bluuebunny/crossref_metadata_2025 huggingface dataset:
Generalized diatomics-in-molecules potential energy surfaces for H3 and H4
Authors: Eaker Charles W. , Allard Lee R.
Published: February 2003

The generalized diatomics-in-molecules method has been applied to calculate potential energies of H3 and H4. This modification of the diatomics-in-molecules equations significantly improves the calculated energies for nonlinear geometries of these systems.
Collisional pair polarizabilities calculated by exchange perturbation theory using electric field variant basis sets
Authors: Hilton Peter R. , Oxtoby David W.
Published: February 2003

The use of exchange perturbation theory to calculate collisional pair polarizabilities is investigated. Both conventional and electric field variant basis sets are considered for the cases of helium and hydrogen atom pairs. It is shown that high quality atomic descriptions are required to obtain reliable results from the method. The inclusion of exchange is found to be important and the method leads to closer agreement with Hartree–Fock results than does dipole–induced-dipole theory. The significance of recent configuration interaction calculations for hydrogen and helium atom pairs is also discussed.
Vibrationally induced nuclear quadrupole coupling in tetrahedral and octahedral molecules
Authors: Hougen J. T. , Oka T.
Published: February 2003

Hyperfine splittings arising from the presence of a quadrupolar nucleus at the center of a molecule belonging to the point group Td or Oh (e.g., 189OsO4 or 235UF6) are symmetry forbidden to a high degree of approximation. Nevertheless, quadrupole splittings can be induced by either vibrational or rotational distortions of the molecule, i.e., by distortions similar to those responsible for inducing electric dipole moments in Td molecules. Such hyperfine splittings have recently been observed in several laboratories using laser saturation spectroscopy. In this paper we investigate theoretically the quadrupole splittings induced by excitation of doubly and triply degenerate vibrations in Td and Oh molecules. We find that much of the vibration–rotation formalism already present in the methane literature can be applied with only minor changes to the induced-quadrupole-coupling problem, and that rather simple theoretical relationships can be derived between the quadrupole splitting and the tetrahedral or octahedral vibration–rotation splitting of a given level. Values for the scalar and tensor contribution to the quadrupole coupling constant have been derived from the experimental data for 189OsO4 reported by Bordé et al.
Isoelectronic molecules—The fourteen electron sequence of diatomics
Authors: Laurenzi Bernard J.
Published: February 2003

A general theory of isoelectronic diatomic molecules is developed and numerical calculations are performed for the fourteen electron sequence. Calculated values of equilibrium bond distances, force constants, and dissociation energies compare well with experimental values and the global behavior of each quantity is displayed as a function of the nuclear charges. The screening effect of the electrons is seen to play the key role in determining the regions of stability on the nuclear charge plane.
Molecular dynamics and the phase transition in the naphthalene–tetracyanobenzene charge-transfer complex as studied by 1H NMR and triplet state EPR
Authors: Ripmeester J. A. , Reddoch A. H. , Dalal N. S.
Published: February 2003

1H NMR second moments and static and rotating frame relaxation time measurements were performed on the crystalline naphthalene–tetracyanobenzene charge-transfer complex between ∼50 and 150 K. The results are consistent with a model where the random jumping of naphthalene molecules between equivalent orientations becomes nonrandom below the phase transition at 72±1 K. Below the transition the two orientations must be inequivalent, with the molecules settling in the one of lower enery to yield a phase which ideally is completely ordered at 0 K. The barrier between the two orientations is 8.0 kJ/mol above Tc and 6.1 kJ/mol below Tc. The statistical average energy difference between the orientations increases with decreasing temperature from near zero at Tc and is 7.2 kJ/mol at 62 K. The EPR spectrum of the triplet naphthalene trap becomes weaker at higher temperature and disappears by 90 K. It shows no evidence of the phase transition. The loss of hfs at 55 K reveals a process, presumably thermal detrapping, which is 50 times faster than the naphthalene reorientation. Anthracene traps reorient some five times faster than naphthalene molecules at low temperatures but the two rates become comparable at higher temperatures. Naphthalene triplet excitons show processes only about four times as fast as the naphthalene reorientation. Evidently the triplet EPR spectra do not directly probe the molecular dynamics of the host lattice or the phase transition.
Raman and infrared spectra of crystalline dinitrogen tetroxide
Authors: Andrews Bruce , Anderson Anthony
Published: February 2003

Raman and infrared spectra of polycrystalline samples of N2O4 at low temperatures have been recorded. The number and activities of the fundamentals are compared with the predictions of group theory, based on the known molecular and crystal geometries. Several additional peaks are attributed to Fermi resonances of combinations with fundamentals. A new low frequency internal mode and five lattice modes have been observed.
Length dependence of the second-order optical nonlinearity in conjugated hydrocarbons
Authors: Dulcic A. , Flytzanis C. , Tang C. L. , Pépin D. , Fétizon M. , Hoppilliard Y.
Published: February 2003

Experimental results and a theoretical model are presented which give strong evidence that the ratio β/L2μM is a constant for a large class of disubstituted conjugated hydrocarbons, β is the second-order polarizability, μM the mesomeric dipole moment, and L the length of the hydrocarbon chain.
Spectra and structure of small ring compounds. XLII. Microwave spectrum of 2,5-dihydrothiophene
Authors: Durig J. R. , Li Y. S. , Durig D. T.
Published: February 2003

The microwave spectrum of 2,5-dihydrothiophene CHCHCH2SCH2 has been recorded from 18.0 to 39.0 GHz. Only a-type R branches were observed and assigned for the ground and five excited states of the ring puckering motion. The ground state rotational constants are A=6621.59±0.12, B=4798.42±0.03, and C=2885.27±0.01 MHz. These rotational constants are shown to be consistent with the reasonably proposed structural parameters for this molecule. The quantity Ia+Ib−Ic=6.487 U Å2 is shown to arise from the four out-of-plane hydrogen atoms and the heavy atom skeleton is, therefore, shown to be planar. The dipole moment was determined from the Stark effect to be ‖μa‖=‖μt‖=1.75±0.01 D. These results are compared to the corresponding quantities for similar molecules.
Time-dependent multiphoton absorption and optical Kerr effect in liquid CS2
Authors: Ho P. P. , Lu P. Y. , Alfano R. R.
Published: February 2003

Multiphoton absorption processes in liquid CS2 were observed when the power density of 1.05 μm ps laser pulse exceeded 4 GW/cm2. Laser pulse distortion was achieved involving the combination of multiphoton absorption and optical Kerr effect. These effects can affect the overall response kinetics of Kerr shutters for extracting temporal information on chemical systems.
Variation of chemical shielding with intermolecular interactions and rovibrational motion. VI. 19F in SF4 and COF2
Authors: Jameson Cynthia J. , Jameson A. Keith , Wille S.
Published: February 2003

The 19F nuclear resonance frequency in SF4 and COF2 molecules were measured as a function of density ρ and temperature T in the gas phase. In the virial expansion σ(T, ρ)=σ0(T)+σ1(T)ρ+..., the effect of intermolecular interactions on nuclear magnetic shielding is given by σ1(T) for low densities, and the average of the shielding over vibration and rotation is observed as σ0(T)–σ0(300 K). For SF4, the measurements are made in the fast-exchange limit, so that the average over the Berry-pseudorotation type exchange is observed. The average shielding terms are as follows: [σ0av(T)−σ0av(300 K)] =−4.332×10−3(T−300)−1.3465×10−5(T−300)2+1.623×10− 8 (T−300)3 ppm for COF2 and −1.297×10−2(T−300)−7.047×10−6 (T−300)2 ppm for SF4. σ1(T)=−(1.987±0.14)×10−2 +8.907×10−5 (T−300) −7.983×10−7 (T−300)2 ppm amagat for COF2 and −(8.35±1.5)×10−3 ppm/amagat for SF4. These functions are defined over the temperature range 230–380 K for COF2 and 300–380 K for SF4.
A single crystal EPR study of the 3ππ* benzaldehyde in benzoic acid host
Authors: Nagaoka Shin-ichi , Hirota Noboru
Published: February 2003

We have made a single crystal EPR study of the 3ππ* benzaldehyde (BA) in benzoic acid (BAC) host. It was shown that there are two different sites for BA in BAC host. The orientations of BA in these sites were determined from the analysis of the hyperfine structure (hfs). The detailed analysis of the hfs shows the following spin distributions. Site 1: ρ2=0.09, ρ4=0.33, ρ6=0.12, and ρ7≲0.03; and Site 2: ρ2 =0.09, ρ4=0.33, ρ6=0.09, and ρ7=0.06. Here positions 2 and 6 are ortho and 4 is para to the formyl group. Carbon 7 is the formyl carbon. A very small value of ρ7 is a striking difference to a very large ρ7 in the BA anion. We have made a UHF calculation of the spin densities. The results of the UHF calculation are in good agreements with the experimental results. It is shown that the spin distribution in the π* orbital in the 3ππ* benzaldehyde is very different from that in the 3nπ* state obtained from an INDO calculation.
Generalized isotope rules for molecular vibrations with application to A2B4 planar molecules
Authors: Wolfram T. , Asgharian A.
Published: February 2003

The Green’s function method for the analysis of molecular vibrations is employed to derive generalized isotope rules for the relationships between the harmonic frequencies of a molecule and those of its isotopically substituted derivatives. The theory is applied in detail to the isotopes of A2B4 planar molecules with D2h symmetry. The isotopes treated include A′2B′4, A2B′4, AA′B4, as-A2B2B′2, cis-A2B2B′2, trans-A2B2B′2, AA′B′4, and AB′3B, where the primes indicate isotope atoms. Numerical results are obtained for the isotopes of ethylene (C2H4) and compared with prior results.
Semiempirical formula for the electric field dependence of geminate ion recombination fluorescence
Authors: Baird J. K. , Bullot J. , Cordier P. , Gauthier M.
Published: February 2003

A semiempirical formula is derived which governs the electric field dependence of the fluorescence produced upon recombination of geminate ions. The formula is applied to some new data on the fluorescence which follows the ultraviolet irradiation of solutions of N,N,N′,N′-tetramethylparaphenylenediamine (TMPD) in tetramethylsilane and 2,2-dimethylbutane. From the two-parameter-least-squares lines which result from the application of the formula, we determine a number of derived quantities. Among these are Φip the quantum yield for photoionization of TMPD, and P(0,0) the probability of dissociation of the electron–ion pair produced by the photoionization. The values of P(0,0) are found to be nearly equal to the ratio Gfi/(Ggi+Gfi) where Gfi and Ggi are the free ion and geminate ion yields, respectively, obtained in 60Co γ irradiations of the pure solvents. The product of Φip and P(0,0) is Φe0, the free electron quantum yield. The values of Φe0 obtained from the fluorescence measurements are compared with those obtained from separate measurements of the photocurrent. Other quantities derived from the semiempirical formula include the first three moments of a function related to the distribution of electron–ion pair distances in the liquid.
Landau free energy form at the F point of the R3̄c structure
Authors: Hatch Dorian M. , Decker D. L.
Published: February 2003

A free energy form compatible with the F point of the trigonal D63d(R3̄c) symmetry is constructed. It is a function of the order parameter, elastic strain, and the stress. The resulting free energy exhibits a close formalistic analogy to the stressed cubic perovskite structures and their associated multicriticality.
A theory of ion binding and phase equilibria in charged lipid membranes. I. Proton binding
Authors: Copeland Bruce R. , Andersen Hans C.
Published: February 2003

A statistical mechanical theory is developed for the binding of protons to charged, acidic phospholipid membranes in contact with an aqueous solution, and the effects of such ion binding on phase transitions within a membrane are considered. Poisson–Boltzmann approximations are used to derive a law of mass action that contains an additional ionic strength dependent factor not found in the laws of mass action used by earlier workers. An equation is obtained for the equilibrium area of a membrane, and expressions are developed for a number of important thermodynamic properties, including the lipid chemical potential. These results are used to obtain equations for the membrane phase transition temperature. A correspondence between the lipid chemical potential and the chemical potential of protonated lipid is employed to explain the form of these equations. Calculated curves of transition temperature versus pH are presented and discussed for a number of different monovalent salt concentrations, and the results of the theory are compared with results of previous theories.
Model for the tricritical point of the chain-melting transition in a monomolecular layer of amphiphilic molecules
Authors: Firpo J. L. , Dupin J. J. , Albinet G , Baret J. F. , Caillé A.
Published: February 2003

A model for the tricritical point of the first order chain melting transition in a monomolecular layer of long chain amphiphilic molecules is presented. It is based on the two state model of chain melting where the surface pressure and intramolecular degrees of freedom enter through an effective internal field. Two order parameters are introduced: the degree of the intramolecular isomerization η and the nematic parameter Δ characterizing the anisotropic distribution of the orientation of the molecules on the plane of the interface. A coupling between these two order parameters is sufficient to transform, at a tricritical point, the high temperature second order nematic phase transition on Δ to a first order transition on Δ and η at lower temperature. Based on these results, an alternative allocation of the phases for the two-dimensional monomolecular system is proposed.
Surface tension and contact angle of a liquid–solid interface
Authors: Sullivan D. E.
Published: February 2003

The contact angle between a liquid film and a solid substrate is treated in the van der Waals model used previously by the author to discuss gas adsorption on solid surfaces. The model accounts for the occurrence of three types of wetting behavior, i.e., complete wetting, partial wetting, and nonwetting, as well as for transitions between these different regimes. According to this model, the contact angle depends on two parameters, namely the reduced temperature T/Tc, where Tc is the critical temperature of the liquid, and εw/kTc, where εw is the minimum in the liquid–solid interaction potential. For high-energy surfaces, corresponding to large εw/kTc, the contact angle decreases on increasing temperature; the opposite behavior is predicted to occur on sufficiently low-energy surfaces. With some hypotheses on how εw depends on the nature of the solid and liquid phases present, one can calculate the variation in contact angle with liquid species for a given solid at fixed temperature. The theory is compared with contact-angle results for n-alkanes on teflon surfaces; qualitative agreement is found. The contrasts between the present model and previous theories for the contact angle, in particular those of Good and Girafalco, Navascués and Berry, and Cahn, are discussed. Whereas Cahn predicts that the transition between different wetting classes is first-order in nature, we find that it is actually of second order.
ESR studies of 0−2 adsorbed on Ti supported surfaces: Analysis of motional dynamics
Authors: Shiotani M. , Moro G. , Freed J. H.
Published: February 2003

Temperature-dependent ESR spectra of O−2 adsorbed on Ti ions supported on porous Vycor glass were observed over the range 4.2 to 400 °K. These spectra were obtained under normal high vacuum conditions as well as under UHV conditions (P⩽10−9 Torr) and are very well resolved. It was observed that the line position of the g̃ tensor component that is perpendicular to the internuclear axis of O−2 remained constant with temperature, whereas the other two components of the g̃ tensor shift in position with temperature, and are accompanied by drastic line shape changes. This observation indicates that the molecular motion of O−2 on the surface is highly anisotropic, consisting essentially of planar rotation about the axis perpendicular to the internuclear axis of O−2 and parallel to the normal to the surface. Furthermore, the observation of nonequivalent 17O hfs of O−2 suggests that the internuclear axis of O−2 might be tilted slightly from the surface and/or one oxygen is closer to the Ti4+. The ESR line shapes were simulated for the different possible models: Brownian diffusion, jump diffusion (from weak jump to strong jump), approximate free diffusion, and discrete jump. It was found that the theoretical spectra calculated using the model of weak jump rotational diffusion best fit the observed spectra in the temperature range below 57.4 °K. However, in the temperature range above 57.4 °K, although the Brownian diffusion model seems the best among the models used, none of the present models used could successfully reproduce the observed line shapes. The rotational correlation time τR∥ was found to range between 10−5 sec (below 14.5 °K) and 10−9 sec (263 °K). The values of τR∥ depend strongly on the model used in the lower temperature range, but were essentially independent of model above 100 °K. The activation energy for rotational diffusion was estimated to be 0.5 kcal/mole above 100 °K. The line shape below 15 °K is independent of temperature, although the O−2 spectrum appears to exhibit residual motional effects. This observation suggests that coherent quantum mechanical motion is predominant below 15 °K. This matter is discussed in some detail, and the appropriate theory to investigate quantum effects on the motional dynamics is outlined including possible isotope effects on the motion. Spectral observation of possible interaction between C2H4 and O−2 on the surface is presented. Also discussed are the techniques for preparing samples with strong well-resolved signals and for removing the other types of O−2 signals, which do not show significant temperature-dependent spectral changes. The show.
Theory of rate processes at metal surfaces. II. The role of substrate electronic excitations
Authors: Metiu Horia , Gadzuk J. W.
Published: February 2003

A theoretical description of reactive molecular processes at metallic surfaces is formulated, following the Feschbach theory of nuclear reactions. Special attention is directed towards: (i) the role of substrate electron-hole pair excitations, both as a heat bath and as an entity permitting access to reaction channels not open in the absence of an excitation continuum; (ii) the importance of nuclear Franck–Condon factors; (iii) substrate induced quasiadiabatic electronic transitions on the incident atoms or molecules; and (iv) incorporation of substrate dynamic structure factors into the theory. Simple illustrative examples are presented which demonstrate the physical content of the formal theory and its relationship to various simplified models.
Propensity for preserving polarization in rotationally inelastic molecular collisions
Authors: Khare V. , Kouri D. J. , Hoffman D. K.
Published: February 2003

The CS approximation is employed to calculate scattering amplitudes for He–CO, Ne–HD, and He–H2 collisions.(AIP)
Ground state structures of sulfur dicyanide and its cation
Authors: So S. P.
Published: February 2003

A b initoio LCAO-MO SCF calculation on S(CN) and S(CN)2t have been carried out, using minimal STO-3G basis sets. The calculations predict a bent SCN linkage for S(CN)2 in its ground electronic state.(AIP)
On j
z-preserving propensities in molecular collisions. I. Quantal coupled states and classical impulsive approximations
Authors: Khare V. , Kouri D. J. , Hoffman D. K.
Published: February 2003

The occurrence of jz-preserving propensities in atom–linear molecule collisions is considered within the contexts of the quantum mechanical CS approximation and of a classical model collision system. The latter involves an impulsive interaction which is the extreme limit of the class of potentials for which the CS approximation is expected to be valid. The classical model results in exact conservation of jz along a ’’kinematic apse.’’ Quantum mechanically, the CS approximation is reformulated in a manner that clearly shows the relationship between the ℓ̄ choice and the degree and direction of jz preservation. Away from the forward direction, the simplest choice obeying time reversal symmetry ℓ̄=(ℓ+ℓ′)/2, is shown to result in a propensity for preserving jz along a ’’geometric apse’’ which coincides with the kinematic apse in the energy sudden limit, and for nonenergy sudden systems only differs significantly from it close to the forward direction.
Vibrational deactivation of oxygen ions in low velocity 02+(X2Πg,υ=1)+02(X 3Σg−,υ=0) collisions
Authors: Moran T. F. , McCann K. J. , Cobb M. , Borkman R. F. , Flannery M. R.
Published: February 2003

The deactivation of 02+(X 2Πg, υ=1) ions in collisions with 02(X 3Σg−,υ=0) molecules has been examined using multistate impact parameter eikonal and orbital treatments. Cross sections for the formation of various product states in the charge exchange and direct scattering channels have been computed for ions with 0.5 to 8.0 eV c.m. kinetic energies. The relative probabilities for forming products in given vibrational states at the higher kinetic energies are similar for the eikonal and orbital approaches. At energies below several eV it is necessary to employ the multistate orbital treatment which takes explicit account of the strong ion–molecule scattering. Cross sections for reaction channels leading to de-excitation and/or excitation of the product 02+(X 2Πg,v=1) ions have been computed for both charge exchange and direct scattering processes. The channels leading to vibrationally deactivated 02+(X 2Πg, v=0) product ions are strongly favored at low velocities over the excitation processes in the charge exchange as well as in the direct scattering channels.
Theoretical treatment of the X 1Σ+, A 1Σ+, and B 1Π states of LiH
Authors: Partridge Harry , Langhoff Stephen R.
Published: February 2003

A b initio self-consistent-field plus configuration-interaction calculations are reported for the X 1Σ+, A 1Σ+, and B1Π states of LiH using a 22σ12π7δ function Slater basis set. The resulting dissociation energies, with the experimental values in parentheses, are De (X 1Σ+)=19 972 (20 288) cm−1, De(A 1Σ+)=9042 (8682) cm−1, and De (B 1Π)=284 (288) cm−1. This is the first ab initio treatment to quantitatively account for the binding in the B 1Π state. Calculated dipole moments and electronic transition moments for the X 1Σ+–A 1Σ+, X 1Σ+–B 1Π, and A 1Σ+–B 1Π band systems are in excellent agreement with existing theoretical and experimental data. Radiative transition probabilities and lifetimes, including both the bound→bound and bound→free contributions, are computed for all vibrational levels of the A 1Σ+ and B 1Π states. Comparison with previous results using experimentally based potentials provides insight into the sensitivity of the radiative lifetimes to the detailed nature of the uppermost region of the potentials. Our calculated lifetimes for the lower vibrational levels of the A 1Σ+ state are within the experimental uncertainty. Our calculated lifetimes for the three vibrational levels of the B 1Π state are in excellent agreement with those of Zemke and Stwalley (values in parentheses), increasing with v′ from 11.3 (11.3) nsec at v′=0, to 17.0 (17.0) nsec at v′=1, and then to 23.5 (24.0) nsec at v′=2.
The electronic structure and dipole moment function of PbO (X 1Σ+)
Authors: Basch Harold , Stevens W. J. , Krauss M.
Published: February 2003

Valence multiconfiguration self-consistent-field (MC–SCF) calculations of the energy and dipole moment function curves of PbO (X 1Σ+) have been performed using effective core potentials (ECP) to replace the core electrons in the Hamiltonian. The present results are encouraging considering the small basis set and configuration list. This list is identical to that obtained in previous calculations on CO, and should be adequate for all other Group IV/VI molecules. The agreement of the calculated dipole moment at Re for PbO with the experimental value for the ground vibrational state is good. The shape of the dipole moment function for a molecule with a heavy atom has been obtained and this shape is seen to be comparable to that for CO.
Molecular structure of ArDF: An analysis of the bending mode in the rare gas–hydrogen halides
Authors: Keenan M. R. , Buxton L. W. , Campbell E. J. , Legon A. C. , Flygare W. H.
Published: February 2003

The rotational spectrum of the ArDF van der Waals molecule has been assigned using pulsed Fourier transform microwave spectroscopy in a Fabry-Perot cavity with a pulsed supersonic nozzle as the molecular source. The DF nuclear spin–nuclear spin and D nuclear quadrupole coupling constants are 12.5(11) and 194.7(20) kHz, respectively. These constants yield equivalent structural information and are used together with the rotational constant to obtain the structure of ArDF. The rotational spectrum of ArHF was also re-examined and the HF spin–spin constant found to be 49.0(19) kHz. The structure of ArHF was then obtained from the rotational and HF spin–spin constants. Finally, the vibrationally averaged angles in 14 complexes of the type X–H(D)Y (X=Ar, Kr, or Xe; Y=F, Cl, or Br) are calculated using a multipole potential and compared with experiment. The agreement between the observed and calculated angles is very good.
On the interpretation of 3d
N ions photoemission spectra. I. A comparative study for the valence band of FeO, CoO, and NiO
Authors: Grenet G. , Jugnet Y. , Duc Tran Minh , Kibler M.
Published: February 2003

This paper combines some aspects of the theory of level splitting and of the theory of photoemission intensity and thus provides a unified framework for a quantitative interpretation of the valence band photoemission spectra of 3dN ions in complexes and/or solids. The approach to the position of the photoemission peaks lies on a simplified LCAO–MO model, namely, the Koide–Pryce model involving ε2 terms. Two mechanisms are proposed for the intensity of the photopeaks : Both of them take configuration interaction into account but they differ in that one of them includes additional interference processes between the ejection of t2 and e photoelectrons and the other one does not. The formalism is applied to FeO, CoO, and NiO.
Infrared double resonance of SF6 with a tunable diode laser. II. Line shape analysis
Authors: Reiser C. , Steinfeld J. I. , Galbraith H. W.
Published: February 2003

Two- and three-level diode–infrared laser double resonance experiments in SF6 are modeled in terms of a superposition of noninteracting pairs of levels. The Mollow or Hänsch–Toschek line shape functions give a good representation of the observed spectra, with all features describable as either saturated or Stark-modulated transitions. No evidence for intramolecular coupling is seen at v3=1 of SF6.
A quantum-mechanical study of the collinear collision CO2+X (X=He, Ne, Ar)
Authors: Jolicard G. , Durand G. , Chapuisat X.
Published: February 2003

The collinear collision of CO2 with He, Ne, and Ar is studied. The effect of the intramolecular anharmonicity of CO2 on the vibrational excitation is investigated. Exact quantum-mechanical transition probabilities are calculated and compared with the results of an analytical theory based on the N-coupled harmonic oscillator model. Due to the strong intramolecular anharmonicity of CO2, the transition amplitudes which involve several really or virtually coupled intramolecular levels are deeply affected so that the harmonic theoretical results are of no use even as zero-order solutions. Each transition amplitude is shown to use a few channels only so that a strong interference effect results as a consequence of the slowly varying nature of the phases which perturb the levels.
State resolved rotational relaxation of CO in the free jet expansion of He–CO mixtures
Authors: Bassi D. , Boschetti A. , Marchetti S. , Scoles G. , Zen M.
Published: February 2003

We describe an infrared molecular beam spectrometer which uses a semiconductor diode laser and has been used to study the rotational relaxation of CO molecules in the free jet expansion of pure CO and a 90% He–10% CO mixture. The data for the mixture are analyzed by means of a kinematical method based on the use of the master equation and empirically scaled state to state rate constants. Information of the He–CO rotationally inelastic rate constants are obtained. The possibility of using free jet expansion to study rotational inelastic collisions is discussed.
Multiple transition states in unimolecular reactions: A transition state switching model. Application to the C4H8 +⋅ system
Authors: Chesnavich Walter J. , Bass Lewis , Su Timothy , Bowers Michael T.
Published: February 2003

A transition state switching model is developed for use in systems where more than one transition state occurs along the reaction coordinate. The model is cast in the perspective of both the unified statistical theory (UST) of Miller and of variational transition state theory. The basic assumptions are those common to transition state theory and RRKM–QET. A reaction branching analysis leads to reaction probabilities for a number of potential surfaces and appropriate expressions are delineated for both unimolecular and bimolecular reactions. The theory is developed from a microcanonical viewpoint and rigorously conserves both energy E and angular momentum J. Comparison is made with experimental data for the C4H8 +⋅ system where absolute unimolecular rate constants and branching ratios have been measured as a function of energy, bimolecular rate constants, and branching ratios measured at room temperature (the ethylene ion–molecule reaction), the lifetime of C4H8 +⋅ measured when formed by the ethylene ion–molecule reaction, and product kinetic energy distributions measured. The principal conclusions of the work are (i) a multiple transition state switching mechanism is required if the experimental data is to be adequetely fit by theory; (ii) at least two transition states (i.e., dividing surfaces at points of minimum flux along the reaction coordinate) naturally occur in unimolecular reactions, one a tight (i.e., configurational) TS occuring near the unimolecular reactant and the second an orbiting (i.e., centrifugal) TS occuring near the bimolecular products; (iii) tight transition states naturally occur at energies E‡ less than the threshold energy for reaction E0 when the system energy E is significantly greater than E0; (iv) the relative abundance of the three product channels C2H4 +⋅+C2H4, C3H5 ++CH3⋅, and C4H+7+H⋅ are strong functions of both energy and angular momentum in the reactant C4H8 +⋅ ion.
Photochemical hole burning: A means to observe high resolution optical structures in phycoerythrin
Authors: Friedrich J. , Scheer H. , Zickendraht-Wendelstadt B. , Haarer D.
Published: February 2003

We have investigated the lowest electronic transitions of phycoerythrobilin using laser-induced low temperature photochemistry. Narrow band irradiation into the absorption bands of the native chromophore leads to photochemical holes of the width of about 1–2 cm−1. We attribute the hole burning mechanism to reversible proton rearrangement processes. The high resolution of the experiments allows the resolution of well-defined substructures in the broad absorption bands which we interpret as being due to discrete vibrational states or due to a nonresonant energy transfer mechanism within the highly ordered chromophore–protein configuration.
Competition between dissociative and nondissociative decay of excited perdeutero-2,4-hexadiyne cations
Authors: Dannacher Josef , Stadelmann Jean-Pierre , Vogt Jürgen
Published: February 2003

The breakdown diagram of perdeutero-2,4-hexadiyne cation is reported and compared with the results on 2,4-hexadiyne cation, presented earlier. The competitive depletion of the first excited electronic state ? 2Eu by dissociative and nondissociative processes is quantified and related to the radiative decay ? 2Eu→? 2Eg reported in the literature. Combining the present data with the known lifetime of the excited state, the radiative and nonradiative rate constants are estimated for the lowest vibrational levels of the ? 2Eu state.
The electron spin resonance spectrum of the 2P1/2 state of atomic iodine
Authors: Lilenfeld H. V. , Richardson R. J. , Hovis F. E.
Published: February 2003

The F=3, mF=−3→F=2, mF=−2 ESR transition of I(2P1/2) was detected. Comparison of the experimental results at several transition frequencies with a theoretical calculation yielded a best-fit g value of 0.6654±0.004. Using these experimental results, the Einstein A coefficient for the 2P1/2→2P3/2 magnetic dipole transition was calculated to be 7.87 s−1.
Spectroscopic B
v constants for Rydberg states of D3
Authors: Carney Grady D.
Published: February 2003

Theoretical spectroscopic Bv values for several low lying levels of vibration in 14 Rydberg electronic states of D3 are reported. Addition of excitation increments δBV to the B0 (zero-point state) reported previously allowed calculation of Bv=B0+δBv for Rydberg states of D3. These are the first theoretical predictions for these quantities to become available.
Dissociation energies of molecules with very heavy atoms from mass spectrometry
Authors: Pitzer Kenneth S.
Published: February 2003

For Pb2 and BiLi, it is shown that relativistic effects yield large spin-orbit splittings and thus reduce the electronic partition function by significant amount. Since the partition function is required for dissociation energy calculations from mass spectrometric data, caution is advised in choosing the electronic partition function.
Response to the critique to the paper ‘‘The role of shallow traps on the mobility of electrons in liquid Ar, Kr, and Xe’’
Authors: Ascarelli G.
Published: February 2003

In response to the critique of the previous papers by Freeman and some of the comments by Schnidt et al., it is shown that the electric field dependence of the drift velocity is inconsistent with the hot electron model. The information obtained from the addition of polyatomic impurities to the liquified rare gases is inconsistent with a picture of an average electron energy that is a large fraction of 1 eV. In the following paper it will be shown that the field dependence of the trapping rate constant of electrons with SF6 and O2 can be quantitatively explained with no reference to hot electrons. (AIP)
Electric field dependence of the capture rate constant of electrons by SF6 and O2 dissolved in either liquid Ar or Xe
Authors: Ascarelli G.
Published: February 2003

It is shown that the electric field induced variation of the rate constant for the capture of electrons by SF6 and O2 dissolved in Ar and Xe can be quantitatively explained by considering a two step recombination: Initially, the electron is captured in an excited state and thereafter it decays to the ground state. The field depencence of the rate constant arises because the electric field changes the binding energy of the excited state, thus changing the probability of thermal ionization. (AIP)
A drift chamber study of the reaction ArH++H2 →H+3 + Ar and related reactions
Authors: Rakshit Asit B. , Warneck Peter
Published: February 2003

Ion–molecule reactions in argon– hydrogen mixtures were studied as a function of total and partial pressures, using a drift chamber mass spectrometer. The rate coefficient of the title reaction was found to depend on pressure, decreasing as the total pressure is increased from a value of 3.5×10−10 at low pressure to 3.5×10−11 cm3  molecule −1 s−1 at pressures greater than 0.8 Torr. This behavior is explained by the presence of vibrationally excited ArH+ ions formed in the reaction Ar+ + H2, and their deactivation by collisions with argon atoms. The thermalized ArH+ ions react with hydrogen more slowly than the excited ArH+ ions first formed. Rate coefficients for these processes and for several other ion reactions occurring in argon–hydrogen mixtures are given.
Rate constants for deactivation of N2(A) v′=0,1 by O2
Authors: Piper L. G. , Caledonia G. E. , Kennealy J. P.
Published: February 2003

The removal of N2(A 3Σu+, v′=0,1) by O2 has been studied in a room temperature discharge-flow apparatus by monitoring the temporal decay of the 0,6 and 1,10 bands of the Vegard–Kaplan system. Rate constants for the reaction with O2 are (2.3±0.4) and (4.1±0.7)×10−12 cm3 molecule−1 s−1 for v′=0 and 1, respectively. The rate constant measured for v′=0 is about 35% lower than commonly accepted values for the rate constant for quenching N2(A) by O2. This discrepancy is shown to result from the fact that most other studies used a tracer to monitor the N2(A) number density. The rate of decay of the tracer reflects both the decay of the v′=0 and v′=1 levels of N2(A) and its value is midway between the decay rates of the individual vibrational levels.
SCF and CI potential energy curves for H2–H+2 interactions
Authors: Borkman R. F. , Cobb M.
Published: February 2003

Potential energy curves have been computed using SCF and CI techniques for ground and excited state interactions of H2 with H+2. A total of seven planar and two nonplanar interaction geometries have been studied. In all cases the H2 and H+2 bond lengths were restricted to their equilibrium values, while the intermolecular H2–H+2 distance varied from 0.5 to 12 a.u. Ground state binding energies were found to range from 0.4 to 1.6 eV in the SCF calculations and from 0.8 to 1.9 eV in the complete single-and-double excitation CI calculations (SD–CI). In both SCF and SD–CI calculations, the most stable geometry was found to be the perpendicular orientation, with the H+2 molecule lying on a line bisecting the H2 bond axis. Potential curves were also computed for the charge-transfer excited state of H2–H+2, and these results are discussed in terms of their implications for the dynamics of charge exchange reactions.
Mechanism of the H+O3 reaction
Authors: Chen Maynard M. L. , Wetmore Ross W. , Schaefer Henry F.
Published: February 2003

The H+O3 reaction has played an important role in the evolution of modern chemical kinetics. Here certain aspects of the HO3 potential energy hypersurface have been investigated using nonempirical molecular electronic structure theory. For the qualitative purposes of the present study, most wave functions were of the self-consistent-field (SCF) variety, constructed from a double zeta basis set of contracted Gaussian functions. Two low energy pathways were established for the reaction. The first involves a coplanar transition state with a nearly linear H–O–O arrangement. The second possible mechanism allows the hydrogen atom to descend perpendicularly upon the ozone molecule. The two mechanisms are evaluated in light of the current experimental understanding of the H+O3 reaction.
Linear response theory of the heat of transport of electrolyte solutions: Electrostatic contributions
Authors: Kahana Paz Y. , Lin Jeong-long
Published: February 2003

The electrostatic contribution to the heat of transport of very dilute electrolyte solutions is determined from the linear response theory. The model considered is that of charged hard spheres immersed in a medium of solvent molecules which screen the Coulomb potential between ions by the dielectric constant D. Furthermore, we assume that the fluctuation in phase that determines the momentum of the ion is just the Brownian motion. The Brownian motion approximation simplifies the calculation and enables us to evaluate the time integral. The Brownian motion approximation also effectively decouples the various diffusion fluxes and thus the contributions of the cross coefficients to the heat of transport are not calculated in this treatment. The limiting slope for the heat of transport predicted in an earlier work by Helfand and Kirkwood [J. Chem. Phys. 32, 857 (1960)] agrees with ours in the limit where the hard sphere diameter σ→0.
On the extended Gibbs relations and nonlinear irreversible thermodynamics
Authors: Eu Byung Chan
Published: February 2003

The consequences of the extended Gibbs relation previously derived from the Boltzmann kinetic theory are further considered and various extensions of equilibrium thermodynamic relations are made into the realm of nonlinear irreversible thermodynamics. Especially, local and global stability theories are presented along with the stability criteria.
Ionic transport in nematic liquid crystals: A study of viscous and dielectric frictions
Authors: Hérino Roland
Published: February 2003

The ionic mobilities of different species have been determined in two nematic solvents, in order to specify the relative importance of the dielectric friction and of the viscous friction in the mechanisms limiting the mobility in a liquid. The determination of the mobilities of a series of ions of different size shows the Stokes’ law can account for the observed mobility changes with the ionic radius. The study of the temperature dependence of the drift mobility in both nematic and isotropic phases shows that no discontinuity is observed at the nematic to isotropic transition, and the activation energy of mobility is found to be equal to the activation energy of viscosity. All experimental results give evidence that the dielectric friction mechanism, as described by the Fuoss–Boyd–Zwanzig model, is not the predominant factor affecting the ionic mobilities. It is concluded that the viscous friction does play the leading part in the mechanisms limiting the mobility, and that the Stokes–Einstein model can be considered as a first satisfactory approach for the description of ionic mobility in the nematic solvents.
The mutual diffusion coefficient in a dilute hard sphere gas mixture
Authors: Eder O. J. , Lackner T.
Published: February 2003

The mutual diffusion coefficient is calculated in the ’’successive uncorrelated binary collision approximation’’ (SUBC approximation). It is shown that the ’’single collision approximation’’ agrees with the first Chapman–Enskog approximation and how higher approximations can be obtained by including more information on the collision processes. One of the long standing puzzles of mean free path theory—the fact that only collisions between unlike molecules contribute to mutual diffusion—is solved quite naturally within the SUBC approximation.
Hyperfine splitting of EPR spectra of paramagnetic centers created by gamma irradiation of cesium trihydrogen selenite
Authors: Gade S. , Johnson B. , Halvorsen A. , Knispel R.
Published: February 2003

The EPR spectra of gamma irradiated cesium trihydrogen selenite at 25 °C were analyzed to determine g and hyperfine tensor principal values and direction cosines. The observed spectra were from a single type of center with g11, g22, and g33 equal to 1.995, 2.031, and 2.007 (±0.002), respectively. Hyperfine splitting from 133Cs and from 77Se was observed with A11, A22, A33 values of 11.9, −7.7, −7.2 G and 263, ±85, ±100 G, respectively. The principal axes of g and the 77Se hyperfine tensor are nearly aligned with the special directions of one face of one of the SeO3 pyramids, indicating that the center is an SeO−2 radical.
Far infrared spectroscopy and empirical intermolecular potential for α- and γ-N2 under pressure
Authors: Fondère F. , Obriot J. , Marteau Ph. , Allavena M. , Chakroun H.
Published: February 2003

The far infrared spectrum of solid N2 has been investigated under pressure in both the α and γ phases. Measurements of pressure shifts and integrated intensities have been performed and the Grüneisen parameters are derived. From experiments carried out on a thick crystal sample it is deduced firstly that librational modes have no measurable intensities and, secondly, that the Tu translational mode has a structure. A series of semiempirical calculations has been performed. Emphasis has been placed on the derivation of a unique intermolecular potential valid both in the α and γ phases which allows calculating with a sufficient accuracy a large number of physical quantities, i.e., translational and librational frequencies, Grüneisen parameters, cohesive energy, elastic constants, dispersion curves, and frequency shifts under pressure.
ESR and ENDOR studies of radicals produced in hydroxyproline single crystals by x irradiation at low temperature
Authors: Nelson William H. , Nave Carl R.
Published: February 2003

X irradiation of hydroxyproline CH2CHOHCH2CHNH+2COO− at ∼15 K produces a complex ESR dominated by a two-line absorption resulting from the carboxyl anion. ENDOR shows the interaction to be from a nonexchangeable proton similar to those observed in other amino acids, with coupling constants A1=67.9±0.3 MHz, A2=58.9±0.2 MHz, and A3=56.3±0.3 MHz. The results indicate also that the anion is unprotonated at the carboxyl group. When the crystal is warmed to 77 K, the ESR rapidly transforms to one dominated by a three-line absorption, and further warming of the crystal to ∼125 K leads to rapid transformation of the ESR to one dominated by an eight-line pattern. Both absorptions are found to be the result of a radical formed by rupture of the Cα–N bond, a reaction similar to deamination in other amino acids. The transformation at 125 K is evidently the result of an irreversible conformation change. All interactions are found to result from nonexchangeable protons. At 77 K, Aα1=−89.6±0.2 MHz; Aα2=−55.3±0.5 MHz; Aα3=−26.6±0.4 MHz; Aβ1=86.3±0.3 MHz; Aβ2=74.6±0.3 MHz; Aβ3=73.2±0.4 MHz. At 125 K the couplings are Aα1=−88.9±0.7 MHz; Aα2=−55.2±1.3 MHz; Aα3=−25.4±1.3 MHz; Aβ11=130.5±0.4 MHz; Aβ21=120.9±0.4 MHz; Aβ31=118.0±0.3 MHz; Aβ12=29.0±0.3 MHz; Aβ22=18.2±0.4 MHz; Aβ32=17.1±0.4 MHz.
Rotation–vibration CARS spectra of the 2ν2 mode in CO2 gas
Authors: Roland C. M. , Steele William A.
Published: February 2003

Polarized and depolarized CARS band shapes are reported for the 2ν2 band of the Fermi resonance diad in room temperature CO2 gas at a series of densities. Analysis of the spectra yields vibrational relaxation times and the shift of the band center with density. Comparison of the depolarized spectra with the J diffusion model for reorientation shows that there is good agreement. Effective collision frequencies are obtained for various densities. The frequency shifts, the inverse vibrational relaxation time and the collision frequency are all linear functions of density except at the highest densities. Binary collision cross sections are calculated for angular momentum and for vibrational relaxation. The advantages of these and other CARS measurements relative to more conventional spectroscopies are briefly discussed.
The ESR spectra of NSO2−4 in (NH4)2SO4
Authors: Bailey C. Edward
Published: February 2003

The slow decay of NH+3 and NH+2 centers in ammonium sulfate produce a new, very stable, paramagnetic center that has been identified as NSO2−4. This center has g values of gx=2.012, gy=2.011, and gz=2.007 with the corresponding hfi constants Ax= Ay=0.0 and Az=25.2 G. No change in this center is observed on passage of the crystal through the ferroelectric phase transition.