bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic structure of the N4+ molecular ion
Authors: de Castro Sandra C. , Schaefer Henry F. , Pitzer Russell M.
Published: February 2003

The N4+ ion is an important species in the chemistry of the atmosphere. Here N4+ has been studied theoretically using the methods of ab initio molecular quantum mechanics. There is considerable complexity involved in the theoretical study of N4+ due to (a) the fact that N2+ has two low-lying electronic states, X 2Σg+ and A 2Πu and the order of these is reversed within the Hartree–Fock approximation and (b) there are six low-lying electronic states of N4+. Results are first presented at the self-consistent-field (SCF) level of theory using a double zeta (DZ) basis set N(9s 5p/4s 2p). Both Koopmans’ theorem and direct positive ion calculations in both D2h (rectangle) and C2v (regular trapezoid) symmetry suggest only a single (out of six) substantially bound electronic state, the 2B2u(D2h) or 2A1(C2v) state. Because the D2h SCF wave function necessitates a compromise description of the N2+N2+ asymptote, the predicted dissociation energy is artificially large, although in reasonable agreement with experiment. Polarization functions were added to the basis set and all three geometrical parameters examined to locate the C2v equilibrium structure, which lies 19.9 kcal below the dissociation limit N2+N2+(2Σg+). Similar theoretical methods were applied to the T-shape geometry, with the constrained equilibrium structure bound by 24.2 kcal. The linear conformation represents the absolute minimum on the N4+ potential energy surface, lying 30.4 kcal below N2+N2+. The latter dissociation energy agrees well with the experimental value of ∼26 kcal. In studying the linear 2Σ+ state, the surprising result was found that the 2Σg+ restricted Hartree–Fock wave function is unstable with respect to the removal of the g/u element of symmetry. The 2Π state is stable in this respect, and as a result, the 2Σg+ state of N2+ falls closer to the 2Πu state in this mildly unrestricted form of Hartree–Fock theory.
Inequalities among atomic expectation values
Authors: Gadre Shridhar R. , Matcha Robert L.
Published: February 2003

Some new inequalities involving various 〈rn〉 and 〈pn〉 atomic expectation values have been derived using theorems by Pólya and Szegö. Using these inequalities, bounds on one expectation value can be obtained in terms of other expectation values. These bounds have been tested numerically using expectation values computed with wave functions of varying quality. Bounds obtained using Hartree–Fock and correlated wave functions are found to be fairly tight for small atomic systems considered. However, those obtained for neutral Thomas–Fermi atoms are not close to the correct Thomas–Fermi expectation values. The inequalities discussed here form a basis for obtaining bounds on unknown expectation values using known values of related expectation values.
Theoretical studies of the ozone molecule. I. A
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o MCSCF/CI potential energy surfaces for the X 1A1 and a 2B2 states
Authors: Wilson C. Woodrow , Hopper Darrel G.
Published: February 2003

The potential energy surfaces for the X 1A1 and first-excited a 3B2 states of ozone are surveyed with accurate ab initio electronic structure calculations. An efficient technique for using the multiconfiguration self-consistent field (MCSCF) method for polyatomics has been established for these computations, and is described in some detail. The method involves a geometry-dependent flow of configurations between the MCSCF wave function and a small configuration interaction (CI) wave function. The configurations are constructed in a set of 8 core, 7 valence, and 4 ’’virtual’’ MCSCF orbitals, which are expanded in a double-zeta plus polarization one-electron basis. The vertical and adiabatic excitation energies, uncorrected for zero-point energies, are presently obtained as 1.21 and 0.74 eV, respectively. These results are in agreement with other quantitative theoretical studies which are now available for comparison. Furthermore, the present results indicate that O3(a 3B2) has an O2–O binding energy Eb=0.4 eV and that there is no barrier relative to its adiabatic asymptote, O(3P)+O2(X 3Σ−g). These energies are estimated to be accurate to within 0.2 eV. Thus, we conclude that O3(a 3B2) is a bound state of ozone. Similar MCSCF/CI calculations are reported for the ring state. The implications of this work for ozone chemistry are discussed. The existence of a second bound state, relative to the ground state atom–diatomic asymptote, may have important consequences with regard to the interpretation and modeling of ozone formation from O(3P) and O2(X 3Σ−g): the a 3B2 state, as well as the X 1A1 state, may be involved.
On the condensation sticking coefficient: A quantitative discussion
Authors: Davidovits Paul , Faist Michael B.
Published: February 2003

A model is presented for the calculation of the sticking coefficient for two colliding particles. The coalescing process is treated as a two step event. First, the various pathways for the formation of a collision complex are identified and their lifetimes are calculated. Second, the mechanisms for the stabilization of the activated complex are discussed and characteristic times associated with these mechanisms are derived. The sticking coefficient is formulated as a function of these characteristic times. The resulting expression contains the explicit dependence on the mass of the colliding particles. As a specific numerical example, the sticking coefficient is calculated for the homogeneous condensation of vaporized coal ash.
A stochastic analysis of the homogeneous nucleation of vapor condensation
Authors: Gillespie Daniel T.
Published: February 2003

It is argued that condensational growth and evaporative decay of a molecular cluster are intrinsically stochastic processes, properly described by condensation and evaporation ’’probability’’ rates rather than ’’number’’ rates. This implies that the pretransitional evolution of a pure, supersaturated vapor should be mathematically described as a continuous-time/discrete-state Markovian random walk. A ’’master equation’’ is derived for a simplified version of this random walk. It is shown to lead, on the one hand, to the approximate formulas of classical steady-state nucleation theory, and, on the other hand, to an exact (but complicated) formula for the probability distribution of the system nucleation time. An alternate ’’pedestrian’’ approach to random walks is then described, and is shown to lead to a simple, approximate way of calculating the average time to system nucleation. This calculation also provides additional insight into the relationship between the approximate results of classical steady-state nucleation theory and the exact results of the master equation treatment. Finally, the pedestrian approach is used to estimate the mean nucleation time for a cluster growth mechanism that is energetically more realistic than the commonly used cluster growth mechanism.
Activity series for B.E.T.-like multilayer adsorption with interparticle interaction
Authors: Hamilton Chris , Poland Douglas
Published: February 2003

Activity series are given for B.E.T.-like multilayer adsorption with interactions between all nearest neighbor particles. At the condensation activity (unit fugacity), the activity series are used to construct low temperature series. Analysis of the low temperature series indicates that there is a critical temperature above which the average height of the adsorbed phase goes to infinity at unit fugacity.
Collisions of UF−6 ions with Ar, Xe, SF6, and UF6
Authors: Annis B. K. , Stockdale J. A. D.
Published: February 2003

Electrostatic energy analysis has been performed on the products of collisions of up to 200 eV (lab) UF−6 ions with Ar, Xe, SF6, and UF6. Negative ions of UF6 containing internal excitation energy up to ∼5 eV have been observed and their distributions as functions of scattering angle and internal excitation recorded. The behavior of the fragment ions F− and UF−5 was investigated and found to be in accord with a simple statistical model of the collision dynamics.
Muonium in the condensed phases of Ar, Kr, and Xe
Authors: Kiefl R. F. , Warren J. B. , Marshall G. M. , Oram C. J. , Clawson C. W.
Published: February 2003

The first observations of Mu (μ+e−) in the solid and liquid phases of Ar, Kr, and Xe are reported. The fraction of muons injected into the liquid phases of Ar, Kr, and Xe, which form observable Mu, are 97%±29%, 57%±10%, 43%±9%, respectively. The corresponding fractions in the solid phases are 91%±9%, 99%±10%, and 79±25%. The spin relaxation rate of Mu in solid Xe is a factor of 10 larger than in the liquid. The effect is attributed to the nuclear magnetic dipole moments of 129Xe and 131Xe which are less effective in relaxing Mu in liquid Xe where motional narrowing is enhanced. The hyperfine splittings of Mu in solid Kr and solid Ar have been measured to an accuracy of 0.1% and are found to be in agreement with the vacuum hyperfine splitting.
An investigation of the line and band variation of spectral line shifts
Authors: DePristo Andrew E.
Published: February 2003

A study of the vibrational band and rotational line dependence of spectral lines shifts is presented. This application is based upon the earlier formal scaling theoretical treatment of these processes. Two points are considered in some detail: (1) the accuracy of the energy corrected sudden scaling relationship for phase sensitive properties; and (2) the systemization, extrapolation, and inversion of experimental line shifts, and the interpretation of the information provided by such data.
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o treatment of electronically inelastic K+H collisions using a direct integration method for the solution of the coupled-channel scattering equations in electronically adiabatic representations
Authors: Garrett Bruce C. , Redmon Michael J. , Truhlar Donald G. , Melius Carl F.
Published: February 2003

We calculate the adiabatic potential energy curves and nonadiabatic first-derivative couplings for the X, A, and C 1Σ+ states of KH by an ab initio one-electron pseudopotential formalism. The splitting of the X and A curves at the avoided crossing is in good agreement with experiment. The ab initio results are used to calculate the electronically inelastic transition probabilities and cross sections for K+H collisions at low energies by R matrix propagation in the adiabatic representation with exponential sector transformations. Since this method has never been applied before, we made an extensive study of its convergence properties and efficiency. We found it to be a convenient, accurate, and efficient method. The cross sections are changed by about a factor of two when the potential curves are changed by a different treatment of the KH+ core, but only by about 1% when the assumptions about the nonadiabatic second-derivative coupling terms are altered. Our estimate of the 42P→ 42S quenching cross section at 0.022 eV relative translational energy is 2–4×10−4 a20. This increases to 8–10×10−4 a20 by 1.1 eV. The emphasis in this article is on testing and evaluating the new method for solving the scattering problem rather than on the cross sections themselves.
Laser-induced fluorescence of the C2H3O radical
Authors: Inoue Gen , Akimoto Hajime
Published: February 2003

Laser-induced single vibronic level fluorescence of the C2H3O radical is reported. The 0–0 band of the fluorescence lies at 28 798 cm−1 (347.2 nm). The C–O and C–C stretching, and the C–C–O bending vibrational energies are 1560, 1150, and 530 cm−1, respectively, in the ground electronic state, and 872, 1110 and 432 cm−1, respectively, in the excited state. The lifetime of the excited state is 0.8 μsec. The double bond nature of C2H3O is more dominant in the C–O bond than in the C–C bond in the ground state. The C2H3O radical is found in the C2H4+O reaction, as well as in the CH3CHO+F reaction.
Excimer formation in argon, krypton, and xenon discharge afterglows between 200 and 400 K
Authors: Wieme W. , Lenaerts J.
Published: February 2003

The decay of the VUV continuum radiation has been measured in the afterglow of a pulsed dc discharge in argon, krypton, and xenon at pressures between 1 and 100 Torr and temperatures between 200 and 400 K. By comparing the results with those obtained in absorption studies, it is inferred that the precursor to the R*2 excimers is the R* (1s5) metastable state. The reaction rates for the formation of excimers R*2 in three-body collisions are obtained as a function of temperature T: The three-body reaction rate can also be represented in an Arrhenius plot, yielding activation energies Ea(Ar) =−0.007 eV, Ea(Kr) =−0.020 eV, and Ea(Xe) =−0.016 eV. The destruction of the R*(1s5) state by two-body collisions results either in the formation of R*2 excimers or in the transition to the nearby R*(1s4) resonance state. Reaction rates for these processes could not be obtained separately, but the resulting total two-body reaction rate is given in graphical form in the paper.
Penning and associative ionization in crossed-beam Na/Na collisions assisted by strong resonant laser fields
Authors: Weiner J. , Polak-Dingels P.
Published: February 2003

We observe the production of Na2+ and Na+ arising from single collisions between crossed beams of sodium atoms when a laser field is tuned near the Na(3p 2P3/2) and Na(3p 2P1/2) transitions. Measurements of ion intensity vs laser intensity show that at moderately high power true laser-induced processes dominate over purely collisional effects. Relative intensity of mass-selected ions produced at either member of the Na resonance doublet shows conclusively that Na+ does not arise simply from photodissociation of Na2+ but must result from a direct, laser-induced collisional ionization.
Xe2* laser photolysis of OCSe at 172 nm: Se(1S) production and quenching
Authors: Trott Wayne M. , Rice James K. , Woodworth Joseph R.
Published: February 2003

Energetic and kinetic properties of OCSe photodissociation at 172 nm have been studied using a Xe2* laser photolytic driver which delivered excitation energies of up to 60 mJ/cm2. Photoelectron production in the OCSe/Se(1S) medium was directly measured by analysis of microwave transmission data. At high laser fluence, significant concentrations of electrons (≳1.5×1013 cm−3) are produced by a consecutive process of Se(1S) photoionization, superelastic electron heating, and subsequent electron impact ionization of residual Se(1S); however, quenching by electrons can be effectively controlled by adding an electron-attaching diluent such as SF6. The OCSe photodissociation cross section and Se(1S) photoionization cross section at 172 nm were determined to be (1.0±0.15)×10−16 and (3.4±1.2)×10−19 cm2, respectively. Rate constants for Se(1S) quenching by electrons, the parent molecule and Se(3P) were determined to be (1.2±0.3)×10−7, (1.40±0.08)×10−10, and (7±3)×10−11 cm3 sec−1, respectively. The quantum efficiency for Se(1S) production was found to be 0.63±0.17 at a wavelength of 172 nm. Relative rates for collision-induced emission from Se(1S0) to Se(1D2) by N2, H2, SF6, CO, Ar, and Xe have been measured.
Average singlet–triplet coupling properties of biacetyl and methylglyoxal using quantum beat spectroscopy
Authors: Chaiken J. , Gurnick M. , McDonald J. D.
Published: February 2003

We have observed quantum beats in the reversible intersystem crossing of simple α-dicarbonyls, and we have analyzed them to obtain information concerning the density of interacting states and the average intramolecular coupling energy. We have analyzed most of the biacetyl quantum beats using a method based on the properties of random matrices which is described elsewhere and we present the results here. We also present an analysis based on perturbation theory using the Fourier transforms of the quantum beats which is appropriate for understanding the methylglyoxal quantum beats. The density of vibrationally hot triplet states which interact with excited singlet states that are connected to the ground electronic state via optical excitation (∼22 000 cm−1) is found to increase with the amount of vibrational excitation in the accessible singlet state at roughly the same rate as the overall density of triplet vibrational stress increases. We find satisfactory agreement between the density obtained from the quantum beats and that calculated using well known analytical formulas and direct state counting. The density of interacting states increases with the rotational quantum number of the initially excited singlet rovibronic state. The average value of the magnitude of the spin–orbit interaction is 1–10 MHz independent of the amount of vibrational–rotational excitation present. Radiationless transitions in these highly excited molecules are evidently not subject to any overriding selection rules other than spatial symmetry, and conservation of total energy, total angular momentum, and nuclear spin. The biacetyl quantum beats are collisionally quenched with helium at the same rate as the overall fluorescence. The cross section for this process is roughly 300±200 Å2.
Statistical analysis of polyatomic quantum beats using the properties of random matrices
Authors: Chaiken J. , Gurnick M. , McDonald J. D.
Published: February 2003

We have devised a numerical method of analyzing polyatomic quantum beats for the case when so many states are coherently excited that the properties of individual states are inaccessible. The method involves ensemble averaging singlet character distributions obtained by diagonalization of random matrices and using these distributions in simulations of observable quantum beats. By ensemble averaging the simulated quantum beats, we can correlate the average parameters which describe the observable fluorescence decays to the molecular parameters that determine the form of the random matrices. We have found that when very complicated quantum beat effects are observed, extra information is available which relates the overall width of the singlet character distribution to a third observable parameter beyond the two parameters that have traditionally been employed to characterize biexponential fluorescence. In the absence of hyperfine structure this method allows the determination of the width of the singlet character distribution even when that width is greater than the width of the excitation source. This is important since our numerical studies also show that the density of nonradiatively coupled states and the root mean square interaction energy determine the width of the distribution of singlet character in a very simple and general fashion. We believe this method can be easily modified to describe other quantum beat phenomena and can presently be used to specify conditions for specific molecules under which quantum beats are observable.
Zero quantum NMR in the rotating frame: J cross polarization in AXN systems
Authors: Chingas G. C. , Garroway A. N. , Bertrand R. D. , Moniz W. B.
Published: February 2003

An analysis of liquid state cross polarization in AXN spin systems is presented. A two-level geometrical formalism derived from symmetry considerations provides a closed form description of the quantum dynamics for arbitrary N. A parallel discussion using a classical vector model explains the significance of the quantum mechanical solutions and illustrates the nature of the spin–spin correlations accompanying magnetization transfer. General expressions for the J cross polarization A signal with and without X decoupling are given. A discussion of RJCP and PCJCP pulse sequences introduced previously shows how manipulation of Hartmann–Hahn mismatch can advantageously modify cross-polarization dynamics. The quantum formalism demonstrates that efficient polarization transfer is equivalent to population inversion of an ensemble of fictitious spins subjected to different rf field intensities, and that pulse sequences for cross polarization can be constructed by analogy with conventional spin inversion techniques.
Brillouin scattering from liquid CCl4 at high pressures
Authors: Asenbaum A. , Hochheimer H. D.
Published: February 2003

Brillouin spectra were obtained from liquid carbon tetrachloride at pressures ranging from 1 to 1500 bar and at three temperatures (298, 323, and 348 K). Using Mountain’s theory the relaxation time τv of the bulk viscosity and the energy relaxation time τE=Cv⋅τv/(cv−ci) were derived from the experimental spectra. At constant temperature τE decreases with increasing density, and at constant density τE decreases with increasing temperature. From τE the effective collision diameter was calculated for hard spheres (HS): σHS=5.24±0.05 Å.
Optical and dielectric functions of liquid hexamethylphosphoric triamide between 2 and 25 eV
Authors: Birkhoff R. D. , Heller J. M. , Hubbell H. H. , Painter L. R.
Published: February 2003

The reflectance of liquid hexamethylphosphoric triamide has been determined between 2 and 9 eV by the semicylinder method and between 10 and 25 eV by the double ionization chamber method. Kramers-Kronig analysis of the data gave the optical and dielectric functions, the energy loss functions, and the effective number of electrons, all as a function of photon energy. Structure in the absorption spectrum and the imaginary part of the complex dielectric function at 7.5 and 11.5 eV are associated with excitation of π and σ electrons, respectively. The σ→σ* excitation is shifted in the energy loss function −Im(1/ε) to 21 eV, indicating an appreciable degree of collective behavior. Of the total 68 valence electrons, the number of electrons taking part in optical processes and energy loss processes increases to 38 and 25, respectively, at 25 eV.
Sulfur K x-ray absorption spectra of FeS, FeS2, and Fe2S3
Authors: Sugiura Chikara
Published: February 2003

The K absorption spectra of sulfur in FeS, FeS2 (pyrite), and Fe2S3 have been measured with a 50 cm bent-quartz-crystal vacuum spectrograph. The spectra of FeS and Fe2S3 are similar to each other but are fairly different from that of FeS2. The FeS2 spectrum is discussed in terms of two theoretical calculations: the energy band for FeS2 (pyrite) and the molecular orbital (MO) of the S22− ion. The spectra for FeS and Fe2S3 are explained by the application of Terakura’s theory and the first absorption band is attributed to the transitions to the empty antibonding states formed by the redistribution of the sulfur 3p bands under the influence of the iron 3d bands.
High pressure luminescence studies of metalloporphyrins in polymeric media
Authors: Politis T. G. , Drickamer H. G.
Published: February 2003

High pressure luminescence measurements have been made on free base tetraphenylporphyrin (H2–TPP), as well as on Zn–TPP, Mg–TPP, Cr–TPP, and Cu–octaethylporphyrin (Cu–OEP), in PMMA and PS. In the case of H2–TPP, Zn–TPP, and Mg–TPP the efficiency of fluorescence and phosphorescence (Zn–TPP) was pressure dependent as was the relative energy of the excited singlet and triplet states. The relative transfer rates have been calculated, and it was shown that the fluorescence efficiency is limited by intersystem crossing while the phosphorescence efficiency is limited by both radiationless rates S1→T1 and T1→S0. The rate S1→T1 scaled very well with the S1–T1 energy difference, and the rate T1→S0 with the T1–S0 energy difference. In the case of Cu–OEP and Cr–TPP only phosphorescence was observed from two states. At low pressure phosphorescence comes mainly from the high energy excited state (2T1), while at high pressures it comes from the low energy excited state (4T1). The excitation maximum was found to be strongly dependent on pressure. The large red shift of the excitation maximum causes changes in the feeding of 2T1 and 4T1 states, which result in an apparent increase of the radiative rates. Some experimental results on three phthalocyanines are also briefly presented and discussed.
A quadratically convergent MCSCF scheme using Fock operators
Authors: Das G.
Published: February 2003

A quadratically convergent formulation of the MCSCF method using Fock operators is presented. Among its advantages the present formulation is quadratically convergent unlike the earlier ones based on Fock operators. In contrast to other quadratically convergent schemes as well as the one based on generalized Brillouin’s theorem, this method leads easily to a hybrid scheme where the weakly coupled orbitals (such as the core) are handled purely by Fock equations, while the rest of the orbitals are treated by a quadratically convergent approach with a truncated virtual space obtained by the use of the corresponding Fock equations.
The intermolecular arrangement in the plastic crystal (phase Ia) of carbon tetrachloride studied by x-ray diffraction
Authors: Nishikawa Keiko , Tohji Kazuyuki , Murata Yoshitada
Published: February 2003

The x-ray diffraction pattern of the CCl4 plastic crystal (fcc), consisting of halos and a few sharp peaks, was observed by using an energy-dispersive method at −30 °C and the halo pattern was analyzed. The observed intensity was explained by the ’’head–tail head–tail model.’’ This arrangement is consistent with the local structure of the bcc cluster model presented by us for liquid CCl4.
An application of the osmotic equation to a polyelectrolyte solution
Authors: Iwasa K.
Published: February 2003

An attempt to use the osmotic equation in deriving the osmotic coefficient and activity coefficients in a polyelectrolyte solution is reported. In spite of some problems which appear in the process, it is possible to derive valid limiting behavior of polyelectrolytes.
The relaxation spectrum for a polymerization reaction of a linear polymer
Authors: Kjaer Knud Hindhede
Published: February 2003

The model An+Am?An+m for the polymerization of a linear polymer has already been treated in the case where all the reactions were ’’equal’’ giving an explicit expression for the relaxation spectrum. In the present paper this model is generalized. The relaxation spectrum for this generalized model is brought on a form which is suited for numerical calculations. The main features of the spectrum are obtained explicitly by the use of perturbation theory.
Estimation of <p≳ and <p−1≳ from atomic electron densities
Authors: Pathak Rajeev K. , Gadre Shridhar R.
Published: February 2003

Semi-classical phase space considerations are used in order to find a relation between single particle density ρ(r) and single particle momentum density χ(p). From this relation, momentum expectation value 〈p〉 is deduced. (AIP)
Local electron–electron energy density functionals
Authors: Gázquez José L. , Robles Juvencio
Published: February 2003

The total electron–electron interaction energy is derived using an electron gas model for inhomogeneous systems with a finite number of electrons. (AIP)
Rotational excitation and radiative lifetimes of N2+
Authors: Mahan Bruce H. , O’Keefe Anthony
Published: February 2003

The N2+ first negative system (B 2Σ+u–X 2Σ+g) has been recorded by laser by induced fluorescences of N2+ ions confined in an rf trap. The ions were produced by electron impact and had an initial rotational distribution close to the room temperature distribution of the target N2 gas. By admitting Ar gas at pressures 10 to 100 times in excess of the N2 pressure, variable numbers of N2+–Ar collisions were allowed to occur before the N2+ distribution was interrogated by the laser. Since the N2+ was translationally energetic, being driven by the trap fields, a rapid and extensive T to R energy transfer was observed. In this way, high rotational states of N2+ were populated and the radiative lifetimes of these states were measured. No evidence for a perturbation by 4Σu+ states was found in the region (N = 31, 32) where previous studies have indicated a possible perturbation. Evidence for enhanced charge transfer from N2+ X 2Σ+g to Ar was found for N2+ in its first vibrational level.
A stochastic approach to the theory of intramicellar kinetics. III. The homogeneous system limit
Authors: Hatlee Michael D. , Kozak John J.
Published: February 2003

Photoinduced reactions when confined to the interior (or perhaps the immediate vicinity) of a micellar assembly are known to exhibit remarkable kinetic effects (e.g., drastically modified reaction rates, transition to pseudo-first-order behavior for reactions of any molecularity) which have been well documented over the last decade. In earlier papers in this series we have shown how a stochastic approach can be mobilized to describe the dynamics of reactions in compartmentalized, distributed systems. A master equation was derived and solved both for the case of irreversible and reversible photoinduced reactions. In this contribution we study the asymptotic properties of the stochastic master equation for (irreversible and reversible) intramicellar reactions and show how the correct homogeneous-system (macroscopic) kinetic description is reached in the limit of large micellar volume and attendant molecular occupancy. We display numerically the kinetic behavior at intermediate stages in the taking of this limit and, for reversible reactions, show how the ’’apparent’’ equilibrium constant Q for a reaction carried out in a compartmentalized, distributed system relaxes to the ’’canonical’’ equilibrium constant K when the homogeneous system limit is reached. The simulations reported have considerable relevance to recent work on microemulsions and in the concluding section we discuss, from the vantage point of the theory presented in this paper, the possible kinetic effects observable experimentally in these systems.
Formation of CHn molecules in excited electronic states by electron capture collisions of CH+n ions with metal atoms
Authors: Gellene Gregory I. , Williams Brian Wesley , Porter Richard F.
Published: February 2003

Neutralized ion beam experiments with 5 keV CH+n ions have provided information on the nature of the electronic state of the neutrals formed by electron capture from metal atom targets (Na, K, Mg, and Zn). A general trend from formation of excited dissociative states toward stable electronic states is noted as the ionization potential of the target metal is increased from 4.4 eV (K) to 9.6 eV (Zn). Within experimental uncertainties fragmentation energies of the dissociating state formed in electron capture collisions of CH+4 or CH+3 with Na, K, or Mg atoms are independent of the ionization potential of the target. Results of our measurements show that CH4 and CH3 are formed exclusively in these reactions in the 3T2 and 2A1′ states, respectively. The dissociating state of CH2 formed by electron transfer to CH+2 from Na or K atoms is identified as the nonoptical 3B2 state which is known only from theoretical calculations. Analysis of neutralized CH+ beam profiles indicates that CH+ ions formed by electron impact of CH4 or CHCl3 are a mixture of singlet and triplet states. Analyses based on fragmentation energies indicate that when CH+ ions are neutralized by electron transfer from Na or K, the resulting states of CH are most probably 2Σ+ and 4Π. Vertical electron affinities corresponding to the formation of CH4(3T2), CH3(2A1′), CH2(3B2), and CH(2Σ+) from their respective cations are 5.2±0.3, 3.9±0.1, 4.1±0.3, and 5.4±0.1 eV, respectively.
Transition matrix theory of molecular reactive scattering
Authors: Choi B. H. , Poe R. T. , Sun J. C. , Tang K. T. , Yung Y. Y.
Published: February 2003

A two-potential formalism of the transition matrix (T matrix) theory suitable for the molecular reactive scattering is developed. In general, there are two terms in the T matrix. The second term vanishes only under certain special conditions. The reactive T matrix can be expressed in terms of a potential where a large nonreactive portion is subtracted from the full interaction potential. A summary of a variety of calculations based on T matrix theory is given and the approximations are traced back to the original exact expressions. The accuracies of the approximations are analyzed. To obtain accurate results, strong distortions in the exit channel as well as in the entrance channel should be included in the wave function. Based on the present analysis with a two-potential formalism, better approximation schemes within the framework of the T matrix theory are proposed.
Expansion approach to photodissociation dynamics. II. Correction formula for linewidth and numerical examples for HCN
Authors: Takatsuka Kazuo , Gordon Mark S.
Published: February 2003

A correction formula for energy disposal and linewidth of photodissociation dynamics is proposed. This formula was derived from an identity similar to the Kato identity of the scattering theory. Thanks to the correction formula the resultant linewidth turns out to be more accurate and stable even when a relatively poor basis set is used. Numerical examples for assumed direct dissociation of HCN, DCN, and TCN and for the predissociation of HCN and DCN are presented.
Second quantization and averaging: Fermi resonance
Authors: Sanders Jan A.
Published: February 2003

We show how to compute from the perturbed classical Hamiltonian the eigenvalues of the quantized problem in the case of resonance. This is done by averaging the classical system up to a given order and then applying second quantization on the reduced, averaged equations. The results show agreement with numerical calculations with an application to a model problem, analyzed in Noid et al. (Ref. 2).
A theoretical study on the reactivity and spectra of H2CO and HCOH. A dimeric model for nonzero pressure formaldehyde photochemistry
Authors: Kemper M. J. H. , Hoeks C. H. , Buck H. M.
Published: February 2003

The reactivity and spectra of formaldehyde isomers and dimeric complexes between them are studied with ab initio methods. A large number of complexes between H2CO, trans-HCOH, cis-HCOH is calculated. Infrared and Raman spectra of (H2CO)2 are calculated with relatively simple methods using spectroscopic masses and scaled force constants. In this way, the structure of dimers in matrices can be deduced. Hydroxycarbene (HCOH) plays a key role in a model that explains a large number of experimental facts of the nonzero pressure photochemistry. Hydroxycarbene forms complexes with H2CO; the stabilization is due to classical hydrogen bonds. HCOH is a new example of an ambiphilic carbene. Addition products are formed from HCOH⋅⋅⋅H2CO complexes. The calculations show that, in agreement with matrix experiments, glycoaldehyde and methanol are easily formed. The formation of trans-HCOH occurs through a dimeric interaction with the shifting hydrogen. This bimolecular process is 9.6 kcal/mol (6–31G*) in favor of the unimolecular conversion. cis-HCOH might be formed via a nonplanar transition state, where also stabilization at the carbenic center is possible. When higher concentrations of HCOH are available, a hydrogen exchange mechanism easily transfers hydroxycarbene back to H2CO. Several experiments are suggested in this paper; notably, molecular beam and isotopic-mixture experiments will give useful information. The involvement of HCOH in the light-induced formose reaction is suggested.
A general theory of the spectroscopic properties of partially ordered ensembles. II. Two-vector problems
Authors: Nairn John A. , Friesner R. , Sauer Kenneth
Published: February 2003

We have expanded on the results of an earlier paper [J. Chem. Phys. 72, 221 (1980)] which deals with a method for determining the response of a static, partially ordered ensemble of molecules to various types of electromagnetic probes. In this paper we consider types of spectroscopy whose response depends on the location of two vectors in an axis system fixed with respect to the molecule. Examples of such spectroscopies discussed in detail include fluorescence polarization, photoselection linear dichroism, Raman spectroscopy, and two-photon absorption. We outline the kinds of structural information available from polarization experiments on partially ordered ensembles.
Spectroscopy of benzil–bibenzyl clusters. Site selection in three-component mixtures
Authors: Kohler Bryan E. , Loda Richard T.
Published: February 2003

Site selection experiments have been performed on the benzil molecule in a series of two- and three-component solutions. Successful selection and the concomitant spectral narrowing have been obtained for both benzil–bibenzyl–n-decane (BBD) and benzil–bibenzyl–3-methylpentane (BB3MP). It appears that these three-component systems contain microcrystalline inclusions of bibenzyl lightly doped with benzil. This result is unusual both because of the aggregation phenomena and because the selection which occurs in the singlet manifold is preserved through intersystem crossing to give significantly sharpened triplet–singlet emission for a molecule which experiences large geometry changes upon excitation. Of the two three-component systems, BB3MP gives by far the narrower spectra. This observation is consistent with a reduction in the phonon density of states over that of the benzil–bibenzyl mixed crystal.
The hyperfine structure of the calcium monohalides
Authors: Bernath P. F. , Pinchemel B. , Field R. W.
Published: February 2003

Intermodulated fluorescence spectra of the A 2Π–X 2Σ+ and B 2Σ+–X 2Σ+ systems of Ca79Br, Ca81Br, and CaI have been recorded. 79Br, 81Br, and 127I magnetic and electric quadrupole hyperfine structure was observed. The following parameters were determined (in MHz, 1σ uncertainties in parentheses):                       with eQq set equal to zero in all X 2Σ+ states. The observed hyperfine structure of CaF, CaCl, CaBr, and CaI may be interpreted in terms of halide orbital polarization rather than ionic–covalent mixing of M+X− and MX configurations.
Observation of the set of the three sublevel phosphorescence spectra. III. External heavy atom effect on the phosphorescence spectra of quinoxaline and 2,3-dichloroquinoxaline
Authors: Yamauchi Seigo , Saigusa Hiroyuki , Azumi Tohru
Published: February 2003

The sublevel phosphorescence spectra of quinoxaline and 2,3-dichloroquinoxaline are obtained in a light atom host such as naphthalene-d8 and 1,2,4,5-tetramethylbenzene and in a heavy atom host such as 1,2,4,5-tetrabromobenzene. Only the radiative decays, but not the nonradiative decays, are susceptible to external heavy atom effect. For the phosphorescence from the in-plane sublevels significant heavy atom effect is observed for the 0–0 and some vibronic bands involving nontotally symmetric vibrations. For the phosphorescence from the out-of-plane sublevel, external heavy atom effect exists but the enhancement is very slight. These features are interpreted satisfactorily in the framework of a theory. The external heavy atom effect observed in 1,4-dibromobenzene differs considerably, and the origin of the discrepancy is discussed.
Picosecond phase relaxation experiments: A microscopic theory and a new interpretation
Authors: Oxtoby David W.
Published: February 2003

We present a microscopic theory to determine the correlation function measured by picosecond phase relaxation experiments for vibrational modes in liquids. We show that the traditional separation of line broadening into ’’homogeneous’’ and ’’inhomogeneous’’ contributions is only valid in the limiting case where there is true time scale separation into very fast (generally subpicosecond) and very slow (50–100 psec) processes. We discuss recent experimental work on vibrational dephasing in simple liquids like benzene which have found inhomogeneous contributions to the isotropic Raman line shapes; we suggest a possible alternative explanation involving weak coupling to a degree of freedom (perhaps rotational) relaxing on a time scale of a few picoseconds.
The dichroic ratio of pleochroic dyes dissolved in nematic liquid crystals
Authors: Osman M. A. , Pietronero L. , Scheffer T. J. , Zeller H. R.
Published: February 2003

We discuss the factors which determine the dichroic ratio R of pleochroic dyes dissolved in nematic liquid crystals. For cylindrical dye molecules the order parameter and the dichroic ratio can be calculated from a generalization of the Maier–Saupe theory. In planar dyes such as anthraquinones the order tensor strongly deviates from cylindrical symmetry. The direction of the optical transition moment does in general not coincide with one of the principal axes of the order tensor. We calculate the reduction of R by this effect. The direction of the optical transition moment can be calculated and we discuss the optimization of substituents in anthraquinones with respect to angle between transition moment and molecular axis. Vibrational effects on the dichroic ratio are shown to be restricted to thermally excited vibrations in the electronic ground state and are small in rigid molecules.
Theory of nonlinear transport processes in a dilute gaseous mixture
Authors: Eu Byung Chan
Published: February 2003

The modified moment method is applied for studying nonlinear transport processes in a dilute gaseous mixture described by the Boltzmann equation. The method supplies a variational functional which may be regarded as a nonlinear generalization of the Rayleigh–Onsager variational functional for linear irreversible processes. A variational principle is formulated with the variational functional, which yields a set of nonlinear equations for fluxes as the condition for the functional to be an extremum. By solving the set iteratively, we obtain nonlinear constitutive relations and transport coefficients which reduce to their linear counterpart as thermodynamic gradients decrease in magnitude. The Knudsen gas limits of the transport coefficients obtained are also briefly discussed, which show a slightly different behavior from that predicted by the collisionless Boltzmann equation.
Pressure tensor in lamellarly structured fluids
Authors: Ottino J. M. , Davis H. T.
Published: February 2003

The physics of lamellarly structured fluids with small striation thickness scales can be successfully described by molecular theory arguments. Developments are based on the molecular theory of thin films (Rusanov and Kuni) and integral density function expansions. Results show that lamellarly structured fluids present local normal stresses as well as macroscopic pressure tensor values which are a function of striation thickness scales, molecular attractive parameters, and surface tension effects. This analysis should shed light on small scale effects during fluid mechanical mixing of fluids, rheology and stability of structured materials, as well as an organized summary of restrictions of macroscopic models on a structured continuum scale.
The oxidation state of dispersed Rh on Al2O3
Authors: Rice C. A. , Worley S. D. , Curtis C. W. , Guin J. A. , Tarrer A. R.
Published: February 2003

An extensive infrared investigation of the CO/Rh/Al2O3 system has revealed the presence of eight different CO/Rh species including two which have not been observed previously. It has been shown that Rh loading on an alumina support is more critical than reduction temperature in effecting infrared spectral changes in the 1800–2200 cm−1 region. The oxidation state of Rh for the various CO/Rh species has been discussed; it has been postulated that for several of the species the oxidation state of Rh is greater than O. Furthermore, this work indicates that there is significant atomic dispersion on Rh/Al2O3 catalysts prepared from RhCl3⋅3H2O/Al2O3 by a procedure described originally by Yang and Garland. These catalysts retain appreciable amounts of chlorine even following hydrogenation at 673 K.
A finite element method with local trigonometric basis for close coupling equations
Authors: Askar Attila
Published: February 2003

A method is presented for the solution of close coupling equations. The method formulates the scattering problem (i) as a set of boundary value problems, (ii) uses a variational principle, and (iii) used local trigonometric basis within the spirit of the finite element method. The use of local trigonometric basis as compared with the traditional polynomial basis in finite elements offers a distinct superiority in approximating oscillary wave functions.
Quantum molecular dynamics in intense laser fields: Theory and applications to diatomic molecules
Authors: Leasure Steven C. , Milfeld Kent F. , Wyatt Robert E.
Published: February 2003

The quantum dynamics of vibration–rotation excitation in diatomic molecules in intense laser fields is investigated. The Floquet method is used in solving the equations-of-motion for a Hamiltonian explicitly time-dependent. This method requires computation of the time-displacement propagator only over the first optical cycle of the laser field, and is accomplished both numerically and with Magnus approximations. A number of features of single and multiphoton absorption in the LiH, CO, IBr, and HF molecules are studied as functions of the laser intensity and frequency. Average photon absorption spectra are studied with respect to power broadening, dynamic Stark shifts and line shapes. In some cases effective two-state perturbative results accurately agree with the numerical results. In addition, rotational distributions in IBr following two-photon absorption are found to have both thermal components for nonresonant states and structured nonthermal components for states nearly in resonance with the field. Finally, the intensity and temperature dependence of a two-photon laser induced isotopic enrichment in CO is studied. The 14CO to 13CO enrichment ratio is found to decline with increasing laser intensity and initial rotational temperature.
Use of polarization measurements in evaluating cascade contributions to optical excitation functions
Authors: McConkey J. W.
Published: February 2003

Recent developments in theory and experimental measurements of rotational line polarization fractions of diatomic molecules following electron impact are used to show how in some instances cascade free optical excitation functions can be derived without additional measurements of the cascading contribution. The Lyman system of H2 is presented as an example and some previously conflicting excitation cross-section measurements obtained by different techniques are reconciled.
Reactive cross section as a function of reagent energy. II. H(D)+HBr(DBr)→H2(HD,D2)+Br
Authors: Hepburn J. W. , Klimek D. , Liu K. , Macdonald R. G. , Northrup F. J. , Polanyi J. C.
Published: February 2003

A crossed molecular beam study has been made of reactive cross section as a function of collision energy Sr(ET) for all isotopic variants of the abstraction reaction H′+H″Br→H′H″+Br. The apparatus incorporates, for reagent preparation, a supersonic source of variable-energy H or D atoms, and, for product detection, a tunable vacuum ultraviolet laser to obtain laser-induced fluorescence of Br. The cross-section functions indicate that the threshold energy for reaction is <1 kcal/mol. At enhanced collision energy of ET = 7 kcal/mol, the observed order of reactivity in the isotopic series designated (H′,H′′) was (D,H)≳(D,D)≳(H,H)≳(H,D). As noted in a previous report from this laboratory [Int. J. Chem. Kinet., Laidler Festschrift (in press)] the favorable kinematics for (D,H) as compared with (H,D) can be understood in terms of lengthened interaction time for D atom reaction (compared with H) and diminution in the time required for HBr (compared with DBr) to rotate into the preferred alignment for reaction. The effect is illustrated here in terms of a simple model of reaction. The experimental data obtained in this work at low collision energy, in conjunction with 300 K rate constants obtained by others, suggest that close to threshold, kinematic effects are supplanted by threshold effects, yielding Sr(H,D)≳Sr(D,H), the inverse of the principal isotope effect at enhanced collision energy.