifugal decoupling cross sections for rotationally inelastic Ar–HCl collisions
Authors: Pattengill M. D.
Published: February 2003

Comparisons of planar trajectory and classical mechanical calculations are presented for the rotationally inelastic Ar–HCl collisions.(AIP)
Dipole correlations in conducting media
Authors: Fulton Robert L.
Published: February 2003

It is shown that the lack of static long range correlation of permanent dipoles in a conducting medium follows simply from the assumption that the phenomenological coefficients are short ranged. The long range correlation which develops in time can be looked at as being induced by a dipole formed by ions which shield a static dipole.
Effect of centrifuging on fluctuations in the critical liquid–liquid region
Authors: Alder B. J. , Alley W. E. , Beams J. W. , Hildebrand J. H.
Published: February 2003

This paper presents a reassessment of an earlier paper on the elevation of the critical temperature, tc, of liquid–liquid mixtures in a centrifugal field and in a uniform pressure field. Two systems are used: one whose components have nearly the same density, the other whose densities differ widely. We find that with the first pair pressure raises tc by the same amount whether applied by external compression or by a centrifuge, where the centrifugal force produces the same pressure. With the second pair there is a small additional increase in tc which we attribute to stretching of the range of the thermal fluctuations. We confirm this effect and its magnitude by a theoretical analysis.
Substituent effects in benzene cation spectroscopy. I. Perturbation theory
Authors: Dunbar Robert C.
Published: February 2003

The spectroscopy of substituted benzene radical ions is considered within the context of perturbation methods successfully developed for neutrals. Expressions are given for substituent perturbation of the two spectroscopic peaks in the visible–uv spectral region; the positions of the peaks are perturbed by both first- and second-order terms, and the intensities by second-order terms. As an example, the information on ion state positions in the methylbenzenes from photoelectron spectroscopy and ionization potential measurements is discussed and parametrized in relation to the perturbation expressions.
Substituent effects in benzene cation spectroscopy. II. Photodissociation spectra of methylbenzene ions
Authors: Teng Harry Ho-I , Dunbar Robert C.
Published: February 2003

Photodissociation spectra have been obtained for the complete series of methylbenzene cations. The spectra are similar in pattern, showing a visible peak and a uv peak, and the positions, widths, and intensities of the peaks are reported for all 13 ions. The peak positions, and to some extent intensities, are discussed in terms of the perturbation expressions derived in the preceding paper. Substituent effects on the visible peak positions are reasonbly well accounted for by the simplest first-order inductive perturbation model. However, the uv peak positions are in extreme disagreement with the predictions of this simplest model. It is shown that second-order perturbations can account for nearly all of the uv peak shifts, and a successful quantitative model is described, although the complexity of possible second-order effects precludes any definitive conclusion about the uv peak situation.
Generalized Langevin theory of gas/solid-surface dynamics: A formulation for thermal desorption
Authors: Doll J. D.
Published: February 2003

Thermal desorption is examined using a combination of recently developed generalized Langevin methods and techniques related to Bennett’s induced trajectory approach. The combined methods appear well suited for the treatment of the dynamics of infrequent, many-body events.
On the heat of formation of diimide
Authors: Foner S. N. , Hudson R. L.
Published: February 2003

The heat of formation of trans-diimide (N2H2) has been determined mass spectrometrically by combining the ionization potential of N2H2 with the appearance potential of N2H+2 in the reaction N2H4+e→N2H+2+H2+2e. The measurements are I.P.(N2H2) =9.65±0.08 eV, in agreement with the photoelectron spectroscopic value, and A.P.(N2H+2) =10.75±0.08 eV leading to ΔH°fo(N2H2) =52.4±2 kcal/mole and ΔH°f298(N2H2) =50.7±2 kcal/mole. Studies of the reaction N2H4+e→N2H+2+2H+2e have provided confirmatory information on this energy assignment and suggest that excess energy in these reactions is very low or zero. The results on the heat of formation of diimide are in good agreement with our earlier work and in remarkable concordance with recent theoretical calculations, but are in marked disagreement with a very recent determination using data from the reaction N2H2+e→N+2+H2+2e. We have been unable to confirm the experimental results reported for this reaction. The measured value of ΔH°fo(N2H2) together with some auxiliary data allows the calculation of several important N–N and N–H bond dissociation energies.
Subpicosecond vibrational relaxation in Ca2 in rare gas solids
Authors: Bondybey V. E. , Albiston Carolyn
Published: February 2003

The ground state vibrational levels of Ca2 exhibit broadening due to fast vibrational relaxation. The relaxation in the upper state is considerably slower and in krypton extensive unrelaxed emission is observed. The Ca2 molecular potential and vibrational constants exhibit large matrix shifts due to interaction with the lattice.
Pressure dependence of the reaction of He2(3Σ) metastable molecules with Ar
Authors: Lee F. W. , Collins C. B. , Pitchford L. C. , Deloche R.
Published: February 2003

This paper reports the re-examination and analysis of data characterizing the excitation transfer reaction of metastable He2(3Σu) helium molecules with argon atoms. Data were originally obtained from measurements of the transient optical absorption of the 4650 Å, 3Σu→3πg helium transition in high pressure, e-beam afterglows, and in low pressure, electric discharges. Rate coefficients have been obtained from the data and resolved into components describing the bimolecular and termolecular reactions of He2(3Σu) with Ar. Values of 1.5×10−10 cm3 sec−1 and 2.4±0.8×10−30 cm6 sec−1, respectively, were found. The former corresponds to a reaction probability of 0.18 and the latter represents the first measurements of a termolecular excitation transfer reaction of He2(3Σu).
A classical trajectory study of collisional energy transfer in thermal unimolecular reactions
Authors: Stace A. J. , Murrell J. N.
Published: February 2003

Classical trajectory calculations have been carried out to study energy transfer in atom–triatom collisions. Collisions between O3 and He, Ar and Xe have been studied at energies corresponding to the temperature range 300–2500 K and between H2O and Ar in the range 2500–10 000 K. The results allow a comparison between the multistep and strong collision models of unimolecular decay. For both O3 and H2O the multistep model gives good agreement with experimental results and the energy transfer characteristics of He, etc. conform to those expected for an inefficient heat-bath gas. Rotational energy transfer is found to be more efficient than vibrational and energy transfer in general is sensitive to the R−6 term in the atom–triatom potential.
A
b i
n
i
t
i
o effective core potentials for molecular calculations. II. All-electron comparisons and modifications of the procedure
Authors: Hay P. Jeffrey , Wadt Willard R. , Kahn Luis R.
Published: February 2003

Recently methods have been developed [L. R. Kahn, P. Baybutt, and D. G. Truhlar, J. Chem. Phys. 65, 3826 (1976)] to replace the core electrons of atoms by ab initio effective core potentials (ECP). Valence-electron calculations are reported on the ground states of HF, F2, HCl, LiCl, and Cl2 using ECP’s for F and Cl, and the results are compared to all-electron calculations. Significant discrepancies are found (as much as 20 kcal/mol for De and 0.3 bohr for Re in the case of Cl2) between the two sets of calculations. The original formulation of generating ECP’s can lead to artificial attractive tails in the potential at large values of r which leads to potential energy curves which are too attractive. The procedure of generating ECP’s is modified to remove the long-range attractive tails. Excellent agreement between the valence-electron and all-electron calculations is then obtained.
Pressure and temperature dependence of molecular motion in organic plastic crystals
Authors: Ross S. M. , Strange J. H.
Published: February 2003

The proton NMR relaxation times T1 and T1ρ have been measured as a function of temperature and pressure in the plastic crystals cyclohexane, hexamethylethane (HME) and hexamethyldisilane (HMDS) for pressures up to 2.8 kbar. The T1 data provide information on the molecular reorientation. This motion is only weakly dependent on pressure. The reorientational activation volumes for the three materials studied were found to be temperature dependent and generally less than 10% of the molar volume Vm. The T1ρ data have been analysed in terms of the translational molecular self-diffusion, using a weak-collision theory. A value of 1.1Vm was obtained for the activation volume for self-diffusion in HME, independent of pressure and temperature over most of the range studied. Diffusional activation volumes for the other two substances were found to be dependent on both pressure and temperature, ranging from 0.9 to 0.3Vm in cyclohexane and from 0.55 to 0.2Vm in HMDS. The observed temperature and pressure dependences of the activation enthalpies and volumes are discussed in relation to the mechanisms of the molecular motion. The complexity of the diffusive motion appears to correlate with the entropy of fusion of the material. While the dominant defect responsible for self-diffusion is usually thought to be a relaxed vacancy, there is some evidence to suggest that, in parts of the plastic phase, translational motion may proceed by more than one mechanism.
The uncoupled symmetric stretching frequency of H3+
Authors: Dykstra Clifford E. , Gaylord Arthur S. , Gwinn William D. , Swope William C. , Schaefer Henry F.
Published: February 2003

The ab initio D3h potential energy surface for H3+ has been calculated using the method of self-consistent electron pairs (SCEP). The symmetric stretching frequency is estimated to be 3347.3±0.5 cm−1 from these calculations.(AIP)
Chemical relaxation pulses and waves. Analysis of lowest order multiple time scale expansion
Authors: Collins Michael A. , Ross John
Published: February 2003

We analyze the propagation of pulses and wave trains in a chemical reaction–diffusion system far from equilibrium whose kinetics are characterized by multiple time scales. The previously derived equations describing this system in the limit of widely separated time scales are employed. For a two-component system we derive lowest order expressions relating the velocity of a propagating front to the chemical kinetics. These expressions are used to obtain the structure of solitary pulses. The mechanics of this pulse propagation is further investigated with simple integral transform methods in a multiple time scale system whose kinetics contain linear flux. Finally, we derive the structure of wave trains in these systems and the relation between their velocity and frequency in terms of the chemical kinetics.
Brillouin scattering and refractive index measurements in molten Li2SO4
Authors: Aronsson Robert , Knape H. E. Gunilla , Torell Lena M.
Published: February 2003

Molten Li2SO4 has been investigated to determine the hypersonic velocity by use of Brillouin spectroscopy and the refractive index by an interferometric method. The hypersonic velocity and its temperature dependence has been determined at phonon frequencies around 13 GHz and in the temperature range 860–1000 °C. Values of the refractive index and its temperature dependence are presented in a temperature interval of ∼60 °C above the melting point at a wavelength of 4880 Å.
Electronegativity: The density functional viewpoint
Authors: Parr Robert G. , Donnelly Robert A. , Levy Mel , Palke William E.
Published: February 2003

Precision is given to the concept of electronegativity. It is the negative of the chemical potential (the Lagrange multiplier for the normalization constraint) in the Hohenberg–Kohn density functional theory of the ground state: χ=−μ=−(∂E/∂N)v. Electronegativity is constant throughout an atom or molecule, and constant from orbital to orbital within an atom or molecule. Definitions are given of the concepts of an atom in a molecule and of a valence state of an atom in a molecule, and it is shown how valence-state electronegativity differences drive charge transfers on molecule formation. An equation of Gibbs–Duhem type is given for the change of electronegativity from one situation to another, and some discussion is given of certain relations among energy components discovered by Fraga.
Energy transfer in solid Xe and Kr doped with N2O
Authors: Monahan K. M. , Rehn V.
Published: February 2003

Photoluminescence excitation, emission, and decay spectra were obtained for solid Xe:N2O and Kr:N2O mixtures using vacuum ultraviolet synchrotron radiation as the excitation source. The principal emitting species were determined to be XeO(1D) and KrO(1S) excimers. Fluorescence decay measurements indicate that the XeO(1D) emission arises from two excited states. Analysis of the emission and excitation spectra leads to the conclusion that XeO(1D) production follows nonradiative energy transfer to N2O from self-trapped excitons in the xenon host.
Photodissociation and photodetachment of molecular negative ions. V. Atmospheric ions from 7000 to 8400 Å
Authors: Smith G. P. , Lee L. C. , Cosby P. C. , Peterson J. R. , Moseley J. T.
Published: February 2003

Measurements of the photodestruction cross sections for a number of parent and first hydrate negative ions of atmospheric importance have been extended to the wavelength range from 7000 to 8400 Å, using a drift tube mass spectrometer and a tunable dye laser. Most of these ions do not dissociate or detach at these wavelengths; upper limits smaller than 1.0×10−19 cm2 are established for the photodestruction cross sections of O−3, O−2⋅H2O, O−3⋅H2O, CO−3, CO−4, CO−4⋅H2O, HCO−3, HCO−3⋅H2O, O−2⋅NO, O−2⋅NO⋅H2O, and NO−2⋅H2O. Thresholds for the photodestruction of CO−3⋅H2O and O−4 are observed at photon energies of 1.67 eV and 1.57 eV, respectively, and the cross sections for these ions have been measured at higher photon energies.
Electrical effects during condensation and phase transitions of ice
Authors: Onsager Lars , Staebler David L. , Mascarenhas Sergio
Published: February 2003

Condensation potentials and corresponding electrical currents are observed during the isothermal growth of amorphous, cubic, and hexagonal ice from the vapor phase. Upon heating of the condensate a thermally stimulated current (TSC) spectrum is observed with the application of any external electric field. Current peaks in the TSC spectrum are shown to be of two types: depolarization currents related to dipolar relaxation processes in the condensate and peaks due to phase transitions. Nearly all depolarization (∼90%) occurs in the amorphous phase via relaxation processes. Shifts in the maximum temperature of the peaks are observed for deuterated water for both types of peaks, and are on the order of 5–10 K. Models are proposed for the electrical effects observed during condensation, phase change, and depolarization.
Orthogonal vector analyses of distributions of self-avoiding and limited order random walks on lattices
Authors: Wall Frederick T. , Seitz William A. , Chin John C.
Published: February 2003

When self-avoiding random walks on a lattice are used to simulate flexible macromolecules the dimensional distributions can be regarded as continuous only when the number of steps is large. For walks of a small number of steps, on the other hand, the lattice effect is noticeable and any analysis of dimensional distributions should be treated accordingly. The distributions of end-to-end separations of lattice walks can be precisely expanded in terms of Krawtchouk orthogonal vectors which are discrete counterparts of the Hermite orthogonal functions. It is shown how an analysis using Krawtchouk vectors can be carried out and the results are applied to various distributions. In particular, it is shown how the coefficients in the orthogonal vector expansion depend upon the order of self-exclusion and the length of the walk. Finally, the transition to the continuous Hermite representation for longer walks is illustrated.
The effects of multiperturber interactions on the sodium–rare gas excimer bands
Authors: West W. P. , Shuker P. , Gallagher Alan
Published: February 2003

The extreme wing (600<λ<820 nm) of the sodium D lines perturbed by xenon and krypton has been measured in fluorescence at perturber densities between 2×1019 and 3×1020 cm−3. At these perturber densities the observed fluorescence results mostly from the NaXe* (NaKr*) molecule in the wavelength range of 600<λ<720 nm (600<λ<680 nm) and from the NaXe2* (NaKr2*) molecule in the range 740<λ<820 nm (700 <λ<820 nm). The fluorescence emission coefficient is obtained in normalized units, allowing quantitative comparison to theory. The quasistatic line-broadening theory, including multiple-perturber interactions, is used to calculate the density dependence of the spectra which would result from trimer potential surfaces given by scalarly additive pair interactions. This approximation explains some of the observatons; discrepancies between this model and the experimental results are discussed.
Structure of the αγ interface
Authors: Widom B.
Published: February 2003

We study the structure and thickness of the interface between two phases, α and γ, when they are, or are nearly, in equilibrium with an intermediate third phase β. If β is not present in bulk, but if it would have been in equilibrium with α and γ had it been present, then the αγ interface consists of a film of β of a thickness that diverges logarithmically with the total sample height. Far from a critical point of any of the phase equilibria, that β film will typically be about 100 Å thick. If, as a bulk phase, β is just slightly less stable than α and γ, by an amount measured by a dimensionless distance δ in the composition space, then when the total sample height is infinite the thickness of the β film at the αγ interface diverges proportionally to ln(1/δ). The composition and composition-gradient profiles through the αγ interface for small δ are calculated and displayed graphically.
Double and triple resonance studies of rotational relaxation in NH3–He and NH3–H2 collisions
Authors: Morita Norio , Kano Satoru , Shimizu Tadao
Published: February 2003

Collision-induced transitions of NH3 in the collisions with He and H2 are studied by the laser-microwave double and triple resonance methods. The results are summarized in the preference rules. It is confirmed that Δk=±3n transitions occur, while ortho–para conversion is still strictly forbidden.
Molecular versus atomic secondary ion emission from solids
Authors: Morgan A. E. , Werner H. W.
Published: February 2003

Mass-selected ion currents resulting from 6 keV Ar+ bombardment of specially prepared samples of PbF2, PbCl2, PbBr2, PbI2, PbO, NaCl, and Bi2O3 containing small specified amounts of metallic elements have been measured using either 0–20 or 40–60 eV secondary ions. With either energy range the yield of M+ ions from a given matrix is inversely proportional to the exponential of the ionization potential EM of the element M. Also, the ratio of the monohalide or monoxide MX+ current to the corresponding M+ current is directly proportional to the exponential of the bond strength D (M–X) of the molecular ion; this result is utilized to obtain hitherto unknown bond strengths, thus allowing some estimates of monoxide ionization potentials to be given. MX+ emission is therefore substantial in those instances when (a) the bonding between M and X is strong and (b) the ionization potential of MX is low. These remarks seem to apply to molecular ion emission in general. Substantial MX+ emission leads to a reduction in M+ emission, suggesting competition between these two processes. A simple model for the production of these molecular and atomic ions is proposed.
Molecules with hindered internal rotation. II. A symmetry determined investigation of the nonbonded ligand interaction
Authors: Meinköhn Dirk
Published: February 2003

In a class of ethane derivatives, defined by the ethane skeleton which permits internal rotation, the symmetry properties of an expression in a scalar molecular variable are used to investigate model conceptions of the nonbonded ligand interactions.
Electron attachment kinetics in flames. II. Dissociative attachment to HCl
Authors: Miller William J. , Gould Robert K.
Published: February 2003

This work comprises the first direct determination of the kinetics of Cl− formation in flames. The experiments involve mass spectrometric measurements of negative ion concentrations as functions of flow time in the burned gases of well-characterized laboratory test flames seeded with small quantities of alkali metals and chlorine compounds. The results are consistent with the two-body dissociative attachment mechanism e+HCl→Cl−+H with a rate coefficient well represented by the Arrhenius expression k=6×10−11 exp(−10 100/T) ml molecule−1 s−1 over the temperature range 1730 to 2475 K. The flames are operated at pressures of 100 and 760 Torr. Electron concentrations were measured using a microwave resonance cavity method and positive ion profiles (total charged species) were determined with electrostatic probes. H-atom concentration profiles required to take account of the reverse detachment process are well known for the 760 Torr flames; in the low pressure flames they were obtained using a mass spectrometric negative ion tracer technique involving the known ion–molecule chemistry of molybdenum additives and confirmed via optical absorption measurements of [OH]. Contributions to Cl− formation from three-body attachment processes are not observed.
Statistical–mechanical theory of membrane transport for multicomponent systems: Passive transport through open membranes
Authors: Mason E. A. , Viehland Larry A.
Published: February 2003

Equations for transport through membranes are derived from basic principles of statistical mechanics, with the classical–mechanical Liouville equation as the starting point. General fluid-dynamic equations are first obtained, following the methods of Bearman and Kirkwood and of Snell, Aranow, and Spangler; a quasicontinuum or coarse-graining assumption then allows these equations to be inverted to a Stefan–Maxwell form in which the diffusion and thermal diffusion coefficients can be given an experimental interpretation. The remaining assumptions pertain specifically to membranes. The membrane is taken as one component of the mixture, held fixed in space, and the structure of the membrane is assumed to lead only to a geometrical space-filling role. Finally, the assumption of nonseparative viscous flow allows the transport equations to be decoupled from the details of membrane structure. The results have the same final form as those obtained earlier by heuristic generalization of gas transport equations. Comparison is made with previously proposed phenomenological equations for membrane transport, including the Onsager (Kedem–Katchalsky) linear laws, the frictional model, the Stefan–Maxwell diffusion model, a hydrodynamic model, and Nernst–Planck equations. It is shown in what sense these different formulations are equivalent, where terms have been omitted, and how such omission can be compensated by reinterpretation of the transport coefficients. Effects of membrane heterogeneity and heteroporosity are briefly discussed.