bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoionization dynamics of excited molecular states: D2 C 1Πu
Authors: Pratt S. T. , Dehmer P. M. , Dehmer J. L.
Published: July 2002

The three photon resonant, four photon ionization spectra of D2 via the C 1Πu, v′=0–4 levels are reported. The photoelectron spectra for all five bands and the photoelectron angular distribution via the C 1Πu, v′=0–3←X 1∑+g, v″=0, Q(3) transitions are also reported. The photoelectron spectra are compared with theoretical Franck–Condon factors determined using accurate potential curves for the D2 C 1πu and D+2 X 2∑+g states. As in the case of H2, the photoelectron spectra display non-Franck–Condon behavior, and the photoelectron angular distributions exhibit strong v+ dependences. The relation of the present results to earlier results on H2 is discussed.
Comparison of DODCI fluorescence depolarization in glycerol and ethylene glycol: Effect of orientational correlation on excitation transport
Authors: Anfinrud Philip A. , Struve Walter S.
Published: July 2002

Time-correlated single-photon counting was used to obtain anisotropic fluorescence profiles for DODCI in ethylene glycol. Rotational diffusion of DODCI in this solvent was characterized at a concentration of ∼10−6 M, where minimal electronic excitation transport occurs during the dye fluorescence lifetime. The fluorescence depolarization due to excitation transport at higher concentrations was separated from that due to rotational diffusion; analysis of our data yields excellent agreement with current transport theories. The discrepancies observed between these theories and our earlier DODCI experiments in glycerol [J. Chem. Phys. 86, 2689 (1987)] appear to originate from orientational correlation of neighboring chromophores in that solvent; glycerol exhibits remarkable coherence in its total radial distribution function out to separations as large as 25 Å at temperatures between 250 and 350 K [Z. Naturforsch. Teil A 36, 1045 (1981)].
Convergence properties of coherent state path integrals from statistical mechanics
Authors: Stolze Joachim
Published: July 2002

Coherent state path integral representations for matrix elements of density operators are compared to various formulations of coordinate space path integrals discussed recently [R. D. Coalson, J. Chem. Phys. 85, 926 (1986)]. The convergence properties of finite-dimensional approximations to these path integrals are tested for the harmonic oscillator. It is found that at low temperatures the coherent state path integrals converge much better than the coordinate space path integrals and thus should be preferred in numerical (e.g., Monte Carlo) calculations of low-temperature properties.
Direct measurements of formation rate constants of allylic radical from hot olefins formed by internal conversion. I
Authors: Nakashima Nobuaki , Ikeda Noriaki , Shimo Nobuo , Yoshihara Keitaro
Published: July 2002

The photofragmentation of 2,3,3-trimethyl-1-butene (TMB), 2,3-dimethyl-2-pentene (DMP), and tetramethylethylene (TME) has been studied in the gas phase by ArF (193 nm) laser flash photolysis. For the first time, the formation rate constants to the common product of trimethylallyl (TMA) radical have been measured directly under collision free conditions. The C–C bond (the C–H bond for TME) at the position β to the double bond dissociates and TMA radical is formed. The formation rate constants of the TMA radical are found to be 5.6×107 s−1 for TMB, 2.7×107 s−1 for DMP, and 1.7×107 s−1 for TME. These rate constant values can be explained in terms of the RRKM theory by assuming that the absorbed photon energy is redistributed in all the vibrational modes. Formation of the TMA radical is reduced by adding a foreign gas, however, a small amount of the radical was still detected under a high pressure. The unquenched processes are interpreted in terms of predissociation.
Studies of chemical reactivity in the condensed phase. III. Direct measurement of predissociation and geminate recombination times for iodine in solution
Authors: Smith D. E. , Harris C. B.
Published: July 2002

Results are presented which clarify the time scales for predissociation and geminate recombination in the iodine photodissociation/recombination reaction. Direct comparison of absorption features following A state excitation with those following B state excitation suggests that predissociation may be much more rapid (≤1 ps) than previously thought, and that geminate recombination is also very rapid (≤2 ps). Both of these results lead to the conclusion that curve crossing in solution is extremely facile and not the rate-limiting step in recombination. An important consequence of this is that molecular dynamics simulations seem to give reasonable results for recombination dynamics even when curve crossing is treated by oversimplified models. Finally, it is shown that in order to properly interpret the early time bleach kinetics it is necessary to consider dynamics within the ground state population remaining after excitation as well as excited state dynamics.
Surface diffusion of hydrogen on carbon-covered Ru(001) surfaces studied using laser-induced thermal desorption
Authors: Mak C. H. , Koehler B. G. , Brand J. L. , George S. M.
Published: July 2002

The effects of surface carbon on the surface diffusion of hydrogen on Ru(001) were studied using laser-induced thermal desorption techniques. The surface mobility of hydrogen decreased by approximately a factor of 60 as a function of increasing surface carbon coverage from θC=0 to θC=0.42 monolayer at T=300 K. The observed reduction of hydrogen surface mobility vs surface carbon coverage was consistent with the trapping of hydrogen atoms by carbide species on the Ru(001) surface. A simple trapping model suggests that the potential energy wells of the carbon trap sites are ΔE>2.4 kcal/mol deeper than regular hydrogen adsorption sites. This estimate is also consistent with the results of Monte Carlo simulations.
Effect of weakening the hydrogen bond on the angular geometry of H2CO⋅⋅⋅HX: Evidence from the rotational spectrum of H2CO⋅⋅⋅HCN
Authors: Goodwin Elizabeth J. , Legon A. C.
Published: July 2002

The ground-state rotational spectra of three isotopic species (H2CO⋅⋅⋅HC14N, H2CO⋅⋅⋅HC15N, and H2CO⋅⋅⋅DC14N) of the formaldehyde–hydrogen cyanide dimer have been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. Analysis of the spectra yielded rotational constants, centrifugal distortion constants, and 14N-nuclear quadrupole coupling constants which for the species H2CO⋅⋅⋅HC14N have the following values: A0=91.2(7.5) GHz, B0=1785.6217(10), C0=1749.7215(10) MHz, ΔJ =4.76(2) kHz, ΔJK=−6.1181(7) MHz, δJ =2.58(1) kHz, χaa=−4.381(3) MHz, and χbb =2.141(4) MHz. The spectroscopic constants have been interpreted to establish a planar geometry for the dimer in which HCN forms a hydrogen bond to O in H2CO. The angle between the C=O bond and HCN symmetry axis is θ=138.1 deg, and the intermolecular distance from O to C is r(O⋅⋅⋅C)=3.271 Å. The difference in the angle θ between H2CO⋅⋅⋅HCN and H2CO⋅⋅⋅HF is discussed on the basis of a simple model for the hydrogen bond in these dimers.
Multiphoton ionization of vinylchloride, trifluoroethylene, and benzene at 193 nm
Authors: Rossi Michel J. , Helm Hanspeter
Published: July 2002

The multiphoton ionization–fragmentation pattern of vinylchloride, trifluoroethylene and benzene has been measured at 193 nm using time-of-flight mass spectrometry and gas chromatographic analysis. Relative cross sections for multiphoton ionization and the laser power dependence have been determined for the three species at 193 nm at low laser fluence (<10 mJ/cm2) and relative cross sections for multiphoton ionization of benzene have been obtained at 193 and 248 nm. The dominant fragmentation and ionization channels in the two substituted ethylenes are discussed.
Harmonic force field and Raman trace scattering intensity parameters for cyclohexane
Authors: Gough K. M. , Murphy W. F.
Published: July 2002

Absolute Raman trace scattering cross sections have been measured for gaseous cyclohexane-h12, cyclohexane-d12, and the axial and equatorial isomers of cyclohexane-d11. A set of ∂ᾱ/∂Sj intensity parameters were determined from the experimental cross sections. The quadratic vibrational force field required for this analysis was obtained by scaling an ab initio force field to fit well determined vibrational frequencies. The unexpected intensity ratio observed for the CH stretching bands of the two conformers of d11 is explained by a difference in the ∂ᾱ/∂rCH value for the axial and equatorial CH bonds. The transferability of Raman intensity parameters is considered in light of recent results.
Photochemical timing. Application to intramolecular vibrational redistribution in t-stilbene
Authors: Petek Hrvoje , Yoshihara Keitaro
Published: July 2002

Photochemical timing, a spectroscopic method for measuring time-gated fluorescence spectra with picosecond time resolution, is demonstrated in a study of intramolecular vibrational redistribution in the S1 state of supersonically cooled t-stilbene. In the photochemical timing technique, two different color, nanosecond laser pulses are used: the first one prepares the molecule in an S1 vibronic state, while the second one quenches S1 fluorescence by excitation from the S1 state, to a dark Sn state. With this technique, the lifetime of t-stilbene is reduced to <60 ps; this allows observation of fluorescence from the initially excited state in preference to fluorescence from the states populated by vibrational redistribution.
General classical variational treatment of the rates of bimolecular exchange and unimolecular reactions involving three bodies
Authors: Koeppl Gerald W.
Published: July 2002

The classical variational theory of chemical reaction rates gives the rate as the equilibrium flux of systems through a trial surface in the phase space of the reaction system. The surface divides the phase space into reactant and product regions and is varied to obtain a least upper bound for the rate of product formation. For bimolecular reactions of the type A+BC→AB+C and the high pressure limit of unimolecular reactions of the type ABC→products, we have derived expressions which give the canonical and microcanonical rate coefficients for the most general dividing surface defined by internal configuration space coordinates. For bimolecular reactions, we have also derived an expression for the energy dependent mean reaction cross section for the most general dividing surface. Expressions for the rate coefficients and mean reaction cross section for any of the more restricted formulations of the dividing surface used in earlier work, and more flexible ones, can be obtained by the substitution of two terms in the appropriate general equation; namely, the partial derivatives of the function that defines the surface with respect to the internal coordinates that define the surface. For example, the flexibility of the surface can be improved systematically by introducing terms in a power series expansion of the surface. The application of a simplex algorithm to determine the coefficients (variational parameters) in an expansion of the surface that includes terms up to the second and third order may give a surface which is close to the best one possible. The minimization procedure corresponds to a search of the potential-energy function for a reaction coordinate that satisfies the variational condition. The variationally determined dividing surface identifies regions of configuration space in which the potential energy must be accurate in order to obtain accurate classical rate coefficients. The

calculus of variations was applied to the general equations for the rate coefficients to obtain differential equations which give the best dividing surface defined by internal coordinates for the canonical and microcanonical formulations of transition state theory. The corresponding rate coefficients or mean reaction cross section are as close as possible to convergence with the results of classical trajectory calculations. The flux can thus be minimized completely by obtaining a numerical solution for the differential equations which define the best surface or nearly minimized for a series approximation to the best surface. Computational studies are required to determine the more tractable method. By using an appropriate generating function, the procedure described in this work can be extended to reactions involving more than three bodies. The variationally determined dividing surface, reaction coordinate, and reaction surface Hamiltonian could provide a basis for a semiclassical theory of reaction rates.
X-ray studies of concentrated aqueous solutions
Authors: Ludwig K. F. , Warburton W. K. , Fontaine Alain
Published: July 2002

Concentrated aqueous solutions of three transition metal bromides (ZnBr2, CuBr2, and NiBr2) and an alkali bromide (RbBr) have been studied with differential anomalous scattering (DAS) and extended x-ray absorption fine structure (EXAFS). The aq-ZnBr2 solutions exhibit considerable inner-shell ion complexing with the formation of tetrahedral complexes about the Zn2+. In aq-CuBr2, the Cu2+ has an octahedral coordination shell. Most of the anions are bound directly to the cations in both solutions. In contrast, there are only a few Ni–Br nearest neighbors in aq-NiBr2. Instead, cations and anions share hydrating water molecules. Preliminary data show that any ion complexing in aq-RbBr must be weak. These results are in good agreement with published thermodynamic studies.
Quantum and classical studies of the dissociation dynamics of H2 and its isotopes on Ni
Authors: Chiang Chao-Ming , Jackson Bret
Published: July 2002

A two-dimensional quantum mechanical model is used to study the dissociative adsorption of H2 and its heavier isotopes on Ni(100). Dissociation probabilities are computed as a function of molecular kinetic energy for H2 , D2 , T2 , and a hypothetical heavier isotope. It is demonstrated how the variation of the zero point energy with mass strongly influences the dynamics. A qualitative agreement with recent experimental results for H2 and D2 is obtained. Quasiclassical trajectory calculations are performed for the same systems. By comparison with the exact quantum calculations, the classical probabilities for H2 and D2 are shown to be too large at low kinetic energies. For molecules heavier than T2 , classical dynamics are shown to be adequate. The sources of error in the classical simulations are discussed.
CO2 laser–microwave double resonance spectroscopy of NH3: Precise measurement of dipole moment in the ground state
Authors: Tanaka Keiichi , Ito Hajime , Tanaka Takehiko
Published: July 2002

CO2 laser–microwave double resonance (LMDR) spectroscopy with an intense electric field was applied to the NH3 molecule. Using infrared transitions in the ν2 band for pumping, 76 and 37 LMDR signals originating from 25 and 16 inversion transitions in the ground vibrational state were observed for 14NH3 and 15NH3, respectively. The applied electric field was calibrated with a precision of 10 ppm by a new method using the infrared–infrared double resonance signals of PH3, which have been calibrated against the OCS LMDR signals. The observed signals were analyzed to yield the precise dipole moment with rotational dependence, which was expanded as μ0+μJJ(J+1) +μKK2+⋅⋅⋅. The coefficients determined are, with one-standard-deviation uncertainties in the parentheses, μ0=1.471 932(7) D, μJ=1.8736(34)×10−4 D, and μK =−3.5550(47)×10−4 D for 14NH3, and μ0=1.471 964(11) D, μJ =1.9244(77)×10−4 D, and μK =−3.5903(98)×10−4 D for 15NH3. The absolute accuracy of the dipole moment is, however, limited to 50 ppm as determined by the uncertainty of the OCS dipole moment. The effective dipole moments for the J,K=1, 1 and 1,0 levels of 14NH3 calculated from the present coefficients are consistent with the results of a molecular beam electric resonance experiment. The μJ and μK coefficients are interpreted in terms of Watson’s θαβγ constants, resulting in a good agreement between the observed and theoretical values. The consistency between the two isotopic species is also confirmed.
The B̃ 4Σ−u–X̃ 4Σ−g electronic spectrum of C+2
Authors: Maier John P. , Rösslein Matthias
Published: July 2002

The B̃ 4Σ−u–X̃ 4Σ−g electronic transition of C+2 has been studied in the gas phase by laser excitation spectroscopy. The rotational analysis of ten vibronic bands of 12C+2(v′,v″≤3) and seven of 13C+2(v′,v″≤2) has been carried out. The spectroscopic constants Te, Be, αe, ωe, ωexe are obtained for 12C+2 and 13C+2 in the X̃ 4Σ−g and B̃ 4Σ−u states. The lifetimes of C+2 in selected levels of the B̃ 4Σ−u state have been measured and the oscillator strength f0–0 =0.023(3) is derived.
Nuclear spin relaxation and quasicoherent fluctuations in ordered fluids: A different expression for the high-frequency cutoff
Authors: Vold Regitze R. , Vold Robert L.
Published: July 2002

An expression for the contribution to nuclear spin relaxation rates in liquid crystals from fluctuations in the nematic order has been rederived using an ellipsoidal integration volume for the director modes. When the anisotropic elastic constants, viscosities, and cutoff wavelengths are replaced by effective, isotropic values this expression reduces to the familiar ‘‘one-constant’’ formula, a result which cannot be recovered from expressions based on a cylindrical integration volume. The frequency dependence of the spectral density of motion in various limits as well as practical applications to studies of nematic phases is discussed.
Two-dimensional Fourier transform ESR correlation spectroscopy
Authors: Gorcester Jeff , Freed Jack H.
Published: July 2002

We describe our pulsed two-dimensional Fourier transform ESR experiment and demonstrate its applicabilty for the double resonance of motionally narrowed nitroxides. Multiple pulse irradiation of the entire nitroxide spectrum enables the correlation of two precessional periods, allowing observation of cross correlations between hyperfine lines introduced by magnetization transfer in the case of a three-pulse experiment (2D ELDOR), or coherence transfer in the case of a two-pulse experiment (COSY). Cross correlations are revealed by the presence of cross peaks which connect the autocorrelation lines appearing along the diagonal ω1=ω2. The amplitudes of these cross peaks are determined by the rates of magnetization transfer in the 2D ELDOR experiment. The density operator theory for the experiment is outlined and applied to the determination of Heisenberg exchange (HE) rates in 2,2,6,6-tetramethyl-4-piperidone-N-oxyl-d15 (PD-tempone) dissolved in toluene-d8. The quantitative accuracy of this experiment is established by comparison with the HE rate measured from the dependence of the spin echo T2 on nitroxide concentration.
Multiple ionization of benzene clusters by ultraviolet radiation
Authors: Hahn M. Y. , Schriver K. E. , Whetten R. L.
Published: July 2002

Efficient multiple ionization of large benzene clusters has been observed following irradiation of a supersonic cluster beam by ArF-laser pulses at low fluences. The mechanism of multiple ionization is shown to be distinct from that believed to predominate in electron impact or synchrotron ionization experiments. The fluence dependence is instead interpreted by considering independent ionization of molecules within the cluster. The efficiency of multiple ionization over a range of laser fluence can be predicted quantitatively using a recently proposed exciton annihilation model, and explains the 106 enhancement of the double ionization rate over the molecular value. The critical sizes N*(z) for stability of a z-charged drop with respect to Coulomb fission are N*(2)=23, N*(3)=52, and N*(4)=92 and can be accounted for by liquid drop models. However, the fission of doubly charged clusters could not be detected in dynamical mass spectrometry experiments on the 10−6 to 10−4 s time scale. The surprising critical aspect of the fission rate is discussed, and arguments are made in favor of the Gay–Berne dynamical mechanism.
High-pressure NMR study of the dynamical solvent effects on the rotation of coordinated ethylene in (π-C5H5)Rh(C2H4)2
Authors: Xie Chuan-Liang , Campbell Douglas , Jonas Jiri
Published: July 2002

Preliminary results of the high-pressure NMR study show that the rate of rotation of coordinated ethylene groups in (π-C5H5)Rh(C2H4)2 dissolved in benzene and chloroform solvents increases with increasing pressure. The rate constants obtained in different solutions correlate with the solvent shear viscosity, and the viscosity dependence of the rate constant indicates inertial behavior in agreement with the prediction of the Skinner and Wolynes model.
Variational–perturbational treatment for the polarizabilities of conjugated chains. I. Theory and linear-polarizabilities results for polyenes
Authors: de Melo C. P. , Silbey R.
Published: July 2002

We use a perturbative density matrix treatment to investigate the behavior of the static polarizabilities α of linear conjugated chains CNHN+2, with respect to the chain length and to the presence of neutral and charged conjugation defects of soliton and polaron type. The molecules are described by the Pariser–Parr–Pople Hamiltonian; both closed and open shells are treated. It is shown that both the longitudinal component of α and the orientationally averaged linear polizability scale as different powers of N in each case. It is also shown how the present treatment can be considered as a generalization of standard variation–perturbation methods for the Hartree–Fock case.
Potential energy surface morphology and the variation of dissociative chemisorption probabilities with kinetic energy and angle: N2/W(110)
Authors: Kara A. , DePristo Andrew E.
Published: July 2002

Stochastic molecular dynamics simulations are used to calculate the dissociation probability S0 of N2 on W(110). The results obtained are in good agreement with the experimental data both in magnitude and dependence on incident kinetic energy and angle. The dependence of S0 on the total kinetic energy, in distinction to the normal kinetic energy, is interpreted in terms of the narrow and steep transition state region and the attractive well of the potential energy surface.
Theoretical study of the electron affinities of Cu, Cu2, and Cu3
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Taylor Peter R.
Published: July 2002

Modified coupled pair functional (MCPF) calculations are shown to yield the electron affinities of Cu, Cu2, and Cu3 to nearly equal accuracy. The structural results for Cu−3, together with the experimental negative ion photoelectron results, show conclusively that Cu−3 is linear in its (1Σ+g) ground state. The weak feature in the Cu−3 photoelectron spectrum at an electron binding energy of 1.5 eV is due to ionization from the 3A′2 (equilateral triangle) excited state of Cu−3.
Electron–ion recombination rate constants in gaseous, liquid, and solid argon
Authors: Shinsaka Kyoji , Codama Mitsufumi , Srithanratana Tipaporn , Yamamoto Motohiko , Hatano Yoshihiko
Published: July 2002

Electron drift mobilities μe and electron–ion recombination rate constants kr in gaseous, liquid, and solid argon have been measured by the analysis of transient current induced with the irradiation of an x-ray pulse as functions of temperature and external electric field strength except solid. The effect of external dc electric field on kr up to ∼3×10−3 Td (1 Td=10−17 V cm2/molecule) was examined in gaseous and liquid phases. The observed kr values in both liquid and gas phases were found to be much smaller than those calculated by the reduced Debye equation. The deviation, which is bigger in gas than in liquid, has been compared with recent theoretical studies1–5 which were presented to explain our previous data on the kr values for methane.6 It has been concluded that the recombination also in liquid and gaseous argon is not a usual diffusion-controlled reaction. In the solid phase the observed kr values were almost in agreement with those calculated by the reduced Debye equation. The electron–ion recombination rate constants in both liquid and gas phases increased almost linearly with an external dc electric field up to a critical electric field strength below which electron mobility was nearly constant, and decreased with further increase in the electric field where electron mobility also decreased.
Inelastic collisions of OH(X 2Π) with para-H2: Λ-doublet and hyperfine-structure transitions
Authors: Corey Gregory C. , Alexander Millard H.
Published: July 2002

In this article we report Λ-doublet and hyperfine-structure resolved close-coupled and coupled-states cross sections for collisions of OH (X 2Π) with para-H2 at thermal collision energies relevant to kinetic modelling of OH emission in astrophysical sources. Our calculations predict a degree of selective excitation of the Λ-doublet levels in agreement with earlier work of Schinke and Andresen a a higher collision energy. That is, collisions of OH with H2 lead to preferential population of the lower Λ-doublet (ε=+1,e) in the Ω=3/2 manifold, while the upper Λ doublet (ε=−1,f) is preferentially populated in the Ω=1/2 manifold. In contrast to a purely stastistical model, we observe here that, for a given JΩε→JΩε′ transition, the hyperfine resolved cross sections are consistent with a strong ΔF=0 collisional propensity rule. This collisional propensity is a manifestation of a fundamental tendency to conserve the orientation of the rotational angular momentum vector during collisions of molecules with closed-shell targets. This propensity rule implies that cross sections for transitions across a hyperfine doublet will be small.
The temperature-dependent distribution of relaxation times in glycerol: Time-domain light scattering study of acoustic and Mountain-mode behavior in the 20 MHz–3 GHz frequency range
Authors: Yan Yong-Xin , Cheng Lap-Tak , Nelson Keith A.
Published: July 2002

A time-domain light scattering study of acoustic and Mountain modes in glycerol is reported. By using light-scattering angles between 0.89° and 88.9°, a wide range of acoustic frequencies is sampled. The data also yield information about time-dependent density responses to stress and to heat (the latter is the time-dependent thermal expansion). These responses are associated with the Mountain mode and provide additional information about structural relaxation dynamics. A theoretical framework is presented which can treat these experiments as well as ultrasonics and specific heat spectroscopy. The time or frequency dependences of the elastic modulus, heat capacity, and pressure response to temperature change are all accounted for and appear to be significant. The experimental results are fit best with a distribution of relaxation times which is somewhat less asymmetric than a Cole–Davidson distribution. The width of the distribution (on a logarithmic frequency scale) does not change significantly in the 200–300 K temperature range.
Surface diffusion of H, D, and T on a metal surface: The role of metal motions in the kinetic isotope effects
Authors: Truong Thanh N. , Truhlar Donald G.
Published: July 2002

Canonical variational transition state theory and a multidimensional semiclassical tunneling method were used to calculate the surface diffusion rate constants of H, D, and T on the (100) surface of a model metal, nominally Cu. In the present study, we especially examined the influence of metal motions on the kinetic isotope effect for this process. We have employed the embedded cluster approach with a cluster size of 28 metal atoms. The adsorbate–substrate and substrate–substrate interactions are modeled by pairwise potential functions. The results show that including the metal zero-point motions has a negligible effect on the kinetic isotope effect predicted by the rigid-lattice model, but the inclusion of nonzero metal vibrational amplitudes in the semiclassical tunneling path has a large effect. We call this phonon-assisted tunneling. In the low-temperature region, where phonon-assisted tunneling is most important, the isotope effects were found to be smaller than those predicted by the rigid-lattice model, whereas at higher temperatures, they are found to be larger than the lattice predictions. This occurs despite the fact that the phonon–adsorbate interactions decrease the effective reduced masses of the adsorbates in the tunneling region.
Kinetics of CH(X 2Π) radical reactions with cyclopropane, cyclopentane, and cyclohexane
Authors: Zabarnick S. , Fleming J. W. , Lin M. C.
Published: July 2002

Multiphoton laser photolysis of CHBr3 and laser-induced fluorescence detection of CH(X 2Π) radicals is utilized for the direct measurement of rate constants for CH reaction with a series of cycloalkanes. Experiments are performed over the temperature range 295 to 670 K and over the pressure range 30 to 300 Torr. The following Arrhenius equations result: CH+c-C3H6, k=(5.30±0.27)×10−11 exp [(1050±40)/RT] cm3 s−1; CH+c-C5H10, k=(2.66±0.33)×10−10 exp [(400±80)/RT] cm3 s−1; CH+c-C6H12, k=(3.07±0.12)×10−10 exp [(300±30)/RT] cm3 s−1. No change in the measured rates with pressure is observed. A mechanism that involves CH radical insertion into the CH bonds on the cycloalkanes followed by rapid decomposition of the excited adduct is invoked to explain the observed kinetic behavior. A correlation of room temperature rate constant vs number of C–H bonds in the reactant is presented.
Curvature elasticity and rotational viscosity of the nematic decylammonium chloride system
Authors: Zhou E. , Stefanov M. , Saupe A.
Published: July 2002

Experimental data are reported on curvature elasticity and rotational viscosity of micellar nematics formed in the decylammonium chloride/NH4Cl/H2O system. Two mixtures of different salt concentrations were studied (DACl:NH4Cl=10:1 and 20:1 by weight). The constants for the bend elasticity and the rotational viscosity were found to vary over nearly three orders of magnitude from 0.2×10−6 to 150×10−6 dyn and from 1 to about 1000 P, respectively. The large range is due to the divergence at the nematic–smectic transition. Both constants diverge with the same exponent. We obtained an exponent of 1.07±0.05 for a weight ratio DACl/NH4Cl of 20 and 0.87±0.02 for a ratio of 10, but an unexpected thermal hysteresis interfered with a reliable determination of the critical properties. Overall, the elastic properties and the divergence is this micellar nematic are similar to those observed in thermotropic nematics.
Condensation of supersaturated vapor. VII. The photoinduced nucleation of o-tolualdehyde and its underlying reaction mechanism
Authors: Chen Chin-Cheng , Katz Joseph L.
Published: July 2002

On illuminating a supersaturated vapor containing a low concentration of o-tolualdehyde with ultraviolet light, nucleation occurs. This paper presents values for the diffusivity of the photonucleator and measurements of the quenching effects of NO on the rate of nucleation. These diffusivities lead to lower limits for the radius of the photonucleators of 4.5 to 8.7 Å, depending on which supersaturated vapor is used. A numerical simulation of the quenching effects of NO, assuming photoinduced nucleation occurs via a radical mechanism, was also made. The measured quenching effect agree qualitatively with those predicted by the simulation. We conclude that the photonucleators are photoproducts resulting from the reaction of photoexcited o-tolualdehyde with other vapor phase molecules. The reaction mechanism depends on the wavelength of the ultraviolet excitation light and the chemical reactivity of the supersaturated vapor, and may be a radical reaction mechanism. In the presence of NO, the reaction mechanism changes.
Energy storage and thermoluminescence in halogen doped solid xenon. III. Photodynamics of charge separation, self-trapping, and ion–hole recombination
Authors: Fajardo Mario E. , Apkarian V. A.
Published: July 2002

The optically accessed excitonic charge transfer states of solid xenon doped with atomic halogens relax by one of two channels: self-trapping of the exciton to form the triatomic molecular exciplex, or self-trapping of the hole which leads to charge separation by the creation of a pair of oppositely charged small polarons. The latter channel leads to long term storage of optical energy. Charged pair storage densities of 1017 cm−3 are routinely realized, and retention times as long as 35 h have been directly verified. The trapped ion–hole pair may recombine either through tunneling or thermal activation of the self-trapped hole. Tunneling leads to temperature independent phosphorescence which decays with a hyperbolic time dependence. Temporally and spectrally resolved phosphorescence and thermoluminescence are used for the characterization of the trapped state energetics and their recombination dynamics. A first order kinetic treatment of the recombination kinetics yields 800(±200) cm−1 as the lattice relaxation energy for the self-trapped hole.
Calculation of isotropic hyperfine coupling constants by the symmetry adapted cluster expansion configuration interaction theory
Authors: Momose Takamasa , Nakatsuji Hiroshi , Shida Tadamasa
Published: July 2002

The isotropic hyperfine coupling constants (HFCCs) are calculated for H2O+⋅, CH2O+⋅, CH3O⋅, CH3NH⋅, CH3CH2⋅, and CH3OCH+⋅3, using the method of SAC (symmetry adapted cluster expansion)-CI. After examining various basis sets, we found that the double-zeta quality basis sets of Dunning are the best among the sets examined. The calculated values agree fairly well with the experimental values. We point out that the conventional configuration selection based on the energy criterion has an inherent limit for the calculation of HFCCs. The influence of molecular vibrations on the HFCCs has been semiquantitatively examined for CH2O+⋅, CH3O⋅, CH3NH⋅, and CH3CH⋅2.
Absorption spectrum of Kr2F(4 2Γ) in the near ultraviolet and visible (335≤λ≤600 nm): Comparison with Kr+2(1( 1/2 )u) measurements
Authors: Geohegan D. B. , Eden J. G.
Published: July 2002

Absolute photoabsorption cross sections for the lowest excited state (4 2Γ) of Kr2F and the ground state (1( 1/2 )u) of Kr+2 have been measured in the visible and ultraviolet (Kr2F: 248, 308, 335–600 nm; Kr+2 : 360–500 nm). As predicted by theory, the Kr2F spectrum is dominated by the 9 2Γ←4 2Γ band which peaks below 340 nm and correlates with the Kr+2 2( 1/2 )g ←1( 1/2 )u transition. However, Kr2F(4 2Γ) and Kr+2 (1( 1/2 )u) both absorb more strongly for wavelengths beyond ∼450 nm than expected, apparently owing to a nonthermalized Kr+2 vibrational distribution. Despite interference from Kr2F stimulated emission between ∼350 and 460 nm, the experimental results show the Kr2F(4 2Γ) and Kr+2 absorption profiles to be similar for λ≥460 nm but diverging rapidly as λ increases from 335 to 360 nm. At 360 nm, the absorption cross sections differ by a factor of 3. Contrary to theoretical expectations, the Kr2F absolute absorption cross section is consistently smaller (typically <40%) than that for Kr+2 which suggests that the impact of F− on the Kr+2 oscillator strengths is significant.
Potential energies for the reaction F+H2→HF+H by the random walk method
Authors: Garmer D. R. , Anderson J. B.
Published: July 2002

A b initio calculations of potential energies have been carried out by the random walk method for a dozen points on the F–H–H potential energy surface in the region of the saddle point of the barrier for the reaction F+H2→HF+H. The fixed-node method was used with an importance sampling wave function consisting of a scaled RHF function multiplied by Jastrow factors. The total electronic energies obtained are lower by about 40 kcal/mol than those from the most extensive variational calculation and only about 10 kcal/mol above the exact values. Recovery of correlation energy is about 96% complete. A modified LEPS surface fitted to the results yields a classical barrier height of 4.5±0.6 kcal/mol, a value near that predicted by several variational calculations and as much as 3 kcal/mol above the most recent extrapolated and semiempirical estimates. The barrier height might be lower for calculations with improved node locations; but, since little room for error due to node locations remains, the results provide evidence for a high barrier.
Reversible electric field induced spectral hole filling in a doped polymer film
Authors: Gu Wei , Hanson David M.
Published: July 2002

The effect of a dc electric field on persistent spectral holes in the absorption spectra of perylene doped polyvinyl butyral films has been measured. This effect is not like a typical Stark effect that is obtained with polymer films doped with polar dye molecules. Instead, a new phenomenon of reversible spectral hole filling is observed. This phenomenon is attributed to the elastic deformation of the interaction potentials of the dopant and the polymer associated with reversible field-induced tunneling in the intrinsic two-level systems. A quantitative theory of the phenomenon is proposed.
Molecular beam spectrum of the highly perturbed C–H stretching region of fluoroform
Authors: Pine A. S. , Fraser G. T. , Plíva J. M.
Published: July 2002

Assignments of the C–H stretching fundamental of fluoroform, CHF3, have been obtained from a spectrum recorded near 3035 cm−1 at sub-Doppler resolution (∼10 MHz FWHM) and low effective temperature (∼4 K) in an adiabatically cooled molecular beam using bolometric detection of molecules excited by a color-center laser. This fundamental band is highly perturbed and has resisted analysis at higher temperatures, even at Doppler-limited resolution, whereas its overtones have been the subject of several studies of intramolecular vibrational relaxation. Under molecular beam conditions, the central Q branch, which dominates the spectrum at room temperature, almost vanishes, while the bulk of the intensity is equally shared by two ‘‘sidebands’’ with Q branches symmetrically displaced by ∼±5 cm−1 from the suppressed central Q branch. This peculiar structure is due to a close anharmonic and Coriolis resonance between the fundamental ν1 (species A1) and the combinations ν±4+ν±5+ν±6 (A1+A2) located only 1.40 cm−1 above ν1, whose origin is at 3033.55 cm−1. The coupling constant of the resonance is k1456/2=3.68 cm−1. Additional severe perturbations by the E components of ν4+ν5+ν6 as well as by ν2+ν4+ν6 have been identified in the spectrum.
Molecular ion spectroscopy: Mass selected, resonant two-photon dissociation spectra of CH3I+ and CD3I+
Authors: Walter K. , Weinkauf R. , Boesl U. , Schlag E. W.
Published: July 2002

We present a new ion spectroscopic technique by which it is possible to investigate larger molecular ions by scanning an entire electronic state and coincidentally measuring and, hence, fixing the mass of the fragment ion observed. The technique involves primary resonance excitation together with secondary absorption followed by dissociation as detection process, employing the absorption of two photons in the ion. This method accesses the nondissociating vibronic levels of an electronic state rather than just the narrow range of predissociating levels, as in one-photon dissociation spectroscopy. Spectra of CH3I+ and CD3I+ over a range of 7000 cm−1, beginning at the origin of the Ã state, were recorded with progression bands up to n=21. These spectra allow for the first time an unambiguous assignment of the Ã←X̃ transition of CH3I+ and CD3I+. The exact positions of the electronic origin as well as of the origin of several progressions and new values for several vibrational constants of methyl iodide cations are reported. Some 300 bands in both ions were measured and assigned with this technique.
Universal properties of linear and ring polymers
Authors: Bishop Marvin , Saltiel Craig J.
Published: July 2002

The ratio of the mean-square radius of gyration to mean-square end-to-end distance for linear chains and the mean-square radius of gyration ratio of ring to linear chains are investigated for two-, three-, four-, and five-dimensional polymers, with and without excluded volume, via Brownian dynamics. Our studies, for the Lennard-Jones repulsive interaction, indicate the universality of these ratios.
Interaction of CO with a Pd(110) surface, studied by low energy electron diffraction, thermal desorption spectroscopy, and Δφ
Authors: He Jian-Wei , Norton P. R.
Published: July 2002

The interaction of CO with a Pd(110) surface has been studied by low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), and work function measurements (Δφ). c(2×2), (4×1), and (2×1) phases were observed in sequence with increasing CO exposure at 130 K. When the (2×1) phase is heated to ∼330 K, it transforms to the (4×2) phase. At ∼285 K, the phases c(2×2), (4×2), and (2×1) appear in sequence with increasing exposure. The stable final surface configuration at this temperature is the coexistance of the (2×1) and (4×2) phases. The transition (2×1)⇄(4×2) is found to be reversible for T≥320 K if the sample is heated and cooled in a CO ambient. The thermal desorption spectra reveal five peaks labeled as α1, α2, α3, β1, and β2, respectively. The sharp α3 peak is found to be associated with the phase transition (2×1)→(4×2) which we believe involves Pd-atom displacements. The desorption of the α3 state CO follows fractional order kinetics (<0.3). It is also possible that the (4×2) phase involves surface reconstruction. A work function change of 1085±10 mV was measured for a CO saturated surface at 130 K.
Improved infinite order sudden cross sections for the Li+HF reaction
Authors: Laganà A. , Garcia E. , Gervasi O.
Published: July 2002

Reactivity of the Li+HF system has been calculated by adapting a light–heavy–light infinite order sudden computational scheme. The improvement of the numerical procedure as well as the restructuring of the computational code for use on supercomputers has allowed a successful extended comparison of calculated vs measured cross sections.
Rotational rainbows in the vibrational predissociation of ICl–He complexes
Authors: Waterland Robert L. , Skene John M. , Lester Marsha I.
Published: July 2002

Strongly peaked ICl fragment rotational distributions are observed following vibrational predissociation of ICl–He B state complexes containing two or three quanta of ICl vibrational excitation. The nascent rotational distributions of the ICl product exhibit two distinct maxima, occurring at j=7 and j=16. A theoretical analysis demonstrates that the two maxima are due to rotational rainbows, arising from the He atom scattering off of the I and Cl ends of the ICl molecule. The vibrational predissociation of ICl–He B(vB=2) is simulated with a semiclassical scattering theory which is analogous to that developed by Schinke [J. Chem. Phys. 85, 5049 (1986)] for direct photodissociation. Vibrational predissociation is modeled as a rotationally inelastic ‘‘half-collision,’’ following deactivation of the ICl vibration. The final rotational angular momentum of the ICl fragments is determined from exact classical trajectories and in a sudden limit on a model potential energy surface. The calculated ICl product rotational distribution, like the experimentally observed distributions, is bimodal.
Monte Carlo simulations of polyelectrolytes: Isolated fully ionized chains with screened Coulomb interactions
Authors: Carnie Steven L. , Christos George A. , Creamer Trevor P.
Published: July 2002

We present Monte Carlo simulation results for an isolated polyelectrolyte chain where the charged groups interact via a screened Coulomb interaction of Debye–Hückel form. The chain consists of hard spheres on a backbone with fixed bond angles and either free rotation or a torsional potential that produces hindered rotation. Parameters have been chosen to roughly mimic polyacrylic acid (PAA). We give results for the mean square end-to-end distance 〈r2〉, mean square radius of gyration 〈s2〉, and finite-chain persistence length for chains of up to 320 beads and salt concentrations ranging from 0.1 to 0.0001 mol dm−3. Both 〈r2〉 and 〈s2〉 show power law behavior for chains larger than 80 beads at 0.1 mol dm−3 —the scaling exponents are intermediate between rigid rod and self-avoiding walk values. No scaling behavior can be observed at lower concentrations because of the limitations on the chain lengths that can be simulated. At 0.1 and possibly at 0.01 mol dm−3 the infinite-chain persistence length can be obtained by extrapolation of the distribution of angles of the chain bonds relative to the central bond. These values are in rough agreement with the worm-like chain calculations of Skolnick, Fixman, and Odijk. Our calculations would be expected to give larger values for the persistence length than theories which include ionic degrees of freedom, such as the Poisson–Boltzmann calculations of Le Bret and Fixman. At 0.1 mol dm−3, however, their results are larger than ours, a result we find puzzling.
Vibrational, rotational, and tunneling dependence of vibrational predissociation in the HF dimer
Authors: Pine A. S. , Fraser G. T.
Published: July 2002

Vibrational predissociation linewidths have been resolved in the two H–F stretching bands of the HF dimer using an optothermal (bolometer-detected) molecular-beam color-center laser spectrometer. In addition to the strong vibrational mode dependence reported earlier by several groups, we observe a substantial K-rotational and tunneling dependence to the longer-lived mode ν1, which is associated with the ‘‘free-H’’ stretch. The predissociation linewidths (FWHM in MHz) for this vibration are 6.4(5) for K=0+, 9.5(5) for K=0−, 10.2(5) for K=1+, and 11.8(5) for K=1−, where the +/− superscripts refer to the symmetric/antisymmetric tunneling states. The J dependence (at low J) is negligible compared to the K dependence. The K=0 levels of the ‘‘bound-H’’ stretch have tunneling-independent widths of 330(30) MHz. Extraneous broadening due to saturation effects was observed and corrected for in these measurements.
Vibronic coupling calculations of vibrational circular dichroism intensities using floating basis sets
Authors: Freedman Teresa B. , Nafie Laurence A.
Published: July 2002

The concept of atomic orbital basis sets that float with the nuclear positions is extended to include floating with the nuclear velocities. With such basis functions the contribution to vibrational dipole and rotational strengths from electronic motion that perfectly follows the nuclear motion can be calculated from the equilibrium ground state electronic wave function. Contributions from electronic current due to charge redistribution are determined from vibronic summations that require nuclear displacement derivatives of ground state linear combination of atomic orbitals (LCAO) coefficients. In addition, for the dipole velocity and magnetic dipole operators, a contribution from charge flow at constant electron density enters. The relationships between this formulation, other vibronic coupling models, and approximate ground state models for vibrational circular dichroism intensities are described.
Spin relaxation of I>1 nuclei in anisotropic systems. I. Two-dimensional quadrupolar echo Fourier spectroscopy
Authors: Furó István , Halle Bertil , Wong Tuck C.
Published: July 2002

The response, in the frequency domain, of half-integral spin I>1 nuclei in anisotropic systems to the quadrupolar echo (QE) pulse sequence (π/2)x−τ−(π/2)y is investigated, with particular emphasis on the effects of quadrupolar relaxation. Using a state multipole formalism, we derive the frequency spectra produced by one- or two-dimensional Fourier transformation. Powder samples as well as uniformly and partially oriented samples are considered. The 2D QE experiment produces, in the F1 dimension (Fourier transformation with respect to τ), spectra which provide information about the static quadrupole coupling even if only the central line in the QE spectrum (obtained by Fourier transformation of the induction signal after the echo) is detected. For partially oriented and (incompletely dephased) powder samples, the F1 spectra allow the motional spectral densities to be determined with little or no effect of the inhomogeneous quadrupole coupling, which severely complicates the analysis of conventional (single-pulse) and QE spectra. For completely dephased powder samples (large splittings), the last of the multiple spin echoes gives rise to a triplet QE spectrum (for any I≥3/2), the satellite decay in which yields a transverse relaxation rate.