bluuebunny/crossref_metadata_2025 huggingface dataset:
Intramolecular hydrogen bonding. VI. The phosphorescence of 9-hydroxyphenalenone in an N-hexane Shpol’skii matrix
Authors: Gillispie Gregory D.
Published: July 2002

Luminescence spectra have been measured for 9-hydroxyphenalenone and its hydroxy-deuterated form in an n-hexane Shpol’skii matrix. Fluorescence spectra from the zero-point level in S1 agree well with those reported for neon matrix by Bondybey et al. [J. Chem. Phys. 80, 5432 (1984)]. However, there is no measurable unrelaxed emission when higher S1 vibrational levels are excited; vibrational relaxation is much faster in n-hexane than in solid neon. Vibrationally resolved phosphorescence spectra are the main new results reported here. The phosphorescence quantum yield for 9-DPO is an order of magnitude stronger than for 9-HPO and the origin band is red shifted by the deuterium substitution. It is proposed that the lowest triplet state is nonplanar.
Test of Hubbard–Onsager–Felderhof’s dielectric friction theory for ion rotation in solution
Authors: Nakahara M. , Ibuki K.
Published: July 2002

Hubbard–Onsager–Felderhof’s (HOF) dielectric friction theory for rotational motion of ions is tested by using NMR orientational relaxation times recently measured for such polyatomic ions as ClO−4,SO2−4,PO3−4, etc. in D2O and CD3OD at infinite dilution. The HOF theory combined with the bulk solvent properties is qualitatively successful in explaining how rotational friction coefficients for ions with a high surface charge density depend on ion size and valence, temperature, and solvent. In the case of a low surface charge density, the continuum model shows its inherent limitations in the treatment of ion dynamics, rotational or translational. It turns out that the HOF theory serves an important theoretical framework for the interpretation of the semiempirical relation between orientational relaxation times of ions and the solvent viscosity divided by temperature.
Sensitive quantum state selective detection of H2O and D2O by (2+1)-resonance enhanced multiphoton ionization
Authors: Meijer Gerard , ter Meulen J. J. , Andresen Peter , Bath Anette
Published: July 2002

The first observation of (2+1)-REMPI of H2O and D2O is reported. With the use of a high power tunable excimer laser radiating at 248 nm, the H2O and D2O molecules are ionized after resonant two-photon absorption into the predissociated C̃ 1B1 state. The clearly observable peaks in the (2+1)-REMPI spectra are all identified and can be used for sensitive state selective detection. Parent molecular fluorescence excitation spectra (C̃ 1B1→Ã 1B1) were remeasured over an increased spectral range, and are remarkably the same as the (2+1)-REMPI spectra. Furthermore the OH/OD (A 2Σ+, v′=0→X 2Π, v″=0) photofragment fluorescence excitation spectra were measured, and these spectra do not show any nonresonant background as stated before. Additional fluorescence bands starting from the vibrationally excited A 2Σ+, v′=1 state were observed. Simulation of the observed REMPI and fluorescence excitation spectra yields the branching ratios for the predissociation, ionization, and fluorescence processes.
Molecule–corrugated surface scattering calculations using the close coupling wave packet method
Authors: Mowrey R. C. , Bowen H. F. , Kouri D. J.
Published: July 2002

Numerically exact quantum calculations for scattering of N2 off a model corrugated rigid lattice are reported. The computations were done using the recently developed close coupling-wave packet method for treating quantum scattering. Results for all energies in the range 0.010–0.040 eV are obtained from a single wave packet propagation. The calculations included even N2 rotor j states j=0–12, all mj’s, and sufficient grid points to describe diffraction states −8≤m≤8, −4≤n≤4, for a total of 13 923 channels. The computations, which required a total of 230 minutes of computer time, were carried out on the CRAY2 supercomputer at the University of Minnesota Supercomputer Center under National Science Foundation support.
Loss of tensile strength in liquids without property discontinuities: A thermodynamic analysis
Authors: D’Antonio Michael C. , Debenedetti Pablo G.
Published: July 2002

The thermodynamically consistent behavior of any fluid whose tensile strength exhibits a maximum with respect to temperature (tensile instability) is derived for the case where the isochore corresponding to the fluid density at such a maximum is single branched (i.e., a metastable solution exists only for temperatures higher than the tensile instability temperature). The resulting thermal and volumetric picture is considerably simpler than for the recently derived behavior corresponding to the case where the tensile instability isochore admits metastable solutions both above and below the tensile instability temperature (double-branched limiting isochore). Density extrema are inseparable from tensile strength maxima: a tensile instability is, in fact, the low-pressure intersection of a spinodal curve and a locus of density extrema.
Vibrational relaxation in fluids: A many body scattering formalism
Authors: Dardi Peter S. , Cukier R. I.
Published: July 2002

We derive an expression for the vibrational energy relaxation rate constant for dilute diatomic molecules in a structureless fluid. Our approach is based on a many-body scattering formalism within the Markov approximation. Using a quantum scattering formalism allows us to formally separate the inelastic part of the problem from the bath dynamics. We assume that the vibrational transition rate is small, and accordingly we treat the inelastic potential as a perturbation. Also, we assume that the translational motion of the diatom and the bath can be treated classically. The separation of the inelastic interaction from the bath dynamics allows the bath motion to be written in terms of a classical time correlation function of the bath density relative to the diatom. The bath, though, evolves under two Hamiltonians; one with the diatom in its initial state and the other with the diatom in its final state. A method is introduced to approximate this time correlation function in terms of single Hamiltonian correlation functions. We discuss the approximations inherent in our method and also those in the independent binary collision (IBC) model.
Polarized and depolarized Raman spectra of liquid carbon disulfide in the pressure range 0–10 kbar. II. Reorientational and vibrational relaxation
Authors: Ikawa S. , Whalley Edward
Published: July 2002

The effect of pressure on the reorientational and the vibrational phase correlation times of liquid carbon disulfide has been determined up to 10 kbar at 295 K from the widths of the isotropic and anisotropic components of the Raman ν1 band. The reorientational correlation time increases with pressure from 1.3 ps at 1 bar to 9 ps at 10 kbar, and the vibrational dephasing time decreases from 19 ps at 1 bar to 5 ps at 10 kbar. The reorientational correlation time is linear in the shear viscosity, and the slope agrees well with the hydrodynamic estimate based on slip at the boundary and the assumption that the carbon disulfide molecule is prolate. The correlation times reported in Ref. 20 at 164–310 K at ambient pressure are the same function of the viscosity divided by the temperature as our values, so showing that the correlation time varies in the same way with the viscosity divided by the temperature whether the viscosity is varied by varying the temperature at constant pressure or by varying the pressure at constant temperature. The correlation time extrapolated to zero viscosity agrees well with the correlation time of a classical free rotor. Schweizer and Chandler’s kinetic model for vibrational dephasing reproduces the dephasing time within the experimental precision over the experimental pressure range, whereas the hydrodynamic model agrees well with experiment at atmospheric pressure but predicts a dephasing time at 10 kbar that is 2/5 of the experimental value.
Equilibrium structure of liquid wetting layers
Authors: Tarazona P. , Telo da Gama M. M. , Robert M.
Published: July 2002

A phenomenological theory of the liquid wetting layer is presented. Its predictions are compared to those of earlier simpler models as well as to recent experimental results. Special attention is given to the critical region and to the crossover from the complete wetting regime to the critical wetting regime.
On the nonadditivity of the second-order exchange-dispersion energy in the interaction of three helium atoms
Authors: Bulski M. , Chal/asiński G.
Published: July 2002

Explicit formulas for the nonadditivity of the exchange-dispersion energy in the interaction of three closed-shell systems are derived under the assumption that the electron correlation within the noninteracting systems is neglected. The idea of applying many centered basis sets is presented and its usefulness in the investigation of nonadditive effects is discussed. Numerical calculations of the exchange-dispersion nonadditive effect have been performed for the interaction of three ground-state He atoms in the equilateral triangle configuration within the trimer centered basis set scheme. The nonadditivity of the exchange-dispersion energy has been found to be positive and much smaller than that due to the first-order exchange-repulsion in the region under consideration. However, because of the cancellation of the first-order and triple-dipole nonadditive effects it becomes much more important around Re and, e.g., at R=6 a0 it accounts for ∼90% of the total nonadditive effect. In contrast to the Jansen prediction [Adv. Quantum Chem. 2, 119 (1965)], the relative second-order three-body interaction energy has been found to be much smaller than the first-order one. A damping function has been proposed for evaluation of the nonexpanded triple-dipole energy. The role of various contributions to the total nonadditivity of the interaction energy is discussed. The values of the two-body effects are also presented.
Rotational energy level structure of stretching vibrational states in some small symmetrical molecules
Authors: Halonen Lauri
Published: July 2002

The vibrational dependence of various vibration–rotation parameters in XY2, XY3, and XY4 molecules with C2v, C3v, and Td point groups has been investigated between normal mode and local mode limits. The results at these two limiting cases differ drastically from each other. The rotational constants of the two lowest vibrational states in each overtone manifold (v=v1+v3=constant) are equal for v≥2 in the local mode limit. The effective Coriolis constants between these vibrational pairs disappear but the vibrationally off-diagonal H22 resonance terms remain important as the local mode limit is approached. The vibrational dependence of the rotational constants of the stretching vibrational states in the symmetrically substituted acetylenes and in normal silane has been analyzed. It is shown that it is necessary to couple levels by quartic anharmonic resonance terms in the normal coordinate space in order to explain the vibrational behavior of the rotational constants.
van der Waals clusters of pyridazine and isoquinoline: The effect of solvation on chromophore electronic structure
Authors: Wanna J. , Bernstein E. R.
Published: July 2002

van der Waals clusters of pyridazine and isoquinoline with CH4, NH3, H2O, and CH3OH are generated in a supersonic molecular jet expansion and investigated by two-color time-of-flight mass spectroscopy. As is the case for the other diazine systems, no spectra could be observed for pyridazine (H2O)n or (CH3OH)n clusters. Both chromophore molecules are reported to have close lying, vibronically coupled S1 and S2 excited states: nπ* for pyridazine and nπ* (S1) and ππ* (S2) for isoquinoline. Cluster spectra for pyridazine methane and ammonia clusters do not favor the presence of two nπ* transitions in the S1←S0 transition region but rather suggest that the ‘‘S2 origin’’ is a vibronic feature of the S1←S0 transition. Isoquinoline clusters that are only weakly or not at all hydrogen bonded (CH4 and NH3) display a complicated spectrum indicative of S1 (nπ*)–S2 (ππ*) vibronic coupling and not the usual shifted isolated molecular spectrum. Isoquinoline clusters with substantial hydrogen bonding (H2O and CH3OH) display relatively simple spectra indicative of only a single electronic transition S2 (ππ*)←S0 in the region and no interstate vibronic coupling. These results are compared and contrasted with each other and the spectra of the other diazine clusters. Potential energy calculations are also employed to help understand the clustering in these systems.
Product yield-detected ESR on magnetic field-dependent photoreduction of quinones in SDS micellar solution
Authors: Okazaki M. , Sakata S. , Konaka R. , Shiga T.
Published: July 2002

Transient free radicals in the magnetic field-dependent photoreduction of quinones (menadione or anthraquinone) in a sodium dodecyl sulfate (SDS) micellar solution, were converted to stable nitroxide radicals by the ‘‘spin trapping’’ technique with or without the microwave irradiation. Upon irradiating the microwave at 160 mW, the product yield (‘‘spin adduct’’ of the alkyl radical generated from SDS molecule) decreased by up to 14% at certain magnetic fields in a resonant manner. Although only one component of the postulated radical pair was converted to the spin adduct, the decrease in the yield as a function of external magnetic field revealed the ESR spectra of both component radicals of the radical pair, i.e., the semiquinone radical and the alkyl radical from SDS. This experiment not only gives the direct evidence for the radical pair model, but also suggests the possibility for this method to be applied in controlling the chemical reactions by the microwave. A simple calculation was made to simulate the observed ‘‘product yield-detected ESR.’’ Agreements were achieved semiquantitatively between the observed reductions in the spin adduct yields and those calculated. The estimated exchange interaction between the component radicals in the radical pair of the present systems was lower than 0.3 mT.
Photodissociation measurement of rovibrational energies and populations of molecules in fast beams
Authors: Helm Hanspeter , Cosby Philip C.
Published: July 2002

Using as examples H2 and H+2, we show that position- and time-sensitive detection of the two correlated photofragments can be used to determine the rovibrational energies and populations of molecules in a fast beam by observing the product distribution from bound–free photodissociation.
The prediction of nuclear quadrupole moments from a
b i
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i
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i
o quantum chemical studies on small molecules. I. The electric field gradients at the 14N and 2H nuclei in N2, NO, NO+, CN, CN−, HCN, HNC, and NH3
Authors: Cummins Peter L. , Bacskay George B. , Hush Noel S. , Ahlrichs Reinhart
Published: July 2002

Electric field gradients (efg’s) at the nitrogen nuclei in N2, NO+, NO, CN, and CN− and at the nitrogen and hydrogen nuclei in HCN, HNC, and NH3, calculated using ab initio quantum chemical methods, are reported. Employing extensive Gaussian basis sets, the efg’s were computed at the self-consistent field (SCF), singles and doubles configuration interaction [CI(SD)], and coupled pair functional (CPF) levels of theory as the expectation values of the efg operator and also as the energy derivatives of the appropriate perturbed Hamiltonian using the finite field method. Corrections due to zero point vibrational motions were also calculated. The effect of basis set incompleteness on the calculated efg’s, together with the experimental nuclear quadrupole coupling constants, are used to estimate the 14N and 2H nuclear quadrupole moments, and to test the quality of the correlated wave functions generated by the CI(SD) and CPF methods. The recommended values, on the basis of the present calculations, are 2.05±0.02 and 0.29±0.01 fm2, respectively, for the 14N and 2H quadrupole moments.
An operator approach to functional sensitivity analysis in reactive molecular scattering
Authors: Shi Shenghua , Rabitz Herschel
Published: July 2002

The effect of infinitesimal functional variations in a scattering potential on reactive transition amplitudes is studied. A formalism is derived for the functional sensitivity coefficients, i.e., the functional derivatives of the reactive transition amplitudes with respect to the variation of a potential. The resultant coefficients provide a means to access the significance of any region of the potential surface upon the scattering cross section. The formal theory is presented for reactive scattering with variations allowed in the internal Hamiltonian as well as the scattering potential. Both local and nonlocal Hamiltonians are treated. A clear relationship is shown to exist between the structure of the scattering wave function and the regions of significance in the Hamiltonian. The application of the formalism to one-dimensional barrier penetration and reactive scattering of collinear H+H2 shows that the most important region of the potential for the tunneling is the classical turning point region while the most responsible region for the resonance scattering in H+H2 is the saddle point region. In addition broad regions of significant potential sensitivity are also found adjacent to these specific locations.
Contribution of small ions to the scattered intensity in the hypernetted chain approximation: Application to micellar solutions
Authors: Derian Paul-Joel , Belloni Luc , Drifford Maurice
Published: July 2002

The HNC approximation is applied to the primitive model of asymmetrical electrolytes that mimic charged micellar solutions. The structure factors Sij(q) are calculated for two or three component systems (polyions, counterions, and coions). The scattered intensity I(q) is obtained by two approaches: The first one considers the structural system where the structural charge of the polyion is equal to the aggregation number. The second one defines an effective system with a larger and less charged effective polyion which contains ‘‘condensed counterions.’’ We show the equivalence between both approaches in different cases and particularly in the case where the counterions contribute significantly to the scattered intensity. An application is presented: the investigation of a micellar solution of octyltrimethylammonium bromide with x-ray scattering. In this case, the high scattering length density of bromide ions contributes strongly to the total scattered intensity. The experimental curve which presents a large intensity at high scattering vectors can be fitted with the structural model. The structural charge, or aggregation number, which is the single fit parameter, is equal to 27±4.
Profile of a fused salt near a wall in the GMSA
Authors: Li S. , Mazo R. M.
Published: July 2002

The integral equations of the generalized mean spherical approximation (GMSA) are solved for the case of a primitive model fused salt near a wall. In particular, the charge density profile near the wall is determined as a function of the charge on the wall. The input data are the bulk pair correlation functions of the ions determined by the GMSA. The results are presented in graphical form. There are, at the present time, no simulation data or experiments with which to compare the results.
A screened potential molecular-orbital calculation of the π-electron systems of metalloporphin, metallochlorin, and metallobacteriochlorin
Authors: Sekino Hideo , Kobayashi Hiroshi
Published: July 2002

The excitation energies and oscillator strengths of the lowest (π,π*) excited states of metalloporphin, -chlorin, and -bacteriochlorin were calculated by the screened potential molecular-orbital theory. The screened potential incorporates instantaneous polarization effects in the molecular Hartree–Fock field. The model reproduced the spectra of the lowest excited states fairly well without use of empirical parameters. The calculated energies of the lowest excited triplets and the Q singlet state were in good agreement with those observed, while the B state was correctly predicted only for the reduced resonance integrals.
Theoretical study of Ca(4s5p 1P)→Ca(4s5p 3P) transitions in collisions with He: Integral cross sections and alignment effects
Authors: Pouilly Brigitte , Alexander Millard H.
Published: July 2002

The fully quantum theory is developed for 1P→3P transitions in collisions of electronically excited (nsn′p) alkaline earth atoms with closed shell atoms. Spin-changing transitions occur by means of the small spin–orbit mixing between the 1P and 3PJ=1 states of the isolated atom, and are facilitated by the crossing between the 1Π and 3Σ potential curves which correlate, respectively, with the 1P and 3P asymptotes. Close-coupled calculations are carried out for the Ca(4s5p)+He system, based on four different choices of the necessary interaction potentials. Particular attention is devoted to the simulation of the recent experiment of Hale, Hertel, and Leone [Phys. Rev. Lett. 53, 2296 (1984)], in which the 1P→3P cross section was determined in a crossed-beam experiment as a function of the orientation of the initially excited 5p orbital. This polarization dependence depends critically on the long-range splitting between the 1Π and 1Σ curves. A fully adiabatic description of the collision dynamics is used to interpret the results of the quantum scattering calculations. No clear-cut theoretical justification is found for the ‘‘orbital following’’ models which have been developed to interpret prior experimental studies of collisions involving excited atoms in P electronic states. Rather, a picture emerges in which the initially selected orientation of the Ca p orbital correlates, at short range, with equal probability to Σ-like and Π-like potential curves. Variations in the 1P→3P cross sections are due to long-range Coriolis coupling between the electronic orbital and nuclear orbital momenta and may reflect quantum interference effects between the Π-like and Σ-like adiabatic potentials.
Solvent isotope effect on mobilities of potassium and chloride ions in water at high pressure. II. A low temperature study
Authors: Ueno M. , Yoneda A. , Tsuchihashi N. , Shimizu K.
Published: July 2002

Limiting molar conductances of the K+ and Cl− ions in heavy and light water have been determined at 5 °C as a function of pressure up to 1500 kgf cm−2 (1 kgf cm−2=0.9807×105 Pa) from the measured conductances and transference numbers of KCl. The residual friction coefficients (Δζ) are obtained for the cation and anion in D2O and H2O by using the determined limiting conductance and the bulk viscosity of solvent, and they are compared with the corresponding values predicted by applying the Hubbard–Onsager (HO) dielectric friction theory at various pressures below 1000 kgf cm−2. At atmospheric and high pressures, the solvent isotope and temperature effects on Δζ observed for the K+ ion are qualitatively in agreement with the theoretical prediction. However, the slightly positive pressure coefficients of Δζ(K+) are obtained in both solvents at 5 °C contrary to the theoretical prediction. Furthermore, Δζ(K+) in D2O at 5 °C is smaller than the theoretical one. These results suggest that another mechanism such as the passing through cavities one plays a more important role in ion migration of the K+ ion at lower temperatures and in D2O. On the other hand, the reverse solvent isotope and temperature effects on Δζ(Cl−), and the negative values of Δζ(Cl−) are found. These anomalies cannot be explained by the continuum theory.
Measurement of the short-time self-diffusion coefficient in dilute and concentrated suspensions: Influence of direct particle interactions
Authors: van Veluwen A. , Lekkerkerker H. N. W. , de Kruif C. G. , Vrij A.
Published: July 2002

The short-time self-diffusion coefficient of two types of monodisperse colloidal particles differing only in interparticle interactions were measured by dynamic light scattering both in dilute as well as in concentrated dispersions. The results indicate that the short-time self-diffusion is remarkably insensitive to direct particle interactions. The dependence on concentration is in good agreement with the theoretical predictions.
Multichannel quantum theory for propagation of second order transition amplitudes
Authors: Singer Sherwin J. , Lee Sungyul , Freed Karl F. , Band Yehuda B.
Published: July 2002

We study the Green’s function representation of second order transition amplitudes for the transition from an initial to a final quantum state that occurs because both are weakly coupled to a manifold of intermediate states. These processes apply to Raman scattering, two-photon absorption, fluoresecence from dissociating molecules, electron stimulated desorption, etc., and the transition amplitudes are called generalized Raman amplitudes. The generalized Raman transition amplitudes are expressed in terms of matrix elements of a multichannel Green’s function whose determination requires the simultaneous generation of the regular and irregular solutions of the multichannel Schrödinger equation for the intermediate state manifold. However, the numerical propagation of the generalized Raman transition amplitudes through classically forbidden regions requires, in effect, the simultaneous propagation (in the same direction) of both the regular and irregular solutions of the intermediate manifold Schrödinger equation, and use of standard multichannel scattering methods lead to numerical instabilities. We introduce new methods for maintaining both the stability and linear independence of the regular and irregular multichannel intermediate manifold eigenfunctions as they are one-way propagated along a reaction coordinate with standard quantum scattering methods. The methods may be used with systems having asymptotically open or closed channels or both in the intermediate state manifold. First order transition amplitudes, such as state selected photodissociation amplitudes to fragment states of the intermediate state manifold, emerge as a by-product of our algorithm, and the computation of the second order generalized Raman amplitudes scales roughly as double the computation time required for the first order amplitudes.
Penning and associative ionization of mercury by collision with metastable rare gas atoms
Authors: Appolloni L. , Brunetti B. , Hermanussen J. , Vecchiocattivi F. , Volpi G. G.
Published: July 2002

The ionization of mercury atoms by thermal energy collisions with metastable neon, argon, and krypton has been studied in a crossed beam experiment. The production of both Hg+ and RHg+ (R=Ne, Ar, Kr) ions has been observed and the collision energy dependence of the cross section for each ionization channel has been measured. The experimental relative cross sections have also been analyzed in terms of an optical model. In all cases the main ionic product is the Hg+ ion. The average relative yield of RHg+ appears to increase going from neon to krypton. The different role of the two spin–orbit states of the metastable krypton atoms has been assessed: The Kr(3P0) atoms can produce both Hg+ and KrHg+ in the whole energy range investigated, while the Kr(3P2) atoms can ionize the mercury only for a collision energy larger than ∼0.15 eV, producing KrHg+ in the energy range between ∼0.15 and ∼0.5 eV, and producing also Hg+ at energies larger than ∼0.5 eV.
Mo/ller–Plesset perturbation theory for van der Waals complexes bound by electron correlation effects: Ground states of the Ar and Mg dimers
Authors: Chal/asiński Grzegorz , Funk David J. , Simons Jack , Breckenridge W. H.
Published: July 2002

We demonstrate that MPPT through fourth order is suitable for studying van der Waals correlation-bound complexes provided that (a) accurate calculations are carried at the SCF level; (b) efficient basis sets for intersystem correlation effects (i.e., dispersion) are used; and (c) the full counterpoise (CP) method is applied to correct for basis set superposition error (BSSE). Interaction potentials are obtained for Ar2 and Mg2 with extended basis sets that contained up through the f-symmetry functions. For Ar2 the potential is characterized by R≈7.3 a0 and De∼0.34 mhartree and for Mg2 by Re≈7.4a0 and De∼2.1 mhartree. The discrepancies between our potentials and the most accurate semiempirical and experimental results (Ar2:Re∼7.1a0, De ≈0.45 mhartree; Mg2:Re≈7.35a0, De∼1.93 mhartree, are analyzed in detail and attributed to the lack of higher than f-symmetry functions, as well as, in the Mg2 case, to the approximate nature of the MP4 approach.
Curvature-related molecular segregation in lyotropic aggregates of potassium palmitate and benzyl alcohol: A 2H NMR study
Authors: Pope J. M. , Doane J. W.
Published: July 2002

The ternary system potassium palmitate/benzyl alcohol/water has been studied by 2H NMR of selectively 2H labeled amphiphiles. At low temperatures the samples comprise a mixture of cylindrical aggregates (Rα) and pure potassium palmitate gel (Lβ), which transforms first to a liquid crystalline ribbon phase (RBα) and then a lamellar (Lα) phase with increasing temperature. The ribbon phase exhibits strong molecular segregation of the benzyl alcohol molecules into the curved edge of the ribbons. The results enable quantitative measurements of the molecular segregation to be made. The presence of phase transitions within the low temperature cylindrical aggregates and high temperature lamellar phases is also reported.
Effect of high pressure on the exchange interactions in binuclear chromium (III) complexes
Authors: Riesen Hans , Güdel Hans U.
Published: July 2002

Single crystals of [LCr(III) (OH)3Cr(III)L](ClO4)3 (L=1,4,7-trimethyl-1,4,7-triazacyclononane) and [(NH3)5Cr(III)OHCr(III)(NH3)5]Cl5⋅H2O were studied by low-temperature luminescence spectroscopy under hydrostatic pressure up to 42 kbar. Dramatic shifts of the sharp luminescence lines up to 300 cm−1 were observed. The ground state exchange parameters ‖J‖ were found to increase linearly with pressure by d‖J‖/dP=0.7 and 0.15 cm−1/kbar, respectively. Using the shift of the 4A2→4T2 absorption bands as a function of the applied pressure the local compressibilities were estimated. The increase of ‖J‖ with decreasing interatomic distances was rationalized by extended Hückel calculations.
Dynamics of the reactions of Zn(4s4p 3P1) with H2, HD, and D2: Rotational and vibrational quantum-state distributions of ZnH (ZnD) products
Authors: Breckenridge W. H. , Wang Jiang-Hua
Published: July 2002

The complete initial vibrational and rotational quantum state distributions of ZnH(ZnD) products in the reactions of Zn(4s4p 3P1) with H2, HD, and D2 have been determined using a laser ‘‘pump-and-probe’’ technique. The most striking result is that the quantum-state distributions of ZnH (or ZnD) products are essentially unchanged when the mass of the leaving atom is doubled, from H to D. It is suggested that this indicates that simple impulsive bond breaking cannot play a large role in the reaction of Zn(3P1) with H2, and that potential surface anisotropy in the decomposition of bent H–Zn–H insertion intermediates could be responsible for the rotational energy distributions of the products. Similar isotopic results for reactions of Cd(5s5p 3P1), Hg(6s6p 3P1), and O(1D2) with H2, HD, and D2 are noted, and the general implications of the lack of an isotope effect are discussed in detail. The branching ratio of ZnD vs ZnH formation in the reaction of Zn(3P1) with HD was determined to be 1.1±0.2 and it was pointed out that several ‘‘insertion’’ reactions have now been shown to have branching ratios for reaction with HD which are very near 1.0, inconsistent with earlier qualitative arguments that such processes should lead to high branching ratios.
Collisional energy transfer in highly vibrationally excited H2CO(X̃ 1A1)
Authors: Temps F. , Halle S. , Vaccaro P. H. , Field R. W. , Kinsey J. L.
Published: July 2002

Collisional energy transfer has been investigated in highly vibrationally excited H2CO (X̃ 1A1) at Evib≅11 400 cm−1 using the method of stimulated emission pumping–transient absorption spectroscopy (SEP-TAS). Total depopulation and state-to-state rate constants were measured for several rotational levels of the 2444 vibrational state. For H2CO self-relaxation the depopulation rate constant of the 61,5 level was measured to be k0=(3.12±0.13)×10−9 cm3/molecule s. An analysis of the state-to-state data with a simplified master equation approach yielded rate constants k1=(7.7±1.2)×10−10, k2=(1.2±0.3)×10−10, and k3=(0.6±0.3)×10−10 cm3/molecule s for collisions with ΔJ=1, 2, and 3, respectively, and ΔKa=0, ΔKc=ΔJ, the reverse rate constants being given by microscopic reversibility. Thus, the ΔJ=±1 steps account for ∼50% of all inelastic collisions, which can be rationalized in terms of a simple dipole–dipole interaction. At Evib≅11 400 cm−1, where ρvib≅0.42/cm−1, collisions seem to conserve the vibrational character in spite of the fact that the energy gap between adjacent vibrational states is a fraction of the transferred rotational energy.
Angular overlap model for the Jahn–Teller coupling constants in the orbital triplet states of d5 ions: Case of Mn2+ in ZnS and ZnSe
Authors: Parrot R. , Naud C. , Gendron F. , Porte C. , Boulanger D.
Published: July 2002

A theoretical determination of the Jahn–Teller coupling constants of d5 ions in tetrahedral clusters is performed in the framework of the angular overlap model and the quasimolecular approximation. It is shown that the angular model correctly predicts the trend of the preponderant coupling of the 4T1(G) and 4T2(G) levels of Mn2+ in MnS4 and MnSe4 to the E-vibrational effective mode. Finally, the limitations of this model are discussed. In particular, it is shown that more elaborated covalent models are needed to interpret the Jahn–Teller coupling coefficients of the 4E and 4A1 levels of d5 configurations and to determine the influence of the crystal on the clusters.
A new potential energy surface for the CH3+H2↔CH4+H reaction: Calibration and calculations of rate constants and kinetic isotope effects by variational transition state theory and semiclassical tunneling calculations
Authors: Joseph Tomi , Steckler Rozeanne , Truhlar Donald G.
Published: July 2002

We present a sequence of three successively improved new semiempirical potential energy surfaces for the reaction CH3+H2→CH4+H. The semiempirical calibration is based on ab initio electronic structure calculations and experimental thermochemical data, vibrational frequencies, reaction rate constants, Arrhenius parameters, and kinetic isotope effects (KIE’s). To compare to the experimental kinetic data we apply variational transition state theory and semiclassical estimates of tunneling probabilities. We also provide detailed factorization analyses of the KIE’s to illustrate the way in which various surface features contribute to the overall KIE’s, and we discuss the substantial difficulties in attributing specific kinetic results to isolated potential energy surface features. Each of the three new surfaces, called J1, J2, and J3, has a thinner barrier than the one before. In addition, we provide one example, called surface J2A, showing the effect of making the barrier even thinner than on the best surface. The best surface yields rate constants for the forward and reverse reaction, activation energies, and KIE’s that are consistent with most of the available experimental data.
Laser probing of the rotational alignment of N+2 drifted in helium
Authors: Dressler Rainer A. , Meyer Henning , Leone Stephen R.
Published: July 2002

Results are presented on laser-induced fluorescence studies of the rotational alignment of N+2 drifted in helium. The alignment which is caused by collisions of the ions with the helium buffer gas is observed in the uniform electric field of a drift tube. The angular momentum vectors of the ions are preferentially aligned perpendicular to the electric field vector. At a drift field of 14 Td, corresponding to a collision energy of 52 meV (c.m.), a quadrupole moment A(2)0=−0.11±0.03 is determined for the N=10 rotational state. This yields an approximate population ratio of 2:3 for finding molecules with rotational angular momentum vectors parallel and perpendicular to the electric field vector, respectively. In addition to the alignment studies, a detailed characterization of the drift tube using laser-induced fluorescence detection of N+2 is presented. Theoretical results for the determination of alignment parameters using saturated laser-induced fluorescence are presented.
Enhanced photochemistry on silver surfaces
Authors: Wolkow R. A. , Moskovits M.
Published: July 2002

Pyrazine and triazine are found to photofragment on rough silver surfaces to form graphitic carbon as a major product. The rate-determining step is found to be biphotonic. The enhanced photochemistry is shown to take place most efficiently directly at the metal surface rather than a short distance above the surface as would be expected on the basis of a competition between nonradiative relaxation of the excited molecule and excitation by the surface-enhanced electric field. Moreover, it was found possible to increase the photolysis rate further by overcoating the adsorbed molecules with an appropriate number of layers either of the photoreagent itself or of another molecule, tuning thereby the localized surface plasmon of the rough surface into resonance with the exciting laser. We suggest that in a previous report [Goncher, Parsons, and Harris, J. Phys. Chem. 88, 4200 (1984)] of enhanced photochemistry at rough surfaces the effect of displacing the photoreagent from the surface using spacer layers was in fact to tune the photochemical rate by the same mechanism. The graphitic photoproduct is speculated to arise from a multiphoton process occuring subsequent to the initial two-photon step, and the apparent absence of nonradiative decay is ascribed to intersystem crossing to a triplet manifold from which the nonradiative decay rate is substantially reduced. These last proposals are presented tentatively and other possibilities are discussed.
Correlated molecular and cumulative atomic multipole moments
Authors: Sokalski W. A. , Sawaryn A.
Published: July 2002

Molecular charge distributions for several neutral and charged molecules (LiH, HF, H2O, NH3, CH3, CH4, OH−, H3O+, N2, CO, HCN, H2CO, C2H2, and N2O) have been decomposed into the sets of correlated cumulative atomic multipole moments (CCAMM). These segmental moments sum up to the corresponding correlated molecular multipole moments (CMMM) and match closely the available experimental values. The corresponding correlation contributions have been compared with basis set extension effects for a number of 6-311G(nd/mp) basis sets with multiple polarization functions. The uncorrelated moments have been presented too, allowing the correction of intramolecular correlation and basis set truncation effects in the electrostatic multipole interaction energies, molecular electrostatic potentials, etc.
Spherical wave close coupling wave packet formalism for gas phase nonreactive atom–diatom collisions
Authors: Sun Y. , Mowrey R. C. , Kouri D. J.
Published: July 2002

In this paper we discuss the use of the total angular momentum representation in the close coupling-wave packet (CCWP-J) method for solving the time dependent Schrödinger equation for inelastic, nonreactive gas phase atom–diatom collisions. This enables the wave packet propagation for the relative motion to be reduced from three dimensions to one. The approach utilizes a close coupling expansion of the wave packet into subpackets labeled by quantum numbers for total angular momentum J, z-component of angular momentum m0, rotor angular momentum j, and orbital angular momentum l. The number of coupled subpackets is less than the number for the plane wave boundary condition CCWP method when J<jmax and they are equal when J≥jmax. The present method requires solving for the time evolution of such coupled subpackets for 0≤J≤jmax +lmax, where lmax is the largest orbital angular momentum for which significant scattering occurs. However, the number of grid points required in the fast Fourier transform portion of the evolution of the wave packet will be far fewer since only a 1D FFT transform is required in the present version of the CCWP-J. All the other attractive features of the CCWP method are common to both the total angular momentum and plane wave representation versions of the CCWP; namely, results are obtained over the range of energies included in the initial packet, the labor scales as the number of rotor states squared, and standard approximation methods may be used in conjunction with the formalism. We also present the l-labeled coupled states or centrifugal sudden wave packet (CSWP) formalism as an example approximate version of this approach. The CCWP-J method is illustrated by application to a model atom–diatom collision problem. The extension to treat collisions involving a vibrating rotor is given in an Appendix. Finally, we compare features of the CCWP-J and CSWP with standard close coupling, CS approximation, and the plane wave basis CCWP methods.
Collisional energy transfer of vibrationally highly excited CS2. II. Temperature dependence of 〈ΔE〉 from experiments in shock waves and laser-heated reactors
Authors: Heymann M. , Hippler H. , Plach H. J. , Troe J.
Published: July 2002

Collisional energy transfer between vibrationally highly excited CS2 molecules and 22 different collision partners has been measured directly by time resolved UV absorption spectroscopy after laser excitation. Experiments at temperatures up to 1200 K were performed in a reactor heated by cw CO2 laser radiation. Experiments at higher temperatures (up to 2400 K) with the collision partner Ar were done with pulsed laser excitation behind reflected shock waves. At all temperatures a marked dependence of the average energy 〈ΔE〉 transferred per collision on the excitation energy is noted. Also the molecular size of the deactivating molecule has a large influence on 〈ΔE〉. The temperature coefficients of 〈ΔE〉∝Tn are small in all cases. They vary between n=−0.6 for n-heptane and n=+1.0 for Ne.
Normal form analysis of multiple bifurcations in incompletely mixed chemical reactors
Authors: Puhl Andreas , Nicolis Grégoire
Published: July 2002

Using the theory of normal forms, we investigate the effects of mixing in a continuous flow stirred tank reactor (CSTR) for a reaction model exhibiting oscillatory behavior in the vicinity of a degenerated bifurcation point (here, a Takens–Bogdanov point). In addition we show without specification of a particular reaction system that, as long as reaction rates remain much slower than the inverse mixing time, incomplete mixing introduces a new bifurcation parameter for nonpremixed feeding conditions, whereas premixed feeding conditions merely lead to a renormalization of flow rate.
Vibrational state mixing of individual rotational levels in 1-butyne near 3333 cm−1
Authors: de Souza A. M. , Kaur Devinder , Perry David S.
Published: July 2002

The high resolution infrared spectrum of jet-cooled 1-butyne is reported for the acetylenic C–H stretch. The absorbance of an F-center laser beam is enhanced by a multiple reflection cell surrounding the free jet. Although the spectrum in the gas phase (300 K) is a featureless rotational contour, at 3 K sharp features are observed. For J′=0, 1, 2, the K′a =1 lines are spectroscopically well-behaved, but the Ka =0 lines are split into multiplets. Analysis of the K′a =1 lines gives the constants (B′+C′)/2=0.143 82(11) cm−1, (B′−C′)/2=0.007 20(11) cm−1, and ν0+A′−A″=3333.0182(9) cm−1. The quantities in parentheses are two standard deviations in units of the last digit. It is concluded that the Ka =0 levels are mixed with the bath of background vibrations by anharmonic matrix elements the order of 0.01 cm−1.
Transient solvation of polar dye molecules in polar aprotic solvents
Authors: Kahlow Michael A. , Kang Tai Jong , Barbara Paul F.
Published: July 2002

Subpicosecond fluorescence spectroscopy of the polar dye molecules coumarin 311 and coumarin 102 has been used to measure the microscopic solvation dynamics of several polar aprotic solvents. The measured solvation times are significantly longer than the longitudinal relaxation times τ1 of the solvents. τ1 is the predicted time for solvation according to dielectric continuum theory. The experiments were made with a newly constructed subpicosecond ultraviolet emission apparatus that takes advantage of recent advances in ultrafast laser technology. The newly developed, time saving procedure [Nagarajan et al., J. Chem. Phys. 86, 3183 (1987)] for measuring the microscopic solvation response functions was used in this research.
Diode laser spectroscopy for monitoring the yield of metastable Cl from photodissociation of simple molecules
Authors: Tiemann Eberhard , Kanamori Hideto , Hirota Eizi
Published: July 2002

Time resolved spectroscopy of the atomic fine structure transition of Cl, 2P1/2–2P3/2 at 882.35 cm−1 is applied to derive relative yields of the metastable level 2P1/2 of Cl(Cl*) from the photodissociation by excimer laser radiation of KrF or ArF. Molecules such as HCl, CH3Cl, CH2Cl2, phenyl chloride, and PCl3 are studied, getting for the first time the possibility of systematic comparison of yields for Cl* and also of earlier results on heavier halogens obtained by other authors using different methods. Most yields determined in the present work are close to a value as calculated from the statistical weights of the two fine structure levels of Cl. This finding is related in a simple diabatic picture to the small spin–orbit interaction in Cl compared to that in iodine. Additionally some rate constants for the collisional quenching of the metastable Cl by the parent molecule are derived.
ESR spectrum of I17O2: Asymmetric hyperfine tensor of 17O
Authors: Byberg J. R.
Published: July 2002

ESR spectra of I 17O2 embedded in solid KC104 exhibit a complicated 17O hyperfine structure, which allows an accurate determination of the hyperfine and quadrupole tensors A(17O) and Q(17O). A(17O) is strongly anisotropic in the molecular plane and contains an asymmetric component. These unexpected features are accounted for in terms of second-order contributions to A(17O) arising from cross terms of the spin–orbit coupling and the orbital and spin dipole hyperfine interaction. The second-order terms also contribute substantially to the hyperfine tensor of the iodine nucleus.
Concentration dependent depolarized Rayleigh scattering of dimethylformamide-h7 and -d7 in CCl4 solution
Authors: Rodriguez R. , McHale J. L.
Published: July 2002

The formulas derived in our previous paper on the first moments of far-infrared and depolarized Rayleigh spectra are applied to the study of dimethylformamide (DMF) and its deuterated derivative in carbon tetrachloride solution. Changes in the peak and average frequencies of the R(ω) representation of the depolarized Rayleigh wing data are measured as a function of concentration and compared to limited far-infrared absorbance spectra of the more dilute solutions. The measured average frequencies of the DRS data agree well with those calculated from the area under the imaginary correlation function divided by time, and with the values calculated from the first term in the series expression for ω̄ in terms of fourth and higher even moments. Collisional contributions to the spectral moments complicate the determination of mean square torques and orientational correlations from the DRS spectra. The first and second moments were found to be larger than the theoretical values for purely reorientational scattering and were fairly independent of concentration. The data suggest that the additional scattering is due to a dipole–induced dipole contribution to the polarizability αi⋅Ti j⋅αj.