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Volume 103, Issue 3, 15 July 1995

Discrete dipole approximation for calculating extinction and Raman intensities for small particles with arbitrary shapes
View Description Hide DescriptionWe present a discrete dipole approximation (DDA) method to determine extinction and Raman intensities for small metal particles of arbitrary shape. The Raman intensity calculation involves evaluation of surface electromagnetic fields, and thus is relevant to surface enhanced Raman scattering(SERS) intensities. We demonstrate convergence of the method by considering light absorption and scattering from an isolated spheroid, from an isolated tetrahedron, from two coupled spheroids, and from a spheroid on a flat surface. We also examine comparisons with traditional T‐matrix methods. Extensions and simplifications of the method in studies of clusters and arrays of particles are presented.

The complex H_{3}N⋅⋅⋅Br_{2} characterized in the gas phase by rotational spectroscopy
View Description Hide DescriptionThe ground‐state rotational spectra of four isotopomers of the complex H_{3}N...Br_{2} have been observed using pulsed‐nozzle, FT microwave spectroscopy. Reaction of NH_{3} and Br_{2} was avoided by using a fast‐mixing nozzle. The isotopomers H_{3} ^{15}N...^{79}Br_{2}, H_{3} ^{15}N...^{81}Br^{79}Br, H_{3} ^{15}N...^{79}Br^{81}Br, and H_{3} ^{15}N...^{81}Br_{2} exhibited symmetric‐top type spectra which were analyzed to give rotational constantsB _{0}, the centrifugal distortional constants D _{ J } and D _{ JK }, the Br‐nuclear quadrupole coupling constants χ(Br_{x}) (x=i, inner or o, outer), and the Br spin‐rotational coupling constants M _{ bb } (Br_{ x }). The distances r(N...Br_{i})=2.72(2) Å and r(Br–Br)=2.335(10) Å in the complex were estimated from the rotational constants while the intermolecular stretching force constant k _{σ}=18.5(5) N m^{−1} was determined from D _{ J }. An interpretation of the magnitude of the χ(Br_{ x }) values and the difference Δχ(Br)=χ(Br_{i})−χ(Br_{o}) allows the conclusion that the extent of intermolecular charge transfer is probably small and that a polarization of Br_{2} equivalent to the transfer of ∼0.1e from Br_{i} to Br_{o} accompanies formation of the complex. The properties of the series of complexes H_{3}N...XY, where XY=Cl_{2}, BrCl, and Br_{2} are compared.

Chemical reaction jet spectroscopy, molecular structure, and the bending potential of the Ã ^{1} A″ state of monofluorosilylene (HSiF)
View Description Hide DescriptionThe jet‐cooled laser induced fluorescence excitation spectrum of the Ã ^{1} A″–X̃ ^{1} A′ band system of HSiF has been observed with the chemical reaction jet technique. Vibrational analysis of the spectrum gave upper state fundamental vibrational frequencies of ν_{1}=1547 cm^{−1}, ν_{2}=558 cm^{−1}, and ν_{3}=857 cm^{−1}. Seven bands in the spectrum were recorded at high resolution and rotationally analyzed, providing excited state molecular constants. The upper state vibrational and rotational bending levels were fitted to a semirigid bender model to obtain the equilibrium geometry and the potential energy barrier to linearity. Due to correlations in the parameters, it was necessary to fix the bond angle at the ab initio value of 114.5°. The resulting fitted model yielded r _{ e }(Si–F)=1.602 Å, r _{ e }(Si–H)=1.548 Å with a potential energy barrier to linearity of 9130 cm^{−1}.

Depolarized Rayleigh scattering from simple molecular liquids: Investigation of transitions in the melt
View Description Hide DescriptionDepolarized Rayleigh scatteringphotometry has been employed to study the temperature dependence of the optical anisotropy for benzene, hexafluorobenzene, and carbon tetrachloride. The depolarized Rayleigh intensity from carbon tetrachloride is entirely due to scattering by pairs of density fluctuations and increases monotonically with temperature. The total depolarized Rayleigh ratio R _{ VH } for benzene and hexafluorobenzene decreases smoothly and monotonically with temperature, in contradiction to a recent report [J. Chem. Phys. 95, 1223 (1991)]. The temperature dependence of the total depolarized Rayleigh ratio can be analyzed in terms of an increasing contribution due to scattering by pairs of density fluctuations and a decreasing contribution due to orientational fluctuations of optically anisotropic molecules. No anomalous behavior is observed anywhere in the liquid range for these three fluids.

Infrared spectroscopy of the CH stretching modes of partially F‐substituted benzenes. II. The fundamental spectrum of fluorobenzene
View Description Hide DescriptionThis work is part of a series aimed to investigate the fundamental and overtone transitions of the CH stretching modes in partially F‐substituted benzenes. In this article we report the study of the fundamental transitions of fluorobenzene (C_{6}H_{5}F) in the wave numbers range from about 3000 to 3150 cm^{−1}. The absorptionspectrum has been measured through both a low‐temperature opto‐acoustic cell and a supersonic‐beam opto‐thermal spectrometer. The opto‐thermal spectrometer enabled us to measure the detailed vibrational structure, providing the basis for an algebraic analysis of the fundamental CH transitions. We show, in particular, how benzene algebraic procedures must be modified for taking into account the effects induced by the substitution of one hydrogen atom with fluorine, i.e., (i) the reduction of symmetry and (ii) the frequency (chemical) shift due to the change of electron distribution. Moreover we include the contribution of anharmonic resonances between unperturbed CH stretching modes and overtone or combination bands of lower energy molecular vibrations.

The interpretation of the c ^{1}Π←a ^{1}Δ excitation spectra of the ArNH complex
View Description Hide DescriptionWe use slightly modified versions of earlier ab initiopotential energy surfaces for Ar+NH(a ^{1}Δ) and Ar+NH(c ^{1}Π) to investigate the bend–stretch levels of ArNH complexes, both within an adiabatic bender model and in full variational calculations. The positions of these levels are used to reinterpret the fluorescence excitation spectra reported previously [R. W. Randall, C.‐C. Chuang, and M. I. Lester, Chem. Phys. Lett. 200, 113 (1992)]. The first 15 observed bands can be unambiguously assigned, with near‐quantitative accuracy. The weak anisotropy in the lower electronic state [ArNH(a ^{1}Δ)] complicates the rotational structure, since each band involves overlapped transitions out of several, nearly degenerate bend–stretch levels of the complex.

Vibration–rotation–tunneling dynamics calculations for the four‐dimensional (HCl)_{2} system: A test of approximate models
View Description Hide DescriptionSeveral commonly used approximate methods for the calculation of vibration–rotation–tunneling spectra for (HCl)_{2} are described. These range from one‐dimensional models to an exact coupled four‐dimensional treatment of the intermolecular dynamics. Two different potential surfaces were employed—an ab initio and our ES1 experimental surface (determined by imbedding the four‐dimensional calculation outlined here in a least‐squares loop to fit the experimental data, which is described in the accompanying paper [J. Chem. Phys. 103, 933 (1995)]. The most important conclusion deduced from this work is that the validity of the various approximate models is extremely system specific. All of the approximate methods addressed in this paper were found to be sensitive to the approximate separability of the radial and angular degrees of freedom, wherein exists the primary difference between the two potentials. Of particular importance, the commonly used reversed adiabatic angular approximation was found to be very sensitive to the choice for fixed R; an improper choice would lead to results very much different from the fully coupled results and perhaps to false conclusions concerning the intermolecular potential energy surface.

Determination of the intermolecular potential energy surface for (HCl)_{2} from vibration–rotation–tunneling spectra
View Description Hide DescriptionAn accurate and detailed semiempiricalintermolecular potential energy surface for (HCl)_{2} has been determined by a direct nonlinear least‐squares fit to 33 microwave, far‐infrared and near‐infrared spectroscopic quantities using the analytical potential model of Bunker et al. [J. Mol. Spectrosc. 146, 200 (1991)] and a rigorous four‐dimensional dynamical method (described in the accompanying paper). The global minimum (D _{ e }=−692 cm^{−1}) is located near the hydrogen‐bonded L‐shaped geometry (R=3.746 Å, θ_{1}=9°, θ_{2}=89.8°, and φ=180°). The marked influence of anisotropic repulsive forces is evidenced in the radial dependence of the donor–acceptor interchange tunneling pathway. The minimum energy pathway in this low barrier (48 cm^{−1}) process involves a contraction of 0.1 Å in the center of mass distance (R) at the C _{2h } symmetry barrier position. The new surface is much more accurate than either the ab initio formulation of Bunker et al. or a previous semiempiricalsurface [J. Chem. Phys. 78, 6841 (1983)].

Ab initio potential energy surface for the HCl dimer
View Description Hide DescriptionAn intermolecular potential energy surface for the dimer of hydrogen chloride in the ground state is calculated at the levels of the second‐order (MP2) and fourth‐order (MP4) Mo/ller–Plesset approximations using a large basis set containing bond functions. The surface is characterized by the minimum energy pathway through two equivalent hydrogen‐bonded structures. The hydrogen‐bonded equilibrium geometry has the centers of mass distance R _{ m }=3.78 Å and polar angles θ_{1}=8.0° and θ_{2}=90.0° (at MP2 level). The well depth at the hydrogen‐bonded minimum is V _{ m }=−710.9 cm^{−1} at MP2 and V _{ m }=−643.9 cm^{−1} at MP4 level. The interchange barrier between the two equivalent minima occurs at R=3.68 Å, θ_{1}=θ_{2}=46.0°, with the barrier height of 58.6 cm^{−1} at MP2 and 45.9 cm^{−1} at MP4 level (with the MP2 geometries). These results are in good agreement with a new empirical potential of Elrod and Saykally. Our calculations show that the bonding in the HCl dimer is dominated by the dispersion forces, which is different from the bonding in other classical hydrogen‐bonded systems such as the hydrogen fluoride dimer and the water dimer.

Stabilization of barium dimers on clusters: Reactions of Ba_{2} with Cl_{2} and O_{2} on large argon clusters
View Description Hide DescriptionBarium dimers have been produced and stabilized by the successive pickup of two barium atoms on large argon clusters (average size of 8000). By adding chlorine and oxygen molecules, chemiluminescent reactions of these dimers have been observed. This technique of dimer formation can be extended to higher order clusters to study reactions of cold metallic clusters.

Statistical distribution and stochastic resonance in a periodically driven chemical system
View Description Hide DescriptionThe probability density distribution is studied analytically and by Monte Carlo simulations for a periodically driven chemical bistable system, described by a master equation, for the case of low‐frequency driving. The quasistationary distribution about the stable states is well approximated by the solution of the master equation in the eikonal approximation for large volumes of the system. For a one‐component system both the exponent and the prefactor of the steady distribution are obtained in explicit form, for an arbitrary strength of the driving and for an arbitrary interrelation between the frequency of the driving and the probabilities of transitions between the stable states. The results of the simulations are in good agreement with analytical results. We demonstrate the onset of stochastic resonance for the driving frequency close to the probabilities of fluctuational transitions between the states.

Quantum theory of activated rate processes: A maximum free energy approach
View Description Hide DescriptionA quantum theory of activated rate processes applicable to nonlinear potentials of interaction is developed. The central premise is that the rate is determined by the point of maximal quantum free energy separating reactants and products. The quantum free energy is defined in terms of a quantum centroid potential. The resulting rate expressions reduce to known limits for generalized Langevin equations and their Hamiltonian representation. They also reduce in the classical limit to previous results derived using an optimal planar dividing surface classical variational transition‐state theory. A saddle‐point estimate of the quantum rate leads to a generalization of Wolynes’ high temperature rate expression valid for nonlinear system bath interactions and anharmonic baths. Maximizing the free energy leads to a quantum friction function. Application to realistic systems demands the computation only of centroid densities.

Detachment and charge transfer for collisions of negative ions with ozone
View Description Hide DescriptionCross sections for electron detachment and charge transfer have been measured for collisions of O^{−}, S^{−}, and halogen anions with neutral ozone for laboratory collision energies ranging from 3 to 500 eV. Both electron detachment and charge transfer are found to be large over the entire range of collision energies for O^{−} and S^{−}+O_{3} whereas the cross sections for the halogen anions colliding with O_{3} exhibit threshold behaviors characteristic of endothermic scattering processes.

Transition state dynamics of chemical reactions in clusters: A six‐dimensional study of Ar(ClHCl)
View Description Hide DescriptionCluster effects on transition state dynamics are investigated through a time‐dependent calculation of photodetachmentspectrum of Ar(ClHCl)^{−}. This system is studied by propagating a three‐dimensional grid representation of the wave function in the H atom coordinates, coupled to three one‐dimensional semiclassical Gaussians in the three lowest frequency normal coordinates of Ar(ClHCl)^{−}. Over the 75 fs propagation times considered here, the motions of the three heavy atoms are very small. Therefore, we expect that the coupled quantum/semiclassical propagation will be in quantitative agreement with a full quantum treatment of this system. The couplings between the quantum and semiclassical degrees of freedom and between the three semiclassical degrees of freedom are introduced through the time‐dependent self‐consistent field (TDSCF) approximation. A computational bottleneck in applying the TDSCF approximation to such systems is that propagation of each of the modes requires the evaluation of the average of the potential over the wave functions in the three remaining modes. A solution to this problem for H transfer systems is presented and discussed. Comparisons between the dynamics of Ar(ClHCl) and ClHCl indicate that the presence of the Ar atom produces a hole in the wave function that is otherwise cylindrically symmetric. Possible observable experimental consequences of the introduction of the Ar atom to this system are discussed.

Quartet excited halogen atoms produced in the electron pulse irradiation of rare gases containing halogenated compounds
View Description Hide DescriptionQuartet excited halogen atoms F*(2p ^{4} 3s,^{4} P _{ J }), Cl*(3p ^{4} 4s,^{4} P _{ J }), Br*(4p ^{4} 5s,^{4} P _{ J }), and I*(5p ^{4} 6s,^{4} P _{ J }), where the J’s are 5/2, 3/2, and 1/2, were found to be produced in the electron pulse irradiation of Ne or Ar containing one of SF_{6}, CCl_{4}, CClF_{3}, CBrF_{3}, CBr_{2}F_{2}, and CF_{3}I. The population distribution ratios at the stage of production were 1.0(J=5/2):0.41(J=3/2):0.06(J=1/2) for F* in Ne containing SF_{6}, 1.0(J=5/2):0.27(J=3/2):0.14(J=1/2) for Cl* in Ne containing CCl_{4}, 1.0(J=5/2):0.29(J=3/2):0.2–0.3(J=1/2) for Br* in Ne containing CBr_{2}F_{2}, and 1.0(J=5/2):0.13(J=3/2):0.54(J=1/2) for I* in Ar containing CF_{3}I. The observed ratios considerably differ from those calculated from the Boltzmann distribution law. F*(^{4} P _{5/2}), F*(^{4} P _{3/2}), and Cl*(^{4} P _{5/2}) are mainly produced by the reactions of lowest triplet excited diatomic molecules of neon with SF_{6} and CCl_{4}. Cl*(^{4} P _{3/2}) and Cl*(^{4} P _{1/2}) are produced in a rapid process and deactivated into lower Cl*(^{4} P _{5/2}). Several reaction channels probably contribute to the formation of Br*(^{4} P _{ J }) and I*(^{4} P _{ J }). Rate constants for reactions of triplet excited diatomic molecules of neon or argon with these parent molecules were determined from observed absorption decay curves for Ne_{2} ^{*} or Ar_{2} ^{*} in the presence of parent molecules.

A charge exchange model of sequential fragmentation applied to dissociative double ionization of hydrazoic acid and halogen azides
View Description Hide DescriptionA model of charge separation reactions including charge exchange between fragments is developed to include sequential reactions. The model uses known molecular structures, assumes an exponential distribution of intermediate lifetimes, and requires a mean lifetime as the only disposable parameter. The general predictions and quantitative calculations of relative intensities, peak widths, and peak shapes are shown to agree with photoelectron–photoion–photoion coincidence spectra of HN_{3}, ClN_{3}, and IN_{3} and other small molecules studied previously.

Observation of an indirect pathway in the femtosecond study of alkyl nitrite photodissociation in the S _{1} state
View Description Hide DescriptionThe present work has addressed the question of the dissociation time of four alkyl nitrites upon photoexcitation in the S _{1} state (methyl nitrite, n‐ , and t‐butyl nitrites and i‐amyl nitrite). The time resolvedLaser Induced Fluorescence technique has been used in the femtosecond regime under bulk conditions. The photodissociation has been initiated at 351 nm by 150 fs pump pulses, and has been probed using a two‐photon process at 467 nm by 200 fs pulses. The LIF signal has qualitatively the same shape for the four nitrites: it passes through a transient peak before reaching a plateau. The two‐photon process that induces the detected fluorescence is nonresonant for detection of the dissociation product NO through the A←X transition. Conversely, the two‐photon process is resonant or quasiresonant for detection of the excited nitrite molecule in the S _{1} level before it dissociates. This leads to an enhanced detection efficiency of the non‐dissociated excited molecule versus that of the NO fragment. A simple kinetic model has been developed to account for this detection scheme. It shows that the transient peak is observable only if the lifetime of the excited molecule is comparable or larger than the temporal width of the lasers. In that case, the model allows the determination of an effective lifetime of the excited molecule from the measured LIF signal. Lifetimes have been found in the range of 125 fs for the four nitrites investigated. This has allowed us to answer a controversy on the dissociation mechanism of methyl nitrite and to confirm that an indirect pathway exists in the photodissociation of this molecule.

Comparison of the open‐shell state‐universal and state‐selective coupled‐cluster theories: H4 and H8 models
View Description Hide DescriptionThe spin nonadapted or partially spin adapted spin orbital multireference (MR) state‐universal (SU) or Hilbert spacecoupled cluster (CC) method is analyzed from the viewpoint of possible spin symmetry breaking or spin contamination and contrasted with the recently introduced state specific (SS), automatically spin adapted, spin‐free unitary group based CC method. It is shown that the partial spin‐adaptation of the two‐determinantal MR SUCC approach can only be effective if the complementary singlet and triplet states have a similar structure in the sense that their weights at corresponding singlet and triplet configurations, having the same orbital occupancies, are identical or nearly identical. The performance of both SS and SU CC methods, and of corresponding CI method (all truncated at the double excitation level) for the lowest lying open‐shell singlet and triplet states of the H4 and H8 model systems was examined, showing the superiority of the unitary group based CC approach in all studied aspects.

Comment on symmetry of the interaction between an asymmetric rigid rotor and a linear rigid rotor
View Description Hide DescriptionIn fitting an ab initio potential for H_{2}O–H_{2}, Phillips et al. [J. Chem. Phys. 101, 5824 (1994)] excluded certain terms in the angular expansion they believed to vanish because of ‘‘the requirement that the potential is invariant to inversion of all coordinates through the origin.’’ However, there has been some question in the literature as to whether these terms must, in fact, vanish owing to spatial inversion symmetry. By providing counterexamples, it is demonstrated here that this is not required by fundamental spatial symmetry. However, these terms do appear to vanish for realistic molecular interactions and this symmetry may arise from the two‐body nature of the electrostatic Hamiltonian.

A theoretical study of bridged vs atop interactions of Pt_{2} with CO
View Description Hide DescriptionPotential energy curves for the low‐lying electronic states of the Pt_{2}CO complex are studied using the Mo/ller–Plesset second‐order perturbation theory (MP2) and the complete active space multiconfiguration self‐consistent field method (CASSCF). Multireference singles and doubles configuration interaction (MRSDCI) computations that included up to two million configurations were also made. The results for Pt_{2}CO are compared with experimental results for chemisorption of CO on a Pt surface. The atop and bridged bondings of CO on the Pt‐surface are modeled using potential energy curves for the ground state linear and bridged Pt_{2}CO structures. It is shown that the atop interaction proceeds without a barrier while the bridge interaction has to surmount a barrier, even though the bridge bonding leads to a more stable equilibrium complex. The calculated vibrational frequencies at the MP2 level for Pt_{2}CO and Pt_{3}CO are compared with the experimentally determined values for different chemisorptive sites. The differences between the atop and bridged chemical bonds are discussed using the Mulliken population analysis. The spin–orbit effect is studied utilizing a relativistic configuration interaction (RCI) approach.