bluuebunny/crossref_metadata_2025 huggingface dataset:
Mobilities and longitudinal diffusion coefficients for Li+ ions in Ar, Kr, and Xe at room temperature
Authors: Takebe M. , Satoh Y. , Iinuma K. , Seto K.
Published: February 2003

The mobility of Li+ ions has been measured in Ar, Kr, and Xe at room temperature over the E/N range 6–218 Td in Ar, 8–216 Td in Kr, and 8–228 Td in Xe using a drift tube without mass spectrometer. The pressures used were 0.3–2.0 Torr for argon and 0.5–2.0 Torr for krypton and xenon. It is found that mobilities can be deduced from arrival time spectra which have been affected by reactions. The zero-field reduced mobilities are found to be 4.56±0.05 cm2/V s for Ar, 3.65±0.04 cm2/V s for Kr, and 2.77±0.03 cm2/V s for Xe. These mobility values are higher than the Langevin values by 2.3%, 4.9%, and 3.4%, respectively. The longitudinal diffusion coefficients were also measured and compared with values calculated using the generalized Einstein relations and the present mobility data. The experimental values are somewhat higher than the calculations in all cases.
Correlations for dilute hard core suspensions
Authors: Ackerson Bruce J. , Fleishman L.
Published: February 2003

The generalized Smoluchowski equation is solved exactly for two particles interacting via a hard core potential in one, two, and three dimensions without the hydrodynamic interaction. From this solution coherent and incoherent intermediate scattering functions, memory functions, and transport coefficients are calculated. While many results relating to mutual diffusion are known, new results relating to self-diffusion are presented. These are compared with other exact and approximate results. The ’’hydrodynamic regime’’ for dynamic light scattering is discussed.
On a model chemical system related to absolutely asymmetric synthesis
Authors: Iwamoto Kazutoshi , Senō Manabu
Published: February 2003

The behavior of a model chemical system related to absolutely asymmetric synthesis is investigated. This model system includes stereospecific and autocatalytic processes. Especially, the effect of incomplete stereospecific discrimination on the behavior is examined in detail. This chemical system can provide an absolutely asymmetric synthesis even though the stereospecific processes distinguish incompletely the d and l forms of the isomers. Moreover, the role of spatial diffusion is discussed, and it is shown that, if a local excess of either optically active isomer is created by chance, it would be grown and spread in space, annihilating another isomer.
Rotational rainbows in diatom(solid) surface scattering
Authors: Schinke Reinhard
Published: February 2003

Performing the classical limit of the coordinate-representation-sudden approximation of Gerber et al. [J. Chem. Phys. 73, 4397 (1980)], we discuss rainbow effects in diatom-surface scattering. Under special conditions, which are stated in this article, rainbows can be classified into surface rainbows and rotational rainbows. The latter are expected to be common features of diatom-surface scattering provided: (i) the collision is impulsive and (ii) many rotational states are energetically open. Simple analytic expressions for the rainbow states are derived using a repulsive model potential and the dependence on collision and potential parameters is discussed. The predictions are all substantiated by calculations performed within the sudden approximation and using this model potential.
Unique determination of the He2 ground state potential from experiment by use of a reliable potential model
Authors: Feltgen R. , Kirst H. , Köhler K. A. , Pauly H. , Torello F.
Published: February 2003

Measured backward glory oscillations of integral 4He2 and 3He2 scattering cross sections are evaluated by use of an improved semiclassical backward glory formula yielding the energy dependence of the s phases, which allows the calculation of the He2 potential in the region 1.83–2.12 Å via Miller’s semiclassical inversion method. A physically realistic two-parameter potential model, which uses all ab initio data available with sufficient accuracy, is fitted to the inversion result, giving in a unique way the entire helium pair potential function. A well with a depth of 10.74 K at 2.975 Å is obtained, which supports a bound state for 4He2 very near to the dissociation limit. Via construction, the potential model also gives results for individual interaction energy terms in the symmetry adapted perturbation scheme for the He2 interaction. Calculations with the determined potential reproduce the various experimental data available for helium. The best ab initio He2 potentials available today converge towards that potential.
Multiphoton ionization of methyl iodide and selective stepwise ionization of methyl radicals
Authors: Danon J. , Zacharias H. , Rottke H. , Welge K. H.
Published: February 2003

The collisionless multiphoton ionization of methyliodide in the isotopic forms CH3I and CD3I has been investigated with pulsed laser light of two wavelengths: fixed-frequency 266 nm of high intensity (∼1×107 W/cm2) and tunable around 215 nm of relatively lower intensity (∼2×106 W/cm2). The 215 nm was chosen because the methyl radicals CH3 and CD3 absorb there resonantly through the (0,0) (B̃ 2A′1←X̃ 2A′′2) band transition with the B̃ state being very heavily predissociated. Objectives have been: (1) the ionization mechanism of CD3I to form ion fragments D+, CD+, CD2+, CD3I+, and I+, obtained with 266 nm alone, and 266 nm together with tunable 215 nm, (2) the possibility of influencing the MPI by selective excitation of an intermediate (CD3, CH3), and (3) detection and spectroscopy of the methyl radical by two-photon ionization through the decaying B 2A′1 state as intermediate step. Mass spectra have been taken and measurements have been made with respect to the abundance of fragments, power dependence, sensitivity of CH3, CD3 detection and excitation–ionization spectra of the methyl radicals.
On exact hydrogenic Compton profiles
Authors: Mukhopadhyaya S. , Ray S. N. , Talukdar B.
Published: February 2003

Using nonrelativistic Coulomb functions in the spherical polar coordinates, atomic Compton profiles are calculated in the form factor or exact hydrogenic (EH) approximation. A three-term recurrence relation is shown to facilitate a straightforward evaluation of the Gaussian hypergeometric functions which characterize the bound–free matrix elements under consideration. Numerical data for EH L-shell profiles of elements from 10Ne to 14Si and 18Ar are compared with those obtained in the impulse approximation. While the impulse profile is symmetric about the profile center, the EH profile exhibits energy defect. Based on analysis of the present data, the dynamical origin of such defects is investigated.
Rotationally adiabatic treatment of Λ-doubling in CH+ A 1Π
Authors: Helm H. , Cosby P. C. , Saxon R. P. , Huestis D. L.
Published: February 2003

A new approach is presented to calculate the vibrational and rotational dependence of the Λ-type splitting of 1Π levels in a diatomic molecule. We derive rotationally adiabatic potentials which include the off-diagonal interactions, between Born–Oppenheimer states, arising from the Coriolis Hamiltonian. Eigenvalues of these rotationally adiabatic potentials are then obtained separately for the two parity components of a Λ-doublet. We apply this technique to the A 1Π state of CH+. Calculations using an ab initio matrix element 〈1Π‖L+‖1Σ〉 show the experimentally observed vibrational and rotational dependence of the Λ splitting. A small adjustment of the magnitude of the ab initio matrix element was required to obtain absolute agreement.
Quantitative measurements of very weak H2O absorption lines by time resolved intracavity laser spectroscopy
Authors: Stoeckel Frédéric , Melieres Marie-Antoinette , Chenevier Marc
Published: February 2003

Time resolved quasicontinuous intracavity dye laser absorption spectroscopy (ICLAS) experiments have been made without and with absorbing species inside the cavity. In the absence of an absorber, a near Gaussian shape with a half-width decreasing as the square root of the time has been found for the spectrum. With atmospheric water vapor as the absorber, absorption lines in the 600 nm range appear on the spectrum, and the absorption intensity increases with time. This time evolution leads to a quantitative determination of the absorption coefficient of the lines. We confirm that ICLAS follows the Lambert–Beer law with an equivalent length of the cell given by Leq = ct (c is light velocity and t is the time between the laser turn on and the time of observation). This experiment result is in good agreement with time-dependent theories.
Microwave, infrared, and Raman spectra, structure, vibrational assignment, and normal coordinate analysis of disilanyl cyanide
Authors: Durig J. R. , Brletic P. A. , Church J. S. , Li Y. S.
Published: February 2003

The microwave spectra of SiH3SiH2 12C14N and SiH3SiH2 12C15N have been recorded from 18.0 to 26.5 GHz. Only a-type transitions were observed and R-branch assignments have been made for the ground vibrational state. The rotational constants were found to have the following values: for SiH3SiH2 12C14N, A = 8996.72±5.91, B = 2203.95±0.05, and C = 1844.03±0.05 MHz; for SiH3SiH2 12C15N, A = 8896.08±5.70, B = 2145.15±0.04, and C = 1798.63±0.03 MHz. From a diagnostic least-squares adjustment to fit the six rotational constants, the following structural parameters were obtained: r(Si–Si) = 2.332±0.014 Å; r(Si–C) = 1.841±0.015 Å; r(C≡N) = 1.156±0.010 Å; and ∢SiSiC = 107.4±0.1°. These parameters are compared to the corresponding ones in some other silanes and cyanide molecules. The infrared (2500 to 80 cm−1) and the Raman (2500 to 10 cm−1) spectra of the solid phase have been recorded for disilanyl cyanide-d0 and -d5. Additionally, the infrared spectrum of the gaseous phase and the Raman spectrum of the liquid phase were recorded and qualitative depolarization values were obtained. All of the normal modes have been assigned based upon band contours, depolarization ratios, and group frequencies but the assignment of the SiH3 torsional mode must be considered tentative. A normal coordinate calculation has been carried out by utilizing a modified valence force field to calculate the frequencies and the potential energy distribution. These results are compared to similar quantities in some corresponding molecules.
Raman spectroscopy of supercooled water
Authors: Bansil R. , Wiafe-Akenten J. , Taaffe J. L.
Published: February 2003

We report Raman spectroscopic measurements on supercooled water down to a temperature of −20 °C. The spectral region investigated is the uncoupled OD stretching vibration (2000–3000 cm−1) from a 5% solution of D2O in H2O. The results are consistent with an interpretation in terms of two categories of OD bonds, those that are hydrogen bonded and those that are essentially free. The fraction of H-bonded OD bonds varies from 0.9 to 0.95 as the temperature decreases from 0 to −20 °C. We observe that the frequency of the bonded OD component decreases with temperature at a slightly slower rate in the supercooled region than in the normal liquid region. With decreasing temperature, a decrease in width of this component band is observed, and this decrease appears to have a much larger slope in the supercooled region as compared to that observed previously in the normal temperature region.
The microwave spectrum and molecular structure of the acetylene–HF complex
Authors: Read W. G. , Flygare W. H.
Published: February 2003

The microwave spectrum of the acetylene–HF complex in the ground vibrational state has been assigned using a pulsed Fourier transform microwave spectrometer, employing a Fabry–Perot cavity and a pulsed supersonic nozzle as the molecular source. Acetylene–HF is T shaped, probably having C2v symmetry at equilibrium with the HF subunit, making a hydrogen bond to the midpoint of the acetylene triple bond. The spectroscopic constants for various isotopes of acetylene–HF are
Excitation spectroscopy of the acetylene Ã–X̃ transition in the 220 nm wavelength region
Authors: Abramson Evan , Kittrell Carter , Kinsey James L. , Field Robert W.
Published: February 2003

The fluorescence excitation spectrum of the Ã 1Au–X 1Σg+ transition of acetylene has been examined in the 220 nm wavelength region. Sharp lines were observed at frequencies that agree with earlier studies using absorption spectroscopy, and most lines previously reported as blended were well resolved. Time-resolved fluorescent decays following excitation in four of the peaks revealed suprisingly long lifetimes (2–5 μs in the low-pressure limit). Two of the levels exhibited modulation in their fluorescent decays.
Vibrational dephasing and frequency shifts of polyatomic molecules in solution
Authors: Schweizer Kenneth S. , Chandler David
Published: February 2003

A theory for the effects of repulsive and spatially slowly varying attractive forces on the vibrational frequency and dephasing of polyatomic molecular liquids is developed. Thermodynamic state dependence of these features is of particular interest because of the competition between the two types of forces. Solvent shifts of vibrational lines are computed. Proper separation of the rapidly and slowly varying branches of the intermolecular potentials leads to a separation of time scales that allows the dephasing relaxation to be computed using a combination of binary collision and mean field ideas. The question of homogeneous vs inhomogeneous broadening of the isotropic Raman line is addressed. The theory is applied to several polyatomic fluids with special emphasis on the isothermal density dependence of the spectral features. Agreement with experiment is good. The slowly varying attractive forces are found to play a significant role in determining the Raman linewidth for all the systems studied. Comparison with previous theories is made.
The quenching mechanism of electronically excited Rydberg states of nitric oxide
Authors: Asscher Micha , Haas Yehuda
Published: February 2003

Two photon excited fluorescence (TPEF) is used to measure the quenching cross sections of some NO Rydberg states by a number of molecules. The cross sections obtained are quite large, particularly for the C 2Π and D 2Σ states, often significantly exceeding the gas kinetic value. It is suggested that a mechanism common to many collision partners involves an ion pair intermediate. Using this model, the quenching efficiency is related to the electron affinity and to the weakest bond energy of the quencher.
Identification of a high energy state of I2 by ion dip spectroscopy
Authors: Cooper D. E. , Wessel J. E.
Published: February 2003

A new high energy electronic state of I2 has been characterized by means of ion dip spectroscopy, a two-color multiphoton ionization technique. The method yields rotationally resolved spectra analogous to excited state absorption spectra. The new state is characterized by an extensive 37 cm−1 vibrational progression, an internuclear separation of about 5.3±0.2 Å, and a term energy about 70 000 cm−1, near the ionization limit. Bonding is significantly weaker than in the previously investigated ion-pair states. The new results help to establish the pathway of the multiphoton ionization process in I2.
The dielectric relaxation time of supercooled water
Authors: Bertolini D. , Cassettari M. , Salvetti G.
Published: February 2003

The complex dielectric constant of liquid water was measured at 9.61 GHz down to −18 °C by means of a two resonant cavities apparatus. The static dielectric constant of bulk samples was also measured at 27.5 MHz down to −16.5 °C using a resonant circuit technique. From the analysis of the experimental results it follows that water in the metastable region has practically a single dielectric relaxation time τ. An analysis of the dynamic properties of water using our results and available data in literature, is presented. The main result is that self-diffusion DS, shear viscosity η, and τ below 0 °C are related to the same mechanism. For T≳0°C another mechanism affecting η rises.
New electronic states in CH3, observed using multiphoton ionization
Authors: DiGiuseppe T. G. , Hudgens Jeffrey W. , Lin M. C.
Published: February 2003

We report the first observation of the npe′ (2E′) and npa″2 (2A″2) electronic states in gas phase methyl radicals using the technique of resonance enhanced multiphonon ionization (REMPI). These previously unobserved states were identified in our REMPI spectrum as two-photon allowed resonance transitions leading to ionization. These electronic states arise from the excitation of the ground state electron in CH3 to np Rydberg orbitals. Transitions into these states from the ground 2A″2 electronic state are forbidden in one photon; however, they are allowed in a two-photon absorption. (AIP)
State-selective photodissociation dynamics of CF3NO
Authors: Jones R. W. , Bower R. D. , Houston P. L.
Published: February 2003

The vibronic states of NO are examined after photolysis of CF3No cooled by supersonic expansion. (AIP)
Vibrational relaxation of highly excited diatomics. II. HCl(v⩽7)+20 quenchers
Authors: Berquist B. M. , Dzelzkalns L. S. , Kaufman F.
Published: February 2003

120 vibrational relaxation rate constants kv,v−1Q are measured at 298 K for HCl (v⩽7) collisions with six diatomics: N2, CO, NO, HCl, DCl, and HF; three triatomics: CO2, N2O, NO2; and eleven polyatomics: CH4, C2H6, C3H8, iso-C4H10, CH3Cl, CH3F, CF4, SF6, CF3Cl, CF2Cl2, and CFCl3. For most quenchers, kQ increases with increasing v independent of whether the vibrational energy gap is reduced or increased. Large collisional relaxation probabilities for high v, of order 0.1 to 1 gas kinetic, are encountered for 11 of 20 quenchers. For Q=HC1 (v=0), the relaxation is shown experimentally to be mainly V–R,T for high v. The rates reflect a mix of effects due to dipole and dispersion interactions, energy gap, rotation, and complex formation, and present interesting problems of interpretation.
Beam maser investigation of inelastic scattering of NH3. I. Cross sections for inversion transitions induced by polar gases
Authors: Klaassen D. B. M. , Reijnders J. M. H. , ter Meulen J. J. , Dymanus A.
Published: February 2003

Cross sections for transitions across the inversion doublets (J, K) = (1, 1), (2, 2), (3 ,3), and (6, 6), of NH3 in collisions with NH3, CF3H, and CH3F have been measured in a molecular beam maser. The results are in good agreement with values calculated both in Anderson’s theory and in first-order Born approximation. A comparison is made with results of line broadening and transient experiments.
Multiquantum vibration–vibration transitions in H 35Cl(n)+H 37Cl(0) and H 35Cl(n)+H 79Br(0)
Authors: Shin H. K.
Published: February 2003

Multiquantum transitions in the vibration–vibration energy exchange process H 35Cl(n)+H 37Cl(0)→H 35Cl(0)+H 37Cl(n) are shown to be quite efficient near room temperature. This result is due to the presence of strong molecular attraction and the condition of near resonance. At 300 K, the probability for n = 1 is 0.085, while that for n = 5 is 0.014, the difference being not great. For the nonresonant collision of H 35Cl(n)+H 79Br(0)→H 35Cl(0)+H 79Br(n), the probability for n = 1 is found to be very small compared to the near-resonant case and, furthermore, the probability for n = 2 is two orders of magnitude smaller than that for n = 1.
Electronic structure and ESR parameters of (CrOCl4)−
Authors: Sunil K. K. , Harrison J. F. , Rogers Max T.
Published: February 2003

The self-consistent field Xα multiple-scattering (SCF–MS–Xα) method has been employed to study the electronic structure of (CrOCl4)−. The principal components of the g and hyperfine interaction tensors characterizing the electron spin resonance spectra of (CrOCl4)− have been computed using the wave functions obtained by both the SCF–MS–Xα and extended Hückel methods. Equations relating the g- and A-tensor components to the molecular orbitals have been derived for d1 complexes of c4v symmetry to include contributions from both occupied and unoccupied b1 levels as well as ligand spin–orbit coupling terms. Some difficulties in using the results from the extended Hückel molecular orbital model to compute ESR parameters have been discussed.
An SCF–MS–Xα study of a series of d1 transition metal oxohalo complexes
Authors: Sunil K. K. , Harrison J. F. , Rogers Max T.
Published: February 2003

A detailed study of the electronic structures of a series of d1 transition metal oxohalo complexes (MOXn)m−, where M = V, Nb, Cr, Mo, and W and X = F, Cl, and Br (n = 4 or 5), has been carried out by the SCF–MS–Xα method. The results provide values of the d–d transition energies, and also give some understanding of the similarities and differences in bonding characteristics of the penta- and hexacoordinated complexes, and of the observed trends in the g and metal hyperfine interaction tensors in these series. The utility of the SCF–MS–Xα method in predicting trends in bonding and in ESR parameters in transition metal complexes is discussed and the results compared with those from ab inito methods.
Classical, diatomic molecule, kinetic theory cross sections. III. Rigid body models
Authors: Curtiss C. F.
Published: February 2003

Expressions are developed for the generalized or kinetic theory cross sections of a gas of diatomic molecules, modeled as rigid, cylindrically symmetric bodies. The rigid bodies are described by ’’supporting functions’’ and the expressions reduced to forms involving integrations over the final translational energy, the final rotational energies of each molecule, and five angles. The expression for any particular cross section may then be obtained in the form of a double integral; as an example such a result is obtained for the ’’viscosity’’ cross section. As a more detailed example, an explicit expression for this cross section for the ’’loaded sphere’’ model is then obtained to second order in the displacement of the load from the geometric center; this result is consistent with the earlier result obtained as the classical limit of the quantum expression.
The nonlinear dielectric effect of α,ω-dibromoalkanes
Authors: Khanarian G.
Published: February 2003

The nonlinear dielectric effect (NLDE) is the change of the dielectric constant of a medium when a strong electric field is applied across it. In this paper, we show the relationship of the NLDE to 〈μ4〉, where μ is the dipole moment of a flexible molecule and 〈 〉 denotes the configurational average in the absence of an applied electric field. We calculate the NLDE of the α,ω-dibromoalkanes using the rotational isomeric state approximation and matrix multiplication methods of Flory. Good agreement is found between our calculations and the experimental results reported by Chelkowski.
The rotational Brownian motion of a nonspherical body and its application to the theory of dielectric relaxation
Authors: Morita Akio
Published: February 2003

The conditional probability density function in angular velocity space for a symmetrical body is obtained exactly both from the stochastic differential equations governing the rotational Brownian motion and from the Fokker–Planck equation. Furthermore, it is shown how the Laplace transform of the angular velocity correlation function for an asymmetrical body may be calculated from the Fokker–Planck equation. At the same time, the Laplace transform of the dipole correlation function for the symmetrical body is calculated both from a stochastic integrodifferential equation and Fokker–Planck–Kramers equation. Also, the Laplace transform of the dipole correlation function for the asymmetrical body is obtained using the Fokker–Planck–Kramers equation. Also, the dipole correlation function as an explicit function of time is calculated numerically for a symmetrical body based on the Fokker–Plank–Kramers equation, and is compared to that obtained from the free rotational model and to a single exponential decay function with the Debye relaxation time.
Absorption cross sections and line shape for the NO3(0–0) band
Authors: Marinelli W. J. , Swanson D. M. , Johnston H. S.
Published: February 2003

The absorption cross section of the nitrate free radical has been scanned by a tunable dye laser with 0.05 nm resolution (FWHM) between 613 and 672 nm, and portions of the region were scanned at high resolution (200 MHz) with the laser in single-frequency mode. These data, combined with other recent measurements, give a peak cross section at 661.9 nm of 1.90×10−17 cm2; and the integrated cross section over the 0–0 band from 14 910 to 15 290 cm−1 is 2.02×10−15 cm. Two recent studies that were said to disagree because of different peak cross sections are shown to agree in terms of integrated absorption cross section over the 0–0 band. It is confirmed that the spectrum is diffuse even under very high resolution. The 0–0 band is well approximated by two closely spaced Lorentzian functions.
Photolysis of NaClO3 and KClO3 at 26 K studied by optical and ESR spectroscopy
Authors: Bjerre Nis
Published: February 2003

Two types of defects are formed when crystalline NaClO3 and KClO3 are photolyzed at 26 K with monochromatic light of a wavelength corresponding to the fundamental absorption. One type of defects absorbs around 380 nm and has an ESR spectrum characteristic of trapped O2. The other type of defects absorbs around 270 and 580 nm and has no ESR spectrum. The two types of defects are interconverted reversibly by bleaching within the respective absorption bands. Chemical analysis and isotopic labeling indicates that the defects consist of fragments of a single chlorate ion. The 380 nm absorption is assigned to charge transfer in the complex [ClO−,O2]; the 270 and 580 nm absorptions are assigned to [Cl−,O3]. Both the optical spectra and the mechanism of photolysis are represented by a model based on the enthalpies of formation of various configurations of one chlorine and three oxygen atoms.
Spectrum of acetylene fluorescence excited by single XUV photons
Authors: Schmieder Robert W.
Published: February 2003

The spectrum of visible emission from photofragments of acetylene excited by single 16.85 eV photons has been recorded for the first time. The spectrum is dominated by the Swan and Deslandres-d’Azambuja bands of C2 and the 431.5 nm band of CH. The yields of these emissions are of the order 10−3 photons per absorbed incident photon. The experimental conditions suggest that the emission results from primary C*2 and CH* photofragments.
Theoretical and experimental pressure shifts in some select rotational transitions of the methyl cyanide molecule
Authors: Messer J. K. , Khoobehi B. , Roberts J. A.
Published: February 2003

The Anderson–Tsao–Curnutte–Frost and Murphy–Boggs theories of linewidth and line shift are briefly discussed, and the significance of certain ’’resonance’’ functions is noted. Power series approximations for these functions, which facilitate numerical evaluation of linewidths and line shifts using these two theories, are given and discussed. For completness, experimental line shift parameters for several rotational transitions in the CH3C14N and CH3C15N molecules are presented with a comparison of the theoretical results derived using the Murphy–Boggs and the modified Anderson theories.
Influence of intermolecular energy transfer on photoacoustic spectra
Authors: Schneider S. , Coufal H.
Published: February 2003

It is demonstrated that the photoacoustic spectrum of a strongly fluorescing compound which gives a poor photoacoustic response is very sensitive to the presence of another compound with low fluorescence quantum yield. Deliberate addition of such compounds can assist in detection and identification of compounds of low ’’photoacoustic yield.’’ It is also shown that photoacoustic spectroscopy is well suited to the study of energy transfer to acceptor molecules, which have a negligible fluorescence quantum yield.
Dynamical polar solvent effects on solution reactions: A simple continuum model
Authors: van der Zwan G. , Hynes James T.
Published: February 2003

The influence of polar solvent dynamics in solution reactions is investigated for a simple model. In this model, a charge is subject to a chemical free energy barrier, the successful crossing of which constitutes reaction. The retarding influence of the time dependent solvent polarization is described at the continuum Debye relaxation level. The reaction rate constant k is determined as a function of the barrier curvature, the charge-solvent interaction strength, and the solvent polarization relaxation time. The reduction of k with increasingly slower solvent relaxation is found to depend sensitively on the magnitude of charge-solvent interactions. When the latter are weak, the behavior of k differs qualitatively from a standard Kramers prediction. The analogous reaction problem for a dipole is discussed briefly. Directions for a more realistic treatment of polar solvent effects on reaction rates are described.
Photodissociation of formaldehyde in a molecular beam
Authors: Ho Pauline , Bamford Douglas J. , Buss Richard J. , Lee Yuan T. , Moore C. Bradley
Published: February 2003

Formaldehyde photopredissociation was studied under collisionless conditions by the technique of crossed laser and molecular beams. Detection of the molecular product CO after excitation of H2CO near the S1 origin gives strong support to the sequential coupling model for fast nonradiative decay of S1 states through broadened S0 levels to the continuum. For H2CO excitation at 283.9 nm, formation of the radical product HCO dominates dissociation to molecular products by at least one order of magnitude.
Rovibronic species, overall allowed species, and nuclear spin statistical weights for symmetric top molecules. II. Point groups Cnv and Cnh (n≤6)
Authors: Weber Alfons
Published: February 2003

The method described in I [A. Weber, J. Chem. Phys. 73, 1952 (1980); 74, 4754 (1981)] has been used to derive the rovibronic species, overall allowed species, and the nuclear spin statistical weights for symmetric top molecules belonging to the point groups Cnv or Cnh with n⩽6. Rules are presented by means of which the rovibronic species of Cnv and Cnh molecules are obtained from the results given in I. The overall species and the nuclear spin statistical weights are given in a new set of tables.
The structure of dinitrogen tetroxide N2O4: Neutron diffraction study at 100, 60, and 20 K and ab initio theoretical calculations
Authors: Kvick Å. , McMullan R. K. , Newton M. D.
Published: February 2003

Single crystal neutron diffraction studies are reported for cubic dinitrogen tetroxide (space group Im3; Z = 6). The crystals were grown from NO2 vapor in situ on the diffractometer by precise cryostatic control. The lattice parameter, measured at seven temperatures, increases from 7.6937(6) Å at 20 K to 7.7925(6) Å at 140 K. The nuclear positional and thermal parameters were refined using diffraction data (sin ϑ/λ⩽0.79 Å−1) measured at 20, 60, and 100 K. Final fit indices R(F2) are 0.028, 0.034, 0.037, respectively. The observed N–N bond length and O–N–O angle are invariant between 20 and 100 K at values 1.7562(±4) Å and 134.46(±6)° with individual e.s.d.’s of 0.001 Å and 0.1°. The observed N–O bond length increases linearly from 1.1855(9) Å at 100 K to 1.1893(5) Å at 20 K; the extrapolated zero-point value is 1.191 Å. Ab initio self-consistent field calculations using a two-configuration wave function which allows partial occupation of the σ* MO (antibonding with respect to the two nitrogens) yields a reasonable value for the N–N bond length: 1.80 Å is obtained when the 4-31G basis is employed, and an estimate for the effect of polarization functions leads to a corrected value of 1.74 Å.
A function useful for analyzing single-step one-dimensional Markov processes
Authors: Gillespie Daniel T.
Published: February 2003

We introduce, for a single-step, one-dimensional Markov process, the quantity T+∞(x), defined as the fully relaxed mean waiting time for the occurrence of the transition x→x+1. We show that T+∞(x) is just as easy to calculate from the given transition probability rates as is the fully relaxed state probability function P∞(x). We further show that many interesting first-passage quantities, particularly those describing transitions and fluctuations in bimodal systems, can be expressed simply and compactly in terms of the functions T+∞(x) and P∞(x).
Multiphoton processes of charge carrier generation in thianthrene crystals
Authors: Samoć Marek , Samoć Anna , Williams Digby F.
Published: February 2003

Photoconductivity in crystals of thianthrene has been studied by a standard dc method and by exciting with pulses of weakly absorbed light from a dye laser. The dc photoconductivity exited in the range 300–400 nm depends on the light intensity to the 1.5 power and is attributed to the interaction between singlet and triplet excitions. The S–T interaction coefficient is estimated to be 2×10−11 cm3 s−1. Pulsed photoconductivity in the range 450–640 nm, which follows a cubic light intensity dependence can be explained as arising from photoionization of two-photon produced singlet excitions. This hypothesis is corroborated by a comparison of the action spectrum of the two-photon absorption coefficient with that of the three-photon generation of charges. The photoionization cross section of a singlet excition is found to be ∼10−23 cm2. In the range 385–440 nm, the pulsed photocurrent depends on the square of the light intensity, where carrier generation occurs through the photoionization of triplets formed by a direct S0–T1 absorption or photoionization of singlets formed by absorption within the Urbach tail of the first absorption band.
Infrared laser stimulated desorption of pyridine from KCl surfaces
Authors: Chuang T. J.
Published: February 2003

Molecular desorption by resonant vibrational excitation with an infrared laser has been observed in the KCl–C5H5N5 system at 95 K. The desorption yield per laser pulse as a function of the laser frequency, the laser intensity, and the adsorbed overlayer thickness has been measured. the results indicate that the resonant absorption of the IR photons probably does not promote immediate desorption of the excited molecules and the process of energy transfer to the surrounding molecules and substrate phonons is likely to play a major role. (AIP)
Laser spectra of jet-cooled ions and ion clusters
Authors: Heaven Michael , Miller Terry A. , Bondybey V. E.
Published: February 2003

Laser induced fluorescence is measured for cations of hexafluorobenzene, pentafluorobenzene, 1,3,5-trifluorobenzene, 1,3,5-trichlorabenzene, and dimethly-diacetylene. (AIP)
A
b i
n
i
t
i
o effective spin-orbit operators for use in atomic and molecular structure calculations. Results for CH, OH, SiH, CO+, CO, and SiO
Authors: Stevens Walter J. , Krauss Morris
Published: February 2003

A b initio effective spin-orbit operators, based on relativistic effective core potentials, are used to determine the spin-orbit coupling constants for CH(X 2Πr), OH(X 2Πi), SiH(X 2Πr), CO+(A 2Πi), CO(a 3Πr), and SiO(b 2Πr). In comparison with experimental and ab initio values all electron values are very favorable.
The crystal ionization energy of PTS
Authors: Arnold S.
Published: February 2003

The ionization energy of the polydiacetylene solid PTS has been measured by the Millikan–Pope–Arnold method (MPA). This energy is found to be 5.2±0.1 eV in contrast with ESCA results published previously (7±1 eV). The present results are consistent with recent ab initio calculations. The yield is found to go approximately as the maximum electron kinetic energy cubed.
A comparison of V
N and V
N−1 potential results in many-body calculations of spectroscopic parameters
Authors: Ahmad S.
Published: February 2003

A comparison of Vn and VN−1 potential results in many-body calculations of spectroscopic parameters for Be is presented. (AIP)
Photoionization of the neon–rare gas dimers NeAr, NeKr, and NeXe
Authors: Pratt S. T. , Dehmer P. M.
Published: February 2003

The relative photoionization cross sections for the heteronuclear neon–rare gas dimers NeAr, NeKr, and NeXe were measured at a wavelength resolution of 0.15 Å for NeAr and 0.28 Å for NeKr and NeXe in the energy region between the molecular ionization threshold and the atomic 2P01/2 ionization limit of the heavier rare gas atom. The dimer ionization potentials are 15.685±0.004 eV for NeAr, 13.950±0.003 eV for NeKr, and 12.094±0.004 eV for NeXe. Combining these values with the known values of the atomic ionization potentials and the neutral ground state dissociation energies yields values of the A 2Σ+1/2 ionic ground state dissociation energies of 0.079±0.004 eV for NeAr, 0.055±0.003 eV for NeKr, and 0.041±0.004 eV for NeXe. Molecular Rydberg structure, which shows strong similarities to the atomic structure in the region of the autoionizing Beutler–Fano resonances, has been partially analyzed in terms of Rydberg series converging to the B 2Π1/2 state of the dimer ion. Vertical binding energies for the individual Rydberg states were determined based on the assumption that the Rydberg states dissociate to a neon atom in the 1S0 ground state plus a rare gas atom in an ns′[1/2]01 or nd′[3/2]01 excited state. Values of the vertical binding energies of the B 2Π1/2 ionic state were determined from the series limits and are 0.022±0.002 eV for NeAr, 0.018±0.004 eV for NeKr, and 0.014±0.002 eV for NeXe.
Kinetic studies of Ar2F* in electron-beam excited Ar–NF3 mixtures
Authors: Boewering N. , Sauerbrey R. , Langhoff H.
Published: February 2003

The fluorescence of ArF* and Ar2F* produced in e-beam exited mixtures of Ar, NF3, and He was investigated. Ar2F* is mainly generated via ArF* by ionic channels and by Ar*2. However, also the direct production from Ar*2 was observed. Rate constants for the production and quenching of Ar2F* are deduced from the measurements. Due to the long exciting pulse of 45 ns, the fluorescence efficiency became rather low and was determined to be 0.1 Ar2F* photons/excited argon atom at optimum conditions.
On the choice of phase in the CS and IOS approximation
Authors: Snider R. F.
Published: February 2003

With the recognition that the angular momentum representations of unit position and momentum directional states must have different but uniquely related phases, the previously presented expression of scattering amplitude in terms of IOS angle dependent phase shifts must be modified. This resolves a major disagreement between IOS and close coupled degeneracy averaged differential cross sections. It is found that the phase factors appearing in the differential cross section have nothing to do with any particular choice of decoupling parameter. As a consequence, the differential cross section is relatively insensitive to the choice of CS decoupling parameter. The phase relations obtained are also in agreement with those deduced from the Born approximation.
Analysis of the effects of high pressure on the spectra of molecular crystals
Authors: Berry David E. , Tompkins Robert C. , Williams Ferd
Published: February 2003

The failure of the Drickamer, Frank, and Slichter (DFS) theory, as modified by Okamoto and Drickamer to fit the experimental data on the pressure-dependent spectra of aromatic materials is analyzed quite generally. Possible explanations are (a) different coordinates are important for the optical and pressure effects, (b) anharmonicity is improperly included in the modified theory, and (c) the omission of the direct effect of pressure on the adiabatic potential is responsible. We show, in the analysis of (a), that a many-coordinate harmonic system can be reduced to a one-coordinate model with pressure-dependent force constants and, in the analysis of (c), that the data on the aromatic compounds cannot differentiate between the modified DFS theory and a similarly modified Curie, Berry, and Williams (CBW) theory, which includes the direct effect of pressure on the adiabatic potentials. Therefore, we conclude that (b) is responsible for the failure, verifying Tompkins’ earlier conclusion. We have also generalized his consistency tests and clarified the type of data needed to test the respective validities of the DFS and of the CBW theories.
Vibrational spectra of the chlorine monofluoride ClF in cryogenic solutions
Authors: Naulin Christian , Lambard Jacques , Bougon Roland
Published: February 2003

The infrared and Raman spectra of ClF in solution in liquid Ar, O2, and N2 are reported. The line profile evolution with temperature has been analyzed from the point of view of molecular dynamics, using the Fischer–Laubereau model for the Raman isotropic lines, and the Frenkel–Rakov, Debye, Gordon, and Bartoli–Litovitz models for the IR spectra. Spectroscopic data are discussed using these models in terms of Lennard-Jones ClF-X interaction potentials for binary mixtures (X = Ar, O2, N2). An unexpected behavior of N2 is pointed out from spectra of ternary ClF–N2–X mixtures (X = Ar, O2).
A note on the assignments of Rydberg states of COS
Authors: Wu C. Y. Robert
Published: February 2003

The assignment of Rydberg states of COS is reported. (AIP)
‘‘Ring-current’’ effects on 1H-NMR chemical shifts in linear acenes
Authors: Haigh C. W. , Mallion R. B.
Published: February 2003

It is pointed out that application of the traditional ’’ring-current’’ picture gives rise to qualitatively similar predictions of limiting trends in the 1H-NMR chemical shifts of large linear acenes as those based on Blustin’s recent ’’π-bond’’ model.
Semiclassical approximation in the coherent states representation
Authors: Weissman Yitzhak
Published: February 2003

This paper presents a semiclassical theory for the computation of matrix elements of the type 〈u‖v〉 when either one or both ‖u〉 and ‖v〉 are coherent states (in different representations). Our results can be considered as an extension of Miller’s semiclassical theory [Adv. Chem. Phys. 25, 69 (1974)]. Such an extension has been presented also by Heller [J. Chem. Phys. 66, 5777 (1977)]. We were able to simplify considerably some of Heller’s results by exploiting the canonical properties of the classical coherent variables. This enabled us to relate the elements 〈u‖v〉 to certain generalized, complex generator functions in a manner that is very similar to the relations that appear in the original Miller’s theory. The advantages that are inherent in the coherent states representation are illustrated in a few elementary examples. We were able to derive an excellent approximation to the eigenstates of the harmonic oscillator which is valid even for the ground state. Furthermore, we have demonstrated that it is possible to describe semiclassically dynamical processes that are classically forbidden by real time trajectories in a certain generalized phase space.
Electronic charge density and bonding in two NiAs-type solids: TiS and VS
Authors: Nakahara Jan , Franzen Hugo , Misemer David K.
Published: February 2003

The electronic band structure and charge densities for two transition metal monosulfides TiS and VS, with the NiAs-type structure have been calculated using the band theoretical Green’s function technique. The results of these calculations are interpreted in terms of chemical bonding and stabilization effects in the solid. It is found that the charge density of the transition metal d-like electrons is maximized in the direction through centers of triangular faces of the octahedrally coordinated sulfur atoms, rather than along the c axis between adjacent metal atom layers, a distribution which was originally believed to stabilize the NiAs-type structure.
A systematic procedure for extracting fragment matrices for the method of diatomics-in-molecules from a
b i
n
i
t
i
o calculations on diatomics
Authors: Kuntz P. J. , Schreiber J. L.
Published: February 2003

A computationally practical prescription is given for defining the fragment matrices which constitute the input to the method of diatomics-in-molecules (DIM). It is shown how a mixed-state energy matrix e and the mixing coefficient matrix c for each required diatomic manifold may be obtained from nonempirical calculations on a small number of states within the manifold. Apart from the eigenvalues and eigenvectors for these states, the only extra information required is the overlap of the DIM basis with the diatomic wave functions. It is suggested to define the DIM basis in terms of the eigenfunctions of selected atomic states so that any type of ab initio calculation may be used to compute the input fragment matrix b = cec−1. The procedure is illustrated by application to the molecule BeFH for which minimum and saddle point properties are computed for a variety of DIM bases. It is also shown how empirical information can be introduced into the DIM formalism in a reasonable fashion.
Lattice statistics for highly heterogeneous populations of rigid rectangular particles
Authors: Herzfeld Judith
Published: February 2003

DiMarzio’s lattice model is used to calculate the configurational entropy of a population of rigid rectangular parallelepipeds which is arbitrarily heterogeneous in both lengths and widths. An analytical expression is obtained which is a product of a small number of terms and is readily applicable to statistical thermodynamic calculations. The limit of infinitesimal lattice cell size is considered as an approximation of a continuum description. In this continuum limit, the alignment transition for monodisperse nematogen is found to occur at a lower density, and with a smaller density discontinuity, than is predicted by the lattice model in its original form. Although, at a given density, the alignment and the pressure are generally greater in the continuum description than in the discrete description, the alignment and pressure are less at the transition in the continuum case than they are at the transition in the discrete case. Whereas the original lattice model predicts that no transition occurs for particles of axial ratio less than 3.65, in the continuum limit the transition disappears only when the axial ratio is less than 3.06. Alternative approaches for treating solvents are discussed.
A theoretical model for phase transitions in lipid monolayers
Authors: Zuckermann M. J. , Pink D. A. , Costas M. , Sanctuary B. C.
Published: February 2003

The statistics of Guggenheim are used in conjunction with a ten-state conformational model (originally formulated for phase transitions in lipid bilayers) to predict gel–fluid and fluid–vapor phase boundaries for lipid monolayers. The theoretical model is a model of interacting dimers on a two-dimensional lattice with each dimer representing a lipid molecule with two acyl chains and one polar head. The acyl chains are assumed to interact via anisotropic van der Waals interactions. The interaction between the polar heads is simulated by an attractive isotropic interaction with coupling constant K0. The analysis of Costas and Sanctuary is then combined with this model so as to obtain self-consistently the fraction of occupied and vacant lattice sites for the monolayer at different pressures. Surface pressure-area isotherms and coexistence curves are obtained for fixed van der Waals interactions and several values of K0. It is shown that a fluid–vapor phase boundary occurs for values of K0 above a critical value. The method is also applied to the calculation of the change in packing fraction at the main gel–fluid (liquid crystal) transition of lipid bilayers. One of our principal results is that some of the qualitative features of Π-A isotherms for lipid monolayers may now be asociated with particular values of K0.