bluuebunny/crossref_metadata_2025 huggingface dataset:
Extrinsic photoconductivity shadowing (EPS) as a method to study triplet energy migration in molten naphthalene
Authors: Prock Alfred , Walker J. Kennedy , Melman Paul
Published: February 2003

Temperature dependence around the melting point of the long-range triplet diffusion length in molten naphthalene was measured by a steady state extrinsic photoconductivity shadowing (EPS) method. Photoinjection was shown to be bipolar and its relation to the nonuniform field profile, which is determined by the dark current, was elucidated. The calculated effect of this field profile on a pulse experiment designed to measure triplet lifetime and other kinetic parameters is presented. The EPS method is based on a precisely defined light intensity distribution and derives its accuracy from the ability to determine optical component positions by diffraction techniques.
Calculation of NMR chemical shifts. II. Comparison of the SOS and FP methods
Authors: Fukui H. , Yoshida H. , Miura K.
Published: February 2003

A comparison of the sum-over-states method and the finite perturbation method used for the calculation of chemical shifts in the category of the perturbation theory is made.(AIP)
A measurement of the hyperfine structure of C17O
Authors: Frerking Margaret A. , Langer William D.
Published: February 2003

The electric quadrupole and magnetic dipole coupling constants for C17O have been determined from observations of the J = 1→0 emission lines in an interstellar cloud with sufficiently small velocity dispersion such that all three components are clearly resolved. The new values we derive are somewhat different from previous determinations based on partly resolved spectra and are eQq = 4337±56 kHz and CI = −30.4±1.2 kHz.
A nonseparable quantum mechanical transition state theory
Authors: Pollak Eli
Published: February 2003

A quantal transition state bound on the rate of chemical reactions in any dimension is derived. The magnitude of the bound is dependent on the location of a dividing surface and thus is variational. Analysis of the quality of the bound in one and two dimensions shows that the quality of the bound improves with increasing dimensionality of the problem. The new theory requires only the computation of diagonal density matrix elements and so should prove to be computationally useful.
Single and double ionization of nitric oxide by electron impact from threshold up to 180 eV
Authors: Kim Y. B. , Stephan K. , Märk E. , Märk T. D.
Published: February 2003

Electron impact ionization of nitric oxide was studied as a function of electron energy up to 180 eV. A double focusing mass spectrometer in combination with an improved electron impact ion source (Stephan et al.) was used, alleviating the problems of ion extraction from the source and the transmission of the extracted ions through the mass spectrometer system. Relative partial ionization cross section functions were measured for the processes NO+e→NO++2e and NO+e→NO2++3e. An accurate determination of the cross section ratio q(NO2+/NO)/q(NO+/NO) was made. In addition, absolute partial ionization cross sections for the production of NO+ and NO2+ were determined by normalizing the relative cross sections with two different methods. Using nth root extrapolation an appearance potential of NO2+(X 2Σ+) was determined. The result is compared with previous measurements and theoretical calculations.
VUV fluorescence following photodissociation of N2O at 193 nm
Authors: Zavelovich J. , Rothschild M. , Gornik W. , Rhodes C. K.
Published: February 2003

Photolytic studies performed at 193 nm demonstrate that NO in the highly excited D(v = 1,5) and E(v = 0) states is generated from N2O during irradiation in three sequential steps involving photodissociation, chemical reaction, and photoexcitation. The resulting NO fluorescence (160–230 nm) was analyzed with a system of rate equations, and the temporal behavior, intensity dependence, and pressure dependence were found to be consistent with a simple kinetic model. The quenching coefficient of NO by N2, Ar, and N2O were determined in this analysis to be qN2 = (2.7±0.8)×10−11 cm3 sec−1, qAr = (6.6±1.4)×10−11 cm3 sec−1, and qN2O = (1.5±0.4)×10−10 cm3 sec−1. Finally, dramatic changes in the spectral distribution of the ultraviolet NO fluorescence due to collisions with He were observed, which contrasts with the absence of spectral redistribution in collisions involving N2, Ar, and N2O.
On the theory of the Stern–Volmer coefficient for dense fluids
Authors: Baird J. K. , McCaskill J. S. , March N. H.
Published: February 2003

In phenomenological theories of fluorescence quenching in dense fluids, one assumes that the Stern–Volmer coefficient K has a dependence upon a limited number of parameters. Denoting by K0 the value of K when the quencher molecules are infinitely dilute, our starting point is that K0 = K0(D,τ,k,R), where D is the sum of the diffusion coefficients of the fluorophore and quencher, k is a rate constant, τ is the unquenched lifetime of the fluorophore, and R is the range of interaction between fluorophore and quencher. We then find by dimensional analysis that K0 can be simplified to K0 = DRτ⋅F (k/DR,Dτ/R2), where F(x,y) is a function of just two variables. Furthermore, if S is a function specifying the region of configuration space in which fluorophores and quenchers interact [G. Wilemski and M. Fixman, J. Chem. Phys. 58, 4009 (1973)], we find that K0 depends only upon an average of S over the steady state fluorophore–quencher pair correlation function. Under ordinary conditions, k/DR and Dτ/R2 will be large. We note that in this limit K0 approaches the Smoluchowski value 4πDRτ. Correction terms to the Smoluchowski value are explicitly computed, however, when S is a delta function or a step function. Inasmuch as both forms lead to corrections tending in the same direction, it may be difficult to distinguish them by experiment. Thus, we conclude that future theoretical work should focus on a more realistic form for S.
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o third order effective valence shell Hamiltonian calculations for first row diatomic hydrides
Authors: Sun Hosung , Sheppard Maurice G. , Freed Karl F.
Published: February 2003

A b initio effective valence shell Hamiltonian ℋv calculations are presented through third order for the correlation energies of CH, NH, and OH at their equilibrium internuclear distance. The valence shell consists of the 2σ, 3σ, 1π, and 4σ orbitals. This represents the first quasidegenerate third order many-body perturbation calculation of ℋv for molecules with more than two valence electrons. As in the atomic case, a single ℋv calculation provides correlation energies for all neutral valence states as well as for the positive and negative ion valence states of the hydrides. Calculated vertical excitation energies are in good agreement with those obtained from large configuration interaction calculations. Four-electron effective interactions, present in the third order expansion of ℋv, are evaluated. These four-electron terms make significant contributions to the energies of many states. Technical difficulties are discussed which are related to ℋv calculations with large valence spaces that violate the quasidegeneracy requirements of the theory. One method which alleviates the associated divergence problems is shown to be useful in third order calculations.
On the use of corresponding orbitals in the calculation of nonorthogonal transition moments
Authors: Lengsfield Byron H. , Jafri Jawed A. , Phillips Donald H. , Bauschlicher Charles W.
Published: February 2003

Full valence and first-order CI wave functions are invariant with respect to unitary transformations among the valence orbitals. We exploit this degree of freedom and show that by transforming the valence orbitals into a corresponding orbital basis, nonorthogonal transition moment calculations become an easily managed task. Sample full valence calculations on several states of O+2 and OF are also presented.
Coriolis interactions and infrared intensities in fluoroform
Authors: Kondo Shigeo , Saëki Shinnosuke
Published: February 2003

Infrared intensities of the fundamental bands of the CHF3 molecule were remeasured. The Coriolis interaction between ν3 and ν5 bands as well as that between ν6 and ν5 bands was analyzed to determine the sign of the perturbation. As a result, the relative signs of all the (∂p/∂Qi)’s in this molecule have been determined without using the CNDO/2 method or an ab initio calculation. An empirical rule on the sign of the CF bond intensity parameters is proposed by the name of ’’CF-bond criteria’’, according to which a negative dipole moment is enhanced in the direction of the displacements of the F atoms in both the CF stretching and CF deformation modes. Further, a possibility is suggested to extend this rule to all halogen atoms.
Intramolecular vibrational relaxation in jet-cooled phthalocyanine
Authors: Fitch Pamela S. H. , Haynam Christopher A. , Levy Donald H.
Published: February 2003

We have observed the dispersed fluorescence spectrum of free base phthalocyanine cooled in supersonic free jet. The spectrum consists of a single strong feature and some weaker red shifted structure. The frequencies of the single strong feature and the red shifted structure change only slightly as a function of the exciting frequency. This indicates that the potential surface does not change greatly upon electronic excitation and that the strong fluorescence transitions are Δv = 0. For excitation at frequencies greater than 720 cm−1 above the origin, the features in the fluorescence spectrum are broadened although the absorption features remain narrow. This is taken to indicate a mixing of background vibrational levels leading to intramolecular vibrational relaxation that is fast compared to the fluorescence lifetime. The intramolecular vibrational relaxation in phthalocyanine is compared to that observed in a recent series of experiments on alkylbenzenes.
Infrared emission spectroscopy of the products of UV-dissociated chloroethylenes
Authors: Moss Mary G. , Ensminger Mark D. , McDonald J. D.
Published: February 2003

The chloroethylenes are known to undergo dissociation under ultraviolet light to form chloroacetylenes. We report here the infrared emission spectra of the products of 193 nm dissociation of vinyl chloride, trans- and cis-dichloroethylene, vinylidene chloride, and trichlorothylene. The spectra reported here represent molecules that have undergone from four to 40 collisions. The most prominent features of these spectra are that the products are very highly vibrationally excited. The energy partitioning possibilities that are consistent with these results are discussed.
The high temperature generation and ESR investigation of ScH2, YH2, and YD2 radicals in rare gas matrices: A nonequilibrium vaporization technique for solid samples
Authors: Knight L. B. , Wise M. B. , Fisher T. A. , Steadman J.
Published: February 2003

The ScH2, YH2, and YD2 radicals have been generated from metal hydride powders at high temperatures by a nonequilibrium vaporization technique. The MH2 radicals were isolated in rare gas matrices at 12 K and investigated by electron spin resonance (ESR). The vaporization procedure has potential applications for the production of other vapor species which are difficult to generate by conventional means. The electronic structure, geometry, and ground state of these dihydride radicals were estimated from the ESR results and compared with recent theoretical conclusions. The metal ’’d’’ electron was found not to participate significantly in the M–H bond. These findings for MH2 are compared with recent results for ScF2 and YF2. The magnetic parameters for ScH2 are g∥ = 1.980 (1), g⊥ = 1.987 (1), A∥(45Sc) = −212 (2) MHz, A⊥(45Sc) = −128, ‖Aiso‖(H) = 12 (2). For YH2, g⊥ = 1.991  (1), A⊥(89Y) = −40 (10) MHz, ‖A⊥‖ (H) = 13 (2) MHz.
The structure of N2O–HF
Authors: Joyner C. H. , Dixon T. A. , Baiocchi F. A. , Klemperer W.
Published: February 2003

The rotational spectra of four isotopes of the weakly bound complex N2O–HF have been measured by molecular beam electric resonance spectroscopy. The following spectroscopic constants and structural parameters were derived from the observations:                               In the average structure, the N2O axis makes an angle of ∼47° with a line rc.m. drawn between the centers of mass of the two submolecules. HF is hydrogen bonded toward the oxygen on N2O to form an angle of ∼31° with the center of mass line.
Far infrared laser magnetic resonance spectrum of NF(a 1Δ)
Authors: Davies P. B. , Temps F.
Published: February 2003

The NF radical in its a 1Δ metastable state has been detected by far infrared laser magnetic resonance spectroscopy at 513 μm. The spectrum arises from the J = 7→8 rotational transition, and yields an accurate measurement for this spacing of 19.55172 cm−1.
Theory of resonance Raman scattering beyond the Condon approximation for a system with two modes of different symmetry
Authors: Henneker W. H. , Siebrand W. , Zgierski M. Z.
Published: February 2003

A theoretical study is presented of resonance Raman excitation profiles and depolarization dispersion curves for a molecule with two Raman-active normal modes of vibration and three electronic states. The modes are taken to be of different symmetry, one totally and the other non-totally symmetric. The three electronic states are the ground state and a pair of excited states coupled through the non-totally symmetric mode. The energy gap separating the pair is allowed to vary from zero (Jahn–Teller limit, strong coupling) to large values (Herzberg–Teller case, weak coupling). The totally symmetric coordinate is allowed to assume different equilibrium values not only between the ground state and the resonant state, but also between the two coupled excited states. As a result, the effective coupling depends indirectly on the totally symmetric coordinate, causing breakdown of the Condon approximation. The effect of this breakdown is demonstrated by calculating excitation profiles for all one and two quantum transitions involving the two modes. The profiles show progression of the totally symmetric mode with or without superimposed non-totally symmetric quanta. If the Condon approximation breaks down, the intensity distribution in these progressions generally depends on the presence of such quanta. For weak coupling, the effect should be most readily observable in combination bands. For strong coupling, it should also be observable in totally symmetric fundamentals and overtones. Special attention is given to systems with resonance–preresonance interference and to intermediate-coupling systems.
Vibrational dephasing in liquid ethane-d6
Authors: Wilde Richard E. , Chang Ta-Chau
Published: February 2003

The spontaneous Raman spectrum of liquid C2D6 has been obtained. The ν2 and ν3 isotropic bands have been analyzed by a memory-functions procedure. The analysis suggests that the vibrational dephasing arises from rotation about the symmetry axis. The tumbling of the symmetry axis is much slower, and in the case of the ν3 band leads to ’’homogeneous narrowing.’’ The 2ν2 overtone bandwidth was found to have a first-power dependence on the vibrational quantum number. The depolarization ratio of the ν3 band is 0.14; the ν2 band has a depolarization ratio less than 0.02.
Monte Carlo trajectory calculations of the energy of activation for collision-induced dissociation of H2 by Ar as a function of rotational energy
Authors: Blais Normand C. , Truhlar Donald G.
Published: February 2003

Rate constants and activation energies for selected initial rotational levels, thermally averaged over vibrational states and translational energies, are calculated for Ar+H2→Ar+H+H by the Monte Carlo quasiclassical trajectory method. The results show that activation energies for high rotational quantum numbers exceed those estimated from centrifugal barrier heights. To characterize the rotational-level model of diatomic dissociation, we tabulate rate constants, activation energies, and other properties of dissociative collisions as functions of initial rotational quantum number j for conditions of thermal vibrational and translational degrees of freedom at 4500 K. Under equilibrium conditions, dissociation from a given j level is shown to occur primarily from the topmost v state of that j level.
Intermolecular and intramolecular V→V transfer in the vibrational predissociation of some polyatomic van der Waals molecules
Authors: Beswick J. A. , Jortner Joshua
Published: February 2003

In this paper we present a theoretical study of vibrational predissociation (VP) on the ground state potential surface of van der Waals complexes consisting of polyatomic molecules. The intermolecular interaction was represented by separate contributions of atom–atom interactions, which were expressed in terms of Morse potentials. The VP process was handled utilizing the distorted wave approximation to describe a zero-order Hamiltonian, which is separable in the intramolecular and in the intermolecular motion, while the residual interaction, which induces the reactive process, corresponds to the deviations of the intermolecular interaction from its value at the frozen intramolecular equilibrium configurations of the two constituents. Model calculations of the VP dynamics were performed for the He⋅⋅⋅N2O complex and for the (N2O)2 dimer, where the N2O unit is initially excited to the (001) vibrational state. These calculations were performed for the linear configuration as well as for the T-shaped configuration of the complexes. We have considered three VP channels; (1) the V→T process, (2) the intramolecular V→V+T process and (3) the intermolecular V→V+T process. For He⋅⋅⋅N2O channels (1) and (2) prevail, while for (N2O)2 all three channels are open. For collinear He⋅⋅⋅N2O a second-order process of type (2) dominates the VP, while for the T-shaped He⋅⋅⋅N2O complex the first-order process (1) is not efficient. In both cases the VP lifetimes, τ is τ∼10−5–10−6 sec. For the collinear (N2O)2 dimer a V→V+T process of type (2) dominates, with τ∼10−2–10−3 sec, the inefficiency of this process being attributed to a large mass effect. For the T-shaped (N2O)2 dimer the most effective VP channel involves simultaneous intermolecular and intramolecular V→V+T process with low ΔE = 355 cm−1 translational energy, which is characterized by τ∼10−5–10−6 sec. These results provide guidelines for the analysis of the infrared spectra of the (N2O)2 complex.
Study of the interactions of atomic and molecular oxygen with O2 and N2 by scattering data
Authors: Brunetti B. , Liuti G. , Luzzatti E. , Pirani F. , Vecchiocattivi F.
Published: February 2003

Absolute total scattering cross sections for the O2–O2, O2–N2, O–N2, and O–O2 systems are measured in the thermal energy range. A glory structure is present for O2–O2, O2–N2, and O–N2. For O–N2 this structure appears to be partially quenched due to the anisotropy of the 3P oxygen atom. For O–O2 the structure is almost completely quenched because of the presence of a manifold of interactions. From the analysis of the cross section data, information about the interaction potential of all these systems of atmospheric interest is obtained. For O2–O2 a potential able to reproduce also the spectroscopic and thermophysical data is proposed.
Nonequilibrium velocity distribution and reaction rate in ion–molecule reactions
Authors: Koura Katsuhisa
Published: February 2003

The nonequilibrium velocity distribution and reaction rate in fast ion–molecule reactions with non-Langevin reaction cross sections are studied using the Monte Carlo simulation in steady-state systems where ions are diluted in heat-bath reactant molecules and ion sources compensate the loss of ions due to the reaction. The non-Langevin reaction cross sections are taken as the power and exponential dependences on the collision energy and the elastic cross section is taken as the hard-sphere model. The ion velocity distribution and temperature and the reaction rate constant indicate significant deviations from equilibrium.
Spectrum and energy levels of Pr3+ in ThBr4
Authors: Conway J. G. , Krupa J. C. , Delamoye P. , Genet M.
Published: February 2003

The strong features in the absorption spectrum and the laser excited fluorescence spectrum have been interpreted as arising from levels of Pr3+ in the D2d symmetry site of ThBr4. 43 energy levels have been fitted to the parameters with an rms deviation of 61 cm−1. The values of the crystal field parameters are B20=260.0, B40=−644.2, B44=929.2, B60=1089.0 , and B64=240.6 cm−1. The presence of other crystal symmetry sites is observed.
Pressure dependence of the absorption spectrum of β-carotene
Authors: Ho Z. Z. , Moore T. A. , Lin S. H. , Hanson R. C.
Published: February 2003

The pressure dependence of the absorption spectrum of β-carotene in pentane: isopentane solvent has been measured in the pressure range up to 60 kbar. A large red shift and broadening of the vibronic peaks is observed. The 0–0 peak appears to gain intensity relative to the 0–1 peak.
Vibronic relaxation in molecular mixed crystals: Pentacene in naphthalene and p-terphenyl
Authors: Hesselink Wim H. , Wiersma Douwe A.
Published: February 2003

Picosecond photon echo techniques are used to measure directly vibronic relaxation times in the first excited singlet state of pentacene in naphthalene and p-terphenyl. In regions of low (<300 cm−1) and high (≳1000 cm−1) vibrational energy, relaxation is fast (τ<2 ps) due to direct phonon emsssion (low energy) and fast vibrational energy redistribution (high energy). In the intermediate region relaxation times vary drastically. Effects of isotope substitution of guest and host suggest intramolecular relaxation in this region. Changes in local environment are shown to influence the relaxation rate quite drastically, indicating large host-induced anharmonic coupling between different vibrations.
Relaxation equations for multispin systems. II
Authors: Sanctuary B. C. , Selwyn L.
Published: February 2003

The Redfield equations are derived quite generally for multispin systems of arbitrary number of spins with arbitrary spin magnitudes. The spin lattice coupling Hamiltonian is assumed to arise from random fluctuations only. The extreme narrowing limit is not taken.
NMR study of compressed supercritical water
Authors: Lamb W. J. , Jonas J.
Published: February 2003

The proton spin–lattice relaxation time T1 in water has been measured as a function of pressure in the temperature range 150 to 700°C. This study focuses on the supercritical region (tc=374°C) where the spin–rotation interaction mechanism dominates the observed proton relaxation rate. Since water is an asymmetric top molecule, the analysis of the experimental data involves a number of simplifying assumptions discussed in detail. The experimental finding that in supercritical water the spin–rotation relaxation time T 1SR is a linear function of density ρ, up to relatively high densities (ρ≃ 1.5 ρc) provides rationale for analysis of the NMR experimental data in terms of a model used for dilute gases. The T 1SR data are analyzed on the basis of the assumption that the collision modulated spin–rotation interactions can be described by a single correlation function which is an exponential function of time. Using this procedure, we find that T 1SR/ρ αT−2, i.e.T 1SR/ρ exhibits a stronger temperature dependence than that found (T 1SR/ρ αT−3/2) for many polar and nonpolar gases. The calculated effective cross sections for the transfer of angular momentum σeff which show strong temperature dependence (σeff αT−1.5) are several times larger than the kinetic cross sections. By assuming applicability of expressions derived for isotropic reorientation of spherical-top molecules and using the effective spin–rotation interaction constant as obtained from microwave measurements, we are able to calculate the angular momentum correlation time τJ, over the range of temperatures and densities studied. In the supercritical region τJ⩾τΘ, where τΘ is the reorientational correlation time, and the estimated mean angle of reorientation ΔΘ¯ is in the range 50° to 800°. The T 1SR data are also interpreted in terms of the modified rough hard sphere (RHS) model which for ρ<2ρc takes into account the effect of attractive forces. We find that 1/T 1SR is a linear function of the Enskog relaxation time τE. The experimental ratio of τE/τJ reflecting the efficiency of angular momentum transfer shows density and temperature dependence, in agreement with expectation. There is no quantitative agreement between τE/τJ calculated and observed because of the oversimplified treatment used. An empirical relationship between T 1SR and the shear viscosity in the supercritical compressed water is established, namely, the T 1SR is a linear function of the logarithm of shear viscosity. In addition, the relaxation data in the temperature range 150 to 350°C is also reported and analyzed. In this specific temperature range, both the dipolar (intra- and intermolecular) and the spin–rotation interaction mechanisms contribute to the observed proton relaxation rate.
Contribution of specifically adsorbed ions, water, and impurities to the surface enhanced Raman spectroscopy (SERS) of Ag electrodes
Authors: Pettinger Bruno , Philpott Michael R. , Gordon Joseph G.
Published: February 2003

Surface enhanced Raman scattering (SERS) has been observed from silver electrodes for water (H2O and D2O) in the frequency region of the librational, bending, and stretching modes. Simultaneously, SERS has been observed for halide ions and some organic impurities. The appearance of SERS from water and halide ions under the circumstances of the experiment is attributed to the formation of surface complexes involving silver adatoms, halide ions, and water molecules.
Helical polymer circular dichroism: Transition matrix theory for polypeptides
Authors: Rabenold David A.
Published: February 2003

A Green operator technique used within a linear response framework is employed to present a transition matrix theory of long helical polymer circular dichroism. Specializing to a polypeptide, simple closed form expressions are obtained for the circular dichroism. The formulation clearly shows (1) how the ΠΠ* exciton system gives rise to the ’’helix band’’ and ’’radial bands’’; (2) how the coupling of the ΠΠ* and nΠ* transition densities gives rise to a band centered at the unperturbed nΠ* transition frequency which is balanced in rotational strength by bands at the ΠΠ* perpendicular and parallel polarized transition frequencies; and (3) how interaction of these low energy transitions with high energy transitions can alter all of the above mentioned bands.
Optical modes of t
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s-polyacetylene
Authors: Schügerl F. B. , Kuzmany H.
Published: February 2003

The Raman spectra of trans-polyacetylene excited with various laser wavelength (4579, 4880, 5145, 5682, and 6471 Å) show the lines at 1100 cm−1, 1500 cm−1 in (CH)x, and 860 cm−1 and 1400 cm−1 in (CD)x broadened. From a valence-force-field calculation of the optical modes, force constants in agreement with results from ab initio molecular orbital theories are obtained. It is shown that the broadening of the Raman lines may be interpreted as contributions to the scattered light from defect free polyene chains with various length. The experimentally observed spectra of the modes and the corresponding linewidths are explained by increasing the C=C stretch force constant from 7.7 mdyn/Å (infinite chain) up to 9.5 mdyn/Å (short chain). From the C=C double bond stretch force constant a bond order between 0.67 to 0.85 was found.
EPR measurements of an alternate NO2−− defect in irradiated potassium azide
Authors: Willis James S.
Published: February 2003

The angular dependence of the EPR spectra of irradiated single crystals of potassium azide has been measured at room temperature. As a result, by comparison of spectroscopic splitting factors and hyperfine splittings, one of the defects present can be identified as an NO2−− radical existing in an orientation different from that previously reported. Crystal symmetry and the symmetry of the angular dependence of the EPR spectra indicate that this alternate orientation is such that the axis which is perpendicular to the plane of the NO2−− radical lies in either the [110] or the [11̄0] direction of the crystal.
Resonance Raman scattering of benzene and benzene-d6 with 212.8 nm excitation
Authors: Ziegler L. D. , Hudson B.
Published: February 2003

The first resonance Raman spectra of benzene and deuterobenzene are reported. Resonance excitation with the second excited singlet transition (1B1u) is obtained with 212.8 nm radiation. These resonance Raman spectra are dominated by the overtones and combinations of the vibronically active e2g modes in this dipole forbidden transition, as well as the fundamental, overtone, and combination of ν1, the a1g ring breathing mode. All of the observed features, including relative intensities and depolarization ratios, are fully rationalized within the context of the vibronic theory once the 3N-6 multidimensionality of the vibrational sublevels is exposed. The ν8 e2g mode is found to be the most vibronically active mode in the 1B1u transition, in agreement with all calculations. However, in contrast to these calculations, the Raman analysis finds that ν9, not ν6, is the next most vibronically active e2g mode in this transition. These results parallel the observed pattern of vibronic intensities in the 3B1u→1A1g phosphorescence spectra.
The infrared spectrum of ammonia hydrate: Explanation for a reported ammonia phase
Authors: Sill Godfrey , Fink Uwe , Ferraro John R.
Published: February 2003

A number of anomalous spectra of solid NH3 deposited from the vapor phase have appeared in the literature. These spectra have been ascribed to a new phase of NH3. In the experiment reported here these anomalous spectra were reproduced by depositing a thin film from a mixture of gaseous NH3 and H2O and annealing this film at a temperature of 162 °K. The thin film spectra showed excellent agreement with recent data on NH3⋅H2O. The anomalous ’’NH3’’ spectra are, therefore, seen to be caused by H2O contamination of solid NH3 with formation of NH3 hydrate.
Silicon-29 cross-polarization magic-angle sample spinning spectra in amorphous silicon–hydrogen films
Authors: Reimer J. A. , Murphy P. Dubois , Gerstein B. C. , Knights J. C.
Published: February 2003

Magic-angle sample spinning cross-polarization 29Si NMR spectra are reported for plasma-deposited amorphous silicon-hydrogen films. The data show (i) confirmation of the identification of polyhydride (SiH2)n species as seen by infrared spectroscopy and (ii) a large degree of chemical shift dispersion due to disorder in the silicon lattice.
The polarized Raman spectrum of the argon diatom
Authors: Frommhold Lothar , Proffitt Michael H.
Published: February 2003

The first measurement of the polarized Raman spectrum of the argon diatom are reported. The existing theoretical models of the diatom are discussed. (AIP).
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U(2) symmetry for linear polyenes
Authors: Herrick David R.
Published: February 2003

This paper describes a novel approximate SU(2) symmetry for valence-shell π-electron correlation in unsaturated hydrocarbons CnHn+2. The symmetry is related to accidental near-degeneracy and vibrationlike excitations in a Hückel resonance picture. Diagonalization of the CI energy matrix within each manifold splits the degeneracy and gives configuration-mixed states, including collective one- and two-electron excitations thought to be important in the first excited 1A−g level of butadiene. The approximate SU(2) symmetry for the electron correlation which causes the splitting is found with a Pariser–Parr–Pople Hamiltonian. Coupled SU(2) representations have Casimir invariants that are near-constants of the motion, and CI mixing coefficients are identified as Clebsch–Gordan coefficients. Exact SU(2) symmetries are found for a two-electron zero-differential-overlap repulsion operator, and for a many-electron PPP Hamiltonian with harmonic two-body repulsion terms. Polyene spectra computed with an Ohno–Coulomb repulsion are classified with new term symbols 2S+1(J)Pν that contain quantum numbers for spin(S), pseudorotation angular momentum (J), particle-hole exchange (P), and ’’quasispin’’ (ν), much like the usual spectroscopic term symbols with seniority for atoms and nuclei. Selection rules for CI matrix elements and radiative transitions are derived from tensor operators and the Wigner–Eckart theorem, and show that in a zeroth approximation two-photon excitation of the first 1A−g level is a forbidden process. The SU(2) approach for near-degeneracy CI is applied to the problem of photoelectron spectroscopy to predict shake-up for butadiene and hexatriene. The group SU(2) is also used to interpret exciton correlation.
CEPA calculations on open-shell molecules. IV. Electron correlation effects in B1 Rydberg states of H2O
Authors: Staemmler Volker , Jaquet Ralph , Jungen Martin
Published: February 2003

We have calculated the vertical electronic excitation energies of ten 1b1→a1 excited states of H2O belonging to different Rydberg series. The calculations have been carried out with the FRC, SCF, PNO–CI, and CEPA approximations in order to investigate the electron correlation of Rydberg states. We report on correlation contributions to the electron binding energies, to the quantum defect of Rydberg series, and to the singlet–triplet splitting energies. In addition we make predictions for the dipole moments of the Rydberg states based on SCF calculations. Comparison of our results with experiment allows the assignment of transitions belonging to a np series and two nd series and it suggests an analysis for the 1b1→ns1B1 Rydberg series.
Application of perturbation theory to the crystal–melt interface
Authors: Bonissent Alain , Abraham Farid F.
Published: February 2003

The Abraham–Singh perturbation theory is applied to the case of the interface between an fcc crystal and its melt. The starting point is the density distribution of a hard-sphere liquid near a hard wall. The perturbation theory corrects the hard-sphere fluid density distribution by accounting for the soft repulsive nature of the crystal face with respect to the fluid atoms, this interaction being approximated by its two-dimensional Boltzmann average potential. Account is taken of the thermal motion of the crystalline molecules at the interface. The theoretical density distribution is in good agreement with that obtained from a molecular dynamics experiment, for both (100) and (111) faces.
A ‘‘RISM’’ theory for multicomponent molecular fluids
Authors: Lombardero M. , Enciso E.
Published: February 2003

The Chandler–Andersen RISM theory is extended to multicomponent molecular fluids made up of molecules consisting of fused hard spheres. Following these authors the diagram technique, as formulated by Morita and Hiroike, is used to obtain the RISM equation for mixtures. This equation is solved for a binary mixture formed by hard spheres and homonuclear hard-sphere diatomics; the correlation functions obtained are used to investigate structural properties of Ar/N2 mixtures. The behavior of the correlation functions with density, molecular geometry, and concentration in an arbitrary atomic/diatomic mixture are also examined.
On the onset of chaotic motion in deterministic systems
Authors: Ramaswamy Ramakrishna , Marcus R. A.
Published: February 2003

In the present paper the classical counterpart of the quantum avoided crossing method for detecting chaos is described using classical (Lie-transform) perturbation theory and a grid of action variables. The results are applied to two systems of coupled oscillators with cubic and quartic nonlinearities. The plots of energy of members of the grid versus the perturbation parameter provide a visual description for predicting the onset of chaos.
Solvation forces in simple dense fluids. II. Effect of chemical potential
Authors: van Megen W. J. , Snook I. K.
Published: February 2003

The grand canonical ensemble Monte Carlo method is used to calculate the distribution of liquid molecules and the solvation force between two parallel solid surfaces separated by a simple liquid. Neither this distribution nor the solvation force are very sensitive to changes in the chemical potential (i.e., bulk density) of the interstitial liquid.
Computer simulation of a grand canonical ensemble of rodlike molecules
Authors: Yoon Kyonggeun , Chae Dong Ghie , Ree Taikyue , Ree Francis H.
Published: February 2003

A Monte Carlo method for simulating a grand canonical ensemble of semi-infinite lattice–gas systems is proposed. This method evaluates the largest eigenvalue of the Kramers–Wannier matrix by means of Markov chains. The method is applied to two-dimensional lattice–gas systems of dimers, trimers, and tetramers (having the length-to-width ratios of two, three, and four, respectively) as well as to molecules with equal side lengths. No first-order isotropic-to-aligned phase change is found for the rodlike systems. The tetramer system, however, indicates a probable higher-order phase change.
Time-dependent diffusion coefficients
Authors: Schurr J. M.
Published: February 2003

It has been noted in the literature that the intensity autocorrelation function of polarized light scattered from a flexible macromolecule should in principle exhibit an initial deviation from exponential decay, even when the fluctuation wavelength is large compared to the molecular dimensions. However, it is shown here that the amplitude of the initial more rapid component rigorously vanishes in the long-wavelength limit; consequently, this effect will be extraordinarily difficult to observe experimentally for noninteracting freely flexing macromolecules. As a particular example, it is shown that the initial diffusion coefficient D00, obtained from the first cumulant of the dynamic structure factor of the Rouse–Zimm model, actually differs somewhat from the limiting value D∞ prevailing at long times, but in the limit K2=0 it is the latter which characterizes the entire relaxing amplitude, as only a vanishing amplitude is associated with former more rapid component.
A theoretical model in organic cosmochemistry
Authors: Espinosa-Müller A. , Bravo N. A.
Published: February 2003

A generalized scheme for molecular force field calculations has been applied through a thermodynamic criterion, to the occurence of a number of equilibrium reactions. These reactions could take place in a reducing or nearly reducing dense media (e.g., in a Jovian planetary atmosphere). A procedure of calculation is given that represents a general methodology which can be used as an efficient tool in this kind of theoretical research. In this regard the equilibrium formation reaction of some simple precursor molecules as well as the synthesis of complex biochemical compounds are studied. Extensive input data together with a great deal of output thermodynamical information are reported.
Dissociative and double photoionization of CO from threshold to 90 Å
Authors: Masuoka Toshio , Samson James A. R.
Published: February 2003

Partial cross sections for molecular photoionization (CO+), dissociative photoionization (C+ and O+), and dissociative double photoionization (C2+) in CO have been measured from their thresholds to 90 Å using techniques of mass spectrometry. The results are compared with data reported previously. Several peaks observed in the cross section curves for dissociated fragments are tentatively assigned by comparing with those in the photoelectron spectra reported for CO. It is concluded that the shoulder in the total absorption cross section curve between 400 and 90 Å results solely from the dissociative ionization processes.
Theoretical study of hydrogen-bonded trimers. Three-body nonadditive interactions between ammonia molecules
Authors: Novaro O. , Cruz S. , Castillo S. , Kol/os W. , Leś A.
Published: February 2003

The ineraction energy of the system (NH3)3 was calculated for different geometrical arrangements and mutual orientations between the ammonia monomers. The calculations used the SCF LCAO MO method with a near-minimal set of Gaussian functions. The analysis of the three-body terms shows that although nonadditive contributions are comparatively small, they do play a role in the relative stability of the different structures for the trimer. The long-range induction nonadditive terms are evaluated, turning out to be substantially less dominant than for water trimers. The present results are related to existing experimental data on ammonia polymers and solid ammonia crystals.
A simple theoretical model for the van der Waals potential at intermediate distances. III. Anisotropic potentials of Ar–H2, Kr–H2, and Xe–H2
Authors: Tang K. T. , Toennies J. Peter
Published: February 2003

A simple semiclassical theory of the van der Waals potential, which uses no empirical constants, was proposed and tested on atom–atom systems in an earlier paper [J. Chem. Phys. 66, 1496 (1977)]. In a following paper [J. Chem. Phys. 68, 5501 (1978)] it was successfully applied to the prediction of the radial v0(R) and anisotropic parts v2(R) of the van der Waals potential expanded as V(R,γ)=v0(R)+v2)R)× P2(cos γ) (R is the distance between centers of mass and γ is the angle between R and the molecular axis) of He–H2 and Ne–H2 for which all the necessary ab initio input data were available. In the present paper the method is applied to the heavier systems Ar–H2, Kr–H2, and Xe–H2 for which the ab initio data are not available. The dispersion terms were estimated using the precise combining rule described in II. The anisotropic Born–Mayer parameters for the repulsive potentials were estimated by successive application of the Gilbert–Smith combining rules. The necessary input was the ab initio repulsive anisotropy in He–H2 and the repulsive parameters for Ar–Ar, Kr–Kr, and Xe–Xe, which could be estimated from the experimental potential well parameters. The resulting Born–Mayer parameters were adjusted slightly to agree with the experimental v0 potential parameters. The v2 potentials predicted in this way are presented and compared with the best available experimental potentials of Le Roy and co-workers and Zandee and Reuss. Cross section anisotropy factors have also been calculated for direct comparison with the measured orientation dependence of integral cross sections measured by Reuss and co-workers. For all systems the agreement with experiment is very good and within the experimental error. Finally, a new law of corresponding states which predicts that the reduced shapes of the spherical symmetric and anisotropy potentials are identical is proposed.
Quasiclassical trajectory studies of the H+H2 reaction on an accurate potential energy surface. II. Effect of initial vibration and rotation on reactivity
Authors: Barg G.-D. , Mayne H. R. , Toennies J. P.
Published: February 2003

Classical trajectory calculations have been carried out on the semiempirical Porter–Karplus and the accurate Siegbahn–Liu–Truhlar–Horowitz potential energy surfaces for the H+H2 (v, j) reaction. The results reveal that initial vibration in the diatom increases reactivity at a given translational energy, and broadens the final rotational and angular distributions. Initial rotation reduces reactivity near threshold, but the effect decreases far from theshold. Initial rotation broadens the final rotational but not the angular distribution. The results on both surfaces are similar, with total cross sections larger for the Porter–Karplus surface. The trends are in qualitative agreement with most available quantum mechanical calculations.
Analytical solution of a new integral equation for triplet correlations in hard sphere fluids
Authors: Parrinello M. , Giaquinta P. V.
Published: February 2003

Use is made of an equivalence between the calculation of triplet correlation functions and a molecular problem. The molecular problem is then tackled within the framework of the reference interaction site model. As a result, an integral equation for an angular average of the triplet correlation function is obtained. This equation is then analytically solved for a hard sphere fluid. A closed form expression for the result in the case in which two of the atoms in the triplet are in contact is given. The theory is compared with computer experiments and it is shown to be a significant improvement over the superposition approximation at high densities.
The structure and thermodynamics of a solid–fluid interface
Authors: Toxvaerd So/ren
Published: February 2003

Two sets of molecular dynamics calculations have been performed for systems of 952 Lennard-Jones particles placed up to surfaces with and without lattice structure. The calculations are performed for different pressures corresponding to dense fluid states at a temperature above the critical temperature. Only little lattice order is found in the interface and only in the contact layer direct up to the lattice surface. The excess thermodynamic functions, e.g., the surface tension, however, are found to be affected by the particle and lattice structure of the surface.
Interaction of atoms with solid surfaces: Energy transfer in hyperthermal collisions of Li+ with W(110)
Authors: Micha David A.
Published: February 2003

A recently developed many-body approach to atom–polyatomic collisions at hyperthermal energies is applied to scattering of light ions by metal surfaces. Following a brief description of the scattering model, which can in principle describe surface diffraction, surface rainbows, and phonon excitation, we concentrate on energy transfer processes. Cross sections are related to atom–pair correlation functions of the target surface, which are expressed in terms of the normal vibrational modes of a solid slab. The correlation functions are calculated within a short-time expansion which gives Gaussian distributions for energy transfer probabilities. A simple surface model of Einstein anisotropic oscillators is worked out in detail. Results of calculations for scattering of Li+ by W(110) at kinetic energies of several eV show good agreement with experimental values of most probable final energies and of distribution widths for several angles and initial ion energies.
Surface widths of simple liquids and an empirical law of freezing
Authors: Mon K. K. , Stroud D.
Published: February 2003

On the basis of a simple formula relating the surface width l to the liquid compressibility and surface tension, we conjecture that l is a universal multiple of the interlayer spacing at the triple point. The hypothesis is shown to be in accord with available data for 25 simple liquids, with an rms deviation of about 25%.
A quantum mechanical study of the collinear Li+FH reaction
Authors: Walker Robert B. , Zeiri Yehuda , Shapiro Moshe
Published: February 2003

A quantum mechanical collinear model of the LiFH reaction is studied using the semiempirical potential energy surface of Zeiri and Shapiro. Accurate reaction probabilities are presented over the total scattering energy range from 0.5 to 1.6 eV. At 1.6 eV, three HF reactant vibrational states are energetically accessible, and 15 LiF product vibrational states are accessible. The reaction probabilities show extensive resonance features over the energy range considered, and there is considerable evidence for the formation of a long-lived collision complex from the initial v=1 state of HF.
Dynamics of hydrogen atom and proton transfer reactions. Symmetric case
Authors: Babamov V. K. , Marcus R. A.
Published: February 2003

A method is devised for treating the transfer of a light particle such as a hydrogen atom or a proton between two heavy particles, for the case of a collinear symmetric reaction AH+A→A+HA. Polar coordinates (ρ,ϑ) with origin at the origin of the usual mass-weighted skewed-angle potential energy contour plot are employed to simplify the calculation. Symmetric and antisymmetric vibrationally adiabatic states are obtained at each ρ. They provide an effective potential for the ρ motion and are used to obtain the elastic adiabatic phase shifts for the ρ motion and from them the transfer probability. The ρ motion corresponds approximately to that of the heavy particles and the ϑ to that of the light one. Semiclassical limiting expressions are obtained and provide a description of light particle tunneling at low energies for heavy–light–heavy particle systems. Numerical results are given and the present approximation is good for the systems investigated.