bluuebunny/crossref_metadata_2025 huggingface dataset:
Radiation-induced color centers in p
a
r
a-terphenyl
Authors: Kawakubo Tetsuya
Published: February 2003

P a r a-terphenyl single crystals irradiated with g rays at liquid nitrogen temperature have been investigated by electron paramagnetic resonance and optical absorption experiments. In the optical absorption spectra at 77 K, obtained with the light polarized to the a, b, and c′ axes of the crystals, four absorption bands were detected both in the longer wavelength region and in the shorter one. These absorptions were compared with the transition energies and moments of 2-,3-,4-, and 9-hydroterphenyl radicals calculated using SCF–CI molecular orbital theory. The 707 and 637 nm bands which are polarized along the c′ axis and stable at room temperature were assigned to 2- and 9-hydroterphenyl radicals, respectively. The very weak transitions of 716 and 604 nm were ascribed to the transitions of 4- and 3-hydroterphenyl radicals, respectively. The four bands in the shorter wavelength region were assigned to the four kinds of hydroterphenyl radicals considering the results of the thermal and optical annealing experiments. Upon annealing at room temperature, growths of four c′-polarized clear absorption bands at 497, 536, 567, and 583 nm were observed. These bands were attributed to the absorptions of four kinds of hydrogen added terphenyl dimers judging from the different annealing behaviors and theoretical results. The EPR spectrum at room temperature exhibits triplet–triplet characteristics with splittings of 41 and 8.5 G at a rotation angle of 15 °. This spectrum was attributed to the signals arised from 2- and 9-hydroterphyl radicals.
The Q-branch rotational spectrum in the ground state of CD4, studied by laser–rf double resonance spectroscopy
Authors: Kreiner W. A. , Robiette A. G.
Published: February 2003

Near coincidences between CO2 and N2O laser lines and transitions in the n2 and n4 infrared bands of CD4 have been investigated using laser–rf double resonance spectroscopy. Six rf transitions have been assigned to ground state DJ = 0 rotational transitions of CD4, with J values of 7, 7, 10, 19, 19, and 21. A least squares fit of the observations using quartic and sextic tensor distortion constants only yields the parameters Dt = 32649.7±1.7 Hz, H4t = −1.9747±0.0044 Hz, and H6t = 1.1027±0.0016 Hz.
Resonance effects on two photon absorption processes of molecules
Authors: Fujimura Y. , Lin S. H.
Published: February 2003

Effects of resonance on two photon absorption of molecules have been investigated. Using the expression for the photon processes which was derived by using the density matrix method in the Markoff approximation, the intensity of the two photon absorption can be separated into simultaneous, sequential, and their mixing terms. The last term vanishes in the case of zero pure dephasing constant between the initial and final electronic states. In order to study the resonance effects on the two photon absorption for a multilevel molecular system, an expression for the transition probability of the two photon absorption processes is derived in the Born–Oppenheimer basis set. Analytical expressions for both simultaneous and sequential processes are obtained for the displaced harmonic oscillator model. A temperature dependent simultaneous two photon absorption probability for the multimode case is formulated. Some model calculations of the vibrational structures in the low temperature limit for the molecular system are performed to illustrate the resonance effect.
Excited-state dynamics of the isolated ultracold ovalene molecule
Authors: Amirav A. , Even U. , Jortner Joshua
Published: February 2003

In this paper we report the results of an experimental study of the energetics and the dynamics of the first two electronically excited singlet states S1 and S2 of the ovalene molecule (C32H14) seeded in supersonic beams of Ar and of Kr. By an optimal choice of the stagnation pressure (100–300 Torr) of the Ar and Kr carrier gases expanded through a 200 m nozzle, efficient rotational–vibrational cooling of the large molecule was accomplished, while the spectrum was not obscured by the formation of van der Waals complexes. We have interrogated the energy-resolved fluorescence action spectrum, the time-resolved fluorescence, as well as the energy-resolved fluorescence of the bare, ultracold, isolated, large molecule. The first spin-allowed 1S0(1A1g)→1S1(1B3u−) transition with an origin at 21 449 cm−1, exhibits a sparse, well-resolved, vibrational level structure at low excess vibrational energies, EV, while the higher energy range of S1 reveals a congested level structure corresponding to overlapping resonances within a single electronic manifold. The time-resolved decay lifetimes of photoselected single vibronic levels of S1 are in the range 1.7–2.2 msec, exhibiting a very weak energy dependence on EV and being close to the pure radiative lifetimes of S1. The second S0(1A1g)→S2(1B2u) transition of ovalene reveals an irregular, closely-spaced structure, resulting from strong interstate coupling between the S2 and the S1 electronic states, whose electronic origins are separated by ∼1800 cm−1. The strength function for the scrambling of the S2 origin with the background S1 manifold was extracted from the spectroscopic data, providing information on the energetics and on the energy dependence of interstate coupling. The time-resolved decay lifetimes of the scrambled S2 molecular eigenstates are longer by about two orders of magnitude than the pure radiative lifetimes of S2, as estimated from the integrated oscillator strength, manifesting the dynamic consequences of the intermediate level structure. The energy-resolved fluorescence resulting from excitation into the S2 state exhibits emission in the S1→S0 energy region, which is in accord with the large dilution factor for the S2–S1 scrambling. The structure of the energy-resolved emission spectrum may provide some information regarding intrastate vibrational energy redistribution in this large molecule.
Calculation of ESR spectra and related Fokker–Planck forms by the use of the Lanczos algorithm
Authors: Moro Giorgio , Freed Jack H.
Published: February 2003

The applicability of the Lanczos algorithm in the general ESR (and NMR) line shape problem is investigated in detail. This algorithm is generalized to permit tridiagonalization of complex symmetric matrices characteristic of this problem. It is found to yield very accurate numerical solutions with at least order of magnitude reductions in computation time compared to previous methods. It is shown that this great efficiency is a function of the sparsity of the matrix structure in these problems as well as the efficiency of selecting an approximation to the optimal basis set for representing the line shape problem as distinct from actually solving for the eigenvalues. Furthermore, it is found to aid in the analysis of truncation to minimize the basis set (MTS), which becomes nontrivial in complex problems, although the efficiency of the method is not very strongly dependent upon the MTS. It is also found that typical Fokker–Planck equations arising from stochastic modeling of molecular dynamics have the property of being representable by complex–symmetric matrices that are very sparse, so calculation of associated correlation functions can be very effectively implemented by the Lanczos algorithm. It is pointed out that large problems leading to matrices of very large dimension can be efficiently handled by the Lanczos algorithm.
IR photochemistry: A unified approach for single-channel reactions. I. Theory and computational examples
Authors: Baldwin Alan C. , Barker John R.
Published: February 2003

A model for infrared multiphoton decomposition of polyatomic molecules based on an energy-grained master equation (EGME) shows remarkably uniform behavior over a wide range of physically reasonable model parameters. These parameters encompass the size of the molecule, the reaction threshold energy, the variation of the decomposition rate constant above the threshold, and the functional dependence of the infrared absorption process on the internal energy of the molecule. The results are shown to be uniformly well represented by a cumulative log–normal distribution function for reaction yield versus time or fluence. A complementary approach based on the distribution of first passage times for a Markovian stochastic process in discrete state space and continuous time shows that the yield behavior of the EGME can be conveniently calculated without solution of the full set of equations. The uniform behavior of the model results yields a powerful method for presenting and analyzing experimental data.
Interfacial tensions and susceptibilities in a model three-phase system
Authors: Ramos-Gómez F.
Published: February 2003

A system with three phases in equilbrium near the tricritical point is considered. The interfacial tension and susceptibilities of the system are discussed according to the model of Griffiths2.
Cusps, θ trajectories, and the complex virial theorem
Authors: Moiseyev Nimrod , Friedland Shmuel , Certain Phillip R.
Published: February 2003

The conditions are discussed under which cusps in ϑ trajectories of complex resonance energies correspond to eigenvalues which satisfy the complex virial theorem. (AIP)
Energy transfer in atom–surface scattering: Sum rules and velocity distributions
Authors: Levine R. D. , Silbey R.
Published: February 2003

Sum rules are derived for simple scattering process of an atom with a surface. The sum rules are based on information theoretic approach to collision processes. The results agree with the experimental measurement of direct inelastic scattering of rare gas atoms from metal surfaces. (AIP)
Electric field effects on propagating BZ waves: Predictions of an Oregonator and new pulse supporting models
Authors: Schmidt S. , Ortoleva P.
Published: February 2003

A theory of electrical effects on BZ wave propagation is presented. An asymptotic reduction of the mechanism of Field, Körös, and Noyes (FKN) results in two models: one which assumes irreversibility of all reactions is the Oregonator of Field and Noyes except that the rate constants are different from theirs; the new model is quite different but consists of the same three chemical species as the Oregonator. Asymptotic (multiple length scale) techniques are used to find approximate solutions to the reaction–transport wave equations which, since two of the species are charged, are parametrized by the applied electrical field. The velocity is found to exhibit multiplicity and possible hysteretic effects in both models although the new reversible model adheres more closely to observations on the zero field waves. The results show that the electric field response of a wave is a rigorous test of the proposed reaction mechanism.
An exact model calculation of the diffusion controlled intramolecular rate constant
Authors: Battezzati M. , Perico A.
Published: February 2003

An exact calculation of the diffusion controlled first-order intramolecular rate constant k1 is given for the harmonic spring model and for any type of reaction sink in the frame work of the Wilemski–Fixman theory. For a one parameter spherically symmetric sink the resulting exact expansion for k−11 in terms of the sink parameter γ, k−11 = (√π/2Λγ)+(ln2−1)/Λ+0.5(√πγ/Λ)+⋅⋅⋅, gives the same γ→0 behavior of other known asymptotic calculations obtained for simpler diffusion controlled reaction theories. Also, the mean first passage time for end-to-end contact may be evaluated and results coincident to the third term in the expansion with the exact result given by the first passage time approach theory. These results support the vicinity of the Wilemski–Fixman closure approximation.
The production of O(3P) and ground state OH in the reaction of hydrogen atoms with ozone
Authors: Finlayson-Pitts B. J. , Kleindienst T. E. , Ezell M. J. , Toohey D. W.
Published: February 2003

The production of significant concentrations of O(3P) in the gas phase reaction of hydrogen atoms with ozone in a fast flow discharge system at 1.1±0.1 Torr total pressure in Ar and at room temperature has been reported earlier. These yields were observed to be independent of the concentration of known deactivators of OH† (v = 9) such as O2 and CO2. We report here the results of detailed studies of the yields of O(3P) and ground state OH(X 2πi)v = 0 (OH) using resonance fluorescence under a wide variety of reaction conditions. It is shown that the highest yields of O(3P), 27±2% of the calculated loss of O3 are observed when H is present in excess and the flow tube is coated with a halocarbon wax. The yields are substantially lower when O3 is present in excess and increases as [O3]0 increases. In addition, in an excess of either reagent, the O(3P) yields are lower when the walls of the flow tube are coated with boric acid, which is known to deactivate the vibrationally excited OH† produced in the initial reaction of H with O3. Addition of NO decreased the yield of O(3P), and increased the yield of OH. These results suggest that secondary reactions of OH†, likely with H and O3, are responsible for the production of O(3P), and that a second primary reaction channel, H+O3→HO2+O(3P) [Eq. (lb)] accounts for ?2% of the net reaction.
An electron pair operator approach to coupled cluster wave functions. Application to He2, Be2, and Mg2 and comparison with CEPA methods
Authors: Chiles Richard A. , Dykstra Clifford E.
Published: February 2003

A method for obtaining coupled cluster expansions with double substitutions (CCD) utilizing the electron pair operator approach of self-consistent electron pair (SCEP) theory is presented. A fairly simple operator is developed in this method and its calculation increases the expense over a typical variational configuration expansion only moderately. With this method, large basis set calculations have been performed on the weakly interacting dimers He2, Be2, and Mg2. Comparison calculations have been performed with various types of coupled electron pair approximations (CEPA), which may be viewed as approximations to coupled cluster theory, and with modification of the internal orbitals in the CCD treatment.
Theoretical prediction of the potential curves for the lowest-lying states of the C2+ molecular ion
Authors: Petrongolo Carlo , Bruna Pablo J. , Peyerimhoff Sigrid D. , Buenker Robert J.
Published: February 2003

A b initio MRD–CI potential curves have been calculated for C2+ in its first 16 electronic states and vertical transition energies Tv have been computed for a number of higher-lying species, all of which correlate with the first dissociation limit C(3Pg)+C+(2Pu). The ground state of this molecular ion is found to be X 4Σg− while the first excited state is 1 2Πu, with a calculated Te value of 0.84 eV. On the basis of this work the C2 I.P. value known experimentally is ascribed to the a 3Πu→1 2Πu process while the transition involving both ground states appears to be difficult to detect experimentlly. Thus, the measured De value for C+2 should involve fragmentation of the 1 2Πu states as well. A comparison with previous calculations which attempt to estimate the correlation energies of the various C+2 states in a semiempirical manner shows very large discrepancies, both in the transition energies themselves and in the ordering of these states. Finally the assignment for the Meinel experimental band system at 4.98 eV as a 2Σ−g←2Πu transition in C+2 is not supported by the present theoretical study.
Theoretical prediction of the potential curves for the lowest-lying states of the CSi+ and Si2+ molecular ions
Authors: Bruna Pablo J. , Petrongolo Carlo , Buenker Robert J. , Peyerimhoff Sigrid D.
Published: February 2003

Large-scale ab initio CI calculations have been carried out for the prediction of the potential curves of the electronic states of the isovalent CSi+ and Si2+ molecular ions which dissociate into C (3Pg)+Si+(2Pu) and Si (3Pg)+Si+(2Pu), respectively. The present results have been compared with previous findings for C2+ and the various parent neutral molecules, and the ionization potentials of the neutral species and the dissociation energies of all these diatomic species have been calculated. The ground and the first excited states of the ions are invariably π2(u)σ(g) X 4Σ(g)−, π3(u)–π(u)σ2(g) 1 2Πu, and π2(u)σ(g) 1 2Δ(g). The relative stabilities and potential curves of the various electronic states are markedly influenced by silicon substitution, as is the relative stability of the key σ(g) and π(u) MO’s. An increase in the number of Si atoms progressively favors double σ(u) occupation and leads to a lowering of the energy spacings between corresponding electronic and vibrational states, while π(u) ionization gradually becomes preferred over that of σ(g). The calculated ionization potentials and dissociation energies confirm, in general, the indirect experimental estimates of these quantities and allow for a more realistic assignment for the appearance potential of Si2+.
Some formal properties of the MTGLE representation of generalized Brownian motion theory
Authors: Adelman S. A.
Published: February 2003

The molecular time scale generalized Langevin equation (MTGLE) representation of generalized Brownian motion theory [S. A. Adelman, Adv. Chem. Phys. 44, 143 (1980)] is reformulated to yield a more rigorous and general theory. Key results include: (i) The theory is extended so as to be valid for an arbitrary real scalar process ṙ0(t) whose dynamics are governed by an arbitrary Hermitian Liouville operator L and whose statistics are governed by an arbitrary inner product 〈 〉; (ii) a rigorous development of the equivalent harmonic chain representation of ṙ0(t) is presented; (iii) the dynamical and statistical properties of the equivalent chain are rigorously shown to be formally identical to the dynamical and statistical properties of physical harmonic chains obeying classical Boltzmann statistics; (iv) the concept of clamping of equivalent chain atoms is developed for both dynamics and statistics in a rigorous manner. The clamping concept plays a role in the MTGLE theory somewhat analogous to the role played by successive projection operations in the Mori theory; (v) the MTGLE random forces Ṙp(t) are shown to be nonstationary processes if the inner product is 〈 〉. A new inner product 〈 〉r0,r1,⋅⋅⋅rp−1 is developed which renders the random forces stationary; (vi) the normal modes of the equivalent chain are shown to be simply related to the eigenfunctions of L; (vii) the MTGLE fluctuation–dissipation theorems are shown to be direct consequences of the orthonormality of the eigenfunctions of L; (viii) the connection between the MTGLE and Mori representations is made.
Theory of nonlinear transport processes and irreversible thermodynamics in multicomponent dense fluids
Authors: Eu Byung Chan , Okamura Y.
Published: February 2003

The kinetic equation for a single component dense fluid reported previously is extended to a multicomponent dense fluid and the evolution equations are derived therefrom for macroscopic fluxes. The extended Gibbs relation is derived for dense mixtures from the entropy balance and evolution equations by applying the modified moment method. The evolution equations yield nonlinear transport equations whose solutions in terms of thermodynamic forces give nonlinear constitutive relations as well as nonlinear transport coefficients. The nonlinearity and nonlinear couplings between different irreversible processes generally tend to decrease the transport coefficients according to the results obtained. The non-Newtonian shear viscosity is a typical example.
Laser magnetic resonance of the v = 1←0 transition of the PD (3Σ−) radical
Authors: Uehara Hiromichi , Hakuta Kohzo
Published: February 2003

Sixteen LMR spectra for the low N (N′, N″⩽3) transitions of the vibration (v = 1←0) band of PD (X 3Σ−) were observed by using seven 12C 16O laser lines. Analysis has yielded rotational, spin–spin, and spin–rotation constants for the v = 0 and 1 states and the band origin. The band origin ν0 has been determined to be 1653.2858(36) cm−1.
ESR of the 6Σ CrCu molecule at 4 °K
Authors: Van Zee R. J. , Weltner W.
Published: February 2003

The CrCu molecule has been formed in a solid krpton matrix at 4 °K by codepositing Cr and Cu atom beams. Isotopically enriched Cr 65Cu was also trapped. X-band ESR spectra indicated that the molecule has a 6Σ ground state with a small zero-field splitting D = −0.007(5) cm−1. Other derived magnetic parameters are g⊥ = 2.0030(10), A⊥(63Cu) = 856(9), A∥(63Cu) = 631 (60), and ‖eqQ(63Cu)‖ = 310(120) MHz. The relatively large quadrupole coupling constant is compared with that in other copper-containing molecules and in other transition-metal diatomics.
Heat capacity and thermodynamic functions of β-242Pu2O3 from 8 to 350 K. Contributions to the excess entropy
Authors: Flotow Howard E. , Tetenbaum Marvin
Published: February 2003

The heat capacity of β-242Pu2O3 was determined by a quasiadiabatic method from 8 to 350 K. The preparation of a single-phase hexagonal plutonium sesquioxide by hydrogen reduction of 242PuO2 at 2250 K is described. A λ-type heat capacity anomaly with its peak at 17.65 K was found and this anomaly is associated with an antiferromagnetic transition which is reported by McCart et al. in the preceding paper. An excess entropy of β-242Pu2O3 at 298.15 K is evaluated and shown to approach Sexcess = 2R ln6 = 29.79 J K−1 mol−1. In the same manner the previously published experimental entropy of 242PuF3 at 298.15 K was shown to contain an excess entropy contribution approaching Sexcess = R ln6 at 298.15 K. For each compound it was concluded that the excess entropy associated with antiferromagnetic ordering is R ln2 per mol of Pu+3 ions. For β-242Pu2O3 at 298.15 K the heat capacity C°p, entropy S°, enthalpy increment H°(T)−H°(0), and the Gibbs energy divided by temperature [G°(T)−H°(0)]/T are, respectively, (116.98±0.47) J K−1 mol−1, (163.02±0.65) J K−1 mol−1, (22 572±90) J mol−1, and (−87.31±0.35) J K−1 mol−1. The standard Gibbs free energy of formation of Pu2O3 at 298.15 K is calculated from available experimental data.
Ionicity and paramagnetism of strong organic charge-transfer complexes
Authors: Soos Z. G. , Keller H. J. , Ludolf K. , Queckbörner J. , Wehe D. , Flandrois S.
Published: February 2003

Ionic charge-transfer (CT) complexes were prepared by mixing neutral solutions of the potent electron acceptor TCNQF4 (7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane) with the strong donors M2P(5,10-dihydro-5,10-diemethylphenazine) and TMPD(N,N,N′,N′-tetramethyl-p-phenylenediamine). M2P–TCNQF4 [complex (1)] and TMPD–TCNQF4 [complex (2)] were obtained as deep blue solids, the latter only in polycrystalline form. Complex (1) crystallizes in the triclinic space group P1̄ with a = 7.002 (2), b = 8.787(2), c = 9.551(3), α = 108.53(2), β = 99.68(2), γ = 91.88(2), and Z = 1. The planar M2P+⋅ and TCNQF−4⋅ radical ions form mixed regular stacks. The structure refinement gave R = 0.039 and Rw = 0.033 for 1044 counter data. The ionicity γ and structure of mixed ⋅⋅⋅D+γA−γD+γA−γ⋅⋅⋅ stacks are closely related to the paramagnetic susceptibility χ(T). The two TCNQF4 complexes are shown to be the first highly ionic (γ≳0.9) systems. χ(T) for complex (1) follows a Heisenberg antiferromagnetic chain with J = 175 cm−1 for T≳122 K, below which χ(T) decreases steeply as expected for a dimerization transition. χ(T) for complex (2) can be fit to a power law T−α, with α = 0.75±0.10, and also indicates γ≳0.9. Both complexes (1) and (2) are on the paramagnetic side of the paramagnetic–diamagnetic interface, while the finite activation energy for paramagnetism in the less ionic M2P–TCNQ and TMPD–TCNQ complexes puts them on the diamagnetic side. The elementary excitations of these related CT complexes with different ionicities are modeled.
Molecular dynamics of infrequent events: Thermal desorption of xenon from a platinum surface
Authors: Grimmelmann Erik K. , Tully John C. , Helfand Eugene
Published: February 2003

Molecular dynamics techniques are employed to compute the rate of thermal desorption of Xe atoms from a platinum surface at temperatures between 100 and 2000 °K. A generalized Langevin approach is used to substantially reduce the number of degrees of freedom required to achieve an accurate description of the desorption process. A variety of techniques developed to improve the feasibility of simulating slow processes are explored. Using a ’’compensating potential’’ procedure, desorption rates can be computed efficiently even at temperatures for which the residence time of Xe on the surface is of the order of 1 sec (1014 vibrational periods).
Can photochemistry be enhanced on rough surfaces?
Authors: Nitzan Abraham , Brus L. E.
Published: February 2003

Molecular absorption is shown to be substantially enhanced near rough metallic and dielectric surfaces. The conditions under which this may lead to enhanced photochemical processes are investigated.(AIP)
On rate constants for dissociation of SF6 after multiple photon absorption
Authors: Poppe D.
Published: February 2003

The dissociation of SF6 by an intense IR laser is investigated theoretically. The approach is based upon a potential model for SF6. The dynamics are calculated using classical mechanics and Monte Carlo averageing. It is shown that, within the statistical errors, the logarithm of the reactant yield depends linearly on the energy fluence of the laser.
Raman spectrum of pressure compacted fused silica
Authors: Walrafen G. E. , Krishnan P. N.
Published: February 2003

Raman spectra of compacted fused silica are presented. Fused silica was sujected to a pressure of 9.0 GPa for 1 hr at 23 °C. The density of the compacted silica was 2.40 g cm−3 compared to 2.22 g cm−3 for the uncompacted glass. Various interesting features of the spectrum are discussed. (AIP)
Two-photon spectra of substituted benzenes
Authors: Goodman Lionel , Rava Richard P.
Published: February 2003

A perturbation approach to the two-photon spectra of monosubstituted benzenes for the 1B2(Lb)←1A1 transition is developed. The theory predicts that inductive substituents are ineffective in making the benzene 1Lb←1A two-photon spectrum allowed (opposite to the effect on the one-photon spectrum), but weakly mesomeric substituents are much more effective. Intensity measurements have been made which show that the allowed part of the toluene two-photon spectrum is much stronger than in fluorobenzene, and that the halobenzene intensity ordering is Br≳Cl≳≳F, in direct contrast with results found for one-photon spectra and in agreement with the theory. The two-photon spectrum of aniline is reported and found to be identical with the one-photon spectrum where they can be compared. This result also comes out of the theory, where the Franck–Condon part of the one- and two-photon spectra for strongly mesomeric substituents are predicted to be identical.
Ionization potential of the NH free radical by mass spectrometry: Production of ground state and electronically excited NH by F-atom reactions
Authors: Foner S. N. , Hudson R. L.
Published: February 2003

Sequential F-atom reactions with ammonia were used to produce NH radicals in the X 3Σ− ground state and the a 1Δ excited state. Ionization potential measurements on both of these species lead to the determination I(NH) = 13.47±0.05 eV, indicating that the error limits in at least one of the previous studies was seriously underestimated. The F-atom reactions also produced ground and electronically excited N atoms. Efforts to obtain an independent measurement of the heat of formation of NH using dissociative ionization of NH3 were unsuccessful, apparently because of excess energy involved in the ionization process. Some important N–N and N–H bond dissociation energies have been updated.
Translational energy dependence of reaction mechanism: Xe++CH4→XeH++CH3
Authors: Miller G. D. , Strattan L. W. , Cole C. L. , Hierl P. M.
Published: February 2003

The dynamics of the exoergic ion–molecule reaction Xe+(CH4,CH3)XeH+ were studied by chemical accelerator techniques over the relative translational energy range 0.2 to 8 eV. Results of the kinematic measurements are reported as scattering intensity contour maps in Cartesian velocity space. Center-of-mass angular and energy distributions, derived from these maps, provide information on the reaction mechanism and on the partitioning of available energy between internal and translational modes in the products. The results suggest that reaction proceeds via the formation of a long-lived complex at low collision energies (below 0.5 eV) and by a direct mechanism approaching spectator stripping at higher energies.
Level to level differential cross sections for vibrationally–rotationally inelastic collisions of Na2 with Ar
Authors: Serri J. A. , Becker C. H. , Elbel M. B. , Kinsey J. L. , Moskowitz W. P. , Pritchard D. E.
Published: February 2003

We report measurements of differential cross sections of rotationally (Δj = 15 to 79)–vibrationally (Δv = 1) inelastic collisions in ground electronic state Na2 with Ar on a level to level basis. The experiment is performed using crossed supersonic molecular beams and two dye lasers, one for tagging the initial level (v = 0, ji = 7), and the second for detection of the final level (v = 1, jf = 7+Δj) and scattering angle ϑ. A description of the experimental apparatus and technique is given. The differential cross sections all exhibit a pronounced rise at some minimum angle to a maximum followed by a gradual decline out to ϑ = π. The angular positions of the rise and peak increase nearly linearly with Δj. The Δv = 1, Δj integral cross sections are roughly constant in size even though the inelasticity (ΔE) varies over a large fraction of the available initial kinetic energy (∼2400 cm−1); by comparison, the Δv = 0, Δj integral cross sections, over a similar range in Δj, follow a power law dependence in ΔE. The qualitative features of both the differential and integral cross sections appear consistent within the framework of a classical, sudden collision on a sharply repulsive potential.
Half-projected Hartree–Fock calculations on several small molecules
Authors: Lengsfield Byron H. , Phillips Donald H. , Schug John C.
Published: February 2003

The half-projected Hartree–Fock (HPHF) method is examined with respect to its ability to obtain molecular correlation, describe molecular potential energy surfaces, and provide a one particle basis for more elaborate treatments. The equivalence, aside from questions of efficiency, of two different HPHF algorithms is demonstrated. The results of calculations on H2O, C2, N2, and CH2 indicate that the performance of the HPHF method, with spin projection in appropriate cases, is roughly equivalent to limited MCSCF treatments. In particular, (1) a small but important fraction of the correlation energy, (2) qualitatively correct potential energy surfaces, and (3) good one particle orbital bases are obtained.
Core-electron binding energies from self-consistent field molecular orbital theory using a mixture of all-electron real atoms and valence-electron model atoms
Authors: Quinn Charles M. , Schwartz Maurice E.
Published: February 2003

The chemistry of large systems such as clusters may be readily investigated by valence-electron theories based on model potentials, but such an approach does not allow for the examination of core-electron binding energies which are commonly measured experimentally for such systems. Here we merge our previously developed Gaussian based valence-electron model potential theory with all-electron ab initio theory to allow for the calculation of core orbital binding energies when desired. For the atoms whose cores are to be examined, we use the real nuclear changes, all of the electrons, and the appropriate many-electron basis sets. For the rest of the system we use reduced nuclear charges, the Gaussian based model potentials, only the valence electrons, and appropriate valence-electron basis sets. Detailed results for neutral Al2 are presented for the cases of all-electron, mixed real–model, and model–model SCF–MO calculations. Several different all-electron and valence electron calculations have been done to test the use of the model potential per se, as well as the effect of basis set choice. The results are in all cases in excellent agreement with one another. Based on these studies, a set of ’’double-zeta’’ valence and all-electron basis functions have been used for further SCF–MO studies on Al3, Al4, AlNO, and OAl3. For a variety of difference combinations of real and model atoms we find excellent agreement for relative total energies, orbital energies (both core and valence), and Mulliken atomic populations. Finally, direct core-hole-state ionic calculations are reported in detail for Al2 and AlNO, and noted for Al3 and Al4. Results for corresponding frozen-orbital energy differences, relaxed SCF–MO energy differences, and relaxation energies are in all cases in excellent agreement (never differing by more than 0.07 eV, usually by somewhat less). Taken as a whole, the study clearly demonstrates the accuracy of the mixed real–model theory.
Relativistic electronic structure of UO2++, UO2+, and UO2
Authors: Wood J. H. , Boring Michael , Woodruff Susan Beatty
Published: February 2003

We have calculated the one-electron energy level structures of the isolated species UO2++, UO2+, and UO2 in their linear forms, using a self-consistent relativistic multiple scattering model with Xα exchange. For UO2++, we have used the molecular orbitals of the ground state muffin-tin potential to calculate the energies of a few of the many-electron excited states; these afford a somewhat more fundamental basis for comparison with experiment than do the one-electron energies, and their determination also sheds some light on the coupling scheme (ω–ω vs Λ–Σ) to be expected in these systems. These excited state energies agree well with the experimental absorption data. The calculated one-electron excitation energies for UO2++ are in reasonable agreement with the observed onset of absorption. The behavior of the corresponding one-electron binding energies as a function of U–O distance supports a different interpretation of the XPS than that given by Veal et al. Considering the three species at a common bond distance of 2.0 Å, we find that each contains roughly three 5f electrons; UO2++ has three bonding 5f ’s, UO2+ has two bonding 5f ’s and one nonbonding (localized) 5f, and UO2 has one bonding 5f and two nonbonding 5f ’s. This agrees with XPS which exhibits two localized f electrons in UO2 and none in UO2++.
Circular intensity differential scattering of light by helical structures. III. A general polarizability tensor and anomalous scattering
Authors: Bustamante Carlos , Tinoco Ignacio , Maestre Marcos F.
Published: February 2003

Numerical calculations of the circular intensity differential scattering of light by oriented helical structures made of units with general polarizability tensors are presented. The effects on the scattering patterns of both absorptive and dispersive properties of the units are illustrated. The differential scattering and the total scattering both show anomalous scattering phenomena; the differential scattering pattern is asymmetric when the wavelength of incident light is within an absorption band. Equations for biaxial polarizabilities are used to derive the symmetry properties of the differential scattering pattern and to show how this symmetry can be used to determine the right- or left-handed sense of the helical structure. The wave-length dependence of the scattering pattern is obtained for a Lorentzian polarizability.
Intramolecular vibrational energy transfer in 1B2 aniline induced by collisions with H2O and CH3F
Authors: Vandersall Mark , Chernoff Donald A. , Rice Stuart A.
Published: February 2003

Single vibronic level fluorescence spectroscopy and lifetime measurements were used to study intramolecular vibrational energy transfer in low pressure gas samples of 1B2 aniline, induced by collisions with H2O and CH3F. Overall rates of depopulation of the initially prepared levels, as well as rate constants for specific mode-to-mode transfers, have been measured for the initially excited levels 00, 10b1, and I1. The observed rates and patterns of energy transfer show both a high collisional efficiency for these polyatomic colliders and mode-to-mode specificity; they also indicate that hydrogen bonding interactions are unimportant.
A correlation diagram model for interpreting propensity rules in collision induced vibrational relaxation
Authors: McDonald Daniel B. , Rice Stuart A.
Published: February 2003

Propensity rules for collision induced vibrational energy transfer are derived by a correlation diagram technique similar to that used by Woodward and Hoffman to treat symmetry restrictions in concerted reactions. A rough quantification is accomplished by an ad hoc generalization of the modified SSH breathing sphere model. Predicted relative transfer rates agree qualitatively and, in some cases, quantitatively with those observed for collisions of benzene, aniline, and pyrazine with inert atoms.
The Ramsauer–Townsend effect in molecular systems of electron-spin-polarized hydrogen and helium and their isotopes
Authors: Lim T. K. , Larsen Sigurd Y.
Published: February 2003

From arguments based on the De Boer quantum parameter and accurate phase shift calculations, we predict that the Ramsauer–Townsend effect occurs in numerous molecular systems involving electron-spin-polarized hydrogen and helium and their isotopes. Our work raises prospects of more precise determination of the two-body potential parameters of these systems as well as exciting quantum behavior in their macroscopic states.
Recurrent precipitation and Liesegang rings
Authors: Keller Joseph B. , Rubinow Sol I.
Published: February 2003

A mathematical formulation is presented of Wilhelm Ostwald’s supersaturation theory of Liesegang ring formation. The theory involves diffusion of two reactants toward one another, their chemical reaction to form a product, the reverse reaction, and diffusion of the product. When the product concentration reaches a certain supersaturation value, it begins to precipitate. It is shown that this theory can lead to recurrent precipitation, resulting in rings or bands of precipitate. Conditions under which this occurs are determined. In addition, the locations and times of formation of the bands are calculated and shown to agree with experimental results.
Dynamic light scattering from macromolecules: The separation of translation from rotational and deformational modes
Authors: Rallison J. M. , Leal L. G.
Published: February 2003

We consider the theory of dynamic light scattering from dilute solutions of macromolecules that undergo simultaneous translational, rotational, and conformational diffusion. Although the coupled diffusion problem is very difficult, we show that a relatively simple approximate theory is possible provided that the time scale for adjustment of the spatial position of the molecule is long relative to that characteristic of changes in the orientational and deformational configurations. After first developing a theoretical framework for the general case, we consider the case of rigid-rod macromolecules which was previously studied by other authors. In the present analysis we use a multiple time scale asymptotic technique which renders the solution of the coupled diffusion problem uniformly valid for both long and short times following some prescribed initial conformation. The resulting expression for the light scattering spectrum consists of a series of discrete exponentials in time (Lorentzians in frequency) rather than the more complex forms which were obtained by Maeda and Saito and Schaeffer et al. The latter can, in fact, be obtained from the present theory in the limit of small correlation times. Comparison is made between the theoretically derived scattering spectrum and two available data sets for TMV. The indicated degree of anisotropy of the translational diffusion coefficient is the same for the two sets of data, but considerably smaller than expected from electron micrographs of TMV.
A semiempirical Xα calculation of the K
V
V Auger line shape of O2
Authors: Dunlap B. I. , Mills P. A. , Ramaker D. E.
Published: February 2003

The O KVV line shape of O2 is analyzed using Xα one-electron populations and empirically derived atomic matrix elements. A simple method is presented for including the broadening effects of final state nuclear vibrational motion. The energies, widths, and intensities of the major features are qualitatively accounted for in this treatment.
Universal strong coupling equation of state for inverse power potentials
Authors: Rosenfeld Y. , Baram A.
Published: February 2003

A variational model is studied with the Percus–Yevick hard sphere g(r). Using the PY virial entropy, it predicts a universal expression for the potential energy in the strong coupling regime U/NKBT = amΓ+bnΓ1/4−0.5 for all inverse power potentials.
Structural and magnetic properties of Co(urea)2Cl2⋅2H2O: A two-dimensional Ising system with hidden canting
Authors: Carlin Richard L. , Joung Kyong O. , van der Bilt A. , den Adel H. , O’Connor C. J. , Sinn E.
Published: February 2003

The characterization of Co(urea)2Cl2⋅2H2O is reported. The crystal structure at ambient temperatures yields the following information: the chlorine, water, and urea group are coordinated, the urea being bound via the oxygen, to form an octahedral environment about the metal atom. Intermolecular hydrogen bonds between the coordinated chlorine and water ligands produce parallel infinite sheets which provide a two-dimensional exchange pathway. The sheets are separated by the urea ligands and may be considered as magnetically isolated from one another. Crystal data: space group P21/c, Z = 2, a = 7.397 (3) Å, b = 7.795(1) Å, c = 9.764(2) Å, β = 116.23(5) Å, V = 505 Å3, ρcalc = 1.87 g cm−3, ρobs = 1.88 g cm−3, R = 4.1% for 692 reflections. A λ anomaly in the specific heat was observed at 2.585±0.005 K, and the critical parameters are characteristic of the two-dimensional Ising model. The zero-field susceptibilities have been measured over the temperature interval 1.2–30 K, and are consistent with antiferromagnetic ordering at 2.52±0.05 K. The easy axis lies parallel to the monoclinic a axis, while hidden canting was observed parallel to the b axis. All the data have been analyzed consistently by the two-dimensional Ising model with the parameters ga = 7.0±0.1, J/kB = −2.1±0.05 K.
Theoretical studies of the angular distributions of oxygen atoms ejected from an ion bombarded c(2×2) overlayer of oxygen on Ni(001). I. Effect of geometry
Authors: Kapur Shukla , Garrison Barbara J.
Published: February 2003

A detailed molecular dynamics study has been performed in order to determine the factors controlling the ejection directions of adsorbate atoms due to 600 eV Ar+ ion bombardment. The specific system studied is oxygen which dissociatively adsorbs to form a c(2×2) overlayer on Ni(001). A fourfold bridge, an atop or linear and twofold bridge bond, as well as several heights of the oxygen atoms above the surface were investigated. The angular distributions are shown to be influenced by both the bonding site and the height of the oxygen adsorbate. There are distinguishing characteristics that can be ascribed to each of these three bonding sites.
Self-avoiding walks with span limitations. II. The mean square radius of gyration
Authors: Barr Ronnie , Brender Chava , Lax Melvin
Published: February 2003

Results are reported on the average radius of gyration <S2N(L)≳L for self-avoiding walks of size N steps having span L generated on the simple cubic lattice. Limiting behavior was fitted to expressions of the form <S2N(L)≳L∼ANγ(L) where γ(L) was found to vary from γ(O)∼1.50 to γ(N)∼1.20. In addition, the limiting ratio <S2N(L)≳L/<R2N(L)≳L was investigated. It was found that <S2N(O)≳L/<R2N(O)≳L →0.136, whereas <S2N(N)≳L/<R2N(N)≳L →0.155. In the interval O⩽L⩽N, γ(L)→1.0, and <S2N(L)≳L/<R2N(L)≳L →0.167, indicating a conformational change in the chains to a more compact randomlike configuration.
Reduced equations of motion and reduced S matrix for scattering processes
Authors: Mukamel Shaul
Published: February 2003

Reduced equations of motion (REM) are derived for the reduced density matrix of some of the degrees of freedom taking part in a scattering process. The derivation is made making use of the Zwanzig–Mori projection operator technique and utilizing a partial time ordering prescription (POP) which results in simple REM which contain no convolution in time. The present formulation is convenient for the direct calculation of reduced information on scattering events since it avoids the necessity of calculating the complete S matrix. The entire dynamical information relevant for the present description is expressed in terms of a hierarchy of correlation functions which in turn depend on the potential surface only in the region of interest. We further develop an exponential approximation for the reduced tetradic scattering S matrix in Liouville space, which forms a convenient framework for systematic approximations.
Vibrational rainbows
Authors: Drolshagen Gerhard , Mayne Howard R. , Toennies J. Peter
Published: February 2003

We extend the theory of inelastic rainbows to include vibrationally inelastic scattering, showing how the existence of vibrational rainbows can be deduced from collinear classical scattering theory. Exact close-coupling calculations are carried out for a breathing sphere potential, and rainbow structures are, in fact, observed. The location of the rainbows generally agrees well with the classical prediction. In addition, the sensitivity of the location of the rainbow to changes in the vibrational coupling has been investigated. It is shown that vibrational rainbows persist in the presence of anisotropy. Experimental results [R. David, M. Faubel, and J. P. Toennies, Chem. Phys. Lett. 18, 87 (1973)] are examined for evidence of vibrational rainbow structure, and it is shown that vibrational rainbow theory is not inconsistent with these results.
The origin of the infrared multiphoton induced luminescence of chromyl chloride
Authors: Tsao Jeffrey Y. , Bloembergen N. , Burak Itamar
Published: February 2003

We have studied systematically the luminescence induced when CrO2Cl2 is infrared multiphoton excited by a moderate fluence (0–15 J/cm2) CO2 laser. This includes the dependence of the full luminescence spectrum on CO2 laser fluence, and for various spectral regions of the luminescence their dependence on CO2 laser wavelength and fluence, their intramolecular decay rates, and using a time-of-flight technique the translational temperatures associated with their sources. These measurements show that two species contribute to the luminescence. In addition, a chemical identification of these species is made by preceding the IR laser pulse with a UV laser pulse to photochemically prepare the CrO2Cl fragment, and isolate its contribution to the luminescence. Strong evidence is obtained in support of both CrO2Cl fragment and CrO2Cl2 parent contributions to the luminescence. The relative contributions of the two to the luminescence and an absolute yield for the IR multiphoton induced dissociation of CrO2Cl2 into CrO2Cl as functions of CO2 laser fluence are estimated.