bluuebunny/crossref_metadata_2025 huggingface dataset:
The vibrational energy pattern in acetylene (IV): Updated global vibration constants for C212H2
Authors: El Idrissi M. I. , Liévin J. , Campargue A. , Herman M.
Published: July 2002

All 253 vibrational levels in the ground electronic state of C212H2 with assigned rotational structure reported in the literature from absorption, stimulated emission pumping, and dispersed laser induced fluorescence spectroscopic investigations are gathered. They cover the range up to 18 915 cm−1. Some 219 of these energies are simultaneously fitted using the same so-called Cluster model based on the emergence of three constants of the motion, as previously used to deal with the vibrational energy levels up to 12 000 cm−1 [Abbouti Temsamani and Herman, J. Chem. Phys. 103, 5931 (1995)]. Thirty-nine vibrational constants are produced. The rms value of the fit is 0.81 cm−1. Principal rotational constants are predicted for all levels, which satisfactorily compare with the experimental results. Problems are demonstrated to concern a fraction of the 34 remaining levels only. Thus, the adequacy of the model is fully confirmed. The remaining problems are discussed and globally attributed to problems of a vibration–rotation nature. A comprehensive listing of calculated vibrational energies for all k sublevels up to 15 000 cm−1, including predicted Bv values and assignments is made available.
A molecular dynamics quantum Kramers study of proton transfer in solution
Authors: Antoniou Dimitri , Schwartz Steven D.
Published: July 2002

We present a quantum study of a proton transfer reaction AH–B⇌A−–H+B in liquid methyl chloride, where the AH–B complex corresponds to phenol-amine. We use the same intramolecular potentials that were used in two earlier studies of this system [H. Azzouzz and D. Borgis, J. Chem. Phys. 98, 7361 (1993); S. Hammes-Schiffer and J. C. Tully, J. Chem. Phys. 101, 4657 (1994).] The former study employed a Landau-Zener approach and a molecular dynamics centroid method, while the latter a surface-hopping method. These studies obtained results that differ by an order of magnitude. In the present work, we first performed a molecular dynamics simulation to obtain the spectral density, which was then used as an input to the method we have developed for the study of the quantum Kramers problem [S. D. Schwartz, J. Chem. Phys. 105, 6871 (1996)]. Thus, in this work both the reaction coordinate and the bath are treated quantum mechanically.
A novel, general method of analyzing magnetic circular dichroism spectra and magnetization curves of high-spin metal ions: Application to the protein oxidized rubredoxin, Desulfovibrio gigas
Authors: Oganesyan Vasily S. , George Simon J. , Cheesman Myles R. , Thomson Andrew J.
Published: July 2002

We have developed a general theoretical approach for analyzing the intensities of magnetic circular dichroism (MCD) spectra of paramagnetic species with S>1/2 in the nonlinear regions of temperature and magnetic field. The method takes full advantage of the irreducible tensor method in order to obtain maximum simplification from symmetry. The approach, which is based on a detailed treatment of spin-orbit coupling and Zeeman interaction in terms of the symmetry properties of basis sets of wave functions, factorizes contributions into bands with Gaussian and derivative shapes in order to extend earlier treatments based on the so-called linear field limit. The method is applied to analyze and fit the form of the MCD spectra and the MCD magnetization curves of pseudo-tetrahedral high-spin Fe(III), S=5/2, in the protein rubredoxin from Desulfovibrio gigas, a representative of a family of iron–sulphur proteins. This treatment provides for the first time a satisfactory fit of these curves over a temperature range between 1.6 and 10 K and up to magnetic fields of 5 T. We show that the forms of the magnetization curves are strongly dependent on the polarizations of the optical transitions and on both the sign and magnitude of D, the ground state axial zero-field parameter. The sign and magnitude of D is determined to be −0.6 cm−1 with a fixed value of E/D=0.25 obtained from an analysis of electron paramagnetic resonance data. This shows that earlier, simpler fitting procedures were inadequate.
Improving the efficiency of the Herman–Kluk propagator by time integration
Authors: Elran Y. , Kay K. G.
Published: July 2002

A method is presented that reduces the number of trajectories needed to achieve accurate results with the Herman–Kluk method and similar initial value semiclassical propagation techniques that involve integration over phase space. It is shown that a time average over motion may be inserted into these integrals with the result that their numerical convergence is accelerated. This technique is applied to two systems with encouraging results, especially for long-time propagation.
A four dimensional quantum scattering study of the Cl+CH4⇌HCl+CH3 reaction via spectral transform iteration
Authors: Yu Hua-Gen , Nyman Gunnar
Published: July 2002

We present a quantum dynamics study of the Cl+CH4⇌HCl+CH3 reaction using a four-dimensional rotating bond umbrella (RBU) model. A semiempirical potential energy surface is employed, where the zero point energy of the modes not explicitly treated in the RBU calculations is approximately included. The potential gives a vibrationally adiabatic ground state barrier height of 3.48 kcal/mol. The calculations have been performed in hypercylindrical coordinates using a log-derivative method. A single sector hyperspherical projection method has been developed for applying boundary conditions. A guided spectral transform (GST) Krylov subspace method has been constructed to find the eigenstates of the coupling matrix appearing in the coupled channel equations. The results show that the product methyl is rotationally cold for the forward reaction. A pronounced tunneling effect on the rate constants was obtained. The calculated thermal rate constants are 12%–45% smaller than the experimental results over the temperature range 200–800 K for the Cl+CH4⇌HCl+CH3 reaction. State-to-state differential cross sections have been studied at a kinetic energy of 0.159 eV. A detailed comparison with experimental measurements is made.
An infrared spectroscopic and density functional investigation of dinitrogen activation by group IV metal atoms
Authors: Kushto Gary P. , Souter Philip F. , Chertihin George V. , Andrews Lester
Published: July 2002

Reactions of laser-ablated Ti, Zr, and Hf atoms with N2 in excess argon and excess dinitrogen have produced the MN and (N2)xMN molecules in addition to M(μ–N)2M with no dinitrogen bond. Evidence is presented for simple η1–N2 and higher complexes for each metal. The observation of cyclic Ti(N2) and Zr(N2) molecules with N–N stretching frequencies at 1125.9 and 1022.8 cm−1, respectively, indicate significant activation of the dinitrogen bond. The identification of product molecules is based on isotopic substitution and the results of density functional theory frequency calculations.
Determination of interaction potentials of amino acids from native protein structures: Tests on simple lattice models
Authors: van Mourik Jort , Clementi Cecilia , Maritan Amos , Seno Flavio , Banavar Jayanth R.
Published: July 2002

We propose a novel method for the determination of the effective interaction potential between the amino acids of a protein. The strategy is based on the combination of a new optimization procedure and a geometrical argument, which also uncovers the shortcomings of any optimization scheme. The strategy can be applied on any data set of native structures such as those available from the Protein Data Bank. In this work, however, we explain and test our approach on simple lattice models, where the true interactions are known a priori and a Model Protein Data Bank (MPDB) can be generated by identifying proteins as amino acid sequences having a unique ground state conformation among all possible conformations. Excellent agreement is obtained between the extracted and the true potentials even for modest numbers of protein structures in the MPDB. Comparisons with other methods are also discussed.
Protein flexibility in solution and in crystals
Authors: Eastman Peter , Pellegrini Matteo , Doniach Sebastian
Published: July 2002

Characterizing the inherent flexibility of a protein provides an important link between structure and function. In this article, we examine some of the methods used to determine protein flexibility, and address several unanswered questions relating to them. We perform 4 ns simulations of bovine pancreatic trypsin inhibitor (BPTI) in solution and in a crystal. For comparison, we also calculate atomic fluctuations from room temperature x-ray diffraction data by two different methods: single copy refinement with isotropic B-factors, and constrained multiple copy refinement. We reach the following conclusions: (1) Crystal contacts significantly reduce atomic fluctuations, especially in the flexible loop regions. (2) Center of mass motion in the crystal contributes 0.1–0.2 Å to the rms fluctuations, with little variation between parts of the protein. (3) Isotropic B-factors are an accurate measure of atomic motion in the stable parts of the protein, but significantly underestimate motion of the flexible sidechains. (4) Nanosecond scale simulations can obtain a reasonable sampling of backbone atomic motion in the most stable regions, but are still too short to allow flexible regions to explore their full range of motion.
Three-pulse photon echoes for model reactive systems
Authors: Yang Mino , Ohta Kaoru , Fleming Graham R.
Published: July 2002

A theoretical description of the three-pulse photon echo peak shift for model reaction systems is presented. An electronic two-state system with a finite upper-state lifetime and a three-state system in which electronic transitions can occur are considered. A probabilistic argument is employed to incorporate the incoherent transitions. New pathways describing the transition of electronic population are introduced and the nuclear propagator in the electronic population state is written by a convolution integral between those of the nonreactive two-state system weighted by some factors for the electronic transition. The response functions are given by multitime correlation functions and are analyzed by the cumulant expansion method. Some numerical calculations are presented and the influence of incoherent reactions on the peak shift is discussed. Comparison with experimental data confirms the existence of the effects predicted here.
Molecular coordinates for instantaneous normal mode calculations. II. Application to CS2 and other triatomics
Authors: Murry Robert L. , Fourkas John T. , Li Wu-Xiong , Keyes Tom
Published: July 2002

We derive and demonstrate appropriate molecular coordinates for instantaneous normal mode calculations on liquids composed of linear and bent triatomic molecules. Comparisons are made between calculations in atomic Cartesian and molecular coordinates, the latter using both laboratory-frame Euler-angle and molecular-frame rotational coordinates. Results are contrasted for coordinate systems with both rigid (with no internal degrees of freedom) and flexible molecules. The coordinate dependences of the density of states, its breakdown into translational and rotational contributions, participation ratios and instantaneous normal mode (INM) spectroscopic quantities are examined and discussed.
Excited-state reversible geminate reaction. III. Exact solution for noninteracting partners
Authors: Gopich Irina V. , Agmon Noam
Published: July 2002

An analytic solution is derived for the Green function and survival probability of excited-state reversible recombination reactions of noninteracting geminate particles in solution, which have different lifetimes in their bound and unbound states and participate in a competing quenching reaction. The behavior of the three roots of the cubic polynomial, on which this solution depends, is investigated in the complex plane. Two kinds of “complex plane maps” are identified on which three types of transitions may occur. One root may vanish, or two roots coincide, or the three real parts coincide. The first transition leads to a corresponding transition in the long-time asymptotic behavior, which is derived in the sequel. The quenching and lifetime effects result in nonmonotonic dependence of the binding probability on the initial separation distance.
Bosonized squeezed-state coupled-cluster approach to electron correlations in nonlinear spectroscopy
Authors: Chernyak Vladimir , Mukamel Shaul
Published: July 2002

The signatures of excited-state correlations in the third-order optical response of many-electron systems are calculated using a time-dependent quasiparticle picture which maps the optical excitations onto a system of interacting bosons, following a transformation to boson (oscillator) variables. The many-body wavefunction is approximated by a Gaussian wavepacket representing a squeezed state in the quasiparticle space. Unlike the standard coupled-cluster technique, the present approach constitutes a generalized coherent state representation which makes it possible to derive simple closed equations of motion for its parameters. The time-dependent Hartree–Fock approximation is recovered for weakly correlated systems where squeezing is negligible.
Potential surfaces from the inversion of time dependent probability density data
Authors: Zhu Wusheng , Rabitz Herschel
Published: July 2002

This article presents an algorithm to assess the feasibility of inverting probability density data to extract potential surfaces. Such data admit the generation of a noniterative quantum inversion algorithm that does not require the solution of the Schrödinger equation. Tikhonov regularization is employed to manage the singular nature of the problem. The inversion in regular regions has excellent accuracy, and an error analysis also indicates that the potential in the regular regions is stable under perturbations from noisy data. The regular regions of the potential are identified by the algorithm. The algorithm does not require knowledge of the excitation process initiating the evolution of the system. Analysis indicates that the most detailed potential surface information will result from broadband excitation leaving the molecule with significant population in as many quantum states as possible. The inversion algorithm is tested in a simulation for the O–H potential, which shows that the algorithm is very fast and reliable.
Stochastic resonance in the presence or absence of external signal in the continuous stirred tank reactor system
Authors: Hou Zhonghuai , Xin Houwen
Published: July 2002

A two variable model, which has been proposed to describe a first-order, exothermic, irreversible reaction A→B carried out in a continuous stirred tank reactor (CSTR), is investigated when the control parameter is modulated by random and/or periodic forces. Within the bistable region where a limit cycle and a stable node coexist, stochastic resonance (SR) is observed when both random and periodic modulations are present. In the absence of periodic external signal noise induced coherent oscillations (NICO) appear when the control parameter is randomly modulated near the supercritical Hopf bifurcation point. In addition, the NICO-strength goes through a maximum with the increment of the noise intensity, characteristic for the occurrence of internal signal stochastic resonance (ISSR).
Vibrational-rotational energy transfer in H2-H2 collisions. I. Semiclassical decoupling approximation
Authors: Zenevich Vladimir A. , Billing Gert D.
Published: July 2002

A new semiclassical decoupling procedure for rotational projection states in rovibrationally inelastic atom-diatom and diatom-diatom collisions is developed. Computed vibrational self-relaxation rate constants for para-H2 and ortho-H2 are in good quantitative agreement (within a factor of 1.5, except for the lowest temperatures) with experimental data over the investigated temperature range 50–2000 K. This allows us to hope that also more detailed (nonmeasured) rate constants for rovibrational state-to-state transitions in molecular hydrogen, calculated by our new model, are sufficiently accurate for astrophysical applications.
The infrared spectrum of the nitric oxide dimer cation: Problems for density functional theory and a muddled relationship to experiment
Authors: Xie Yaoming , Schaefer Henry F. , Fu Xiao-Yuan , Liu Ruo-Zhuang
Published: July 2002

Ab initio and density functional theory (DFT) methods have been used to study the geometries, vibrational frequencies, and infrared intensities for the trans-, cis-, and gauche-structures of the ONNO+ cation. Five different functionals were employed for comparison. Double-ζ plus polarization (DZP) basis sets and triple-ζ plus double polarization with f functions (TZ2Pf) basis sets were utilized. The ground state of the trans-ONNO cation is of Ag2 symmetry. The prominent infrared absorption is predicted as ∼1900 cm−1 based upon the DFT methods. However, this DFT prediction is suspect since ONNO+ exhibits inverse symmetry breaking, dissociating to the physically absurd limit ON+1/2 plus NO+1/2. This inverse symmetry breaking phenomenon was discussed in an important 1997 paper by Bally and Sastry [J. Phys. Chem. A 101, 7923 (1997)]. Therefore, a higher theoretical level, Brueckner coupled-cluster method was ultimately applied, and the harmonic vibrational frequency of this mode was predicted to be about 1550–1650 cm−1. The important matrix isolation infrared experiments of Jacox et al. [J. Chem. Phys. 93, 7609 (1990)], Lugez et al. [J. Chem. Phys. 110, 10345 (1999)], Hacaloglu et al. [J. Phys. Chem. 94, 1759 (1990)], Andrews et al. [J. Phys. Chem. A 103, 4167 (1999)], and Strobel et al. [J. Phys. Chem. 99, 872 (1995)] are carefully considered.
Four-dimensional quantum scattering calculations on the H+CH4→H2+CH3 reaction
Authors: Yu Hua-Gen , Nyman Gunnar
Published: July 2002

Time-independent quantum scattering calculations have been performed to study the H+CH4→H2+CH3 reaction, using the analytic potential-energy surface developed by Jordan and Gilbert. A rotating bond umbrella (RBU) approximation with the implementation of a guided spectral transform subspace iteration technique has been applied together with a log-derivative method in hyperspherical coordinates. A single sector hyperspherical projection method was used to apply the boundary conditions to extract the S matrix at a large hyperradius. The results show that the H+CH4→H2+CH3 reaction occurs via a direct mechanism. The tunneling effect is pronounced, while there is little recrossing. Vibrational excitation of the C–H stretch and/or the H–CH3 bending modes of CH4 significantly enhance the reactivity. Exciting the umbrella mode of CH4 also enhance the reactivity, although less efficiently. The calculated thermal rate constants are larger than the experimental ones. However, good agreement has been obtained by including a barrier height correction of the potential function to make it agree with ab initio results. Finally, vibrational and rotational distributions of the reaction products are discussed in detail.
Impurity dynamics in binary van der Waals clusters created by pick-up
Authors: Vach Holger
Published: July 2002

We present results from molecular dynamics simulations concerning the creation of binary van der Waals clusters under a very large range of possible experimental pick-up conditions. Special emphasis is put on the dynamical processes occurring during and after the “pick-up” of Ne, SiF4, Kr, and Xe by Arn clusters with n ranging from 53 to 5000 atoms. Both Ne and SiF4 impurities are shown to normally reside in cluster surface states. For certain experimental conditions, however, both dopants may present transient matrix states. Matrix states are found to be the most probable final locations for Kr and Xe dopants under all considered conditions. We show that the dopant penetration depth crucially depends on cluster size, cluster velocity, nature of the dopant, and buffer gas pressure and that the final result is not always predictable from simple equilibrium considerations.
Temperature-dependent total emission spectra of azulene in polymers: Modeling using spectral densities
Authors: Gupta Vinita , Kelley Anne Myers
Published: July 2002

Monochromatically excited total emission spectra have been measured for the S1→S0 transition of azulene in polyethylene, polystyrene, and poly(methylmethacrylate) matrices over a temperature range from 1.4 to 100 K. The spectra in all three polymers exhibit strong zero-phonon lines (excitation of azulene vibrations only) accompanied by well-defined Stokes-shifted phonon sidebands at the lowest temperatures. As the temperature is raised the phonon bands broaden and gain relative intensity at the expense of the zero-phonon lines, and the spectra become qualitatively similar to the room-temperature liquid-phase spectra with sharp Raman lines on a broad fluorescence background. The near-origin-excited data are simulated by calculating the complete emission spectrum as a χ(3) process that assumes no artificial partitioning between “Raman” and “fluorescence.” The internal vibrations of azulene are modeled as simple undamped displaced harmonic oscillators while the intermolecular or matrix phonons are either modeled as a Brownian oscillator or treated as effective spectral densities extracted from published neutron scattering and/or low-frequency nonresonant Raman data in the same polymers. While the qualitative features of the spectra and their temperature dependence are reproduced, none of the spectral densities employed give a fully satisfactory fit to the experimental spectra. The results demonstrate the sensitivity of total emission spectra to the chromophore–matrix interactions, and suggest that the spectral densities describing these interactions are functions not only of the matrix but also of the chromophore involved.
Infrared spectra and density functional calculations of the CrO2−, MoO2−, and WO2− molecular anions in solid neon
Authors: Zhou Mingfei , Andrews Lester
Published: July 2002

Laser-ablated chromium, molybdenum, and tungsten atoms react with O2 in excess neon during condensation to form the MO2 dioxide molecules as major products. In addition, the MO2− anions, formed via capture of ablated electrons by the neutral molecules, were identified from isotopic splittings on their matrix infrared spectra and from density functional theory (DFT) calculations of isotopic frequencies. Evidence is also presented for CrO3 and the CrO3− anion. Doping with CCl4 to serve as an electron trap gave the same neutral molecules and virtually eliminated the anion absorptions, which strongly supports the anion identification.
On the virial series for hard-sphere mixtures
Authors: Wheatley Richard J.
Published: July 2002

The limitations of expanding the pressure of a binary mixture of hard spheres as a power series in the diameters of the spheres is investigated. It is shown that such an expansion cannot give the correct monodisperse virial coefficients, while fulfilling certain exact conditions for diameter ratios of zero and one. Imposing the correct virial coefficients is shown to be difficult, without making substantial changes in the functional form of the pressure equation, for a reasonable choice of high virial coefficients.
Polarizabilities of the rare-gas homonuclear diatoms
Authors: Ceccherini Simone , Moraldi Massimo , Frommhold Lothar
Published: July 2002

We present a simple, analytical model of the interaction-induced polarizability of the homonuclear rare-gas diatoms which contains only one free parameter. Electrostatic and exchange contributions to the pair polarizability are derived under the assumption that the electronic properties of each atom can be simulated by a single effective electron whose average distance from the nucleus is determined by the atomic polarizability. The dipole-induced dipole contribution at short internuclear distances is damped as appropriate for any realistic model. The model permits the reproduction of the available measurements of the interaction-induced, binary depolarized Raman spectra of the rare gases.
Diatomic metal encapsulates in fullerene cages: A Raman and infrared analysis of C84 and Sc2@C84 with D2d symmetry
Authors: Krause M. , Hulman M. , Kuzmany H. , Dennis T. J. S. , Inakuma M. , Shinohara H.
Published: July 2002

Raman scattering and infrared absorption of the C84 and Sc2@C84 isomers 23:D2d were studied at room temperature and 95 K. The results are compared to the response of pristine and doped C60. According to the lower symmetry and the higher number of atoms C84 exhibits much more vibrational modes than C60, in particular at wave numbers above 500 cm−1. For lower energies the vibrational structure of C84 resembles a downshifted and split C60 spectrum. After the encapsulation of two scandium atoms the overall vibrational structure and the number of C84 modes was preserved as a result of the similar geometric structure. From the very good correlation of the C84 and Sc2@C84 cage modes metal to fullerene charge transfer induced shifts could be analyzed. The lines were found less shifted compared to the C60 modes in exohedral doped A6C60 (A=K,Rb,Cs). Increased line widths of low energy cage modes were attributed to an additional intramolecular relaxation channel related to the dynamics of the encapsulated scandium ions. A set of nine new lines with almost complementary Raman and infrared intensities was found for Sc2@C84 below 200, at 246 and at 259 cm−1, and attributed to Sc–C84 vibrations. These vibrations were further identified as Sc–C84 stretching and Sc–C84 deformation modes. The Sc–C84 valence force constant of 1.19 N/cm was derived with a linear three-mass oscillator model for Sc2@C84. Both, the charge transfer induced line shifts and the Sc–C84 valence force constant indicate an effective transfer of approximately two electrons per scandium to the carbon cage. This is in agreement with an electronic state (Sc2.2+)2@C844.4− previously proposed on the basis of x-ray powder diffraction, x-ray photoemission spectroscopy (XPS), and quantum chemical calculations. The unexpected high number of Sc–C84 vibrations is attributed to crystal field and factor group splitting.
Rotational spectrum, structure, and modeling of the HCCH–(OCS)2 trimer: Observation of a polar OCS dimer fragment
Authors: Peebles Sean A. , Kuczkowski Robert L.
Published: July 2002

The rotational spectra of eight isotopomers of the HCCH–(OCS)2 trimer have been assigned by pulsed nozzle, Fourier-transform microwave spectroscopy. The rotational constants and dipole moment components are consistent with a nonplanar, triangular-twisted structure of C1 symmetry which aligns the three monomer axes approximately 3°–27° away from perpendicular to a triangle formed by the center of HCCH and the carbons of OCS. The OCS dimer portion of the trimer has the two OCS monomers aligned in an almost parallel fashion such that the monomer dipole moments reinforce, rather than in the antiparallel arrangement observed in the well-known OCS dimer. This configuration has been obtained using a semiempirical model which employs a distributed multipole representation of the electrostatic interaction and analytical atom–atom terms to describe the dispersion and repulsion terms in the interaction potential.
Rapidly fluctuating anisotropy parameter in the near-threshold photodissociation of NO2
Authors: Monti O. L. A. , Dickinson H. , Mackenzie S. R. , Softley T. P.
Published: July 2002

A novel experiment for near-threshold photodissociation studies is presented. State-selective excitation of the molecular photofragments to high-n Rydberg states is used in a variation of the ion imaging technique, allowing for undistorted detection of slow fragments produced close to the channel dissociation threshold. As a first demonstration of this method, the angular anisotropy parameter β for production of NO (J=17/2) and O 3P2 in the photodissociation of NO2 has been obtained as a function of excess energy. A classical model for β as a function of excess energy is presented, accounting for the decrease of anisotropy in the angular photofragment distribution upon approaching the channel threshold. The experimental values of β fluctuate substantially around the values predicted by the model, indicating strong underlying fluctuations in the state-to-state rate constant. This experiment offers in principle a unique route to measuring state-to-state reaction rate constants in situations where existing time- or frequency-resolved methods are inappropriate.
Water-chain clusters: Vibronic spectra of 7-hydroxyquinoline⋅(H2O)2
Authors: Bach Andreas , Coussan Stéphane , Müller Andreas , Leutwyler Samuel
Published: July 2002

Mass- and isomer-selected S1←S0 resonant two-photon ionization and S1→S0 fluorescence spectra were obtained for the supersonically cooled 7-hydroxyquinoline⋅(H2O)2 cluster. UV/UV-holeburning measurements show that >98% of the spectrum is due to a single “water-chain” cluster isomer, although two different tautomers (7-keto- and 7-hydroxyquinoline), two different rotamers (cis- and trans-hydroxy), and two torsional conformers of the chain are possible. Ab initio calculations of structures and vibrations of five different tautomers/ rotamers/ conformers of this cluster are reported. These predict that the cis-7-hydroxyquinoline⋅(H2O)2 “up/down” water-chain form is the most stable cluster. The experimentally observed S0 and S1 state vibrational frequencies agree well with those calculated for this isomer. We find no evidence for either the trans-rotamer or the keto tautomer clusters. S1←S0 excitation leads to contraction of all three hydrogen-bonds along the hydrogen-bonded water chain, inducing intermolecular stretching vibrations, but no proton transfer.
The excited electronic states and the ionization potential of the AlND3 complex
Authors: Jakubek Zygmunt J. , Simard Benoit
Published: July 2002

The AlND3 complex was studied in a molecular beam by resonant one-color two-photon and two-color two-photon photoionization techniques with a resolution of 0.3 cm−1. Six progressions of bands were observed in the 18 100–26 200 cm−1 (381–552 nm) region. One progression (ωe=316±0.8 cm−1) was assigned to the Al–ND3 stretch (ν3′) in the B̃2A1 state correlating with the 3s 2S term of Al. The origin of the B̃2A1–X̃2E1/2 system was found at 18 532.5±0.7 cm−1. The second (ωe=428±2 cm−1) and third (ωe=594±12 cm−1) progressions were assigned to the Al–ND3 stretch (ν3′) and the bend (ν6′) in the C̃2E state correlating with the 3d 2D term of Al. The origin of the C̃2E–X̃2E1/2 system was found at 21 185±5 cm−1. Assignments of the other three progressions with origins and harmonic wave numbers of 22 667±2 and 358±2 cm−1; 24 382±10 and 432±10 cm−1; and ∼21 871 and ∼1050 cm−1 remain uncertain. The spin-orbit splitting of the X̃2E ground state was determined to be 55.8±0.7 cm−1. Several Rydberg series converging to the v3+=1, 2, and 3 levels of the ground state of the AlND3+ ion were observed. The ionization potential of the AlND3 complex was measured to be 39 710±2 cm−1. The ν3+ fundamental in the ground state of the AlND3+ ion was found to be 325±5 cm−1. To aid the interpretation of our spectra, ab initio optimized structures and vibrational wave numbers for the ground states of various isotopomers of AlNH3 and AlNH3+ were calculated. Also, Al–NH3 potential curves for several low-lying excited states were calculated. In addition, the ionization potential of the AlNH3 isotopomer was measured by the one-photon photoionization efficiency technique. The value of 39 760±10 cm−1 is an improvement on that previously reported by Di Palma et al. [Chem. Phys. Lett. 284, 184 (1998)], which was 180 cm−1 smaller.
Observation of spin-polarized atomic photofragments through the Doppler-resolved Faraday technique
Authors: Korovin Konstantin O. , Picheyev Boris V. , Vasyutinskii Oleg S. , Valipour Hamid , Zimmermann Dieter
Published: July 2002

The angular distribution of spin-oriented Rb photofragments produced by photodissociation of RbI molecules using circularly polarized UV light at 266 nm has been investigated for the first time applying the method of Doppler-resolved paramagnetic Faraday rotation. The observed signals could successfully be explained as the sum of two contributions being due to incoherent and coherent excitation of the RbI molecules. For the ratio of the two corresponding anisotropy parameters we obtain a value of −0.33 as a preliminary result.
Fluorescence interferometry with random phased pulses: Diagonal site disorder and vibrational effects in molecular solids—A theoretical treatment
Authors: Szöcs V. , Tortschanoff A. , Pálszegi T. , Warmuth Ch. , Kauffmann H. F.
Published: July 2002

The technique of COIN (coherence observation by interference noise) relies on the measurement of correlated fluorescence fluctuations subsequent to two-pulse excitation with randomized relative phase [Kinrot et al., Phys. Rev. Lett. 75, 3822 (1995)]. In this paper, a comprehensive theoretical study on the use of random-phase fluorescence interferometry in the measurement of solid-state, molecular coherence is given. Optical response functions in terms of populations expanded to second order in a perturbational approach are presented for a guest/host system that are based on a generalized Bloch–Liouville formalism including inhomogeneous site broadening and harmonic vibrational modes. The theoretical treatment is intended to explain the coherence and coherence loss mechanisms measured, very recently, in the pentacene/p-terphenyl mixed crystal at low phonon temperatures. The expressions derived within the limits of the usual approximations are quite general and valid for both δ- and finite-width pulses. The perturbative solutions reproduce the temperature variation of the experimental COIN interferograms and reveal the effects of inhomogeneous dephasing and impurity–bath coupling, quite satisfactorily.
An ab initio study of the potential energy surface and spectrum of Ar–CO
Authors: Toczyłowski Rafał R. , Cybulski Sławomir M.
Published: July 2002

The two-dimensional potential energy surfaces for the Ar–CO complex have been developed using single and double excitation coupled-cluster theory with noniterative treatment of triple excitations [CCSD(T)]. The most accurate results have been obtained with the augmented correlation-consistent polarized triple zeta basis set (aug-cc-pVTZ) with an additional (3s3p2d2f1g) set of bond functions. The minimum of −104.68 cm−1 has been found at (R,Θ)=(3.714 Å, 92.88°), where R and Θ denote the Jacobi coordinates with Θ=0° corresponding to the linear Ar–OC geometry and Θ=180° to the linear Ar–CO geometry. Dynamical calculations have been performed to determine the frequencies of various rotational and rovibrational transitions. The overall agreement with experiment is good. For example, the calculated frequencies of the intermolecular bending and stretching vibrations, 12.015 and 18.520 cm−1, respectively, agree very well with the experimental values (12.014 and 18.110 cm−1).
Investigation of geometric shell aluminum clusters using the Gupta many-body potential
Authors: Turner Giles W. , Johnston Roy L. , Wilson Nicholas T.
Published: July 2002

A many-body Gupta-type potential, derived by fitting bulk experimental parameters for aluminum, has been used to study geometric shell clusters of aluminum with icosahedral, decahedral, fcc, and bcc packing. The stabilities of fcc-like octahedral clusters are compared with cuboctahedral and regularly truncated octahedral clusters. A stepwise pattern of truncation is observed and truncated octahedra are predicted to remain the preferred fcc-like structure until complete transition to the bulk fcc lattice.
Scheme for state-selective formation of highly rotationally excited diatomic molecules
Authors: Li Jing , Bahns John T. , Stwalley William C.
Published: July 2002

A scheme is proposed for making highly rotationally excited diatomic molecules (“super rotors”) in their ground vibrational and electronic state, e.g., 6Li2X 1Σg+ (v=0,J⩾115) where the rotational energy exceeds the bond strength (E(0,J)−E(0,0)⩾D00). Such levels, while strictly speaking quasibound, have very long tunneling lifetimes (>1011 s for J⩽130), and should have very interesting and unique collisional properties, especially at low temperature. The rotation of the molecules is “spun up” by sequential irradiation by R branch photons in the A 1Σu+–X 1Σg+ bands starting with cold molecules at low J. Spontaneous emission to other vibrational levels is overcome by using a pump laser and its multiple Raman sidebands as in previous work on “spinning down” diatomics.
A first principles study of the acetylene–water interaction
Authors: Tzeli Demeter , Mavridis Aristides , Xantheas Sotiris S.
Published: July 2002

We present an extensive study of the stationary points on the acetylene–water (AW) ground-state potential energy surface (PES) aimed in establishing accurate energetics for the two different bonding scenarios that are considered. Those include arrangements in which water acts either as a proton acceptor from one of the acetylene hydrogen atoms or a proton donor to the triple bond. We used a hierarchy of theoretical methods to account for electron correlation [MP2 (second-order Moller–Plesset), MP4 (fourth-order Moller–Plesset), and CCSD(T) (coupled-cluster single double triple)] coupled with a series of increasing size augmented correlation consistent basis sets (aug-cc-pVnZ, n=2,3,4). We furthermore examined the effect of corrections due to basis set superposition error (BSSE). We found that those have a large effect in altering the qualitative features of the PES of the complex. They are responsible for producing a structure of higher (C2v) symmetry for the global minimum. Zero-point energy (ZPE) corrections were found to increase the stability of the C2v arrangement. For the global (water acceptor) minimum of C2v symmetry our best estimates are ΔEe=−2.87 kcal/mol (ΔE0=−2.04 kcal/mol) and a van der Waals distance of Re=2.190 Å. The water donor arrangement lies 0.3 kcal/mol (0.5 kcal/mol including ZPE corrections) above the global minimum. The barrier for its isomerization to the global minimum is Ee=0.18 kcal/mol; however, inclusion of BSSE- and ZPE-corrections destabilize the water donor arrangement suggesting that it can readily convert to the global minimum. We therefore conclude that there exists only one minimum on the PES in accordance with previous experimental observations. To this end, vibrational averaging and to a lesser extend proper description of intermolecular interactions (BSSE) were found to have a large effect in altering the qualitative features of the ground-state PES of the acetylene–water complex.
Photodissociation of HCN at 157 nm: Energy disposal in the CN(A 2Π) fragment
Authors: Guo Jingzhong , Eng R. , Carrington Tucker , Filseth S. V.
Published: July 2002

Photodissociation of HCN at 157 nm was found to produce only H and CN(A 2Π) through analysis of laser-induced fluorescence experiments. The CN(A 2Π) vibrational populations decrease monotonically from v=0 to 3, represent 19±1% of the available energy and are in good agreement with a prior distribution of 9500 cm−1 of energy. The rotational distributions of the four vibrational levels are very similar, represent 6.5% of the available energy and are approximately Boltzmann with temperatures near 900 K. The rotational distribution of CN(A 2Π,v=0) is compared to several highly simplified reference distributions and approximate agreement is found with a phase space calculation.
Orientational states, phase transitions, and spectra of vibrational excitations for two-dimensional systems with quadrupole interactions
Authors: Rozenbaum V. M. , Lin S. H.
Published: July 2002

Consideration concerns a two-dimensional system of molecules fixed by an adsorption potential at lattice sites of a square lattice at a definite angle θ to the surface normal. Among lateral intermolecular interactions including quadrupole, dispersion, and repulsive, the former are regarded as predominating. At small θ, the Hamiltonian of the system is shown to reduce to that of the thoroughly studied two-dimensional dipole system with renormalized interaction constants. The hierarchy of ground-state orientational phases depending on the angle θ is established in two particular cases: for free azimuthal rotation and for four discrete orientations of projections of long molecular axes along square-lattice axes. The latter case is included in the diagram of temperature phase transitions. The results obtained adequately describe the properties of the 2×1 phase of CO/NaCl(100). The structure of the observed ground state and the orientational phase transition temperature are reproduced correctly. The spectrum of orientational vibrations calculated for this system is characterized by the band half-width which is consistent in the order of magnitude with those found from experimental temperature dependences of spectral line shifts and widths for local C–O vibrations dephased on orientational vibrations.
The predissociation mechanism of the B̃ 2A′ state of HCO via the conical intersection with the X̃ 2A′ state
Authors: Manaa M. Riad
Published: July 2002

We use large-scale multiconfiguration self-consistent field/configuration interaction calculations to characterize the predissociation mechanism of the B̃ 2A′ state of HCO through conical intersection with the X̃ 2A′ ground state. We locate two regions of intersection: the first represents a highly bent HCO that is 8 kcal/mol energetically lower than the B̃ state minimum, with a barrier height of 26 kcal/mol. Energy points on the B̃ 2A′ potential surface connecting these extrema were also calculated. This region emphatically illustrates the feasibility of a nonradiative decay mechanism consistent with latest experimental findings of purely vibronic coupling mechanism. The second region of intersection represents a confluence of three linear Σ+2–2Π states crossings, 53 kcal/mol below the B̃ state minimum. A barrier about 21 kcal/mol above the B̃ state equilibrium structure is located and assigned to the entrance channel of H–CO(3Π).
The role of segment interactions in pattern recognition between random heteropolymers and disordered surfaces
Authors: Srebnik Simcha
Published: July 2002

Recent studies have shown that preferential adsorption of random heteropolymers on disordered multifunctional surfaces occurs when the statistics describing the monomer sequence and the statistics describing the distribution of sites on the surface are matched in a certain way. The polymers undergo a sharp transition from weak to strong adsorption, indicative of pattern recognition. In this work, we continue to study the behavior of random heteropolymers as they adsorb on disordered surfaces using a nonreplica mean-field model that accounts for distinct and competitive interactions both among the polymer segments as well as between the polymer segments and sites on the surface. We find that strong interactions between polymer segments and between segments and sites on the surface are dominated by energetically favorable contacts among the segments and between segments and surface sites, respectively. Our results indicate that the polymers strongly adsorb in conformations that allow for a very small number of contacts between the segments, implying that the polymers strongly adsorb in relatively flat and stretched conformations.
A Monte Carlo study on the effect of excluded volume interactions on the scattering from block copolymer micelles
Authors: Svaneborg Carsten , Pedersen Jan Skov
Published: July 2002

Effects of excluded volume interaction on the form factor of a block copolymer micelle model have been investigated by performing Monte Carlo simulations. The micelles are modeled as a corona of semi-flexible chains tethered to a spherical core. Simulated form factors are analyzed using the model proposed by Pedersen and Gerstenberg. A slightly modified model is presented, in which chains consists of a radially pointing rigid rod, onto which a Gaussian chain is attached. The straight section emulates chain stretching near the micelle core. Both models are fitted to the simulation data using two parameters, that describes the individual chains: the radius of gyration, and the average center-of-mass distance to the micelle core. Based on a comparison between parameters obtained from fits, and those obtained directly from the simulation, it is concluded that the models provide good estimates for the radius of gyration and the chain center-of-mass distance for a low surface coverage, while systematic deviations are observed for high surface coverage, where chains begin to overlap, and excluded volume interactions becomes significant.
Effect of hydrodynamic interactions on the irreversible deposition of colloidal particles: Deposition algorithm and simulations
Authors: Jones R. B. , Kutteh Ramzi
Published: July 2002

A Stokesian dynamics constraint algorithm for the irreversible deposition of colloidal particles onto a surface is presented. This algorithm can also be used to simulate a variety of nondeposition phenomena and with any method for computing the hydrodynamic interactions among the particles. In particular, a multipole scheme we recently described for computing the mobility matrix of colloidal particles, with or without the presence of a wall, is used here in conjunction with the new deposition algorithm to perform Stokesian dynamics irreversible deposition simulations on a group of 75 colloidal particles. Deposition simulations are performed with and without the hydrodynamic effect of the wall (substrate) being included in the mobility matrix. The results of the simulations lead to important qualitative conclusions on the effect of hydrodynamic interactions on the deposition process, in particular the hydrodynamic effect of the wall. In addition, the simulations serve to illustrate the performance of the new deposition algorithm.
Total cross sections for dissociative electron attachment in dichloroalkanes and selected polychloroalkanes: The correlation with vertical attachment energies
Authors: Aflatooni K. , Burrow P. D.
Published: July 2002

Electron attachment into the lowest unoccupied molecular orbital of a typical polychloroalkane in the gas phase forms a temporary negative ion in which the impinging electron resides on a combination of local C–Cl σ* orbitals. Because of the antibonding character of these orbitals, these anions may dissociate, producing Cl− with cross sections that vary enormously over the chloroalkane family. In this work, we present absolute total dissociative electron attachment (DEA) cross sections for 33 of these compounds, and we show that the peak values of the cross sections correlate strongly with the vertical attachment energies (VAEs) for formation of the lowest anion states at the equilibrium geometries of the neutral molecules. This behavior is a consequence of the remarkably monotonic dependence of the resonance widths of the temporary anion states on VAE over the range 0.42<VAE<3.45 eV. Finally, we note also the strong connection between the s-wave attachment process at 0 eV in these compounds and the VAEs associated with the nearby anion states.