bluuebunny/crossref_metadata_2025 huggingface dataset:
State-resolved, three-dimensional product recoil velocity spectroscopy
Authors: Ni Hong , Serafin Joseph M. , Valentini James J.
Published: July 2002

We describe a conceptually and experimentally simple approach for quantum-state-resolved measurement of the full three-dimensional recoil velocity distribution of the products from photodissociation or photoinitiated chemical reaction. The method uses pulsed lasers to determine two components of the recoil velocity vector, by spatial displacement of a probe laser beam relative to a photolysis laser beam, so we call this method POSTS, for position sensitive translational spectroscopy. The third component of the velocity vector is obtained from Doppler selection, ion time-of-flight mass spectrometry, spatial masking of a detector, or use of a one-dimensional array detector. POSTS requires only a single probe laser, and it is not essential that this laser have a narrow frequency bandwidth. Its TOF measurements can be made with very high resolution on a spatial scale as small as 0.1 cm. POSTS will work with all atomic and molecular species having any magnitude of recoil velocity, and with most pulsed-laser detection techniques. We demonstrate the capabilities of the POSTS method by velocity measurements on the H atoms from photodissociation of HI, and HCl molecules from the vibrational predissociation of (HCl)2. In the latter case the high resolution capabilities of POSTS allow a determination of the bond dissociation energy of the HCl dimer to an accuracy of ±1 cm−1 from TOF measurements on a spatial scale of only 0.1 cm.
Non-Markovian Brownian motion in a viscoelastic fluid
Authors: Volkov V. S. , Leonov A. I.
Published: July 2002

A theory of non-Markovian translational Brownian motion in a Maxwell fluid is developed. A universal kinetic equation for the joint probability distribution of position, velocity, and acceleration of a Brownian particle is derived directly from the extended dynamic equations for the system. Unlike the extended Fokker–Planck equation which corresponds to Mori–Kubo generalized Langevin equation and provides only with calculations of one-time moments, the universal kinetic equation obtained gives complete statistical description of the process. In particular, an exact generalized Fokker–Planck equation in the velocity space valid for any time instant is derived for the free non-Markovian Brownian motion. It shows that both the ‘‘master telegraph’’ and the respective kinetic equations, obtained in the molecular theory of Brownian motion, are type of approximations. The long and short time behavior of velocity and force correlations for a free Brownian particle is investigated in the general case of a nonequilibrium initial value problem. A corresponding diffusion equation in the coordinate space, and the generalized Einstein relation between the diffusion coefficient and the mobility are derived.
Effect of ‘‘blockiness’’ on the thermal transition of the terephthalate–adipate copolymers
Authors: Madkour Tarek M.
Published: July 2002

Now, it is well established that sequence distributions in copolymers greatly affect their thermal transitions. Random and block terephthalate–adipate copolymers, for instance, are known to have different thermal transitions while maintaining the same chemical structures. While no theory was able to fully predict the characteristics of the thermal transition of copolymers, modeling techniques have shown good reliance in simulating the copolymeric chains and related physical properties. This is done by generating representative sequences and studying the packing of such sequences into crystalline regions in multichain systems. The quantities of particular interest were the nature of the sequence distribution in random and block copolyesters, the degree of crystallinity, melting points, the standard free energy of fusion, interfacial free energies, and the moduli (calculated on the basis of the crystallites acting as cross-links in an elastomeric structure). The work is also capable of predicting the size and the distribution of the crystals and the minimum sequence length required for crystallization.
Correlated static-exchange interaction for electron–molecule scattering: Case study for LiH and H2
Authors: Pal Sourav , Sabane Sampada C.
Published: July 2002

The correlated static-exchange potential for LiH and H2 molecules has been studied using many-body coupled cluster technique. A general trend has been observed. Its importance to the low energy scattering of electrons from these diatomic targets has been pointed out.
New time-dependent methods in quantum scattering
Authors: Zhu Wusheng , Zhao Xinsheng , Tang Youqi
Published: July 2002

The present paper developed two kinds of new time-dependent methods in quantum scattering calculation. One method is to combine a split-operator method with a differential method to achieve better convergent property than a pure differential method and better adaptive property in a multidimensional case than the pure split-operator method. Another method is to adopt a desired time-dependent variational principle to achieve variational correction in numerical calculations, which can improve the result from direct numerical evaluation with little additional consumed time.
Geometry, vibrational frequencies and bonding mechanism of NO adsorbed on Cu(111)
Authors: Illas F. , Ricart J. M. , Fernández-García M.
Published: July 2002

We report theoretical results for the bonding of NO to the threefold (fcc) position of the Cu(111) surface. Both, N- and O-down orientations have been considered. Our study is based on several cluster models and ab initio wave functions. We found that the NO/Cu(111) interaction is dominantly ionic in nature, with a small but noticeable π covalent contribution for the N-down orientation only. The ionic nature of the bond has important consequences as an avoided crossing between two electronic states of ionic and neutral character. The existence of this avoided crossing interaction indicates that the adsorption process is nonadiabatic. Finally, the analysis of the vibrational frequencies of adsorbed NO predicts that a O-down species may exist in the NO/Cu(111) system. This is the first time that an O-down species is proposed in Cu surfaces.
The coupled electronic oscillators vs the sum-over-states pictures for the optical response of octatetraene
Authors: Chen Guanhua , Mukamel Shaul , Beljonne David , Brédas Jean-Luc
Published: July 2002

A coupled electronic oscillator (CEO) analysis of the third harmonic generation (THG) spectrum for octatetraene is presented. The dominant oscillators and their couplings are identified using tree diagrams. The correspondence between the dominant oscillators in the CEO picture and the relevant excited states in the sum-over-states (SOS) description is demonstrated. The important channels in the SOS are related to the dominant oscillator pathways in the CEO picture.
Molecular adsorption of alkanes on platinum surfaces: A predictive theoretical model
Authors: Stinnett James A. , Madix Robert J.
Published: July 2002

The adsorption probabilities of methane and propane on Pt(111), and propane on Pt(110)-(1×2) have been successfully predicted for a wide range of incident energies and angles with classical stochastic trajectory simulations, using a pairwise additive Morse methyl–platinum potential previously developed from the measured trapping probabilities of ethane on Pt(111). These predictions, along with those for ethane adsorption on Pt(110)–(1×2), comprise a unified model for the molecular adsorption of alkanes on platinum surfaces. The simulations show the initial trapping probabilities of methane and propane on Pt(111) are determined to within approximately 10% by the fate of the first bounce. They also indicate that at normal incidence on Pt(111) energy conversions from perpendicular translational motion to both cartwheeling rotation and lattice phonons play increasingly important roles in increasing the trapping probability as the alkane increases in size and molecular weight. For methane itself excitation of parallel translational momentum after the first bounce serves as the most effective energy storage mechanism which facilitates trapping, whereas for propane cartwheel rotational motion plays the dominant role. Excessive excitation of these modes of motion, however, can cause scattering on subsequent bounces by reconversion of the energy into perpendicular translational energy. Collisions of methane with the hollow and bridge sites on the Pt(111) surface appear less effective in trapping than do atop sites. The simulations also suggest excitation of the C–C–C bending mode of propane has little effect on the trapping of propane on platinum surfaces for beam energies below 55 kJ/mol.
A molecular dynamics model of the amorphous regions of polyethylene oxide
Authors: Lin Bin , Boinske P. T. , Halley J. W.
Published: July 2002

We report a molecular dynamics model of the amorphous regions of polyethylene oxide for use in study of the ionic transport mechanisms in this polymer when it is used as an electrolyte in batteries. The model is produced by beginning with a molecular dynamics of dimethyl ether which we have reported earlier and ‘‘polymerizing’’ it computationally by successively choosing pairs of unbonded methyl groups and changing the forces to describe the chemical bond in the polymer. This is not intended to be a fully realistic simulation of the actual chemical polymerization process, but we argue that it produces a useful model of a sample of the amorphous polymer. We report structural and dynamical properties of the resulting model, in which we have adopted torsion forces reported by Krimm and coworkers to fit the observed vibrational spectrum.
Evidence of small odd-numbered dianionic carbon cluster beams from a cesium-sputter negative ion source
Authors: Calabrese D. , Covington A. M. , Thompson J. S.
Published: July 2002

Evidence of odd-numbered Cn2− (n=7–13) cluster beams produced in a cesium-sputter negative ion source has been observed. The minimum lifetime of smallest dianion, C72−, is estimated to be at least 55 μs. An experiment to investigate the structure of these dianions is also described.
Geometry and time scale of the complex rotational dynamics of amorphous polymers at the glass transition by multidimensional nuclear magnetic resonance
Authors: Heuer A. , Leisen J. , Kuebler S. C. , Spiess H. W.
Published: July 2002

We present a new multidimensional NMR exchange experiment, three dimensional difference correlated exchange spectroscopy (DICO), which allows one to extract detailed information about the time scale as well as the geometry of rotational processes simultaneously. In contrast to experiments presently available the geometric information specified by a dynamic order parameter is independent of a specific model. Among others full isotropisation can be directly seen from the spectrum via a symmetry criterion. We apply this method to study the chain motion of polystyrene slightly above the glass transition. The DICO spectra prove that a typical chain segment performs small angle fluctuations as well as large angle jumps which can neither be described as random jumps nor tetrahedral jumps. A typical jump angle can be estimated to be of the order of 30°−40°.
Evidence of nonadditive many-body terms in the water potential
Authors: De Santis Alberto , Rocca Dario
Published: July 2002

The zero density limit of g(r) of heavy water has been calculated exploiting both ab initio and effective pair potentials. The molecular dynamic simulation and the TIP4P effective potential have been also used to reproduce the g(r) at two density states of the steam along the T=580 K isotherm. The results are compared with the experimental data at nearly the same two state points. The intensity of the experimental g(r) increases with the density while that of the computed ones decreases. Furthermore, the zero density g(r) obtained from effective pair potentials severely overestimate the experimental g(r) at the lower density. The discrepancies are very strong and provide direct evidence of nonadditive many-body effects. Quantum mechanical studies on clusters of water molecules, available in literature, allow to give a reliable explanation of the behavior of the experimental g(r).
Quantum mechanical tunneling through a time-dependent barrier
Authors: Ge Jiu-Yuan , Zhang John Z. H.
Published: July 2002

We present a numerical investigation of quantum mechanical tunneling process in a time-dependent (fluctuating) barrier using a one dimensional model of Eckart barrier. The tunneling probability is calculated for two cases in which (1) the height of the barrier is undergoing harmonic oscillation with frequency ω and (2) the location of the barrier is undergoing harmonic oscillation with frequency ω. It is observed in both cases that the quantum mechanical tunneling probability exhibits a maximum as a function of the oscillating frequency ω between the low and high frequency limits. The physical origin and process underlying this resonantlike phenomenon are proposed in this paper based on the current model study.
The influence of high-frequency modes on ultrashort pulse absorption initiated processes
Authors: Kobrak Mark N. , Hiller Eric M. , Rice Stuart A.
Published: July 2002

Many molecules studied by ultrashort pulse spectroscopic techniques contain modes which have both short and long periods relative to the time scale of excitation. The influence of the short period (high frequency) modes on the ultrashort pulse absorption process is usually neglected. In this paper we examine the effect of the high frequency modes on the energetics of ultrashort pulse absorption, and construct a method for calculating observables for polyatomic systems containing both high and low frequency modes. We compare our results with experimental data for the iodobenzene system.
Scattering of gases from aligned liquid crystals: Collision-induced loss of order at the gas–liquid interface
Authors: Randall Karen L. , Fan Y. B. , Michel Oliver , Goh M. Cynthia , Donaldson D. J.
Published: July 2002

An atomic beam of helium is scattered from the surface of an aligned nematic liquid crystal film (4′-pentyl-4-cyanbo biphenyl, 5CB). Collisions of atoms with the surface initiate a transient loss of order in the film. The magnitude of this change depends on the momentum direction of the atomic beam with respect to the liquid crystal alignment director. The effect is significantly more pronounced when the incoming beam is perpendicular to the director than when it is parallel. This anisotropy may be due to a more efficient alignment-changing torque exerted on the 5CB molecules by perpendicular collisions with He.
Quasielastic helium atom scattering measurements of microscopic diffusion of CO on the Ni(110) surface
Authors: Bertino M. F. , Hofmann F. , Steinhögl W. , Toennies J. P.
Published: July 2002

The microscopic diffusion of CO on the Ni(110) surface has been studied by quasielastic helium atom scattering. From the temperature dependence of the energetic broadening of the quasielastic peak measured at a parallel wave vector near the Brillouin zone boundary, the activation energies for diffusion have been determined to be Ediff=57±4 and 35±4 meV for diffusion parallel to the rows (〈11̄0〉) and perpendicular to the rows (〈001〉), respectively. The activation energies are a factor 2–6 smaller and the preexponential diffusion coefficients orders of magnitude larger than obtained in recent laser measurements of chemical diffusion coefficients, indicating that diffusion over distances from about a few Ångstroms up to 30 Å is much faster than over the much larger distances probed in macroscopic diffusion measurements. The difference is attributed to the impeding effect of step edges or impurities on the latter measurements.
Energy switching approach to potential surfaces: An accurate single-valued function for the water molecule
Authors: Varandas A. J. C.
Published: July 2002

A novel scheme is suggested to construct a global potential energy surface by switching between representations which are optimal for different energy regimes. The idea is illustrated for the electronic ground state of water for which we use as switched functions the many-body expansion potential of Murrell and Carter [J. Chem. Phys. 88, 4887 (1984)] and the polynomial form of Polyansky, Jensen, and Tennyson, [J. Chem. Phys. 101, 7651 (1994)]. By also modifying the former to reproduce the Coulombic behavior at the collapsed molecular limits for vanishingly small interatomic distances and approximately account for the long range forces, the new potential energy surface has been given double many-body expansion quality. The result is a global H2O potential energy surface which has spectroscopic accuracy and may be used for studies of reaction dynamics.
Structure and vibrations of catechol(methanol)1 in the S and S1 state
Authors: Gerhards M. , Perl W. , Schumm S. , Henrichs U. , Kleinermanns K.
Published: July 2002

The structure as well as the inter- and intramolecular vibrations of the catechol (methanol)1 cluster are investigated both experimentally and theoretically. By using resonant two-photon ionization (R2PI) and dispersed fluorescence (DF) spectroscopy, the vibrational transitions of the S0 and S1 state are obtained. In order to find the corresponding vibrations of the S0 and S1 state, DF spectra are recorded by pumping the electronic origin and the most intense vibrations of the R2PI spectrum. According to ab initio calculations performed at the Hartree–Fock level [6-31G(d,p) basis], including MP2, BSSE, and ZPE (zero point energy) corrections, the most stable structure turns out to be translinear. The calculated vibrational frequencies are in close agreement to the experimental values. Since the catechol(methanol)1 cluster has no symmetry, all intermolecular fundamental vibrations of the S0 and S1 state spectra can be assigned. A large number of combination bands and overtone vibrations are observed in the low frequency region (<200 cm−1) of the S0 and S1 state spectra. From the DF spectrum obtained by pumping the low frequency ρ1 vibration, it can be concluded that the catechol(methanol)1 cluster undergoes a geometry change in the S1 state. The OH…O bond turns out to be nonplanar with respect to the aromatic plane. This confirms the results reported for catechol and the catechol(H2O)1 cluster.
Determination of diabatic coupling potentials from the inversion of laboratory inelastic scattering data: Application to C4++He→C2++He2+
Authors: Boyd Robert , Ho Tak-San , Rabitz Herschel
Published: July 2002

Experimental inelastic cross section data are used to successfully recover the diabatic coupling potential for the C4++He system via a general iterative inversion algorithm based on first-order functional sensitivity analysis and Tikhonov regularization. The coupling potential was found to converge to a curve with a distinctly different shape from that of an earlier fitted-parameter model potential. Cross sections calculated from the new potential reproduce the relative heights and shapes of the peaks in the experimental cross sections much better than those obtained from the parametrized potential.
A theoretical approach for modeling reactivity at solid–liquid interfaces
Authors: Stefanovich Eugene V. , Truong Thanh N.
Published: July 2002

We present a new general methodology capable of modeling chemical reactions at solid–liquid interfaces called CECILIA (combined embedded cluster at the interface with liquid approach). The main idea is to combine the embedded cluster molecular orbital or density functional methods for describing interactions at the surface of a solid with the dielectric continuum approach for modeling a liquid. More details are given on how to apply this methodology to model processes at the ionic solid–water interface. Geometries and adsorption binding energies of H2O, NaCl, Na+, and Cl− at the NaCl(001)–water interface are calculated using this approach and compared with those at the NaCl(001)–vacuum interface.
The spectroscopy of hexafluorothioacetone, a blue gas
Authors: Clouthier Dennis J. , Joo Duck-Lae
Published: July 2002

The vibrational and electronic spectra of hexafluorothioacetone, (CF3)2CS, a novel blue gas, have been studied. Ab initio calculations of the vibrational properties of CF3COF, CF3CSF, and (CF3)2CO were used to establish the feasibility and effectiveness of using theoretical predictions in the analysis of the spectra of perfluorinated compounds. These predictions have allowed us to obtain revised interpretations of the spectra of trifluoroacetyl fluoride and trifluorothioacetyl fluoride that are consistent with both experiment and theory and have allowed us to confirm a previous theoretical and experimental study of the spectrum of hexafluoroacetone. Similar calculations on hexafluorothioacetone predicted a ground state of C2 symmetry, with the CF3 groups staggered in an antieclipsed configuration and a pattern of vibrational frequencies similar to that of hexafluoroacetone. The gas phase and argon matrix infrared spectra and the Raman spectrum of hexafluorothioacetone were analyzed with the aid of the ab initio predictions and 20 of the 24 fundamentals were assigned. The blue color of the compound originates from very weak T1–S0 (800–675 nm) and S1–S0 (725–400 nm) transitions in the visible due to the n–π* electron promotion. Promotion of an electron from the π to the π* orbital gives rise to a very strong electronic transition in the 230–190 nm region of the ultraviolet. No emission was observed on laser excitation of hexafluorothioacetone in the visible.
An investigation of the influence of solute size and insertion conditions on solvation thermodynamics
Authors: Cann N. M. , Patey G. N.
Published: July 2002

In this paper we examine the influence of solute size and insertion conditions on solvent structural changes and excess thermodynamic properties in the infinite dilution limit. A general integral equation approach which can be applied under arbitrary conditions is given and isothermal-isochoric and isothermal-isobaric insertions are discussed in detail. Scaling relationships valid in the large solute limit are determined for both structural and thermodynamic properties. This is done by considering macroscopic thermodynamic relationships and explicit evaluation of low solvent density expansions of pair correlation functions. The hypernetted-chain and reference hypernetted-chain closure approximations are used to obtain numerical results for the insertion of hard sphere solutes of varying diameter into hard sphere, dipolar hard sphere and water-like solvents. The results obtained give a good deal of insight into the nature of solvation of inert solutes. It is shown that for all three solvents the excess properties are very well represented by a function obtained by summing terms proportional to the solute volume, surface area and diameter. One would expect such a result for large solutes, but here we show that this expression extrapolates all the way down to solutes comparable in size to the solvent particles. Further, it is shown that both the numerical value, and, more importantly, the physical interpretation of the excess thermodynamic properties strongly depend on the insertion conditions. Under all insertion conditions the chemical potential is a local property in the sense that it is completely determined by solute-solvent correlations which are important only in the immediate vicinity of the solute. However, this is not true of the excess energy, enthalpy and entropy which all contain nonlocal contributions arising essentially from changes in the actual or effective solvent density depending on the insertion conditions. We demonstrate that the nonlocal contributions can be very significant and therefore the excess energies, enthalpies and entropies often cannot provide useful information about solvent structure near solutes. This has significant implications for models which attempt to rationalize excess thermodynamics in terms of local solvent structure in the vicinity of solute particles.
A quantum dynamical study of CH overtones in fluoroform. II. Eigenstate analysis of the vCH=1 and vCH=2 regions
Authors: Maynard Andrew , Wyatt Robert E. , Iung Christopher
Published: July 2002

In this series, the multiple time scales and mechanisms of intramolecular vibrational-energy redistribution (IVR) present in the CH overtones of fluoroform is investigated. In part II, we analyze the ab initio vibrational spectrum and dynamics of the vCH=1 and vCH=2 regions, explicitly treating all vibrational degrees of freedom. A wave operator sorting algorithm is used to develop an efficient vibrational basis. Spectral transformation, by a filtered-Lanczos method, is used to accelerate eigenstate analysis. The theoretical spectrum of the fundamental is nontrivial, predicting a close triplet of lines (width <6 cm−1) centered at 3048.3 cm−1 and a weak line 20 cm−1 upfield, due to coupling with a small subsystem of background modes: ν4±+ν5±+ν6±, ν2+ν4±+ν6±, and ν3+2ν5. Furthermore, these background states mix with one another and other nearby background states via resonances ω5≈(ω3+ω6), ω2≈ω5, and (ω4+ω6)≈(ω3+ω5). An effective Hamiltonian is used to assess chromophore–background and background–background coupling. In contrast, the first overtone exhibits no strong background resonances, rather the background modes function as a weakly coupled reservoir. IVR time scales and eigenstate convergence, with respect to the active space, are presented.
A vibrational study of ethanol adsorption on Si(100)
Authors: Eng, Jr. Joseph , Raghavachari Krishnan , Struck Lisa M. , Chabal Yves J. , Bent Brian E. , Flynn George W. , Christman Stan B. , Chaban Ed E. , Williams Gwyn P. , Radermacher Klaus , Mantl Siegfried
Published: July 2002

The adsorption of ethanol-d0, -d3, and -d6 on Si(100) has been studied in the mid- to far-infrared region using surface infrared absorption spectroscopy. The acquisition of infrared spectra in this frequency range (<1450 cm−1) is made possible by using specially prepared Si(100) wafers which have a buried metallic CoSi2 layer that acts as an internal mirror. We find that ethanol dissociatively adsorbs across the Si(100) dimers near room temperature to form surface bound hydrogen and ethoxy groups. Furthermore, the ethoxy groups are oriented such that the C3v axis of the methyl group is nearly perpendicular to the surface, unlike the case for ethoxy groups bound to metal surfaces. This adsorption geometry is deduced on the basis of the surface dipole selection rule, which applies to these Si(100) samples with a buried CoSi2 layer. Ab initio cluster calculations using gradient-corrected density functional methods confirm the proposed adsorption geometry for ethoxy on Si(100) and accurately reproduce the observed normal mode frequencies.
Time domain modeling of spectral collapse in high density molecular gases
Authors: Meinrenken Christoph J. , Gillespie Walter D. , Macheret Sergey , Lempert Walter R. , Miles Richard B.
Published: July 2002

In many cases, the widely used matrix inversion approach to describe the spectral interference in collisionally perturbed molecular spectra is not feasible if the particular molecular interactions do not allow the sudden impact approximation (infinitely short collision duration). To overcome this problem, we present a time domain model that describes collisional broadening and narrowing phenomena without requiring the sudden approximation. The key element of the model is a Monte Carlo type sampling process to quantify the temporal autocorrelation of the molecular dipole moment. The spectrum is then obtained numerically via fast Fourier transform. The model does not require a frequency-dependent relaxation operator; the finite collision duration is simply an adjustable parameter in the time domain process. Our approach, which is generally applicable to any set of transition lines, is derived from concepts of both conventional quantum-mechanical and semiclassical theory of line interference. Coherent transfer effects from rotationally inelastic collisions are described as randomly occurring events which affect frequency, amplitude, and phase of the sampled oscillation. Effects of vibrational dephasing are included as well. To demonstrate its feasibility, we apply the model here to the 2.7 μ absorption spectrum of carbon dioxide diluted in high density air (ρ=43–485 amagat, T=297–754 K). The successful modeling of the experimental data, especially the full collapse of P and R branches at ultrahigh densities, accounts for interbranch mixing and for incoherent effects. The calculations make extensive use of the new Hitran (HITEMP) molecular database. Results include revised estimates for the collision duration of CO2 with nitrogen and oxygen at room temperature.
Calculation and fitting of potential energy and dipole moment surfaces for the water molecule: Fully ab initio determination of vibrational transition energies and band intensities
Authors: Kedziora Gary S. , Shavitt Isaiah
Published: July 2002

Potential energy and dipole moment surfaces for the water molecule have been generated by multireference singles-and-doubles configuration interaction calculations using a large basis set of the averaged-atomic-natural-orbital type and a six-orbital-six-electron complete-active-space reference space. The surfaces are suitable for modeling vibrational transitions up to about 11 000 cm−1 above the ground state. A truncated singular-value decomposition method has been used to fit the surfaces. This fitting method is numerically stable and is a useful tool for examining the effectiveness of various fitting function forms in reproducing the calculated surface points and in extrapolating beyond these points. The fitted surfaces have been used for variational calculations of the 30 lowest band origins and the corresponding band intensities for transitions from the ground vibrational state. With a few exceptions, the results compare well with other calculations and with experimental data.
Space charge and plasma effects in zero kinetic energy (ZEKE)photoelectron spectroscopy
Authors: Villeneuve D. M. , Fischer Ingo , Zavriyev A. , Stolow Albert
Published: July 2002

In photoelectron spectroscopy experiments it is generally assumed that the Coulomb force between charged particles is small compared with external fields, and that the free kinetic electrons will quickly leave the ions. This is the basis of the ZEKE photoelectron spectroscopy. However as the density of charged particles is increased, plasma physics effects begin to become important, and the kinetic electrons become trapped by the net positive charge and move so as to set up a self-field which can cancel any externally imposed electric fields. For high densities, fewer electrons than expected are able to escape the self-field. The production of self-consistent electric fields is studied by means of particle-in-cell plasma simulations and by N-body trajectory calculations, and simple expressions are derived for when plasma physics effects become significant. An experimental illustration of plasma effects in ZEKE is presented.
Spectroscopic characterization of the excited valence MgXe(3dδ 3Δ1), MgXe(3dπ 3Π), and MgXe(“3dσ” 3Σ+) van der Waals states
Authors: Kaup John G. , Breckenridge W. H.
Published: July 2002

The Mg(3s3dσ 3DJ)⋅Xe(3Σ+), Mg(3s3dπ 3DJ)⋅Xe(3Π0+,0−), and Mg(3s3dδ 3DJ)⋅Xe(3Δ1) excited states have been characterized via resonance enhanced two-photon ionization (R2PI) spectroscopy of transitions from the long-lived Mg(3s3pπ 3PJ)⋅Xe(3Π0+,0−), metastable states of the MgXe van der Waals molecule. Because the excited Mg(3d) orbital is quite diffuse and the Xe atom can approach along the nodal axis of the aligned 3d orbital, minimizing repulsion, the MgXe(3s3dδ 3Δ1) state (Do=3160±150 cm−1) is even more strongly bound than the MgXe+ core ion (for which Do=2848±150 cm−1). The MgXe(3dσ 3Σ+) state (Do=1262±150 cm−1) and the MgXe(3dπ 3Π0−) state (Do=1229±150 cm−1) are much less bound. However, the potential curves of these two states are quite different, and it is suggested that the MgXe(“3dσ” 3Σ+) state is bound only because of substantial mixing of Mg(4pσ) Rydberg character into the wave function. Interesting spin–orbit and spin–spin effects, detected and analyzed from the rotational structure of the vibrational bands, are attributed to mixing of some Xe character into molecular orbitals nominally of Mg* excited state character.
Predicting electron affinities with density functional theory: Some positive results for negative ions
Authors: Tschumper Gregory S. , Schaefer Henry F.
Published: July 2002

The atomic electron affinities of the eight first row (H,Li,…,F) atoms as well as the adiabatic electron affinities of 12 first row diatomic and 15 first row triatomic molecules were determined using six different density functional or hybrid Hartree–Fock/density functional methods. The 35 species were selected for having relatively well-established experimental electron affinities. Harmonic zero-point vibrational energy corrected electron affinities are also reported for the diatomic and triatomic molecules. Equilibrium geometries and harmonic vibrational frequencies are given for the 27 molecules and their anions as determined with each density functional method. Discussion focuses on comparison of theoretical and experimental electron affinities. For the atomic, diatomic, and triatomic electron affinities the average absolute error is reported for each exchange–correlation functional. Since many of the molecular anion structures and vibrational frequencies are unknown, the work suggests new experimental directions.
Calculations of the excitation energies of all-trans and 11,12s-dicis retinals using localized molecular orbitals obtained by the elongation method
Authors: Kurihara Youji , Aoki Yuriko , Imamura Akira
Published: July 2002

In the present article, the excitation energies of the all-trans and the 11,12s-dicis retinals were calculated by using the elongation method. The geometries of these molecules were optimized with the 4-31G basis set by using the GAUSSIAN 92 program. The wave functions for the calculation of the excitation energies were obtained with CNDO/S approximation by the elongation method, which enables us to analyze electronic structures of aperiodic polymers in terms of the exciton-type local excitation and the charge transfer-type excitation. The excitation energies were calculated by using the single excitation configuration interaction (SECI) on the basis of localized molecular orbitals (LMOs). The LMOs were obtained in the process of the elongation method. The configuration interaction (CI) matrices were diagonalized by Davidson’s method. The calculated results were in good agreement with the experimental data for absorption spectra. In order to consider the isomerization path from 11,12s-dicis to all-trans retinals, the barriers to the rotations about C11–C12 double and C12–C13 single bonds were evaluated.
Photodissociation of Cl2O at 248 and 308 nm
Authors: Moore Teresa A. , Okumura Mitchio , Minton Timothy K.
Published: July 2002

Molecular beam studies of Cl2O photolysis at 248 and 308 nm have been repeated and the analysis refined. At 248 nm, three distinct dissociation pathways that led to Cl+ClO products were resolved. At 308 nm, the angular distribution was slightly more isotropic than previously reported, leaving open the possibility that Cl2O excited at 308 nm lives longer than a rotational period.
The identification of In2O in the gas phase by high resolution electronic spectroscopy
Authors: Lakin Nicholas M. , van den Hoek Ger , Beattie Ian R. , Brown John M.
Published: July 2002

The molecular species In2O has been identified in the gas phase as a product of the high temperature reaction between water and indium (850 °C) or between indium trioxide (In2O3) and indium (950 °C) by the observation of an electronic transition in the near-ultraviolet. The spectra are simplified by supersonic cooling of the sample in a free jet expansion after it is formed. The vibrational structure shows that the molecule has a very similar geometry in the two states involved while the 18O/16O isotope shift suggests that the molecule is only slightly nonlinear in the excited electronic state. Rotational structure can be resolved at high resolution and shows an intensity alternation; the molecule thus has a symmetric In–O–In arrangement.
Sideband optical–optical double resonance Zeeman spectroscopy. III. Analysis of composite lines and selective detection
Authors: McCarthy Michael C. , Field Robert W.
Published: July 2002

Sideband optical–optical double resonance Zeeman spectroscopy is applied here to studies of the electronic spectrum of gas-phase PtH. Even though each rotational transition is a composite line with closely spaced isotope and hyperfine structure, the sub-Doppler Zeeman spectrum of each feature has been recorded and analyzed using a systematic, quasi-two-dimensional approach. Sub-Doppler radio frequency magnetic resonances, in addition to their diagnostic power to assist rotational and electronic assignment, can also be used to detect selectively other transitions with the same Zeeman properties; if a fixed magnetic field is applied and the sub-Doppler signal recorded as the laser is scanned, spectral features which share the common Zeeman splitting in either the lower or upper state can be detected. Several examples illustrating the selectivity of this scheme are given.
Effect of pressure on lattice modes and electronic excitations of 9,10-diiodoanthracene crystals
Authors: Brillante A. , Della Valle R. G. , Ulrich C. , Syassen K.
Published: July 2002

We have measured Raman, optical absorption, and fluorescence spectra of 9,10-diiodoanthracene (DIA) crystals as a function of hydrostatic pressure. Unlike analogous anthracene derivatives with similar quasi-one-dimensional stacking of molecules, the ambient pressure crystal structure remains stable up to at least 20 GPa. Lattice dynamics calculations, based on intermolecular potentials, reproduce well the structural parameters at zero pressure and the pressure dependence of the intermolecular Raman-active phonons. An explanation is given for the unusual stability of the one-dimensional molecular packing in DIA.
The effect of high pressure on the dynamics of doped organic glasses: A study by spectral hole-burning
Authors: Creemers T. M. H. , Koedijk J. M. A. , Chan I. Y. , Silbey R. J. , Völker S.
Published: July 2002

We report time-resolved spectral hole-burning experiments on bacteriochlorophyll-a (BChl-a) doped into the glass triethylamine (TEA) at ambient pressure (Δp=0) and at a pressure of Δp=3.4 GPa. We observe a number of remarkable effects: (a) from the change in the temperature dependence of the “effective” optical homogeneous linewidth Γhom′, we conclude that local order is introduced in TEA under high pressure; (b) from the change in the time dependence of Γhom′, we conclude that spectral diffusion is induced by “downhill” energy transfer among BChl-a molecules (at a concentration of c∼5×10−4 M) within the S1↔S0 0-0 band, both at ambient and high pressure; and (c) from the wavelength dependence of Γhom′, we conclude that an energy barrier (ΔE∼135 cm−1 at Δp=0, and ΔE∼70 cm−1 at Δp=3.4 GPa) has to be crossed in order to excite two-level systems (TLSs) of the TEA host which, subsequently, leads to spectral diffusion. We present a qualitative molecular model for the interpretation of these effects.
Hydrogen-bonding in glycine and malonaldehyde: Performance of the Lap1 correlation functional
Authors: Sirois S. , Proynov E. I. , Nguyen D. T. , Salahub D. R.
Published: July 2002

The conformational equilibrium of gaseous glycine presents a severe challenge to quantum chemistry and, in particular, to density functional theory (DFT), due to the presence of internal hydrogen bonds. We present new DFT results for the structure and the energetics of glycine and malonaldehyde, using the recently developed nonlocal exchange-correlation functionals BLap1 and PLap1. A comparative analysis is made with the results of the generalized-gradient-approximation (GGA) schemes Becke–Perdew (BP86) and Becke–Lee–Yang–Parr (BLYP), the hybrid Hartree–Fock (HF)-DFT methods B3PW91 and B3LYP, with post-HF methods, and with the available experimental data. Our BLap1/TZVP and PLap1/TZVP values for the energy margin between the two lowest conformers of glycine (0.84 and 1.05 kcal/mol, respectively) are within the experimental error bars (1.0±0.5 kcal/mol). MP2 underestimates this energy difference by about 0.5 kcal/mol, BLYP and the hybrid methods are off by about 0.9 kcal/mol. The optimized geometry of malonaldehyde is improved with the Lap1 functionals, compared to the GGA results reported previously. Improvement over the GGA is also achieved for the energy barrier for the internal proton transfer in malonaldehyde. Recent high quality post-HF (G2) calculations of Barone and Adamo1 yield 4.4 kcal/mol. The best GGA (PP86) value of 2.1 kcal/mol is seriously underestimated. The best B3LYP estimate is 3.0 kcal/mol.1 Our BLap1/TZVP (3.28 kcal/mol) and PLap1/TZVP (4.56 kcal/mol) values are in the correct energy range, the latter being, in fact, very close to the G2 estimate.
Evidence for the B12−2A1 electronic transition in chlorine dioxide from resonance Raman depolarization ratios
Authors: Reid Philip J. , Esposito Anthony P. , Foster Catherine E. , Beckman Robert A.
Published: July 2002

The resonance Raman depolarization ratios of chlorine dioxide (OClO) dissolved in cyclohexane are measured and analyzed to establish the existence of a A12 excited state that is nearly degenerate with the optically stronger, A22 excited state. The depolarization ratio of the symmetric stretch fundamental transition is measured at several excitation wavelengths spanning the lowest-energy electronic transition centered at ∼360 nm. The depolarization ratio of this transition reaches a maximum value of 0.25±0.04 directly on resonance suggesting that scattered intensity is not derived from a single excited state. The depolarization ratios are modeled utilizing the time-dependent formalism for Raman scattering. This analysis demonstrates that the observed Raman depolarization ratios are derived from contributions of two excited states of A12 and A22 symmetry to the observed scattering. The results presented here support the emerging picture of OClO excited-state reaction dynamics in which photoexcitation to the A22 excited state is followed by internal conversion from this state to the A12 surface. Both the role of the A12 state in the photochemistry of OClO and the importance of this state in modeling resonance Raman intensities are discussed.
The mechanism of formation and infrared-induced decomposition of HXeI in solid Xe
Authors: Pettersson Mika , Nieminen Janne , Khriachtchev Leonid , Räsänen Markku
Published: July 2002

Ultraviolet (UV) irradiation of HI-doped xenon matrix dissociates the precursor and leads to the formation and trapping of neutral atoms. After UV photolysis, annealing of the matrix mobilizes the hydrogen atoms at about 38 K. The mobilized hydrogen atoms react with I/Xe centers forming HXeI molecules in a diffusion controlled reaction. The formed molecules can be photolyzed with infrared (IR) irradiation at 2950–3800 cm−1 and quantitatively regenerated thermally. The formation of HXeI from neutral atoms is proved by the quantitative correlation between neutral iodine atoms and HXeI molecules in selective IR photodissociation and thermal regeneration experiments. Kinetic measurements show that the formation of HXeI from atoms is prevented by a potential barrier, which is estimated to be 700 cm−1 in magnitude. The potential barrier is proposed to originate from the avoided crossing between neutral H+Xe+I and ionic (HXe)++I− singlet surfaces. The dissociation energy D0 of HXeI with respect to the top of the potential barrier is estimated to be 2950 cm−1 and De about 4070 cm−1 in solid Xe. The weak IR photodissociation profile of HXeI around 3000 cm−1 is measured by irradiating the sample with tunable IR source and monitoring the changes in the fundamental region. The formation mechanism from neutral atoms is believed to be valid for other similar rare-gas compounds.
Helium pressure broadening of ammonia inversion transitions from 10 to 35 K
Authors: Willey Daniel R. , Timlin Robert E. , DeNardo Patrick G. , Null Paul L. , Pondillo Peter L. , Tyszka Teresa
Published: July 2002

Helium pressure broadening cross sections for the (J,K)=(1,1), (2,2) and (3,3) inversion transitions of ammonia were measured at temperatures from 10 to 35 K. Measurements were taken in a quasiequilibrium cell using the collisional cooling technique. In contrast to the situation at room temperature where there is little state-to-state variation in helium pressure broadening cross sections, at low temperature the (3,3) cross sections were found to be, on average, three times larger than the (1,1) cross sections while the (2,2) cross sections were twice as large as the (1,1) values. The major factor determining the relative size of the low temperature cross sections appears to be the contribution from inelastic collisions into lower energy levels. The relative size of the cross sections thus scales with the number of rotational levels underlying a given (J,K) state. Cross sections were also calculated for comparison with the experimental data using three existing NH3–He potential surfaces. Overall, none of the three surfaces gave consistent agreement with the experimental results for all three observed transitions. The discrepancy between experiment and theory was greatest for the (3,3) data. This may be a manifestation of a previously observed tendency of NH3–He potential surfaces to underestimate the contribution from parity-conserving collisions in calculated state-to-state rates.
Laser velocity modulation spectroscopy of TiCl+: Observation of the A 3Δ(3d2) state and deperturbation of the X 3Φ−A 3Δ complex
Authors: Focsa C. , Pinchemel B. , Féménias J.-L. , Huet T. R.
Published: July 2002

A single mode cw dye laser along with velocity modulation was used to record weak bands of TiCl+ in the spectral region 17 100–18 600 cm−1. The ions were produced in the positive column of an ac glow discharge with a gas mixture of He/TiCl4. These new bands have been identified as linking the upper already known [17.8]3Δ(3d4s) state to respectively the X 3Φ(v=1) state and to a newly observed A 3Δ(v=0) state located 350 cm−1 above the X 3Φ(v=0) state. As suggested by Focsa et al. (J. Chem. Phys. 106, 9044 (1997)) the A 3Δ(3d2) state was found to be responsible for the perturbations observed in the X 3Φ(3d2) state. The spin-orbit components of the A 3Δ(v=0) and the X 3Φ(v=1) states revealed interaction leading to homogeneous and heterogeneous perturbations. A set of deperturbed parameters has been determined with the help of a full matrix-based handling of the A 3Δ(v=0) and the X 3Φ(v=0,1) states. Study of the contributions of the basis functions to the eigenvectors of the diagonalized matrix displayed evidence of a reversal of the leading character of the Ω=3 spin-orbit components of the A 3Δ(v=0) and the X 3Φ(v=1) states. A remarkable example of avoided crossing has been observed between the A 3Δ1(v=0) and the X 3Φ2(v=1) spin-orbit components.
Analysis of the x-ray absorption spectra of linear saturated hydrocarbons using the Xα scattered-wave method
Authors: Väterlein P. , Fink R. , Umbach E. , Wurth W.
Published: July 2002

The Xα scattered-wave (Xα-SW) method was used to calculate the C K x-ray absorption (NEXAFS) spectrum of propane which previously was used as a model system for larger saturated hydrocarbons. The absorption resonances below the ionization threshold are analyzed in detail and characterized with respect to their valence and/or Rydberg-character. The shape resonances above the ionization threshold were investigated by calculating the eigenchannel wave functions involved in the formation of the resonances. The applicability of the results to the spectra of long alkane chains is studied, in particular for the determination of molecular orientation from their polarization dependence.