bluuebunny/crossref_metadata_2025 huggingface dataset:
The orientation of pyridine on Pd(110): An angle-resolved photoemission study
Authors: Netzer F. P. , Rangelov G. , Rosina G. , Saalfeld H. B.
Published: July 2002

The orientation of pyridine on Pd(110) has been studied by angle-resolved UV photoelectron spectroscopy (ARUPS) with use of synchrotron radiation in conjunction with low-energy electron diffraction (LEED) and work function measurements. The pyridine molecules in the saturation adlayer are arranged into an ordered c(4×2) LEED structure. The strong anisotropies of photoemission into the [001] and [11̄0] azimuths suggest azimuthal orientation. Analysis of ARUPS in terms of polarization-dependent symmetry-derived selection rules provides strong support for a near-parallel η6 configuration, and a surface bonding via the aromatic π electron system. Experiments utilizing photoemission into the mirror planes of the (110) surface allow to specify the azimuthal orientation of the molecules, viz., with the N atom aligned along the [001] direction. In combination with the ordered LEED pattern a structure model for the pyridine adlayer is presented.
The valence isoelectronic molecules CCO, CNN, SiCO, and SiNN in their triplet ground states: Theoretical predictions of structures and infrared spectra
Authors: DeKock Roger L. , Grev Roger S. , Schaefer Henry F.
Published: July 2002

We have completed theoretical studies on the 14-valence–electron molecules CCO, CNN, SiCO, and SiNN in their 3Σ− linear ground states. The theoretical methods that we employed were ab initio self-consistent field (SCF), configuration interaction including single and double excitations from the dominant configuration (CISD), and complete active space (CASSCF). The extent of bonding of the C or Si atom with the CO or N2 ligand is significantly different in each case. The strongest bond is formed between the C and N atoms in CNN and the weakest between the Si and N atoms in SiNN. Only for CNN does the resultant geometry vary significantly with the theoretical method. Our theoretical harmonic frequencies are compared to the experimental infrared spectra reported for these molecules in matrix isolation studies. The theoretical and experimental results are easily brought into harmony for CCO and SiCO, but not for CNN and SiNN.
Thermal electron attachment to NO. I. The mechanism and the three-body rate constants
Authors: Shimamori H. , Hotta H.
Published: July 2002

Mechanism of thermal electron attachment in pure NO and NO–M mixtures (M: He, Ne, Ar, H2 , CO2 , and n-C4H10 ) at room temperature has been studied by using a pulse radiolysis–microwave cavity technique. The pressure dependence of the attachment rates indicates that a collisional electron detachment from NO− competes with electron attachment at relatively low pressures but three-body attachment dominates at higher pressures. The three-body rate constants in pure NO is (6.5±0.2)×10−31 cm6  molecule−2  s−1 and those values for the other gases as third bodies range from 1×10−32 (for He) to 3.6×10−30 cm6 molecule−2 s−1 (for n-C4H10), and no systematic correspondence can be seen between the values obtained here and those estimated previously from the data of reverse (electron detachment) processes.
On the evaluation of non-Born–Oppenheimer interactions for Born–Oppenheimer wave functions. V. A body fixed frame approach. Applications to isotope effects on equilibrium geometries and the adiabatic correction for the X 1Σ+ state of LiH
Authors: Jensen James O. , Yarkony David R.
Published: July 2002

The evaluation of the total second derivative nonadiabatic coupling matrix element H(J,I,R)=〈ψJ(r;R)‖∑i(−1/2 Mu)(∂2/∂R2i )ψI(r;R)〉r is considered. Here ψJ(r;R) is the adiabatic Born–Oppenheimer electronic wave function which in this work will be approximated by a large-scale CI wave function developed from an MCSCF reference space. For diatomic and triatomic systems the computational effort associated with the evaluation of H(J,I,R) can be reduced considerably by the use of a body fixed frame approach. In this approach costly evaluation of the derivative wave function with respect to noninternal degrees of freedom in the space fixed frame is replaced by the evaluation of matrix elements of many electron operators including the mass polarization operator (total electronic linear momentum squared) and the L2 operator (total electronic orbital angular momentum squared). The equivalence of the body fixed frame and space fixed frame results leads to valuable diagnostic equations which provide stringent tests of the derivative methodology used to evaluate the remaining second derivatives with respect to internal coordinates. The methods presented here are applied to the benchmark systems BeH+ and LiH. The Born–Oppenheimer diagonal correction or adiabatic correction (AC) is evaluated for the X 1∑+ state of these systems and used to consider the effect of isotopic substitution on equilibrium geometries. For the X 1∑+ state of LiH a troubling discrepancy exists between the AC determined by advanced theoretical and experimental techniques.

For R≲Re the AC determined directly with specialized CI wave functions and the experimental value inferred from a detailed spectroscopic analysis of the A→X emission agree. However, for R>Re theory and experiment disagree qualitatively. For R≲Re our results are consistent with the previous work. For R>Re our results are in accord with the experimentally derived AC.
Autoionization dynamics in the valence-shell photoionization spectrum of CO
Authors: Hardis J. E. , Ferrett T. A. , Southworth S. H. , Parr A. C. , Roy P. , Dehmer J. L. , Dehmer P. M. , Chupka W. A.
Published: July 2002

Autoionizing Rydberg series in the valence-shell spectrum of CO have been studied by determining the high resolution relative photoionization cross section of cooled CO in the energy region 14.0–20.0 eV and by determining the vibrational branching ratios and the photoelectron angular distributions for production of CO+ X 2Σ+, v+=0–2 in the energy region 16.75–18.75 eV. Of particular interest are three prominent spectral features between 17.0 and 17.5 eV that result from interactions involving Rydberg series converging to the excited A 2Π and B 2Σ+ states of the ion. The results are discussed in the context of recent two-step multichannel quantum defect theory calculations by Leyh and Raseev (the following paper).
Parametrization of a Heitler–London valence bond Hamiltonian from complete-active-space self-consistent-field computations: An application to chemical reactivity
Authors: Bernardi Fernando , Olivucci Massimo , McDouall Joseph J. W. , Robb Michael A.
Published: July 2002

A simple practical procedure is proposed for the transformation of a complete-active-space self-consistent-field (CAS-SCF) wave function to Heitler–London valence bond space via the construction of an effective (Heisenberg) Hamiltonian. With such a procedure, the computed Heitler–London parameters Q and Kij can be used for a posteriori rationalization of the bonding effects in molecular structures. In particular, the diabatization of the adiabatic potential surface, first proposed by Evans can now be accomplished in a rigorous way. The technique is applied to some previously computed transition structures for cycloaddition reactions.
Pressure effects on kinetics and decay processes in krypton after selective photoexcitation
Authors: Audouard E. , Laporte P. , Subtil J.-L. , Damany N.
Published: July 2002

Kinetics associated with the decay of the 5s[3/2]01 (3P1) excited state of krypton is investigated using selective excitation, in the density range 2.5×1018 to 375×1018 atoms cm−3 (pressure from 104 to 1.5×106 Pa). The vacuum ultraviolet (VUV) one-photon excitation around 123.6 nm is provided by monochromatized light from a laser-generated rare gas plasma. The basic experimental data are obtained through the time-resolved second continuum excimer fluorescence at 145 nm. When initial conditions give purely 3P1 atomic excitation a single slow component is observed at densities up to 15×1018 cm−3, corresponding to the 1u,0−u, v=0 excimer radiative decay (265 ns). From 15 to 60×1018 cm−3, a second fast component shows up, corresponding to the 0+u(3P1), v=0 excimer radiative decay. Above 60×1018 cm−3 the intensity ratio between the two components remains constant (F/S≊0.21). Behavior rather similar to that previously reported for argon and xenon is found. In particular one concludes that the 3P2 atomic state plays a major role as an intermediate state in the 1u,0−u dimer formation whereas possible short-distance attractive g states do not seem to play any significant role. A self-consistent analysis leads to a detailed quantitative description of the kinetics. In particular, it is found that dimer formation times from 3P1 and 3P2 atomic states are in the ratio 3/1. In the considered density range collisional rates are found to vary quadratically.
Infrared spectroscopy of mass-selected Cs(CH3OH)+N N=4–16
Authors: Liu Wen-Long , Lisy James M.
Published: July 2002

The infrared spectra of mass-selected cesium cations solvated by 4–16 methanol molecules have been obtained from vibrational predissociation spectroscopy. The solvated ions were prepared in a molecular beam apparatus equipped with a quadrupole mass spectrometer. A cw CO2 laser operating in the 9.6 μ region was used to excite the methanol C–O stretch in the solvated ions. Depletion of the mass-selected solvated ion signal due to vibrational predissociation was used to generate the infrared absorption spectra. This work represents the first infrared study of metal ion–solvent clusters. The spectra display a clear variation with the number of solvents throughout the range of 4–16. A single peak at ∼1031 cm−1 steadily grew in intensity from four to ten methanol molecules. The presence of additional methanol molecules beyond ten did not affect the intensity of the 1030 cm−1 peak, but a new spectral feature became observable to higher frequency. The spectroscopic data are consistent with a first solvation shell size of ten methanol molecules. This is the first direct experimental observation of a filled solvent shell associated with the cesium cation. The C–O stretching frequencies of the methanol molecules are compared with existing experimental results involving neutral clusters.
TMPD–CA revisited: Ionicity, stack dimerization, and phase transition of a key mixed stack charge transfer crystal
Authors: Girlando A. , Painelli A. , Pecile C.
Published: July 2002

Raman and single crystal polarized reflectance spectra (from 400 to 10 000 cm−1) of the tetramethylparaphenylenediamine–chloranil (TMPD–CA) charge transfer complex are reported. A careful interpretation of the data leads one to estimate for TMPD–CA a degree of ionicity of 0.64 at room temperature, indicating a quasi-ionic ground state. Moreover, we show that the mixed stack formed by TMPD and CA is dimerized at all temperatures. The latter finding is substantiated by an a priori calculation of the infrared conductivity spectrum polarized along the stack axis. The calculation is based on a model taking into account the coupling of the molecular vibrations with the charge transfer excitations through the modulation of both on-site energies and charge transfer integrals; moreover the intrinsic intensity of the infrared active modes is introduced semiempirically. The spectral simulation finds a further application in interpreting the subtle spectral changes observed in correspondence with the TMPD–CA 250 K phase transition: it is suggested that the phase transition is a disorder–order, orientational one. An extensive comparison with the results previously obtained by other experimental techniques allows one to solve early contradictions and definitely clear up the nature of the ground state of this key charge transfer crystal.
Density effects on collision induced Raman scattering in CO2–Ar and CO2–Xe mixtures
Authors: Song X. , Frattini R. , Jonas J.
Published: July 2002

The density dependence of the intensity of the CO2 Raman forbidden ν2 band (667.3 cm−1 ) has been studied in the CO2 –Ar and CO2 –Xe mixtures at 40 and 80 °C. The experimental results show that the band intensity increases linearly with density from 0.057 to 0.166 mol/cm3 which indicates that two-particle collisions dominate the scattering process. The intensity of the ν2 band is higher in the CO2 –Xe mixture than that in the CO2 –Ar mixture, and the experimental intensity ratio for the two mixtures is 2.2±0.2. The very good agreement between the experimental intensity ratio and the ratio calculated theoretically for the dipole–quadrupole (Aα) contribution to the collision induced scattering indicates that the Aα mechanism is responsible for the appearance of the forbidden band.
Cross sections for the anisotropic interaction of NNO with various molecules
Authors: Jameson Cynthia J. , Jameson A. Keith , Hwang Joseph K. , Smith Nancy C.
Published: July 2002

Cross sections for the relaxation of the rotational angular momentum vector of the NNO molecule on collision with CO, Ar, CO2, NNO, Kr, CF4, and Xe, have been measured by 15N spin relaxation in 15N15NO molecules. The relaxation times of the two nuclei are in a ratio 1.86, independent of density, collision partner, or temperature. Except for Ar and CO, the cross sections are larger than the hard sphere cross sections and their temperature dependences range from T−0.8 to T−1.0.
Luminescence quenching in organized assemblies treated as media of noninteger dimensions
Authors: Lianos Panagiotis
Published: July 2002

Various decay models for luminescence quenching in organized assemblies have been derived in relation with the dimensionality and the effective microviscosity of the reaction medium. It has been shown that the most general models should involve the fractal or the spectral dimension of the reaction medium. A decay model has been derived for each major category where organized assemblies may belong. The theory has been successfully tested on some water-in-oil microemulsions below and at the percolation threshold.
Study on conformations of n-alkanes in urea inclusion compounds using CH dipolar switching-angle sample spinning nuclear magnetic resonance and MM2 molecular mechanics calculations
Authors: Imashiro Fumio , Kuwahara Daisuke , Nakai Toshihito , Terao Takehiko
Published: July 2002

The CH dipolar spectra of n-heptane, n-octane, n-nonane, and n-decane in urea inclusion compounds were separately obtained for the α-, β-, and inner-CH2 and CH3 groups using the SASS (switching-angle sample spinning) NMR method. The CH dipolar coupling constant for the CH2 group decreases with the increasing distance from the center of the molecule and with the decreasing chain length. MM2 molecular mechanics calculations suggested that the included molecule can have various conformations, and provided their geometries and populations. The experimental results could be reproduced by using the calculated parameters and assuming the occurrence of dynamical disorder of various conformations in addition to rotation of the included molecule around the long axis. This is the first evidence for the existence of conformations other than all-anti in the included n-alkane molecule; it has long been taken to be in the all-anti conformation. Previous studies which were explained by the all-anti conformation are reinterpreted by dynamical disorder of various conformations.
The statistical mechanics of a liquid of breathing hard spheres
Authors: Desjardins Steven G. , Stratt Richard M.
Published: July 2002

The study of fluctuating internal degrees of freedom of molecules in liquids has progressed significantly in the past decade, but there is still no general understanding of the interplay between intramolecular dynamics and repulsive forces. In particular, the existing theories are largely inappropriate for studying the role of liquid-state packing in determining electronic or vibrational structure. We point out, however, that a liquid of atoms with fluctuating hard-sphere diameters is a model which not only explores just this kind of repulsive-force coupling, but which can be solved analytically within the framework of standard liquid theory by rearranging previous Percus-Yevick results.
Measurements of density and expansion coefficient for the nematic, lamellar smectic, and isotropic phase of micellar systems
Authors: Photinos Panos , Saupe Alfred
Published: July 2002

Measurements of the density and expansion coefficient as a function of temperature are presented for two micellar systems, namely, decylammonium chloride/NH4Cl/water, and cesium perfluorooctanoate/water. The measurements cover the lamellar smectic, the nematic, and the isotropic micellar solution. The measurements show that away from the phase transitions, the density decreases smoothly with increasing temperature. At the nematic-to-isotropic transition the density shows steplike features, indicating a change in the micellar structure at the transition, with denser packing of the amphiphiles in the micelles of the isotropic phase. A similar change is observed in the lamellar to nematic transition, however, the detailed behavior at this transition showed some dependence on the history of the sample. The expansion coefficients decrease at the transitions. Considerable pretransitional behavior is observed about the lamellar-to-nematic transition.
A solution of the gauge origin problem for the magnetic susceptibility
Authors: Geertsen Jan
Published: July 2002

A polarization propagator based theory for calculation of the diamagnetic part of the magnetic susceptibility is presented. Since the paramagnetic term also may be obtained from the propagator it is thus possible to compute both contributions at the same level of approximation, e.g., in RPA, SOPPA, and CCPPA. Contrary to other commonly used procedures the present method gives a total result for the susceptibility, which—even in a limited basis set—is independent of the origin chosen for the vector potential.
Cumulant methods and short time propagators
Authors: Coalson Rob D. , Freeman David L. , Doll J. D.
Published: July 2002

The present paper clarifies a number of issues concerning the general problem of constructing improved short time quantum mechanical propagators. Cumulant methods are shown to be a particularly convenient tool for this task. Numerical results comparing methods based on partial averaging and on gradient approaches are presented for simple model problems and for many particle quantum fluids.
Optimization of kinetic and thermodynamic properties in irreversible open chemical and thermal engines
Authors: Hjelmfelt Allen , Harding Robert H. , Ross John
Published: July 2002

Periodic variations of an external parameter or constraint of open chemical systems have been shown to induce changes in time averaged kinetic and thermodynamic quantities. We examine the effects of the analytic form of the periodic variation on the time averaged quantities and find the maximum changes obtainable through periodic variations. A variational procedure is proposed, based on a Fourier expansion of the form of the periodic perturbation, the laws of thermodynamics, conservation of matter, and the kinetics. The efficiency of power production in a combustion system is examined with this method in a numerical example. A unique maximum in the efficiency is found, with the gains achievable for more complex functions exceeding those for a sinusoidal perturbation. We interpret the changes in efficiency in terms of the magnitude of the response of the system (resonance) and phase shifts between the periodic perturbations and the response of the system. We illustrate the mechanisms of efficiency changes in this system with two examples; one in which the periodic perturbation affects the phase relations and one in which the periodic perturbation affects the magnitude of the response. Finally, we note that multiple attractors may coexist in this system for certain forms of the periodic perturbation, each with a distinct efficiency.
Determination of atomic point charges and point dipoles from the Cartesian derivatives of the molecular dipole moment and second moments, and from energy second derivatives of planar dimers. II. Applications to model systems
Authors: Dinur U. , Hagler A. T.
Published: July 2002

The use of Cartesian derivatives of molecular observables in the analysis of intermolecular interactions is demonstrated through ab initio calculations for some model planar systems. It is shown that the perpendicular Cartesian second derivatives of the total energy extract from the latter the parameters of the atom–atom pairwise interactions and that in the limit of large intermolecular separation these parameters are the corresponding Cartesian derivatives of specific components of the molecular dipole and second moments. The latter can be used to define unique and unambiguous atomic charges and atomic dipoles. Trends in the atomic moments are observed and correlated with the molecular bonding structure. The ability of these atomic multipoles to reproduce the molecular electrostatic potential at the intermediate and close range is also examined and found to be comparable to that of atomic point charges that are derived by fitting the electrostatic potential.
Collisional relaxation of H2CO (Ã 1A2, v4=1, J
K
a ,K
c=132,12) by He, Ar, Xe, and N2
Authors: Temps F. , Halle S. , Vaccaro P. H. , Field R. W. , Kinsey J. L.
Published: July 2002

The collision-induced deactivation of H2CO in a specific rotational level in its first electronically excited singlet state, Ã 1A2, v4=1, JKa,Kc=132,12, has been investigated with a series of colliders, M=He, Ar, Xe, and N2. The target level was populated via the transition Ã←X̃, 410, 132,12←133,11 using a pulsed dye laser. The subsequent relaxation was monitored using the technique of transient gain spectroscopy (TGS) via the transition Ã→X̃, 201301414, 132,12→131,13, which coincides with the line of an Ar+ laser at λ=488.0 nm. The experiments yielded the overall collisional depopulation rate constants (velocity averaged cross sections) for He, Ar, Xe, and N2, respectively: (5.25±0.20)×10−10 cm3/molecule s (39 Å2), (5.22±0.18)×10−10 cm3/molecule s (86 Å2), (5.37±0.29)×10−10 cm3/molecule s (106 Å2), (8.25±0.29)×10−10 cm3/molecule s (125 Å2), where the uncertainties correspond to the 2σ standard deviations.
Potential energy surfaces for Tc+H2 and Ru+H2 reactions
Authors: Balasubramanian K. , Wang J. Z.
Published: July 2002

Potential energy surfaces of 12 electronic states of TcH2 and RuH2 are obtained using a complete active space MCSCF (CASSCF) followed by multireference singles + doubles configuration interaction calculations (MRSDCI). The 6S ground state of Tc has to surmount a barrier of 40 kcal/mol to insert into H2 while the 5F ground state of Ru inserts into H2 with a much smaller barrier of 8.5 kcal/mol. The excited 6D, 4D, and doublet states of Tc atom insert into H2 spontaneously. The ground state of TcH2 is a linear 6Σ+g state arising from the 6S state of the Tc atom, while the ground state of RuH2 is of bent equilibrium geometry with 3A2 symmetry. A nearly degenerate bent state of 3B1 symmetry also exists for RuH2 . The bent minima of TcH2 are at least 0.9 eV above the linear 6Σ+g ground state of TcH2 . The ground state of RuH2 is 35 kcal/mol more stable than Ru(3F) + H2 while the 6Σ+g ground state of TcH2 is 27 kcal/mol more stable than Tc(6S)+H2 . All the bent states of TcH2 and RuH2 are ionic exhibiting strong M+H− polar bonds. The M–H bonds are made of dsp hybrid bonds as evidenced from Mulliken population analyses.
Rotational excitations in K1−x(NH4)xI mixed crystals: A neutron scattering study
Authors: Bostoen C. , Coddens G. , Wegener W.
Published: July 2002

A neutron scattering study was performed on K1−x(NH4)xI mixed crystals (x≤0.44) at temperatures T≤20 K. Our results show in a direct fashion that the NH+4–NH+4 interaction contributes significantly to the crystal-field potential. A quantitative discussion in terms of a locally perturbed field is applied to the x=0.02 results. The barrier increase due to the presence of a single NH+4 ion cannot be ascribed to octopole–octopole interactions alone. A qualitative model, which includes dipole–dipole interactions, is presented. The temperature study on a sample with x=0.28 shows a continuous transition from a quantum regime to a classical regime of reorientational motions.
Reaction kinetics of muonium with the halogen gases (F2, Cl2, and Br2)
Authors: Gonzalez Alicia C. , Reid Ivan D. , Garner David M. , Senba Masayoshi , Fleming Donald G. , Arseneau Donald J. , Kempton James R.
Published: July 2002

Bimolecular rate constants for the thermal chemical reactions of muonium (Mu) with the halogen gases—Mu+X2→MuX+X—are reported over the temperature ranges from 500 down to 100, 160, and 200 K for X2=F2,Cl2, and Br2, respectively. The Arrhenius plots for both the chlorine and fluorine reactions show positive activation energies Ea over the whole temperature ranges studied, but which decrease to near zero at low temperature, indicative of the dominant role played by quantum tunneling of the ultralight muonium atom. In the case of Mu+F2, the bimolecular rate constant k(T) is essentially independent of temperature below 150 K, likely the first observation of Wigner threshold tunneling in gas phase (H atom) kinetics. A similar trend is seen in the Mu+Cl2 reaction. The Br2 data exhibit an apparent negative activation energy [Ea=(−0.095±0.020) kcal mol−1], constant over the temperature range of ∼200–400 K, but which decreases at higher temperatures, indicative of a highly attractive potential energy surface. This result is consistent with the energy dependence in the reactive cross section found some years ago in the atomic beam data of Hepburn et al. [J. Chem. Phys. 69, 4311 (1978)]. In comparing the present Mu data with the corresponding H atom kinetic data, it is found that Mu invariably reacts considerably faster than H at all temperatures, but particularly so at low temperatures in the cases of F2 and Cl2. The current transition state calculations of Steckler, Garrett, and Truhlar [Hyperfine Interact. 32, 779 (986)] for Mu+X2 account reasonably well for the rate constants for F2 and Cl2 near room temperature, but their calculated value for Mu+Br2 is much too high. Moreover, these calculations seemingly fail to account for the trend in the Mu+F2 and Mu+Cl2 data toward pronounced quantum tunneling at low temperatures. It is noted that the Mu kinetics provide a crucial test of the accuracy of transition state treatments of tunneling on these early barrier HX2 potential energy surfaces.
Computer simulation of the aggregation process in self-associating polymer and surfactant systems
Authors: Chakrabarti Amitabha , Toral Raul
Published: July 2002

We present results of numerical simulations of a new model for the clustering process in self-associating polymer systems. We focus on two experimentally interesting systems; telechelic ionomers and reverse micelles. The clustering dynamics introduced by us is an extension of the Eden method, previously used in the context of single particle aggregation. The structure of the aggregates seen in the simulations is qualitatively similar to that observed in experimental systems. The quantitative predictions of the model are found to be similar to those found in simulations of a model based on a diffusion-limited-aggregation-type process.
Electrostatic correlation forces between surfaces with surface specific ionic interactions
Authors: Podgornik Rudi
Published: July 2002

We have studied some consequences of a generalized theory of electrostatic interactions between two macroscopic surfaces immersed in a dilute electrolyte. The ionic interactions close to the two bounding surfaces were supposed to be specifically affected by the presence of the surfaces, leading to a surface contribution to the total free energy density of the system. A functional integral (sine-Gordon) representation of the grand canonical partition function was derived and evaluated in the saddle-point approximation, leading to the repulsive mean-field and an attractive first-order correlation correction to the interaction free energy. The magnitude of the correlation term was investigated in detail and general conditions on the form of the surface-specific free energy density were derived that would lead to an anomalously large exponentially decaying attractive interactions. We present some arguments in favor of the view that this anomalous long-range attraction could well represent a case for the electrostatic nature of the recently measured long-range ‘‘hydrophobic’’ attraction.
Dispersed fluorescence spectra of jet-cooled benzene from the levels near the channel three threshold
Authors: Suzuki Toshinori , Ito Mitsuo
Published: July 2002

Highly resolved dispersed fluorescence spectra from single vibronic levels lying 2594–3084 cm−1 above the 1B2u origin of benzene have been measured in a supersonic jet. The dispersed fluorescence spectra from the levels 6110211, 6117211, 71 lying below the threshold of the third channel consist of very sharp lines, while the spectrum from the 6113 level lying above the threshold contains a large amount of broad emission due to a fast intramolecular vibrational redistribution (IVR) process.
Photodissociation of NCO(X 2Π) radicals
Authors: Liu X. , Coombe R. D.
Published: July 2002

Photolysis of NCO(X 2Π) at 193 nm leads to the production of CN(X 2Σ+ )+O(3 P). The CN(X) was detected by monitoring the CN X 2Σ+ →B 2Σ+ laser-induced fluorescence excitation spectrum. A spectral simulation calculation was used to obtain vibrational and rotational population distributions in the CN fragments. The fractional vibrational populations obtained in this manner are 0.43, 0.32, 0.21, and 0.04 for v=0, v=1, v=2, and v=3, respectively. The near-nascent rotational distributions in the different CN(X) vibrational levels are not characterized by a Boltzmann rotational temperature, but rather are bi-modal with maxima at both high and low N. The high N rotational excitation of the CN fragment suggests the existence of an excited dissociative state of NCO which is bent. A lower limit for the heat of formation of NCO, ΔHf >37 kcal/mol, is derived from the upper limit on the internal excitation of the CN(X) fragments. The spectrum of prompt emission produced by the 193 nm photolysis indicates the existence of a bound excited state of NCO which radiatively relaxes to the A 2Σ+ state. From the spectrum, this state is thought to be linear.
On the forced catalytic oscillator
Authors: Sheintuch M.
Published: July 2002

Catalytic oscillators are usually characterized by dynamic variables that cannot be perturbed directly, by wide separation of time scales and by either soft or hard bifurcation to periodicity. Analysis of a simple relaxation oscillator subject to a square-wave variation in a parameter reveals a structure similar to that known for the circle map. Qualitative analysis of periodic forcing around a hard-bifurcation boundary is also presented. These results are compared with motions obtained by a periodic change in the composition of the environment surrounding a Pt wire catalyzing NH3 oxidation. The unperturbed system exhibits the three features described above. Harmonic quasiperiods and narrow subharmonic bands are mapped in the forced system.
Absolute state-selected and state-to-state total cross sections for the reaction Ar+(2P3/2,1/2)+O2
Authors: Flesch G. D. , Nourbakhsh S. , Ng C. Y.
Published: July 2002

Absolute spin–orbit state-selected total cross sections for the reactions, Ar+(2P3/2,1/2)+O2→O+2+Ar [reaction (1)], O++O+Ar [reaction (2)], and ArO++O [reaction (3)], have been measured in the center-of-mass collision energy (Ec.m.) range of 0.044–133.3 eV. Absolute spin–orbit state transition total cross sections for the Ar+(2P3/2,1/2)+O2 reaction at Ec.m.=2.2–177.6 eV have also been examined. The appearance energies for the formation of O+ (Ec.m.=2.9±0.2 eV) and ArO+ (2.2±0.2 eV) are in agreement with the thermochemical thresholds for reactions (2) and (3), respectively. The cross sections for O+2, O+, and ArO+ depend strongly on Ec.m. and the spin–orbit states of Ar+, suggesting that reactions (1)–(3) are governed predominantly by couplings between electronic potential energy surfaces arising from the interactions of Ar+(2P3/2)+O2, Ar+(2P1/2)+O2, and O+2+Ar.

In the Ec.m. range of 6.7–22.2 eV, corresponding to the peak region of the O+ cross section curve, the cross sections for O+ are ≥50% of those for O+2. The production of O+ by reaction (2) is interpreted to be the result of predissociation of O+2 in excited states formed initially by reaction (1). The formation of charge transfer O+2(ã 4Πu) has been probed by the charge transfer reaction O+2(ã 4Πu)+Ar. The results indicate that in the Ec.m. range of 0.4–3.0 eV charge transfer product O+2 ions are formed mainly in the O+2(ã 4Πu) state. Experimental evidence is found supporting the conclusion that the vibrational distributions of O+2(ã 4Πu) formed in reaction (1) and by photoionization of O2 in the energy range between the O+2(ã 4Πu, v=0) and O+2(Ã 2Πu, v=0) thresholds are similar.

The population of O+(4S) formed by reaction (2) has also been measured by the reaction O+(4S)+N2→NO++N. In the Ec.m. range of 3–44 eV, product O+ ions of reaction (2) are shown to be dominantly in the O+(4S) ground state. At Ec.m.≥14 eV, the retarding potential energy analysis for O+2 shows that more than 98% of the charge transfer O+2 ions are slow ions formed mostly by the long-range electron jump mechanism. Product ArO+ ions are observed only in the Ec.m. range of 2.2–26.6 eV. At Ec.m. slightly above the thermochemical thresholds of reactions (2) and (3), the overwhelming majority of ArO+ and O+ ions are scattered backward and forward with respect to the c.m. velocity of reactant Ar+, respectively. This observation is rationalized by a charge transfer predissociation mechanism which involves the formation of ArO+ and O+ via nearly collinear Ar+–O–O collision configurations at Ec.m. near the thresholds of reactions (2) and (3).
Comparison of theory and experiment on relative stability in a bistable (ZnSe) system
Authors: Harding Robert H. , Paoli Paolo , Ross John
Published: July 2002

We compare theory and experiments on relative stability of the two stable stationary states in optically bistable ZnSe interference filters. We examine the equistability predictions of the master equation, the Fokker–Planck equation, and a deterministic analysis. For this effectively one-variable system there exists a potential for each of these descriptions. We derive a master and Fokker–Planck equation for the fluctuations in temperature expected in our system and find that the three theoretical potentials differ significantly for homogeneous geometries of the system, but the stochastic predictions agree with the deterministic equistability point for inhomogeneous systems. We also examine the effect of external noise, which is independent of the state variable, and find that a very small amount of noise reduces the equistability prediction of the homogeneous master and Fokker–Planck equations to that of the deterministic potential. Noise in the constraints (such as light intensity) results in a new effective potential.
Quantum state-resolved study of the rovibrational excitation of OCS by hot hydrogen atoms
Authors: Zhu Lei , Hershberger John F. , Flynn George W.
Published: July 2002

The vibrationally inelastic scattering of OCS by hot hydrogen atoms was studied using an excimer laser photolysis/diode laser probe technique. Nascent rotational distributions for the 1000 (2062 cm−1) and 0001 (859 cm−1) stretching vibrations are well fit to Boltzmann temperatures of 840±50 and 545±60 K, respectively. The total vibrational excitation probability of the 0001 state was about 2.3±0.3 times that of the 1000 state. Measurement of OCS linewidths immediately after hot atom collisions indicate translational recoil temperatures that vary from 360 to 730 K, with a positive dependence on rotational state. The results are interpreted in terms of repulsive, hard shell collisions with the different stretching vibrations arising from trajectories approaching different ends of the OCS molecule.
Raman spectroscopic investigation of irreversibly compacted vitreous silica
Authors: Walrafen G. E. , Chu Y. C. , Hokmabadi M. S.
Published: July 2002

Polarized, depolarized, isotropic, difference, and reduced Raman spectra were obtained from vitreous silica, irreversibly compacted at 600 °C and 50 kbar, to ambient densities as high as 2.73 g cm−3. The 60 cm−1 ‘‘Boson’’ peak; and, the intense optic, Si–O–Si bending peak at 440 cm−1 including its low-frequency shoulders at ≊210–240 and ≊340 cm−1; were observed to move strongly upward in position, but at two different rates with density rise. The two peak frequencies behave experimentally like spherical transverse acoustic (TA) or shear (S), and spherical longitudinal acoustic (LA) or pressure (P) stress waves of an isotropic elastic solid. In contrast, downward frequency shifts were observed for the transverse optic (TO) 1060 cm−1 and longitudinal optic (LO) 1200 cm−1 modes in the Si–O stretching region, but at absolute rates equal, respectively, to those of the 60 and 440 cm−1 peak frequencies thus linking optic as well as acoustic modes to the elastic modulii.

The TO and LO splitting was shown from a phonon coupling model to result from very strong interaction between the optic modes and the low-frequency S and P modes. The frequency decreases of the TO and LO modes indicate a decrease in the Si–O stretching force constants, related to a decrease in the mean Si–O–Si bridging angle, from ≊144° to ≊120°, inferred from frequency increases at 440 and ≊800 cm−1. A decrease in the mean O–2nd-O distance also occurs, but severe distortion of the SiO4 tetrahedra does not seem probable to densities of 2.73 g cm−3. The dilatational P-mode wavelength ≥5 Å corresponds to volume-related distances, e.g., Si–2nd-O, O–2nd-O, or Si–2nd-Si. The shear mode wavelength is larger (≊13 Å) and similar to the structural correlation length, estimated roughly from x-ray data, where G(r)≊1. Both S and P modes involve small coherence volumes, and both may have negligible group velocities, i.e., standing waves. The low-frequency S scattering is weakly to completely depolarized, but the higher-frequency P scattering may be strongly polarized, falling near or just below (BeF2, B2O3, GeO2, SiO2, ZBLAL) the first very strong minimum in the Raman depolarization ratio, located above the S mode.

This new criterion locates the actual P mode of fused silica within the ≊200–350 cm−1 shoulder region. This region is intense in the density of states, determined by neutron inelastic scattering (NIS). S and P modes are resolved in the Raman spectra of several glasses, but their relation to the low-order spherical acoustic stress waves of isotropic media has, heretofore, not been widely recognized.
Streaming potential: Continuum expression applicable to very small nonuniform ion channels
Authors: Levitt David G.
Published: July 2002

A new expression for the streaming potential of a charged channel is derived. It reduces to the classical result in the large pore limit. It is particularly well suited to application to biological channels—channels that contain only a small number of ions whose motion may be correlated and which are not axially uniform. The derivation is based on a calculation of the volume of water carried by each ion that traverses the channel and makes use of the reciprocal theorem of Lorentz. An estimate of the contribution to the streaming potential of the wide region of biological ion channels is presented. The derivation uses a set of assumptions and conditions that differ significantly from previous derivations and may be useful for nonbiological applications.
Collision-theory treatment of three-body reactions forming N+4, O+4, N2O+2, and N2NO+
Authors: Phillips L. F.
Published: July 2002

Termolecular reactions leading to formation of N+4, O+4, N2O+2, and N2NO+, in the presence of diatomic bath gas, have been modeled by using Langevin-type collision theory to calculate rates of formation and stabilization of the complex ions. The reactions show an interesting trend from RRKM to non-RRKM behavior with decreasing potential well depth for the complex. At the non-RRKM limit the complex lifetime reduces to the duration of an elastic collision between the reactants. The larger negative temperature coefficients of the N2O+2 and N2NO+ reactions result from dynamical constraints on the total rotational energy of the colliding species.
Theoretical study of the radiative lifetime of the A 1∏u state of C2
Authors: Langhoff Stephen R. , Bauschlicher Charles W. , Rendell Alistair P. , Komornicki Andrew
Published: July 2002

We have studied the Phillips (A 1∏u–X 1∑+g ) band system of C2 in order to help resolve the discrepancy between the best theoretical lifetimes and those deduced recently from laser-induced fluorescence (LIF) studies. Our calculated lifetime for the A 1∏u(v′=0, J′=1) state is 13.0 μs, in excellent agreement with the recent calculations of ONeil et al., but considerably shorter than the direct experimental measurements. A measure of the accuracy of the complete-active-space self-consistent-field (CASSCF) multireference configuration-interaction (MRCI) results in this work is obtained by a series of calibration calculations. Convergence in the n-particle space is demonstrated by comparison with full configuration interaction calculations in a double-zeta plus polarization basis, while the convergence of the one-particle basis set is demonstrated by systematically expanding the one-particle basis set up through g angular momentum functions. Furthermore, a coupling of the one- and n-particle spaces is shown to be unimportant by systematically expanding the active space in the CASSCF/MRCI treatment. Given that our theoretical results are estimated to be accurate to about 5%, the LIF lifetimes would appear to contain some systematic error.
Potential-energy surfaces for Tc++H2 and Ru++H2 reactions
Authors: Das Kalyan K. , Balasubramanian K.
Published: July 2002

Geometries, potential-energy surfaces (PES), and some one-electron properties of 12 electronic states of two second row transition metal dihydride ions, namely, TcH+2 and RuH+2 are calculated using complete active space multiconfiguration self-consistent field (MCSCF) followed by multireference single- and double-excitation configuration interaction (MRSDCI) methods. The ground-state Tc+ (a7S, d5s1) ion does not insert into H2. The 5B2 ground state of TcH+2 with geometry re=1.59 Å, θe=49.5° is formed by the spontaneous insertion of Tc+(a5D, 4d6) into H2. The quartet-state Ru+ (a4F) ion arising from the 4d7 configuration inserts spontaneously into H2 to form the ground state (4A2) of RuH+2 with re=1.678 Å and θe=29.2°. RuH+2 in the ground state is better described as a complex of Ru+ (a4F) with H2. The energies of formation of the ground states of TcH+2 and RuH+2 from their dissociated counterparts are calculated as 22.4 and 21.2 kcal/mol, respectively. All the sextet-state PES’s of RuH+2 and some of the quintet-state surfaces (namely, 5A1, 5A2, and 5B1 ) of TcH+2 contain large barriers for insertion. The high-spin linear stationary states of both TcH+2 and RuH+2 are more stable than the low-spin linear states. The polarity of the metal–H bond in the ground state of TcH+2 is found to be opposite to that in RuH+2. The adiabatic ionization potentials of TcH2 and RuH2 are calculated as 7.41 and 6.63 eV, respectively, at the MRSDCI level of theory.
Determination of the differential structure factor of liquid bromobenzene by anomalous x-ray scattering
Authors: Schultz E. , Bertagnolli H. , Frahm R.
Published: July 2002

Liquid bromobenzene (C6H5Br) has been studied with differential anomalous scattering to determine the differential structure factor of the Br atoms. The experiments were carried out at beam line W1 at the Hamburger Synchrotronlaboratorium (HASYLAB). The differential structure factor was checked by comparison with the theoretical intramolecular structure factor calculated from well-known data of the intramolecular geometry of C6H5Br. The differential radial distribution function shows a peak at 3.9 Å, which can be assigned to Br–Br atom pairs.
Infrared spectra of large clusters containing small ether molecules: Liquid, crystalline, and clathrate-hydrate cluster spectra
Authors: Fleyfel Fouad , Devlin J. Paul
Published: July 2002

Large clusters of different structural phases containing the small-ring ether molecules tetrahydrofuran, trimethylene oxide, and ethylene oxide have been prepared and studied spectroscopically using a simple cluster cell based on a small infrared gas cell. Pure ether clusters are formed efficiently upon loading of the precooled cell (80–130 K) to pressures of a few hundred Torr with a gaseous mixture of an ether diluted with N2. For the right choice of temperature the pure ether clusters, which tend to be exceptionally large, initially assume a supercooled liquid structure but convert to the stable crystal structure over the observation time of a few minutes. Approximate nucleation temperatures, for this phase transition, range from 125 K for tetrahydrofuran to 100 K for ethylene oxide. The crystalline-cluster absorption bands, for the ether vibrational modes having large oscillator strengths, are distorted by dominance of the longitudinal mode absorption as expected for ultrathin particles. When water is included as a third component of the gaseous mixture, with a concentration chosen to give cluster compositions appropriate to the respective clathrate hydrate structures, the clusters assume the crystalline clathrate hydrate structures for temperatures close to the optimum values for thin-film clathrate hydrate formation by vapor cocondensation methods.
Generalized reactive flux method for numerical evaluation of rate constants
Authors: Borkovec Michal , Talkner Peter
Published: July 2002

Reactive flux method for numerical evaluation of rate constants is generalized to arbitrary underlying dynamics. The feasibility of the method is illustrated by numerically evaluating rate constants for a one-dimensional jump process and a diffusion process. In both cases, we find excellent agreement with exact results known in these cases.
Reduced dimensionality quantum reactive scattering: H2+CN→H+HCN
Authors: Sun Qiyan , Bowman Joel M.
Published: July 2002

We apply a recently developed, reduced dimensionality quantum theory of diatom–diatom reactive scattering [Q. Sun and J. M. Bowman, Int. J. Quantum Chem., Symp. 23, 115 (1989] to the exoergic H2+CN→H+HCN reaction, for zero total angular momentum. A new semiempirical, three-dimensional potential surface, which is based in part on ab initio calculations of the saddle point properties is also reported. Reaction probabilities for the ground and first excited bending states of HCN are calculated for total energies up to 1.0 and 1.06 eV, respectively. The results show a strong preference for formation of HCN (0vb1) and HCN (0vb2), vb=0 and 1, starting with ground vibrational state reactants. Reaction probabilities for vibrational excitation of H2 or CN are also reported for both bending states of HCN. Vibrational excitation of H2 is found to be far more effective in promoting reaction than vibrational excitation of CN.
Wave function optimization with a fixed sample in quantum Monte Carlo
Authors: Sun Zhiwei , Huang Sheng-yu , Barnett Robert N. , Lester William A.
Published: July 2002

The optimization of trial functions consisting of a product of a single determinant and simple correlation functions is studied. The method involves minimizing the variance of the local energy over a finite number of points (sample). The role of optimization parameters, e.g., sample characteristics, initial trial function parameters, and reference energy, is examined for H2, Li2, and H2O. The extent to which cusp conditions are satisfied is also discussed. The resulting variational Monte Carlo energies 〈ΨT‖H‖ΨT〉 recover 46%–95% of the correlation energy for the simple trial function forms studied. When used as importance functions for quantum Monte Carlo calculations, these optimized trial functions recover 90%–100% of the correlation energy. Time-step bias of the computed quantum Monte Carlo energies is found to be small.