bluuebunny/crossref_metadata_2025 huggingface dataset:
Laser fluorescence state selected and detected molecular beam magnetic resonance in I2
Authors: Yokozeki Akimichi , Muenter J. S.
Published: February 2003

A molecular beam magnetic resonance experiment, using laser fluorescence for state selection and beam detection, has been performed on I2. Twenty ΔF=±1 hyperfine transitions were observed in the v=0, J=13 level of the ground electronic state. The transitions had a FWHM linewidth of 5–15 kHz in the 100–400 MHz region and were observed in zero external field. eqQ, spin rotation, tensor spin–spin, and scalar spin–spin interactions of −2 452 583.7(16), 3.162(8), 1.58(5), and 3.66(3) kHz, respectively, were obtained. Using these X 1Σ state constants, the precise optical data of Hackel, Casleton, Kukolich, and Ezekiel [Phys. Rev. Lett. 35, 568 (1975)] were reanalyzed to obtained hyperfine properties of the v=43, J=12 B 3Π0 electronic state. In the same order as above, these constants are −558 669(8), 190.13(12), −100.2(7), and 0.2(4) kHz. These values for both the X and B states differ significantly from those previously reported. Neither magnetic octupole nor electric hexadecupole interactions were required, however, upper limits for these interactions are established for both electronic states.
On the semiclassical theory of collision-induced vibrational–rotational transitions in molecules
Authors: Miklavc Adolf
Published: February 2003

The ideas which were introduced in our recent theoretical work on collision-induced vibrational–rotational transitions are further developed here so that the resulting theory can be applied without any restrictions on the type of molecule. Moreover, it is shown that the original theory, in which only straight-line trajectories were considered, can be extended to include other trajectories as well. A minor error occuring in the original work is also corrected. As an application, the dependence of the vibrational transition rates of methyl halides upon the mass of the collision partner is studied.
Zeroth order hyperharmonic series in the discrete spectrum of electrically charged particles
Authors: Demiralp Metin
Published: February 2003

For a system of electrically charged particles, the time-independent Schrödinger’s equation written in hyperspherical coordinates is extended to a more general equation by inserting a dummy parameter which will be equated to unity finally. This last equation is reduced to a formal algebraic equation, separating the Hilbert spaces on hyper-radial and hyperangular coordinates into convenient subspaces. By expanding this algebraic equation into the powers of the dummy parameter mentioned above, a parameter related to the total energy of the system is obtained in terms of a serial representation. As an application some approximate numerical evaluations have been done for certain systems of three particles.
Ultrasonic absorption in aqueous binary mixtures. II. p-Dioxan–water at 11° and 25 °C
Authors: Atkinson G. , Rajagopalan S. , Atkinson B. L.
Published: February 2003

Ultrasonic absorption has been measured in the p-dioxane/water system at 25° and 11 °C at nine concentrations over the frequency range 0.3–630 MHz.It is shown that the frequency dependence can be fitted quantitatively by the ’’fluctuation’’ theory of Romanov and Solovev if NMR determined diffusion data are available. The experimentally determined amplitude of absorption agrees very well with that calculated using experimental thermodynamic data and the R–S theory. The ultrasonic relaxation times show the same concentration dependence as the high frequency dielectric relaxation measured by Garg and Smyth. The discussion considers the ultrasonic fluctuations as a dynamic analog of the clathrate hydrate forming proclivities of such solutes.
An irreducible tensor method of deriving the long-range anisotropic interactions between molecules of arbitrary symmetry
Authors: Leavitt Richard P.
Published: February 2003

The long-range, anisotropic interaction energy between two molecules of arbitrary symmetry is derived to second order in perturbation theory by using irreducible tensor methods. The resultant formulas can be used to express the interaction energy to any desired power of R, the separation of the two molecules. Group-theoretical arguments are used to reduce the number of independent parameters in the theory and to determine which parameters are nonzero in particular molecular point groups. The theory is illustrated by being applied to the case of two linear molecules, and the interaction energy is given explicitly for linear molecules up to R−5 for the electrostatic (first-order) interactions and to R−7 for the induction and dispersion (second-order) interactions. Certain discrepancies in the published literature concerning the induction and dispersion interactions of two linear molecules are clarified.
The structure of ionic clusters: Thermodynamic functions, energy surfaces, and SIMS
Authors: Martin T. P.
Published: February 2003

The Helmholz free energy of several stable NaCl clusters was calculated. It is shown that the relative stability of clusters of the same size can change with temperature. For (NaCl)4, the cubic configuration is preferred below 500 K, the eight-ring configuration above. The energy surface was examined in order to determine the energy barrier separating stable configurations and thereby the time needed to establish thermodynamic equilibrium. In one case, (CaF2)5, the barrier is so low as to permit tunneling from one configuration to the other. The relative stability of clusters having the composition NanCl+n-1 was calculated and the results are compared with a recent SIMS investigation. Na5Cl4+ is found to be a particularly stable charged cluster.
An efficient approach to CI: General matrix element formulas for spin-coupled particle–hole excitations
Authors: Tavan Paul , Schulten Klaus
Published: February 2003

A new, efficient algorithm for the evaluation of the matrix elements of the CI Hamiltonian in the basis of spin-coupled ν-fold excitations (over orthonormal orbitals) is developed for even electron systems. For this purpose we construct an orthonormal, spin-adapted CI basis in the framework of second quantization. As a prerequisite, spin and space parts of the fermion operators have to be separated; this makes it possible to introduce the representation theory of the permutation group. The ν-fold excitation operators are Serber spin-coupled products of particle–hole excitations. This construction is also designed for CI calculations from multireference (open-shell) states. The 2N-electron Hamiltonian is expanded in terms of spin-coupled particle–hole operators which map any ν-fold excitation on ν-, and ν±1-, and ν±2-fold excitations. For the calculation of the CI matrix this leaves one with only the evaluation of overlap matrix elements between spin-coupled excitations. This leads to a set of ten general matrix element formulas which contain Serber representation matrices of the permutation group Sν×Sν as parameters. Because of the Serber structure of the CI basis these group-theoretical parameters are kept to a minimum such that they can be stored readily in the central memory of a computer for ν?4 and even for higher excitations. As the computational effort required to obtain the CI matrix elements from the general formulas is very small, the algorithm presented appears to constitute for even electron systems a promising alternative to existing CI methods for multiply excited configurations, e.g., the unitary group approach. Our method makes possible the adaptation of spatial symmetries and the selection of any subset of configurations. The algorithm has been implemented in a computer program and tested extensively for ν?4 and singlet ground and excited states.
Rayleigh–Bénard instability in reactive binary fluids
Authors: Gutkowicz-Krusin Dina , Ross John
Published: February 2003

We study the hydrodynamic stability of reactive, binary fluids in the Bénard geometry, and the Boussinesq approximation, with both the Soret and Dufour effects included. The boundary conditions are rigid, perfectly conducting walls impermeable to mass flow, conditions which are experimentally approachable. For fast reactions, the reactive fluid is less stable against convection than the one-component fluid with the same values of transport and thermodynamic coefficients, due in part to the presence of a ’’chemical boundary layer’’. A slow reaction stabilizes the fluid if the heat of reaction is large and the fluctuations in temperature and concentration relax on the same time scale (gases). When the separation of these two time scales is large (liquids), reaction is always destabilizing. The influence of the Soret and Dufour effects on stability is most pronounced for slow reactions. We provide some interpretations of the role of chemical reactions on hydrodynamic stability.
Direct inversion method for obtaining anisotropic potentials from rotationally inelastic and elastic cross sections
Authors: Gerber R. B. , Buch V. , Buck U.
Published: February 2003

A method is given for the direct inversion of rotationally elastic and inelastic differential scattering cross sections, to yield the underlying intermolecular anisotropic potential surface. The scheme presented applies to molecules of large rotational spacings, for which the inelastic transitions are relatively weak. Semiclassical considerations and the exponential distorted-wave approximation are used in developing the method. Employed also is a peeling transform that, in effect, determines the interaction at each distance range from a different angular range of the data. The method is tested by application to simulated data, calculated for Ne+D2 from a known potential at 0.07 eV collision energy. The inversion procedure recovered the anisotropic part of the interaction to within a few percent error.
Diffusion between cells with semipermeable walls
Authors: Weber Thomas A.
Published: February 2003

A model is presented for the diffusion of noninteracting particles in a lattice of cubic cells with semipermeable walls. This model describes the type A Fickian diffusion of low concentrations of small molecules in molecular sieves or polymer cavities. Exact formulas are presented for the average mean-square displacement as a function of time and for the diffusion constant. The jump frequency of the jump diffusional model is related to the probability of transmission through the cell walls.
L
M
M Auger spectra of selenium and some of its compounds
Authors: Bahl M. K. , Watson R. L. , Irgolic K. J.
Published: February 2003

The energies of the 3s, 3p3/2,1/2, and 3d photopeaks, and of the L2,3 MM Auger peaks have been measured for selenium metal and some of its compounds. The associated relaxation energies were computed by two different methods; first by comparing the measured binding energies with those calculated theoretically employing Koopmans theorem, and second, by a method devised by Shirley and co-workers. The merits of these two methods are examined by comparing measured LMM and LMN Auger energies with those calculated using the predicted relaxation energies. The relative intensities of various Auger multiplets are compared with theoretical values for L–S and intermediate coupling of the final hole states.
Vibrational excitations in soft x-ray emission and core ESCA spectra of NH3
Authors: Ågren H. , Müller J. , Nordgren J.
Published: February 2003

A b initio calculations are performed for the energy surfaces of the ground N ls hole and 3a1 hole states of ammonia in order to investigate the vibrational band profiles in its soft x-ray emission and core ESCA spectra. Both the N ls and 3a1 hole states are found to be planar. In comparison with the ground state, the N–H bond length is significantly shortened for the core hole state but relatively unchanged for the 3a1 hole state. The results lead to a core ESCA spectrum dominated by a progression of closely spaced vibrational peaks of the pure bending mode and to a 3a1 x-ray spectrum with excited stretch–bend combination bands.
The surface photovoltage of polymethine semiconducting films
Authors: Musser M. E. , Dahlberg S. C.
Published: February 2003

Recent solar cell work on Al/merocyanine/Ag sandwich devices has prompted the examination of the surface photovoltage of thin films (∼500 Å) of merocyanine and of four other closely related polymethine type dyes deposited on Ag rear contacts. All of these materials exhibited space charge depletion layers at the front surface with energy band bending changes resulting from a laser pulse ranging from 1 to 100 mV, and corresponding photovoltaic relaxation time constants on the order of 30 msec. The spectral dependence of the observed photovoltage was dominated by the nominally ’’forbidden’’ S0→T1 transition in the near IR. Much less photovoltage is observed at wavelengths corresponding to the allowed S0→S1 transition, even though light of this photon energy is strongly absorbed. This discrepancy between the relative intensity in the absorption and photovoltage spectra could be due to decreased dissociation of the bound excitons, and/or to increased recombination of the free carriers at the organic semiconductor free surface.
Radiative lifetimes and collisional deactivation rate constants of excited Ne(2p 5 3p) states
Authors: Chang R. S. F. , Setser D. W.
Published: February 2003

The radiative decay and collisional deactivation of eight of the ten neutral atomic neon (2p 5 3p) levels have been studied. Neon metastable atoms were generated by a hollow-cathode discharge in a flowing afterglow apparatus and were optically pumped by a pulsed tunable dye laser to selected p states; the decay rates then were determined as a function of neon pressure from 1–8 Torr by analyzing the decay of spontaneous emission from the excited level. The radiative lifetimes of these eight p states have been measured with an uncertainty of ±3%. These lifetimes were combined with the radiative branching ratios from a given p level to assign absolute transition probabilities for the Ne(3p–3s) transitions. Two-body deactivation of the Ne(3p) states by collisions with ground state neon atoms have rate constants in the range of 1–5×10−11 cm3 atom−1 sec−1. Intramultiplet relaxation is the dominant quenching mechanism for the p3, p4, p5, p6, p7, and p8 levels. However, for p9 intermultiplet and intramultiplet relaxation are competitive and for p2 and p10 intermultiplet relaxation is dominant. The relaxation mechanism of these excited Ne(3p) states is discussed.
Single quantum vibrational energy transfer from HCl (v=2) and HBr(v=2)
Authors: Dasch Cameron J. , Moore C. Bradley
Published: February 2003

HCl (v=2) and HBr (v=2) have been directly excited with a pulsed optical parametric oscillator at 295 K. The relaxation in hydrogen halide mixtures proceeds largely by single quantum V–V exchange. Measured total relaxation rates include HCl→HCl 9.5±0.7; HCl→HBr 9.3±0.6; HCl→DCl 1.3±0.2; HBr→HBr 6.2±0.5; and HBr→HCl (1.0±0.2) ×104 Torr−1 s−1. The two quantum exchange rate for HCl (v=2)+ HBr (v=0) is <1×103 Torr−1 s−1. Upper limits on the V–R, T self-relaxation rates of HCl (v=2) and HBr (v=2) are 2× and 1×104 Torr−1 s−1.
The vibrational relaxation of O3 by He, D2 and H2
Authors: Moy Jerry , Mao Chung-Rei , Gordon Robert J.
Published: February 2003

The rate constants for the vibrational relaxation of O3 by He, D2, n-H2, and p-H2 were measured over the temperature range 164 to 413 K. Visible chemiluminescence from the laser enhanced reaction NO+O3(001) was used to monitor the decay of vibrationally excited ozone. D2 was found to relax O3 more efficiently than He, with the ratio of relaxation probabilities PD2/PHe varying inversely with temperature. n-H2 relaxed O3 more efficiently than p-H2, with Pn-H2/Pp-H2 increasing with temperature. Also Pp-H2/PD2 was found to be insensitive to temperature, having a value of 4±1. The data are compared with a semiclassical model which takes into account both VT and VR interactions. The variation of PD2/PHe appears to be due to D2 rotation. The behavior of Pn-H2/Pp-H2 can be explained by a near-resonant VR transition occurring only with o-H2. The Pp-H2/PD2 ratio appears to be simply a mass effect, and can be accounted for with a VT model.
Studies of the reorientational relaxation of pyridine in water by depolarized Rayleigh light scattering
Authors: Wang C. H. , Whittenburg S. L. , Lund P.-A. , Christensen D. H.
Published: February 2003

The depolarized Rayleigh spectra of aqueous solutions of pyridine have been studied using a high-finesse Fabry–Perot interferometer as a function of temperature and concentration. The Rayleigh relaxation times are found to have a complex concentration and viscosity dependence. The classical Stokes–Einstein–Debye equation for molecular reorientation breaks down in this system. The Rayleigh relaxation time of pyridine molecules is not determined by the macroscopic shear viscosity of the solution. The specific interaction due to the formation of hydrogen bonds between pyridine and water molecules plays a very important role in affecting the relaxation time. At a fixed temperature the plot of πray/η versus pyridine concentration shows two maxima at low and high pyridine concentrations. The low concentration maximum is due to the incorporation of pyridine molecules in the water network structure and the high concentration maximum is associated with the formation of individual pyridine–water complexes. The activation energy for the reorientation of pyridine molecules depends on the pyridine concentration. At low pyridine concentration the activation energy corresponds well to the N⋅⋅⋅H–O hydrogen bonding energy. Above 70% volume the activation energy decreases with increasing pyridine concentration, and above this concentration range the reorientational relaxation time becomes less structure controlled.
A condition on the derivative of the potential in the primitive model of an electric double layer
Authors: Blum Lesser , Lebowitz Joel L. , Henderson Douglas
Published: February 2003

It is shown that the potential difference in a primitive model of an an electric double layer must increase as the surface change density on the electrode is increased. (AIP)
Effect of fluctuations in internal coordinates on dimensions of macromolecules
Authors: Mansfield Marc L.
Published: February 2003

The effect of fluctuations in the internal coordinates (bond lengths, bond angles, and bond rotational angles) on the end-to-end length of a macromolecule in a fixed rotational isomeric state is considered. Equations are derived for the first and second moments of the deviations in the end-to-end distance and the end-to-end vector. Approximations are then made for long chains that express these moments in terms of the relative position of the center of mass and chain ends, the radius of gyration, the end-to-end length, and the moment of inertia about the end-to-end axis. The distribution functions of both the end-to-end length and end-to-end vector are examined. The uncertainty in the position of the end of the chain can be a substantial portion of the end-to-end distance. Implications for chain dynamics are also discussed.
Valence bond model potential energy surface for H4
Authors: Silver David M. , Brown Nancy J.
Published: February 2003

Potential energy surfaces for the H4 system are derived using the valence bond procedure. An ab initio evaluation of the valence bond energy expression is described and some of its numerical properties are given. Next, four semiempirical evaluations of the valence bond energy are defined and parametrized to yield reasonable agreement with various ab initio calculations of H4 energies. Characteristics of these four H4 surfaces are described by means of tabulated energy minima and equipotential contour maps for selected geometrical arrangements of the four nuclei.
The reactivity dependence of the recombination probability
Authors: Pedersen J. Boiden
Published: February 2003

An exact relation for the reactivity dependence of the recombination probability in the presence of a scavenger is derived. For a spherical symmetric system with a uniformly distributed scavenger this relation expresses the partially diffusion controlled recombination probability in terms of the totally diffusion controlled recombination probability. The relation is valid for a space-dependent diffusion coefficient and for an arbitrary potential energy between the particles. The relation is proved only for a three-dimensional system, but it appears to be valid also for one- and two-dimensional systems.
Theory of hydrogen bonded chains in bioenergetics
Authors: Nagle J. F. , Mille M. , Morowitz H. J.
Published: February 2003

Three thermodynamic cycles in which hydrogen bonded chains play a central role are presented as models for biological energy transduction. These include (i) a primitive, highly irreversible proton pump, (ii) a simple reversible chemical to electrical energy transducer and (iii) a reversible molecular engine which transduces pH differences to mechanical energy. A careful analysis is given to the free energy diagrams which reveal that the rate limiting step of the proton conduction is similar in the three cases. The limiting rate is essentially the inverse of the equilibration time for a localized proton to delocalize over a free energy barrier. Using basic hopping rates obtained from studies in ice, the kinetic equations are set up and solved by three methods for the simplest case of first passage of the proton. The agreement of the exact solution with a computerized difference equation approximation lends confidence to calculations using the latter method for more complicated cases where the exact method is intractable. For free energy barriers of 250–450 mV (6–10 kcal), equilibration times are rapid (μsec–msec) on a biological time scale. The kinetics of the complete cycles are also investigated and shown to be in agreement with the preceding calculation. For steady state linear transport and the special case when the transport of a single defect type is rate limiting, our kinetic results reduce to those obtained for single-file transport.
Non-Markovian effects on vibrational energy relaxation in liquids
Authors: Abbott Richard J. , Oxtoby David W.
Published: February 2003

We consider the non-Markovian effects on vibrational relaxation which can occur when the bath relaxation rate is comparable to or slower than the relaxation of the vibrational degrees of freedom. We carry out quantum mechanical simulations of a two level system strongly coupled to a bath of known statistical properties (either a Poisson or a Gaussian process) and show that the relaxation is surprisingly insensitive to the nature of the bath. Small oscillations in the relaxation are found for the Poisson simulation which maybe observable in high resolution picosecond experiments. We investigate both cumulant expansion and memory function descriptions of relaxation in the non-Markovian limit and show that neither approach provides quantitatively accurate results.
Rigorous and approximate relations between expectation values of atoms
Authors: Tal Yoram , Levy Mel
Published: February 2003

The Z dependence of some expectation values of local operators, such as rα and ρα−1, is derived within the Thomas–Fermi model for atoms. It is shown to have the general form F (α) =A (α) Zβ(α), where β (α) =aα+b. This equation leads to further relations among the expectation values and the total energy. The parameters A (α) and β (α) are also treated variationally to get the best agreement with the Hartree–Fock expectation values and it is shown that for a certain range of α (depending on the operator) the agreement is quite good. Further, some inequalities relating the aforementioned expectation values to ρ (0), the charge density at the nucleus, are developed. Finally, the N dependence of the Z−1 expansion coefficients is considered and a conjecture by March and White [N.H. March and R.J. White, J. Phys. B 5, 466 (1972)] concerning their asymptotic behavior is proved for ε1(N).
Reflection and absorption spectra of singlet charge transfer excitons in anthracene–PMDA crystals
Authors: Brillante Aldo , Philpott Michael R.
Published: February 2003

The polarized reflection spectra of single crystals of the weak charge transfer (CT) complex anthracene–pyromellitic dianhydride (A–PMDA) have been measured at 2 K. The oscillator strengths and energies of the singlet charge transfer transitions have been determined from the frequency dependent dielectric function derived from the reflection spectrum by means of a Kramers–Kronig transformation. A theory of exciton–phonon coupling appropriate for CT transitions in a mixed stack system is briefly described and used to interpret the derived absorption spectrum. The absorption spectrum consists of a purely electronic zero phonon line (ZPL) and a set of zero lattice phonon lines (ZLPL) corresponding to the excitation of high frequency intramolecular modes of the ions. Each ZLPL, like the ZPL, acts as the origin for several librational modes and for a progression in a lattice mode of approximately 30 cm−1. Exciton–phonon coupling is weakest for the vibrationless ZPL and increases with the energy of the intramolecular vibrational mode of the corresponding ZLPL. Approximate values for the exciton–phonon coupling parameter S are found for the ZPL and each ZLPL using the local excited state model.
Effective intramolecular potentials for molecular bromine in argon. Comparison of theory with simulation
Authors: Pratt Lawrence R. , Chandler David
Published: February 2003

The results of our theory for the effects of condensed phase environments on the intramolecular structure of nonrigid molecules are compared with recent Monte Carlo computer simulations of the effective vibrational potential of the diatomic bromine molecule in liquid argon. To implement our theory we use our previously developed results for hard sphere cavity distribution functions plus a new mean field approximation for the effects of attractive forces on intramolecular structure. The agreement between theory and experiment is good.
Electron attachment to chlorofluoromethanes using the electron-swarm method
Authors: McCorkle D. L. , Christodoulides A. A. , Christophorou L. G. , Szamrej I.
Published: February 2003

Electron attachment rate constants, as a function of pressure-reduced electric field, were measured in mixtures with nitrogen for CCl3F, CCl2F2, and CClF3 using the electron swarm method. From these data total electron attachment cross sections σa(ε) as a function of electron energy ε were determined for the chlorofluoromethanes using the electron-swarm unfolding technique and a new set of electron energy distribution functions for N2. For CCl3F and CCl2F2 three maxima in σa(ε) were found at thermal energy, 0.25 and 0.75 eV and at 0.07, 0.30, and 0.93 eV, respectively. Only one pronounced maximum in σa(ε) for CClF3 was observed at 1.4 eV. The thermal values of the attachment rate constant for CCl3F, CCl2F2, and CClF3 are, respectively, 3.90×108, 4.00×107, and 6.05×103 sec−1 Torr−1. Additionally, comparisons are made between the σa(ε) calculated for these and other molecules using both the new electron energy distribution functions for N2 and those determined earlier. Substituent effects on the number and position of negative ion states and on the magnitude of σa(ε) for the chlorofluoromethanes are discussed.
Electronic energy transport in substitutionally disordered molecular crystals
Authors: Godzik Klaus , Jortner Joshua
Published: February 2003

In this paper we present a theoretical study of incoherent electronic energy transfer (EET) in an impurity band of substitutionally disordered, mixed, molecular crystals. Dispersive diffusion of the electronic excitation was treated by an Average Dyson Equation Approximation (ADEA) to the master equation for EET. The ADEA rests on expressing the Green’s function (GF) for the probability of site-excitations in a single fixed spatial configuration in terms of a Dyson equation with a normalized vertex function and subsequently performing the configurational average of the GF, invoking a decoupling approximation which omits many-site contributions. Explicit expressions were derived for the initial site occupation probability, Po(t), the mean square displacement, Σ2(t), and the time dependent diffusion coefficient D(t). We have explored the relation between the ADEA and the stochastic continuous time random walk (CTRW) model applied by us for EET. We have demonstrated that the ADEA and the CTRW results for Σ2(t) and for D(t) are identical, while Po(t) has a similar structure in both schemes. The ADEA/CTRW scheme was utilized to derive analytical results for Po(t), Σ2(t) and D(t) in an impurity band where the pair-probability of EET is determined by multipolar interactions. From the analysis of asymptotic expansions for D(t) in such cases we conclude that the short time diffusion coefficient has the time dependence D(t)∝t(5−n)/n while the long time diffusion coefficient is independent of t, corresponding to an average superlattice of impurities. Numerical calculations based on the ADEA/CTRW scheme were performed to elucidate the quantitative features of the crossover from dispersive transport to pure diffusive behavior and how it is affected by the impurity concentration and by the range of the multipolar interactions. For short range high order multipolar interactions the effects of dispersive energy transport are expected to prevail over a broad concentration and time domain, being more pronounced than in the conventional case of dipole–dipole coupling.
Diffractive scattering of H atoms from the (001) surface of LiF at 78 K
Authors: Caracciolo G. , Iannotta S. , Scoles G. , Valbusa U.
Published: February 2003

We have built an apparatus for the measurement of high resolution diffractive scattering of hydrogen atoms from crystal surfaces. The apparatus comprises a hydrogen atom beam source, a hexapolar magnetic field velocity selector, a variable temperature UHV crystal manipulator, and a rotatable bolometer detector. The diffraction pattern of a beam of hydrogen atoms scattered by a (001) LiF surface at 78 K has been obtained for different angles of incidence and different orientations of the crystal. The Debye–Waller factor has been measured leading to a surface Debye temperature ϑS=550±38 K. The corrugated-hard-wall-with-a-well model of Garibaldi et al. [Surf. Sci. 48, 649 (1975)] has been used for the interpretation of the intensities of the diffracted peaks. By means of a best fit procedure we obtain a main ’’corrugation’’ parameter ξ0=0.095 Å. By comparison of the data with the theory of Cabrera et al. [Surf. Sci. 19, 70 (1967] at the first order, the strength parameters of a periodic Morse potential have been determined.
Lithium [Li*(2 2P
J)] fine structure transitions and quenching induced by collisions with molecular hydrogen [H2, D2]
Authors: Elward-Berry Julianne , Berry Michael J.
Published: February 2003

Dye laser excitation and interferometric fluorescence analysis techniques were used to measure collision induced quenching and fine structure transition rate coefficients in the 7Li*(2 2PJ)+H2,D2 systems at a temperature of 564 K. The quenching and fine structure transition energy transfer processes all occur at ’’gas kinetic’’ rates, but the magnitudes of cross sections for each process are determined by different electronically nonadiabatic interactions at different ranges of internuclear separation. Fine structure transition cross sections are determined by the forms of long range interaction potentials, which are undoubtedly very similar to those of the isoelectronic Li*(2 2PJ)+He system. Quenching cross sections are determined by short-to-intermediate range interaction potentials, some of which may effectively be coupled by fine structure transitions. Simple dynamical models provide limited insight into these rate processes, suggesting that ab initio potential hypersurface calculations plus refined quantal and/or semiclassical dynamical computations using these hypersurfaces are required for proper treatment of the Li*(2 2PJ)+ H2,D2 systems.
Homogeneous vapor phase nucleation of a large fatty acid: Stearic acid
Authors: Becker Carol
Published: February 2003

Homogeneous nucleation measurements are made on stearic acid vapor using a diffusion cloud chamber and the resulting critical supersaturations compared with the predictions of the classical Becker-Doering theory. These measurements are difficult because of the unusually low pressures and high temperatures required to investigate the involatile stearic acid molecule. The results indicate a definite discrepancy between theory and experiment. Measured critical supersaturations lie 27% above the theory at the lowest temperature and 10% below the theory at the highest temperature. Calculations show that part of this increase in experimental critical supersaturation at the lower temperatures could be due to association in the vapor. There are two additional findings: (1) The usual top plate boundary condition must be corrected for molecules, such as stearic acid, which have low equilibrium vapor pressures in the temperature range of interest. (2) The growth times for stearic acid droplets are at the limits that can be tolerated in a feasible diffusion cloud chamber experiment. This study suggests an area for future investigation. Once the association equilibrium constant for stearic acid vapor is known, the data presented here can be reanalyzed and the extent to which the dimer stabilizes the supersaturated vapor determined.
Theory of polarized fluorescent emission in uniaxial liquid crystals
Authors: Szabo Attila
Published: February 2003

The theory for the fluorescence emission anisotropy [r(t)] of a cylindrical probe in a macroscopically aligned uniaxial liquid crystal (e.g., an oriented membrane) is developed for the situation where the equilibrium orientational distribution of the probes is not random. Expressions are derived for r(0) and r(∞) involving equilibrium averages for the general case where the emission and absorption dipoles and the major axis of the probe are not parallel. The rotational motion of the probe is described as diffusion in a potential which is consistent with the equilibrium distribution (i.e., using the Smoluchowski equation). Both the short time behavior and the time integral of r(t) are explicitly expressed in terms of equilibrium averages. It is shown how these limits can be used to construct useful approximations for the time dependence of r(t), and how the diffusion constant can be obtained for the short time behavior of r(t).
OH(A 2Σ+→X 2Πi) yield from H2O photodissociation in 1050–1370 Å
Authors: Lee L. C.
Published: February 2003

The cross section and the quantum yield for the production of the OH(A 2Σ+→X 2Πi) emission from H2O photodissociation were measured, using synchrotron radiation in the 1050–1370 Å region. In the 1250–1370 Å region, the cross section for the OH emission has a vibrational structure different from the photoabsorption cross section, and the transitions responsible for the production of the OH emission are attributed to the 3a1→4a1 valence transition and the 3a1→3sa1 Rydberg transition. The quantum yields for the production of the OH emission appear weakly at 1366.5 Å and begin to increase sharply at 1362.5 Å. The quantum yields have a maximum value of 11% at 1300 Å and then decrease slowly with decreasing wavelengths. For wavelengths shorter than 1130 Å, the quantum yields vary sharply with photon energies. Various photodissociation processes of H2O in the current wavelength region are discussed.
First passage time approach to diffusion controlled reactions
Authors: Szabo Attila , Schulten Klaus , Schulten Zan
Published: February 2003

Association reactions involving diffusion in one, two, and three-dimensional finite domains governed by Smoluchowski-type equations (e.g., interchain reaction of macromolecules, ligand binding to receptors, repressor–operator association of DNA strand) are shown to be often well described by first-order kinetics and characterized by an average reaction (passage) time τ. An inhomogeneous differential equation is derived which, for problems with high symmetry, yields τ by simple quadrature without taking recourse to detailed cumbersome time-dependent solutions of the original Smoluchowski equation. The cases of diffusion and nondiffusion controlled processes are included in the treatment. For reaction processes involving free diffusion and intramolecular chain motion, the validity of the passage time approximation is analyzed.
ESR of vanadyl ions in Mg(H2O)6H2EDTA single crystals
Authors: Narayana M.
Published: February 2003

An ESR study of single crystals of Mg(H2O)6H2EDTA doped with very small concentrations of vanadyl ions showed the spectra and spin-Hamiltonian parameters to be totally different from those obtained in an earlier study with higher concentrations of VO2+. The results obtained in the present study are comparable with the data of VO(H2O)52+ in other hydrated salts like alums, etc. In low concentrations, VO2+ takes up two preferred orientations along Mg–Ow1 and Mg–Ow2 in the host octahedron. The hyperfine splitting is slightly less than that in other hydrated salts, most probably due to the strong hydrogen bonding involving the metal–water octahedra in Mg(H2O)6H2EDTA.
Vibrational relaxation of acetylene and acetylene–rare-gas mixtures
Authors: Häger J. , Krieger W. , Rüegg T. , Walther H.
Published: February 2003

Vibrational relaxation of acetylene (C2H2) and acetylene–rare-gas mixtures has been investigated using laser-induced fluorescence. Time-dependent fluorescence signals from the vibrational modes ν2 and ν5 have been recorded, following excitation of ν3 and (ν2+ν4+ν5) at 3300 cm−1 with pulses of a tunable optical parametric oscillator. The activation rate of ν2 and ν5 were estimated to be ≳880 ms−1 Torr−1 and ≳330 ms−1 Torr−1, respectively. The following V-T/R relaxation rates have been obtained from the decay of the ν5 fluorescence signal: (12.7±1.1) ms−1 Torr−1 for C2H2 as collision partner, (0.46±0.05) ms−1 Torr−1 for Ne, (0.66±0.06) ms−1 Torr−1 for Ar, (0.67±0.04) ms−1 Torr−1 for Kr, and (0.57±0.04) ms−1 Torr−1 for Xe. Surprisingly small V–V deactivation rates of the CC stretching vibration ν2 have been measured. These are (21.3±1.0) ms−1 Torr−1 for C2H2 collisions, (0.63±0.06) ms−1 Torr−1 for Ne, (0.78±0.08) ms−1 Torr−1 for Ar, (0.93±0.09) ms−1 Torr−1 for Kr, and (0.77±0.09) ms−1 Torr−1 for Xe. Acetylene relaxation mechanisms compatible with these rates are considered.
Equation for the magnetic susceptibility of water
Authors: Day Edmund P.
Published: February 2003

The new precision magnetic susceptibility data of Philo and Fairbank make it possible for the first time to look for a parallel between water’s response to a magnetic field and water’s response to an electric field. These volume susceptibility (κ) data measured at saturated vapor pressure over the temperature interval from 20 to 60 °C are found to satisfy the relationship, −γ−1(∂ln(−κ)/∂T)P=Bχ+Cχ(γ −1). Here γ=−(∂lnρ/∂T)P is the volume expansion coefficient and Bχ and Cχ are independent of temperature. Assuming Bχ and Cχ are also independent of density, we can integrate the above relationship to find This expression for κ describes the same volume magnetic susceptibility data of water to within its experimental uncertainty of two parts in one million. A similar relationship for water’s refractive index was previously shown by H. Eisenberg to fit all of the precision refractive index data of water to within its experimental uncertainty of one part in one million. The constant Cχ=−(∂ln(−κ)/∂T)ρ reflects the unusual temperature dependence of the volume magnetic susceptibility of water. The given expression for κ can be further tested by measuring (∂ln(−κ)/δP)T for water to see if the prediction β−1[∂ln(−κ)/∂P]T=Bχ=1.0213±0.0007 is observed experimentally. Here β=(∂lnρ/∂P)T is the isothermal compressibiltiy. Verification of the expression for κ would establish for the first time a close parallel between the electric polarizability and magnetic susceptibility of water.
The internal optic modes of RbClO3/RbBrO3 mixed crystals
Authors: Nichols Henry F. , Frech Roger
Published: February 2003

The internal optic modes of single crystal RbClO3 and RbBrO3 are observed by Raman scattering and near normal incidence infrared reflection spectroscopy. Vibrational modes are then studied in several mixed crystals in which one anion has been isomorphously substituted for the other. The Raman spectra of microcrystalline powders of both single and mixed crystals are shown to map out an oblique phonon density of states. The mixing of the ν1 and ν3 intramolecular motions is observed in the oblique phonons of the mixed crystals and is followed as a function of relative anion concentration. Residual dynamical interactions apparently persist into the high dilution limit in the mixed crystals for both guest ClO3− in host RbBrO3 and guest BrO3− in host RbClO3. Finally these results are compared with a similar study of NaClO3/NaBrO3 mixed crystals. The near accidental degeneracy of ν1 and ν3 observed in the sodium system also occurs in the rubidium system.
Nodal structure and global behavior of scattering wave functions
Authors: Askar Attila , Cakmak Ahmet S. , Rabitz Herschel
Published: February 2003

The global wave function is obtained for finite element calculations of H+H2. The wave function is a linear combination of linear solutions which are symmetric and antisymmetric to a reflection plane on the potential surface. Boundary conditions are discussed. (AIP)
Photodissociation of Na2 and Rb2: Analysis of atomic fine structure of 2P products
Authors: Rothe Erhard W. , Krause U. , Düren R.
Published: February 2003

Sodium and rubidium dimers are photodissociated with the use of an argon ion laser. The sodium dimers constitute a fraction of a supersonic molecular beam, while the rubidium dimers are part of a vapor. The intensity of the emitted atomic D1 and D2 lines is measured as a function of the laser wavelength. The sodium is excited to the B state, which then dissociates to 2S1/2 and 2Pj atoms. At two of the laser wavelengths the sodium fluorescence originates entirely from the j=3/2 fine structure level. The rubidium, with most laser lines, is excited to the C state which is then predissociated by the B state to yield 2S1/2+2P3/2 atoms. The data are compared with those of other experiments and are analyzed with the use of previously published atom–molecule correlation diagrams. The dissociation proceeds more simply than has been suggested by others.
Charged macromolecules in external fields. I. The sphere
Authors: Fixman Marshall
Published: February 2003

The response of rigid macroions in aqueous solution to an external electric field is considered. The external field, which may be steady or oscillating, induces perturbations in the distribution of coions and counterions, and also in the electrical and solvent velocity fields. The basic equations that describe the various fields are reviewed, and then specialized to the special problem of thin double layers. Boundary conditions are derived which differ from those used in previous work. Previous treatments of the counterion flux into the double layer have either omitted the flux, or taken it to be in phase with the applied field. We find that this flux is large, and has a significant component out of phase. The new boundary conditions make substantial changes in the complex dielectric response. The dominant slow process that controls relaxation is the slow diffusion of neutral salt in the environment of the macromolecule. Large perturbations are induced in the neutral salt concentration by charge distortions in the double layer, and in turn modify the response of the charge. The slow diffusion of the salt gives rise to broad dielectric dispersion. An application is made to the charged sphere, and the results are compared with dielectric response measurements on polystyrene latex spheres, and with the theory of Schwarz.
Raman spectra of thin organic films enhanced by plasmon surface polaritons on holographic metal gratings
Authors: Girlando A. , Philpott M. R. , Heitmann D. , Swalen J. D. , Santo R.
Published: February 2003

The variation in the Raman intensity of a thin film of polystyrene coated onto a Ag metal holographic grating has been measured as a function of the angle of incidence of the exciting laser beam. The Raman signal reaches its maximum intensity at the angle at which the laser excites the plasmon surface polariton mode of the metal directly by coupling through the grating vector. This demonstrates that plasmon surface polariton fields can be exploited to enhance the Raman spectra of very thin films on metal surfaces. The advantages of using holographic gratings in this type of experiment are briefly discussed.
Experimental V–V energy transfer from directly excited CO(v=2) and DCl(v=2)
Authors: Dasch Cameron J. , Moore C. Bradley
Published: February 2003

CO(v=2) and DCl(v=2) have been directly excited with a pulsed optical parametric oscillator. The total deactivation rate constants for CO(v=2) and DCl(v=2) in CO–DCl mixtures at 295 K are determined from the temporally and spectrally resolved infrared fluorescence. These rate constants are interpreted as single quantum V–V exchange rates and have the magnitudes CO→CO 2.7±0.1; CO→DCl 3.7±0.3; DCl→CO 1.4±0.15; DCl→DC1(3.3±0.3)×10−12 cm3 s−1. For the data now available, it appears that for CO–DCl collisions energy transfer is faster for a given vibrational energy difference when the DCl quantum is larger.
Theory of capillary condensation
Authors: Chalupa J. , Huberman B. A.
Published: February 2003

We report the results of a Landau theory of capillary condensation which we have constructed for a liquid or gas in contact with the solid walls of a capillary. For a fixed value of the surface–adsorbate interaction potential we find three phases, which we call wet and full (W.F.), wet and empty (W.E.) and dry and empty (D.E.). We derive the phase diagram in the (ρ,T) plane and observe that the D.E.→W.E. transition induces a coexistence curve whose width depends on the solid surface separation.
Refined calculated ion–molecule association rates
Authors: Herbst Eric
Published: February 2003

Three-body ion-molecule association is discussed. Bates1 proposal of correction factor to the calculated rate constants is evaluated and an alternative correction is proposed. (AIP)
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o potential-energy surface study of several states of the water cation
Authors: Jackels Charles F.
Published: February 2003

Potential energy surfaces of the water cation H2O+ have been studied using configuration–interaction techniques. Double-zeta quality basis sets augmented with polarization functions have been used throughout. The theoretical C2v equilibrium conformations of the ? 2B1, ? 2A1, and ? 2B2 states have been determined. The 2A1 potential surface has a minimum for the linear conformation of the ion, and the 2B2 surface has its minimum at an H–O–H angle of 55.7 °. The vibronically coupled 2A1 - 2B2 (1 2A′ - 2 2A′) set of states has been examined at Cs conformations in both the adiabatic and diabatic representations. This coupling, although too weak to eliminate the C2v minimum in the 2B2 potential surface, is expected to perturb the lower-lying 2B2 vibronic levels. Two dissociation pathways for the 2B2 H2O+ ion (C2v and Cs) are partially characterized.
The infrared spectrum of matrix isolated hydrogen and deuterium
Authors: Warren Jonathan A. , Smith George R. , Guillory William A.
Published: February 2003

The induced infrared spectra of H2 and D2, trapped in matrices of Ar, Kr, N2, CO, have been investigated. It is found that in Ar and Kr, the pure rotation spectrum is always readily observable. Portions of the fundamental region, however, are induced by impurities, while the entire spectrum is impurity induced in N2 matrices. These results are discussed in light of a recent Raman study of this system, and in comparison with several single crystal studies. Effective rotational and vibrational constants for matrix isolated H2 are also given.