bluuebunny/crossref_metadata_2025 huggingface dataset:
Rapid long range intramolecular electron transfer within a steroid molecule with two electron binding groupsa)
Authors: Huddleston R. Kurt , Miller John R.
Published: February 2003

Intramolecular electron transfer has been observed to have occurred in less than 100 ns in a steroid molecule having two distinct electron binding groups separated by distances distributed from 7–11 Å. Experiments were carried out in organic glasses at 77 K with pulse radiolysis techniques to create trapped electrons which were captured by a group on one end of the steroid molecule. Although one of the groups, benzoate, is held to the steroid spacer by a flexible linkage, the rigidity of the glassy matrices prevented movement to alter the initial distance. Interestingly, no effects of distance were seen: all ET processes appeared to have occurred much faster than our 100 ns time resolution, consistent with measurements of the rate of intermolecular electron transfer between the same functional groups in random solutions. Solvation energetics, on the other hand, had a remarkable influence on the extent and direction of electron transfer. A change in solvent polarity was observed to reverse the direction of electron transfer. Evidence was obtained for a distribution of solvation environments for ions in glasses which may be as broad as 0.15 eV.
Solvent influence on photoisomerization dynamicsa)
Authors: Rothenberger G. , Negus D. K. , Hochstrasser R. M.
Published: February 2003

The rate constants of the nonradiative decay process of the excited singlet state of trans-stilbene and trans-1,1′-biindanylidene (‘‘stiff ’’ stilbene) were measured in liquid n-alkanes at room temperature. These data were compared with the two models for isomerization in condensed phases proposed by Kramers and by Grote and Hynes. For trans-stilbene, the agreement with Kramers’ model was unsatisfactory, whereas better agreement was obtained with Grote and Hynes’ model. The data for stiff stilbene agreed well with Kramers’ model in the Smoluchowski limit. These results were explained in terms of the shape of the potential energy barrier which separates the trans from the twisted configuration of the excited singlet state of these molecules. It is suggested that the deviation between theory and experiment is a result of a qualitative difference in the microscopic solvent–solute collisional interaction when the solvent molecules are longer than stilbene. The best fit of theory to experiment for trans-stilbene requires the use of an unphysically low (8 cm−1) frequency for the top of the barrier to isomerization.
Thermal energy charge−transfer reactions: He+2 with N2 and CO
Authors: Endoh Minoru , Tsuji Masaharu , Nishimura Yukio
Published: February 2003

The reactions of He+2 with N2 and CO at thermal energy have been studied by optical emission spectroscopy in a He flowing afterglow. The following emission systems have been observed: N+2(B 2Σ+u− X 2Σ+g) and CO+(B 2Σ+−X 2Σ+, B 2Σ+− A 2Πi, A 2Πi−X 2Σ+). The vibrational and rotational distributions of the products have been determined. A Franck–Condon-like vibrational distribution is obtained for N2+(B) and CO+(B) which are formed through energy-resonant processes. In contrast, the vibrational distribution of CO+(A), which is formed through a nonenergy-resonant process, is shifted to higher vibrational levels in comparison with Franck–Condon-type distribution. Simple model calculations are suggestive of mutual effects of Franck–Condon and energy resonance criteria for the formation of CO+(A) vibrational levels. Rotational excitations are observed in N2+(B) and CO+(B). The effective rotational temperatures of N+2(B: v′=0) and CO+(B: v′=0) are 900±60 and 890±100 K, respectively.
Stochastic theory of intramolecular vibrational energy redistribution and dissociation in the presence of radiation
Authors: Eslava Luis Alejandro , Augustin Stuart D. , Rabitz Herschel
Published: February 2003

A theory for internal vibrational energy redistribution and dissociation in polyatomic molecules in the presence of a strong radiation field is formulated. The fundamental assumption is that a random phase approximation is valid at specific time intervals. This results in the replacement of the Schrödinger equation by a master-type equation, which is further approximated by a Fokker–Planck diffusionlike equation. Energy transfer is described as a flow of probability among the quantum states, and the dissociation dynamics are embodied in the boundary conditions. By virtue of the continuous character of the Fokker–Planck equation, the computational difficulty of its numerical solution depends only on the number of degrees of freedom and not on the number of states. Due to the high density of levels encountered in a polyatomic molecule, this is of paramount importance in reducing the problem to a manageable size. A multiple time scale stochastic formulation, which allows for a mixed quantum-stochastic approach, is also described. No assumptions regarding  the strength of the intramolecular coupling are made, and energy conservation is specifically enforced. The coefficients of the Fokker–Planck equation are shown to be expressible in terms of simple functions of the molecular potential, which involve raising and lowering operators. Finally, the coefficients of the Fokker–Planck equation are calculated using the best available potential information for the case of the ozone molecule in a strong infrared laser field, and their physical significance is discussed.
Excess vibrational energy dependence of quantum beats and fluorescence decay in molecules with intermediate case level structure: 1B1 pyrimidine with 0–1300 cm−1 excess energies
Authors: Saigusa H. , Jameson A. Keith , Lim E. C.
Published: February 2003

We have measured the biexponential decay and quantum beats from jet-cooled pyrimidine excited to 1B1(nπ*). Single vibronic level excitation of the Q branch of ten vibronic levels produces observable quantum beats in the long component decay of almost all of these levels. Although the ratio of the preexponential factors Af/As becomes progressively larger in a theoretically well-understood manner, the quantum beat pattern remains deceptively simple for all the vibronic levels probed 0–1300 cm−1 above the S1 band origin. We attribute this anomaly to a few levels strongly coupled to the optically prepared singlet resulting in the observed beat pattern, while a dense manifold of less strongly coupled levels dominates the characteristic intensities of the biexponential decay.
ESR of bimetallic transition-metal molecules at 4 K. I. CrCu, CrAg, and CrAu
Authors: Baumann C. A. , Van Zee R. J. , Weltner W.
Published: February 2003

CrCu, CrAg, and CrAu molecules have been observed via ESR in solid rare-gas matrices at 4 K. Isotopically enriched 53Cr (I=3/2) was also used in the preparation of CrCu and CrAg with the observation of distinctly different Cr hyperfine splittings, 37 and <1/2 G, respectively. Analysis of the spectra, including the hyperfine and quadrupole interaction with 63,65Cu, 107,109Ag, and 197Au, indicated that CrCu has a 4Σ ground state, while the heavier molecules have the expected 6Σ ground state. Thus, CrCu is proposed to be triply bonded and therefore intermediate in properties between Cr2 and Cu2. The zero-field splitting parameters, D, in the three molecules were found to increase from −0.005(1) to (+)0.44(1) to ≥(+)2 cm−1 with the increasing spin-orbit coupling.
Determination of the spectrum of recoil energies for chlorine atoms generated by the 35Cl(n,γ) 36Cl nuclear process
Authors: Chang J. , Ferro L. J. , Spicer L. D.
Published: February 2003

The recoil energy spectrum for 36Cl produced in the (n,γ) nuclear reaction is calculated from prompt γ-ray spectroscopy data of Spits and Kopecky. The average energy for the recoil 36Cl atoms is found to be 634 eV, whereas the most probable energy is 720 eV if the maximum recoil energy of 1099 eV is excluded. Comparison is made with the corresponding spectrum for the lower energy, recoil 38Cl atoms also generated by thermal neutron capture.
Interpolation of molecular wave functions with a variable geometrical parameter
Authors: Montagnani Raffaele , Riani Pierluigi , Salvetti Oriano
Published: February 2003

An interpolative method for calculating approximate wave functions of molecules with a variable geometrical parameter is presented. This method has been applied to diatomic molecules (variation of the internuclear distance), triatomic molecules (bond angle), and tetratomic molecules such as NH3 (H-molecular axis angle). The interpolation is referred to the coefficients of the basis functions and to the coefficients of the configurations in the case of a many-configuration wave function. The knowledge of the wave function at the extreme points of the interpolation and of the potential energy curve in an approximate form is needed.
Some remarks on the influence of well width on the convergence of perturbation theory for the square-well fluid
Authors: Henderson D.
Published: February 2003

The dependence of the convergence of perturbation expansions upon well width is examined for the square-well fluid. For two- and higher-dimensonal square-well fluids, perturbation expansions converge best for large well width.
Theory of non-Markovian exciton transport in a one dimensional lattice
Authors: Bagchi Biman , Oxtoby David W.
Published: February 2003

We consider non-Markovian exciton transport in a one dimensional system with nearest neighbor interactions. Kubo’s stochastic Liouville equation (SLE) is used to derive a set of coupled equations of motion which are exact for a Poisson bath and which also should give a fairly accurate long time description for a Gaussian bath. The SLE is especially suitable for this problem since both cumulunt expansion and projection operator techniques encounter difficulties when the system is quantum mechanical, the stochastic off-diagonal coupling is strong, and the relaxation is non-Markovian. To elucidate the role of correlations in space and time, we consider two different models: (a) Both diagonal and off-diagonal fluctuations arise from the same bath. (b) All the fluctuations arise from uncorrelated baths. We find that model (a) always gives coherent exciton transport even at long times if the mean off-diagonal coupling J is nonzero. Model (b) gives diffusive behavior at long times, but the diffusion constant is very different in the non-Markovian limit from the prediction of Markovian or weak-coupling theories. In the Markovian limit, the expressions for the mean square displacement agree exactly with those of Haken and co-workers and of Grover and Silbey. Some interesting new features of non-Markovian transport are discussed.
A study of the freezing transition in the Lennard-Jones system
Authors: Bagchi Biman , Cerjan Charles , Rice Stuart A.
Published: February 2003

The freezing of a Lennard-Jones fluid into a face-centered-cubic lattice is examined within the framework of a recently proposed analysis of the nonlinear integral equation that relates the densities of the coexisting liquid and solid phases. The temperature dependent densities of the liquid and solid phases along the coexistence line in the temperature–density plane are calculated and found to be in reasonable agreement with the results of computer simulations.
A Monte Carlo approach for the real time dynamics of tunneling systems in condensed phases
Authors: Behrman E. C. , Jongeward Gary A. , Wolynes Peter G.
Published: February 2003

A Monte Carlo method for evaluating real time path integrals is proposed. We study two-level tunneling systems coupled to many body environments. A dynamical correlation function for a model of quantum coherence shows a plateau indicative of the localization effects of damping.
The quantum mechanical cell model in two dimensions
Authors: Phillips James M. , Bruch L. W.
Published: February 2003

The statistical thermodynamic properties of 2D triangular lattices of neon and argon are calculated for the quantum mechanical version of the non-self-consistent cell model. Energy eigenvalues, in a circularly averaged cell potential, are obtained by the numerical integration of the Schrödinger equation for the Lennard-Jones and Devonshire form of the cell model. A general method is given for treating troublesome singularities in the numerical solution of the 2D Schrödinger equation at low angular momentum. A maximum in the specific heat for highly dilated lattices is observed.
Nonresonant third order susceptibilities for various gases
Authors: Lundeen Thomas , Hou Shih-Yue , Nibler Joseph W.
Published: February 2003

Relative values of the nonresonant third order electric susceptibilities of various gases have been measured by nonlinear optical mixing at 532 and 683 nm. These results have been converted to absolute values by scaling them to the vibrationally resonant susceptibility of H2 which has been calculated from recent Raman cross section and linewidth data. Comparisons are offered with nonresonant electronic susceptibilities by other methods.
Concentration and excluded volume dependence of coherent scattering functions for polymers: Chain conformational space renormalization group
Authors: Freed Karl F.
Published: February 2003

The concentration dependent chain conformational space renormalization group theory is extended to a consideration of the explicit dependence on the strength of the excluded volume interaction in the nonscaling, ‘‘cross-over,’’ region between the single chain Gaussian and self-avoiding limits. Generalized ‘‘scaling laws’’ in this cross-over domain are derived without the use of perturbation theory or ε expansions. In the dilute and semidilute region these generalized scaling laws have distances scaled by the infinite dilution radius of gyration RG(ζ) where ζ is a scaling variable which vanishes in the Gaussian chain limit and tends to infinity in the good solvent limit. Hence, concentrations c are scaled by the overlap concentration c*ζ evaluated from the cross-over RG(ζ). Quantities like the coherent scattering function are then functions of kRG(ζ), c/c*ζ, and ζ with k the momentum transfer vector. The explicit ζ dependence persists through the cross-over regime for all c and is eliminated in the simple scaling limits ζ→0 or ∞. We use the theory to evaluate the coherent scattering functions for a single tagged polymer chain, for a set of tagged chains at concentration cT<c, as well as the full polymer solution. The concentration and excluded volume dependences of a tagged chain radius of gyration and the full solution correlation length are evaluated, and the former is studied as a function of the ratio of the tagged chain molecular weight to that for an average solution chain.
The flexible and the wormlike range in a real polymer chain
Authors: Allegra Giuseppe
Published: February 2003

From the statistics of unperturbed chains with interdependent rotations, a quadratic intramolecular potential of general validity may be derived on the basis of the Fourier configurational representation [G. Allegra, J. Chem. Phys. 68, 3600 (1978)]. Introducing a suitable observational cutoff it is proven that any real chain shows a different elastic behavior whether D is larger (flexible range) or smaller (wormlike range) than a suitable limit. In the case of polyethylene and atactic polystyrene, the latter range is observable for D smaller than ∼25 Å (and larger and ∼8 Å). The flexible range exists even for the ideal, infinitely long Porod–Kratky chain; its neglect leads to an inconsistency between the intramolecular potential and the mean-square chain length.
Collective diffusion, nucleation, and spinodal decomposition in polymer mixtures
Authors: Binder K.
Published: February 2003

The dynamics of concentration fluctuations in binary polymer mixtures AB, A and B being long and flexible chains, is studied near the critical point of unmixing. A ‘‘hydrodynamic’’ theory in close analogy to the treatments of Cahn–Hilliard and Cook for the dynamics of unmixing alloys is developed, and its validity is carefully analyzed. It is shown that homogeneous nucleation for long chains is negligible except in a narrow region of volume fraction φ0 near the spinodal curve φs0 of width (φs0−φ0)/φ0∝N−1, where N is the number of subunits of the chains. For N→∞ the spinodal curve hence is well defined, in contrast to mixtures with short-range forces, and also the linearized theory of spinodal decomposition is predicted to have wider validity. The collective relaxation in the one-phase region is described by a characteristic time involving the collective diffusion constant Dcoll τq=(Dcoll q2)−1, if the wave vector q is smaller than the inverse correlation length ξ−1coll, while in the range ξ−1coll≪q≪R−1 (R being the polymer coil radius) rates τ−1q∝q4 and in the range R−1≪q rates τ−1q∝q2 are predicted. Both the Rouse model and the reptation model for single-chain relaxation are considered, and a comparison with previous treatments is made. One central result, namely that the wave vector qm of maximal growth in spinodal decomposition is typically of the order of qm∼R1, agrees with the results of Pincus. Experimental consequences are briefly discussed.
Application of transition state theory to desorption from solid surfaces: Ammonia on Ni(111)
Authors: Redondo Antonio , Zeiri Yehuda , Low John J. , Goddard William A.
Published: February 2003

We have used transition state theory to derive analytical expressions for the rates of desorption of atoms and molecules (diatomic as well as polyatomic) from solid surfaces. Realistic forms for the three dimensional adsorbate-surface interaction potentials are employed, including surface corrugation. Using potential parameters from a combination of experiment and ab initio calculations we have applied the rate expressions to evaluate the temperature programmed desorption spectra of NH3 from Ni(111). Comparing these curves to the experimental spectra leads to a bond energy of D0=21.0–3.7Θ kcal/mol, where Θ is the fraction of saturation coverage.
Energy up-conversion in LaF3:Nd3+
Authors: Reddy B. R. , Venkateswarlu P.
Published: February 2003

On resonant excitation of the D levels of LaF3:Nd3+ with yellow laser line, ultraviolet fluorescence is detected from L levels. It is found that both sequential two photon excitation (STEP) process and energy transfer up-conversion (ETU) process are responsible for the population drain mechanism from the lower D levels to the higher L levels. A rate equation model has been developed which also supported the ETU process.
Singlet–triplet perturbations in the Ã 1A2 (v=0) state of thioformaldehyde
Authors: Clouthier D. J. , Ramsay D. A. , Birss F. W.
Published: February 2003

Extensive singlet–triplet perturbations have been observed in the 0–0 band of the Ã 1A2–X̃ 1A1 system of H2CS with energy displacements up to about 0.4 cm−1. The singlet–triplet nature of the perturbation has been confirmed by high resolution magnetic rotation studies and by laser excitation studies in the presence of a magnetic field. The J and Ka dependence of the perturbation matrix elements has been shown to be consistent with a vibronic spin-orbit mechanism. The coupling is predominantly to the F1 components of the 4361 level of the ã 3A2 state.
Comparison of calculated and experimental 4f → 4f intensity parameters for lanthanide complexes with isotropic ligands
Authors: Reid Michael F. , Dallara John J. , Richardson F. S.
Published: February 2003

Calculations of 4f → 4f electric dipole intensity parameters are reported for six different lanthanide (III) systems whose solid-state structures and optical spectra have been well characterized. In each case, the ligands are considered to have isotropic electronic charge distributions, and the 4f → 4f intensity parametrization is expressed in terms of the parameters Aλλ±1,p(λ=2, 4, and 6), which have been defined in a previous paper (see paper I). The calculations include contributions to these parameters from both the static-coupling and dynamic-coupling intensity mechanisms. Comparisons between the calculated and empirically determined signs of the Aλλ±1,p parameters suggest that, in most cases, the dynamic-coupling mechanism makes the dominant contributions to the Aλλ+1,p parameters. The empirically determined signs of the Aλλ−1,p parameters correlate with those calculated on the basis of the static-coupling mechanism. For isotropic ligands, the dynamic-coupling mechanism cannot contribute to the Aλλ−1,p parameters. Intrinsic intensity parameters, defined within the context of the mechanism-independent superposition model for lanthanide-ligand interactions, are also derived for each of the six systems examined here. These parameters, denoted herein as Āλλ±1, are defined to be independent of coordination number, coordination geometry, and lanthanide site symmetry.
Raman studies in CuTCNQ: Resonance Raman spectral observations and calculations for TCNQ ion radicals
Authors: Kamitsos Efstratios I. , Risen William M.
Published: February 2003

The Raman spectra of thin film, and powdered crystalline forms of CuTCNQ have been studied and analyzed in connection with their electronic spectra and vibrational normal coordinate analysis and molecular orbital results. Nonresonance and resonance Raman conditions have been obtained by using different sample forms and source wavelengths. Shifts of TCNQ− and TCNQ= vibrational frequencies from those of TCNQ0 due to changes in the charge on the TCNQ moiety have been calculated in terms of molecular orbital parameters and TCNQ0 vibrational frequencies. The resonance Raman enhancement of TCNQ− in the red band system is shown to be of Franck–Condon origin and its resonance Raman spectrum in this region is calculated. Transformation of CuTCNQ to TCNQ0 with different laser lines is explained on the basis of photoinduced electronic transitions.
Angle-resolved thermal and flash desorption of atoms from surfaces: Model predictions within the one-phonon approximation
Authors: Reyes Jesus , Romero Isidro , Goodman Frank O.
Published: February 2003

We present a detailed analysis of the angular features predicted in angle-resolved desorption, both in a steady-state situation (thermal desorption) and in a flash desorption situation. The work is within a distorted-wave Born approximation and a one-phonon approximation for the atom–solid interaction. Departures from the results given by the ordinary equilibrium theory are found in all cases and are examined in terms of the following elements of our model: conservation of momentum parallel to the surface and of energy, the energy distribution of the adsorbed atoms, the phonon populations, and the matrix elements between inital and final states.
A unified quantum model of resonant and direct scattering in elementary chemical reactions
Authors: Shyldkrot Haim , Shapiro Moshe
Published: February 2003

A quantum model for resonant and direct exchange reactions based on Feshbach’s partitioning techniques is presented. The model is formulated in terms of the reactance matrix and hence involves only real arithmetic. Resonances are introduced via a real and symmetric effective Hamiltonian thus avoiding the search for complex eigenenergies and the construction of a bi-orthogonal basis. The resulting equations are reduced to evaluating free–free, bound–free, and bound–bound integrals. Analytic approximations for all these integrals are developed. The model is applied to model H+OH exchange reactions and tested against exact numerical results. The well position and decelerating forces at the classical turning points are shown to determine most of the observed structure in the reactive probabilities.
State selected ion–molecule reactions by a TESICO technique. VIII. Vibronic-state dependence of the cross sections in the reaction NO+(a 3Σ+, v ; b 3Π, v)+Ar → NO+Ar+
Authors: Kato Tatsuhisa , Tanaka Kenichiro , Koyano Inosuke
Published: February 2003

Charge transfer reactions NO++Ar → Ar++NO (1) have been studied by selecting the vibronic states of NO+ using the threshold electron–secondary ion coincidence (TESICO) technique. The vibronic states selected were a 3Σ+, v=0–5 and b 3Π, v=0, for each of which the relative cross sections have been determined at two collision energies 1.4 and 5.8 eV. The cross section for the a 3Σ+ state has been found to show a resonancelike enhancement at v=2. This feature is reproduced fairly well by the simple two-state theory of Rapp and Francis combined with the consideration of the Franck–Condon factors. However, a closer comparison of the theoretical and experimental results over the entire range of v reveals that the reaction cross sections for the a 3Σ+ reactant ion state consist of two components, one in which the vibrational-state dependence is determined simply by the energy defects and Franck–Condon factors, and the other in which the vibrational-state dependence cannot be interpreted by those factors. The cross section for the b 3Π, v=0 state has been found to be much smaller than those for the exoergic reactions of the a 3Σ+ state (v=1–5). This has been attributed to the difference in the nature of the molecular orbitals that play primary roles in the a 3Σ+ and b 3Π reactions.
Collisional deactivation of a selected energy level in S p-difluorobenzene embedded in a dense vibrational field: Absolute rate constants for a variety of collision partners
Authors: Lawrance Warren D. , Knight Alan E. W.
Published: February 2003

Absolute rate constants are reported for collisional relaxation from the 52302 level (εvib=2036 cm−1) of S0(1Ag) p-difluorobenzene into the relatively dense surrounding field of S0 vibrational levels (6–8 per cm−1). The relaxation is induced by a bath of foreign gas at 300 K under bulb conditions. A total of 23 collision partners have been studied. Stimulated emission pumping (SEP) is used to populate the 52302 level selectively. Single vibronic level fluorescence (SVLF) from the S1 level 302, generated by exciting the 5023022 transition at a variable delay time (85–260 ns) after 52302 preparation, is used to monitor the population dynamics of 52302. The equivalent cross sections deduced from the rate constants for vibrational relaxation with foreign gas M range from one quarter of the hard sphere value for M=He to 2.7 times hard sphere for M=(C2H5)2O. These are an order of magnitude larger than the cross sections normally found for vibrational relaxation in ground electronic states of smaller molecules, however, they correlate well with cross sections observed in excited states of related aromatics. The absolute magnitude of the vibrational relaxation rate constants observed for S0 pDFB are well reproduced for all but the lightest collision partners (He, H2, and D2) by a model based on the Lennard-Jones collision rate coefficient. We conclude that the Lennard-Jones collision frequency, or collision frequencies calculated using similar realistic intermolecular potentials, are a good representative model for calculating vibrational relaxation rates in regions of high vibrational state density. Deviation from the commonly used hard sphere rate coefficient is expected to be most marked at low temperatures (<150 K), but even at 300 K, the hard sphere rate is found to underestimate the vibrational relaxation rate by a factor of 2 for vibrationally complex collision partners.
Theory of diatomic molecule photodissociation: Electronic angular momentum influence on fragment and fluorescence cross sections
Authors: Singer Sherwin J. , Freed Karl F. , Band Yehuda B.
Published: February 2003

We present a general theory of the photodissociation of diatomic molecules in the presence of nonadiabatic interactions between dissociative electronic states. Nonadiabatic couplings exert a profound influence on the photodissociation process when the molecule dissociates to atoms with nonvanishing electronic angular momentum, even if there are no ordinary curve crossings. Nonadiabatic interactions in this latter situation couple adiabatic molecular states which would otherwise be degenerate at infinite internuclear separation. Methods are developed for properly including nonadiabatic interactions in an exact or approximate calculation of photodissociation transition amplitudes for the production of the atomic fragments in particular fine structure levels. We derive differential cross sections for photofragment production and general expressions describing the detection of fragments by any secondary process, such as spontaneous emission, occurring during or after the breakup of the diatom. These expressions are then specialized to fragmentation of a diatom into atoms in fine structure states. Detection of photofragments, fluorescence from atoms produced in excited electronic states, or coincidence detection of both fragments and fluorescence, are considered in detail. An approximate treatment of nonadiabatic interactions is developed which is valid when the fragments recede with relative kinetic energy much greater than the magnitude of the nonadiabatic coupling matrix elements. In some cases, the effects of nonadiabatic interactions are still observable in the high kinetic energy limit. Finally, we discuss how the effect of nonadiabatic interactions may be observed experimentally, and how this can be used to amplify the information to be gained from a photodissociation experiment. In particular, it is shown how the polarization of the atomic fragments contains information fingerprinting the participating intermediate dissociative molecular electronic states.
Investigation of the dynamics and energy disposal in the photodissociation of small ion clusters using a high-energy ion beam crossed with a laser beam: Photodissociation of (NO)2+. in the 488–660 nm range
Authors: Jarrold Martin F. , Illies Andreas J. , Bowers Michael T.
Published: February 2003

The results of a study of the dynamics of photodissociation of the (NO)2+. cluster ion to form NO++NO. in the 488–660 nm range are presented. The experiments were performed by crossing a high energy ion beam with a laser beam. Product kinetic energy distributions were derived from the experimental measurements along with information about the angular distribution of the products. Except at the lowest photon energy at which experiments were performed [where the product relative velocity is comparable to the rotational velocity of (NO)2+.], the product angular distributions could be adequately fit by an angular distribution of the form 1+βP2(θ) with a value for β of 1.3. The product kinetic energy distributions are characteristic of dissociation occurring on a repulsive surface. The fraction of the available energy partitioned into relative kinetic energy increases from 22% at 659 nm to 32% at 488 nm. The data on the product angular distributions indicate that photodissociation occurs by a transition to an excited state with a lifetime that is negligible compared with a rotational period, and that the polarization of the transition is predominantly along the NO–NO+ axis in the cluster. The fraction of available energy partitioned into relative kinetic energy can be accounted for by using a simple impulsive model plus vibrational excitation due to changes in equilibrium geometries of the NO. and NO+ moieties.
Plasmon resonance broadening in small metal particles
Authors: Kraus W. A. , Schatz George C.
Published: February 2003

A quantum mechanical method based on the Kramers–Heisenberg dispersion relation is used to evaluate the dielectric response of small metal particles, and thereby to determine the influence of particle size on the widths of the plasmon resonance line shapes. Several different particle shapes are considered (sphere, cylinder, rectangular prism, spherical shell, and cylindrical shell) and for each shape a free electron Schrödinger equation is used to determine conduction band energies and dipole matrix elements. The main emphasis in this work is on particle sizes large enough that only the first order deviations from the infinite size limit are important, and for such sizes we find that the size dependent contribution to the width can be expressed in terms of an effective length Leff. This effective length is found to depend on the direction of the external field relative to the particle symmetry axes, and on the shape of the particle. For compact shapes, Leff is accurately approximated by 0.65 Lav along each principal axis, where Lav is the ratio of particle volume to its projected area along the relevant axis. Comparison with previous classical and semiclassical calculations is considered, and for spherical particles, we find good agreement with the classical surface scattering model, differing by about 16%. More significant differences are found for other shapes, most notably because the classical theory ignores the dependence of resonance width on the orientation of the field relative to the particle.
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o potential energy curves for the low-lying electronic states of the argon excimer
Authors: Yates J. H. , Ermler W. C. , Winter N. W. , Christiansen P. A. , Lee Y. S. , Pitzer Kenneth S.
Published: February 2003

Configuration interaction calculations are reported for the potential energy curves of the argon excimer that arise due to excitation to the 4s and 4p Rydberg molecular orbitals. Effective core potentials were employed to replace the core electrons of the Ar atoms thereby reducing the computational procedure to one for a 16 valence system. Potential energy curves for three excimer states of each of the symmetries 1Σ+g, 1Σ+u, 3Σ+g, 3Σ+u, 1Πu, 3Πu, 1Πg, and 3Πg are reported and compared with those previously computed. Spectroscopic constants and curve maxima are reported where appropriate.
Two-photon Ã←X̃ spectra of benzonitrile and styrene vapors: Tests of perturbation rules
Authors: Chia Lucille , Goodman Lionel , Philis John G.
Published: February 2003

The two-photon vapor Ã←X̃ spectra of benzonitrile and styrene have been studied by fluorescence excitation at room temperature. Because of low frequency internal torsional vibrations, the styrene spectrum was also obtained using multiphoton ionization in a supersonic jet. Despite the totally symmetric symmetry of the excited state, the polarization properties of the styrene Franck–Condon spectrum show that the two-photon electronic intensity mechanism predominantly involves the symmetric tensor and not the scalar tensor. Comparison with the corresponding 1B2←1A1 spectrum of phenylacetylene shows that the cross section for the two-photon spectra of the three molecules are roughly equal (∼102 gm) despite a large dissimilarity in the one-photon spectra. The near equality of the three cross sections in molecules without special symmetry properties confirms both the pseudoparity (±) basis of the Callis–Scott–Albrecht inductive perturbation selection rule, and the rule that orbitally allowed -↔- transitions should be very strong for alternant aromatic hydrocarbon two-photon spectra.
Far-infrared laser magnetic resonance detection and microwave spectroscopy of the PO radical
Authors: Kawaguchi Kentarou , Saito Shuji , Hirota Eizi
Published: February 2003

The PO radical has been detected by far-infrared laser magnetic resonance (FIR LMR) using the CH3OH 369 μm and the CH3OD 305 μm laser lines as sources. The radical was generated by passing the microwave discharge products of H2O or of a H2/O2 mixture over powdered red phosphorus placed in an absorption cell. Subsequently, microwave spectroscopy was applied to observe low-J transitions. The ground-state molecular constants derived from the observed spectra are B=21 899.4915 (33), AeffJ=3.1026(30), D=0.031 120(42), p=188.01(51), qeff=−0.57(29), a=566.16(19), b=227.5(64), c=−415.3(64), and d=751.169(90), all in MHz with three standard errors in parentheses which apply to the last digits of the constants. The mechanism of the reaction generating PO is briefly discussed.
Autoionization processes in sym-trifluorobenzene and hexafluorobenzene: Studies involving threshold photoelectrons and ion fluorescence
Authors: Dujardin G. , Leach S. , Dutuit O. , Govers T. , Guyon P. M.
Published: February 2003

Autoionization in large polyatomic molecules has been studied using synchrotron radiation as a variable energy VUV source. Threshold photoelectron spectra (TPES) of sym-trifluorobenzene and hexafluorobenzene were measured over the 9–29 eV range. Fluorescence quantum yields φF and lifetimes τ were determined for energy selected levels of the cations using the T-PEFCO technique in which coincidences are counted between threshold photoelectrons and ion fluorescence photons. The relative partial cross sections σX̃(E) and σB̃(E) for threshold ionization to the nonfluorescing X̃ and fluorescing B̃ states of 1,3,5-C6F3H+3 and C6F+6 ions are determined. Peaks corresponding to bands of a Rydberg series converging to the C̃ state of C6F+6 are observed in the TPES of C6F6 and result from threshold autoionization processes forming high vibrational levels of the ion B̃ state. An intense underlying continuum in the TPES of 1,3,5-C6F3H3 and C6F6 is interpreted as being due to autoionization from superexcited neutral states to the X̃ ground states of the corresponding cations. The autoionization processes are also discussed in terms of competition between autoionization and ionic core relaxation of superexcited states.
Derived sensitivity densities in chemical kinetics: A new computational approach with applications
Authors: Larter Raima , Rabitz Herschel , Kobayashi Mei
Published: February 2003

Partial differential equations for derived sensitivity densities corresponding to reaction/diffusion problems are developed. The elementary sensitivity densities do not appear explicitly in these equations thus showing that the derived sensitivity densities need not be ‘‘derived’’ from the elementary densities, but may be calculated directly. The derived sensitivities allow for studies of the interrelationship between chemical species concentrations at different locations in space and points in time. In a similar fashion the interactive role of system variables can be examined including the intriguing case of correlation between kinetic and transport effects for controlling the overall reaction. The method is demonstrated for a three-species linear reaction/diffusion problem for which an exact solution is available. The resulting sensitivity densities are found to have an enlightening physical interpretation which may be applied to more complicated reaction/diffusion problems.
Measurement of the relative populations of I(2P01/2) and I(2P03/2) by laser induced vacuum ultraviolet fluorescence
Authors: Brewer P. , Das P. , Ondrey G. S. , Bersohn R.
Published: February 2003

The ground (2P03/2) and first excited (2P01/2) states of iodine atoms can absorb two photons at 304.7 and 306.7 nm, respectively, to reach 2D05/2 and 2D03/2 states. The excited atoms fluoresce twice, emitting an IR and then a VUV quantum. This is the basis of a new method for measuring the relative quantum yields of the two fine structure states at very short times after the atoms are formed. Quantum yields for I* production are reported for a number of alkyl halides and HI upon photodissociation.
Two photon laser induced fluorescence of carbon atoms
Authors: Das P. , Ondrey G. S. , van Veen N. , Bersohn R.
Published: February 2003

Carbon atoms were generated by two photon absorption in carbon suboxide C3O2. Two photon laser induced fluorescence (LIF) excitation spectra using the 3 3DJ′←2 3PJ″ at 287 nm and 4 1P1←2 1D2 at 284 nm transitions were intense showing that LIF is a convenient detection method for carbon atoms in their ground 3P and first excited 1D states. Calculated values of the relative intensities of the seven allowed J′←J″ transitions were in good agreement with experimental values.
The effective delay time in pump-and-probe measurements
Authors: Breckenridge W. H. , Umemoto H.
Published: February 2003

The delay time for secondary collisions is considered for systems in which a reactant is created by one short laser pulse, and a product detected with a second short laser pulse at some later time. Sample calculations for the Mg(1P1)+H2→ MgH+H reaction show that the average (or ‘‘effective’’) delay time for secondary collisions of MgH product is different from and not necessarily linearly related to the delay time between the two laser pulses.
Photoionization studies of Na2Cl and Na2O and reactions of metal clusters
Authors: Peterson K. I. , Dao P. D. , Castleman A. W.
Published: February 2003

Reaction between metal clusters and various reactant gases in a coexpanding nozzle led to the formation of Na2Cl and Na2O. These species were subjected to photoionization using a UV light source, monochromator, and mass spectrometer detection system. Appearance potentials of 95.7 and 116.7 kcal/mol were obtained for Na2Cl and Na2O, respectively. The appearance potential of the Na2Cl lies between the ionization potential of the metal and the electron affinity of chlorine, while that of Na2O is close to the ionization potential of the metal. Using these values and other literature data, energies were derived for the following bonds: Na–NaCl (19.7 kcal/mol), Na+2⋅Cl (115.5), Na2⋅Cl (98.9), and Na+2⋅Cl− (127.8); Na+⋅NaO (56.8), Na+2⋅O (94.2), Na2⋅O (98.6), and Na+2⋅O− (177.2).
Stark effects on the 726 nm 1T1 state of the permanganate ion
Authors: Johnson Lawrence W.
Published: February 2003

The electronic absorption spectrum of LiMnO4⋅3H2O/ LiClO4⋅3H2O mixed crystal is reported near the ‘‘red band’’ system at 726 nm. Stark effect splitting for the 726 nm band is reported. (AIP)
The electronic structure of aromatic molecules adsorbed on Pd(111)
Authors: Netzer Falko P. , Mack J. U.
Published: February 2003

Angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and electron energy loss spectroscopy (ELS) have been used to study the electronic energy level structure of aromatic molecules such as benzene, m-xylene, pyridine, and 2,6-dimethylpyridine adsorbed on Pd(111) at room temperature. Angle resolved He i and He ii spectra were investigated to assign adsorbate derived bands in terms of molecular orbitals of the gas-phase molecules, whereby He ii radiation proved particularly useful for this assignment because deeper lying σ orbitals can be observed. Angle resolved photoemission behavior of benzene is satisfactorily described by an adsorbate complex of C6v symmetry, i.e., consisting of benzene molecules with their ring plane parallel to the surface. It appears that the geometrical structure of the surface does not greatly influence the symmetry properties of the surface molecule. Reduction of the free-molecule symmetry C2v to Cs in the adsorbed state is necessary for m-xylene, pyridine, and dimethylpyridine to account for the relaxation of symmetry based restrictions in normal emission. This can be interpreted for m-xylene and dimethylpyridine in terms of adsorption parallel to the surface, but for pyridine an adsorbate phase of molecules in an inclined geometry, with their molecular plane tilted with respect to the surface, is proposed. The initial state information of UPS is used to discuss the electronic excitation spectra of aromatic molecules on Pd(111). The intramolecular π→π* excitations of the free molecules are not clearly observed in ELS of the adsorbed molecules, but strong charge transfer bands are seen in the loss energy region 2.5–5 eV. Possible reasons for the absence of distinct intramolecular features in adsorbate ELS are discussed. These include pronounced modification of the π orbital structure for strong adsorbate–surface interaction, reduction of transition probabilities due to the dielectric response of the substrate, and interference of the molecular transition with surface plasmon excitation.
Emission and one- and two-photon excitation spectra of the 1 1A
g–2 1A
g origin region of c
i
s–c
i
s octatetraene
Authors: Kohler Bryan E. , West Paul
Published: February 2003

Di-cis 1,3,5,7-octatetraene has been prepared in an n-octane matrix at 10 K and the emission and one- and two-photon excitation spectra have been measured. The di-cis guest fits into the n-octane host in two sites. In both of these sites the 1 1Ag to 2 1Ag origin is strictly one-photon forbidden, two-photon allowed establishing that both sites are centrosymmetric. The 1 1Ag to 2 1Ag and 1 1Ag to 1 1Bu excitation energies in both sites are approximately the same as the corresponding energies for the all-trans isomer in the same host. A vibrational perturbation has been identified and analyzed.
A theoretical study for the dynamic interchromophoric interaction effects upon the susceptibility and hypersusceptibility of chromophore aggregates
Authors: Kamiya Mamoru
Published: February 2003

The dynamic interchromophoric interaction effects upon the susceptibility and hypersusceptibility of the current–charge density of chromophore aggregates are formulated to infinite order and in closed form by means of a decoupling approximation, whereby the induced field operator contained in the dynamic perturbation Hamiltonian is replaced by its averaged form satisfying the generalized Maxwell’s equation for the current-charge source density. This decoupling approximation includes a basic condition that the total momentum and hence the total current of identical charged particles belonging to any chromophore remain as nearly constant in time throughout the dynamic perturbation interactions. The physical meaning of this condition is surveyed by making reference to the local conservation equation which can be derived for the chromophoric current and charge densities. Contributions of the aggregate’s susceptibility and hypersusceptibility to the mean rate of energy absorption caused by a single-frequency external field and to the mean rate difference generated by right and left circularly polarized light are studied in terms of the frequency reversal symmetries of the susceptibility and hypersusceptibility.
Nuclear–nuclear potential energy related to total number of electrons in light molecules
Authors: Mucci J. F. , March N. H.
Published: February 2003

Using self-consistent field calculations for a wide variety of molecules having total number of electrons N, with 10≤N≤32, the nuclear–nuclear potential energy for equilibrium geometry is studied as a function of Δ/ ‖ E ‖, where Δ is defined as E−3/2 Es, with E and Es the ground-state energy and the sum of orbital energies, respectively. The plots for the molecules considered, in the range 10≤N≤22 are accurately represented by Vnn=0.12N2+68 Δ/ ‖ E ‖ a.u. Thus, we can define a series of ‘‘canonical’’ molecules with Δ/E=0, in which Vnn is solely determined by the total number of electrons. Canonical heteronuclear diatomic molecules with nuclear charges Z1 and Z2 have then equilibrium bond lengths given by Re−1=0.12(1+ε)2/ε with ε=Z2/Z1, which for homonuclear molecules gives Re=1.10 Å, near to the value for N2. Empirically, N2 lies at the minimum of the bond length vs N curve for the homonuclear molecules excluding H2. We know of no canonical heteronuclear diatomic molecules, but, nevertheless, two diatomic hydride series obey the canonical type of relationship with a substantially modified constant.