bluuebunny/crossref_metadata_2025 huggingface dataset:
Photophysics of size-selected InP nanocrystals: Exciton recombination kinetics
Authors: Kim S.-H. , Wolters R. H. , Heath J. R.
Published: July 2002

We report here on the size-dependent kinetics of exciton recombination in a III–V quantum dot system, InP. The measurements reported include various frequency dependent quantum yields as a function of temperature, frequency dependent luminescence decay curves, and time-gated emission spectra. This data is fit to a three-state quantum model which has been previously utilized to explain photophysical phenomena in II–VI quantum dots. The initial photoexcitation is assumed to place an electron in a (delocalized) bulk conduction band state. Activation barriers for trapping and detrapping of the electron to surface states, as well as activation barriers for surface-state radiationless relaxation processes are measured as a function of particle size. The energy barrier to detrapping is found to be the major factor limiting room temperature band-edge luminescence. This barrier increases with decreasing particle size. For 30 Å particles, this barrier is found to be greater than 6 kJ/mol—a barrier which is more than an order of magnitude larger than that previously found for 32 Å CdS nanocrystals.
Vibrational dynamics and thermodynamics of Ni(977)
Authors: Kara Abdelkader , Durukanoglu Sondan , Rahman Talat S.
Published: July 2002

We present an analysis of the vibrational density of states and characteristics of some surface vibrational modes of Ni(977), using a real space Green’s function approach with force constants derived from interaction potentials based on the embedded atom method. Changes in the force fields at and near the steps are found to lead to both softening and stiffening of the force constants between the surface atoms, as compared to those on Ni(111). Among the striking features of this vicinal, is a quasi-one-dimensional mode at 3.3 THz that is super localized at the step and a mode at 9.1 THz that has a propagation direction peculiar to this surface. Further, from an examination of the surface thermodynamics of Ni(977) it is seen that only the step and the corner atoms display features that are distinct from those on Ni(111).
Vibrational relaxation of clusters: Relation to potential surface topography
Authors: Vekhter Benjamin , Ball Keith D. , Rose John , Berry R. Stephen
Published: July 2002

Simulations by isothermal and isoergic molecular dynamics show that vibrationally-cold clusters, initially at some high-energy point on their potential surfaces, relax monotonically down to low-lying minima where they are trapped. The most significant point regarding this relaxation is that species with sawtoothlike and staircaselike potentials show the same qualitative behavior, equilibrating their vibrations after each major saddle crossing. This result justifies the use of transition state kinetics for constructing coarse-grained master equations to describe the well-to-well flow of population distributions on complex potential energy surfaces, even in cases such as (KCl)32 and some protein models which have staircase topographies in which large drops in potential energy from one well to the next might suggest nonthermal kinetics could occur.
Nascent rotational and vibrational state distributions of NH(X 3Σ−) and ND(X 3Σ−) produced in the reactions of N(2 2D) with H2 and D2
Authors: Umemoto Hironobu , Asai Takashi , Kimura Yuki
Published: July 2002

Two-photon dissociation of NO was employed to produce metastable atomic nitrogen N(2D) and to study its reactions. A mixture of NO and H2(D2) was irradiated with an intense laser pulse at 275.3 nm which dissociates NO to produce N(2D). Electronically ground state NH(ND) radicals could be detected as products of the N(2D)+H2(D2) reaction. The nascent rotational and vibrational state distributions of NH(ND) were determined by analyzing the laser–induced fluorescence spectra. The nascent vibrational population ratios, NH(v′′=1)/NH(v′′=0) and ND(v′′=1)/ND(v′′=0), were determined to be 0.8±0.1 and 1.0±0.1, respectively. These ratios are larger than the prior ones, but smaller than the recent results of quasiclassical trajectory calculations based on an ab initio potential energy surface. The rotational distributions of NH(ND) were very broad, both for the v′′=0 and v′′=1 levels. These results suggest that there are no specific attacking sites in these reactions.
Unimolecular dissociation dynamics of highly vibrationally excited DCO(X̃ 2A). II. Calculation of resonance energies and widths and comparison with high-resolution spectroscopic data
Authors: Keller Hans-Martin , Stumpf Michael , Schröder Thomas , Stöck Christoph , Temps Friedrich , Schinke Reinhard , Werner Hans-Joachim , Bauer Cornelia , Rosmus Pavel
Published: July 2002

We present a theoretical study of the unimolecular dissociation of DCO in the electronic ground state, X̃ 1A, using a new ab initio potential energy surface. Altogether we have analyzed about 140 resonances up to an energy of ≈1.4 eV above the D+CO threshold, corresponding to the ninth overtone in the CO stretching mode (v2=9). The agreement of the resonance positions and widths with recent stimulated emission pumping measurements of Stöck et al. [J. Chem. Phys. 106, 5333 (1997), the preceding article] is pleasing. The root-mean-square deviation from the experimental energies is only 16 cm−1 over a range of about 16 500 cm−1 and all trends of the resonance widths observed in the experiment are satisfactorily reproduced by the calculations. A strong 1:1:2 stretch–stretch–bend resonance prohibits a unique assignment for the majority of vibrational states.
Vibrational spectra of hexa-atomic silicon-carbon clusters. I. Linear SiC4Si
Authors: Presilla-Márquez J. D. , Rittby C. M. L. , Graham W. R. M.
Published: July 2002

Fourier transform infrared measurements on the spectra of the products of the evaporation of silicon/carbon mixtures trapped in Ar at ∼10 K, combined with the results of ab initio calculations published earlier, and density functional theory (DFT) calculations carried out in the present work, have resulted in the detection for the first time of the linear SiC4Si cluster. Two vibrational fundamentals have been assigned, the C=C stretching mode ν4(σu)=1807.4 cm−1, and the Si–C stretching mode ν5(σu)=719.1 cm−1. The observed frequencies, relative intensities, and isotopic shifts are in very good agreement with the results of the DFT calculations and confirm the previously predicted, linear symmetric geometry for the ground state of SiC4Si.
The 3d Rydberg (3A2) electronic state observed by Herzberg and Shoosmith for methylene
Authors: Yamaguchi Yukio , Schaefer III Henry F.
Published: July 2002

In 1959 and 1961 Herzberg and Shoosmith reported the vacuum ultraviolet spectrum of the triplet state of CH2. The present study focuses on a characterization of the upper state, the 3d Rydberg (3A2) state, observed at 1415 Å. The theoretical interpretation of these experiments is greatly complicated by the presence of a lower-lying A23 valence state with a very small equilibrium bond angle. Ab initio electronic structure methods involving self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), complete active space (CAS) SCF, state-averaged (SA) CASSCF, coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], CASSCF second-order (SO) CI, and SACASSCF-SOCI have been employed with six distinct basis sets. With the largest basis set, triple zeta plus triple polarization with two sets of higher angular momentum functions and three sets of diffuse functions TZ3P(2 f,2d)+3diff, the CISD level of theory predicts the equilibrium geometry of the 3d Rydberg (3A2) state to be re=1.093 Å and θe=141.3 deg. With the same basis set the energy (Te value) of the 3d Rydberg state relative to the ground (X̃ 3B1) state has been determined to be 201.6 kcal mol−1 (70 500 cm−1) at the CCSD (T) level, 200.92 kcal mol−1 (70 270 cm−1) at the CASSCF-SOCI level, and 200.89 kcal mol−1 (70 260 cm−1) at the SACASSCF-SOCI level of theory. These predictions are in excellent agreement with the experimental T0 value of 201.95 kcal mol−1 (70 634 cm−1) reported by Herzberg.
The longer time collapse kinetics of interfacial poly(N-isopropylacrylamide) in water
Authors: Zhu Peng Wei , Napper Donald H.
Published: July 2002

The kinetics of the later stages of the temperature induced collapse transition of swollen poly(N-isopropylacrylamide) (PNIPAM) chains at interfaces in the presence of the surfactant sodium dodecyl sulfate were studied using dynamic light scattering (DLS). The longer time collapse processes were interpreted in terms of the “globule growth” model of Kuznetsov, Timoshenko, and Dawson (KTD) [J. Chem. Phys. 103, 4807 (1995)]. The relaxation times of the collapse transition were determined from the longer time kinetics and were found to decrease with increasing quench temperature above the θ temperature.
Infrared-depletion spectroscopy study on hydrogen-bonded fluorobenzene–methanol clusters
Authors: Djafari S. , Barth H.-D. , Buchhold K. , Brutschy B.
Published: July 2002

Infrared-depletion spectroscopy, a double resonance method combining infrared- with resonant two-photon ionization (R2PI)-spectroscopy has been applied to mixed molecular aggregates of fluorobenzene⋅(methanol)n (Fb⋅MeOH) with n⩽4. From the IR spectra in the region of the OH stretching vibration of methanol it can be shown that the solvent moiety forms subclusters on one side of the aromatic ring. For Fb⋅(MeOH)3 the methanol trimer exhibits a linear as well as a cyclic structure. The different shifts of the UV bands of these two isomeric clusters reveal the diverse character of the hydrogen bonds involved in their formation. Furthermore it can be shown that for both isomeric forms the product anisole+ is formed in equal quantities by an intracluster SN2 reaction following the photoionization of the chromophore. Whereas for Fb⋅(MeOH)2 this reaction is in competition with evaporative fragmentation it is the dominant reaction channel for the Fb⋅(MeOH)3 cluster.
Direct measurement of eigenstate-resolved unimolecular dissociation rates of HOCI
Authors: Wedlock M. R. , Jost R. , Rizzo T. R.
Published: July 2002

Double-resonance overtone excitation prepares HOCl molecules in single rovibrational states above the unimolecular dissociation threshold in the ground electronic state. Detecting the OH dissociation fragments allows us to observe which reactant states lie above or below the dissocation threshold and determine that threshold to be 19 290.3±0.6 cm−1. Dissociation rates from single, well-characterized eigenstates of the parent molecule exhibit fluctuations of more than an order of magnitude.
Study of the methanol trimer potential energy surface
Authors: Mó O. , Yáñez M. , Elguero J.
Published: July 2002

The potential energy surface of methanol trimer has been studied through the use of high-level ab initio calculations and density functional methods. The geometries have been optimized at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. The harmonic vibrational frequencies were obtained at the latter level. The final energies for the most stable (CH3OH)n (n=1,3) clusters were calculated in the framework of the G2(MP2,SVP) theory. For these and all the other structures the final energies were also obtained using the B3LYP/6-311++G(3df,2p) approach. Three local minima have been located. The global minimum corresponds to a cyclic structure with two methyl groups on one side of the O–O–O plane and the third one on the other side. The bowl conformer, where the three methyl groups are on the same side of the O–O–O plane, is predicted to be only 0.8 kcal/mol less stable than the global minimum. The third local minimum, where one of the monomers behaves as a biacceptor is predicted to lie much higher in energy. Other stationary points associated with a systematic flipping of the methyl groups have been also located. These stationary points, which are transition states or saddle points of higher order, are very close in energy to the global minimum, indicating that the potential energy surface of the methanol trimer is very flat and very similar to that reported before for the water trimer. The calculated enthalpies of association for the dimer and the trimer of methanol are in very good agreement with the experimental values when estimated at the G2(MP2,SVP) level, while they are slightly underestimated when the B3LYP approach is used. The most stable trimer has three different O–H stretching frequencies, in agreement with the experimental evidence. The calculated frequency shifts are only in rough agreement with the experimental values. Cooperative effects are significant in the methanol trimer. They are reflected in larger frequency shifts, greater lengthening of the O–H bonds, shorter O⋯O distances than in the dimer. The G2(MP2,SVP) calculated additive interaction energy is also significantly large.
Calculation of resonances of HCO by the artificial boundary inhomogeneity method
Authors: Whittier Gregory S. , Light John C.
Published: July 2002

Resonance states of HCO are calculated for total angular momentum J=0, 1, and 3 using the artificial boundary inhomogeneity (ABI) method of Jang and Light [J. Chem. Phys. 102, 3262 (1995)]. Resonance energies and widths are determined by analyzing the Smith lifetime matrix. A resonance search algorithm and a method for resolving overlapping resonances are described. The accurate prediction of J=3 resonances from J=0 and 1 data is tested with good results for excited stretch resonances and less accurate results for bending resonances, demonstrating the degree of separability of vibration from overall rotation for these quasi-bound states.
Structural relaxation rates near the ice surface: Basis for separation of the surface and subsurface spectra
Authors: Delzeit Lance , Devlin J. P. , Buch V.
Published: July 2002

A differential in the relaxation rates of the ice surface and subsurface during Ostwald ripening near 138 K is the basis for a new approach to determination of the separate surface and subsurface vibrational spectra of ice nanocrystals. The resulting FT-ir spectrum of the ice subsurface matches that previously obtained from adsorbate-induced relaxation of the subsurface, confirming both the form of the surface and subsurface spectra, and the disordered nature of the surface of the nanocrystals. From these data, together with the results of simulation studies, ice emerges as the only molecular crystalline substance for which the surface and subsurface spectra are known and assigned. Semiquantitative rates of the structural relaxations near the surface, for bare ice and ice coated with the adsorbate SO2, have also been determined. Finally, spectral data are presented that track the insertion of SO2 into the vulnerable strained H bonds common to simulated structures of the ice surface.
Bond energies of CaAr+, CaKr+, and CaXe+ from resonant two-color photoionization thresholds
Authors: Kaup John G. , Breckenridge W. H.
Published: July 2002

The ionization thresholds for the metastable Ca(4s4p 3P0)⋅RG(3Π0−,v′′=0) states (RG=Ar, Kr, Xe) have been determined by two-color photoionization via several Ca(4s5s 3S1)⋅RG(3Σ+,v′) intermediate resonance states. From other experimental measurements and ab initio calculations, D0[Ca(4s4p 3P0)⋅RG(3Π0−)]=60±50 cm−1, 120±60 cm−1, and 300±80 cm−1 for RG=Ar, Kr, Xe, respectively, from which D0(CaRG+) are calculated to be 775±50 cm−1, 1244±60 cm−1, and 1811±80 cm−1 for Ar, Kr, Xe, respectively. Bonding in the CaRG+ ions is discussed and compared to the bonding in the analogous BeAr+, MgRG+, and BaAr+ ions.
Fluctuations in an equilibrium hard-disk fluid: Explicit size effects
Authors: Román F. L. , White J. A. , Velasco S.
Published: July 2002

Explicit size corrections in the calculation of the fluctuations in the number of particles in a finite subvolume of a hard-disk fluid composed of a fixed number of particles are considered. The size corrections are obtained on the basis of a Taylor series expansion of the pair distribution function of the N-particle system in powers of 1/N. Analytical density dependent expressions are obtained at low density. These expressions show that not only explicit size effects (due to consideration of a fixed number of particles) but also edge effects that result from considering a finite subvolume must be taken into account. A general density dependence study is also reported by relating the relative fluctuation in the number of particles to the equation of state. Numerical results for the Henderson equation of state are obtained. These theoretical results are compared with Monte Carlo computer simulation results.
Thermal conductivity of dilute solutions of chainlike polymers
Authors: Curtiss C. F. , Byron Bird R.
Published: July 2002

The Curtiss–Bird phase-space kinetic theory of polymers is used to derive an expression for the thermal conductivity of a dilute polymer solution, with the polymers represented as arbitrary bead-spring models. Then the general expression is specialized to Rouse bead-spring chains (with Hookean springs). The resulting expression contains several momentum-space averages as well as the configuration-space distribution function for the polymer chains. Use is made of the authors’ previous work on the solution of the Fokker–Planck equation for arbitrary bead-spring models to evaluate the momentum-space averages. Then two special cases are considered: (a) the Hookean dumbbell model, in a fluid with velocity gradients, and (b) the Rouse chain model, with the fluid at rest. For the latter, the authors’ previous study of the properties of tensor Hermite polynomials is helpful for solving the partial differential equation for the configurational distribution function for a polymer molecule in a fluid with a constant imposed temperature gradient. It is shown how the Gaussian distribution function is distorted in a nonisothermal system, but this distortion contributes only about 5% to the final value of the thermal conductivity. The results for the Rouse chain are compared with those previously obtained for several dumbbell models.
Spontaneous emission and absorption in model aggregates of π-conjugated oligomers
Authors: McIntire Michael J. , Manas Eric S. , Spano Frank C.
Published: July 2002

Spontaneous emission and absorption in an aggregate of π-conjugated oligomers are studied theoretically. Each oligomer is taken as a ring of N carbon atoms with average bond length a, and is treated using the Hückel Hamiltonian with bond alternation parameter δ. Coulombic interactions between rings are treated to first order using an effective Hamiltonian approach. For aggregates in which the oligomer planes are normal to the aggregate axis with in-phase bond alignment, intermolecular interactions cause a blue-shift in the absorption spectrum due to first order interchain excitation transfer. The average absorption frequency increases with N, peaks, and ultimately tends to a small value when the ring diameter Na/π greatly exceeds d, the nearest neighbor separation. At this point the entire spectrum for an aggregate is indistinguishable from the isolated oligomer spectrum. The radiative decay rate γ for an aggregate of M oligomers is superradiant, being M times faster than the decay rate of an isolated oligomer. When Na/π≫d, the superradiant rate increases linearly with δ over most of the range 0⩽δ<1. Near δ=1, however, there is a sharp transition from a one-dimensional superradiant rate (γ∝M) to a two-dimensional rate, with γ scaling as the product MN when δ=1.
Electronically excited states in size selected solvated alkali metal atoms. II. Isotope effects in the spectroscopy of sodium water and sodium ammonia complexes
Authors: Schulz Claus Peter , Nitsch Claudia
Published: July 2002

The change of spectroscopic properties of sodium-water and sodium-ammonia complexes upon deuteration is investigated. The ionization potential of NaD2O is shifted by 70 cm−1 towards lower energies compared to NaH2O. A shift of 81 cm−1 between NaND3 and NaNH3 is observed again towards lower energies. From these shift the vibrational frequencies for the ground state of the neutral and ionic complex have been estimated. The first electronically excited state (Ã2E) of NaND3 has been investigated by resonant two color two photon ionization. The comparison with the formerly observed NaNH3 spectrum enables us to do an unequivocal assignment of the vibrational structure.
Potential energy surface of cyclooctatetraene
Authors: Andrés José L. , Castaño Obis , Morreale Antonio , Palmeiro Raul , Gomperts Roberto
Published: July 2002

We present a theoretical study of the cyclooctatetraene (COT) molecule. Seven COT structures are located on the singlet ground state potential energy surface. Four of them, which present D2d (tub), Cs (bicyclo[4.2.0]octa-2,4,7-triene or BOT), C2h (chair) and D4 (crown) symmetries are stable species, and the other three are transition state structures showing Cs, D4h, and D8h symmetry. We discuss the symmetry of wave functions for these stationary points. Geometries, energies, and harmonic vibrational frequencies of these structures, and energy gaps between singlet–triplet states and low-lying singlets are presented. For the planar D4h and D8h structures, Jahn–Teller and tunneling effects have also been discussed. Ring inversion, bond shifting and valence isomerization reactive channels from the tub COT conformer are discussed from the point of view of the corresponding transition state structures. Where possible, in order to lend support to this theoretical information comparisons with recent transition state spectroscopy data are made.
The infrared spectrum and structure of the nitrous oxide trimer
Authors: Miller R. E. , Pedersen L.
Published: July 2002

Reported here are the first spectroscopic observations of the nitrous oxide trimer. Two rotationally resolved vibrational bands have been observed using the optothermal detection method. The ground vibrational state rotational constants obtained from these spectra are A=0.052 606(40), B=0.043 571(15), and C=0.028 736(7) cm−1. Ab initio calculations are also reported here, at the MP2/6-31+G(2d,2p) level, which yield A=0.054 92, B=0.045 17, and C=0.030 52 cm−1. The structure of the nitrous oxide trimer can be thought of as a slightly distorted dimer, which has a slipped parallel geometry [Z. S. Huang and R. E. Miller, J. Chem. Phys. 89, 5408 (1988)], with a third monomer unit lying above this dimer. This structure is similar to the recently observed nonplanar isomer of (CO2)3 [M. J. Weida and D. J. Nesbitt, J. Chem. Phys. 105, 10,210 (1996)].
An algorithm for the multi-reference configuration interaction method on distributed memory architectures
Authors: Stephan F. , Wenzel W.
Published: July 2002

We have reformulated the multi-reference configuration interaction algorithm for massively parallel architectures with distributed memory, such that the communication effort associated with the matrix element computation is independent of the number of units of the machine. Exploiting the special structure of the MRCI Hilbert space, this formulation allows the evaluation of all matrix elements in six distinct phases—independent of the number of electrons, the number of orbitals, the number of references and of the number of nodes of the computer used. Using an object-oriented approach, we have implemented and tested the a symmetry-adapted determinantal version of the algorithm in C++ on an IBM-SP2 computer. Our initial results for benchmark calculations on N2 and O3 for Hilbert spaces of up to 12×106 determinants indicate that the vector redistribution effort is negligible in actual calculations. The remaining difficulties to perform calculations for significantly larger Hilbert spaces are critically addressed.
Translational diffusion of small and large mesoscopic probes in hydroxypropylcellulose-water in the solutionlike regime
Authors: Streletzky Kiril A. , Phillies George D. J.
Published: July 2002

Quasi-elastic light scattering spectroscopy was used to study the translational diffusion of monodisperse spheres in aqueous 1 MDa hydroxypropylcellulose (HPC) at 25 °C. Probe diameters d spanned 14–455 nm; HPC concentrations were 0⩽c⩽7g/L. Light scattering spectroscopy consistently found spectra having the form g(1)(t)=(1−Af)exp(−θtβ)+Af exp(−θftβf). Here θf and βf refer to the “fast” mode; θ and β describe the “slow” mode. We examine the dependence of θ, β, θf, βf, and Af on d, c, scattering vector q, and viscosity η. β=1 for large probes; elsewise, β and βf are ∈(0,1). The slow mode, with short-lived memory function, is diffusive; for large probes θ≈(dη)−1. The fast mode, with long-lived memory function, appears coupled to polymer chain internal dynamics. Probe behavior differs between “small” and “large” probes. Small probes have diameters d<Rh, Rh being the chain hydrodynamic radius. Large probes have d⩾Rg, Rg being the polymer radius of gyration.
TDMP2 calculation of dynamic multipole polarizabilities and dispersion coefficients for the halogen anions F−, Cl−, Br− and I−
Authors: Hättig Christof , Heß Bernd Artur
Published: July 2002

A systematic ab initio study of the dynamic multipole polarizabilities of the halogen anions F−, Cl−, Br− and I− is presented. The effects of electron correlation are included for the static as well as for the frequency-dependent polarizabilities using time-dependent second-order Mo/ller-Plesset perturbation theory. Large one-particle basis sets, optimized for polarizabilities, are used to obtain results near the MP2 basis set limit. For the anions Br− and I− also scalar relativistic effects are accounted for by means of the spin-free no-pair Hamiltonian Ĥ+sf1. For the static dipole polarizabilities of the anions F− and Cl− we find good agreement with recent correlated ab initio calculations, but for the higher multipole polarizabilities and for the anions Br− and I− the discrepancies relative to previous calculations and empirical estimates are large. The effects of electron correlation on the polarizabilities of these anions are in general extremely large, while relativistic effects are in all four investigated halogen anions only of minor importance. The dynamic polarizabilities of the halogen anions are combined with previously calculated dynamic polarizabilities for rare gas atoms and the molecules CO and N2 to obtain isotropic and anisotropic dispersion coefficients for the corresponding van der Waals complexes.
Germyne, H–C≡Ge–H, and the excited states of 1-germavinylidene, H2C=Ge
Authors: Stogner Steven M. , Grev Roger S.
Published: July 2002

We have used ab initio quantum-mechanical methods to study the electronic states of germavinylidene, H2C=Ge, and the isomerization of ground-state singlet germavinylidene to the less stable trans-bent germyne isomer, H–C≡Ge–H. The electronic states of germavinylidene that we have studied are the ground A11 state, the A21 and A23 states, involving a π to py transition, and the B21 and B23 states obtained from the ground state by an n to py transition. We have also investigated the ground state of the germavinylidene anion, and the two lowest-lying cation states. Our predicted B̃ 1B2–X̃ 1A1 excitation energy agrees well with the recent experimental value. The geometries of the B21 and B23 states of germavinylidene might be expected to be similar, but they differ by 0.1 Å in the Ge–C bond length and 12° in the H–C–H bond angle. This results from the large electron repulsion between the unpaired electrons in the B21 state, which essentially reside on the same atom. The anomalously low value of the CH2 rocking frequency in germavinylidene is rationalized as a second-order Jahn–Teller effect. Finally, the trans-bent germyne isomer requires 7 kcal mol−1 to isomerize to the germavinylidene isomer, which is 43 kcal mol−1 more stable than germyne.
Electronic structure of benzene adsorbed on single-domain Si(001)-(2×1): A combined experimental and theoretical study
Authors: Gokhale S. , Trischberger P. , Menzel D. , Widdra W. , Dröge H. , Steinrück H.-P. , Birkenheuer U. , Gutdeutsch U. , Rösch N.
Published: July 2002

Benzene adsorption on a single-domain Si(001)-(2×1) surface has been studied by thermal desorption spectroscopy (TPD) and angle-resolved photoelectron spectroscopy (ARUPS) using linearly polarized synchrotron radiation. Angle-resolved photoemission spectra for the saturated benzene layer exhibit well-defined polarization and azimuthal dependencies compatible with a flat-lying benzene molecule with local C2v symmetry. Based on these results two structure models are proposed. First-principles density functional cluster calculations have been performed for each of these structures. Total energy minimization and a detailed comparison of the experimental ARUPS spectra with the one-particle spectra of the model clusters leads to a 1,4-cyclohexadienelike adsorption complex with a flat-lying benzene molecule which is di-σ bonded to the two dangling bonds of a single Si–Si surface dimer. Especially, one of the unoccupied 1e2u (π*) orbitals of the free benzene molecule shifts down (by about 3 eV) and evolves into the highest occupied molecular orbital (HOMO) of the chemisorbed molecule.
Extraction of spectral information from a short-time signal using filter-diagonalization: Recent developments and applications to semiclassical reaction dynamics and nuclear magnetic resonance signals
Authors: Pang John W. , Dieckmann Thorsten , Feigon Juli , Neuhauser Daniel
Published: July 2002

Filter-diagonalization [M. R. Wall and D. Neuhauser, J. Chem. Phys. 102, 8011 (1995)] is a new method for extracting frequencies and damping constants from a short-time segment of any time-dependent signal, whether of quantum origin or not. The method is efficient and able to handle signals with, e.g., millions of (possibly overlapping) frequencies, since it concentrates on specific spectral ranges. The method was shown to be a powerful tool for extracting eigenstates and normal-modes, and for reducing propagation times, in several recent works by us, by Mandelshtam and Taylor (who recently introduced the box filter) and by other groups. Here we extend the method in several directions: first, we show how it can be used with a filter of any form. Next, we show how the methodology may be extended to treat multi-dimensional signals, of the type that appears, e.g., in 2-D nuclear magnetic resonance (NMR). Finally, we exemplify the performance of the various filters for two types of signals where the time-reduction property is potentially quite important: 1D NMR and a correlation function from a semiclassical propagation (due to Grossmann) analyzed recently with a box filter. Significant reduction in required signal lengths, compared with direct Fourier transform, are found in both cases.
A soft Hartree–Fock pseudopotential for carbon with application to quantum Monte Carlo
Authors: Greeff C. W. , Lester W. A.
Published: July 2002

We describe the development of an ab initio Hartree–Fock (HF) pseudopotential for C that has neither repulsive r−2 nor attractive Coulomb singularities at the nucleus. This potential is tested in HF calculations on small hydrocarbons and shown to give good accuracy in comparison to all-electron calculations. The new pseudopotential is demonstrated to give superior basis set convergence in HF calculations compared to standard ones with r−2 singularities. Diffusion Monte Carlo (DMC) calculations on C and CH with the new potential show substantial improvements in stability at larger time steps compared with conventional pseudopotentials, with negligible time step dependence of the total energy. The resultant CH bond energy, De=83.3(4) kcal/mol, is in good agreement with experiment and with a similar all-electron DMC calculation.
Coupled cluster calculations of the frequency-dependent second hyperpolarizabilities of Ne, Ar, N2, and CH4
Authors: Hättig Christof , Jo/rgensen Poul
Published: July 2002

The recently developed coupled cluster cubic response approach is applied to the calculation of frequency-dependent second hyperpolarizabilities for Ne, Ar, N2, and CH4. A basis set investigation is performed for the static hyperpolarizabilities at the Hartree-Fock and the coupled cluster singles and doubles level (CCSD) using series of correlation consistent basis sets. For N2 and CH4 we have employed basis set up to, respectively, 326 and 353 basis functions. The role of correlation effects on the dispersion of the hyperpolarizabilities is explored by a comparison of Hartree-Fock, coupled cluster singles (CCS), CC2 and CCSD results for the third harmonic generation. Dispersion curves are calculated at the CCSD level for the dc Kerr effect, degenerate four wave mixing, electric field induced second harmonic generation, and third harmonic generation. For comparison with experiment we have considered zero point vibrational averaging, pure vibrational contributions and corrections for core correlation and connected triples. For all four systems we obtain much better agreement with experiment than in previous ab initio calculations.
Evidence for the OH(A) formation in photo-initiated reaction of HBr⋅N2O cluster
Authors: Ohoyama H. , Sawai T. , Tsuboi S. , Kasai T.
Published: July 2002

The emission spectrum of OH(A-X) is clearly observed in the photo-initiated reaction with the cluster beam of HBr⋅N2O for the first time. The rotational distribution of the product OH(A) is characterized as Boltzmann-type with temperature of 3500±1000 K. The vibrational distribution is found to be 1:0.45±0.15:0.01±0.01 for v=0, 1, and 2, respectively. The product initial distribution observed in this work significantly differs from the one for its corresponding bimolecular reaction of H+N2O. The product OH(A) formed in the cluster reaction is more excited in rotation and less excited in vibration; inversely, vibration is more excited than rotation for the bimolecular reaction. This result suggests new pathways open due to geometrical constraints in the intra-cluster reaction.
Model calculations of resonant vibration to vibration transition probabilities in clusters
Authors: Herman Michael F.
Published: July 2002

Results are presented for the calculated probability for resonant transfer of vibrational excitation energy in clusters of identical molecules. The calculations are performed for two-dimensional clusters in order to allow for calculations on larger clusters. Clusters of 5, 10, and 20 molecules are considered. The probability of resonant transfer is calculated by quantum mechanically propagating the wave function for the vibrational degrees of freedom of the molecules in the cluster, while the rotational and translational degrees of freedom evolve along classical trajectories. The transition probabilities are averaged over a canonical distribution of initial phase space points for the trajectories. If the probability for the transition of a vibrational quantum of energy from one molecule to some other molecule in an N molecule cluster is evaluated as N−1 independent two molecule vibrational problems, the results are found to be in excellent agreement with those obtained from the propagation of the full N molecule vibrational problem. The results also show that quantum coherence effects result in an early time nonlinear behavior in the transition probabilities that persists for several picoseconds in these systems.
Degenerate four wave mixing of pyridazine from a slit nozzle
Authors: Li Hongzhi , Kong Wei
Published: July 2002

Using a pulsed supersonic slit nozzle, the nonfluorescing π*←n transition of pyridazine was investigated. The degenerate four wave mixing (DFWM) spectra showed numerous vibrational bands over a 1200 cm−1 region. Most of these bands were parallel transitions with a strong Q branch and weaker but observable P and R branches. Based on our previous model [H. Li and W. Kong, J. Chem. Phys. 107, 3774 (1997)], these transitions were simulated with success. The polarization dependence of the rotational branching ratios suggested that primary contributions to the DFWM signal were from large spaced gratings formed by ground state molecules. The lack of contributions from excited state gratings and small spaced gratings was attributed to the fast internal conversion process on the S1 surface of pyridazine (0.3–3 ns), the wash-out time due to movements of the sample in a molecular beam, and the duration time of the excitation laser (7 ns). Two vibrational bands showed unexpected enhancement in the P or R branch, but for each band, one adjustment factor was sufficient to reproduce the spectra recorded under all different polarization combinations. Perturbations were observable from the rotationally resolved spectra, however in most cases, rotational progressions did not seem to be affected by the perturbation in terms of both line positions and intensities. A more detailed analysis of the supersonically cooled spectra, together with data from a room temperature gas cell and ab initio calculations, will be necessary to completely interpret the spectroscopy of pyridazine. This paper demonstrates that with the increased sensitivity achievable through a slit nozzle, DFWM is an effective technique for detailed spectroscopic studies, particularly for nonfluorescing species.
New expression for the effective transfer matrix element in long-range electron transfer reactions
Authors: Katz Daniel J. , Stuchebrukhov Alexei A.
Published: July 2002

A new expression for the effective transfer matrix element, TDA, in long-range electron transfer is derived. This expression corrects the second-order perturbation theory estimate by accounting for an infinite number of terms in the perturbation expansion. The correction factors measure the extent of delocalization of the diabatic donor and acceptor states. A simple procedure is devised to adjust the molecule to its transition state, which is the point of avoided crossing of the energies of the adiabatic states. The new expression is used to compute the half-splitting in these eigenenergies, which equals TDA, without recourse to diagonalization. When checked against direct diagonalization for a truncated model of a ruthenium-modified azurin protein, this method located the point of avoided crossing and produced an estimate of the energy half-splitting which agreed with the result of diagonalization with exceptional accuracy.
A two-dimensional quantum crystal: H2 on NaCl(100)
Authors: Grunwald Martin , Ewing George E.
Published: July 2002

Monolayers of H2 on NaCl(100) at 5.2 K have been prepared from both normal and para-enriched hydrogen and examined by polarized infrared spectroscopy. Vibrational features associated with J=0 and J=1 molecules are distinguished. The spectroscopic data are consistent with hydrogen adsorbed to two distinct sites: surface Na+ ions and Cl− ions. For the para-enriched sample a two-dimensional crystal structure is proposed in which J=0 molecules reside on Na+ ions and J=1 molecules cover Cl− sites. The polarization measurements show that all transition dipoles are perpendicular to the NaCl(100) surface. The mechanism for the infrared absorption is through electric field induced transition dipoles.
Soft and hard x-ray Raman scattering by oriented symmetrical molecules: Selection rules, interference, and dephasing mechanisms
Authors: Gel’mukhanov Faris , Privalov Timofei , Ågren Hans
Published: July 2002

Resonant x-ray Raman scattering in chaotical, partial, and fixed oriented symmetric molecules is analyzed for x-ray photon excitation frequencies in both the soft and the hard x-ray regions. Different dephasing mechanisms and their connection with channel interference and the observation of selection rules are investigated. It is predicted that for harder x-ray energies the scattering cross sections become strongly anisotropic and oscillatory due to channel interference. The orientational dephasing is predicted to be an important coherence-blocking mechanism and can be introduced even by zero-point vibrational or librational motions. The connections between selection rules, symmetry and phase factors of the photon wave function, Bragg conditions and the channel interference show that the selection rules may operate for oriented, surface adsorbed, molecules even in the hard x-ray region. The possibility of using the interference effect for structure determination of adsorbates is discussed.
Monte Carlo simulations of polyelectrolytes at charged hard spheres with different numbers of polyelectrolyte chains
Authors: Wallin Torsten , Linse Per
Published: July 2002

The complexation of a charged hard sphere (macroion) with short and oppositely charged polyelectrolytes was studied by the use of a simple model system. Structural data of the macroion–polyelectrolyte complex and the surrounding solution were obtained by the use of Monte Carlo simulation technique. The number of polyelectrolyte chains as well as the linear charge density and the flexibility of the polyelectrolyte were varied, whereas the properties of the macroion were held constant. In all cases, the macroion and a polyelectrolyte carried an equal number of elementary charges. The simulations showed that for flexible and highly charged polyelectrolytes, a tight complex containing the macroion and one polyelectrolyte was formed independent of the number of polyelectrolytes present. However, for flexible but lower charged polyelectrolytes, a looser macroion–polyelectrolyte complex was established, and a complex involving two polyelectrolytes occurred when two or more polyelectrolytes were available. Finally, for the most rigid polyelectrolyte no complex was formed at all. Generally, an intramolecular contraction of the polyelectrolyte occurred upon complexation with the macroion.
Resonance emission spectroscopy of predissociating SO2 C̃(1 1B2): Coupling with a repulsive A11 state near 200 nm
Authors: Ray Paresh C. , Arendt Michael F. , Butler Laurie J.
Published: July 2002

This work investigates the predissociative C̃(11B2) state of SO2 by cooling the SO2 in a pulsed molecular beam and dispersing the emission upon resonant excitation into several different vibronic absorption features in the C̃ state between 197 and 212 nm. Unlike at the lower excitation energies, the dispersed emission spectra at the higher excitation energies are dominated by progressions with odd quanta in the antisymmetric stretch mode ν3 and combination bands with up to six quanta in the bending mode ν2. The formidable intensity for emission into vibrational states with odd quanta in the antisymmetric stretch of the jet-cooled molecule suggests that the intermediate state at high energies in the excited state is of mixed electronic character at nonsymmetric geometries, so the operative components of the transition moment for excitation and emission may be different. We discuss our results by considering the avoided crossing seam and coupling between the C̃(11B2) state and two dissociative states potentially involved in the electronic predissociation, the 3 1A1 and the 1 3A1 states.
An ab initio derived torsional potential energy surface for the cyclic water tetramer
Authors: Graf Stephan , Leutwyler Samuel
Published: July 2002

A four-dimensional intermolecular torsional potential energy surface was calculated for the cyclic homodromic water tetramer at the level of second-order Mo/ller-Plesset perturbation theory, using a large basis set (82 basis functions per monomer). The four torsional angles ωi (i=1–4), which describe the positions of the “free” O–H bonds relative to the planar hydrogen-bonded O⋯O⋯O⋯O framework were varied, while all other internal coordinates were held fixed. Ab initio calculations were performed at 307 points in the asymmetric unit of the torsional space. The points were fitted using the same seven-parameter analytical potential function as previously employed for the torsional surface of the water trimer [Bürgi et al., J. Chem. Phys. 103, 1077 (1995)]. Fits were performed for four interaction energy ranges spanning 1100, 1500, 3500 and 11 500 cm−1 relative to the global minimum, respectively. A number of important stationary points were studied in greater detail and geometry optimized using larger basis sets at the MP2 level. Torsional interconversion pathways are mapped in terms of isopotential surface representations and contour plots. This accurate and rapidly computable potential can be employed to calculate the coupled anharmonic torsional level energies, wave functions and associated molecular properties, presented in the following paper [D. Sabo et al., J. Chem. Phys. 109, 5404 (1998)].
Relaxation kinetics of diffusion-influenced reactions of the type A+B⇄C perturbed by flash photolysis
Authors: Sung Jaeyoung , Shin Kook Joe , Lee Sangyoub
Published: July 2002

When the equilibrium of a reversible association–dissociation reaction, A+B⇄C, is perturbed by photolyzing C molecules, its relaxation kinetics cannot be described by conventional theories. Not only are the concentrations of reactant species displaced from equilibrium, but also the recombination dynamics of A and B molecules becomes quite different from the equilibrium bimolecular reaction. In particular, geminate pairs of A and B molecules photolytically produced in a viscous solution would give an almost singular contribution to the recombination dynamics at short times. Their dynamics needs to be treated distinctively from the recombination dynamics of thermally dissociated molecules. In the present paper, we develop a relaxation kinetic theory that takes account of these features of the reaction system in a unified manner. While most of previous theories are applicable only to the system of an isolated pair of geminate A and B molecules or to the pseudo-first-order case, the present theory is applicable to the second-order case as well. Simple analytic solutions are obtained in the Laplace domain, which are applicable irrespective of the dimensionality d of the reaction system. The present theory reveals an interesting feature of a photolytically perturbed reaction system as noted recently by Yang et al.; that is, relaxation of the concentration deviation to equilibrium follows the t−(d+2)/2 power law at long times instead of the t−d/2 power-law relaxation known for the thermally perturbed reaction system.
Photodissociation spectroscopy of the Ca+–C2H2 and Ca+–C2D2 π complexes
Authors: France M. R. , Pullins S. H. , Duncan M. A.
Published: July 2002

The Ca+–acetylene complex is produced in a molecular beam and studied with mass-selected resonance enhanced photodissociation spectroscopy (REPD). A spectrum with sharp vibrational and rotational structure is measured near the Ca+(2P←2S) transition. Ca+–acetylene has a π-complex structure with a ground state vibrational frequency (Ca+–C2H2 stretch) of ΔG1/2″=169 cm−1 and a dissociation energy of D0″=18.6±5.0 kcal/mol. Isotopic studies performed show that Ca+–C2D2 has a ground state vibrational frequency (Ca+–C2H2 stretch) of ΔG1/2″=167 cm−1. Excited state vibrational activity is observed in the metal-ligand stretch, the C–C and C–H stretches and the cis bend. The C2v symmetry of the complex is confirmed with rotationally resolved spectra for both isotopomers, which yield a Ca+–C2H2 bond distance of r0″=2.80±0.20 Å.
Vibrational relaxation of I2 in complexing solvents: The role of solvent–solute attractive forces
Authors: Shiang Joseph J. , Liu Hongjun , Sension Roseanne J.
Published: July 2002

Femtosecond transient absorption studies of I2–arene complexes, with arene=hexamethylbenzene (HMB), mesitylene (MST), or m-xylene (mX), are used to investigate the effect of solvent–solute attractive forces upon the rate of vibrational relaxation in solution. Comparison of measurements on I2–MST complexes in neat mesitylene and I2–MST complexes diluted in carbontetrachloride demonstrate that binary solvent–solute attractive forces control the rate of vibrational relaxation in this prototypical model of diatomic vibrational relaxation. The data obtained for different arenes demonstrate that the rate of I2 relaxation increases with the magnitude of the I2–arene attractive interaction. I2–HMB relaxes much faster than I2 in MST or mX. The results of these experiments are discussed in terms of both isolated binary collision and instantaneous normal mode models for vibrational relaxation.