bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectroscopy of pendular states: Determination of the electric dipole moment of ICl in the X 1Σ+(v″=0) and A 3Π1(v′=6–29) levels
Authors: Durand A. , Loison J. C. , Vigué J.
Published: July 2002

In this paper, we describe new measurements of the electric dipole moment of ICl A and X states. These measurements are deduced from a Doppler free laser induced fluorescence spectroscopy of the A-X transition and concern the X v′′=0 and 17 vibrational levels of the A state. A complete modelization of the spectra has been developed and the accuracy of these measurements is rather high. We have been able to invert in an approximate manner the vibrational dependence of the A state dipole moment to get its dependence with the internuclear distance r. The observed dependence at large r is well rationalized by the model of an ionic–covalent avoided crossing, and appears to be very close to the result of an ab initio calculation of the same state of IF molecule.
Vibrational energy relaxation of the cyanide ion in water
Authors: Hamm Peter , Lim Manho , Hochstrasser Robin M.
Published: July 2002

The vibrational relaxation time of the cyanide ion in H2O and in D2O was measured by IR-pump–IR-probe experiments. The isotopic composition of the ion was varied in order vary the oscillation frequency of the CN− vibrational mode. In D2O, the vibrational relaxation rate is accelerated from 120 to 71 ps when increasing the vibrational frequency from 2004 cm−1 (13C15N) to 2079 cm−1 (12C14N−). In H2O, time constants between 31 and 28 ps were observed. The systematic dependence of the relaxation rates on the vibrational frequency provides a small portion of the friction spectrum. A significant correlation between vibrational relaxation time of the solute and the IR absorption cross section of the solvent was found, providing experimental evidences for a dominating contribution to vibrational relaxation of Coulomb interactions and the importance of coupling to internal solvent modes. In addition, the infrared bandwidths and the orientational diffusion times are reported. All experimental observables T1, T2, and τ2R related to the vibrational transitions of CN− are now experimentally available and can be compared with model theoretical calculations.
The reactions of silver clusters with ethylene and ethylene oxide: Infrared and photoionization studies of Agn(C2H4)m, Agn(C2H4O)m and their deuterated analogs
Authors: Koretsky Geoffrey M. , Knickelbein Mark B.
Published: July 2002

Ethylene and ethylene oxide react readily with silver clusters at 70 K to form the addition complexes, Agn(C2H4)m and Agn(C2H4O)m, respectively. The infrared photodissociation spectra of Agn(C2H4)m and Agn(C2D4)m (n=3–7) recorded in the 9–11 μm region show several characteristic vibrational bands of ethylene lying near their gas phase frequencies. Photoionization spectroscopy studies reveal that the ionization potentials (IPs) of the complexes decrease monotonically with adsorption of additional ethylene molecules. Together, these results imply that as on macroscopic silver surfaces, ethylene adsorbs molecularly to small silver clusters, with a net donation of electron density into the underlying cluster. Similarly, silver cluster–ethylene oxide complexes display IPs that decrease with increasing adsorbate coverage. The infrared depletion spectra of Agn(C2D4O)m complexes reveal a single feature at 949 cm−1, assigned to the ν4(a1) fundamental of C2D4O. These results verify that ethylene oxide adsorbs molecularly to silver clusters, with the oxygen atom oriented toward the silver cluster.
Femtosecond two-dimensional spectroscopy from anharmonic vibrational modes of molecules in the condensed phase
Authors: Okumura K. , Tanimura Y.
Published: July 2002

We have developed a theory of the fifth-order off-resonant spectroscopy to study the effect of anharmonicity of molecular vibrational modes. The anharmonicity, as well as nonlinear dependence of polarizability on nuclear coordinates, can be the origin of the fifth-order Raman signal. A profile of the signal varies depending on the relative importance of the two effects—the anharmonicity and the nonlinearity. The anharmonicity of a potential can be distinguished from the other effects such as the nonlinearity or the inhomogeneity of vibrational modes. In order to carry out calculations analytically, we employ the multimode Brownian oscillator model and treat anharmonicity as perturbation to the harmonic vibrational modes. A simple analytical expression for the fifth-order polarization is obtained through a diagrammatic technique, called Feynman rule on the unified time path. Physical pictures for the analytical expression are given for a single mode system through numerical calculations and through double-sided Feynman diagrams. Applications to CHCl3 and CS2 are made where the third-order experiments are used to extract parameters. In the CS2 case, the theoretical fifth-order signals are compared with recent experiment, which suggests some sign of anharmonicity.
Quantum-mechanical scattering of an atom from a rigid hemisphere on a flat surface
Authors: Choi B. H. , Tang K. T. , Toennies J. P.
Published: July 2002

The scattering of a monoenergetic beam of atoms by a hard hemisphere on a hard flat surface is formulated quantum mechanically. Differential and integral cross sections are calculated as a function of incident beam energy and incident angle and compared with the familiar results of scattering from a free hard sphere. Two types of interference processes are identified. In addition to the previously discussed off-specular reflection symmetry interferences, Rayleigh oscillations are found in the integral cross sections. They result from the interference of specular scattering from the hemisphere and from the flat surface. Their phases are found to be independent of the angle of incidence when plotted versus the normal component of the incident wave vector.
Vibrational dephasing mechanisms in liquids and glasses: Vibrational echo experiments
Authors: Rector K. D. , Fayer M. D.
Published: July 2002

Picosecond vibrational echo studies of the asymmetric stretching mode (2010 cm−1) of (acetylacetonato)dicarbonylrhodium(I) [Rh(CO)2acac] in liquid and glassy dibutyl phthalate (DBP) (3.5 K to 250 K) are reported and compared to previous measurements of a similar mode of tungsten hexacarbonyl [W(CO)6]. The Rh(CO)2acac pure dephasing shows a T1 dependence on temperature at very low temperature with a change to an exponentially activated process (ΔE≅400 cm−1) above ∼20 K. There is no change in the functional form of the temperature dependence in passing from the glass to the liquid. It is proposed that the T1 dependence arises from coupling of the vibration to the glass’s tunneling two level systems. The activated process arises from coupling of the high-frequency CO stretch to the 405 cm−1 Rh–C stretch. Excitation of the Rh–C stretch produces changes in the back donation of electron density from the rhodium dπ orbital to the CO π* antibonding orbital, shifting the CO stretching transition frequency and causing dephasing. In contrast, W(CO)6 displays a T2 dependence below Tg in DBP and two other solvents. Above Tg, there is a distinct change in the functional form of the temperature dependence. In 2-methylpentane, a Vogel–Tammann–Fulcher-type temperature dependence is observed above Tg. It is proposed that the triple degeneracy of the T1u mode of W(CO)6 is broken in the glassy and liquid solvents. The closely spaced levels that result give rise to unique dephasing mechanisms not available in the nondegenerate Rh(CO)2acac system.
The adsorption of sulfur on Rh(111) and Cu/Rh(111) surfaces
Authors: Rodriguez José A. , Chaturvedi Sanjay , Kuhn Mark
Published: July 2002

The reaction of S2 with Rh(111) and Cu/Rh(111) surfaces has been investigated using synchrotron-based high-resolution photoemission, thermal desorption mass spectroscopy and ab initio self-consistent-field calculations. At 100 K, the adsorption of S2 on Rh(111) produces multilayers of Sn species (n=2–8) that desorb between 300 and 400 K, leaving a film of RhSx on the sample. S2 dissociates upon adsorption on clean Rh(111) at 300 K. An adsorption complex in which S2 is bridge bonded to two adjacent Rh atoms (Rh–S–S–Rh) is probably the precursor state for the dissociation of the molecule. The larger the electron transfer from Rh(111) into the S2(2πg) orbitals, the bigger the adsorption energy of the molecule and the easier the cleavage of the S–S bond. On Rh(111) at 300 K, chemisorbed S is bonded to two dissimilar adsorption sites (hollow and probably bridge) that show well separated S 2p binding energies and different bonding interactions. Adsorption on bridge sites is observed only at S coverages above 0.5 ML, and precedes the formation of RhSx films. The bonding of S to Rh(111) induces a substantial decrease in the density of d states that the metal exhibits near the Fermi level, but the electronic perturbations are not as large as those found for S/Pt(111) and S/Pd(111). Cu adatoms significantly enhance the rate of sulfidation of Rh(111) through indirect Cu↔Rh↔S2 and direct Cu↔S–S↔Rh interactions. In the presence of Cu there is an increase in the thermal stability of sulfur on Rh(111). The adsorption of S2 on Cu/Rh(111) surfaces produces CuSy and RhSx species that exhibit a distinctive band structure and decompose at temperatures between 900 and 1100 K: CuSy/RhSx/Rh(111)→S2(gas) +Cu(gas)+S/Rh(111).
Vibronic dependence of the B̃ state lifetimes of CH3I and CD3I using femtosecond photoionization spectroscopy
Authors: Baronavski A. P. , Owrutsky J. C.
Published: July 2002

Tunable deep UV femtosecond photoionization spectroscopy with single photon excitation and wavelengths longer than 192 nm has been used to determine predissociation-mediated excited state lifetimes for many vibronic levels of the B̃ (6s[2] Rydberg) state of CH3I and CD3I. These include states with vibrational excitation in the ν1, ν2, ν3, and ν6 modes. We have previously reported lifetime measurements for the origin bands [Chem. Phys. Lett. 222, 335 (1994)]. The vibronic and isotopic dependences presented here qualitatively agree with various aspects of results from two indirect measurements. Our results corroborate the counterintuitive result from the resonance Raman work by Wang and Ziegler [J. Chem. Phys. 95, 288 (1991)] that the level singly excited in the C–I stretching mode (31) dissociates more slowly (we measure ∼4.0 ps for both CH3I and CD3I) than the vibrationless levels (1.38 and 1.90 ps, respectively). In contrast to the resonance Raman results and similar to those from resonance enhanced multiphoton ionization linewidth studies by Syage [Chem. Phys. Lett. 212, 124 (1993)], we find a faster predissociation rate upon excitation in the ν6 mode. The lifetimes are considerably longer than those measured for the higher lying 6p and 7s Rydberg states by femtosecond mass-resolved photoionization studies by Janssen et al. [Chem. Phys. Lett. 214, 281 (1994)]. In that case, a faster dissociation rate was measured for the 31 compared to the vibrationless level. This work provides further evidence of the multidimensional nature of the B̃ state predissociation mechanism and an opportunity to critically test high level calculations of the Rydberg state photodissociation dynamics.
Bonding and structure of Y3C2 and Y3C2+
Authors: Yang Dong-Sheng , Zgierski Marek Z. , Hackett Peter A.
Published: July 2002

Vibrational frequencies of yttrium normal modes of triyttrium dicarbide neutral and cation have been determined from pulsed field ionization-zero electron kinetic energy photoelectron spectra. The yttrium stretching mode has a frequency of 228 cm−1 in the ion. The two deformation modes have frequencies of 82 and 24 cm−1 in the neutral and a degenerate frequency of 86 cm−1 in the ion. Geometries and ground electronic states have been determined by the comparison of the experiments and theoretical calculations. The cluster has a trigonal bipyramid structure with the ion symmetry of D3h and the neutral C2ν. The ground state is identified as 2B1 for the neutral and 1A1′ for the ion. While the Y–Y bonding has a covalent nature, the Y–C bonding has considerable ionic character. Both the Y–Y and Y–C bonds are stronger in the ion than in the neutral. The enhancement of the Y–Y bonding upon ionization is largely due to the removal of the antibonding 5s character from the yttrium atoms and the Y–C due to the increase of the electrostatic attractions between the yttrium and carbon atoms.
Positronium chemistry by quantum Monte Carlo. I. Positronium-first row atom complexes
Authors: Bressanini Dario , Mella Massimo , Morosi Gabriele
Published: July 2002

The stability of the ground and excited states of Positronium–atom complexes [A, Ps], Ps =[e+,e−], has been explored for A=Li, B, C, O, F using variational and diffusion Monte Carlo techniques. From the numerical results of our simulations it turns out that the ground state of the complexes [Li, Ps]2S, [C, Ps]3S, [O, Ps]1P, and [F, Ps]2S is stable against the break up in the two neutral fragments A and Ps, while the ground state of [B, Ps]2P has an energy above the same dissociation threshold. As to the excited states, the only possible candidate, [F, Ps]2P, has a total energy statistically equal to the lower dissociation threshold, i.e. it does not seem to be stable against the dissociation.
Guided ion beam studies of the reactions of Crn+ (n=2–18) with O2: Chromium cluster oxide and dioxide bond energies
Authors: Griffin James B. , Armentrout P. B.
Published: July 2002

The kinetic energy dependence of the reactions of Crn+ (n=2–18) with O2 are studied in a guided ion beam mass spectrometer. A variety of CrmO2+, CrmO+, and Crm+ product ions, where m⩽n, are observed, with the dioxide cluster ions dominating the products for all larger reactant cluster ions. Reaction efficiencies are near unity for all but the smallest clusters. The energy dependence of the product cross sections is analyzed in several different ways to determine thermochemistry for both the first and second oxygen atom binding to chromium cluster ions. These values show little dependence on cluster size for clusters larger than three atoms. The trends in this thermochemistry are discussed and compared to bulk phase oxidation values.
Micro- and macrophase separation in blends of reversibly associating one-end-functionalized polymers
Authors: Huh J. , ten Brinke G.
Published: July 2002

Phase diagrams for reversibly associating one-end-functionalized chain molecules (with an emphasis on hydrogen bonding) are determined by computer simulations of a cubic lattice model employing canonical and grand canonical Monte Carlo methods. Due to the relatively short chain lengths used, the stability of the homogeneous state is strongly enhanced compared to mean-field random phase approximation predictions. Characteristic phenomenon such as reappearing phases and macrophase separation into two phases, at least one of which is microphase separated, are observed and discussed.
Theoretical study of the rovibrational energy spectrum and the numbers and densities of bound vibrational states for the system HCO+/HOC+
Authors: Mladenović Mirjana , Schmatz Stefan
Published: July 2002

Large-scale calculations of accurate energy levels for the system HCO+/HOC+ in its electronic ground state are reported. The rovibrational levels are calculated for total angular momentum J=0, 1, and 2 by means of the discrete variable representation of the angular coordinate in conjunction with a distributed Gaussian basis for the radial degrees of freedom. A new analytical potential energy surface is used which is based on high level ab initio calculations [CCSD(T)/cc-pVQZ]. The rovibrational energy spectra of HCO+ and HOC+, as well as of the isotopomers DCO+ and DOC+, are analyzed in detail up to the ground state adiabatic isomerization barrier at 28 798 cm−1. Spectroscopic parameters calculated for low lying vibrational states show distinct differences between HCO+ and HOC+. A total number of 6042 bound states up to the first classical dissociation limit (H++CO) at 51 621 cm−1 and a density at the threshold of 0.52/cm−1 are estimated for J=0. Semiclassical phase space integration yields nearly identical results for the numbers and densities of bound states for the given surface. An empirical model employing Morse oscillators, an explicit treatment of the hindered rotation, and stretch-bend coupling is used to simulate the quantum mechanical data.
Extended capillary wave theory and the ellipsometric coefficient
Authors: Stecki J.
Published: July 2002

The extended capillary wave theory (ECW) proposed by Robledo et al. and formulated in terms of the unknown direct correlation function C of the two-phase system with a planar interface between liquid and its vapor, is applied to our simulation data on the full inhomogeneous two-point correlation function H from which C is also obtained. The required projection, C̄(q), is shown and discussed. The coefficient of q4, the apparent bending coefficient, was inequivocally negative, in agreement with the view that the true bending coefficient may be absent in one-component interfaces. The ellipsometric coefficient diverged and its calculation still required an ultraviolet cutoff. It is thus demonstrated that ECW is but the first step and further terms, some discovered earlier, as well as mode–mode coupling, are required. These are discussed.
Bimodality of the viscoelastic response of a dense liquid and comparison with the frictional responses at short times
Authors: Bhattacharyya Sarika , Bagchi Biman
Published: July 2002

While the time dependence of the friction on a tagged particle in a dense liquid has been investigated in great detail, a similar analysis for the viscosity of the medium and the interrelationship between the two has not been carried out. This is despite the close relation always assumed, both in theoretical and experimental studies, between friction and viscosity. In this article a detailed study of the time and frequency dependencies of the viscosity has been carried out and compared with those of the friction. The analysis is fully microscopic and is based on the mode coupling theory (MCT). It is found that for an argonlike liquid near its triple point, the initial decay of the viscosity occurs with a time constant of the order of 100 fs, which is close to that of the friction. The frequency dependent viscosity shows a pronounced bimodality with a sharp peak at the low frequency and a broad peak at the high frequency; the usually employed Maxwell’s relation fails to describe the peak at the high frequency. A surprising result of the present study is that both the bare and the MCT values of viscosity and friction individually sustain a ratio which is close to the value predicted by the Stokes relation, even when Navier–Stokes hydrodynamics itself seems to have little validity.
Effect of inert sites on the kinetic oscillations in the catalytic CO oxidation on Pt(100)
Authors: Chávez Francisco , Vicente Luis , Perera Aurélien , Moreau Michel
Published: July 2002

The effect of inert sites in the global oscillations in the oxidation of CO on Pt(100) for both random and clustered inert sites is simulated by use of the cellular automaton technique. The cellular automaton rules account for the structural phase transformations of the Pt substrate, the reaction kinetics of the adsorbed phase and diffusion of adsorbed species. The introduction of a fraction θd of inert sites reduces the extent of the oscillatory region on the bifurcation diagram. The effect of added impurities on the transition from oscillatory to CO poisoned state is found to be quite different depending on the initial position on the bifurcation diagram. Points located near the rightmost branch on the bifurcation diagram will reach inert state through a very abrupt transition, whereas points located far from it undergo a much softer transition. Clustered impurities tend to soften the transition, and higher values of θd are needed to trigger the transition to the poisoned state. The study of a mean-field model without the oscillatory kinetics explains some of the basic features of the effect of increasing densities of impurities on the catalytic surface, for instance the dependence of the location of the transition point θd on the adsorption rate of CO.
Quantum stereodynamics of the Li+HF(v,j) reactive collision for different initial states of the reagent
Authors: Lara Manuel , Aguado Alfredo , Roncero Octavio , Paniagua Miguel
Published: July 2002

The effect of the reagent initial state excitation on the reactive cross section in the Li+HF(v,j) collision is analyzed for v=0, 1 and j=0, 1, 2 and 3. A wave packet treatment is used within the centrifugal sudden approximation on a global potential energy surface recently proposed [Aguado et al., J. Chem. Phys. 107, 10085 (1997)]. The reaction cross-section for v=0 is in good agreement with the available experimental data, and for low j shows oscillations as a function of the translational energy which are due to the structure of the transition state. For v=1 the reaction cross-section increases by a factor of 10–50 with respect to that of v=0. The influence of the alignment of the initial angular momentum on the reaction cross section is studied.
Solid–fluid equilibrium for a molecular model with short ranged directional forces
Authors: Vega C. , Monson P. A.
Published: July 2002

The phase diagram of a system of hard spheres with short-range tetrahedral association has been determined by computer simulation and theory. The fluid phase and two solid phases were considered. One of these solid phases is a low-density solid closely related in structure to ice Ic and the other is a high-density solid closely related in structure to ice VII. At high temperatures freezing occurs into the high-density solid whereas at low temperatures freezing occurs into the low-density solid. At an intermediate temperature a triple point is found where the fluid coexists with the two solids simultaneously. Although the low-density solid melts to a high-density fluid, this transition is found to be metastable with respect to the transformation into a high-density solid. This is evidence that short-range tetrahedral attractive forces are not in and of themselves sufficient to explain the anomalous melting of water. Our results indicate that vapor–liquid equilibrium for the model is preempted by solidification. Monte Carlo simulation results for the fluid phase are described successfully by Wertheim’s theory whereas those of the solid phases are described qualitatively by the cell theory.
Crystallization of a polymer on a surface
Authors: Doye Jonathan P. K. , Frenkel Daan
Published: July 2002

We have studied the structure and free energy landscape of a semiflexible lattice polymer in the presence of the surface of a polymer crystal. At low temperatures coexistence of two-dimensional integer-folded crystals is observed. As the temperature is increased there is a transition from these crystalline configurations to a disordered coil adsorbed onto the surface. The polymer then gradually develops a three-dimensional character at higher temperatures. We compute the free energy as a function of increasing crystallinity and compare with the free energy profiles assumed by the Lauritizen–Hoffman surface nucleation theory of polymer crystallization. Our free energy profiles exhibit a “sawtooth” structure associated with the successive formation of chain folds. However, in the early stages of crystallization our profiles significantly deviate from those assumed by surface nucleation theory because the initial nucleus is not a single stem but two incomplete stems connected by a fold. This finding has significant implications for the theoretical description of polymer crystallization.
The puzzle of poly(ethylene oxide) aggregation in water: Experimental findings
Authors: Faraone A. , Magazù S. , Maisano G. , Migliardo P. , Tettamanti E. , Villari V.
Published: July 2002

Aqueous solutions of poly(ethylene oxide) were investigated using the ultrasonic technique, photon correlation spectroscopy (PCS) and nuclear magnetic resonance (NMR), in a wide range of molecular weight (from ethylene glycol to poly(ethylene oxide) 4 000 000 Da). Ultrasonic data reveal that the mixing process is not ideal and show that the polymer–water interaction strength increases with the polymerization degree. PCS and NMR, on the other hand, furnish a free particle diffusion coefficient which satisfies a unique scaling law from 8000 to 4 000 000 Da and demonstrates the good solvent nature of water. These experimental findings indicate that polymer–polymer aggregation processes are not an inherent property of these systems.
The unimolecular dissociation of vinylcyanide: A theoretical investigation of a complex multichannel reaction
Authors: Derecskei-Kovacs Agnes , North Simon W.
Published: July 2002

Ab initio molecular orbital calculations were performed toward the determination of the potential energy surface for the unimolecular ground-state dissociation of vinylcyanide. Reaction pathways for the three- and four-center elimination reactions of HCN and H2, as well as migration and radical elimination channels of H and CN, were examined. MP2 gradient geometry optimizations and QCISD(T) single point energy calculations were performed for all the relevant product species and transition states. The results are compared to the analogous unimolecular dissociation of vinylchloride which has been theoretically investigated by Morokuma and co-workers [J. Chem. Phys. 100, 8976 (1994)]. The unimolecular rates for all reaction channels have been calculated using Rice–Ramsperger–Kassel–Marcus (RRKM) theory employing ab initio transition state energies and MP2 vibrational frequencies. Our calculations indicate that the elimination of H2 and HCN preferentially proceed via three-center transition states. We also find that H-migration reactions are rapid, suggesting that substantial H-atom scrambling precedes dissociation. This result is important for the interpretation of several isotopic substitution measurements which are all consistent with exclusive three-center elimination of H2 and HCN.
Improving self-consistent field convergence by varying occupation numbers
Authors: Rabuck Angela D. , Scuseria Gustavo E.
Published: July 2002

One problem commonly encountered in quantum chemistry calculations is the convergence of the self-consistent field (SCF) iteration process. Numerous cases are known in which calculations (both Hartree–Fock and density functional theory), even when using extrapolation techniques, converge extremely slowly or do not converge at all. Many of these cases include molecules that contain transition metals. In this paper, we study two techniques that fractionally occupy orbitals around the Fermi energy during the SCF cycles. These methods use fractionally occupied orbitals to aid in the iterative process, but the occupations at convergence are forced to be ones and zeros. We show how using these fractionally occupied orbitals improves convergence for a number of difficult cases and that there is no significant overhead in the number of SCF cycles for molecules that easily converge with standard techniques.
Quantum proton transfer with spatially dependent friction: Phenol-amine in methyl chloride
Authors: Antoniou Dimitri , Schwartz Steven D.
Published: July 2002

In a recent paper [D. Antoniou and S. D. Schwartz, J. Chem. Phys. 110, 465 (1999)] we calculated the reaction rate for a proton transfer reaction in liquid methyl chloride. In that work, we used a spectral density obtained from a molecular dynamics simulation as input to a quantum Zwanzig Hamiltonian which we solved using our exponential resummation method. In the present paper we perform a similar calculation, allowing for a position dependent friction using the method of G. Haynes, G. Voth, and E. Pollak [J. Chem. Phys. 101, 7811 (1994)]. Compared with the results of our previous work, we found that including spatial dependence to the friction led to enhancement of the reaction rate and to reduction of the H/D kinetic isotope effect.
Variational calculations of rovibrational energies of CH4 and isotopomers in full dimensionality using an ab initio potential
Authors: Carter Stuart , Shnider Heather M. , Bowman Joel M.
Published: July 2002

We report variational calculations of vibrational energies of CH4, CH3D, CH2D2, CHD3, and CD4, using a new version of “Multimode” and a high quality ab initio quartic force field of Lee and co-workers [T. J. Lee, J. M. L. Martin, and P. R. Taylor, J. Chem. Phys. 102, 254 (1995)]. Convergence of the vibrational energies with respect to several numerical aspects of the calculation is examined. The converged vibrational energies are compared with experiment and with previous perturbation theory calculations. Fermi resonance analyses of the fundamentals are presented for all isotopomers. Rovibrational calculations are also reported for CH4 and compared with experiment.
Electronic accessibility of dissociation channels in an amide: N,N-dimethylformamide photodissociation at 193 nm
Authors: Forde Nancy R. , Butler Laurie J. , Abrash Samuel A.
Published: July 2002

Measurement of the photofragment velocity and angular distributions from the photodissociation of N,N-dimethylformamide at 193 nm in its πnbπ* absorption evidences three competing dissociation channels: HCON(CH3)2→HCO(X̃ 2A′)+N(CH3)2(X̃ 2B1); HCO(X̃ 2A′)+N(CH3)2(Ã 2A1); and HCONCH3+CH3. (H atom eliminations are not probed.) These products are formed in a ratio of 0.15±0.04:0.49±0.09:0.36±0.07, determined by use of trimethylamine as a calibrant molecule. Nitrogen–carbonyl bond fission occurs on a rapid time scale with an angular distribution of the dissociation products given by β=1.2±0.2. Excited state N(CH3)2 products are formed quasidiabatically from the initial planar geometry, whereas symmetry-breaking vibrations allow one-electron matrix elements to couple the initial electronic configuration to the ground state N(CH3)2+HCO channel. Competition of nitrogen–methyl bond fission is evidence of the strong coupling between the πnbπ* excitation and the nitrogen–methyl reaction coordinate; ab initio calculations confirm that the electronic excitation is not localized on the N–C=O moiety. We also include here an advance report of the excited state energy of the N(CH3)2(Ã 2A1) radical, which is found to be 1.59 eV.
High-resolution ultraviolet laser spectroscopy on jet-cooled benzene molecules: Ground and excited electronic state polarizabilities determined from static Stark effect measurements
Authors: Okruss M. , Müller R. , Hese A.
Published: July 2002

Static electric polarizabilities of the S0(1A1g) ground electronic and the vibronically excited 61 S1(1B2u) state of benzene were measured by applying ultraviolet (UV) laser Stark spectroscopy. The experimental setup consisted of a frequency doubled tunable narrow-band continuous wave (cw) laser system operating at 259 nm, a molecular beam apparatus, and a capacitor capable of generating strong static and homogeneous electric fields up to 250 kV/cm. Experimental linewidths of less than 30 MHz were achieved for the rotational transitions, and Stark displacements of typically several 10 MHz could be detected. Stark patterns of 16 rotational lines were recorded and analyzed under different field strengths. Validity of second-order perturbation theory was confirmed by the observed effects, and the diagonal components of the polarizability tensors could be adjusted by a weighted least-squares technique. Results were applied to calculating Stark spectra and comparing them to experimental data. Good agreement could be found, except for an obscure singular perturbation within one rotational line. The new polarizabilities were compared to both experimental and theoretical results in the literature. Ground state data are in general agreement, whereas no comparable experimental work addressing the excited electronic state exists. Finally, polarizabilities of the beginning five polyacenes were compiled and compared with the benzene data.
Two-dimensional vibrational spectroscopy. II. Ab initio calculation of the coherent 2D infrared response function of CHCl3 and comparison with the 2D Raman response function
Authors: Park Kisam , Cho Minhaeng , Hahn Sangjoon , Kim Dongho
Published: July 2002

The two-dimensional vibrational spectroscopy involving two infrared (IR) pulses and a single optical pulse, which was theoretically proposed recently [J. Chem. Phys. 109, 10559 (1998)], is studied by using the ab initio calculation method for CHCl3. By obtaining the first- and second-order derivatives of the molecular dipole moment as well as the polarizability, the coherent 2D IR response function and its spectrum are calculated with an assumption that the vibrational dynamics can be described by the Brownian oscillator model. The origin of each peak in the entire coherent 2D IR spectrum is discussed in detail, and is directly compared with the coherent 2D Raman scattering spectrum of CHCl3. This comparison demonstrates the complementary nature between the coherent 2D IR and Raman spectroscopies. A brief discussion on the coupling patterns is also presented.
Structures of hydrogen-bonded clusters of benzyl alcohol with water investigated by infrared-ultraviolet double resonance spectroscopy in supersonic jet
Authors: Guchhait Nikhil , Ebata Takayuki , Mikami Naohiko
Published: July 2002

The structures of the benzyl alcohol and its hydrogen-bonded clusters with water have been investigated by infrared-ultraviolet double resonance vibrational spectroscopy along with ab initio molecular-orbital calculations. Characteristic shifts of the OH stretching vibrations of the benzyl alcohol site as well as the water sites were found, which are quite useful to determine the cluster structures. For bare benzyl alcohol, a planar conformer having no intramolecular hydrogen bond is dominant in the jet. On the other hand, the dominant species becomes a gauche-type conformer in the benzyl alcohol–(H2O)n (n=1–4) hydrogen-bonded clusters. In these clusters, the π-hydrogen bond between the phenyl plane and the OH group of the water cluster site is possible. Detailed intermolecular hydrogen bonding structures are discussed by comparing the observed spectra with the simulated spectra obtained by ab initio calculations.
Energetic and structural features of the CH4+O(3P)→CH3+OH abstraction reaction: Does perturbation theory from a multiconfiguration reference state (finally) provide a balanced treatment of transition states?
Authors: Roberto-Neto Orlando , Machado Francisco B. C. , Truhlar Donald G.
Published: July 2002

The stationary points of the CH4+O(3P)→CH3+OH abstraction reaction have been identified at the fully optimized reaction space (FORS) level. For three sets of geometries (FORS plus unrestricted and restricted-open-shell Møller–Plesset second order perturbation theory), single-point calculations by unrestricted Møller–Plesset fourth order perturbation theory (UMP4), by unrestricted coupled cluster theory with single and double excitations and a quasiperturbative treatment of fourth- and fifth-order triple-excitation terms (CCDS(T)), and by multireference Møller–Plesset second order perturbation theory (MRMP2) were also performed for the classical barrier height and energy of the reaction. Calculations carried out at the MRMP2/cc-pVTZ//FORS/cc-pVTZ level predict values for the forward vibrationally adiabatic barrier height and for the energy of the reaction at 0 K equal to 10.3 and 2.0 kcal/mol, respectively. This is in excellent agreement with experiments that show values of the activation energies in the range of 9–12 kcal/mol (at temperatures below 1500 K) and an energy of reaction equal to 1.8 kcal/mol. Expectation values of Ŝ2, where Ŝ is total electron spin, and also the values the coefficients of the configuration state functions show that the reactants and the products of this reaction are well described by single-configuration reference states but that the transition structure has a much higher multiconfigurational character. We conclude that MRMP2 may provide some light at the end of the tunnel in the long-standing quest for method that includes nondynamical and dynamical correlation in a balanced way in the electronic wave function of open-shell transition states.
Temperature dependent reaction coordinates
Authors: Elber Ron , Shalloway David
Published: July 2002

Temperature-dependent reaction-coordinates are investigated using Brownian dynamics. A functional of the reaction coordinate, which does not have explicit time dependence, is derived. The path that minimizes the functional is defined as the reaction coordinate. The optimal coordinate varies from the steepest descent path at zero temperature to a straight line connecting “reactants” and “products” at high temperatures. An estimate of the time scale of the process is an output of the optimization. A numerical example is provided and adjustments for the Stratonovich calculus are discussed.
Simulation of nonadiabatic wave packet interferometry using classical trajectories
Authors: Donoso Arnaldo , Kohen Daniela , Martens Craig C.
Published: July 2002

In this paper, we describe the application of our recently developed multistate semiclassical Liouville equation method for modeling molecular dynamics on multiple coupled electronic states [C. C. Martens and J.-Y. Fang, J. Chem. Phys. 106, 4918 (1997); A. Donoso and C. C. Martens, J. Phys. Chem. 102, 4291 (1998)] to problems where electronic coherence effects play a dominant role. We consider a model problem involving the simultaneous evolution of wave packets on two coupled electronic states. We analyze the problem qualitatively from both quantum and semiclassical perspectives using perturbation theory, and identify the roles played by coupling strength and relative phase of the initial wave packets. We then perform trajectory-based simulations on a two-state one-dimensional model problem and compare the results with those of exact quantum calculations. In marked contrast with most current methods for modeling nonadiabatic dynamics with classical trajectories, the semiclassical Liouville method is found to be capable of treating even dominant electronic coherence effects in a consistent and accurate manner.
Vibrational energy transfer in OH A 2Σ+ between 195 and 295 K
Authors: Steffens Kristen L. , Crosley David R.
Published: July 2002

Vibrational energy transfer (VET) v′=1→0 and electronic quenching of v′=1 and 0 in the A 2Σ+ electronically excited state of the OH radical has been studied over the temperature range 195 to 295 K. The colliders investigated were N2, O2, and CO2. Laser-induced fluorescence experiments were conducted in a flow cell with photolytic production of OH; both fluorescence intensity and time decay measurements were made. The VET cross sections are found to increase with decreasing temperature, suggestive of attractive force interactions in the VET process.
Broadening, shifting, and line asymmetries in the 2←0 band of CO and CO–N2: Experimental results and theoretical calculations
Authors: Predoi-Cross Adriana , Bouanich J. P. , Benner D. Chris , May A. D. , Drummond J. R.
Published: July 2002

We have measured the room temperature, widths, pressure shifts, and line asymmetry coefficients for many transitions of the first overtone band of CO and CO perturbed by N2. The broadening coefficients were obtained with an accuracy of about 1%. The pure CO profiles have been fitted by a Voigt profile while the CO–N2 spectral profiles have been fitted with a Lorentz and an empirical line shape model (HCv) that blends together a hard collision model and a speed-dependent Lorentz profile. In addition to the Voigt, Lorentz, and HCv models, we have added a dispersion profile to account for weak line mixing. The line broadening and shift coefficients are compared to semiclassical calculations employing a variety of intermolecular interactions. The line asymmetry results are compared to line mixing calculations based on the energy corrected sudden (ECS) model. The results indicate that effects other than line mixing also contribute to the measured line asymmetry.
Temperature dependence of cryochemical H-tunneling reactions
Authors: Ivanov G. K. , Kozhushner M. A. , Trakhtenberg L. I.
Published: July 2002

The temperature dependence of tunneling transition of the atomic particle in solids is studied near absolute zero. The different mechanisms of the temperature dependence are considered. They are the medium reorganization, the potential barrier parameters modulation, and the under-barrier friction The nonadiabatic effects are also considered. The rate constant K is described by formula ln K=ln K0+C4T4+C5T5+C6T6+C8T8 at low temperatures. It was conducted through the comparison of theory with the experimental data from the article of Kumada et al. [Chem. Phys. Lett. 261, 463 (1996)]. It turned out that good agreement takes place if one takes into account the quantum properties of the hydrogen crystal with the assumption of the dominated role of medium reorganization.
Kinematics and dynamics of conformal collineations in relativity
Authors: Mason D. P. , Maartens R.
Published: July 2002

Anisotropic fluids in general relativity that admit a conformal collineation, a generalization of a conformal motion, are considered. By investigating the kinematic properties of such fluids, and then using the field equations, some recent results on the restrictions imposed by a conformal collineation symmetry are generalized.
The Bondi metric and conformal motions
Authors: Di Prisco A. , Herrera L. , Jimenez J. , Galina V. , Ibáñez J.
Published: July 2002

It is shown that there exist no physically significant solutions of the Einstein vacuum field equations, except the trivial one (i.e., Minkowski), for axially and reflection symmetric space-times (Bondi metric) admitting a one-parameter group of conformal motions globally defined on S2.
Factorization of the wave equation in a nonplanar stratified medium
Authors: Weston Vaughan H.
Published: July 2002

The wave equation is considered for a stratified medium where the stratifications are in the form of a family of nested C2 surfaces along which the velocity c is constant (c varying only in a direction normal to the surfaces). On each surface c is constant, the solution u of the wave equation is decomposed into an outgoing wave component u+ and an incoming wave component u−. The associated outgoing and incoming wave conditions are expressed in terms of integral operators (kernels being time-dependent single and double layer potential type terms) operating on u and the normal derivative ∂u/∂n on each surface. Using the decomposition the scalar wave equation is split into a vector system involving the components u+ and u−, the vector system decoupling in a region where c is constant. Such a splitting is useful for the inverse problem where a reflection operator relating the outgoing wave to an incoming wave can be defined, and this in turn can be used to determine c.
Conformal symmetries and fixed points in space-time
Authors: Hall G. S.
Published: July 2002

A general study is made of conformal vector fields on a four-dimensional Lorentz manifold with particular emphasis being laid on the structure of the zeros (critical points) of such vectors fields. The implications for general relativity are investigated and a discussion of conformal vector fields in generalized plane wave space-times is given. An attempt is made to clarify the well-known theorem of Bilyalov and Defrise-Carter.
A search for integrable bilinear equations: The Painlevé approach
Authors: Grammaticos B. , Ramani A. , Hietarinta J.
Published: July 2002

The possibility of the existence of new integrable partial differential equations is investigated, the tools of singularity analysis. The equations treated are written in the Hirota bilinear formalism. It is shown here how to apply the Painlevé method directly under the bilinear form. Just by studying the dominant part of the equations, the number of cases to be considered can be limited drastically. Finally, the partial differential equations identified in a previous work [J. Hietarinta, J. Meth. Phys. 28, 1732, 2096, and 2586 (1987); 29, 628 (1988)] as possessing at least four soliton solutions, are shown to pass the Painlevé test as well, which is a strong indication of their integrability.
Symmetry inheritance of conformal Killing vectors
Authors: Saridakis E. , Tsamparlis M.
Published: July 2002

Kinematics and dynamics in general relativity are clearly defined by considering the kinematical and dynamical variables and the identities and constraints they have to satisfy. Symmetries (collineations) are one important form of constraint that can be classified into various levels from Killing vectors to curvature collineations. A method is presented for writing the field equations for a general spacelike collineation ξa orthogonal to the four-velocity ua of the matter fluid and arbitrary matter. Applications are considered for spacelike conformal Killing vectors and matter described either by a perfect fluid or by an anisotropic fluid. The concept of symmetry inheritance is generalized to the case of a conformal Killing vector (CKV). It is shown that the symmetry of a proper spacelike CKV orthogonal to the four-velocity ua is not inherited by any type of matter.
Asymptotics of gravitational collapse of scalar waves
Authors: Gómez R. , Winicour J.
Published: July 2002

Through a combination of analytic and numerical techniques, the formation of a black hole by a self-gravitating, spherically symmetric, massless scalar field is investigated. The evolution algorithm incorporates a Penrose compactification so that the Bondi mass, the news function, and other radiation zone limits can be obtained numerically. The late time behavior recently established by Christodoulou is confirmed and new asymptotic relations for late time and for large amplitude limits are derived. For example, it is shown, that the Bondi mass MB and the scalar monopole moment Q satisfy the asymptotic relation MB∼ π‖Q‖/√2 at high amplitudes. It is found that the scalar monopole moment decays exponentially during black hole formation in contrast to the perturbation theory result for a power law decay rate in an Oppenheimer–Snyder background. It is demonstrated that the Newman–Penrose constant for the scalar field is globally well defined and has significant effects.
Positon solutions of the sine-Gordon equation
Authors: Beutler R.
Published: July 2002

The concept of positons is introduced for the sine-Gordon equation exploiting its covariance under Darboux transformations. Positon solutions represent potentials in the associated linear system corresponding to a vanishing reflection coefficient and a transmission coefficient identically one. The mutual interaction of positons and the interaction of positons with solitons is considered.
Quadrics on complex Riemannian spaces of constant curvature, separation of variables, and the Gaudin magnet
Authors: Kalnins E. G. , Kuznetsov V. B. , Miller Willard
Published: July 2002

Integrable systems that are connected with orthogonal separation of variables in complex Riemannian spaces of constant curvature are considered herein. An isomorphism with the hyperbolic Gaudin magnet, previously pointed out by one of the authors, extends to coordinates of this type. The complete classification of these separable coordinate systems is provided by means of the corresponding L matrices for the Gaudin magnet. The limiting procedures (or ε calculus) which relate various degenerate orthogonal coordinate systems play a crucial role in the classification of all such systems.
Gauge transformations of constrained KP flows: New integrable hierarchies
Authors: Kundu Anjan , Strampp Walter , Oevel Walter
Published: July 2002

Integrable systems in 1+1 dimensions arise from the KP hierarchy as symmetry reductions involving square eigenfunctions. Exploiting the residual gauge freedom in these constraints new integrable systems are derived. They include generalizations of the hierarchy of the Kundu–Eckhaus equation and higher-order extensions of the Yajima–Oikawa and Melnikov hierarchies. Constrained modified KP flows yield further integrable equations such as the hierarchies of the derivative NLS equation, the Gerdjikov–Ivanov equation, and the Chen–Lee–Liu equation.