bluuebunny/crossref_metadata_2025 huggingface dataset:
Collective coordinates for nuclear spectral densities in energy transfer and femtosecond spectroscopy of molecular aggregates
Authors: Chernyak Vladimir , Mukamel Shaul
Published: July 2002

A theory for Frenkel exciton dynamics in molecular aggregates which incorporates coupling to vibrational motions (intramolecular, intermolecular and solvent) with multiple spectral densities of arbitrary nature and interpolates between the coherent and the incoherent limits is developed. A rigorous procedure for identifying the relevant collective nuclear coordinates necessary to represent a given set of spectral densities is obtained. Additional coordinates are required as the temperature is lowered. Exciton dynamics is calculated by following the evolution of wavepackets representing the electronic density matrix in the collective coordinates phase space. The signatures of excitonic and nuclear motions in ultrafast fluorescence spectroscopy are explored using a hierarchy of reduction schemes with varying numbers of collective coordinates.
Extensive ab initio study of the OH+HCN reaction: Low lying electronic states of the stationary points on the 2A′ surface
Authors: Palma A. , Semprini E. , Stefani F. , Talamo A.
Published: July 2002

We found many stationary points (minima and transition states) for the title reaction on the 2A′ surface at unrestricted Hartree–Fock self-consistent field (UHF-SCF) level with two different basis sets. Stable adducts, as suggested by previous experimental works, have been ascertained and several reaction paths are obtained through intrinsic reaction coordinate (IRC) calculations. A link to the HNC+OH reaction is possible. Multiconfiguration SCF (MC-SCF) calculations have been carried out for the addition reaction with the lowest energy barrier in order to eliminate the spin contamination error on these geometries. Correlation energy at the stationary points was estimated via a perturbative scheme, Mo/ller–Plesset at fourth order (MP4) which does not seem adequate for such a system, and via multireference double configuration interaction (MR-DCI) with extrapolation to full CI values for ground and first excited states. Electronic excitations may open some reaction channels.
On the scattering phenomena for various kinds of polarized light in a nonpolar fluid composed of chiral molecules
Authors: Lee Dong J. , Kim Kyoung-Ran
Published: July 2002

The explicit result of the coefficients of the Mueller matrices, which express the Stokes parameters of single and double scattered lights in term of the parameters of the corresponding incident light, are obtained for a nonpolar fluid composed of chiral molecules with the aid of the Ornstein–Zernike form for the correlation function of density fluctuations. The explicit results show that any higher order multiple scattering satisfies the principle of symmetry and law of reciprocity. From the matrices some scattering phenomena for six kinds of completely polarized light in a nonpolar achiral fluid are discussed in the cases of far from and near the critical point. While the degrees of circularity (DCs) give identical information with the circular intensity differences (CIDs) in the region far from the critical point, the critical DCs for circularly polarized lights is quite different from the critical CIDs. The theoretical result of DCs for the circularly polarized lights indicates that the DCs can be other effective experimental tools to measure the critical scattering of a fluid.
Accurate relativistic energies of one- and two-electron systems using Gaussian wave functions
Authors: Cencek Wojciech , Kutzelnigg Werner
Published: July 2002

Gaussian wave functions with optimized nonlinear variational parameters are applied to evaluate clamped-nucleus nonrelativistic energies as well as the lowest order relativistic corrections for the ground states of the hydrogen atom, hydrogen molecular ion H+2, helium atom, and hydrogen molecule. The two-electron functions used depend explicitly on the interelectronic distance r12 but do not describe the cusp properly. Despite this, for H+2 and H2 the results are more accurate than ever reported, and for He they are inferior only to the best calculations employing Hylleraas-type expansions. It is demonstrated that, contrary to a common opinion, Gaussian wave functions are very well suited for high-accuracy relativistic computations even in the Breit–Pauli approximation, provided that the nonlinear parameters are optimized with respect to the nonrelativistic energy.
Equilibrium and kinetic phenomena in a stiff homopolymer and possible applications to DNA
Authors: Kuznetsov Yu. A. , Timoshenko E. G. , Dawson K. A.
Published: July 2002

We study the bead-and-spring model of a stiff chain using a self-consistent mean-field approach. For high stiffness parameter the system may undergo a transition to the phase in which the globule acquires a toruslike shape. The phase diagram of the model contains one second- and two first-order transitions meeting at a bicritical point. The stability of the toroidal conformation and scalings of the torus geometry are analyzed. We investigate different kinetic regimes after an instantaneous quench between the extended coil, torus and the spherical globule phases. The kinetic laws that govern these conformational changes are obtained.
Charge transfer within He clusters
Authors: Callicoatt Berton E. , Mar David D. , Apkarian V. A. , Janda Kenneth C.
Published: July 2002

Mass spectrometric measurements of clusters containing N helium atoms and an NO molecule yield probabilities for charge transfer from He+ to NO that range from 0.8±0.1 for 〈N〉=540 to 0.02±0.02 for 〈N〉=15 000. From this we estimate that the He+ hole hops approximately 70 times before localization at a He+2 core.
Morphology-dependent stimulated Raman scattering imaging. I. Theoretical aspects
Authors: Aker Pamela M. , Moortgat Philip A. , Zhang Jian-Xiang
Published: July 2002

Morphology-dependent stimulated Raman scattering is a nonlinear laser Raman spectroscopy that relies on using the cavity modes associated with micron-sized structures to enhance optical signal generation. Since different cavity modes occupy different regions in space, location-specific spectra can be generated. Here we outline the physics behind morphology-dependent stimulated Raman scattering imaging and model the signal generation process. The results show that a simple tailoring of the illumination conditions allows signal from single, but selectable, cavity modes to be generated. Thus, this nonlinear Raman technique can be used to image spatial variations in chemical composition and/or molecular structure within microstructures.
Two-pulse laser control of bond-selective fragmentation
Authors: Amstrup Bjarne , Henriksen Niels E.
Published: July 2002

We elaborate on a two-pulse (pump-pump) laser control scheme for selective bond-breaking in molecules [Amstrup and Henriksen, J. Chem. Phys. 97, 8285 (1992)]. We show, in particular, that with this scheme one can overcome the obstacle of intramolecular vibrational relaxation. As an example, we consider an ozone molecule with isotopic substitution, that is, 16O16O18O. It is shown that asymmetric bond stretching can be created in simple (intense) laser fields. We predict that an alternating high selectivity between the channels 16O+16O18O and 16O16O+18O can be obtained when such a non-stationary vibrating ozone molecule is photodissociated with short laser pulses (∼10–15 fs) with a time delay corresponding to half a vibrational period (∼17 fs).
Ab initio study of NO2. V. Nonadiabatic vibronic states and levels of the X̃ 2A1/Ã 2B2 conical intersection
Authors: Leonardi Erminia , Petrongolo Carlo , Hirsch Gerhard , Buenker Robert J.
Published: July 2002

We have computed 1500 nonadiabatic levels of the X̃ 2A1/Ã 2B2 conical intersection of NO2, up to 18 700 cm−1. By using a bond lengths–bond angle Hamiltonian, the molecular states have been expanded in a diabatic electronic basis and in primitive, optimized, and Born–Oppenheimer vibrational basis functions. We have optimized the diabatic potentials with respect to 191 observed bands up to 10 000 cm−1, with a root mean square deviation (RMSD) of 17.8 cm−1, and 691 nonadiabatic bands up to 15 000 cm−1 and 1060 up to 17 000 cm−1 have been converged within 1.9 and 4.4 cm−1, respectively, by using 6117 basis functions per symmetry, and several states have been assigned. Up to 9500 cm−1 we have essentially found 2A1 vibrational states, some of them mixed by the Delon–Jost resonances. The nonadiabatic coupling then begins near the 2B2 (0,0,0) origin, which we assign to an electronically mixed band at 9747 cm−1, and gradually increases via the interaction between bending states of 2A1 and 2B2. The vibronic mixing is more important above 12 000 cm−1, where both electronic species contribute to several nonadiabatic states, but the 2B2 bending progressions can be followed up to about 16 000 cm−1, since they give rise to clumps of strongly mixed vibronic bands. Above 16 000 cm−1 finally, the nonadiabatic interactions are very strong, masking all the vibrational progressions of both electronic states, and giving a fully chaotic spectrum which follows a Wigner-type distribution. Our results thus explain the beginning and the development of the 2A1/2B2 nonadiabatic interaction, from the regular far-infrared region up to the chaotic yellow portion of the spectrum. They are in good agreement with the available experimental data, allowing the assignment of several observed bands up to 16 000 cm−1, and increase remarkably the number of known NO2 vibronic levels.
Probing three-body intermolecular forces: Near-infrared spectroscopy of Ar2HF and Ar2DF van der Waals modes
Authors: Farrell John T. , Nesbitt David J.
Published: July 2002

Four intermolecular vibrational states of the weakly bound complexes Ar2HF and Ar2DF have been studied via high-resolution infrared spectroscopy. The vibrations are accessed as combination bands built on the v=1 HF or DF intramolecular stretch. These van der Waals vibrational states correlate adiabatically with j=1 motion of a hindered HF/DF rotor, corresponding to librational motion either in, or out of, the molecular plane. The vibrational origins of the Ar2HF in-plane and out-of-plane bends are 4008.9665(24) and 4035.174 41(86) cm−1, respectively, which are 62.374 and 88.582 cm−1 above the origin of the intermolecular ground state in the vHF=1 manifold. For Ar2DF, the in-plane and out-of-plane origins are 2939.836 63(4) and 2967.101 29(5) cm−1, respectively, which correspond to intermolecular bending frequencies in the vDF=1 manifold of 44.852 and 72.117 cm−1. Two-dimensional angular calculations are presented which solve for the hindered rotor HF/DF eigenfunctions and eigenvalues on a pairwise additive potential constructed using a rigid Ar2 framework; the predicted bending frequencies reproduce the correct energy ordering of the vibrational levels, but are systematically greater than experimentally observed. Rigorous full five-dimensional theoretical calculations of the intermolecular vibrational frequencies by Ernesti and Hutson [Phys. Rev. A 51 239 (1995)] on the full pairwise additive surface are found to be as much as 11% higher than the experimental values, indicating the presence of three-body repulsive contributions to the true angular potential. Inclusion of conventional three-body dispersion and induction terms can only account for a minority (≊1/3) of the observed discrepancies. The majority (≊2/3) of the vibrational shifts can be attributed to three-body ‘‘exchange’’ effects, i.e., the strongly anisotropic interaction of the HF/DF dipole with an exchange quadrupole formed by Ar–Ar. Inclusion of all three nonadditive terms (dispersion, induction, and exchange) improves the agreement with experiment by up to an order of magnitude.
Spectroscopic characterization of the excited Mg(3s3d 3D
J)⋅Ar(3Π), Mg(3s3d 3D
J)⋅Ar(3Δ), and Mg(3s4p 3P
J)⋅Ar(3Π) van der Waals states
Authors: Massick Steven , Breckenridge W. H.
Published: July 2002

The Mg(3s3dπ 3DJ)⋅Ar(3Π), Mg(3s3dδ 3DJ)⋅Ar(3Δ), and Mg(3s4pπ 3PJ)⋅Ar(3Π) excited states have been characterized via resonance enhanced two-photon ionization (R2PI) spectroscopy of transitions from the long-lived Mg(3s3pπ 3PJ)⋅Ar(3Π0+,0−) metastable states of the MgAr van der Waals molecule. Because the outer excited Mg(3d) and Mg(4p) orbitals are (similarly) quite diffuse and the Ar atom can approach along the nodal axes of each of the aligned orbitals, minimizing repulsion, the Mg(3s3dδ 3DJ)⋅Ar(3Δ) and Mg(3s4pπ 3PJ)⋅Ar(3Π) states are both very strongly bound (D0=1140±40, 1250±60 cm−1, respectively), approaching the bond energy of the MgAr+ ‘‘core’’ ion (for which D0=1240±40 cm−1). In contrast, the Mg(3s3dπ 3DJ)⋅Ar state is more weakly bound (D0=290±40 cm−1), although it has a greater bond strength than the Mg(3s3pπ 3PJ)⋅Ar(3Π0−) lower state (for which D0 is 160±40 cm−1). The effective spin–orbit constant of the Mg(3s3dπ 3DJ)⋅Ar(3Π) state is much larger than expected from the Mg(3s3d 3DJ) multiplet splittings, and also increases in magnitude as v′ decreases; possible reasons for this are discussed. It is suggested that the predissociation of the Mg(3s3dπ 3DJ)⋅Ar(3Π0+) state [the Mg(3s3dπ 3DJ)⋅Ar(3Π0−) state does not predissociate] is caused by a curve-crossing with the repulsive Mg(3s3dσ 1D2)⋅Ar(1Σ0++) state, which lies at a lower asymptotic energy because of substantial mixing of Mg(3p3p 1D2) character into its wave function.
NO μ-v-j correlations in the photofragmentation of 2-chloro-2-nitrosopropane
Authors: Uberna Radoslaw , Hinchliffe Robert D. , Cline Joseph I.
Published: July 2002

The trajectory of NO X 2ΠΩ=1/2,3/2(v=0) produced after excitation of 2-chloro-2-nitrosopropane S1 Ã 1A″ is studied by polarized 1+1′ resonance-enhanced multiphoton ionization with time-of-flight mass spectrometry detection. The correlations among the NO velocity v, angular momentum j, and the S1 Ã 1A″←S0 X̃ 1A′ [n(N),π*(N=O)] transition dipole μ of the parent molecule are measured. The dissociation occurs by internal conversion to the ground S0 state or intersystem crossing to the T1 ã 3A″ state and the observed speed distribution of the NO fragments is bimodal. There is no evidence for μ-v-j correlations for the products associated with the slow component of the speed distribution. For the higher speed component, the v-j correlation is a function of the NO rotational state, j, and is described by the bipolar moment β00(22). The average value of β00(22) is −0.17(±0.02) at j≊11.5–22.5, more than five times larger than predicted by statistical phase space theory for dissociation on the S0 X̃ 1A′ surface, indicating a moderate preference for a perpendicular orientation of the NO rotational angular momentum and velocity vectors. The v and j vectors are nearly uncorrelated for low NO rotational states (j≤6.5). The μ-v correlation described by the β20(20) bipolar moment does not change with j and its average value is −0.04(±0.01) at j≊12.5–19.5. The average value of the β20(02) bipolar moment describing μ-j correlation is 0.04(±0.02) at j≊7.5–24.5. These results are consistent with a mechanism involving both impulsive force due to a barrier along the reaction coordinate on the T1 ã 3A″ surface and forces resulting from the C–N–O bending and NO torsion about the C–N bond.
Surface vibrations of argon clusters by helium atom scattering
Authors: Buck U. , Krohne R. , Lohbrandt P.
Published: July 2002

In a crossed molecular beam arrangement helium atoms are scattered from argon clusters which are generated in the size range of n̄=23 to n̄=4600 by adiabatic expansion with conical nozzles. The inelastic energy transfer is detected by time-of-flight analysis of the scattered helium atoms with a resolution of better than 1 meV. The angular dependence allows to distinguish between single and multiphonon excitations. The measured peaks of the averaged transferred energy in the single phonon regime decrease with increasing average cluster size and converges to the values of the surface phonons of solid argon. This behavior is explained by treating the data as collective breathing vibrations of a solid sphere.
A semi-empirical scheme for generalized valence bond calculations on water complexes
Authors: Wang Youliang , Gunn John R.
Published: July 2002

A method is introduced for carrying out semi-empirical energy calculations using a localized valence-bond wavefunction. Chemical bonds are described by correlated electron pairs, with interactions between pairs treated iteratively with a classical approximation at large distance. The model consists of a minimal basis of localized hybrid valence orbitals with semi-empirical approximation for the electron integrals and core repulsions. Parameters are determined for oxygen and hydrogen which are fit to the MP2/6-31G** potential surfaces of H2O and (H2O)2 with 0.5 kcal/mol accuracy. Results are shown for other neutral and charged water clusters, including a water chain of up to 200 H2O molecules for which the calculation shows O(N1.5) scaling with size.
Diffusion on a flexible surface
Authors: Gustafsson Stefan , Halle Bertil
Published: July 2002

The self-diffusion of a particle on a flexible surface, such as a fluid bilayer membrane or a macroscopic interface, is analyzed theoretically in order to relate the macroscopic diffusion coefficient DM, describing displacements in a laboratory-fixed plane, to the intrinsic surface diffusion coefficient DS and to the configurational statistics of the surface. An exact result for DM is obtained for a rapidly fluctuating surface. For a static surface, rigorous bounds on DM are established and an effective medium approximation is derived that should remain accurate for strongly disordered surfaces. With the aid of these results, classical self-diffusion measurements can be used to study the configuration, bending rigidity, and interactions of flexible membranes and interfaces.
On the Ginzburg temperature of ionic and dipolar fluids
Authors: Weiss V. C. , Schröer W.
Published: July 2002

Critical fluctuations in fluids are investigated within the framework of the generalized van der Waals theory. The square-gradient term—added to the Landau expansion of the Helmholtz free energy density—is obtained following a procedure similar to that originally proposed by van der Waals in the theory of surface tension, however replacing the Heaviside step function originally used by an approximative pair distribution function. Representative for ionic fluids we choose the restricted primitive model (RPM) and treat it within the Debye–Hückel theory, thus neglecting effects of ion pairing. The other approximative extreme—complete ion pairing resulting in a fluid of hard dipolar dumbbells—is mimicked by a fluid composed of dipolar hard spheres (DHS). For this case we use the Onsager reaction field and the second pressure virial coefficient. We calculate the amplitudes of the correlation length and the Ginzburg temperatures, and find (in reduced quantities) ξ0*=3.50 and ΔTGi*=0.0087 for the ionic system, and ξ0*=0.82 and ΔTGi*=1.63 for the dipolar fluid. For calibration we compute the same quantities for simple neutral fluids and obtain ξ0*=0.50 and ΔTGi*=2.89 for a Sutherland fluid (hard core term plus attractive r−6-potential) and ξ0*=0.43 and ΔTGi*=8.50 for a square-well fluid. The result of a smaller Ginzburg temperature for the ionic fluid than for nonionic fluids in a treatment that neglects ion pairing is clearly at variance with the results of other groups. The correlation length in the low-density limit obtained from our approach has the same functional dependencies as the Lee–Fisher expression, but differs by a numerical factor of 5.7.
Two-dimensional wave packet studies of photon-stimulated desorption of NO from a metal surface induced by single and multiple electronic excitations
Authors: Guo Hua
Published: July 2002

The photon-stimulated desorption dynamics of nitric oxide (NO) from the Pt(111) surface is investigated using time-dependent quantum mechanics. Empirical potential energy surfaces with two degrees of freedom (desorption and NO vibration) are employed for both the neutral and anionic states. The influence of the time delay between transitions is investigated for desorption with single and double excitation/deexcitation cycles. An improved Monte Carlo sampling scheme is proposed for processes undergoing multiple excitations. For desorption involving a single excitation, the calculated vibrational and kinetic energy distributions of the desorbed NO agree very well with experimental observations. When desorption is driven by multiple excitations, higher vibrational excitation is found for NO.
Discrepancies in turbidity measurements in the ionic binary mixture triethyl n-hexyl ammonium triethyl n-hexyl boride in diphenyl ether
Authors: Wiegand S. , Levelt Sengers J. M. H. , Zhang K. J. , Briggs M. E. , Gammon R. W.
Published: July 2002

We have repeated and extended turbidity measurements on a new sample of the ionic mixture triethyl n-hexyl ammonium triethyl n-hexyl boride (N2226B2226) in diphenyl ether in a temperature range above its consolute point. Previous measurements on this system by some of us indicated mean-field critical behavior over the range of reduced temperatures 10−4<t<10−1. Our new measurements do not reproduce those reported before. The transmission of the new sample is substantially lower, and the turbidities up to a factor of 3 higher than those of the sample used in the previous work. The correlation-length amplitude is approximately 40% larger than that found for the previous sample. Unlike the earlier results, the new data do not display a substantial range of mean-field critical behavior. In our investigation of this irreproducibility, we have carefully reviewed the earlier experiment, and we report here an effect not previously noted: a time dependence of the turbidity of the earlier sample. It is likely that the discrepancy is due to impurity effects, but we do not understand their origin.
Connecting quantum state resolved scattering data directly to chemical kinetics: Energy transfer distribution functions for the collisional relaxation of highly vibrationally excited molecules from state resolved probes of the bath
Authors: Michaels Chris A. , Flynn George W.
Published: July 2002

An energy transfer probability distribution function, P(E,E), for the collisional relaxation of a highly vibrationally excited donor molecule (C6F6, pyrazine) is constructed for the first time from experimental data on the bath (CO2) energy gain. A prescription for mapping bath quantum state resolved data onto P(E,E) is described in detail. Analysis of earlier experimental data allows a calculation of the high ΔE=E−E region (−7000 cm−1<E−E<−1500 cm−1) of P(E,E) for the above systems. Comparison of the P(E,E) functions reveals that C6F6 is a more efficient donor molecule than pyrazine, in agreement with previous experiments and trajectory calculations. In addition, resonance like structures in the P(E,E) functions arising from long range force mediated, V–V excitation of the carbon dioxide ν3 mode are discussed. These results indicate that accurate P(E,E) functions can be determined from experiments involving probes of the bath energy gain. This technique can be expected to provide stringent tests of current energy transfer theory and can, in principle, be used in conjunction with measurements of thermal kinetics to obtain energy dependent unimolecular rate constants, kE.
Molecular dynamics simulations of elementary chemical processes in liquid water using combined density functional and molecular mechanics potentials. I. Proton transfer in strongly H-bonded complexes
Authors: Tuñón I. , Martins-Costa M. T. C. , Millot C. , Ruiz-López M. F.
Published: July 2002

The first molecular dynamics (MD) simulation of a chemical process in solution with an ab initio description of the reactant species and a classical representation of the solvent is presented. We study the dynamics of proton (deuterium) transfer in strongly hydrogen-bonded systems characterized by an energy surface presenting a double well separated by a low activation barrier. We have chosen the hydroxyl-water complex in liquid water to analyze the coupling between the reactive system and the environment. The proton is transferred from one well to the other with a frequency close to 1 ps−1 which is comparable to the low-frequency band associated to hindered translations, diffusional translation and reorientation of water molecules in water. The proton transfer takes place in 20–30 fs whereas the solvent response is delayed by about 50 fs. Therefore, the reaction occurs in an essentially frozen-solvent configuration. In principle, this would produce a barrier increase with respect to the equilibrium reaction path. However, solvent fluctuations play a substantial role by catalyzing the proton transfer. The solvent relaxation time after proton transfer has been evaluated. Since it falls in the same time scale than the reactive events (0.6 ps) it substantially influences the proton dynamics. The present study is intended to model charge transfer processes in polar media having a low activation barrier for which many reactive events may be predicted in a MD simulation. The case of reactions with large activation barriers would require the use of special techniques to simulate rare events. But still in that case, hybrid QM/MM simulations represent a suitable tool to analyze reaction dynamics and non-equilibrium solvent effects in solution chemistry.
Density functional study of structure and bonding in lithium clusters Lin and their oxides LinO
Authors: Jones R. O. , Lichtenstein A. I. , Hutter J.
Published: July 2002

Density functional (DF) calculations have been performed for lithium clusters Lin and their monoxides LinO with up to ten atoms. There are numerous stable structures, and new isomers have been found in each family. The structural patterns of the homonuclear and oxide clusters are quite distinct. The combination of DF calculations with molecular dynamics (MD) sheds light on the observed pseudorotation of Li3 and Li5. We compare with available experimental data and discuss the bonding and structural patterns in the clusters and their oxides, which are often described as “hyperlithiated.”
Detection of metastable triplet acetylene produced by intersystem crossing from the excited Ã(1Au) state
Authors: Suzuki Toshinori , Shi Yang , Kohguchi Hiroshi
Published: July 2002

Triplet metastable species produced by intersystem crossing from the Ã(1Au) state of acetylene has been detected by the sensitized phosphorescence method. A sensitized phosphorescence signal was observed from vibronic levels lying lower than the potential energy barrier for dissociation in the ã state suggested previously, but was not observed from levels higher than this barrier. The lifetimes of triplet states produced by intersystem crossing from the V3K1(J′=2) and V4K1(J′=2) levels were estimated to be 100 and 80 μs.
Two-dimensional deuterium magic-angle-spinning nuclear magnetic resonance of paramagnetic compounds: Separation of paramagnetic and quadrupole interactions
Authors: Spaniol Thomas P. , Kubo Atsushi , Terao Takehiko
Published: July 2002

A new two-dimensional H2 magic-angle-spinning NMR experiment to study paramagnetic compounds is presented. It can be carried out by the two-pulse sequence (90°)–(t1)–(180°)–(t2, acquisition) or the three-pulse sequence (90°)–(t1)–(54.7°)–(τ)–(125.3°)–(t2, acquisition). The latter has the advantages over the former in that pure-absorption spectra can be obtained, and also finite pulse-width effects are less apparent. The intensities of the two-dimensionally spread sidebands are calculated as a function of the paramagnetic and the quadrupole coupling parameters, the time-evolution of the spin system during the rf pulse being involved. The method is applied to paramagnetic coordination compounds with selectively deuterated acetate groups: Mn2O(O2CCD3)2{(C3H3N2)2BH}2 and Pr(O2CCD3)3⋅H2O. The principal values of the two interaction tensors can be separately estimated from the projections of the two-dimensional spectrum onto the corresponding diagonals. These values are further refined by simulations of the two-dimensional spinning sideband pattern, which also allows us to determine the mutual orientation of the two tensors. The paramagnetic shift tensors are calculated on the basis of the crystal structures and are compared with those obtained by the experiments. The effect of the bulk magnetic susceptibility on the observed paramagnetic shift tensor is discussed. Some methods to remove this effect are suggested.
The photodissociation dynamics of OClO between 306 and 370 nm: Fragment translational energy release and recoil anisotropy
Authors: Furlan Alan , Scheld Heiner A. , Huber J. Robert
Published: July 2002

The photodissociation OClO(Ã 2A2)→ClO(X̃ 2Π)+O(3P) was studied at wavelengths between 306 and 370 nm using photofragment translational energy spectroscopy. The flight time distributions and anisotropies of the recoiling fragments were measured with the photolysis wavelength tuned to 10 maxima of the structured absorption spectrum, corresponding to a vibronic excitation of the parent molecule with 9–18 quanta in the symmetric stretching coordinate on the Ã 2A2 surface. The translational energy distributions show that the ClO fragments are created in highly inverted vibrational state distributions which become extremely broad [v(Cl–O)∼1–15] with increasing excitation energy. The large fraction of vibrationally hot ClO fragments produced–particularly at λ<325 nm–could enhance various thermodynamically unfavorable atmospheric reactions in connection with ozone depletion. The main mechanistic features of the dissociation process, which account for the almost constant average translational energy and linearly increasing vibrational energy of ClO as a function of the excitation energy, can be interpreted, to a first approximation, as vibrational predissociation on the Ã 2A2 potential energy surface involving a relatively late exit barrier. From the measured translational energies the barrier height is estimated to be about 48 kJ/mol.
Photofragment imaging of HNCO decomposition: Angular anisotropy and correlated distributions
Authors: Sanov A. , Droz-Georget Th. , Zyrianov M. , Reisler H.
Published: July 2002

Photodissociation of jet-cooled isocyanic acid has been examined by photofragment ion imaging of H(D) from H(D)NCO and CO from HNCO, and by laser induced fluorescence (LIF) of NH(a 1Δ) from HNCO. Only modest recoil anisotropy is observed in the H+NCO channel at 243.1 nm (β=−0.13±0.05), while the D+NCO channel at approximately the same wavelength reveals no anisotropy (β=0.00±0.05), confirming that the dissociation of H(D)NCO from the opening of the H(D) channel proceeds via vibrational predissociation on the S0(1A) surface. In contrast, substantial anisotropy (β=−0.66±0.08) is observed in the NH(a 1Δ)+CO channel at 230.1 nm, but this value can correspond to dissociation on either S0 or S1. The photolysis region between 243 and 230 nm thus appears important in providing clues to the dissociation mechanism and the competition between different potential energy surfaces. At 217.6 nm, product state distributions exhibit clear dynamical biases. CO is produced in both ν=0 and ν=1, while NH(a 1Δ) distributions correlated with different rovibrational levels of CO, although different in shape, are always cold, consistent with the global NH distribution measured by LIF. The NH distributions indicate dissociation on S1(1A′′), and can be described by Franck–Condon mapping of transition state wave functions in the HNC bending coordinate without additional torque, implying little anisotropy in the potential along that coordinate. On the other hand, a larger torque is manifest in the CO rotational distribution. Although at 217.6 nm the dissociation is likely to be dominated by decomposition on S1, competition with radiationless decay is still manifest. From analysis of the CO photofragment velocity distribution at 230.1 nm, the NH(a 1Δ)+CO dissociation threshold is determined at 42 765±25 cm−1.
A Monte Carlo study of subsurface effect on catalytic surface reactions
Authors: Khan K. M. , Yaldram K. , Khalifeh J. , Khan M. A.
Published: July 2002

The effect of the subsurface in monomer–dimer (CO-O2) surface reaction is studied by Monte Carlo simulation. The presence of the subsurface adds some extra reaction steps to the usual three. It is shown that the presence of the subsurface favours the enhancement of the steady reactive state by reducing appreciably the poisoning of the surface by the adsorbed dimer atoms. Under certain conditions the usual second order phase transition in CO−O2 reaction can be eliminated. The role played by the two way diffusion of adsorbed dimer atoms between surface and subsurface in the variation of steady reactive state is clearly illustrated.
Finding transition states using contangency curves
Authors: Ulitsky Alex , Shalloway David
Published: July 2002

We present a new method for approximating reaction paths and transition states of conformational transitions in polyatomic systems. The method uses the quasi-harmonic properties of the metastable state to construct a bi-Gaussian model for the energy landscape. Its reaction path is approximated by the equipotential contour cotangency (contangency) curve which connects the two equilibrium states with the saddle point lying between them. Unlike the reaction path, the contangency curve has an explicit analytic definition which makes it a useful starting point for conventional reaction path calculations. The method is illustrated for the conformational transition between the chair and twisted-boat isomers of cyclohexane.
Dressed states of molecules and microwave–infrared double-resonance spectroscopic techniques employing an electric quadrupole focusing field
Authors: Lee Chung Yi , Pate Brooks H.
Published: July 2002

High-sensitivity, microwave–infrared double-resonance measurements can be made in molecular-beam spectrometers employing a single state-focusing device. The key feature of the double-resonance technique is the achievement of large signal modulations of infrared signals using microwave transitions, even in cases where the infrared transition cannot be saturated. A series of measurements is presented that shows that the technique is based on the transition moment and state-focusing properties of dressed molecular states in the presence of a strong microwave field. Using a state-focusing device, the spectroscopic measurements are doubly sensitive to the composition of the dressed states. The technique can be extended to other types of spectroscopy, such as electronic spectroscopy and the spectroscopy of weakly bound complexes.
Photodissociation dynamics of jet-cooled H2O and D2O in the non-Franck–Condon regime: Relative absorption cross sections and product state distributions at 193 nm
Authors: Plusquellic David F. , Votava Ondrej , Nesbitt David J.
Published: July 2002

Quantum state distributions for nascent OH and OD fragments generated by Franck–Condon “forbidden” 193 nm photodissociation of H2O and D2O are reported, with the two isotopomers initially prepared in their zero-point vibrational and lowest ortho/para nuclear spin allowed rotational states (i.e., JKaKc=101 and 000 in a 3:1 ratio for H2O and 1:2 ratio for D2O) by cooling in a slit supersonic expansion. Product state distributions are probed via OH/OD laser-induced fluorescence (LIF) with cylindrical mirror collection optics optimized for the slit expansion geometry, which makes photodissociation studies feasible with cross sections as low as ≈10−26 cm2. The OH and OD fragments are formed exclusively in v=0, but with highly structured quantum state distributions in rotational, Λ-doublet, and fine structure levels (2Π3/2+, 2Π1/2+, and 2Π3/2−) that exhibit qualitatively different trends than observed in previous jet photolysis studies at 157 nm in the Franck–Condon “allowed” regime. The relative OH/OD fragment yields at 193 nm indicate a 64±10 times greater propensity for OH vs OD bond cleavage in H2O than D2O, which is more than three-fold smaller than predicted from full three-dimensional quantum scattering calculations on ground (X̃ 1A1) and first excited state (Ã 1B1) potential surfaces. One-dimensional semiclassical calculations of the Franck–Condon overlap matrix elements confirm these discrepancies to be considerably outside uncertainties associated with the ground and excited state potential surfaces. These results indicate that the photodissociation dynamics for this benchmark system are not yet fully understood and suggest either non-Born–Oppenheimer effects or contributions from other electronic surfaces may be important in the extreme non-Franck–Condon photolysis regime.
Optimal energy estimation in path-integral Monte Carlo simulations
Authors: Janke Wolfhard , Sauer Tilman
Published: July 2002

We investigate the properties of two standard energy estimators used in path-integral Monte Carlo simulations. By disentangling the variance of the estimators and their autocorrelation times we analyze the dependence of the performance on the update algorithm and present a detailed comparison of refined update schemes such as multigrid and staging techniques. We show that a proper combination of the two estimators leads to a further reduction of the statistical error of the estimated energy with respect to the better of the two without extra cost.
The Ã 2A1–X̃ 2B1 transition of H2O+ in the near infrared region
Authors: Huet T. R. , Hadj Bachir I. , Destombes J.-L. , Vervloet M.
Published: July 2002

A titanium sapphire laser spectrometer along with velocity modulation detection was used to record, between 12 000 and 13 400 cm−1, the absorption spectrum of the Ã 2A1–X̃ 2B1 electronic transition of H2O+ produced in an ac glow discharge with a gas mixture of He/H2O. The Σ(0,7,0)–(0,0,0) transition has been observed at 13 411 cm−1 and revised rotational assignments are presented. The Δ(0,7,0)–(0,0,0) and Π(0,6,0)–(0,0,0) transitions have been observed for the first time, at 13 331 and 12 494 cm−1, respectively. The vibrational assignment follows the vibrational numbering of Brommer et al. [J. Chem. Phys. 98, 5222 (1993)]. The rotational analysis was performed using the combination differences technique. Strong perturbations are observed, they are tentatively explained in term of rovibronic interactions. A set of effective molecular parameters has been obtained for each vibrational level of the Ã state. The values of the spin–orbit parameter are reported, they illustrate the vibronic coupling between the two electronic states.
Photodissociation of rovibrationally excited C2H2: Observation of two pathways
Authors: Schmid R. P. , Arusi-Parpar T. , Li R.-J. , Bar I. , Rosenwaks S.
Published: July 2002

C 2 H 2 is prepared in the 2030000 (five quanta of C–H stretch) vibrational state and photodissociated by 243.135 nm photons that also probe the H photofragments via (2+1) resonance-enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The production of H atoms is greatly enhanced upon rovibrational excitation. The REMPI action spectrum shows the characteristic features of a Σu+–Σg+ band and mimics the absorption spectrum, except that the R(13) line intensity is an order of magnitude higher than that expected for a Boltzmann distribution. The maximum translational energy of the H atoms obtained from dissociation of the regularly distributed rotational states is 0.67±0.10, whereas for R(13) it is 1.34±0.10 eV. The observed intensities and linewidths indicate the existence of two photodissociation pathways following the preparation of C2H2, where the C2H fragment is produced in two different states. In the R(13) pathway an additional bent state is prepared, or an accidental coincidence resonance is involved.
Spatial organization in the A+B→0 reaction under confined-scale mixing
Authors: Reigada R. , Sagués F , Sokolov I. M. , Sancho J. M. , Blumen A.
Published: July 2002

We consider the kinetics of the two-dimensional, stoichiometric A+B→0 reaction under confined-scale turbulent mixing and concentrate on the interplay between the kinetic patterns and the spatial organization of the system. We study the properties of the arising clusters and of the reaction zones, both in the presence and in the absence of mixing. We show that the two- point correlation function CAB(r)=〈cA(r′+r)cB(r′)〉/〈cA(r)〉2 is closely related to the effective reaction rate, while the functional form of the quartic correlation function Q(r,t)=〈cA(r′,t)cB(r′,t)cA(r′+r,t)cB(r′+r,t)〉/〈cA2(r,t)cB2(r,t)〉 is connected to the geometry of the reaction zones. We pay special attention to the occurrence of time windows of fast (classical) concentration decay even when the reactants show strong segregation.
Semiclassical theory of time-dependent curve crossing problems
Authors: Teranishi Yoshiaki , Nakamura Hiroki
Published: July 2002

It is shown that the newly completed accurate semiclassical theory for time-independent curve crossing problems can be usefully utilized to study various time-dependent curve crossing problems. Quadratic time-dependent problems can be solved exactly with use of the theory developed for the time-independent linear potential model. Furthermore, accurate and compact semiclassical theory can be formulated for general curved potentials. Even diabatically avoided crossing cases can be nicely treated. Multi-level problems can also be handled without difficulty with use of a new method to evaluate the necessary basic parameters directly from adiabatic potentials on the real axis in the fully diagonalized adiabatic representation. This method does not require a search for complex crossing points in the multi-level system, which is practically very difficult especially when the number of levels exceeds three.
Toward resolution of the silicon dicarbide (SiC2) saga: Ab initio excursions in the web of polytopism
Authors: Nielsen Ida M. B. , Allen Wesley D. , Császár Attila G. , Schaefer Henry F.
Published: July 2002

The long-standing problem of the topography, energetics, and vibrational dynamics of the ground-state surface of SiC2 is systematically investigated by means of the gamut of state-of-the-art electronic structure methods, including single-reference correlation techniques as extensive as the coupled-cluster singles and doubles method augmented by a perturbative triples term [CCSD(T)], the Brueckner doubles method (BD) with analogous contributions from both triple and quadruple excitations [BD(TQ)], and second-through fifth-order Mo/ller–Plesset perturbation theory (MP2–MP5), as well as the multiconfigurational complete-active-space self-consistent-field [CASSCF(12,12)] approach. The one-particle basis sets for these studies ranged from Si[6s4p1d], C[4s2p1d] to Si[7s6p4d3f2g1h], C[6s5p4d3f2g1h]. The methodological analysis resolves the polytopism problem regarding the mercurial potential energy surface for the circumnavigation of Si+ about C2− in silicon dicarbide, whose topography is shown to exhibit almost all conceivable variations with level of theory. It is concluded that the X̃ 1A1 global minimum of SiC2 is a T-shaped (C2v) structure connected monotonically to a linear transition state 5.8 kcal mol−1 higher in energy, thus ruling out any metastable linear isomer. Previously undocumented bent transition states and L-shaped minima are encountered at relatively high levels of theory, but ultimately these stationary points are shown to be spurious. High-level focal-point thermochemical analyses yield D0(Si–C2)=151 kcal mol−1, and hence a substantial revision is made in the heat of formation, viz., ΔHf,0∘(SiC2)=+155 kcal mol−1. A complete quartic force field about the T-shaped minimum is determined at the CCSD(T) level with the aug-cc-pVTZ (Si[6s5p3d2f],C[5s4p3d2f]) basis set and then employed in a preliminary probe of contours for large-amplitude motion, anharmonicity of the vibrations, and zero-point effects on the molecular structure.
Empirical potential energy surface for Ar⋅SH/D and Kr⋅SH/D
Authors: Korambath Prakashan P. , Wu Xudong T. , Hayes Edward F. , Carter Christopher C. , Miller Terry A.
Published: July 2002

Experimental data from vibrationally and rotationally resolved laser induced fluorescence experiments have been used to produce potential energy surfaces (PES) for the excited Ã 2Σ+ states of the Ar⋅SH and Kr⋅SH van der Waals complexes. This was done using a potential energy functional form first suggested by Bowman and co-workers [J. Phys. Chem. 94, 2226, 8858 (1990); Chem. Phys. Lett. 189, 487 (1992)] for Ar⋅OH/D. A discrete variable representation (DVR) of the vibration–rotation Hamiltonian was used in combination with the implicitly restarted Lanczos method and sequential diagonalization truncation (SDT) of the DVR Hamiltonian. This approach takes advantage of the sparseness of the DVR Hamiltonian and the reduced order of the SDT representation. This combination of methods greatly reduces the amount of computational time needed to determine the eigenvalues of interest. This is important for the determination of the PES that results from minimizing the difference between the experimental and theoretically predicted values for the vibronic energy levels and their corresponding rotational constants. In addition this procedure was helpful in assigning the absolute vibrational quantum numbers for the deuterated species for which less experimental data was available. Plots of the calculated wavefunctions corresponding to various experimentally vibronic bands indicate that these states sample regions of the PES from 0 degrees, where the hydrogen atom is closest to the rare gas atom, to approximately the saddle point, near the T-shaped configuration. As a result this region of the surface is determined accurately whereas the region of the PES around 180 degrees, corresponding to the sulfur atom being closest to the rare gas atom, is determined only qualitatively.
Roles of central and terminal carbon atoms in infrared and Raman intensities of polyenes: Analysis of atomic polar and polarizability tensors
Authors: Lee Jin Yong , Mhin Byung Jin , Kim Kwang S.
Published: July 2002

No theoretical and experimental information has been available for the role of each atom in infrared (IR) and Raman spectra of polyene derivative systems. Analyzing atomic polar and polarizability tensors at each carbon site in the C=C bond fixed frame, we find that in IR intensities, the motions of carbon atoms in the proximity of a substituent are important, while in Raman intensities the carbon motions in the central region are important. The strong IR and Raman intensities appear for the concurrent C=C stretch/contraction motion in nonsymmetric polyenes.
Probing the accuracy of pseudopotentials for transition metals in quantum Monte Carlo calculations
Authors: Flad Heinz-Jürgen , Dolg Michael
Published: July 2002

The accuracy of scalar-relativistic energy-consistent small-core pseudopotentials has been tested in quantum Monte Carlo calculations for the first-row transition metals Sc, V, Ti, and Cr. We have calculated atomic ionization and excitation energies using variational and pure diffusion quantum Monte Carlo methods as well as coupled cluster theory with nearly complete basis sets up to g functions. On the basis of the results we are able to estimate the magnitude of various errors related to the use of pseudopotentials in quantum Monte Carlo calculations for transition metals. From a comparison of coupled cluster valence-only and all-electron results, respectively, of the experimental data we estimate the errors due to the semilocal pseudopotential to be at most 0.1 eV. The average error of pure diffusion quantum Monte Carlo results with respect to experimental data amounts to 0.2 eV and arises mainly from errors due to the semilocal pseudopotential, its localization and the fixed-node approximation used in the quantum Monte Carlo calculations. The errors appear to be closely related to the occupation of the 3d shell.