bluuebunny/crossref_metadata_2025 huggingface dataset:
Equilibrium and nonequilibrium solvation and solute electronic structure. II. Strong coupling limit
Authors: Kim Hyung J. , Hynes James T.
Published: July 2002

The formulation developed in the preceding paper [H. J. Kim and J. T. Hynes, J. Chem. Phys. 93, 5194 (1990)] is applied to describe the electronic structure and spectroscopic features of a model symmetric electron–donor–acceptor solute system D−A⇌DA− in solution in the strong coupling limit. In this limit, the electronic coupling is sufficiently strong to overcome the localizing influence of the solvent polarization, and two stable delocalized solute electronic states are found in the presence of either nonequilibrium or equilibrium solvation. The nonlinear influence of the equilibrated solvent electronic polarization and of exchange contributions to the solute electronic distribution incorporated in the theory lead to several consequences absent in standard descriptions. Among these are the necessity of two solvent coordinates to describe the system, and the prediction of solvent-dependent spectral shifts and the appearance of solvent relaxation dynamics after a Franck–Condon transition between the delocalized electronic states. Estimates of the magnitude of these new effects are provided, and the possibility for their experimental observation is briefly discussed.
The CO+O2 reaction on metal surfaces. Simulation and mean-field theory: The influence of diffusion
Authors: Mai J. , von Niessen W. , Blumen A.
Published: July 2002

A computer simulation for the heterogeneous catalyzed oxidation of CO is presented. The simulation includes adsorption, CO diffusion, reaction, and CO2 desorption. It is found that a first-order phase transition occurs at yCO=y2 (yCO is the mole fraction of CO in the gas phase). In the interval [ y2,1], the catalyst is almost completely covered with CO, i.e., the catalyst is poisoned. The value of y2 is a function of the adsorption/diffusion ration. For no CO diffusion, Ziff, Gulary, and Barshad [Phys. Rev. Lett. 24, 2553 (1986)] found y2=0.525. In this paper, for A/D=1/40, y2=0.650. In a mean-field ansatz with infinite diffusion rate, one obtains y2=0.666. With a linear stability analysis, the dependence of y2 on different initial coverage values can be explained. An initial coverage of oxygen does not influence the value of y2, but with increasing initial coverage of CO, the value of y2 decreases. It will be shown that oscillations are not possible in this simple reaction system. The adsorbed O atoms form large clusters that are found to be fractal in nature. The fractal dimension Df is equal to 1.88. A percolation transition at yCO=0.520 is observed, which corresponds to a critical oxygen coverage of ΘO,crit.=0.561. At this oxygen coverage, an infinite cluster percolates through the lattice. The value of ΘO,crit. is nearly independent of the A/D ratio. In correlated percolation simulations, it is found that ΘO,crit.=0.559, which is in good agreement with the value obtained from the reaction system.
Spectroscopic evidence for high symmetry in (benzene)13
Authors: Easter David C. , Whetten Robert L. , Wessel John E.
Published: July 2002

The (benzene)13 cluster and its isotopic derivatives have been formed in a dilute benzene/helium jet, and investigated by mass-selective R2PI spectroscopy in the region of the B2u ← A1g 000 and 610 transitions. Each band exhibits resolved fine structure, which differs greatly between forbidden (000) and allowed (610) bands. The main features, identified by spectral shift, are proposed to correspond to distinct molecular sites, whose symmetries are deduced by comparing spectra of the allowed and induced vibronic bands. The isotopic substitution of one C6H6 molecule into an otherwise deuterated cluster results in a considerable spectral simplification, due in part to highly nonrandom labelling. The results appear to rule out the crystallographic structure, while being consistent with the compact noncrystallographic structure computed by van de Waal.
The reorientation of t-butyl groups in butylated hydroxytoluene: A deuterium nuclear magnetic resonance spectral and relaxation time study
Authors: Polson James M. , Fyfe J. D. Dean , Jeffrey Kenneth R.
Published: July 2002

Deuterium nuclear magnetic resonance (NMR) spectra and spin–lattice relaxation times were determined in order to study the dynamics of t-butyl groups in butylated hydroxytoluene. The results are consistent with a model first proposed by Beckmann et al. [J. Magn. Reson. 36, 199 (1979)], where there is an inequivalence between the methyl groups within each t-butyl group. While two methyl groups reorient rapidly relative to the whole t-butyl rotation, the remaining methyl group is more restricted in its motion, reorienting at a rate comparable to that of the t-butyl group itself. The spin–lattice relaxation data show two T1 minima, the high temperature minimum (40 °C) corresponding to the combined t-butyl and ‘‘slow’’ methyl rotations, and the low temperature minimum corresponding to ‘‘fast’’ methyl group rotation. Using an explicitly defined T1 fitting function, the T1 data yield activation energies of 2.2 and 6.0 kcal/mol for the fast methyl and t-butyl rotations, respectively, both in agreement with Beckmann’s values obtained from proton T1 experiments. It was also possible to simulate the low temperature deuterium NMR spectra from T=−160 °C to T=−80 °C using the aforementioned dynamical inequivalence between the t-butyl methyl groups. While the fast methyl group rotation was in the motional narrowing region for T>−160 °C, it was possible, from the simulations, to determine the t-butyl exchange rates to within 10%. The jump rates are remarkably close to the values predicted from the T1 results. Above −80 °C, the spectra could not be simulated, implying that a third motion must be present to further alter the high temperature line shapes. The effective axial asymmetry of the T>−20° spectra indicates that the additional motion involves a two site exchange.
Inertial effects in solvation dynamics
Authors: Chandra A. , Bagchi B.
Published: July 2002

The dynamics of solvation of newly created charged species in dense dipolar liquids can proceed at a high speed with time constants often in the subpicosecond domain. The motion of the solvent molecules can be in the inertial limit at such short times. In this paper we present a microscopic study of the effects of inertial motion of solvent molecules on the solvation dynamics of a newly created ion in a model dipolar liquid. Interesting dynamical behavior emerges when the relative contribution of the translational modes in the wave-vector-dependent longitudinal relaxation time is significant. Especially, the theory predicts that the time correlation function of the solvation energy can become oscillatory in some limiting situations. In general, the dynamics becomes faster in the presence of the inertial contribution. We discuss the experimental situations where the inertial effects can be noticeable.
Limitations on measuring solvent motion with ultrafast transient hole burning
Authors: Jong Kang Tai , Yu Jongwan , Berg Mark
Published: July 2002

A number of limitations to hole burning in the liquid phase are identified. As spectral diffusion becomes more rapid, a point is reached where the narrowest hole width no longer measures the homogeneous linewidth. Spectral congestion of the solute is also shown to limit the fastest detectable spectral diffusion rate. An optimal pulse length is found which allows observation of the most rapid spectral diffusion. In addition, the coherence effect observed when the pulses overlap is shown to have resonances at both the excited and ground state vibrational spacings. The coherence effect greatly resembles the hole burning spectrum, but contains no dynamical information. Because of these limitations, hole burning may not be observed even when the homogeneous spectrum is well resolved and spectral diffusion is slow. Experimental confirmation is found in the hole burning spectrum of iodine in hexane, which shows no hole burning despite having a narrow homogeneous linewidth.
Infrared spectroscopy of vibrationally excited HONO2: Shedding light on the dark states of intramolecular vibrational energy redistribution
Authors: Fleming Patrick R. , Li Mengyang , Rizzo Thomas R.
Published: July 2002

Infrared predissociation spectroscopy of nitric acid subsequent to vibrational overtone excitation reveals vibrational state mixing of the highly excited levels and probes the character of the coupled dark states. A visible dye laser excites the 4←0 or 5←0 OH stretch transition and an optical parametric oscillator promotes the molecules above the dissociation threshold by a ΔvOH=1 transition from the excited level. Scanning the optical parametric oscillator frequency while monitoring the predissociation products via laser-induced florescence produces an infrared spectrum of the excited molecules. Although the 4νOH vibrational overtone band consists of a single clean rotational contour that falls directly on a Birge–Sponer plot, the infrared transitions from this level indicate that the zeroth-order bright state is extensively mixed. On the average, the zeroth-order bright state is only a minor component of the eigenstates at this energy. The largest collective contribution is from zeroth-order states that have zero quanta of OH stretch.
NO(X 2Π) product state distributions in molecule–surface collision-induced dissociation: Direct inelastic scattering of n,i-C3F7NO from MgO(100) at Eincident≤7.0 eV
Authors: Kolodney E. , Powers P. S. , Hodgson L. , Reisler H. , Wittig C.
Published: July 2002

Molecule–surface collision-induced dissociation (CID) has been studied for n-C3F7NO and i-C3F7NO molecular beams scattered from MgO(100) at incident kinetic energies (Eincident) up to 7 eV. The NO fragment was detected state selectively using two-photon, two-frequency ionization, and rotational and spin–orbit distributions are reported for several Eincident values. State and angle-resolved signals were integrated to give CID yields, which increased sharply with Eincident . In most cases, rotational excitation could be described by separate temperatures for each spin–orbit state. The upper 2∏3/2 state was underpopulated relative to statistical predictions (e.g., for n-C3F7NO at Eincident =5.0 eV, the spin–orbit temperature was ∼170 K, while Trot was ∼500 K). The CID results are compared to NO state distributions derived from the photodissociation of expansion-cooled molecules under collision-free conditions, at different energies (E°) above D0. These distributions were measured for both n-C3F7NO and i-C3F7NO up to E°∼4500 cm−1, and rotational excitation within each spin–orbit state was statistical, except at E°≥3000 cm−1. As with CID, a low [2∏3/2]/[2∏1/2] ratio was observed, and the reaction mechanism is probably unimolecular decomposition via the lowest triplet surface T1 with little or no exit channel barrier. The pronounced similarities between the CID and photodissociation results suggest that common reaction mechanisms may be operative. All of the CID results are compatible with direct inelastic scattering followed by unimolecular reaction on the S0 and/or T1 potential surfaces.
The coupling between librational motions and conformational transitions in chain molecules. A phenomenological analysis
Authors: Moro Giorgio J.
Published: July 2002

By analyzing the barrier crossing problem according to the Kramers theory, one derives that a conformational transition leads to reorientations in a small portion only of the chain. Therefore one must consider the librational motions, i.e., the fluctuations of the internal coordinates about the minima of the intramolecular potential, in order to describe the relaxation to the equilibrium after a transition. A phenomenological treatment is proposed for the model system of the rotor chain. It is shown that a conformational transition generates reorientations of the chain units, which propagate with increasing time delays. Since the time delays can be comparable or even greater than the lifetimes of the conformational states, the coupling between the librational and the conformational motions must be taken into account in order to describe the rotational relaxation in long chain molecules. This problem requires a generalized master equation (GME) for the distribution function on both the the torsional angle displacements and the conformational states. An approximate method of solution of the GME is presented, which allows the calculation of the orientational correlation functions.
Rhombic C4. Does it contain the shortest nonbonding C–C distance?
Authors: Lammertsma Koop , Güner Osman F. , Sudhakar Pamidighantam V.
Published: July 2002

In this ab initio molecular orbital study the bonding properties between the inverted carbons in rhombic C4 and [1.1.1]propellane are discussed. Rhombic C4, its protonated forms C4H+ and C4H2+2, and its linear isomer have been studied with the 6-311G(2d,p) basis set and all electron correlation using Mo/ller–Plesset perturbation theory at second order. The rhombic form of C4 is estimated to be essentially isoenergetic with the linear form; the estimated harmonic vibrational frequencies for both are compared with experimental data. The molecular orbital and topological one-electron density analyses show that the cross-ring bond of rhombic C4 is topologically unstable (at various levels of theory) which suggests a frozen transition for C–C bond making/breaking. At MP4(SDTQ) this cross-ring distance is only 1.546 Å. The stabilizing properties of rhombic C4 (and C4H+, and C4H2+ ) are compared with those of the well known [1.1.1]propellane, which at MP2/6-31G* has a longer cross-ring bond.
Some investigations of symmetry and extremal properties of molecular electron momentum densities
Authors: Gadre Shridhar R. , Limaye Ajay C. , Kulkarni Sudhir A.
Published: July 2002

It is known that the molecular electron momentum density γ(p) is inversion symmetric. In this article, the generalized repercussions of this fact on the symmetries of γ(p) are discussed. A comprehensive study of the symmetry transformation from positional to momentum density due to the introduction of inversion operator i in the symmetry point group of a molecular system (in r space) has been presented. The symmetry enhancement in C3v and Td point groups is brought out via the respective electron momentum density contour maps. Further, the extremal properties of γ(p) have been studied for CH4, H2O, HCN, NH3, C2H2, C2H4, C2H6, and CO molecules. The local maxima are observed at p=0 and elsewhere, whereas the minima and saddle points are invariably found to be at p=0 for the molecules studied here.
High resolution laser spectroscopy up to the dissociation limit of the NaK B 1Π state, and predissociation near the dissociation limit
Authors: Kasahara Shunji , Baba Masaaki , Katô Hajime
Published: July 2002

Doppler-free high resolution spectrum of the B 1Π(v′,J′)–X 1Σ+(v″,J″) transitions of 23Na39K was measured by the optical–optical double resonance (OODR) polarization spectroscopy. The transition lines up to the v′=43 level, which was estimated to be 1.8 cm−1 below the dissociation limit, were observed. The potential energy curve for internuclear distance from 3.25 to 15.6 Å was calculated by the RKR method, and the inverse-power coefficient was determined by analyzing the long-range RKR turning points. The B 1Π state dissociates to Na(3s2S1/2)+K(4p2P3/2) atoms without a potential hill near the dissociation limit. The dissociation energy was determined to be 1324.3±0.3 cm−1 from the LeRoy–Bernstein plots. Remarkable line broadenings were observed for transitions higher than the dissociation energy to Na(3s2S1/2)+K(4p2P1/2) atoms. This is identified as originating from the predissociation to Na(3s2S1/2)+K(4p2P1/2) atoms. The predissociation is shown to be caused by a spin–orbit interaction between the B 1Π and (2)3Σ+ states, and the potential curves are expected to cross around the inner turning point of the B 1Π (v′=34) level.
Phase transitions at liquid–solid interfaces: Padé approximant for adsorption isotherms and voltammograms
Authors: Blum L. , Huckaby Dale A.
Published: July 2002

A Padé approximant that is the natural extension of Langmuir’s adsorption isotherm is used to study the adsorption in the liquid–solid interface. The coefficients of this approximant are generated by a recursion relation and can be computed from the fugacity series in closed form. We apply this approximant to the underpotential deposition of metals on an electrode, and obtain voltammograms that show the sharp spikes seen in recent experiments.
Interrogating the vibrational relaxation of highly excited polyatomics with time-resolved diode laser spectroscopy: C6H6, C6D6, and C6F6+CO2
Authors: Sedlacek Arthur J. , Weston Ralph E. , Flynn George W.
Published: July 2002

The vibrational relaxation of highly excited ground state benzene, benzene d6, and hexafluorobenzene by CO2 has been investigated with high resolution diode laser spectroscopy. The vibrationally hot polyatomics are formed by single photon 248 nm excitation to the S1 state followed by rapid radiationless transitions. It has been found that in all cases less than 1% of the energy initially present in the polyatomics is deposited into the high frequency mode of CO2 (ν3). An investigation of the CO2(0001) nascent rotational distribution under single collision conditions reveals that very little rotational excitation accompanies vibrational energy transfer to the ν3 mode. The CO2(ν3) rotational states can be described by temperatures, Trot, as follows: C6H6, Trot =360±30 K; C6D6, Trot =350±35 K and C6F6, Trot =340±23 K. An estimate of 〈ΔE〉ν3, the mean energy transferred to the CO2 ν3 mode per collision, suggests that as the availability of low frequency modes in the excited molecule increases, less energy is deposited into the high frequency mode of CO2. Finally, evidence is presented suggesting that even at moderate laser fluences, the two-photon ionization of benzene can lead to substantial CO2 ν3 excitation via electron+CO2 inelastic collisions.
A crossed-beam study of the state-resolved integral cross sections for the inelastic scattering of OH(X 2Π) with CO and N2
Authors: Sonnenfroh David M. , Macdonald R. Glen , Liu Kopin
Published: July 2002

State-resolved cross sections for the inelastic scattering of OH(X 2Π) with CO and N2 have been measured in a crossed molecular beam apparatus over the energy range from 0.5 to 5.5 kcal mol−1. The magnitude and shape of the excitation functions for the same OH final states were virtually independent of the two targets except near threshold. Both spin–orbit conserving and spin–orbit changing transitions have been investigated. Cross sections for spin–orbit changing transitions of large ΔN were approximately the same magnitude as those for spin–orbit conserving transitions. For small ΔN, however, the spin–orbit conserving transitions became more favorable. The Λ-doublet ratio, Π(A′)/Π(A″), in the Π3/2 (F1) manifold differed from unity only for the N2 target for which the ratio became increasingly larger than unity as N′ increased. The observed fine-structure effects were nearly the same as those from the corresponding (reverse) reactive collisions, and a close connection between the inelastic and reactive events has been conjectured.
Rotational diffusion of liquid toluene in confined geometry
Authors: Liu G. , Mackowiak M. , Li Y. , Jonas J.
Published: July 2002

The effects of geometrical confinement on the molecular dynamics of liquid toluene in porous sol–gel silica glasses have been studied by the NMR method. The NMR deuteron spin–lattice relaxation times for toluene-d8, ortho-toluene-d1, and para-toluene-d1 were measured as a function of temperature for bulk and confined liquids. The rotational diffusion tensors for overall, internal, and anisotropic rotational diffusion have been determined. The effect of geometrical confinement results in a considerable decrease of the rotational diffusion constants and leads to a lowering of the symmetry of the anisotropic rotational diffusion tensor.
Mixed quantum-classical studies of H2 dissociation on metals: Dependence upon molecular geometry and dimensionality
Authors: Cruz Astrid J. , Jackson Bret
Published: July 2002

A mixed quantum-classical model is used to compute the probability for dissociation and rotational excitation for H2, HD, and D2 scattered from a Ni surface. The vibrational coordinate, the polar orientation angle, and the center of mass translation of the molecule normal to the surface are treated quantum mechanically using 3D spectral grid/fast Fourier transform techniques. The remaining degrees of freedom are treated classically. The dissociation probabilities are computed as a function of molecular kinetic energy and compared with those determined in a previous 2D study. An increase in rotational excitation coincides with an increase in dissociation as predicted by a recently developed analytical model. The dependence of the dissociation and rotational excitation probabilities on initial internal molecular state, molecular orientation, and surface impact site are also examined.
Separation of collisional and vibrational variables in chemical reactions: Decoupling surface in phase space
Authors: Someda Kiyohiko , Nakamura Hiroki
Published: July 2002

A method for decoupling the collisional and vibrational variables in the collision process is developed. The present procedure of decoupling is based on a canonical transformation in phase space. The basic principle employed is the ‘‘maximal decoupling condition’’ [∂H/∂u]u=v=0=[∂H/∂v]u=v=0=0, where H is the Hamiltonian and u and v represent new vibrational coordinates and momenta. ‘‘Decoupling surface’’ is defined as the surface determined by u=v=0. The partial differential equation to be satisfied by this decoupling surface is derived. This partial differential equation can be solved easily by utilizing classical trajectories. Local vibrational frequencies along the decoupling surface are defined and their stability analysis is shown to provide the criterion for the separability. ‘‘Adiabatic approximation’’ which assumes the conservation of locally defined actions of the new vibrational variables leads to an ‘‘effective Hamiltonian’’ that describes the collision process under a given set of the initial vibrational actions. The present theory is applied to the collinear H+H2→H2+H chemical reaction. Analysis of the local frequencies shows that the instability occurs in the following two regions depending on the collision energy: (1) when the collision energy is just above the activation energy, the instability occurs around the saddle point of the potential energy surface and (2) at the higher collision energies, this unstable region disappears, but another one appears in the region where the reactive trajectories touch the ridge of the potential just after passing through the saddle point. In these unstable regions, bifurcation of the trajectories occurs. Analysis of the effective Hamiltonian can indicate the existence of a resonance structure corresponding to a temporal trapping of trajectory. This resonance structure is analogous to that in the coupled-oscillator problems and we can expect that the so-called irregular motion, or a stochastic layer, exists around the separatrix connected to this resonance structure.
Green’s functions in quantum transition state theory
Authors: Stuchebrukhov A. A.
Published: July 2002

Tunneling corrections to the classical transition state theory rate are considered for multidimensional anharmonic potentials. A theory is proposed for systematic accounting for the reaction barrier anharmonicity and associated curvature of the reaction path in terms of thermodynamic Green’s functions. A corresponding diagrammatic technique is developed for calculation of the rate constant. The theory allows summation of an infinite number of perturbation terms in a self-consistent manner. It is shown that as a result of self-consistent treatment, a renormalized potential function of the barrier can be introduced in place of the original potential which is the average of the latter over the thermal de Broglie wavelength of the tunneling particle. Even simple approximations for the rate calculated for such averaged potential barrier automatically takes into account a large body of anharmonic and quantum effects. The harmonic approximation of the renormalized potential coincides with the ‘‘best fit’’ quadratic potential surface introduced in the paper of G. Voth, D. Chandler, and W. H. Miller [J. Chem. Phys. 91, 7749 (1989)]. The theory also provides a method for calculation of further anharmonic corrections to this renormalized harmonic approximation.
Inherent structures in polyatomic liquids: Simulation for Si2F6
Authors: Weber Thomas A. , Stillinger Frank H.
Published: July 2002

A molecular dynamics study has been carried out on a model for the polyatomic liquid Si2F6. A wide range of temperatures was examined, as well as a 19% density variation. Transferrable two-atom and three-atom interaction functions were combined to represent the system potential energy, and to allow for the possibility of chemical rearrangements. It was discovered that the model liquid is quite stable at modest temperature and pressures, in agreement with experiment; furthermore, it exhibits a temperature-independent mean inherent structure, as has been found for simpler atomic liquids. However, the imposition of strong compression along with high temperature causes the Si2F6 fluid to undergo dissociation, atom exchange, and polymerization reactions. The result of such chemical degradation is a significant shift in the mean inherent structure away from its pure-liquid temperature-independent form.
Theory of liquid-state activated barrier crossing: The instantaneous potential and the parabolic model
Authors: Adelman Steven A. , Muralidhar R.
Published: July 2002

This paper gives a theoretical treatment of liquid-phase activated barrier crossing that is valid for chemical reactions which occur on typical (e.g., high activation barrier) potential-energy surfaces. This treatment is based on our general approach [S. A. Adelman, Adv. Chem. Phys. 53, 61 (1983)] to problems in liquid-phase chemical dynamics. We focus on the early-time regime [times short compared to the relaxation time of 〈F̃(t)F̃〉0, the fluctuating force autocorrelation function of the reaction coordinate] in which the solvent is nearly ‘‘frozen.’’ This regime has been shown to be important for the determination of the rate constant in the molecular-dynamics simulations of model aqueous SN2 reactions due to Wilson and co-workers. Our treatment is based on a generalized Langevin equation of motion which naturally represents the physics of the early-time regime.

In this regime the main features of the reaction dynamics are governed by the instantaneous potential WIP[y,F̃], which accounts for the cage confinement forces which dominate the liquid-phase effects at early times, rather than by the familiar potential of mean force. The instantaneous potential is derived from the t→0 limit of the equation of motion and its properties are developed for both symmetric and nonsymmetric reactions. The potential is then shown to account for both the early-time barrier recrossing processes found to determine the transmission coefficient κ in the SN2 simulations and the dependence of these processes on environmental fluctuations modeled by F̃. Making the parabolic approximation for the gas-phase part of WIP[y,F̃] yields the following result for the transmission coefficient: κ=ω−1PMFx+=ω−1PMFωMIP[1+ω−2 MIPΘ̂(x+)]1/2≠ ω−1PMFω MIP[1+ (1)/(2) ω−2MIPΘ̂(ωMIP)], where ωMIP and ωPMF are, respectively, the barrier frequencies of WIP[y,F̃=0] and of the potential of mean force, and where Θ̂(x+)=∫∞0 exp(−x+t)Θ(t)dt with Θ(t)≡(kBT)−1〈F̃(t)2F〉0. This result for κ, which is equivalent to a result of Grote and Hynes, but which more naturally represents the physics of the early-time regime, permits a straightforward interpretation of the variation of the transmission coefficients for the model SN2 systems.
Complete description of two-photon (1+1′) ionization of NO deduced from rotationally resolved photoelectron angular distributions
Authors: Leahy David J. , Reid Katharine L. , Zare Richard N.
Published: July 2002

Time-of-flight photoelectron spectroscopy has been used to record energy-resolved photoelectron angular distributions (PADs) following (1+1′) resonance-enhanced multiphoton ionization (REMPI) of NO via the vi=1,Ni=22 rovibrational level of the A 2∑+ state. The PADs corresponding to single rotational states of the resulting molecular ion show a strong dependence on the change in ion core rotation ΔN(≡N+−Ni) and also on the angle between the linear polarization vectors of the two light beams. Broken reflection symmetry [I(θ,φ)≠I(−θ,φ)] is observed when the polarization vectors of the two light beams form an angle of 54.7°. A fit to the PADs provides a complete description of this molecular photoionization, namely, the magnitudes and phases of the radial dipole matrix elements that connect the intermediate state to the ‖lλ〉 photoelectron partial waves (Refs. 1 and 2). This information is then used to predict unobserved quantities, such as ion angular momentum alignment and the full three-dimensional form of the PADs.
On the validity of partitioning techniques for molecular infrared-multiphoton excitation
Authors: Jolicard Georges , Grosjean Alain
Published: July 2002

A wave operator treatment of the molecular infrared-multiphoton excitation is presented. The Bloch’s wave operator concept is used as a tool to define a model space around the initial state and to build inside this space the effective Hamiltonian which drives the dynamics. Simple applications made with a rotator coupled to an intense laser field demonstrate the validity of this formulation.
Crystallization of polyethylene and polytetrafluoroethylene by density-functional methods
Authors: McCoy John D. , Honnell Kevin G. , Schweizer Kenneth S. , Curro John G.
Published: July 2002

Molecular density-functional theory is extended to address the crystallization of chemically realistic polymers. The polymer (RISM) reference interaction site model integral-equation approach is employed to calculate the liquid-state structural information required as ‘‘input’’ into our density-functional theory. The single-chain structure is described by the rotational isomeric state model, and the accuracy of both the theoretically calculated single-chain and liquid structures have been verified by direct comparison with Monte Carlo simulation and x-ray scattering, respectively. The driving forces for the crystallization of polymers are found to be completely different from those in monatomic systems and can be understood in terms of an effective ‘‘chain-straightening force’’ (which results from chain packing) combined with a background attractive potential. Remarkably, the predicted melting temperatures for polyethylene and polytetrafluoroethylene at atmospheric pressure are within a few degrees of the experimental values, and the density–temperature phase diagrams are also in good agreement with experiment. Chemically unrealistic, coarse-grained models of polymer structure appear to be inadequate for the crystallization phenomenon, which is found to be quantitatively sensitive to interchain attractive forces and melt compressibility. The aspect ratios in polyethylene and polytetrafluoroethylene melts at the phase transition are predicted to be virtually identical.
Energetics and dynamics in the dissociative photoionization of PF3 at 21.2 eV
Authors: Reynolds David J. , van Kleef Eddy H. , Powis Ivan
Published: July 2002

The first five electronically excited states of PF+3 generated by He i photoionization are found to dissociate exclusively to PF+2 with dynamically distinct mechanisms. A pronounced correlation is observed between the direction of ejection of the photoelectron and the photofragment ion which issue from individual photoionization events, from which it is inferred that both the photoelectron emission and the subsequent unimolecular ion dissociation are highly anisotropic processes. Measurement of the kinetic energy which accompanies PF+2 formation at its experimental onset is used to identify the preferred thermochemical threshold value for this ion.
The lowest excited 1A2 and 1B1 states of ozone: Two conical intersections and their impact on photodissociation
Authors: Braunstein M. , Hay P. J. , Martin R. L. , T Pack R.
Published: July 2002

We report results of ab initio configuration-interaction calculations on the lowest excited 1A2 and 1B1 states of ozone near their C2v minima and also at several nearby Cs geometries, where these two states become the lowest 1A″ states. We find that in C2v symmetry these two states have a seam intersection very near the 1B1 minimum which becomes a conical intersection for small distortions to Cs geometries. The influence of this intersection on the Wulf and Chappuis bands of ozone is discussed and a new interpretation of these bands is presented. We also demonstrate the existence of a second conical intersection involving these same two states for D3h symmetries, where they become degenerate components of a 1E″ state.
Rotationally resolved photoelectron spectroscopy of the 2Σ− Rydberg states of OH: The role of Cooper minima
Authors: de Beer E. , de Lange C. A. , Stephens J. A. , Wang Kwanghsi , McKoy V.
Published: July 2002

We have measured rotationally resolved photoelectron spectra of the OH radical using (2+1) resonance enhanced multiphoton ionization spectroscopy via the D 2Σ−(3pσ) and 3 2Σ−(4sσ) Rydberg states. For the D 2Σ−(3pσ) state, we observe primarily ΔN=even distributions of ionic rotational states, in contrast to the ΔN=odd distribution expected for ionization of a 3pσ Rydberg electron. The observations are described quantitatively by ab initio calculations which predict a Cooper minimum in the 3pσ→kπ(l=2) channel, whose occurrence determines the ΔN=even ion rotational distribution. In contrast, the 3 2Σ−(4sσ) photoelectron spectra reveal a broad distribution in rotational levels, arising from greater l mixing in the higher Rydberg orbital and much weaker Cooper minima in the continuum.
Rotational, fine, and hyperfine structure in the high-resolution electronic spectrum of ArOH and ArOD
Authors: Chang Bor-Chen , Yu Lian , Cullin David , Rehfuss Brent , Williamson James , Miller Terry A. , Fawzy Wafaa M. , Zheng X. , Fei S. , Heaven Michael
Published: July 2002

A number of vibrational bands of the Ã 2Σ+↔X̃ 2Π electronic spectrum of both ArOH and ArOD have been investigated by laser induced fluorescence with a high-resolution, pulsed laser system yielding linewidths ≲250 MHz in the UV. This spectrum not only displays completely resolved rotational structure, but also fine and hyperfine structure. The hyperfine constants and precise interatomic distances derived from the rotational constants provide a very interesting picture of the electronic and geometric structure of the complex. The bonding is incipiently chemical in the Ã state with clear evidence for at least some electronic reorganization between Ar and the open-shell OH radical in the complex. Conversely, the X̃ state appears to be bound almost solely by physical van der Waals interactions characteristic of systems containing only closed-shell species.
Vibrational relaxation of highly excited toluene
Authors: Toselli Beatriz M. , Brenner Jerrell D. , Yerram Murthy L. , Chin William E. , King Keith D. , Barker John R.
Published: July 2002

The collisional loss of vibrational energy from gas-phase toluene, pumped by a pulsed KrF laser operating at 248 nm, has been observed by monitoring the time-resolved infrared fluorescence from the C–H stretch modes near 3.3 μm. The fragmentation quantum yield of toluene pumped at 248 nm was determined experimentally to be ∼6%. Energy-transfer data were obtained for 20 collider gases, including unexcited toluene, and analyzed by an improved inversion technique that converts the fluorescence intensity to the bulk average energy, from which is extracted 〈〈ΔE〉〉, the bulk average amount of energy transferred per collision. Comparisons are presented of these results with similar studies of benzene and azulene, and with the time-resolved ultraviolet absorption study of toluene carried out by Hippler et al. [J. Chem. Phys. 78, 6709 (1983)]. The present results show 〈〈ΔE〉〉 to be nearly directly proportional to the vibrational energy of the excited toluene from 5000 to 25 000 cm−1. For many of the colliders at higher energies, the energy dependence of 〈〈ΔE〉〉 is somewhat reduced. A simple method is described for obtaining good estimates of 〈ΔE〉d (the energy transferred per collision in deactivating collisions) by carrying out an appropriate least-squares analysis of the 〈〈ΔE〉〉 data. The values of 〈ΔE〉d are then used in master-equation calculations to investigate possible contributions from ‘‘supercollisions’’ (in which surprisingly large amounts of energy are transferred) in the deactivation of toluene.
The ground state and excited d-hole states of CuAu
Authors: Bishea Gregory A. , Pinegar Jacqueline C. , Morse Michael D.
Published: July 2002

Resonant two-photon ionization spectroscopy has been applied to jet-cooled diatomic CuAu. Eight band systems have been observed, rotationally resolved, and analyzed. The ground state is X 1Σ+ in symmetry, deriving from the 3d10Cu5d10Auσ2 molecular configuration. Its bond length has been determined as r0=2.3302±0.0006 Å (1σ error limits). The first excited state is the a 3Σ+ state, which derives from the 3d10Cu5d10Auσ1σ*1 molecular configuration. This state possesses a nominal bond order of zero, but is nevertheless bound by D0=1157±150 cm−1 and has a bond length of 2.428±0.028 Å. It is observed through the a 3Σ+–X 1Σ+ transition which is very weak, with f≊5×10−5. Seven higher energy band systems are also observed, most of which correlate asymptotically to d-hole states described by the Cu 3d104s1, 2S+Au 5d96s2, 2D5/2, Cu 3d94s2, 2D5/2+Au 5d106s1, 2S, and Cu 3d94s2, 2D3/2+Au 5d106s2, 2S separated atom limits. The D 0+ excited state, however, displays a very large oscillator strength in transitions with the ground state (f≊0.11), and it is argued that this state corresponds to the ion pair state Cu+Au−. Future experiments are planned to test this hypothesis by measuring the permanent electric dipole moment of this state.
The application of integral equation theories to fluids of nonspherical particles near a uniform planar wall
Authors: Bérard D. R. , Patey G. N.
Published: July 2002

A general reduction of the Ornstein–Zernike equation is given for molecular fluids near a planar wall. This allows integral equation approximations such as the hypernetted-chain or reference hypernetted-chain (RHNC) theories to be solved numerically for such systems. Dipolar hard sphere fluids near a hard wall are considered in detail and RHNC solutions are obtained. The results are compared with previous calculations for curved surfaces. The RHNC result for the asymptotic behavior of the wall–solvent pair correlation function at large separations is derived and compared with expressions given by classical continuum theory and by exact analysis.
The scattering at zero angle in contrast variation
Authors: Teubner Max
Published: July 2002

The scattering of neutrons, x-rays, or light by a polydisperse system of particles is studied. As is well known, for fixed wave vector q the scattering intensity I(q) is a quadratic function of the scattering length density of the external medium. The minimum of the parabola is the minimal scattering intensity Imin(q). It is shown that Imin(0) is, for a certain large class of particles, directly proportional to the width of the size distribution [so that Imin(0)=0 for strictly monodisperse particles]. Explicit formulas are presented which permit the measurement of the size distribution for spheres, disks, rods, and similar particles.
State-selected reactive scattering. II. He+H+2→HeH++H
Authors: Pollard J. E. , Johnson L. K. , Cohen R. B.
Published: July 2002

The endoergic reaction He+H+2→HeH++H is investigated in a molecular-beam experiment as a function of H+2 vibrational energy at c.m. collision energies between 0.3 and 1.9 eV. Reactant ions generated by resonantly enhanced four-photon ionization are impulsively accelerated to collide with a beam of He. Time-of-flight velocity distributions of HeH+, measured at one laboratory angle, yield the differential cross section at c.m. angles θ=0° and 180°. A shift from ‘‘He rebound’’ to ‘‘H+ stripping’’ behavior accompanies the enhancement in the cross section as the H+2 vibrational energy increases, which matches previous studies at higher collision energy. Small-impact-parameter events produce HeH+ with less recoil velocity (more internal energy) than those at large impact parameters. Within the limits of sensitivity and resolution (ΔE≊0.15 eV), definitive resonance features in the collision energy dependence of dσ/dω are not evident. Improvements in the technique to enable such observations are suggested.
Quadrupole echoes and the 127I nuclear hexadecapole interaction in KI
Authors: Ni Q. W. , Sears R. E. J.
Published: July 2002

A quadrupole echo method for searching for the nuclear hexadecapole interaction (HDI) is described. A sequence of three radio frequency pulses applied at times 0, τ, and nτ (n>1) gives a simple pattern of nuclear spin (5)/(2) echoes when only the nuclear Zeeman and quadrupole interactions are present. Calculations for n=2.5, 3.0, 4.0, and 5.0 show that these patterns undergo an elaborate change when the HDI is also present and that these echo patterns are unique to the HDI. Three pulse echoes were used to search for the 127I HDI in a single crystal of KI but, for this sample, a two pulse sequence was found to be more effective. For KI, 2.8 kHz is the estimated magnitude of the 127I hexadecapole coupling constant, eHV(4)/h, where eH is the 127I electric hexadecapole moment and V(4) is a fourth derivative of the electrostatic potential at the nuclear site. This is the smallest HDI measured in a solid so far.
Proteins and glasses: A comparative study of spectral diffusion phenomena
Authors: Zollfrank J. , Friedrich J. , Vanderkooi J. M. , Fidy J.
Published: July 2002

We compare the spectral diffusion broadening of mesoporphyrin substituted horseradish peroxidase with the chromophore doped host glass. Whereas the broadening in the glass is smooth and follows a power law with an exponent close to (3)/(2), there is a a steplike broadening in the protein which occurs around 14 K. The observed phenomena are interpreted on the basis of a two domain interaction of the chromophore with its environment.
Observation of continuum absorption spectra in far wings of the Yb resonance line broadened by He, Ne, Ar, Kr, Xe, and Yb
Authors: Ueda K. , Sonobe O. , Chiba H. , Sato Y.
Published: July 2002

We have determined reduced absorption coefficients in far wings of the Yb (6s2)1S0–(6s6p)1P1 line broadened by He, Ne, Ar, Kr, Xe, and Yb. We find a prominent blue-satellite peak for all perturbers and undulations between the satellite peak and the line center for Ne, Ar, Kr, and Xe. Analyzing the satellite band with the aid of the unified Franck–Condon method, we obtain the position and the height of the extremum on the difference potential-energy curves concerned. We have also observed collision-induced-dipole absorption bands associated with the transition (6s2)1S0–(6s5d)3D2 of atomic Yb for Ne, Ar, Kr, and Xe perturbers.
Internal-state distribution of recombinative hydrogen desorption from Si(100)
Authors: Kolasinski Kurt W. , Shane Stacey F. , Zare Richard N.
Published: July 2002

We have measured vibrational- and rotational-state distributions for H2, D2, and HD thermally desorbed from the monohydride phase on Si(100) surfaces using resonance-enhanced multiphoton ionization detection. The ν=1 to ν=0 population ratio is roughly 20 times higher than that predicted by Boltzmann statistics at the surface temperature, Ts≊780 K. In contrast, the average rotational energies of the desorbed molecules are significantly lower than kTs, exhibit no isotopic dependence within experimental error, and are not significantly different in the ν=0 and ν=1 vibrational states. In the vibrational ground state, we find 〈Erot〉 =345±83 K, 451±77 K, and 332±57 K for H2, HD, and D2, respectively. The degree of vibrational excitation suggests that the H–H interatomic distance in the transition state is elongated compared with the gas-phase equilibrium bond distance. The low average rotational energy clearly rules out recombination from a highly asymmetric transition state or recombination from high-impact-parameter collisions. Our data may be interpreted as resulting from a preference for reactive trajectories that impart little angular momentum either through the effects of the corrugation of the potential-energy hypersurface or through the collision leading to the transition state, followed by prompt desorption of the newly formed molecular hydrogen from Si(100). We propose that pairing on Si dimers occurs prior to desorption; various models are discussed regarding the desorption mechanism subsequent to pairing.
Low-energy vibrations of NH3 and CO adsorbed on Fe(110)
Authors: Toennies J. P. , Wöll Ch. , Zhang G.
Published: July 2002

The low energy vibrations of NH3 and CO molecules adsorbed on Fe(110) have been investigated with inelastic He-atom scattering. For ammonia an energy loss of 10 meV was observed in the time-of-flight spectra and is assigned to the frustrated translations. Previously, this mode had only been observed for adsorbed CO molecules. For CO on Fe(110) the energy of the frustrated translations is 4.6 meV and the observed linewidth is smaller than 0.6 meV.
High resolution electronic spectroscopy of Ne⋅OH
Authors: Chang Bor-Chen , Cullin David W. , Williamson James M. , Dunlop James R. , Rehfuss Brent D. , Miller Terry A.
Published: July 2002

The high resolution electronic spectrum of Ne⋅OH has been recorded in a supersonic free jet expansion using the laser-induced fluorescence technique. From an analysis of the spectrum which yields rotational constants, we are able to obtain Ne⋅OH bond lengths in several vibrational (hindered rotor) levels of the excited state and the vibrationless level of the ground state. We also measure the Fermi contact constant in the Ã state which is, unlike Ar⋅OH, insignificantly perturbed from the value in the OH monomer. However, we now measure a parity doubling of the X̃ state rotational levels which is tenfold larger than the upper limit we established for such an interaction in Ar⋅OH. We interpret these latter measurements to imply weaker and more isotropic bonding in Ne⋅OH compared to Ar⋅OH in both electronic states.
Complete alignment and orientation of atoms and molecules by stimulated Raman scattering with temporally shifted lasers
Authors: Band Y. B. , Julienne P. S.
Published: July 2002

Some simple schemes for obtaining complete alignment and orientation of atoms and molecules using stimulated Raman scattering with temporally shifted laser fields is proposed. (AIP)
A mechanism for growth on diamond (110) from acetylene
Authors: Belton David N. , Harris Stephen J.
Published: July 2002

We have proposed a detailed chemical kinetics mechanism for the addition of C2H2 to a (110) diamond surface, which is the fastest growing face. The model contains no adjustable parameters and is based on the hypothesis that diamond surface chemistry may be understood in analogy with gas-phase hydrocarbon chemistry. We calculated a growth rate of 0.03 μm/h, which gives order-of-magnitude agreement with experiments and suggests we have a feasible mechanism for growth on (110) surfaces.
Electronic structures and photoelectron spectra of Si−3 and Si−4
Authors: McMichael Rohlfing Celeste , Raghavachari Krishnan
Published: July 2002

Vibrationally resolved photoelectron spectra of Si−3 and Si−4, recently reported by Kitsopoulos, Chick, Weaver, and Neumark, are interpreted using ab initio quantum chemical calculations of the ground and excited electronic states of the corresponding neutral clusters. The calculated electron affinities as well as the low-lying excitation energies agree within 0.1–0.2 eV of the experimental values, thus confirming the theoretically predicted structures of neutral and anionic Si3 and Si4 reported previously.