bluuebunny/crossref_metadata_2025 huggingface dataset:
Dissociative photoionization of CH3SSCH3 in the region of ∼8–25 eV
Authors: Chiang Su-Yu , Ma Chien-I , Shr Der-Jr
Published: July 2002

The dissociative photoionization of CH3SSCH3 has been investigated in the photon energy range of ∼8–25 eV with a molecular beam/photoionization mass spectrometry/threshold photoelectron spectrometry system using synchrotron radiation as an ionization source. For dissociation above photon energy of 11.5 eV, six fragment ions of CH3+, C2H3+, SH3+, HCS+, S2+, and CH2S2+ were reported for the first time. The photoionization efficiency spectra for the parent ion and for 12 observed fragment ions, CH3+, C2H3+, SH3+, HCS+, CH2S+, CH2SH+, CH3SH+, CH3SH2+, CH3SCH2+, S2+, CH2S2+, and CH2S2H+, were measured; their branching ratios as a function of photon energy were derived. Ionization energy of 8.20±0.04 eV for CH3SSCH3 and the appearance energy for each fragment ion were determined from the onsets of the photoionization efficiency spectra. Based on the appearance energy and existing thermochemical data, plausible structures of the fragment ions and their neutral counterparts are proposed. Fragmentation mechanisms that involve H migration and structural rearrangement in the dissociative photoionization processes are discussed.
Relaxation behavior in model compounds of poly(aryl-ether-ketone-ketone) as revealed by dielectric spectroscopy
Authors: Ezquerra T. A. , Zolotukhin M. , Privalko V. P. , Baltá-Calleja F. J. , Nequlqueo G. , Garcı́a C. , de la Campa J. G. , de Abajo J.
Published: July 2002

The relaxation behavior of a series of ether-ketone oligomers, considered as model compounds of poly(ether-ketone-ketone), was studied by means of dielectric spectroscopy. The dynamics of the α relaxation of ether-ketone model compounds as compared with that of poly(arylether-ketone-ketone) (PEKK) (50/50), shows up differences which can be attributed to the variation of inter- and intramolecular correlations with the chain length. Model compounds exhibit a nearly similar degree of cooperativity regardless the differences in Tg values. The PEKK (50/50) polymer exhibits stronger cooperativity than the oligomers suggesting that in poly(ether-ketone-ketone)s molecular motions above Tg extend to more than one monomeric unit.
A new one-parameter progressive Colle–Salvetti-type correlation functional
Authors: Tsuneda Takao , Suzumura Toshihisa , Hirao Kimihiko
Published: July 2002

We analyze the Colle–Salvetti correlation functional and propose a new simplified correlation functional called the one-parameter progressive (OP) functional. It contains only one semiempirical parameter with no adjusted fundamental constant. The functional can be adapted to any kind of exchange functional. Different from generalized gradient approximation type functionals, OP is not derived to obey necessary conditions of the exact correlation functional. However, it is proved that OP satisfies all these conditions if the exchange part is chosen to obey the conditions of the exact exchange functional. Compared to conventional correlation functionals, OP is found to give more accurate correlation energies for atoms, helium through argon, and equivalent chemical properties for the G2 set of molecules.
A hybrid method for solutes in complex solvents: Density functional theory combined with empirical force fields
Authors: Eichinger M. , Tavan P. , Hutter J. , Parrinello M.
Published: July 2002

We present a hybrid method for molecular dynamics simulations of solutes in complex solvents as represented, for example, by substrates within enzymes. The method combines a quantum mechanical (QM) description of the solute with a molecular mechanics (MM) approach for the solvent. The QM fragment of a simulation system is treated by ab initio density functional theory (DFT) based on plane-wave expansions. Long-range Coulomb interactions within the MM fragment and between the QM and the MM fragment are treated by a computationally efficient fast multipole method. For the description of covalent bonds between the two fragments, we introduce the scaled position link atom method (SPLAM), which removes the shortcomings of related procedures. The various aspects of the hybrid method are scrutinized through test calculations on liquid water, the water dimer, ethane and a small molecule related to the retinal Schiff base. In particular, the extent to which vibrational spectra obtained by DFT for the solute can be spoiled by the lower quality force field of the solvent is checked, including cases in which the two fragments are covalently joined. The results demonstrate that our QM/MM hybrid method is especially well suited for the vibrational analysis of molecules in condensed phase.
Chlorine atom formation dynamics in the dissociation of CH3CF2Cl(HCFC-142b) after UV laser photoexcitation
Authors: Brownsword Richard A. , Schmiechen Patricia , Volpp Hans-Robert , Upadhyaya Hari P. , Jung Young Jae , Jung Kyung-Hoon
Published: July 2002

The dynamics of chlorine atom formation after UV photoexcitation of CH3CF2Cl(HCFC-142b) in the gas phase was studied by a pulsed laser photolysis/laser-induced fluorescence (LIF) “pump-and-probe” technique at room temperature. The parent molecule was excited at the ArF excimer laser wavelength (193.3 nm) and nascent ground state Cl(2P3/2) and spin-orbit excited Cl*(2P1/2) photofragments were detected under collision-free conditions via laser induced fluorescence in the vacuum ultraviolet spectral region. Narrow-band probe laser radiation, tunable over the wavelength range 133.5–136.4 nm, was generated via resonant third-order sum-difference frequency conversion of dye laser radiation in Krypton. Using HCl photolysis at 193.3 nm as a source of well-defined Cl(2P3/2) and Cl*(2P1/2) concentrations, values for the total Cl atom quantum yield (ΦCl+Cl*=0.90±0.17) and the [Cl*]/[Cl] branching ratio 0.39±0.11 were determined by means of a photolytic calibration method. From the measured Cl and Cl* atom Doppler profiles the average relative translational energy of the fragments could be determined to be 125±25 kJ/mol. The corresponding value fT=0.48±0.10 of the fraction of total available energy channeled into product translational energy was found to be (within experimental uncertainty) in agreement with the result fT=0.39 of a dynamical simulation assuming a repulsive model for single C–Cl bond cleavage. Both the measured total Cl atom quantum yield and the energy disposal indicates that direct C–Cl bond cleavage is a primary fragmentation mechanism for CH3CF2Cl after photoexcitation at 193.3 nm.
Ab initio calculations for the anharmonic vibrational resonance dynamics in the overtone spectra of the coupled OH and CH chromophores in CD2H–OH
Authors: Quack Martin , Willeke Martin
Published: July 2002

We report high level ab initio calculations (treating correlation by second order Möller–Plesset perturbation theory, MP2) of a five-dimensional normal coordinate subspace of the potential and electric dipole hypersurfaces of the Cs conformer of dideuteromethanol, CD2HOH. Accurate vibrational variational calculations are carried out using a discrete variable representation (DVR) for the five anharmonically coupled modes (three coupled CH stretching and bending modes and the OH stretching and high frequency OH bending mode). The overtone spectra of the OH chromophore are calculated and analyzed in detail with respect to their anharmonic resonance dynamics leading to short time intramolecular vibrational redistribution (IVR) via the close resonance coupling of 5νOH (5ν1) with 4νOH+νCH(4ν1+ν2), as previously observed and assigned experimentally. While the assignment of the resonance is confirmed by the ab initio calculation, a sequence of calculations including various subspaces (two-dimensional to five-dimensional) lead to the conclusion that the resonance contains important contributions from coupling to the various bending modes, not just involving the CH– and OH stretching modes. Furthermore, even in the two-dimensional subspace the effective coupling constants k1112 and k1222 characterizing the resonance are not identical with the anharmonic potential constants C1112 and C1222 in the Taylor expansion of the potential, but rather an expansion to sixth order is needed to describe the resonance quantitatively. A similar conclusion holds true with other low order perturbation expansions of the resonance coupling, involving sequences of cubic couplings to other modes. We furthermore predict important resonances between OH stretching and OH bending also involving CH bending modes, which contribute to IVR at higher levels of excitation.
Optical properties of planar colloidal crystals: Dynamical diffraction and the scalar wave approximation
Authors: Mittleman Daniel M. , Bertone Jane F. , Jiang Peng , Hwang Kevin S. , Colvin Vicki L.
Published: July 2002

We present a quantitative comparison between two analytic theories for the propagation of electromagnetic waves in periodic dielectric structures. These theories have both been used extensively in the modeling of optical spectra of colloidal crystals exhibiting photonic band gap behavior. We demonstrate that dynamical diffraction theory is equivalent to the scalar wave approximation, in the limit of small dielectric contrast. This equivalence allows us to place quantitative limits on the validity of dynamical diffraction, relative to the predictions of the more accurate scalar wave theory. We also note that dynamical diffraction is often applied with boundary conditions which neglect the strong interference between the incident and diffracted waves within the periodic medium. These boundary conditions lead to expressions for the transmission spectrum which cannot be generalized to the case of normal-incidence propagation. We provide a corrected form for these expressions, and use them in comparisons with experimental spectra. Excellent agreement between theory and experiment is obtained for the widths of optical stop bands, for both positive and negative values of the dielectric contrast. These are among the first quantitative comparisons between theoretical and experimental optical spectra of colloidal photonic crystals.
State-by-state assignment of the bending spectrum of acetylene at 15 000 cm−1: A case study of quantum-classical correspondence
Authors: Jacobson Matthew P. , Jung Christof , Taylor Howard S. , Field Robert W.
Published: July 2002

Techniques of quantum, semiclassical, and nonlinear classical mechanics are employed to investigate the bending dynamics of acetylene, as represented by a recently reported effective Hamiltonian [J. Chem. Phys. 109, 121 (1998)], with particular emphasis on the dynamics near 15 000 cm−1 of internal energy. At this energy, the classical mechanics associated with the bending system is profoundly different from that at low energy, where normal mode motions (trans and cis bend) dominate. Specifically, at 15 000 cm−1, classical chaos coexists with stable classical motions that are unrelated to the normal mode motions; these high-energy stable bending motions include those that we call “local bend” (one hydrogen bending) and “counter-rotation” (the two hydrogens undergoing circular motion at opposite ends of the molecule), as well as more complicated motions which can be considered hybrids of the local bend and counter-rotation motions. The vast majority of the bending quantum eigenstates near 15 000 cm−1 have nodal coordinates which coincide with the stable periodic orbits, and thus can be assigned semiclassical quantum numbers representing the number of nodes along the stable classical motions.
Dipole and quadrupole polarization in ionic systems: Ab initio studies
Authors: J̈emmer Patrick , Wilson Mark , Madden Paul A. , Fowler Patrick W.
Published: July 2002

The results of electronic structure calculations of the induced dipoles and quadrupoles on an anion in a locally distorted rocksalt crystal are described. Such information is of interest in the construction of ionic interaction potentials and for modeling the dielectric behavior of ionic materials. The systems included in the study are LiF, NaF, KF, LiCl, NaCl, KCl, LiBr, MgO and CaO. The results are used to characterize short-range contributions to the induced multipoles— those not included in a point-charge, point-polarizable ionic model (the “asymptotic” model). It is shown that these short-range effects are large, opposing and sometimes reversing the asymptotic contribution. The representation of the short-range effects in a computationally tractable form, suitable for use in computer simulations, is examined. A pairwise additive form, containing a steplike function of the interionic separation, is found to provide a good representation of the ab initio data for both dipoles and quadrupoles. Furthermore, the parameters involved in the fitted functions transfer from one material to another in a physically transparent and predictable way.
Fluorine atom abstraction by Si(100). I. Experimental
Authors: Tate M. R. , Gosalvez-Blanco D. , Pullman D. P. , Tsekouras A. A. , Li Y. L. , Yang J. J. , Laughlin K. B. , Eckman S. C. , Bertino M. F. , Ceyer S. T.
Published: July 2002

In the interaction of low energy F2 with Si(100) at 250 K, a dissociative chemisorption mechanism called atom abstraction is identified in which only one of the F atoms is adsorbed while the other F atom is scattered into the gas phase. The dynamics of atom abstraction are characterized via time-of-flight measurements of the scattered F atoms. The F atoms are translationally hyperthermal but only carry a small fraction (∼3%) of the tremendous exothermicity of the reaction. The angular distribution of F atoms is unusually broad for the product of an exothermic reaction. These results suggest an “attractive” interaction potential between F2 and the Si dangling bond with a transition state that is not constrained geometrically. These results are in disagreement with the results of theoretical investigations implying that the available potential energy surfaces are inadequate to describe the dynamics of this gas–surface interaction. In addition to single atom abstraction, two atom adsorption, a mechanism analogous to classic dissociative chemisorption in which both F atoms are adsorbed onto the surface, is also observed. The absolute probability of the three scattering channels (single atom abstraction, two atom adsorption, and unreactive scattering) for an incident F2 are determined as a function of F2 exposure. The fluorine coverage is determined by integrating the reaction probabilities over F2 exposure, and the reaction probabilities are recast as a function of fluorine coverage. Two atom adsorption is the dominant channel [P2=0.83±0.03(95%, N=9)] in the limit of zero coverage and decays monotonically to zero. Single atom abstraction is the minor channel (P1=0.13±0.03) at low coverage but increases to a maximum (P1=0.35±0.08) at about 0.5 monolayer (ML) coverage before decaying to zero. The reaction ceases at 0.94±0.11(95%, N=9) ML. Thermal desorption and helium diffraction confirm that the dangling bonds are the abstraction and adsorption sites. No Si lattice bonds are broken, in contrast to speculation by other investigators that the reaction exothermicity causes lattice disorder.
Excited-state reversible geminate recombination with quenching in one dimension
Authors: Kim Hyojoon , Shin Kook Joe , Agmon Noam
Published: July 2002

We obtain the exact Green function for excited-state geminate reversible dissociation with two different lifetimes and quenching in one dimension, in the absence of a potential of interaction. We consider the ground-state and irreversible reactions as special limits, and investigate the long time asymptotic behavior in comparison with that observed in three dimensions.
Single molecule thermal rotation and diffusion: Acetylene on Cu(001)
Authors: Lauhon L. J. , Ho W.
Published: July 2002

A variable temperature scanning tunneling microscope was used to directly observe the thermally induced rotation of a single acetylene molecule between two equivalent orientations on Cu(001) above 68 K. Measurements of the rotation rate as a function of temperature yielded an energy barrier of 169±3 meV and a preexponential factor of 1011.8±0.2 s−1. Thermal diffusion of individual acetylene molecules was monitored above 178 K by single molecule tracking. A diffusion barrier of 0.53±0.01 eV and a preexponential factor of 1013.6±0.2 s−1 were determined by measuring the hopping rate as a function of temperature.
Direct observation of rotational transitions of the CO–CO dimer
Authors: Xu Yunjie , Jäger Wolfgang
Published: July 2002

Measurements of five pure rotational transitions of a mixed isotopomer of the CO–CO van der Waals dimer with a Fourier transform microwave spectrometer in the frequency range from 3 to 19 GHz are reported. For symmetry reasons, pure rotational transitions in the vibrational ground state are not accessible in the symmetric 12C16O–12C16O species, and the mixed 12C16O–13C16O isotopomer was studied instead. The observed lines were identified as belonging to the 12C16O–13C16O dimer by isotopomeric variation of the sample composition, monitoring of the microwave excitation pulse conditions, and comparison of the measured frequencies with those predicted in a recent infrared study. [M. D. Brookes and A. R. W. McKellar, J. Chem. Phys. (submitted).]
On the applicability of the classical nucleation theory in an Ising system
Authors: Shneidman V. A. , Jackson K. A. , Beatty K. M.
Published: July 2002

Large-scale dynamic Monte Carlo simulations of a lattice gas on a 2000×2000 square lattice with a Glauber-type spin flip dynamics were performed. The results are discussed in the light of classical nucleation theory (CNT) which can be fully specified for the problem due to the availability of exact values for the interfacial energy of a large nucleus, known from the Onsager solution. Several alternative (field-theoretic or nonclassical) descriptions were also considered. Special attention was paid to the pre-exponential in the cluster distribution function and to the finite-size corrections to the interfacial energies which are required in order to comply with observations. If taken literally, the CNT produces large errors when predicting either the cluster distribution function or the nucleation rate. However, at intermediate temperatures (up to 0.7 Tc) the correspondence can be substantially improved by considering the low-temperature properties of small clusters and adjusting the pre-exponential. At higher temperatures the CNT is in qualitative disagreement with the simulations. Several explanations for this, including coagulation between clusters, are proposed.
Diffusional behavior of n alkanes in the rotator phase as studied by pulse field-gradient spin-echo H1 NMR method
Authors: Yamakawa Hideyuki , Matsukawa Shingo , Kurosu Hiromichi , Kuroki Shigeki , Ando Isao
Published: July 2002

Self-diffusion coefficients, D, of n-C21H44, n-C23H48, n-C24H50, n-C28H58, n-C32H66, and n-C34H70 were measured in the rotator phase and the liquid phase over a wide range of temperatures by using pulse field-gradient spin–echo H1 NMR method, in order to clarify diffusional behavior of the n-alkanes in the rotator phase. From these experimental results it was found that the self-diffusion coefficients of these n-alkanes in the rotator phase and the liquid phase are decreased with a decrease in temperature with different slopes, and at the transition temperature in going from the liquid phase to the rotator phase do not change with a transitionally large decrease. From the Arrhenius plot of diffusion coefficient against temperature, the activation energies for diffusion of n-C24H50, n-C32H66, and n-C34H70 in the rotator phase were obtained to be 19.9, 33.9, and 35.9 kcal/mol, respectively, and on the other hand those in the liquid phase to be 4.8, 4.8, and 5.1 kcal/mol, respectively. Further, the corresponding values of n-C21H44, n-C23H48, and n-C28H58 in the liquid phase were obtained to be 4.7, 4.6, and 4.9 kcal/mol, respectively. It was found that the activation energy for diffusion of n-alkanes in the rotator phase is increased with an increase in the carbon number and is much larger than that in the liquid phase. Further, it was found that in the rotator phase of a n-C24H50 single crystal, the diffusion coefficients in direction perpendicular and parallel to the long chain axis were determined to be 2.70×10−6 and 1.64×10−6 cm2/s, respectively.
Nondissociative low-energy electron attachment to c-C7F14 and C6F6: Intermediate lifetimes
Authors: Finch C. D. , Parthasarathy R. , Hill S. B. , Dunning F. B.
Published: July 2002

Nondissociative electron attachment to c-C7F14 and C6F6 is investigated by measuring the velocity distribution of K+ ions formed via electron transfer in collisions with velocity selected K(np) Rydberg atoms and by measuring the decay of the product negative ions during passage to a detector. Collisions with C7F14 lead to the production of free electrons and of long-lived (τ>100 μs) C7F14−* ions. The free electrons, which result from autodetachment, are all created within a very short time period (τ≲15 ps) following initial electron capture. Collisions with C6F6 result in the formation of C6F6−* ions that have a broad range of lifetimes against autodetachment. A possible reaction model involving competition between autodetachment and intramolecular vibrational relaxation is discussed.
Ultrafast photodissociation studies of acetyl cyanide and acetic acid and unimolecular decomposition rates of the acetyl radical products
Authors: Owrutsky J. C. , Baronavski A. P.
Published: July 2002

Unimolecular decomposition rates for acetyl radical following the photodissociation of acetyl cyanide and acetic acid near 193 nm have been studied using ultrafast mass-resolved photoionization spectroscopy. In both cases, the parent decays with an instrumentally limited lifetime, while the acetyl radical behaves in a manner consistent with an RRKM mechanism, in contrast to our previous results on acetone. It is necessary to convolute the population distribution with the microcanonical RRKM rates in order to achieve this agreement. We have also undertaken an ab initio study of the excited states of acetyl cyanide to clarify the assignments of these states. The state excited at 193 nm arises from a π→π* transition with a calculated transition velocity dipole moment oriented at an angle of 57° with respect to the C–C≡N bond, resulting in an anisotropy parameter of −0.22. This is in reasonable agreement with the previous data of North et al. [J. Phys. Chem. A 101, 9224 (1997)]. The apparent RRKM behavior of the acetyl radical formed by the photodissociation of acetic acid and acetyl cyanide indicates that acetyl radical produced by the photodissociation of acetone at 193 nm may exhibit “extrinsic non-RRKM” effects, i.e., dynamic bottlenecks or mode specific effects.
Quantum dynamics with dissipation: A treatment of dephasing in the stochastic Schrödinger equation
Authors: Makarov Dmitrii E. , Metiu Horia
Published: July 2002

The behavior of a quantum system coupled to a reservoir is usually described by an equation for the reduced density matrix. If the dephasing and the change of population in these equations can be described phenomenologically, then it is possible to develop a stochastic Schrödinger equation to describe quantum dynamics with dissipation. In this article, we propose a method for treating dephasing, within the stochastic Schrödinger equation, for many-level systems.
FT-ICR study on hydrogenation of niobium cluster cations Nbn+ (n=2–15) in seeded supersonic jet and multiple-collision-induced dissociation of NbnHm+ hydrides
Authors: Vakhtin Andrei B. , Sugawara Ko-ichi
Published: July 2002

Hydrogenation of niobium cluster cations Nbn+ (n=2–15) in a seeded supersonic jet of H2/He and multiple-collision-induced dissociation (MCID) of the resulting NbnHm+ hydrides have been studied using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The nascent NbnHm+ hydrides trapped in the FT-ICR cell have broad m distributions with no apparent prevalence of odd or even m. A pulse of argon applied to the trapped clusters causes a dramatic squeezing of the initial m distribution (through the collision-induced removal of weakly bound H2 molecules), favoring several particular hydrides for each cluster size n, e.g., Nb7H8+, Nb7H11+, and Nb7H12+ for n=7. The maximum m values of these stable hydrides are close to the stoichiometric composition of NbH2 for the clusters with n<13, and approach that of NbH at larger n. The hydrides observed in our experiments are different from the products of the Nbn++H2 reactions performed in the FT-ICR cell at room temperature, which show only even and strongly n-dependent m values. The MCID of the NbnHm+ clusters occurs through the sequential desorption of H2 molecules yielding NbnH+ and Nbn+ as final dissociation products for odd and even m, respectively. Based on the experiments on the MCID of Nb12H18+, an explanation is suggested for different reactivities of the Nb12+ clusters toward H2 in the ICR and fast-flow-reactor experiments.
Comment on “Molecular dynamics simulations in the grand canonical ensemble: Formulation of a bias potential for umbrella sampling” [J. Chem. Phys. 110, 8295 (1999)]
Authors: Mezei Mihaly
Published: July 2002

The cavity-biased grand canonical ensemble Monte Carlo simulation method has been compared with a particular implementation of the grand canonical ensemble molecular dynamics method by examining the fluctuations during simulations of equivalent length. The Monte Carlo run exhibited an order of magnitude faster fluctuation than the molecular dynamics run.
Double-resonance spectroscopy of the high Rydberg states of HCO. IV. Vibrational autoionization dynamics as a function of bending amplitude
Authors: Robinson Jason D. , Foltynowicz Robert J. , Grant Edward R.
Published: July 2002

Double-resonant ionization-detected absorption experiments extend an investigation of the autoionization dynamics of HCO to include the second overtone of the bend. In these experiments, first-photon transitions to the 3pπ 2Π Rydberg state select single rovibrational levels for second-photon scans of vibrationally autoionizing high Rydberg series that converge to specific rovibrational limits of HCO+. Line shapes reflect coupling widths that join discrete states built on vibrationally excited cores with underlying, vibrationally relaxed continua. Scans of series converging to HCO+ with one and two quanta of bend show narrow linewidths, reflecting relatively long autoionization lifetimes. However, for cores excited to the second harmonic of the bend, certain series abruptly broaden, indicating ultrafast decay. The implications of these results are discussed in terms of a qualitative extension of multichannel quantum defect theory to triatomic molecules.
Microsolvation of the methyl cation in neon: Infrared spectra and ab initio calculations of CH3+–Ne and CH3+–Ne2
Authors: Dopfer Otto , Olkhov Rouslan V. , Maier John P.
Published: July 2002

Rotationally resolved infrared photodissociation spectra of the degenerate asymmetric C–H stretch vibration (ν3) of the CH3+–Ne and CH3+–Ne2 ionic complexes have been recorded. The rotational structure and vibrational frequencies are consistent with π-bound cluster geometries, where the Ne ligands are attached to either side of the 2pz orbital of the central C atom of the methyl cation, leading to C3v and D3h symmetric structures for the dimer and trimer. The intermolecular bonds in the ground vibrational state are characterized by averaged separations of Rc.m.=2.30 Å in the dimer and 2.34 Å in the trimer. The origins of the ν3 band are blueshifted by 11.5 and 21.5 cm−1 compared to the monomer frequency, indicating that vibrational excitation is accompanied by a small and additive destabilization of the intermolecular bond. Ab initio calculations at the MP2/aug-cc-pVTZ# level confirm that the π-bound configurations correspond to the global minimum structures for both the dimer (De=958.5 cm−1, Re=2.1347 Å, θe=91.4°) and the trimer (De=745.4 cm−1, Re=2.2322 Å, θe=90°). The calculated intermolecular potential energy surface of the dimer is characteristic for a disk-and-ball complex and reveals significant angular-radial coupling, which accounts for the large discrepancy between the vibrationally averaged and calculated equilibrium intermolecular separations, Rc.m.−Re≈0.17 Å. The comparison of the ionic CH3+–Rg dimers (Rg=He, Ne, Ar, Kr, Xe) with the isoelectronic CH3X molecules (X=H, F, Cl, Br, I) reveals that chemical bonding onsets with Rg=Ar and increases with the size of the Rg atom.
Overtone spectroscopy of methyl C–H stretch vibration in CH3CF2Cl and CH3CFCl2
Authors: Chen X. , Melchior A. , Bar I. , Rosenwaks S.
Published: July 2002

Photoacoustic spectra of the second (3νCH), third (4νCH), and fourth (5νCH) overtones of the methyl C–H stretches in CH3CF2Cl and CH3CFCl2 were measured. The spectra are characterized by a multiple peak structure of partially resolved triplets and quartets with an anomalous linewidth decrease in the 4νCH region. The results are interpreted in terms of a simplified local mode model for C–H stretching vibrations, including also the stretch-deformation Fermi resonances. The model accounts for most spectral features and allows determination of the time scale for vibrational redistribution.
Infrared spectroscopy of ArOH: A direct probe of the Ar+OH X2Π potential energy surface
Authors: Bonn R. Timothy , Wheeler Martyn D. , Lester Marsha I.
Published: July 2002

An infrared-ultraviolet (IR-UV) double-resonance technique has been implemented to obtain the infrared spectrum of ArOH in the vicinity of the fundamental OH stretch at 2.8 μm. A rotationally resolved spectrum of the fundamental OH stretching band of ArOH is observed at 3567.85(1) cm−1 (origin). A combination band, involving both OH stretch and intermolecular bending excitation, is identified at 3577.00(1) cm−1 (origin). The intermolecular energy of the excited bending state provides a direct measure of the anisotropy of the Ar+OH X2Π interaction potential. The rotational structure of the combination band reveals a large splitting between parity components with the same total angular momentum in the excited bending state [0.69(1) cm−1 for J=32]. The experimentally derived parity splitting is compared with previous experimental and theoretical determinations of this parameter. The parity splitting associated with the excited bending state reflects the change in the intermolecular potential when the pπ orbital containing the unpaired electron of OH lies in or out of the ArOH plane.
Thermodynamic properties of lattice polymers: Monte Carlo simulations and mean-field theories
Authors: Buta Dorel , Freed Karl F. , Szleifer Igal
Published: July 2002

Monte Carlo simulations of a lattice polymer melt are used to determine the thermodynamic properties of the system over a range of monomer volume fractions 0⩽φ⩽0.8 and effective temperatures 3.3⩽T*⩽∞. The simulations consider chains of length M=40 and M=100. The thermodynamic quantities analyzed are the chemical potential, the entropy, the specific heat, the isothermal compressibility, the internal energy, and the pressure. Canonical and grand canonical ensemble methods are employed as independent checks of the simulations for the chemical potential and the pressure. The predictions of Flory–Huggins (FH) theory, lattice cluster theory (LCT), and Guggenheim’s random mixing and quasichemical approximations are compared with the simulations. The comparisons greatly extend prior demonstrations of very large errors in the simple FH approximation and show the major improvements provided by Guggenheim’s approximations and the LCT.
Energy decomposition analysis of intermolecular interactions using a block-localized wave function approach
Authors: Mo Yirong , Gao Jiali , Peyerimhoff Sigrid D.
Published: July 2002

An energy decomposition scheme based on the block-localized wave function (BLW) method is proposed. The key of this scheme is the definition and the full optimization of the diabatic state wave function, where the charge transfer among interacting molecules is deactivated. The present energy decomposition (ED), BLW-ED, method is similar to the Morokuma decomposition scheme in definition of the energy terms, but differs in implementation and the computational algorithm. In addition, in the BLW-ED approach, the basis set superposition error is fully taken into account. The application of this scheme to the water dimer and the lithium cation–water clusters reveals that there is minimal charge transfer effect in hydrogen-bonded complexes. At the HF/aug-cc-PVTZ level, the electrostatic, polarization, and charge-transfer effects contribute 65%, 24%, and 11%, respectively, to the total bonding energy (−3.84 kcal/mol) in the water dimer. On the other hand, charge transfer effects are shown to be significant in Lewis acid–base complexes such as H3NSO3 and H3NBH3. In this work, the effect of basis sets used on the energy decomposition analysis is addressed and the results manifest that the present energy decomposition scheme is stable with a modest size of basis functions.
Hydrodynamic slippage inferred from thin film drainage measurements in a solution of nonadsorbing polymer
Authors: Horn Roger G. , Vinogradova Olga I. , Mackay Michael E. , Phan-Thien Nhan
Published: July 2002

Thin film drainage measurements are presented for submicron films of an “ideal elastic” or Boger fluid, which is a high molecular weight polymer solution in a high viscosity solvent. The measurements are made in a surface force apparatus, with the fluid being squeezed between two mica surfaces in a crossed cylinder geometry and the film thickness measured as a function of time to study its drainage behavior. No equilibrium surface forces are detected in this system, indicating that the polymer is nonadsorbing. The effect of fluid elasticity is predicted to make drainage more rapid in a Boger fluid than for the equivalent Newtonian fluid. Qualitatively this is what is observed for films less than 600 nm thick, but the drainage is even more rapid than predicted for the elastic fluid. To account for this, it is suggested that slippage is occurring at the fluid–solid interfaces, and the data is analyzed in terms of a simple slip model. The slip length required to fit the data is in the range 30–50 nm.
Fluorescence lifetimes and predissociation processes in the B̃ 2A′ state of CCH
Authors: Chiang Whe-Yi , Hsu Yen-Chu
Published: July 2002

Fluorescence lifetimes have been measured under supersonic jet conditions for various N, K-rotational levels of three vibrational levels (T, T+ν2, and T+ν3) of the B̃ 2A′ state of CCH. It was found that the lifetimes of levels T+ν2 and T+ν3 are significantly shorter than that of level T. Furthermore, the lifetimes vary in a complicated way from one rovibronic level to another, indicating the presence of local perturbations. The previously reported CC–H bond energy ranges from 95.6 to 116.3 kcal/mol, which leads to a large uncertainty in assigning the onset of the dissociation process, CCH (X̃ 2Σ+)→C2(X 1Σg+)+H(2S1/2). In this work, fluorescence excitation spectra of photofragment C2 via D 1Σu+←X 1Σg+ and d 3Πg←a 3Πu have been recorded to determine the predissociation processes of the B̃ state. Assuming no barrier is present in the dissociation process of the X̃ and Ã states, the upper bound of D00(CC–H) was estimated as 39 388±7 cm−1(≃112.62 kcal/mol). Hence, the lifetime quenching phenomenon observed in the B̃ state is primarily due to predissociation via the dissociative continuum of either the X̃ or the Ã state of CCH.
Bond-breaking in quantum state selected clusters: Inelastic and nonadiabatic intracluster collision dynamics in Ar–H2O→Ar+H(2S)+OH(2Π1/2,3/2±;N)
Authors: Votava Ondrej , Plusquellic David F. , Myers Tanya L. , Nesbitt David J.
Published: July 2002

High-resolution vibrationally mediated IR+UV photodissociation methods are used to investigate the dynamics of H–OH bond breaking in quantum state selected H2O and Ar–H2O van der Waal complexes prepared in a slit supersonic jet expansion. This capability is based on the following strategy: (i) Specific rovibrational quantum states of the Ar–H2O dimer and H2O monomer are optically selected in the second overtone (vOH=3←0) region with an injection seeded, Fourier transform limited (Δν≈160 MHz) optical parametric oscillator. (ii) Selective H–OH bond cleavage of the vibrationally excited H2O subunit in the cluster is achieved by 248 nm or 222 nm UV photolysis. (iii) Multibody collision dynamics between the H, OH, and Ar photofragments are probed via laser induced fluorescence (LIF) on the asymptotic OH rotational, lambda-doublet and spin–orbit distributions. Comparison between cluster (Ar–H2O) and monomer (H2O) data explicitly samples the influence of the Ar “solvent” on the UV photolysis dynamics and in particular highlights the dominant role of intracluster collisions as the fragments recoil. Most importantly, the OH fine structure distributions are found to be dramatically different for Ar–H2O vs H2O photolysis, indicating the major contribution of nonadiabatic events in the photofragmentation dynamics.
Structure determination of propyne and 3,3,3-trifluoropropyne on Cu(111)
Authors: Toomes R. L. , Lindsay R. , Baumgärtel P. , Terborg R. , Hoeft J.-T. , Koebbel A. , Schaff O. , Polcik M. , Robinson J. , Woodruff D. P. , Bradshaw A. M. , Lambert R. M.
Published: July 2002

The technique of scanned-energy photoelectron diffraction has been used to obtain detailed information on the adsorption site and internal structure of propyne (CH3CCH) and 3,3,3-trifluoropropyne (CF3CCH) on Cu(111). Both molecules are bound to the surface via the acetylenic unit in a site analogous to that previously found for ethyne on Cu(111). In particular, the acetylenic unit is parallel to the surface in a cross-bridging position such that one of the C atoms is above a fcc hollow site while the other is above a hcp hollow site, giving a C–C bond length of 1.47 Å. The methyl/trifluoromethyl group is strongly tilted away from the surface and is attached with equal probability to the C atoms in the fcc and hcp hollow sites. The molecular plane is perpendicular to the surface. The fluoro group of CF3CCH is preferentially oriented in the cis conformation. From the LEED pattern and the calculated coverage of 0.25 ML, a possible overlayer arrangement for propyne and 3,3,3-trifluoropropyne on Cu(111) has been proposed. For this overlayer the orientation of the F3 group can be explained by intermolecular steric repulsion.
Quantitative measurement of guest–host interactions in supramolecular systems: A comparative Brillouin scattering study of the Dianin’s compound clathrand and two of its isostructural clathrates
Authors: Sandstedt C. A. , Michalski D. , Eckhardt C. J.
Published: July 2002

Single, isomorphous crystals of the guest-free clathrand of Dianin’s compound and two of its clathrates containing two ethanols and a heptanol, respectively, have been studied by Brillouin scattering to obtain their complete elastic constant tensors. The values found for individual elastic constants are related to microscopic interactions of the guests and host cage using the clathrand as the reference. Direct correlation of the interference of the guest heptanol molecule with the hexamer hydrogen-bonded network of the host cage to reduced stiffness of this clathrate is demonstrated. In contrast, the ethanol-containing clathrate is demonstrated to have negligible interaction of the guests on the host cage hydrogen bonding. Additionally, the ethanol guests are shown to provide a buttressing effect that increases the stiffness of the guest–host system in specific directions. The results are shown to be consistent with previous thermal conductivity and solid state NMR studies on these systems. The results demonstrate the use of macroscopic elastic constants in understanding the nature of the microscopic interactions in supramolecular crystals.
The optimized effective potential and the self-interaction correction in density functional theory: Application to molecules
Authors: Garza Jorge , Nichols Jeffrey A. , Dixon David A.
Published: July 2002

The Krieger, Li, and Iafrate approximation to the optimized effective potential including the self-interaction correction for density functional theory has been implemented in a molecular code, NWChem, that uses Gaussian functions to represent the Kohn and Sham spin–orbitals. The differences between the implementation of the self-interaction correction in codes where planewaves are used with an optimized effective potential are discussed. The importance of the localization of the spin–orbitals to maximize the exchange-correlation of the self-interaction correction is discussed. We carried out exchange-only calculations to compare the results obtained with these approximations, and those obtained with the local spin density approximation, the generalized gradient approximation and Hartree–Fock theory. Interesting results for the energy difference (GAP) between the highest occupied molecular orbital, HOMO, and the lowest unoccupied molecular orbital, LUMO, (spin–orbital energies of closed shell atoms and molecules) using the optimized effective potential and the self-interaction correction have been obtained. The effect of the diffuse character of the basis set on the HOMO and LUMO eigenvalues at the various levels is discussed. Total energies obtained with the optimized effective potential and the self-interaction correction show that the exchange energy with these approximations is overestimated and this will be an important topic for future work.
Theoretical analysis of singlet–triplet energy splitting generated by charge-transfer interaction in electron donor–acceptor radical pair systems
Authors: Kobori Yasuhiro , Akiyama Kimio , Tero-Kubota Shozo
Published: July 2002

A theoretical analysis was performed for a prediction of the singlet–triplet energy splitting (J) of electron donor–acceptor radical pairs on the basis of the electron transfer reaction theory. It has been strongly indicated that charge-transfer interaction dominates the J in condensed phase photoinduced electron transfer systems.
Different acoustic wave effects of thickness-extension and thickness-shear mode resonance oscillation on catalytic activity and selectivity of thin Pd and Ag films deposited on ferroelectric LiNbO3 single crystals
Authors: Saito N. , Inoue Y.
Published: July 2002

The two representative vibration modes, thickness-extension and thickness-shear, of resonance oscillation were employed to examine the effects of lattice vibration of acoustic waves on the activity and the selectivity of catalytic reactions. Thickness-extension mode resonance oscillation dramatically increased the activity of acetaldehyde production in ethanol oxidation on Pd and enhanced the selectivity for ethylene production in ethanol decomposition on Ag; only ethylene production was accelerated without changes in acetaldehyde production. On the other hand, thickness-shear mode resonance oscillation promoted neither oxidation nor decomposition of ethanol. From the comparison of lattice displacement, large lattice displacement vertical to the surface is shown to be useful for the activation of catalysis by the metal surfaces.
Rotationally inelastic collisions of OH(X 2Π)+Ar. II. The effect of molecular orientation
Authors: van Beek M. C. , ter Meulen J. J. , Alexander M. H.
Published: July 2002

Orientation effects in inelastic collisions of OH(X 2Π)+Ar at an energy of 746 cm−1 have been studied in a crossed molecular beam experiment. The OH(X 2Π) radicals were prepared in the v=0, Ω=32, J=32, f state by hexapole state selection and oriented with their O-end or H-end towards the Ar atom by a static electric field in the collision zone. The orientation-dependent probability density function has been determined by laser induced fluorescence spectroscopy using a narrow band dye laser and the Stark mixing induced P1′(1) transition. A degree of orientation of 〈cos θ〉=0.55 has been obtained. The steric asymmetry factor has been determined for rotational excitation up to the Ω=32, J=92 and Ω=12, J=52 states. Differences up to a factor of 8 in cross section were found between O-end and H-end impact. In general a slight preference for O-end impact was found for low rotational excitation, while a strong preference for H-end impact was found for excitation to high rotational states. The experimental results are compared to quantum scattering calculations on ab initio potential energy surfaces developed by Degli-Esposti and Werner [J. Chem. Phys. 93, 3351 (1990)] and Klos et al. [J. Chem. Phys. 112, 4952 (2000)]. The agreement between experiment and calculations based on the Klos and Chalasinski potential is excellent for transitions to states of A″ symmetry and good for transitions to states of A′ symmetry.
Dynamic nuclear polarization of diamond. III. Paramagnetic electron relaxation times from enhanced C13 nuclear magnetic resonance signals
Authors: Reynhardt Eduard C. , High Grant L.
Published: July 2002

It is shown that by varying pulse lengths and delay times in electron spin resonance microwave pulse sequences, designed to enhance C13 nuclear magnetic resonance signals in diamond, and measuring the resulting C13 nuclear magnetic resonance signal, the paramagnetic impurity concentration, electron spin–spin and spin-lattice relaxation times in the laboratory and rotating frames, as well as the electron-nuclear interaction time, can be obtained. It is also shown that the maximum C13 polarization at a specific microwave frequency is dependent on the electron spin-lattice relaxation times in the laboratory and rotating frames.
Surface-aligned photochemistry: Photodissociation of H2S adsorbed on LiF(001) studied by Rydberg-atom time-of-flight spectroscopy
Authors: Giorgi Javier B. , Kühnemuth Ralf , Polanyi John C.
Published: July 2002

Photolysis of H2S adsorbed on LiF(001) was studied by means of hydrogen Rydberg-atom time-of-flight spectroscopy. The H-atom translational energy distributions using 193.3 nm radiation showed three main features: a high energy channel attributed to H atoms escaping the surface without collisions (DIR); a vibrational progression from v=0 to v=5 [EL(v=0) and INEL(v>0)]; and a low energy channel attributed to multiple inelastic collisions [INEL(2)]. The vibrational progression can be attributed to two concurrent processes: the primary production of vibrationally excited SH in the dissociative event, and a secondary T→V energy transfer between the outgoing translationally hot H atom and a neighboring H2S(ad) molecule. The coverage dependence and angular distribution of the DIR, EL, and the INEL(v>0) channels has been used to define a minimum set of required H2S(ad) geometries.
Long-time scale spectral diffusion in polymer glass
Authors: Müller J. , Maier H. , Hannig G. , Khodykin O. V. , Haarer D. , Kharlamov B. M.
Published: July 2002

Spectral diffusion (SD) in polymethylmethacrylate (PMMA) doped with free-base tetraphenylporphine is investigated at 0.5–4.2 K on a time scale of 3–106 s via optical hole burning. Two contradictory results—within the framework of the two level system (TLS) model—are obtained. The first one is the absence of aging effects at temperatures near 4 K which puts the upper limit for the TLS relaxation times at tens of minutes. The second one is an intensive superlogarithmic SD on the whole time scale of the experiment, which is evidence for the presence of very slow relaxations, independent of the sample history on a time scale of up to two months. The results presented provide clear evidence of a deviation of SD behavior from the TLS model predictions at moderately low temperatures. The concept of structural relaxations is applied for a qualitative interpretation of the experimental data.