bluuebunny/crossref_metadata_2025 huggingface dataset:
Repulsive forces of liquid N2: Monte Carlo calculations
Authors: Fiorese G.
Published: February 2003

Monte Carlo calculations are employed to investigate a homonuclear diatomic molecular liquid (N2) using an ’’atom–atom’’ intermolecular potential. Shock wave data (the principal Hugoniot) have been used to test the repulsive part of this potential.
Chemisorption and reaction of cyclopropane on the (110) surface of iridium
Authors: Wittrig T. S. , Szuromi P. D. , Weinberg W. H.
Published: February 2003

Thermal desorption mass spectrometry (TDMS), UV photoelectron spectroscopy (UPS), low-energy electron diffraction (LEED), and contact potential difference measurements have been employed to study the interaction of cyclopropane with the reconstructed Ir (110)–(1×2) surface. At an adsorption temperature of 100 K cyclopropane dissociates on the surface with a reaction probability of 0.87±0.1. This overlayer saturates at a coverage of cyclopropane of (2.1±0.2)×1014 cm−2. Upon further exposure, cyclopropane adsorbs molecularly into two different adsorption sites up to a saturation coverage of 6.6×1014 cm−2. The parameters of the desorption rate coefficients based upon Arrhenius constructions for low coverages of the two desorption states are Ed = 6.4±1 kcal mol−1 and νd = 1.0×109±1 s−1, Ed = 8.3±1 kcal mol−1 and νd = 4.5×1010±2 s−1. The work function decreases by 0.5 eV during formation of the dissociated hydrocarbon residue at 100 K and decreases further (a total decrease of 0.7 eV) during the adsorption of molecular cyclopropane.
Photoassociative laser induced fluorescence of XeCl
Authors: Inoue Gen , Ku J. K. , Setser D. W.
Published: February 2003

Following a pulsed discharge in a Xe/Cl2 mixture, free–bound absorption spectra were observed from analysis of the subsequent bound–free (and bound) emission of XeCl(B). In addition, the expected bound–bound XeCl(X–B) absorption followed by bound–free (and bound) emission was observed. The time resolved emission gives ∼10 and ∼150 ns for the radiative lifetimes of the XeCl(B) and XeCl(C) states.
Intramolecular mode mixing in NaClO3 and NaBrO3
Authors: Frech Roger
Published: February 2003

The F symmetry phonons originating in the anion ν1 and ν3 intramolecular vibrations in single crystal NaClO3 and NaBrO3 contain contributions from both kinds of intramolecular motion. This intramolecular mode mixing is described in terms of the intermolecular coupling of vibrationally induced dipoles. The general problem of dipolar coupling in systems in which the intramolecular vibrational polarization directions are not coincident with the crystallographic axes is examined. Finally, the amount of intramolecular mode mixing in NaClO3 and NaBrO3 is calculated and discussed.
Hypervirial perturbative method (HPM) for perturbed Morse oscillators
Authors: Fernández Francisco M. , Castro Eduardo A.
Published: February 2003

The hypervirial perturbative method is applied to perturbed Morse oscillators. Results are compared with previous ones obtained by different authors and several advantages of the present method are pointed out.
Multiconfigurational Hartree–Fock studies of avoided curve crossing using the Newton–Raphson technique
Authors: Olsen Jeppe , Jo/rgensen Poul , Yeager Danny L.
Published: February 2003

A mode controlling scheme for use in second order Newton–Raphson multiconfigurational Hartree–Fock calculations has been developed and applied on the avoided curve crossing for the lowest three states of 4Π symmetry of BO and on the valence-Rydberg mixing in the lowest two states of 3Σ−u symmetry in O2. Convergence of the multiconfigurational Hartree–Fock calculations was easily obtained in all calculations, even where root-flipping occurred. We further demonstrate how an MCSCF calculation may occasionally be reparametrized, that is, the same MCSCF total energy may be obtained using two different sets of configuration lists. One parameter list may give root-flipping, whereas another parameterization may not. This is exemplified in a calculation on the E 3Σ−u state of O2 at 2.13 a.u..
Comparison of the convergence characteristics of some iterative wave function optimization methods
Authors: Shepard Ron , Shavitt Isaiah , Simons Jack
Published: February 2003

The convergence properties of several iterative methods for the optimization of orbitals and configuration mixing coefficients in multiconfigurational electronic wave functions are compared. All of the iterative methods considered here are derived from corresponding approximate energy expressions. These energy expressions are discussed within the context of their suitability for the calculation of noninfinitesimal wave function corrections. A method based on the partitioned orbital Hessian matrix and which uses an approximate super-CI secular equation for the wave function corrections is shown to posses second-order convergence and to have the largest radius of convergence of the methods analyzed in detail in this work for several molecular examples. Particular attention is given to convergence properties for excited states, where the differences between these methods are most significant.
A unitary group formulation of open-shell electron propagator theory
Authors: Born Gregory , Shavitt Isaiah
Published: February 2003

By replacing the usual annihilation and creation operators of second quantization by appropriately normalized fundamental Wigner operators of the unitary group U(n) and by representing the many-electron spin eigenfunctions in terms of the Gelfand–Tsetlin basis for the appropriate irreducible representation, we have succeeded in developing an attractive electron propagator formalism which incorporates closed-shell, open-shell, or multiconfigurational reference states. Matrix element evaluation for the fundamental U(n) Wigner operators is treated, and illustrative three-orbital examples involving reference states of doublet and triplet spin symmetry are presented.
Steady absorption of Brownian particles by an infinite cylinder
Authors: Harris S.
Published: February 2003

We consider the solution of the Fokker–Planck equation for the case of steady Brownian motion in the presence of an infinite adsorbing cylinder. An approximate solution is found using the modified Maxwellian moment method used earlier with success in treating the one-dimensional problem. We again find considerable deviation from the corresponding diffusion equation result in the boundary layer region; however, the spatial extent of the boundary effects is not controlled by a single length scale λ−1∝ (Brownian particle relaxation time × thermal speed) but by λ−1 and the cylinder radius r0. We also consider the inhibition of diffusion near the boundary associated with the failure of Fick’s law in that region.
Low temperature calorimetric, magnetic, and optical studies of dysprosium sesquisulfide
Authors: Taher S. M. A. , Ho J. C. , Gruber John B.
Published: February 2003

Heat capacity, magnetic susceptibility, and optical measurements have been made on γ-Dy2S3 having the Th3P4 bcc defect-type structure. The compound remains paramagnetic down to 2 K, the lowest temperature of measurements. A heat capacity anomaly with its peak at 3.3 K can be interpreted using the Stark levels of the ground state manifold 6H15/2(4f9) of Dy+3 obtained from optical spectra. By subtracting the corresponding Schottky term from the total heat capacity, the lattice contribution is estimated. By further analyzing the temperature dependence of this contribution, in comparison with those for diamagnetic Y2S3, La2S3, and Lu2S3, the limiting Debye temperature value of Dy2S3 is found to be 290 K.
On the role of attractive forces in the cage effect in simple liquids
Authors: Dean David P. , Kushick Joseph N.
Published: February 2003

Molecular dynamics calculations were performed in an attempt to characterize the mechanism by which attractive forces affect the velocity correlation function in simple Lennard-Jones liquids. The relationships between the presence of attractive forces, the rigidity of molecular cages, and the decay of velocity correlations were studied by molecular dynamics runs in which a single mobile particle moved in a rigid cage environment. Our results suggest that the principal role of the attractive forces is to enhance the cohesiveness of the cages and their ability to confine particles within them.
Electrodynamics at a metal surface. III. Reflectance and the photoelectron yield of a thin slab
Authors: Maniv Tsofar , Metiu Horia
Published: February 2003

We use a jellium model with an infinite barrier to minic the dielectric bahavior of a thin slab of a free electron metal. The model includes spatial dispersion (nonlocality) and allows the dielectric response to vary continuously across the interface. We compute the reflectance and the photoelectron yield as functions of frequency, film thickness, and electron mean free path.
CO2 laser tritium isotope separation: Collisional effects in multiphoton dissociation of trifluoromethane
Authors: Takeuchi Kazuo , Inoue Ichiro , Nakane Ryohei , Makide Yoshihiro , Kato Shuji , Tominaga Takeshi
Published: February 2003

Multiphoton dissociation of ppm level CTF3 in CHF3 is reported. The effect of buffer gas pressure on the specific dissociation rates and the isotopic selectivity is investigated. Pulse energy dependence of the dissociation rates is converted to the intrinsic relation between the fractional conversion per pulse and the fluence by means of the new deconvolution technique. By addition of 100 Torr argon, the critical fluence of CTF3 was decreased from 136 to 34 J/cm2, while the intrinsic selectivity was increased from 34 to 580. In the absence of buffer gas argon, the fractional conversion per pulse exhibits two different pressure dependences. Unlike in the case below 15 Torr, the system shows strong pressure dependence and loss of the isotopic selectivity above 15 Torr. Possible application to the tritium removal process is also discussed.
Infrared multiphoton photophysics: Decomposition of CnF2n+1I (n = 1, 2, 3)
Authors: Rossi Michel J. , Barker John R. , Golden David M.
Published: February 2003

The title compounds have been subjected to unfocused radiation from a high-power CO2-TEA laser under collisionless conditions in a mass spectrometric flow system (VLPO/ method). We report absolute photodissociation yields as a function of fluence for CF3I and C3F7I. In the case of CF3I, we find high yields (100%) at fluences ?4 J/cm2, whereas for C3F7I yields level off at 20%–30%, depending on the wavelength. Secondary photolysis of Ċ2F5 and Ċ3F7 at fluences ?2.5 J/cm2 yields CF2 and ĊF3, but no C2F4. Interpretation of data in terms of a log–normal cumulative distribution function of reaction times or fluence suggests that CF3I interacts homogeneously with the laser field at 1070 and 1083 cm−1, but possibly inhomogeneously at 1076 cm−1, whereas C3F7I represents an inhomogeneous interaction case at all wavelengths. Based upon our photodissociation yields and the validity of a master equation representation, we calculate the infrared absorption spectrum of C3F7I as a function of internal energy.
A localized integral equation formulation of molecular scattering
Authors: Saha Samir , Rabitz Herschel
Published: February 2003

A new integral equation formulation is presented for molecular scattering. This is obtained by exactly transforming the common close-coupling theory into an equivalent nonlocal integral equation form which is not a set of coupled discrete basis equations. The integral equation is formally localized by the introduction of a translation operator, thereby leading to a Hamiltonian (or portion of it) that is a Fourier transform of the original kernel. The theory makes no use of Green’s functions and this overall approach has two attractive features. Firstly, an examination of the integral kernels allows the development of optimal quadrature schemes for each problem and, secondly, judicious approximations of the localized Hamiltonian operator lead to very attractive practical equations. As an illustration of the latter point, the method is applied to replace the differential internal Hamiltonian operator by an effective local function which can act to correct the usual sudden approximation without excess computational effort. Various asymptotic limits of the theory are also discussed.
State-to-state dynamics of IR multiphoton absorption in the Ã1A2 state of thiophosgene: Absorption cross sections
Authors: Brenner D. M. , Peters D. W.
Published: February 2003

Infrared multiphoton absorption (IRMPA) in the first excited singlet state of Cl2CS has been studied. Absolute cross sections for absorption to a fixed energy level, the nonradiative limit, from initial levels of increasing energy have been obtained. Although intuition suggests that as the energy required to reach the nonradiative limit decreases the apparent cross section should increase, this is not observed. Instead, the relative probability of absorption from all initial states is nearly the same as that from the ground vibrational level. Because the density of states in Cl2CS (? 1A2) is higher at 3000 cm−1 than in SF6, where intermode coupling is postulated to be important in the dynamics of absorption, the results suggest that at energies ?2500 cm−1 intramolecular relaxation is competitive with absorption. This interpretation is in agreement with predictions from the current theory of IRMPA.
Evidence of a state dependent depletion process in the two-photon fluorescence excitation spectra of saturated amines
Authors: Halpern Arthur M. , Gerrity D. P. , Rothberg L. J. , Vaida V.
Published: February 2003

The two-photon fluorescence excitation (TPFE) spectra of regions of the ? states of two saturated amines 1-azabicyclo [2.2.2]octane (ABCO) and trimethylamine (TMA) are reported. These spectra are compared with the respective one-photon absorption (OPA), one-photon fluorescence excitation (OPFE), and multiphoton ionization (MPI) spectra for both molecules. For ABCO, this comparison clearly indicates major differences in both the vibronic band intensities and the amount of sequence structure present in the TPFE spectrum relative to the MPI, OPA, and OPFE spectra, which are all comparable. The ’’distortions’’ of the TPFE spectrum are interpreted in terms of a laser-induced ? state-dependent depletion process from ? which results in ionization. Pressure effects on the TPFE spectrum imply that the rate of this up-pumping process depends strongly upon the particular vibrational modes excited in the two-photon-induced ?←? transition. A further implication of this interpretation is that the intramolecular vibrational relaxation time T1 of some initially prepared levels is ?100 ps. A kinetic model is presented which illustrates how (small) variations in the ionization cross sections of the ? state can have large effects on the TPFE spectrum but not on the MPI spectrum.
Long range couplings of CḢ2S–SCH3 radicals trapped in dimethyldisulfide single crystals
Authors: Bonazzola L. , Leray N. , Michaut J. P. , Roncin J.
Published: February 2003

The main radical produced in single crystals of dimethyldisulfide by irradiation with γ rays at 77 K and bleaching by daylight is CḢ2SSCH3. The coupling tensors of the protons have been determined. It arises from the experimental results that the electronic configuration about the radical center is planar. There is an unusually large spin density on the HCH3 protons coming from a delocalization of the unpaired spin density over S–S bond. Fair agreement is found between ab initio UHFMO calculations and experiment.
The EPR spectrum of a single crystal of chromium hexacarbonyl doped with Fe(CO)5
Authors: Lionel T. , Morton J. R. , Preston K. F.
Published: February 2003

Single crystals of Cr(CO)6 have been doped with 0.05% Fe(CO)5 and γ irradiated at 77 K. The irradiated crystals revealed EPR spectra at 95 K of a species identified as Fe(CO)5−, present in two modifications A and B. Using Fe(CO)5+ enriched in the isotope 13C, the hyperfine tensors of the five carbon atoms of A were obtained; the unique 13C had a∥ = 21 G and a⊥ = 18 G; the other four had a∥ = 6.6 G and a⊥ = 9.0 G. Measurement of the 57Fe hyperfine interaction indicated the presence of a single Fe nucleus with a∥ = l0 G and a⊥ = 6 G. The two modifiactions A and B correspond to Fe(CO)5+ molecules in which the unique Fe–C bond is parallel to Cr–C(1,2) or Cr–C(3,4,5,6) bonds, respectively, in the pure Cr(CO)6 crystal.
Molecular motion in anisotropic medium. II. The proton relaxation study of CH3CN: The general A3 spin system
Authors: Courtieu Jacques , Lai N. T. , Mayne Charles L. , Bernassau J. M. , Grant David M.
Published: February 2003

The coupled relaxation of an A3 spin system (CH3CN) dissolved in a perdeuterated nematic phase (EBBA-d23) is studied for the first time. The partially averaged dipolar interaction allows one to observe a triplet pattern on which four different spin perturbations have been performed: a selective soft π pulse on either an external or the central transition, and a nonselective π pulse affecting all three transitions with or without spin tickling on one of the external transitions. The last perturbation provides a measure of the time evolution of the doublet components in the central line of the triplet. The relaxation processes are modeled as intramolecular dipole–dipole interactions plus other mechanisms which are treated collectively as external random magnetic fields (RF). A restricted diffusion model is used which takes account of the molecular orientation of the solute molecule. Cross relaxation terms are very important, and RF mechanisms are not negligible even in a perdeuterated nematic phase. The anisotropy of the rotational diffusion is obtained, and R, the rotational diffusion constant of an axis perpendicular to the axis of symmetry of the rotor, is found to be 25 times smaller than in neat liquid. The diffusion constant about the principal axis R∥ is not affected significantly by the liquid crystal. The values of R⊥ and R∥ give correlation times which allow extreme narrowing approximations, but in accordance with theoretical predictions it is demonstrated experimentally that the zeroth, first, and second order projections of the dipolar spectral densities are not equal due to preferential ordering of CH3CN in the liquid crystal. Optimal values of the effective ordering parameter SZZ obtained from dipolar splittings in the spectrum and from the dipolar relaxation parameters differ from each other. This difference may be rationalized on the basis of models which account for the two different physical phenomena.
Molecular motion in anisotropic medium. III. The carbon-13 relaxation study of 13CH3CN: The general A
X3 spin system
Authors: Black Edward P. , Bernassau J. M. , Mayne Charles L. , Grant David M.
Published: February 2003

The coupled relaxation of an AX3 spin system 13CH3CN dissolved in a nematic phase (EBBA) is studied. The incompletely averaged dipolar interactions allow one to observe a quartet in the carbon domain and a sextet in the proton domain. This rather complex proton spectrum is subjected to nine different spin preparations involving the selective inversion of either one or two lines. The time evolution of the spin system is then studied in the 13C spectrum. The relaxation processes are modeled as dipole–dipole interactions, and all other mechanisms are collectively treated as external random fields. A restricted diffusion model is used which takes into account the preferential orientation of the solute molecules. All the results obtained are fully consistent with the data obtained by studying the proton spectrum alone as an A3 spin system.
Electron spin echo studies of the internal motion of radicals in crystals: Phase memory vs correlation time
Authors: Kispert Lowell D. , Bowman Michael K. , Norris James R. , Brown Meta S.
Published: February 2003

An electron spin echo (ESE) study of the internal motion of the CH2 protons in irradiated zinc acetate dihydrate crystals shows that quantitative measurements of the motional correlation time can be obtained quite directly from pulsed measurements. In the slow motional limit, the motional correlation time is equal to the phase memory time determined by ESE. In the fast motional limit, the motional correlation time is proportional to the no motion spectral second moment divided by the ESE phase memory time. ESE offers a convenient method of studying motion, electron transfer, conductivity, etc. in a variety of systems too complicated for study by ordinary EPR. New systems for study by ESE include biological samples, organic polymers, liquid solutions of radicals with unresolved hyperfine, etc. When motion modulates large anisotropic hyperfine couplings, ESE measurements of the phase memory time are sensitive to modulation of pseudosecular hyperfine interactions.
The microwave rotational spectrum, molecular geometry, 14N nuclear quadrupole coupling constants, and H, 19F nuclear spin–nuclear spin coupling constant of the nitrogen–hydrogen fluoride dimer
Authors: Soper P. D. , Legon A. C. , Read W. G. , Flygare W. H.
Published: February 2003

The microwave rotational spectrum of each of four isotopic species of a weakly bound dimer formed between molecular nitrogen and hydrogen fluoride has been detected and assigned, thus allowing unambiguous identification of this new complex. The following spectroscopic constants have been determined through analyses of the spectra:                 An interpretation of these constants leads to the conclusion that the dimer is linear with the weak binding through a hydrogen bond to one of the nitrogen atoms. The difference in the two 14N nuclear quadrupole coupling constants has been shown on the basis of a simple model to require that ∼0.03e is transferred from the unbound to the weakly bound nitrogen atom when HF achieves its equilibrium position in the dimer.
Velocity dependence for the chemi-ionization of Ar, Kr, N2, CO, and O2 by He(2 1S) and He(2 3S) metastable atoms
Authors: Parr T. P. , Parr D. M. , Martin R. M.
Published: February 2003

The velocity dependence of the title reactions was measured using crossed molecular beams and time-of-flight velocity dispersion of the metastable helium atoms. The relative collision energy ranged from 6–10 to 600–800 meV. The singlet cross section energy dependences are all of the ’’repulsive’’ type, i.e., increasing with energy at low energies, implying that the interaction potential well depths are ≲6–8 meV. The triplet cross sections are also generally repulsive, but the Ar, Kr, and O2 cases show evidence of minimums at the lowest energies. These observations are in qualitative agreement with the available interaction potential data. The cross section curves were put on an absolute basis by normalization to thermal quenching rate constants. The cross sections tend to approach the same magnitude at high energies and diverge at low energies. The singlet cross sections increase more rapidly than the triplet cross sections at low energies, with the singlet Ar, Kr, and N2 curves having especially steep slopes. Theoretical cross section curves were calculated for the He(1S,3S)/Ar, Kr cases using interaction potentials available from differential scattering studies. The results are in good agreement with experiment, and confirm the need for unusual low energy repulsive structure in the singlet potentials to explain the collisional energy dependence of chemi-ionization.
The resonance Raman spectra of aqueous phenoxy and phenoxy-d5 radicals
Authors: Beck S. M. , Brus L. E.
Published: February 2003

The time resolved 416 and 395 nm Raman spectra of the title compounds are observed following 266 nm photolysis of phenol. The observed spectra are simple, and can be tentatively assigned by comparison with similar compounds and radicals. The observed frequencies shift with solvent polarity. The spectra are discussed in light of the expected partial quinoid character of the radical.
Saturation spectroscopy with a CO2 waveguide laser: Analysis of the ν6 bands of CH3 79Br and CH3 81Br
Authors: Herlemont F. , Fleury J. , Lemaire J. , Demaison J.
Published: February 2003

About 100 Doppler-free hyperfine resonances arising from rovibrational transitions of methyl bromide have been observed and accurately measured by means of saturation laser spectroscopy using an absorption cell inside the cavity of a CO2 waveguide laser. Most of these transitions have been assigned to the ν6 fundamental bands of CH3 79Br and CH3 81Br. Assignments were deduced from computed spectra and checked with the resolved hyperfine quadrupole structure. An overall fit including recent microwave line frequency measurements, infrared inverted Lamb dip frequencies, infrared laser Stark measurements, and submillimeter emission frequencies yields a set of molecular parameters for the ν6 band of CH3 79Br and CH3 81Br. With respect to previous values, an improvement of one to two orders of magnitude is obtained for all the molecular parameters of the ν6 band. In particular, all the sextic centrifugal distortion constants are obtained significantly from the fit.
Electron spin–lattice relaxation of the (4Fe–4S) ferredoxin from B. s
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s. Comparison with other iron proteins
Authors: Bertrand Patrick , Gayda Jean-Pierre , Rao K. Krishna
Published: February 2003

The temperature dependence of the electron spin–lattice relaxation time T1 of the (4Fe–4S) ferredoxin from Bacillus stearothermophilus is studied in the range 1.2 to 40 K. This dependence is similar to that observed for the (2Fe–2S) ferredoxin from Spirulina maxima and can be interpreted with the same relaxation processes [J.P. Gayda, P. Bertrand, A. Deville, C. More, G. Roger, J.F. Gibson, and R. Cammack, Biochim. Biophys. Acta 581, 15 (1979)]. In particular, between 4 and 15 K, the data are well fitted by a second-order Raman process involving three-dimensional phonons, with a Debye temperature of about 60 K (45 cm−1). This would give an estimation of the highest frequency of the vibrations which can propagate through the three-dimensional proteinic medium. In the highest temperature range (T≳30 K) the results are interpreted with an Orbach process involving an excited level of energy 120 cm−1. This process could be induced by the localized vibrations of the active site. Finally, these results are compared to those recently reported for some hemoproteins [H.J. Stapleton, J.P. Allen, C.P. Flynn, D.G. Stinson, and S.R. Kurtz, Phys. Rev. Lett. 45, 1456 (1980)]. Below 15 K, the temperature dependence of T1 for these samples is similar to that observed for the iron–sulfur proteins and may be interpreted in the same way. Our interpretation is compared to the fractal model proposed by Stapleton et al.
Optical–optical double resonance spectroscopy of molecular ions
Authors: Cosby P. C. , Helm H.
Published: February 2003

Three-level double resonance spectroscopy is employed in a laser-ion coaxial beams photofragment spectrometer to study highly excited molecular ion states. A probe laser is used to label a bound molecular ion state in a single set of its quantum numbers (v″, J″, Ω″) by pumping from it into a discrete, predissociated state. The charged photofragments produced from this dissociated state are monitored as a measure of the population in the labeled level while a second laser interacts with the molecular ion beam upstream from the probe laser. When this second laser is tuned to transitions accessing either radiating or dissociating levels which originate from the labeled lower level, the resulting depletion of ions in this level is reflected by a decrease in the probe laser photofragment intensity. The technique is applied to the O+2b 4Σ−g(v′ = 4) ← a 4Πu(v″ = 5) and f 4Πg(v′ = 1,2,3) ← a 4Πu(v″ = 5) systems. Transitions into the v′ = 1 level and the Ω = 1/2 and −1/2 substates of the f 4Πg are observed here for the first time. The OODR technique promises wide application in molecular ion spectroscopy and in half-collision studies. A profound simplification of molecular spectra is achieved and nonpredissociated states of molecular ions can now be observed with the very high resolution which accompanies the coaxial laser-fast beam technique.
Analysis of the depolarized Rayleigh light scattering spectrum of liquids composed of anisometric molecules: Nitrobenzene
Authors: Gierke T. D. , Alms G. R
Published: February 2003

Analysis of the central component of the depolarized Rayleigh light scattering spectrum of nitrobenzene is complex. Existing experimental data for nitrobenzene, including the concentration and temperature dependence of the Rayleigh linewidth, the concentration dependence of the depolarized Rayleigh intensity, and the translational-rotational coupling parameter, are analyzed using asymmetric top equations to obtain the orientational pair correlation parameters. The results of this analysis and the translational-rotational coupling parameter are used to explain the temperature dependence of the δ parameter introduced by Lempert and Wang [J. Chem. Phys. 72, 3490 (1980)]. Finally, an equation for the depolarized Rayleigh doublet for a liquid composed of asymmetric top molecules (nitrobenzene) is presented. This result is similar to previous results for liquids composed of symmetric top molecules.
Close-coupled and coupled-states calculation of shear viscosity SBE cross sections for the H2–He system
Authors: Liu W.-K. , McCourt F. R. , Fitz D. E. , Kouri D. J.
Published: February 2003

Close-coupled (CC) and coupled-state (CS) calculations of kinetic theory relaxation and production cross sections are presented for the system H2 at infinite dilution in a bath of He. Performing these calculations for the same potential has allowed a test to be made of the efficacy of the CS procedure for use in calculating kinetic theory cross sections, especially those determining the Senftleben–Beenakker effects (SBE) in transport phenomena. The CS procedure is found to work extremely well (better than 1% agreement with CC results) for those cross sections that are predominantly elastic, to work rather well for relaxation cross sections determined either by reorientation of energetically inelastic collisonal events (about 10% agreement with CC results), and to work relatively poorly for production cross sections. In fact, the CS procedure for light molecules fares only marginally better than the IOS procedure for heavier molecules in the calculation of production cross sections. Finally, using only CC results, the sensitivity of the shear viscosity SBE is shown to be sufficient to distinguish easily between two recently obtained ab initio potentials for the H2–He system.
Unimolecular reactions with intense radiation (continued)
Authors: Vatsya S. R. , Pritchard H. O.
Published: February 2003

Unimolecular reactions in the presence of intense radiation fields are discussed. A model is proposed and solved computationally. (AIP)
Semiempirical calculation of diamagnetic susceptibilities of organogermanium compounds
Authors: Gupta R. R. , Kumar M. , Kalwania G. S.
Published: February 2003

A semiempirical model is presented to calculate diamagnetic susceptibilities of organogermanium compounds where the diamagnetic susceptibility is considered to be contributed to by the atoms, bonds, and bond–bond interactions. Semiempirically calculated values agree with the experimental values within 0.75%. (AIP)
Laser photofragment study of ethyl chloride cation at 5145 Å
Authors: Preuninger F. N. , Farrar J. M.
Published: February 2003

We present the product kinetic energy distribution for the photodissociation of the ethyl chloride cation at 5145 Å. The energy partitioning is consistent with a direct dissociation mechanism in which a combination of recoil effects and FrancknCondon effects dispose 85% of the available energy into fragment internal degrees of freedom. (AIP)
On the existence of sulfur superoxide and its formation in the combustion of carbon disulfide
Authors: Myerson Albert L.
Published: February 2003

The existence of sulfur superoxide SOO is debated upon. The isomer is reported to occur during combustion of CS2-O2 and COS-O2 and is apparantly responsible for the continuum absorption in the region 210 to 300 nm. Some commets are presented on SOO absorption. (AIP)
Changes in fluorescence induced by infrared multiphoton excitation of optically excited SO2
Authors: Kimel S. , Feldmann D. , Laukemper J. , Welge K. H.
Published: February 2003

Photophysical properties of excited SO2 were studied by the UV+MPE technique: single-photon UV excitation together with IR multiphoton excitation (MPE). Compared to the fluorescence in the absence of IR irradiation, the fluorescence following MPE showed a decrease of the main fluorescence peaks, accompanied by the appearance of broad fluorescence features to the high-energy side of the excitation frequency. Different CO2 laser lines were about equally effective. The dependence on laser fluence can be accounted for quantitatively using a simple stepladder model for MPE. The cross section for absorption of CO2 laser radiation by SO2, excited to around 32 000 cm−1, was found to be σ = 1.5×10−21 cm2. The effective decay time of the fluorescence with IR-on is increased twofold, compared to that with IR-off. This increase seems to parallel the reported dependence of the lifetimes on excitation frequency. It may be interpreted as due to more efficient coupling of excited vibronic states with highly-excited vibrational states in the ground electronic state.
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o studies of the photodissociation in the first excited states of Ã 1A1 and ã 3A1 of PH3
Authors: Müller Jiri , Ågren Hans , Canuto Sylvio
Published: February 2003

HF and CI calculations were carried out to explain the predissociation process in the first excited Ã 1A1 state of PH3 that is observed experimentally. It is found that the mechanism of the predissociation involves Rydberg-valence transformation leading to the production of PH2(X̃ 2B1, Ã 2A1) and H(2S). This process is similar to that found in the first excited 1A2′′ state of NH3; however, the thermodynamics of the two processes are different. A corresponding study for the PH3(ã 3A1) reveals a major difference from the 3A2′′ of NH3 due to the absence of the Rydberg character of the PH3(ã 3A1) at the equilibrium geometry. In addition, we report HF calculations on the equilibrium bond distances and the angles, the vibrational frequencies and the barriers to inversion of the neutral ground state of PH3(X̃ 1A1), its first singlet and triplet excited states (Ã 1A1 and ã 3A1, respectively), and the ground state ion (X̃ 2A1).
The possibility of a 4He2 bound state, effective range theory, and very low energy He–He scattering
Authors: Uang Yea-Hwang , Stwalley William C.
Published: February 2003

The best available intermolecular potential for helium by Aziz, Nain, Carley, Taylor, and McConville is shown here for the first time to have a 4He2 bound state. Two numerical analyses, namely, eigenvalue solution and effective range theory, are used to support this conclusion. Unlike usual chemically bound species, the binding energy of this very weakly bound level is found to be only 8.3×10−4 K, which is four orders of magnitude smaller than the potential well depth ε = 10.8 K. The scattering length for He+He collisions, determined from effective range theory, is used to calculate the elastic cross section in the very low energy limit. The results (1.878×105 Å2 for 4He+4He and 6.035×102 Å for 3He+3He) are consistent with measurements at the lowest velocities yet attained. In terms of the estimated uncertainties of the parameters of the potential of Aziz and co-workers, it is shown that it is very likely that a bound state of the 4He2 molecule does in fact exist.
Relativistic a
b i
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o molecular structure calculations including configuration interaction with application to six states of TlH
Authors: Christiansen Phillip A. , Balasubramanian K. , Pitzer Kenneth S.
Published: February 2003

A system for the inclusion of spin-orbit coupling along with moderate scale CI in calculations for molecules containing very heavy elements is demonstrated. In this effective potential procedure rigorous ab initio spin-orbit integrals are computed and added to the conventional integral set after the SCF and integral transformation steps of the calculation. This avoids the use of complex coefficients in the integral transformation and yet includes spin-orbit corrections on an equal footing with electron correlation. The diagonalization of the resulting complex CI plus SO matrix requires only about twice the time of a real CI diagonalization. Our present calculations on the two lowest 0+ and 1 states and the lowest 0− and 2 states of TlH indicate that this procedure allows adequate flexibility in the electronic coupling, resulting in bonding curves which are in good agreement with the experimentally established curves. The results also help to understand and to confirm previously conjectural interpretations of other spectral data.
Selective vibrational excitation of polyatomic molecules by stimulated emission pumping: The 523O2 level (εvib = 2036 cm−1) of p-difluorobenzene
Authors: Lawrance Warren D. , Knight Alan E. W.
Published: February 2003

The method of stimulated emission pumping (SEP) is used to populate excited vibrational level (εvib = 2036 cm−1) in the ground state of a medium-sized polyatomic molecule, namely p-diflurobernzene. The state specificity of the SEP process is demonstrated by generating single vibronic level (SVL) fluorescence from an S1 level (302) via absorption from the SEP-prepared level, 523O2. The wavelength resolved SEP-SVLF fluorescence spectrum establishes the integrity of the prepared S0 level and provides a means for monitoring its population dynamics.
A correlated walk model for thermally stimulated depolarization currents in α-keratin
Authors: Blaisten-Barojas Estela , Mascarenhas Sérgio
Published: February 2003

It is shown that the correlated walk model is able to describe all the main features of thermally stimulated currents in α-Keratin fibers. (AIP)
Alkoxy radical radiation products
Authors: Box Harold C. , Budzinski Edwin E.
Published: February 2003

Single crystals of ribitol and 6-methylpurine-riboside were x-irradiated at 4.2 °K. Alkoxy radicals were identified in the radiation products. The g tensor and proton coupling tensors, were determined from ENDOR and ESR studies. (AIP)
Absorption spectrum of trapped electrons in rhamnose single crystals x irradiated at 4 K
Authors: Li A. S. W. , Kevan Larry , Fujimura T.
Published: February 2003

The stable optical absorption of x-ray produced trapped electrons at 4.2 K in rhamnose single crystals has been observed under similar conditions to those in which the electrons have been identified by electron spin resonance. The absorption maximum is at 400 nm which compares with a transient absorption at 500 nm observed at 273 K by pulse radiolysis. The origin of the temperature shift and the electron trapping site are briefly discussed. (AIP)
Identification of NaAr and NaXe satellite bands: A test for calculations of the B 2Σ+ potential
Authors: Woerdman J. P. , de Groot J. J.
Published: February 2003

The emission spectra of high-pressure sodium discharge lamps with Ar and Xe present are reported. The 555-nm bands of NaAr and 560-nm bands of the NaXe were observed. (AIP)
Integral dependent spin couplings in CI calculations
Authors: Iberle Kathleen , Davidson Ernest R.
Published: February 2003

Integral dependent spin-couplings are constructed which yield one spin coupling per product of orbitals and which reproduce the energy through third order. These couplings yield about 98.5% of the correlation energy obtained by using all spin couplings. Extension of this method to multireference zeroth order states requires little extra effort.
Distributions of energy spacings and wave function properties in vibrationally excited states of polyatomic molecules. I. Numerical experiments on coupled Morse oscillators
Authors: Buch V. , Gerber R. B. , Ratner M. A.
Published: February 2003

Vibrational eigenvalues and eigenfunctions in the strongly coupled regime at intermediate and at high vibrational energy content are characterized in terms of distributions, both of observable and of wave function properties. In this regime, state-by-state description in terms of uncoupled modes is both difficult and too detailed; the distributions provide an adequate characterization of the vibrational behavior. We present results from numerical studies of realistic coupled-vibrational problems; the latter include local-mode Morse oscillators coupled by the Wilson interaction. The distributions studied are g(ω) (the distribution of nearest-neighbor energy spacings), G(ω) (the distribution of all energy spacings), π(χ) (the distribution of transition moments), and two different but related wave function expansion coefficient distributions. We find that in the strongly coupled regime, the parameters of the distributions depend only weakly on the Hamiltonian, and, moreover, that simple analytic forms can be found to represent the distributions over a wide range of systems and energies. We conclude that the distributional description of vibrational systems seems potentially very useful and that dynamical elucidation of the distributional parameters seems a desirable aim. Comments are made on the relation to mode mixing, and chaotic-like behavior.
A theoretical study of the reaction surface for the H2O–Li2O system
Authors: Raghavachari Krishnan
Published: February 2003

The reaction surface for the H2O–Li2O system is investigated by ab initio molecular orbital methods. Electron correlation is included by Mo/ller–Plesset perturbation theory calculations to third order. The reaction of H2O wiht Li2O proceeds without any activation to give the cyclic dimer of LiOH. The following 0 K reaction energies are calculated: Li2+H2O→2 LiOH, ΔH°0 = −20.9 kcal/mol, 2 LiOH→Li2(OH)2, ΔH0° = −63.2 kcal/mol, Li2O+H2O→Li2(OH)2, ΔH°0 = −84.1 kcal/mol. These results are in good agreement with the available experimental results.
Molecular dynamics simulation of induced anisotropy. I. Equilibrium properties
Authors: Evans M. W.
Published: February 2003

A simple mechanical torque is applied to 108 C2v triatomics in a molecular dynamics computer simulation. The effect of the torque on equilibrium autocorrelation functions is evaluated for vectors of interest to the analytical theory of molecular diffusion in anisotropic liquids. The results of the simulation do not, in general, support the conventional viewpoint based on Debye’s theory of rotational diffusion. In particular, the angular momentum acf becomes more oscillatory in the anisotropic liquid, which is not the case in theories of liquid anisotropy such as those of Bénoit, Ullman, and others in the literature.
Ditetramethyltetraselenafulvalenium fluorosulfonate: The effect of a dipolar anion on the solid state physical properties of the (TMTSF)2X phase
Authors: Wudl F. , Aharon-Shalom E. , Nalewajek D. , Waszczak J. V. , Walsh W. M. , Rupp L. W. , Chaikin P. , Lacoe R. , Burns M. , Poehler T. O. , Beno M. A. , Williams J. M.
Published: February 2003

The solid state properties of the title compound are described. The material undergoes a metal to insulator transition at 86 K which is not driven by spin density waves. The metal to insulator transition is reflected by very sharp changes in conductivity and magnetic susceptibility.
Photochromic switching in semiconducting films of zinc dithizonate
Authors: Dahlberg S. C. , Reinganum C. B.
Published: February 2003

A photochromic transition between the red (R) and blue (B) forms of zinc dithizonate has been followed experimentally by measuring the photovoltage of this semiconducting film. The wavelength for maximum photovoltage signal is 620 nm, which corresponds to the maximum optical absorption of the photoexcited blue form. This is consistent with the following reactions: Rk1(hν)Bk2(hν)B++e− (observed photovoltage). The reactions dynamics of this photochromic switching were probed with a pulsed tunable dye laser and (k1)−1 and (k2−1) are both faster than 10 ns, the experimental resolution limit. The decay of the sample back to its dark equilibrium has been examined and is consistent with the reactions B+k3R+ and e−+R+k4R, where k3 = 23.66 s−1 and k4 = 4.558 s−1. The validity of this reaction mechanism was checked by repeating the experiments with additional incident background light, and the photovoltaic decay differed as expected theoretically.
A concealed anomaly at 117.5 K in the heat capacity of hexamethylbenzene
Authors: Atake Tooru , Gyoten Hisaaki , Chihara Hideaki
Published: February 2003

Heat capacities of solid hexamethylbenzene were measured in an adiabatic calorimeter between 3 and 300 K with particular attention to the phase transition at 117 K between phase III and phase II. The transition is very likely to be of first order with the temperature 117.5±0.1 K, the heat of the transition 0.99±0.02 kJ mol−1, and the entropy of transition 8.3±0.2 J K−1 mol−1. The reason why the transition has been referred to as a λ transition is because there is an additional thermal anomaly concealed behind the transition, which has a maximum as large as 50 J K−1 mol−1 at 128 K. This anomaly, or hump in the heat capacity curve, was interpreted in terms of rotatory oscillation/hindered rotation of the methyl groups in a potential barrier of the shape V = V3 cos 3ϑ+V6 cos 6ϑ+V9 cos 9ϑ with V3 = 4×10−21 J, V6 = 1×10−21 J, and V9 = 3×10−21 J. The effect of the transition on the hump is merely to cause a small jump and the hump is otherwise continuous beginning in phase III through phase II. A term, excitation index, is proposed to characterize a first order portion of a phase transition.
Photoelectron–photoion coincidence spectroscopy of gas-phase clusters
Authors: Poliakoff E. D. , Dehmer P. M. , Dehmer J. L. , Stockbauer R.
Published: February 2003

A photoelectron–photoion coincidence technique for obtaining the photoelectron spectrum of a single component of a gas-phase mixture has been developed. It utilizes a newly designed instrument which measures the ion mass in coincidence with the photoelectron kinetic energy. Initial experiments were carried out on Xe2 and Xe3 produced in mixture of clusters (plus monomer) in a free-jet supersonic expansion. These measurements determined the photoelectron appearance potential (i.e., the lowest binding energy for which photoelectrons are detected) to be 11.30(5) eV for Xe3. It was also found that fragmentation of cluster ions strongly affects the coincidence spectra. This was investigated by varying the stagnation pressure, and thus the beam composition, in order to assess fragmentation contributions to coincidence spectra of the cluster under study. One case studied in detail indicated the energy levels of Xe3 near the ionization potential of 11.6 eV, corresponding to 0.7 eV of internal energy in Xe+3, fragmented to form Xe+2 and Xe.
129Xe NMR study of xenon adsorbed on Y zeolites
Authors: Ito Taro , Fraissard Jacques
Published: February 2003

Density dependences of NMR signal shifts and widths for 129Xe adsorbed on NaX, NaY, HY, CaY, and CaA zeolites are observed. At zero density, shifts upfield relative to Xe gas are 58±4 ppm for NaX, NaY, and HY, and 83±2 ppm for CaA. Shift/Xe density gradients are 46±2 ppm/1021 atoms/g for NaX, NaY, and HY, 58±2 ppm/1021 atoms/g for CaA. These shifts are explained mainly in terms of collisions between Xe atoms and cage walls and other Xe atoms. They correspond to a very high apparent Xe density (several hundred amagats).