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22 Jun 1998

Volume 108, Issue 24, pp. 9959-10327

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Investigation of a grid-free density functional theory (DFT) approach

Kurt R. Glaesemann and Mark S. Gordon

J. Chem. Phys. 108, 9959 (1998); http://dx.doi.org/10.1063/1.476494 (11 pages) | Cited 12 times

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Density functional theory (DFT) has gained popularity, because it can frequently give accurate energies and geometries. Because evaluating DFT integrals fully analytically is usually impossible, most implementations use numerical quadrature over grid points, which can lead to numerical instabilities. To avoid these instabilities, the Almlöf-Zheng (AZ) grid-free approach was developed. This approach involves application of the resolution of the identity (RI) to evaluate the integrals. The focus of the current work is on the implementation of the AZ approach into the electronic structure code GAMESS, and on the convergence of the resolution of the identity with respect to basis set in the grid-free approach. Both single point energies and gradients are calculated for a variety of functionals and molecules. Conventional atomic basis sets are found to be inadequate for fitting the RI, particularly for gradient corrected functionals. Further work on developing auxiliary basis set approaches is warranted. © 1998 American Institute of Physics.  
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31.15.E- Density-functional theory
02.60.-x Numerical approximation and analysis

Multidimensional harmonic inversion by filter-diagonalization

Vladimir A. Mandelshtam and Howard S. Taylor

J. Chem. Phys. 108, 9970 (1998); http://dx.doi.org/10.1063/1.476495 (8 pages) | Cited 28 times

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We present a new method for harmonic inversion in multi-dimensions, i.e., extracting the wave vectors ωk and amplitudes dk from a signal cn = ∑kdke−inωk, where n defines the multi-index. The method is an extension of the filter-diagonalization method for 1D signals. As such it enables the harmonic inversion in any small wavevector domain Dω by solving a small generalized eigenvalue problem. The computed ωk and dk can then be used to create a high resolution image F(ω) for ω ∊ Dω. The method greatly overperforms the conventional Fourier analysis for a model 2D signal containing as many as 10 000 damped sinusoids with moderate amount of noise. © 1998 American Institute of Physics.
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07.57.Pt Submillimeter wave, microwave and radiowave spectrometers; magnetic resonance spectrometers, auxiliary equipment, and techniques
32.30.Dx Magnetic resonance spectra
33.25.+k Nuclear resonance and relaxation
76.60.-k Nuclear magnetic resonance and relaxation
02.30.Nw Fourier analysis
02.10.Ud Linear algebra
02.10.Xm Multilinear algebra

Removing resonance effects from quantum mechanical vibrational partition functions obtained from perturbation theory

Alan D. Isaacson

J. Chem. Phys. 108, 9978 (1998); http://dx.doi.org/10.1063/1.476496 (9 pages) | Cited 11 times

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This paper compares different methods for removing resonance effects from second-order perturbation theory calculations of vibrational energies in a variety of systems containing from two to six modes. Both the recently proposed method of Kuhler et al. and the standard approach of Nielsen yield stable energy levels even very close to resonance, with the latter giving smaller average errors in such cases. In addition, the method of Kuhler et al. is observed to affect the ground-state energy, unlike the standard approach. This generally worsens the accuracy of the vibrational partition function at room temperature, especially for systems close to resonance. © 1998 American Institute of Physics.
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33.20.Tp Vibrational analysis

Classical trajectory and statistical adiabatic channel study of the dynamics of capture and unimolecular bond fission. V. Valence interactions between two linear rotors

A. I. Maergoiz, E. E. Nikitin, J. Troe, and V. G. Ushakov

J. Chem. Phys. 108, 9987 (1998); http://dx.doi.org/10.1063/1.476497 (12 pages) | Cited 17 times

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The combination of two linear rotors forming linear or nonlinear adducts is treated using standardized valence potentials. Classical trajectory (CT) and statistical adiabatic channel (SACM) calculations are used for the calculation of thermal capture rate constants. At very low temperatures, only SACM applies. At intermediate temperatures SACM and CT approach each other; however, Landau–Zener-type multiple crossings of adiabatic channel potentials introduce local nonadiabaticity which has to be accounted for. The high-temperature transition from globally adiabatic to nonadiabatic (sudden) dynamics is studied by CT. Thermal rigidity factors, accounting for the influence of the anisotropy of the potential on the capture rate constant, are expressed in simple analytical form which facilitates practical applications. The present work complements similar studies on the addition of atoms to linear molecules in standardized valence potentials (part IV of this series). © 1998 American Institute of Physics.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Kh Potential energy surfaces for chemical reactions

Harmonic inversion of time cross-correlation functions: The optimal way to perform quantum or semiclassical dynamics calculations

Vladimir A. Mandelshtam

J. Chem. Phys. 108, 9999 (1998); http://dx.doi.org/10.1063/1.476498 (9 pages) | Cited 43 times

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We explore two new applications of the filter-diagonalization method (FDM) for harmonic inversion of time cross-correlation functions arising in various contexts in molecular dynamics calculations. We show that the Chebyshev cross-correlation functions ciα(n) = (Φα∣Tn(Ĥ)Φi) obtained by propagation of a single initial wave packet Φi correlated with a set of final states Φα, can be harmonically inverted to yield a complete description of the system dynamics in terms of the spectral parameters. In particular, all S-matrix elements can be obtained in such a way. Compared to the conventional way of spectral analysis, when only a column of the S-matrix is extracted from a single wave packet propagation, this approach leads to a significant numerical saving especially for resonance dominated multichannel scattering. The second application of FDM is based on the harmonic inversion of semiclassically computed time cross-correlation matrices. The main assumption is that for a not-too-long time semiclassical propagator can be approximated by an effective quantum one, exp[−itĤeff]. The adequate dynamical information can be extracted from an L×L short-time cross-correlation matrix whose informational content is by about a factor of L larger than that of a single time correlation function. © 1998 American Institute of Physics.
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03.65.Sq Semiclassical theories and applications
31.15.xv Molecular dynamics and other numerical methods
02.10.Ud Linear algebra
02.10.Xm Multilinear algebra

Simple finite field method for calculation of static and dynamic vibrational hyperpolarizabilities: Curvature contributions

Bernard Kirtman, Josep M. Luis, and David M. Bishop

J. Chem. Phys. 108, 10008 (1998); http://dx.doi.org/10.1063/1.476460 (5 pages) | Cited 38 times

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In the static field limit, the vibrational hyperpolarizability consists of two contributions due to: (1) the shift in the equilibrium geometry (known as nuclear relaxation), and (2) the change in the shape of the potential energy surface (known as curvature). Simple finite field methods have previously been developed for evaluating these static field contributions and also for determining the effect of nuclear relaxation on dynamic vibrational hyperpolarizabilities in the infinite frequency approximation. In this paper the finite field approach is extended to include, within the infinite frequency approximation, the effect of curvature on the major dynamic nonlinear optical processes. © 1998 American Institute of Physics.
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42.65.-k Nonlinear optics
42.70.-a Optical materials
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Additional compact formulas for vibrational dynamic dipole polarizabilities and hyperpolarizabilities

David M. Bishop, Josep M. Luis, and Bernard Kirtman

J. Chem. Phys. 108, 10013 (1998); http://dx.doi.org/10.1063/1.476461 (5 pages) | Cited 65 times

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Compact expressions, complete through second order in electrical and/or mechanical anharmonicity, are given for the dynamic dipole vibrational polarizability and dynamic first and second vibrational hyperpolarizabilities. Certain contributions not previously formulated are now included. © 1998 American Institute of Physics.
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33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
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Photoelectron spectra of the C2nH− (n = 1–4) and C2nD− (n = 1–3) anions

Travis R. Taylor, Cangshan Xu, and Daniel M. Neumark

J. Chem. Phys. 108, 10018 (1998); http://dx.doi.org/10.1063/1.476462 (9 pages) | Cited 48 times

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Anion photoelectron spectra of the carbon monohydrides, C2nH− for n = 1–4 and C2nD− for n = 1–3, have been measured. The spectra were recorded at a wavelength of 266 nm (4.657 eV) and yield electron affinities for each species. The spectra are vibrationally resolved, and some of the vibrational modes in the neutral C2nH(D) radicals are assigned. In addition, photoelectron angular distributions allow one to distinguish between photodetachment transitions to the 2Σ+ and 2Π states of the neutrals. The spectra confirm previous work showing that C2H and C4H have 2Σ+ ground states, while C6H and C8H have 2Π ground states. In addition, we observe the low-lying 2Π or 2Σ+ excited states for all four radicals. The photoelectron angular distributions also serve as a probe of vibronic coupling between the 2Σ+ and 2Π states. These effects are particularly prominent in the C2H− and C4H− spectra. © 1998 American Institute of Physics.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.60.+q Photoelectron spectra
31.50.Df Potential energy surfaces for excited electronic states
33.15.Mt Rotation, vibration, and vibration-rotation constants

Correction of repulsive potential energy surface for photodissociation of H2O in the state

Dunyou Wang and John Z. H. Zhang

J. Chem. Phys. 108, 10027 (1998); http://dx.doi.org/10.1063/1.476463 (6 pages) | Cited 1 time

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We present in this paper the application of the IPSVD (inverse perturbation via singular value decomposition) method to correct repulsive potential energy surfaces (PES) for half-scattering problems by directly inverting the experimental spectroscopic data. Specifically in the present model study, we start from the ab initio PES of Engel, Schinke, and Staemmler for the excited state and use the IPSVD method to correct the PES by directly inverting the measured absorption spectrum. The corrected model PES can accurately reproduce the total absorption spectrum in photodissociation of H2O in the state within the energy range of our study. Our model study shows great promise for future application of the IPSVD method to correct multidimensional repulsive potential energy surfaces for half- and full-scattering problems. © 1998 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
82.20.Kh Potential energy surfaces for chemical reactions
02.10.Ud Linear algebra
02.10.Xm Multilinear algebra

Very high resolution spectroscopy of high Rydberg states of the argon atom

F. Merkt and H. Schmutz

J. Chem. Phys. 108, 10033 (1998); http://dx.doi.org/10.1063/1.476464 (13 pages) | Cited 27 times

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Very high resolution spectra of high Rydberg states of the argon atom with principal quantum numbers in the range n = 60–200 have been measured in double-resonance experiments using a high resolution vacuum ultraviolet laser and frequency stabilized millimeter waves. The 250 kHz resolution achieved in the double-resonance spectra enables the determination of accurate effective quantum numbers and the precise measurement of fine-structure intervals in l = 0–3 Rydberg states at n values much beyond 50. The high resolution is also used to detect spectral shifts induced by small electric fields. Analysis of these spectral shifts allows the determination of stray electric fields with uncertainties of less than 1 mV/cm and their compensation to less than 1 mV/cm. The spectra of high Rydberg states are very strongly influenced by experimental conditions and the highest resolution can only be obtained when the stray electric fields are reduced to less than 1 mV/cm and the intensity of the millimeter waves are reduced to much less than 1 μW/cm2. High resolution measurements on the very high Rydberg states probed by pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy are also reported. These spectra are strongly influenced by the inhomogeneous electric field of ions and other Rydberg states located in the photoexcitation region. These fields induce a strong mixing of the optically accessible low-l Rydberg states with nonpenetrating high-l Rydberg states. These results conclusively demonstrate the important role played by ions in PFI-ZEKE spectroscopy. © 1998 American Institute of Physics.
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33.40.+f Multiple resonances (including double and higher-order resonance processes, such as double nuclear magnetic resonance, electron double resonance, and microwave optical double resonance)
33.60.+q Photoelectron spectra
33.70.Jg Line and band widths, shapes, and shifts

Infrared photodissociation spectra of CH3+–Arn complexes (n = 1–8)

Rouslan V. Olkhov, Sergey A. Nizkorodov, and Otto Dopfer

J. Chem. Phys. 108, 10046 (1998); http://dx.doi.org/10.1063/1.476465 (15 pages) | Cited 34 times

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Infrared photodissociation spectra of the ionic complexes CH3+–Arn (n = 1–8) have been recorded in the vicinity of the ν3 asymmetric stretching vibration of the CH3+ monomer. The CH3+–Ar dimer has also been investigated in the spectral range of the first CH stretching overtones, resulting in the characterization of its 2ν1, ν1+ν3, and 2ν3 vibrational states at the level of rotational resolution. The spectrum of CH3+–Ar is consistent with a pyramidal C3v minimum structure of the complex predicted by ab initio calculations at the MP2 level, whereby the Ar atom is attached to the empty 2pz orbital of the CH3+ moiety. The rotationally resolved ν3 spectrum of the CH3+–Ar2 trimer indicates that the two Ar atoms are located on opposite sides of the CH3+ moiety on the C3 axis, with significantly differing intermolecular C–Ar bond lengths. The splittings observed in the trimer spectrum are attributed to a tunneling motion between two equivalent C3v minimum configurations via a symmetric D3h transition state. The spectra of larger clusters (n ≥ 3) lack rotational resolution, however the positions and profiles of the ν3 band suggest that the additional Ar atoms are weakly attached to CH3+–Ar2 trimer, which acts as the effective nucleation center for the cluster growth. The stretching fundamentals of the CH3+ ion core in the CH3+–Arn clusters are intermediate between those of the methyl radical and the methyl cation, implying a substantial charge transfer from the rare gas atoms to the unoccupied 2pz orbital of CH3+. © 1998 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.20.Ea Infrared spectra
34.70.+e Charge transfer

Photodissociation dynamics of OClO at 157 nm

Jim J. Lin, Dennis W. Hwang, Yuan T. Lee, and Xueming Yang

J. Chem. Phys. 108, 10061 (1998); http://dx.doi.org/10.1063/1.476466 (9 pages) | Cited 4 times

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Photodissociation of OClO at 157.6 nm excitation has been investigated using the photofragment translational spectroscopic technique. Two distinctive chemical dissociation channels have been observed; one is the binary dissociation process, OClO+hν→ClO+O; the other one is the triple dissociation process, OClO+hν→Cl+O+O. The branching ratio of the binary dissociation channel to the triple dissociation channel is determined to be 0.59:0.41. Bimodal vibrational distribution of the ClO product has been observed for the OClO→ClO(X 2Π)+O(3P,1D) channel, implying that two distinctive dissociation routes possibly exist in the binary dissociation process. The bimodal distribution is likely caused by the two dissociation pathways from two excited electronic states: the D(2A1) and E(2B1) states of OClO. These arguments are further supported by the results of the anisotropy parameter measurements for the binary dissociation channels. Experimental results also show that the OClO+hν→ClO(X 2Π)+O(1S) and OClO+hν→ClO(A 2Π)+O(3P) channels might also exist in addition to the ClO(X 2Π)+O(3P,1D) channel. In the triple dissociation process, experimental results show that the main product channel is the OClO+hν→Cl(2P)+O(1D)+O(3P) channel, while the OClO+hν→Cl(2P)+O(3P)+O(3P) channel is the minor one. The branching ratio of these two channels is determined to be 0.89:0.11. From the modeling of the time of flight spectra of the O atom product, it is believed that the triple dissociation process of OClO is a simultaneous process within the time scale of one rotation period. Two-photon dissociative ionization process OClO+hν→Cl++O2+e− has also been observed. © 1998 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
82.20.-w Chemical kinetics and dynamics

Spin-polarization in HBr: Comparison between theory and experiment

R. Irrgang, M. Drescher, M. Spieweck, U. Heinzmann, N. A. Cherepkov, and H. Lefebvre-Brion

J. Chem. Phys. 108, 10070 (1998); http://dx.doi.org/10.1063/1.476467 (8 pages) | Cited 2 times

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Measurements of the electron spin polarization parameter A have been performed for HBr in the energy region between the two ionization thresholds, 2Π3/2 and 2Π1/2. Using the multichannel quantum defect theory and formulas derived for the spin parameters in intermediate coupling between Hund’s cases (a) and (e), calculations for both the cross section and the A parameter have been performed. It is shown that the values of A for each Rydberg resonance change with n, following the change from Hund’s case (a) to case (e). In the approximations used in this paper, no simple relation between the sign of A and the value of J+, the rotational angular momentum of the ion core, was found. Values for the calculated angular asymmetry parameter β and the spin polarization parameter ξ are also presented. Since our calculations reproduced only part of the observed resonances, and since the measurements of the A parameter have been performed in relatively narrow energy regions, one could not perform an extensive comparison between theory and experiment. However, some of the peculiarities of the behavior of the A parameter observed experimentally have been qualitatively reproduced by our calculations.© 1998 American Institute of Physics.  
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33.60.+q Photoelectron spectra
31.50.Df Potential energy surfaces for excited electronic states
34.50.Gb Electronic excitation and ionization of molecules

Theoretical assignments of the photo-dissociation excitation spectra of Mg+ ion complexes with water clusters: Multi-reference CI studies

Hidekazu Watanabe and Suehiro Iwata

J. Chem. Phys. 108, 10078 (1998); http://dx.doi.org/10.1063/1.476468 (6 pages) | Cited 9 times

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Implementing the ab initio molecular orbital (MO) method, electronic excitation energies of Mg+(H2O)n clusters have been calculated in order to analyze the photo-dissociation excitation spectra reported by Fuke and his co-workers. Observed bands for n ⩽ 2 are assigned to the s→p transitions of the most stable isomer of Mg+(H2O)n, with all waters directly bonded to the metal ion. For n = 3, n = 4 and 5, the dominant bands are also those of the most stable isomer, [Mg+(H2O)3](H2O)n−3, which has three water molecules in the first hydration shell. There are, however, shoulders and weak peaks in the experimental spectra, and calculations prove that these are due to the s→p transitions of less stable isomers. The calculated spectra strongly indicate the coexistence of a few isomers for n ≥ 3 in molecular beam experiments. © 1998 American Institute of Physics.
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36.40.Qv Stability and fragmentation of clusters
36.40.Wa Charged clusters
33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
31.15.vq Electron correlation calculations for polyatomic molecules
31.15.A- Ab initio calculations
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.15.Bh General molecular conformation and symmetry; stereochemistry

The exact quantum mechanical kinetic energy operator in internal coordinates for vibration of a hexatomic molecule

Susan B. Rempe and R. O. Watts

J. Chem. Phys. 108, 10084 (1998); http://dx.doi.org/10.1063/1.476469 (12 pages) | Cited 15 times

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The full nonrelativistic quantum mechanical vibrational J = 0 kinetic energy operator for a hexatomic molecule, (A,B)-C-D-(E,F), expressed in valence stretch, bend, and torsion internal coordinates is given explicitly. The derivation of the operator is reviewed and some of its properties and possible applications discussed. © 1998 American Institute of Physics.
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33.20.Tp Vibrational analysis

HF dimer: Empirically refined analytical potential energy and dipole hypersurfaces from ab initio calculations

Wim Klopper, Martin Quack, and Martin A. Suhm

J. Chem. Phys. 108, 10096 (1998); http://dx.doi.org/10.1063/1.476470 (20 pages) | Cited 84 times

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The interaction between two HF molecules as a function of all six internal coordinates is reported as calculated for 3284 selected points in configuration space at counterpoise-corrected explicitly correlated levels and fitted to an analytical expression, which is described in detail. The unweighted rms deviation for all 3284 points is 21 cm−1. Empirical refinements are applied through mixing and scaling of the ab initio data, guided by the comparison of multidimensional nuclear quantum energy levels with experimental data. The resulting semiempirical pair potentials (labeled SC-2.9 and SO-3) contain 67 and 61 freely adjusted parameters and are combined with a four parameter monomer potential of generalized Pöschl–Teller type. Various minimum energy paths and cuts are investigated. Major improvements over earlier HF dimer potentials are demonstrated via multidimensional solutions of the nuclear Schrödinger equation. Comparison with other high level ab initio calculations and with various experimental data reveals very good overall consistency. The new potential suggests strong Coriolis coupling in the librational degrees of freedom. Best estimates of stationary points, of the dimer dissociation energy (De = 19.1±0.2 kJ/mol), of the electronic barrier to hydrogen bond exchange (4.2±0.2 kJ/mol), of the electronic barrier to linearity (3.9±0.2 kJ/mol), and of the electronic barrier to hydrogen exchange (175±10 kJ/mol) are inferred. Based on accurate electric dipole functions for the monomer and distortion contributions calculated with a large basis at SCF level, a simple analytical six-dimensional electric dipole hypersurface is presented. © 1998 American Institute of Physics.
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31.15.A- Ab initio calculations
33.15.Fm Bond strengths, dissociation energies
31.15.xr Self-consistent-field methods

Out-of-plane vibrations of NH2 in 2-aminopyrimidine and formamide

W. J. McCarthy, L. Lapinski, M. J. Nowak, and L. Adamowicz

J. Chem. Phys. 108, 10116 (1998); http://dx.doi.org/10.1063/1.476471 (13 pages) | Cited 13 times

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The out-of-plane vibrations of the amino group in primary amines involve its inversion and rotation relative to the rest of the molecule. These two vibrations in 2-aminopyrimidine (see Fig. 1) were investigated with the combined use of matrix-isolation IR spectroscopy and ab initio quantum mechanical methodology. The ab initio methodology was also applied to another primary amine, formamide, for which a gas-phase IR spectra have been previously assigned. Ab initio potential energy surfaces were calculated in internal coordinates, ω and τ, whose displacements mimicked the inversion and internal rotation normal-mode distortions of the amino group, respectively. Vibrations along these two coordinates were considered uncoupled from all other nuclear motions. Total energy was calculated at the second-order Møller–Plesset perturbation theory level at selected values of ω and τ to allow a least-squares fitting of an analytical function depicting the potential energy curves and surface. A numerical procedure for determining the values of the kinetic energy operator in internal coordinates was also implemented to which an analytical function was fit. Vibrational energy expectation values were variationally determined by utilizing products of Gaussian and sinusoidal functions as the basis set. The resultant calculated fundamental transition energies for the coupled inversion and internal rotation vibrations of 2-aminopyrimidine are vinv = 140.6 cm−1 and vrot = 440.3 cm−1, respectively. These theoretical values reasonably match the experimental quantities of v ≈ 200 cm−1 and v ≈ 500 cm−1, and allow firm assignment of these two experimental infrared spectral bands to the inversion and internal rotation vibrations of the amino group in 2-aminopyrimidine, respectively. For formamide (see Fig. 2), the calculated transition energies for the inversion and internal rotation vibrations, vinv = 249 cm−1 and νrot = 602, match the experimental frequencies of ≈ 289 cm−1 and ≈ 602 cm−1, and confirm the accuracy of the theoretical method. © 1998 American Institute of Physics.
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33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Ea Infrared spectra
31.15.A- Ab initio calculations
02.50.-r Probability theory, stochastic processes, and statistics

Laser-induced emission spectroscopy of matrix-isolated carbon molecules: Experimental setup and new results on C3

Ivo Čermák, Markus Förderer, Iva Čermáková, Stefan Kalhofer, Helmut Stopka-Ebeler, Gerold Monninger, and Wolfgang Krätschmer

J. Chem. Phys. 108, 10129 (1998); http://dx.doi.org/10.1063/1.476472 (14 pages) | Cited 10 times

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We have studied small carbon molecules using a matrix-isolation technique. Our experimental setup is described in detail. The carbon clusters were produced by evaporating graphite and trapping the carbon-vapor molecules in solid argon, where molecular growth could be induced by controlled matrix annealing. To identify the produced molecules, absorption spectroscopy in the ultraviolet (UV)-visible and infrared (IR) spectral ranges was applied. Additional characterization of the excited and ground states of the molecules was obtained from emission and excitation spectra. The molecules were excited by a pulsed dye laser system and the emission spectra were recorded with a high-sensitivity photodiode-array spectrometer. We present our measurements on linear C3. The  1Πu excited state of linear C3 was populated by the electronic transition  1Πu←math 1Σg+, and the corresponding excitation spectra of the C3 fluorescence ( 1Πu→math 1Σg+) and phosphorescence ( 3Πu→math 1Σg+) were studied. Comparison of excitation and absorption spectra yielded information on site effects due to the matrix environment. Emission bands in the fluorescence and phosphorescence spectra up to vibrational energies of 8500 cm−1 could be observed. The radiation lifetime of the  1Πu excited state of C3 in solid argon was found to be shorter than 10 ns. The phosphorescence transition  3Πu→math 1Σg+ decays in about 10 ms and its rise indicates fast vibrational relaxation within the triplet system. Our data support a linear ground state geometry for C3 also in solid argon. © 1998 American Institute of Physics.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.20.Lg Ultraviolet spectra
33.20.Kf Visible spectra
33.20.Ea Infrared spectra
33.50.Dq Fluorescence and phosphorescence spectra
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A comparative electron spin echo envelope modulation study of the primary electron acceptor quinone in Zn-substituted and cyanide-treated preparations of photosystem II

Andrei V. Astashkin, Hideyuki Hara, Shigeki Kuroiwa, Asako Kawamori, and Kozo Akabori

J. Chem. Phys. 108, 10143 (1998); http://dx.doi.org/10.1063/1.476473 (9 pages) | Cited 3 times

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The electron spin echo envelope modulation spectra of the reduced primary acceptor quinone, QA, in two preparations of plant photosystem II, have been studied. In one of these preparations the Fe2+ ion in the quinone–iron complex has been substituted by diamagnetic Zn2+. In the other preparation this iron ion has been converted into the diamagnetic state using a potassium cyanide treatment. A comparative analysis of two-dimensional three-pulse electron spin echo envelope modulation spectra has shown similar structure of the binding site of QA in both preparations. Two nitrogen nuclei have been found to contribute to the spectra in both preparations. One of these nitrogens is, most probably, an amino nitrogen in the imidazole ring of histidine 215 of the D2 protein. The other nitrogen has been assigned to the peptide group of alanine 261 of the D2 protein. The numerical simulations of the electron spin echo envelope modulation spectra have shown that both nitrogens are simultaneously bound to QA. © 1998 American Institute of Physics.
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33.35.+r Electron resonance and relaxation
87.15.M- Spectra of biomolecules
33.15.Fm Bond strengths, dissociation energies

Molecular-dynamics simulation of collisional energy transfer from vibrationally highly excited azulene in compressed CO2

C. Heidelbach, I. I. Fedchenia, D. Schwarzer, and J. Schroeder

J. Chem. Phys. 108, 10152 (1998); http://dx.doi.org/10.1063/1.476474 (10 pages) | Cited 16 times

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Results from nonequilibrium molecular-dynamics simulations of collisional energy transfer from vibrationally highly excited azulene in compressed CO2 are compared with experimental results from our laboratory obtained under comparable physical conditions. As observed in the experiment, the cooling rates show a purely monoexponential decay of the excess energy. The influence of the microscopic solvent shell structure on these processes is investigated using the full three-dimensional anisotropic CO2 structure around azulene obtained from the simulation. The analysis shows that local heating effects of any kind do not play a role in our model system. Predictions of the pressure dependence of the energy transfer rates by the isolated binary collision model are compared with results from the simulations using two different definitions of the collision frequency in dense fluids. © 1998 American Institute of Physics.
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34.50.Gb Electronic excitation and ionization of molecules
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.50.Df Potential energy surfaces for excited electronic states

Quantum effects in simulated water by the Feynman–Hibbs approach

Bertrand Guillot and Yves Guissani

J. Chem. Phys. 108, 10162 (1998); http://dx.doi.org/10.1063/1.476475 (13 pages) | Cited 70 times

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Quantum effects in water are investigated by implementing the Feynman–Hibbs effective potential in a molecular-dynamics code. The reference potential chosen for water is a new central force model related to the one proposed in the 1970s by Lemberg and Stillinger [J. Chem. Phys. 62, 1677 (1975)]. The evolution of the thermodynamics, the structure, the diffusivity, and the dynamics in light and heavy water is investigated over a large range of temperature and is compared with experimental data and with classical simulations as well. It is found that quantum effects are significant near ambient conditions and vanish with increasing temperature less drastically than generally assumed. The most affected quantity is the self-diffusion coefficient for which is predicted a marked increase of the isotopic ratio (DH2O/DD2O) in going into the supercooled region. The accuracy of the results and the very low cost in computer time make the Feynman–Hibbs approach a valuable procedure to rapidly estimate the order of magnitude of the quantum contributions to intermolecular properties of water. © 1998 American Institute of Physics.
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61.20.Ja Computer simulation of liquid structure
61.43.Bn Structural modeling: serial-addition models, computer simulation
61.25.Em Molecular liquids

Chemical association and electronic structure: A new theoretical approach

E. Lomba, J. L. López-Martín, and M. Holovko

J. Chem. Phys. 108, 10175 (1998); http://dx.doi.org/10.1063/1.476476 (6 pages)

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Chemical association introduces essential modifications in the microscopic structure of disordered materials, as new species are formed and the density of reactants decreases. These radical changes in the microscopic structure cannot be adequately captured by the pair distribution function, and hence descriptions of the electronic structure at the level of the effective medium approximation break down. We here propose a novel approach closely connected with the single superchain/effective medium approximation, in which the effects of chemical association are explicitly built-in. The new approximation has been tested in a simple minded model of alkali metal, namely a hard sphere fluid with a one-level tight-binding Hamiltonian with transfer matrix elements modeled by Yukawa terms. This fluid is thought to undergo a dimerization process to yield hard dumbbells via an intermediate stage in which a mixture is present. The proposed approach reproduces the band structure obtained by direct diagonalization for various stages along the reaction path, while retaining the simplicity of a linear theory. © 1998 American Institute of Physics.
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82.30.Nr Association, addition, insertion, cluster formation
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)

Analysis of absorption and luminescence spectra of U3+ doped Cs2NaYCl6 and Cs2LiYCl6 single crystals

M. Karbowiak, J. Drożdżyński, S. Hubert, E. Simoni, and W. Strȩk

J. Chem. Phys. 108, 10181 (1998); http://dx.doi.org/10.1063/1.476477 (8 pages) | Cited 24 times

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Uranium(3+) doped single crystals of Cs2NaYCl6 and Cs2LiYCl6 with a 2.0% and 0.1% U3+ concentration have been obtained by the Bridgman-Stockbarger method. Luminescence spectra of the crystals were recorded at 160, 70, and 15 K. The emission bands observed in the visible and near infrared regions have been assigned to transitions from the lowest components of the 4I11/2, 4F3/2, and 4G7/2 multiplets to the crystal-field components of the 4I9/2 ground level. Absorption spectra were recorded from 4 000 to 25 000 cm−1 at 4.2 K. The zero phonon transitions were identified from an analysis of the vibronic side bands as well as in least-squares fits by applying a semiempirical Hamiltonian representing the combined atomic and crystal-field interactions for an ion of the 5f3 configuration and Oh symmetry. Eight energy level parameters were varied simultaneously in the least-squares adjustments yielding a mean error of 57 and 61 cm−1 for U3+:Cs2LiYCl6 and U3+:Cs2NaYCl6, respectively. The analysis of the spectra enabled the assignment of 25 crystal field levels for U3+:Cs2NaYCl6 and 27 for U3+:Cs2LiYCl6. The received total splitting of the ground level is equal to 625 and 595 cm−1, respectively. © 1998 American Institute of Physics.
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78.55.-m Photoluminescence, properties and materials
78.40.Ha Other nonmetallic inorganics

Evidence of pure 1Lb fluorescence from redshifted indole-polar solvent complexes in a supersonic jet

Kurt W. Short and Patrik R. Callis

J. Chem. Phys. 108, 10189 (1998); http://dx.doi.org/10.1063/1.476478 (8 pages) | Cited 18 times

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Two-photon polarized fluorescence excitation and vibronically resolved one-photon dispersed fluorescence spectra of the long-wavelength absorbing van der Waals complexes of indole with water, methanol, and formamide were examined for the purpose of firmly assigning the nature of the lowest singlet excited electronic state. The two-photon spectra for all of these complexes have circular/linear polarization ratios of absorptivity (Ω-values) that show that excitation is to the 1Lb state. Analysis of the Franck–Condon (FC) patterns of the dispersed fluorescence for these indole-polar solvent complexes show that emission is also from the 1Lb state in each case. In the emission spectra, the intensity ratios of the origin and ν26 lines are about 2:1, which is the value expected for 1Lb emission. The ν26 vibration is the most intense nonorigin vibronic line, as expected for 1Lb emission. Finally, there is little or no intensity from the ν8, ν9, and ν10 vibrations, which would be strongly active with emission from the 1La state. These results show unequivocally that for these indole-polar solvent complexes the 1La state does not shift in energy below the 1Lb state—contrary to the interpretation of several recent papers. Further support is given to the assignment of 1Lb emission by spectral simulations. The jet-cooled complexes do not exhibit excited state complex (exciplex) characteristics, wherein the solvent is much more strongly bound in the excited state than in the ground state. © 1998 American Institute of Physics.  
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33.50.Dq Fluorescence and phosphorescence spectra
33.70.Jg Line and band widths, shapes, and shifts
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors

Brillouin spectra and vibrational–translational energy exchange in liquid acetonitrile

Paola Sassi, Giulio Paliani, and Rosario Sergio Cataliotti

J. Chem. Phys. 108, 10197 (1998); http://dx.doi.org/10.1063/1.476479 (8 pages) | Cited 18 times

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Polarized light scattering experiments in the GHz frequency range (Rayleigh–Brillouin spectra) are reported for liquid acetonitrile. Hypersonic velocities and absorption coefficients have been measured as a function of temperature and of transferred wave vector. The plots of these quantities versus frequency show characteristic dispersion which is indicative of relaxation phenomena. The occurring relaxation has been interpreted as due to a thermal process in which the translational energy of molecules is transferred, during the collisions or through dipolar interactions, into internal degrees of freedom of low quanta vibrational motions. The quite strange behavior of the relaxation time with the temperature has been interpreted and compared with the behavior of the relaxation time in liquid chloroform. © 1998 American Institute of Physics.
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78.35.+c Brillouin and Rayleigh scattering; other light scattering
34.50.Ez Rotational and vibrational energy transfer
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