bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantitative analysis of third harmonic generation of XUV radiation in a cell and a cw free jet of carbon monoxide
Authors: Aguillon F. , Lebéhot A. , Rousseau J. , Campargue R.
Published: July 2002

In the theory, the two-photon resonant third-order nonlinear susceptibility of a molecular gas is calculated. The variations of this susceptibility, with the rotational quantum numbers of the resonant states, are given in the case of two singlet states of different symmetries, Σ and Π. In the experiment, third harmonic generation of XUV light (94.5–97.5 nm) is obtained with and without two-photon resonance. The gaseous nonlinear medium is formed by CO as used either in a cell, or in a continuous supersonic free jet zone of silence. The intensity of the third harmonic wave generated is well related to the laser power, phase-matching conditions, two-photon resonances, and rotational temperature, through the nonlinear susceptibility, as described theoretically.
A quasiharmonic model calculation for two non-Markovian far-infrared spectra of diatomic polar molecules in a rare-gas liquid. I. Spectral theory
Authors: Hernández A. Calvo , Velasco S. , Mauricio F.
Published: July 2002

A model consisting in a quantum rigid rotor interacting with a quasiharmonic medium is presented to obtain the far-infrared spectra of diatomic polar molecules dissolved in rare-gas liquids. The absorption coefficient is calculated within the framework of two non-Markovian approaches called TTOC (total time ordered cumulants) and PTOC (partial time ordered cumulants). Both non-Markovian spectra contain the same correlation functions of the diatomic–medium interaction, which is expressed in terms of a Legendre polynomial series truncated up to second order. These correlation functions are calculated in paper II. The absorption cofficients appear as a sum of two spectral contributions. The first one, called secular contribution, is given as a juxtaposition of basic resonances, different in each of two non-Markovian approaches, associated to the absorption lines j→j+1. The second one, called interference contribution, takes into account the existence of a overlapping line effect. The contributions to the absorption coefficients from the P1 and P2 Legendre polynomials of the interaction are explicitly reported.
Effect of the Lamb shift upon ionization potentials or excitation energies of atoms and ions
Authors: Anno Tosinobu , Teruya Hirohide
Published: July 2002

The effect of the Lamb shift upon the ionization potential of the He-like atoms in their ground states is analyzed. It is shown that the Lamb shift of the ground state of the He-like atom tends to twice the same quantity of the H-like atom of the same Z in its ground state when Z→∞. From this observation, it is supposed that the Lamb shift of a given state of an atom or an ion with more than one electron is made up of the one-electron contributions, which can be calculated with the well known formulas for the radiative corrections to the energy levels of one-electron atoms by introducing the concept of the effective nuclear charge. To confirm this supposition, it is shown that the radiative corrections for the energy levels of the 2 1,3S states of the He atom as well as for the fine structure splittings of the 2 3P and 3 3P states of the He atom can be understood along the same line. It is finally concluded that the effect of the Lamb shift upon the ionization potential or the excitation energy may have to be considered only if a 1s electron is involved either in the ionization or in the excitation.
Rotational spectrum and structure of the Ar3–HCl symmetric top
Authors: Klots T. D. , Ruoff R. S. , Chuang Carl , Emilsson Tryggvi , Gutowsky H. S.
Published: July 2002

The Ar3–HCl tetramer has been identified via the observation of its microwave rotational spectrum using the pulsed nozzle Fourier transform technique. The rotational spectroscopic constants of the ground vibrational state have been obtained for the 35Cl/37Cl isotopes from measurements of the J=1→2 to J=4→5 transitions and their hyperfine structure. For Ar3–H35Cl, the values found for B0, DJ, DJK, and χaa are 843.8974(1) MHz, 2.333(2) and 1.818(6) kHz, and 31.006(5) MHz, respectively, and for Ar3–H37Cl, 825.9811(2) MHz, 2.211(5) and 1.940(12) kHz, and 24.481(8) MHz. The cluster is a symmetric top with the HCl along the C3 axis, its H end directed towards the Ar3 trimer. In its torsional oscillations, the HCl makes an average angle of 36.91° with the symmetry axis. The chlorine isotopic substitution leads to values of 3.8510 and 4.0024 Å for the Ar–Ar and Ar–Cl distances. The results of the present study are compared with those reported earlier for the Arm–HF (m=1,2,3) and Arm–HCl (m=1,2) clusters.
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o studies of low-lying 3Σ−, 3Π, and 5Σ− states of NH. I. Potential curves and dipole moment functions
Authors: Goldfield Evelyn M. , Kirby Kate P.
Published: July 2002

Configuration interaction wave functions, potential energy curves, and dipole moment functions have been calculated for the four lowest 3Σ− and the three lowest 3Π states and 5Σ− states of NH. The electronic wave functions were constructed to give a balanced description of valence–Rydberg interactions. Two repulsive states have been identified as important photodissociation pathways. We present spectroscopic constants for the bound states and compare our results to other theoretical and to experimental work. The possible predissociation of the A 3Π state by the 1 5Σ− state is discussed.
Bond polarizability and vibronic coupling theories of Raman optical activity: The electric-dipole magnetic-dipole optical activity tensor
Authors: Escribano Juan R. , Freedman Teresa B. , Nafie Laurence A.
Published: July 2002

A new formulation of the bond polarizability and vibronic coupling theories of Raman optical activity (ROA) is presented in which terms dependent upon the nuclear momentum are introduced for the first time. The new terms appear in the magnetic-dipole electric-dipole optical activity tensor and are directly analogous to the non-Born–Oppenheimer, momentum-dependent terms required in the formulation of the theory of vibrational circular dichroism. Through the use of simplifying approximations in the ROA vibronic coupling theory, a term-for-term correspondence with the bond polarizability theory is established. It is shown that the new terms are smaller than the original terms by the ratio of the vibrational transition frequency to the frequency of the incident radiation. The new terms also contribute oppositely to Stokes and anti-Stokes ROA which provides a basis for their experimental isolation.
The potential of mean torque for flexible mesogenic molecules. Determination of the interaction parameters from carbon–hydrogen dipolar couplings for 4-n-alkyl-4′-cyanobiphenyls
Authors: Emsley J. W. , Fung B. M. , Heaton N. J. , Luckhurst G. R.
Published: July 2002

The potential of mean torque for the flexible molecules comprising liquid crystals which was proposed by Emsley et al. [Proc. R. Soc. London Ser. A 381, 117 (1982)] contains interaction parameters for rigid fragments of the molecule. These are determined for four members of the series 4-n-alkyl-4′-cyanobiphenyls (NCB) with N=5, 6, 7, and 8, at TNI −T=4 K, by comparing calculated values of SCH, the local order parameter for C–H bonds at each position in the alkyl chain, with values determined from dipolar couplings between carbon-13 and protons. The potential of mean torque characterized in this way is then used to calculate vicinal C–H dipolar coupling constants for nuclei in the alkyl chain for 5CB and 6CB; these are found to be in reasonable agreement with experimental values.
On the structure and stability of the H2O− ion
Authors: Werner Hans-Joachim , Mänz Ullrich , Rosmus Pavel
Published: July 2002

Potential energy surfaces of the three lowest bound electronic states (1 2A′, 2 2A′, and  2A″) of H2O− have been investigated by ab initio calculations using highly correlated electronic wave functions. Minima resulting from ion–quadrupole interactions between the O−(2P) and H2(1Σ+) fragments were found for linear 2Σ+ and 2Π O−⋅⋅⋅H2 structures. The corresponding dissociation energies amount to about 0.2 eV. The 2Σ+ (2A′) minimum is separated by a barrier from the H2O+e continuum, while the 2Π (2A″) state is connected via a similar barrier to another local minimum of 2Π symmetry originating from the ion–dipole interaction of OH(X 2Π) and H−(1S). Hence, in accordance with recent experimental findings of de Koning and Nibbering, the H2O− ion is found to be a bound species in ion–quadrupole or ion–dipole cluster structures.
The dipole–quadrupole hyperpolarizability of helium
Authors: Fowler P. W.
Published: July 2002

At the Hartree–Fock level the value of B, the dipole–quadrupole hyperpolarizability of He, is −6.583 a.u. (−11.17×10−63 C3 m4 J−2). Electron correlation increases the magnitude of B by an estimated 10%.
Selective vibrational excitation and mode conservation in H++CO2/N2O inelastic and charge transfer collisions
Authors: Niedner G. , Noll M. , Toennies J. P.
Published: July 2002

Total angular distributions and vibrationally resolved time-of-flight spectra have been measured for H++CO2/N2O at collision energies of 9.8 and 30 eV and scattering angles up to θ=15°. Results are available for the scattered protons as well as for H atoms from charge transfer collisions into the electronic ground states of CO+2/N2O+. For both systems, the H+ and H product channels exhibit practically identical total angular distributions with marked rainbow structures in the CO2 case. The time-of-flight distributions, on the other hand, reveal strongly selective excitation of the ν3 fundamental modes and their overtones for both target molecules and both product channels. In addition, at each scattering angle, the ν3 transition probability distributions for CO2 and N2O are remarkably similar to those for CO+2 and N2O+, respectively. The dominance of the ν3 mode excitation in the neutral molecules is in accord with what is expected from the combination of dipole- and valence-type interaction mechanisms on the lower H++CO2/N2O potential energy surfaces. Excitation of the same mode with nearly the same distributions in the charge transfer channel is explained by Franck–Condon selection rules, which favor transitions between identical vibrational states of either CO2 and CO+2 or N2O and N2O+.
Fourth order many-body Green’s function: Application of a simplified model to the ionization energies of N2
Authors: Tomasello Pasquale
Published: July 2002

In the framework of the recently proposed ADC (algebraic diagrammatic construction) approximation scheme to the one particle Green’s function (1p-GF), a study of the fourth order approximation on the valence shell ionization spectrum of N2 with two different basis sets is presented. The effects of the various fourth order terms on the structure of the 1p-GF are discussed. It is shown that some conclusions of this work agree with the limited experience existing on the subject, while some others are new. The effect on the structure of the satellite lines of some fourth order contributions, which have been ignored in the literature so far, are shown to be relevant in order to achieve even qualitatively a good description of the inner valence shell ionization processes, especially in the case of a breakdown of the molecular orbital picture of the ionization. As a result of this investigation a computational model of the fourth order approximation to the 1p-GF is proposed. It is expected to be more reliable than the existing ones. This model should be computationally tractable at least for di- and triatomic first-row molecules. Finally, the complete valence shell spectrum of N2 within an extended basis set is calculated and a comparison is made with the experiment as well as with other theoretical results.
Molecular beam measurement of cross sections of collision induced reorientation of molecule
Authors: Matsushima Fusakazu , Shimizu Tadao , Honguh Yoshinori
Published: July 2002

The cross sections for collision induced transitions of molecules between magnetic sublevels slightly separated by a weak Stark field are directly measured with a method of infrared–infrared double resonance in a molecular beam. The ΔM=±1,±2 transitions in the J=K=2 rotational levels in the ground and the ν3 excited vibrational states of CH3F are studied as functions of the energy separation between the M sublevels. It is shown that the cross sections do not have a significant dependence on the energy separation which is less than 1 GHz, and their magnitudes are of a comparable order to the pressure broadening parameters of the infrared absorption lines. The results are compared with other ‘‘indirect’’ measurements of the cross sections.
Vibrational spectra and assignments, normal coordinate analyses, a
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o calculations, and conformational stability of the propenoyl halides
Authors: Durig J. R. , Berry R. J. , Groner P.
Published: July 2002

The infrared and Raman (3500–30 cm−1) spectra of gaseous and solid propenoyl bromide, CH2 CHCBrO, have been recorded. The Raman spectrum of the liquid has been obtained and qualitative depolarization values have been measured. The fundamental asymmetric torsion for both the s-trans and s-cis conformers has been observed along with several hot transitions. From these transitions the potential function governing the internal rotation has been determined and the s-trans to s-cis barrier is 1861 cm−1 (5.32 kcal/mol), the trans conformer being more stable by 158±20 cm−1 (452±57 cal/mol). All normal modes have been assigned for the s-trans conformer and many of those for the s-cis conformer, based on band contours, depolarization values, and group frequencies. A normal coordinate calculation has been carried out by utilizing a modified valence force field to calculate the frequencies and the potential energy distribution for both conformers. Temperature studies of the Raman spectra of gaseous propenoyl fluoride and chloride and of the liquid phases of propenoyl chloride and bromide have been used to obtain the enthalpy differences between the s-trans and s-cis conformers for these compounds. For the bromide and chloride the s-trans conformer is clearly the more stable form but the data are inconclusive for the fluoride. Complete equilibrium geometries have been determined for both rotamers of the fluoride

and chloride by ab initio Hartree–Fock gradient calculations employing both 3-21G and 6-31G* basis sets. The structural parameters are compared to those suggested from microwave and/or electron diffraction studies of these molecules. Both calculations are consistent with the s-trans conformation being thermodynamically preferred for the fluoride molecule and the s-cis conformation for the chloride, which is contrary to the experimental data for the chloride. The 3-21G basis set was used to obtain the ab initio force constants and frequencies for the s-trans and s-cis conformers of both the fluoride and chloride with fixed scaling, variable scaling, and without scaling factors. These results required several reassignments of fundamentals for both conformers of both molecules. Improved vibrational data for both the fluoride and chloride were very valuable for the reassignments. These results are compared with the corresponding quantities for some similar molecules.
Analysis of 27Al nuclear quadrupole interaction effects on electron spin echo modulation in disordered systems
Authors: Goldfarb Daniella , Kevan Larry
Published: July 2002

The effects of 27Al (I=5/2) nuclear quadrupole interaction on the electron spin echo modulation (ESEM) in the case of S=1/2 in disordered systems are analyzed using exact diagonalization of the spin Hamiltonian. The Hamiltonian parameters used in the calculations are those expected in Cu+2-exchanged zeolites. The g anisotropy is taken into account and the ESEM obtained using the spherical model approximation is compared with a trigonal correlated structure. It is found that in the 0.32–0.34 nm range of distances between the paramagnetic center and the Al nuclei all parameters related to the quadrupole tensor, namely the quadrupole interaction constant, the asymmetry parameter and the relative orientation with respect to the g tensor, have a substantial effect on the modulation depth.
Optimization of a distributed Gaussian basis set using simulated annealing: Application to the solvated electron
Authors: Sprik M. , Klein M. L.
Published: July 2002

A distributed Gaussian basis set is employed to calculate the energy eigenvalues and eigenstates of an electron solvated in a molecular fluid. A combination of two techniques is proposed to optimize the basis functions. The first consists of treating the centers of the Gaussians as dynamical variables whose optimal distribution for a given solvent configuration is obtained by the technique of simulated annealing. In addition, the Gaussian basis functions are multiplied by Jastrow factors which suppress excessive overlap within the repulsive cores of the solvent molecules. The method is applied to a series of rigid solvent configurations sampled from a path integral Monte Carlo simulation of an electron solvated in ammonia. A careful comparison is made with averages derived from the path integral calculations. The two methods are found to agree within 10% for the ground state energy. From the sample of different solvent configurations the adiabatic fluctuation in the energy eigenvalues and the density of states is analyzed and compared to experimental data.
Observation of three-photon resonances in NMR under four-field irradiation
Authors: Goelman G. , Zax D. B. , Vega S.
Published: July 2002

Experimentally achievable conditions where only weak off-resonant fields are used to excite coherence in a two-level system are explored. A brief review of previous theoretical results useful in treating multifrequency irradiation sequences is provided. Using an approximation method based on the Floquet formalism we derive an expression for the effective field strength as a function of the phases and amplitudes of the applied irradiation fields. Some of these results are rederived and/or explained in a simplified manner. Under conditions of four-frequency irradiation, three-photon excitation of a two-level system in NMR is experimentally demonstrated on the spin-1/2 system of the 31P nucleus in phosphoric acid. The behavior of the three-photon resonance is explored as a function of all important experimental parameters. The results of these tests conform closely to the theoretical predictions. Possible uses of multifrequency irradiation schemes in NMR are suggested.
Generalized Langevin equation approach for the rotational relaxation of a molecule trapped in a 3D crystal. I. Theoretical considerations
Authors: Delgado V. , Breton J. , Girardet C.
Published: July 2002

The stochastic classical trajectory method is developed for the interpretation of the orientational relaxation of a diatomic molecule trapped in a rare gas matrix. The primary system formed by the molecule and four neighboring atoms is described as a 2D effective dynamical system, while the bath formed by the remaining degrees of freedom has the spatial (3D) dimension. Special emphasis is devoted to the determination of the viscous terms (connected to the molecule rotation and translation and to the four atoms vibrations) which are responsible for the various channels of energy dissipation in the bath. A Monte Carlo numerical procedure is applied in paper II to the dynamics of CO and CH3F molecules trapped in an argon crystal.
Derivative Numerov–Cooley theory. A method for finding vibrational state properties of diatomic molecules
Authors: Dykstra Clifford E. , Malik David J.
Published: July 2002

The difference equation method for finding one-dimensional vibrational state wave functions and energies which is usually referred to as the Numerov–Cooley method has been extended to molecular properties which are defined as energy derivatives. By straightforward differentiation of the Schrödinger equation with respect to embedded parameters, for instance, electric field components, a difference equation procedure has been developed that finds derivative vibrational wave functions and derivatives of the electronic–vibrational state energies of diatomic molecules. Representative test calculations are reported.
Laser IR polarization spectroscopy at surfaces
Authors: Bethune D. S. , Williams M. D. , Luntz A. C.
Published: July 2002

A novel form of laser polarization spectroscopy is described which allows vibrational spectra of submonolayers of some adsorbates on metal surfaces to be observed. As a demonstration of its potential, spectra for CO adsorbed on a Pt(111) single crystal were obtained using a continuously tunable pulsed IR laser. The temperature dependence of the lineshape for the CO stretch in an on-top adsorption site was measured and interpreted in terms of exchange-coupling dephasing. Since sensitivity achieved with this spectroscopy is not yet as high as that obtainable with conventional IRRAS, the issue of noise sources and sensitivity in the laser spectroscopy is examined in some detail, and possibilities for future enhancements are discussed.
Defects in doped polyacetylene: An a
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o infrared and Raman spectroscopy of solitons
Authors: Villar H. O. , Dupuis M. , Clementi E.
Published: July 2002

We report complete vibrational spectra, including IR and Raman intensities of C22H24, C21H+23, and C12H++14, as models for pure and doped polyacetylene. The results provide definitive evidence for charged solitonic rather than bipolaronic defects.
On the nature of the orientational transition of monolayer N2 on graphite
Authors: Kuchta B. , Etters R. D.
Published: July 2002

The character of the orientational transition of monolayer N2 on graphite is investigated using a Monte Carlo method with deformable boundary conditions and a mean field approach involving the rotational susceptibility. The calculated specific heat and various order parameters predict a transition temperature TC=25 K, in good agreement with experiment. The results indicate, however, that the transition from static herringbone order to disorder takes place in stages over a 6–7 K temperature range, in which a local mode involving the rotation of individual molecules through a planar angle of π is activated, followed by a cooperative oscillation through π/2 in which the two herringbone sublattices are interchanged. The transition is identified by a loss of long range order. Above TC the correlation length is reduced to LC=4.2±0.5 Å at T=28 K. It is observed that quadrupolar short ranged correlations persist above TC to near melting.
Exact solution for the pair distribution function of a one-dimensional equilibrium fluid of hard rods with arbitrary size distribution
Authors: Burgos E. , Bonadeo H.
Published: July 2002

An exact solution for the pairwise distribution function of a 1D equilibrium fluid of hard rods of arbitrary size distribution is presented. Discrete size distributions for finite numbers of rods and in the thermodynamical limit, and continuous size distributions are considered. Examples showing the effect of shape and width of the size distributions and of density on the pair distribution functions are given, and the case of chains of sticking rods is discussed.
Wave packet studies of gas–surface inelastic scattering and desorption rates
Authors: Jezercak Michael , Agrawal P. M. , Smith Charles B. , Raff Lionel M.
Published: July 2002

A previously formulated semiclassical wave packet method is used to investigate the importance of different surface phonon modes and the Debye surface temperature upon inelasticity in atomic gas–surface collisions. Desorption rates are calculated as a function of potential-well depth and the rate law for the process is examined. The incident beam is represented by a quantum mechanical wave packet whose momentum distribution is nearly square. This wave packet is coupled to a three-dimensional model lattice through a time-varying potential field obtained by solution of the classical motion equations for the lattice. Calculated final-state momentum and energy distributions are found to be strongly dependent upon the particular surface phonon mode into which the initial lattice energy is partitioned. In general, energy transfer occurs predominantly to and from those modes for which the lattice atom in the impact region have motion in the direction of the momentum vector of the incoming wave packet. The inelasticity of the collision is found to increase as the lattice force constants and the surface Debye temperature decrease. The peak spacings in the final-state momentum and energy distributions are found to correlate well with the surface phonon frequencies. Desorption is found to be well described by a first-order rate law for small potential-well depths. For larger well depths, the first-order decay plots begin to show an increasing amount of curvature. Desorption rate coefficients obtained from the slopes of the decay plots show an approximate exponential dependence upon the potential-well depth.
A generalized regular solution model of a liquid supported monolayer of long chain amphiphile molecules
Authors: Popielawski Jan , Rice Stuart A.
Published: July 2002

We describe a generalized regular solution model of a liquid supported monolayer of long chain amphiphile molecules. The model includes the effects of chain flexibility in the surface layer and in the space outside the surface layer, but the neglect of chain–chain interactions outside the surface layer restricts the domain of applicability of the analysis to small to modest surface coverage (up to the critical density). The analysis leads to self-consistent equations for the distribution of molecular configurations and the surface density and pressure. An examination of available data for pentadecanoic acid on water (pH=2) leads to the strong inference that even at infinite dilution individual molecules take up configurations with a large part of the chain outside the surface layer. This inference, and others drawn from the theory, are supported by Monte Carlo simulations reported in the accompanying paper. An extension of the model includes the influence of aggregation of amphiphile molecules on the properties of the monolayer.
Comparison between local space approximation and finite cluster treatments of chemisorption on metals
Authors: Matos Maria , Kirtman Bernard , DeMelo Celso P.
Published: July 2002

The local space approximation (LSA) method is applied to a model Hamiltonian describing the chemisorption of a hydrogen atom onto a transition metal (0,0,1) surface. In the LSA method a finite cluster defined by a local (orbital) space is properly embedded into the complete system. Without embedding the calculated binding energy, ΔEb oscillates strongly as the size of the cluster increases, and even when 200–250 metal atoms are included the convergence is unsatisfactory. On the other hand, the LSA treatment yields rapid monotonic convergence. Using a systematic buildup of the local space the error in ΔEb is 4.4 kcal/mole out of 80.5 for a 10 metal atom cluster; that error decreases steadily to 1.8 kcal/mole for a cluster containing 30 metal atoms. Extrapolation reduces the error for all clusters between 18 and 71 metal atoms by about 0.6 kcal/mole.
Semidilute good solvent solutions of polymers in a box. Finite size corrections by renormalization group methods
Authors: Cherayil Binny J. , Freed Karl F.
Published: July 2002

Renormalization group methods are used to study the influence of confinement on the free energy of mixing of two different homopolymers in dilute through semidilute solutions as a function of concentration, molecular weight, and A−A, B–B, and A–B polymer second virial coefficients in order to aid in the extrapolation of Monte Carlo simulation data to the thermodynamic limit. The theory uses the Edwards continuum model for semidilute solutions with screening and fully treats the effects of excluded volume in marginal to good solvents. The confined system is taken to have periodic boundary conditions, which are widely applied in simulation work, but other boundary conditions may readily be used. The zeroth order distribution function of the confined polymer is represented as an eigenexpansion in the polymer modes. Analytically continued summation formulas are used to extract the leading finite size corrections to the solution free energy, which is then expressed solely in terms of experimental measurables. The transcription to experimental variables is facilitated by a new determination of the exact first order crossover dependence of the free energy on all three excluded volume interaction parameters.
Theory of multiphoton processes: General formalism
Authors: Fain Benjamin , Boeglin A. , Lin S. H.
Published: July 2002

A general framework for description of quantum systems in strong external fields is established. The most important application of the theory is concerned with the multiphoton ionization of atoms and molecules. The adiabatic approximation is developed. The Hamiltonian of the system is diagonalized by a time-dependent unitary transformation, which generates an interaction energy vanishing for very slow fields. A general analysis of the photoionization rates is performed. A specific model is considered: one-dimensional motion in the δ-function potential and external harmonic electric field. This model allows the exact solution for time-dependent eigenvalues and eigenfunctions. These solutions are compared with the conventional free electron approach to the ionization states.
Determination of population, alignment, and orientation using laser induced fluorescence with unresolved emission
Authors: Kummel Andrew C. , Sitz Greg O. , Zare Richard N.
Published: July 2002

A method is presented for determining the population, A{0}0+, the alignment moments, A{2}q± and A{4}q±, and the orientaion moments, A{1}q± and A{3}q±, for a ground state distribution of diatomic molecules probed by 1+1 laser induced fluorescence. General expresssions are developed for all rotational branches as a function of the rotational quantum number for excitation with linearly, circularly, or elliptically polarized light. Specific expressions are evaluated for the case in which the emission is unresolved and collected independent of its polarization and for the case in which the emission is unresolved but is analyzed with a polarizer. When the emission is collected independent of its polarization, the real polarization moments, A{k}q±, cannot be independently determined and only the apparent moments, A{k}q± (app), can be measured, explicit expressions for the apparent moments in terms of the real moments are presented. However, for the case in which the excitation light is created by passing linearly polarized light through a quarter-wave plate and the emitted light is analyzed with a quarter-wave plate and a linear polarizer, the real alignment and orientation moments can be independently determined.
The onset of nonrotating wave effects for reactive collisions in a laser field
Authors: Peploski James C. , Eno Larry
Published: July 2002

A number of aspects of the breakdown of the rotating wave approximation (RWA) in laser modified reactive collision processes are examined. An analysis is presented into the factors which are responsible for the breakdown and our conclusions are similar to those in a recent article by Last, save with regard to the effect of varying laser field strength. We also consider ways in which one can correct for the onset of RWA breakdown. The schemes considered all depend upon suppression (by matrix transformation) of low order couplings to the non-RWA channels. Application of the schemes and general investigation of non-RWA behavior is made for a simple model problem involving a single nuclear degree of freedom. We find that the results of the calculations lend support to our identified breakdown factors. Of the correction schemes, one is clearly to be preferred, although a second may also have merit. Finally, we discuss the experimental motivation for studies in a non-RWA regime and also consider a number of experimental and theoretical complications which may accompany RWA breakdown.
Monte Carlo simulation of orientational effects on direct energy transfer
Authors: Berberan-Santos M. , Prieto M. J. E.
Published: July 2002

The donor ensemble excitation decay is obtained by a Monte Carlo simulation for the three-dimensional case. Donor and acceptor reorientational motions are explicitly considered within the isotropic diffusion model. Allowance is made for donor and acceptor different rotational rates. The orientation dependent part of the decay is fully contained in the factor 〈(∼(κ2))1/2〉 which is the ensemble average of the square root of the finite time average of the usual orientational factor κ2. The time dependence of 〈(∼(κ2))1/2〉 is evaluated by a Monte Carlo simulation procedure for several relative mobilities of the donor–acceptor pair and finally captured in a simple, one-parameter formula.
Bound states embedded in the continuum of H+3
Authors: Berblinger M. , Pollak Eli , Schlier Ch.
Published: July 2002

Recently, Kennedy and Carrington found new quasibound states of H+3, which lie up to 1 eV above the dissociation limit with lifetimes as long as 1 μs. In an effort to understand the structure of these states, we investigate classically bound states embedded in the dissociative continuum of this molecule. In the first part, we assume J=0, and specialize to one of the two symmetries, C∞V or C2V. Poincaré surfaces of section are used to demonstrate the existence of a small region of bound phase space in these 2D problems, but stability analysis of the periodic orbits show that most of them are unstable in 3D. We conclude that J=0 or, more generally, low J states cannot explain the experiments. In the second part we treat the case J>0. A total angular momentum centrifugal barrier provides a classically rigorous boundary, which separates the phase space into two parts: a dissociative and a bound region. Wells and double wells exist. Trajectories in these wells show quasiperiodic or chaotic character, depending on the total angular momentum, and on the energy relative to the bottom of the well. Quantally, these states can dissociate by tunneling. One finds long lifetimes in qualitative agreement with the experiments. The volume of the bound part of the phase space is determined by Monte Carlo integration. Typically, several thousand resonance states are found for any J between 20 and 50. This suffices (in principle) to explain the very large number of experimentally observed lines.
Vibrational deactivation mechanisms for O+2(v=1) colliding with Kr
Authors: Tosi Paolo , Ronchetti Marco , Laganá Antonio
Published: July 2002

The dynamics of the Kr–O+2(v=1) deexcitation process has been investigated in detail by performing classical trajectory calculations and related graphical studies. The consequence of the trapping at the chemical well, the nature of the intermediate complex, and the effect of reaching turning points have been examined for these atom-charged diatom collisions. The impact of all that on the modeling of the quenching process has also been discussed.
A two-dimensional model for a light particle tunneling system
Authors: Okuyama Shoichi , Oxtoby David W.
Published: July 2002

We present a model calculation for barrier crossing rates of a light particle such as hydrogen, lithium, or a light interstitial in a symmetric double-well potential. At higher temperatures migration takes place through thermally activated processes, while the dynamics are dominated by tunneling at lower temperatures. We construct a model two-dimensional system that consists of a symmetric double-well potential for the high frequency, light particle mode and a harmonic oscillator for the lower frequency mode coupled closely to the reactive mode. This model system is, in turn, immersed in a classical bath. The reaction rate is calculated without assuming thermal equilibrium either for the light particle mode or for the nonreactive mode. Two distinct symmetries of coupling are considered for tunneling. The energy relaxation rate within each well is shown to have a significant effect on the rate in the high temperature limit. The temperature dependence of low temperature tunneling-dominated rates is determined by the type of coupling and by a parameter that describes the displacement of the nonreactive mode.
State-to-state reaction dynamics: A crossed molecular beam study of the reaction CH(2Π1/2;N=1)+D2→CD(2Π1/2,3/2;N′)+HD
Authors: Liu K. , Macdonald R. Glen
Published: July 2002

The title reaction was studied in a new crossed molecular beam apparatus. Laser-induced-fluorescence was used to characterize both the reactant CH radical and the product CD radical in a state-resolved manner. Two types of dynamical information are reported here: (1) the excitation function for a specific reaction product state and (2) the product rotational state distribution at a well-defined collision energy.
A new technique for state-to-state studies of unimolecular reactions
Authors: Luo X. , Rieger P. T. , Perry D. S. , Rizzo T. R.
Published: July 2002

A double-resonance technique which combines infrared excitation of a fundamental vibration of a light atom oscillator with vibrational overtone excitation of that same oscillator selectively prepares molecules in individual quasibound vibrational–rotational states at energies above their dissociation threshold. A third laser probes individual states of the product fragments via laser induced fluorescence. We present our initial results applying this technique to study the 6νOH vibrational overtone spectroscopy and unimolecular dissociation dynamics of hydrogen peroxide (HOOH). The double-resonance approach simplifies the congested vibrational overtone spectrum of HOOH, and the linewidth of resolved features places a lower limit on the lifetime of approximately 7 ps.
Electric microfield distribution in charged fluids: Plasmas, ionic and dipolar mixtures
Authors: Vericat F. , Rosenfeld Y. , Blum L.
Published: July 2002

The electric microfield distribution in fluids of particles of arbitrary size, shape, charge, and charge distribution is studied. The Morita–Iglesias formalism is extended to include angular dependent interactions. The results of the mean spherical approximation are discussed using an exact relation for the second moment of the microfield distribution, obtained in this work. In the Onsagerian strong coupling limit the sum rule for the second moment simplifies, and becomes identical to the mean spherical approximation result for the second moment.
Reaction cross sections and threshold energy in the reaction of isotopically pure H atoms and ethane
Authors: Nicholas John E. , Vaghjiani Ghanshyam L.
Published: July 2002

Reaction cross sections and threshold energy have been determined for the first time for the isotopically pure reaction, H+C2H6→H2+C2H5. Taking measured reaction probabilities from photochemical studies of the isotopically pure reaction and from xenon moderated systems, calculations described here of H atom collision densities in Xe by a stochastic method allowed the evaluation of reaction cross sections. These rise from the threshold energy at 43±2 kJ mol−1 to a maximum value of ∼8×10−3 nm2 in the vicinity of 100 kJ mol−1. The excitation function in this energy range is close to a line-of-centers function. The cross section values per C–H bond are higher than for the isotopically mixed H+C2D6 reaction, similar to the primary C–H bonds in C3H8, but lower than for secondary C–H bonds in C3H8. All these values are small compared with the total collisional cross section.
Bimolecular reactions with a reactive site on a spherical particle: A Hamiltonian formulation
Authors: Cotes N. J. , Sceats Mark G.
Published: July 2002

A formulation is presented for the bimolecular reaction rate between a small reactant molecule A in bulk with a reactive site B on the surface of a large particle E. The approach allows a direct comparison of this rate with that between A and B in bulk. The direct interaction potential between A and B is modeled by a central force potential VAB(R) which is assumed to be unchanged upon binding of B. The reaction rate is evaluated by reduction of the problem to a one-dimensional barrier crossing problem on the effective potential Ṽ(R)=V(R)−kT ln[ f (R)], where the entropic term accounts for the constraints of binding on the surface. The solution applies for arbitrary mean free path, and its limits are applicable to reactions in solutions (diffusion regime) and in gas phase (free molecule regime).
Fine-structure inelastic differential cross sections and B 2Σ potentials for the potassium rare gas interaction
Authors: Düren R. , Hasselbrink E. , Hillrichs G.
Published: July 2002

Differential scattering cross sections for fine-structure inelastic collisions of potassium in its first excited state with various rare gases (Ne, Ar, Kr, and Xe) have been measured. This crossed molecular beams experiment uses cw-dye lasers for the excitation of the incident potassium beam and the detection of the fine-structure inelastic scattered potassium atoms. The collision energy has been varied between 92 and 199 meV. The differential cross sections exhibit for small collision energies Stueckelberg oscillations, which are due to interference of scattering on the attractive A 2Π and the repulsive B 2Σ potential. For higher collision energies these oscillations are missing at large angles. It is demonstrated that with the A 2Π potential known from other sources the repulsive B 2Σ potential can be determined. A shoulder in this repulsive potential is found to be responsible for the absence of the interference oscillations at higher scattering energies.
Semiclassical Gaussian wave packet dynamics for collinear reactive scattering
Authors: Kress Joel D. , DePristo Andrew E.
Published: July 2002

The dynamics of collinear exchange reactions is treated within a semiclassical Gaussian wave packet (GWP) description. Two extensions to the previous methodology of GWP dynamics are presented. The first involves the evaluation of the interaction picture wave function propagators directly via the GWP solution to the time-dependent Schrödinger equation. The second involves use of a sum of GWPs to represent the initial translational plane wave on a finite interval. As usual, the vibrational wave function is also represented as a sum of GWPs, where the expansion coefficients are chosen to yield the desired initial vibrational state. The limitations of the previous GWP dynamical methods as applied to the reactive scattering problem are pointed out and the solutions provided by the present extensions are discussed. Results for the H+H2 collinear exchange reaction using the Porter–Karplus II potential energy surface are shown.
Properties of the solid–liquid interface layer of growing ice crystals: A Raman and Rayleigh scattering study
Authors: Halter P. U. , Bilgram J. H. , Känzig W.
Published: July 2002

The interface layer in front of an ice crystal growing into the melt has been studied simultaneously by Raman and Rayleigh scattering. The Raman spectra of the light scattered from the interface layer material give evidence for intermolecular dynamics which cannot be detected in either bulk phase. The Rayleigh scattering data are in agreement with the data of Böni et al. [Phys. Rev. A 28, 2953 (1983)], who have discovered the interface layer. For the first time it has been possible to measure the thickness of the layer as a function of the growth rate. At high growth rates enhanced Rayleigh scattering originating from the interface layer of a thickness of about 3 μm has been observed. With decreasing growth rate the thickness of the layer increases, reaching up to 200 μm at zero growth rate, and at melting the layer stays back in the water. From the linewidth of the Rayleigh scattered light we deduce a diffusion constant Di≈2×10−8 cm2/s. Raman spectra show that the intermolecular structure in the interface layer is essentially water-like. Compared to water additional intensity has been observed in the polarized Rayleigh wing, indicating a new polarized and unshifted line with a linewidth that corresponds to a relaxation time τ0≈4×10−13 s. It is tentatively assigned to the decay of a symmetric, intermolecular structure occurring in the interface layer. Slight differences found in the O–H stretching band can be interpreted as a small increase of the average number of H bonds per water molecule in the interface layer as compared to bulk water. The O–H stretching band looks as if the water in the interface layer has a temperature which is about 3 deg colder as it actually is.
Medium effects on valence and low-n Rydberg states: NO in argon and krypton
Authors: Morikawa E. , Köhler A. M. , Reininger R. , Saile V. , Laporte P.
Published: July 2002

Absorption and total fluorescence spectra of NO transitions perturbed by argon and krypton at various densities from the dilute gas to the low temperature solid are reported for photon energies 6≤hν≤9 eV. The valence transitions shift monotonically to lower photon energies with increasing perturber number density. The Rydberg transitions to n=3 (A 2Σ+, C 2Π, D 2Σ+) exhibit a moderate red shift at low perturber densities followed by an increasing blue shift with increasing density. Both valence and Rydberg transition energies vary smoothly and continuously with perturber number density even upon the liquid–solid phase transition. For fluid argon the experimental energy shifts are analyzed employing the semiclassical theory for line broadening.
CH(A, B–X ) fluorescence from vacuum ultraviolet excitation of CH3
Authors: Ye Chao , Suto Masako , Lee L. C.
Published: July 2002

CH3 was produced by the reaction of Cl or F with CH4 in a flow tube. The CH(A 2Δ, B 2Σ−–X 2Πr) fluorescence was observed from photoexcitation of CH3 at 157 nm by a single-photon excitation process and at 193 nm by two-photon excitation. No emission was detected at 248 nm. The fluorescence cross section at 157.5 nm was estimated to be about 8×10−19 cm2 with an uncertainty of factor 3. The CH2 emission is negligibly weak. The photodissociation process for the absorption continua of CH3 is discussed.
One-electron diatomics in momentum space. V. Nonvariational LCAO approach
Authors: Koga Toshikatsu , Matsuhashi Toshiyuki
Published: July 2002

The Fock transformation reduces the momentum-space Schrödinger equation for one-electron diatomics to a homogeneous Fredholm-type integral equation with a degenerate kernel. Two nonvariational linear combination of atomic orbitals (LCAO) solutions to this integral equation are rederived in a systematic manner and their mutual relation is clarified. The upper boundness of these nonvariational solutions is proved explicitly. Relative superiority of the two solutions is discussed theoretically based on their convergence rates. The momentum-space LCAO method is shown to reproduce the exact electronic energy for the hydrogen molecular ion at various internuclear distances.