bluuebunny/crossref_metadata_2025 huggingface dataset:
Radiative transition probalities for all vibrational levels in the X 1Σ+ state of HF
Authors: Zemke Warren T. , Stwalley William C. , Langhoff Stephen R. , Valderrama Giuseppe L. , Berry Michael J.
Published: July 2002

Recent analyses have led to an experimentally based potential energy curve for the ground state of HF which includes nonadiabatic corrections and which joins smoothly to the long-range potential at an accurately determined dissociation limit (De=49 362±5 cm−1). Using this potential curve and a new ab initio dipole moment function (in excellent agreement with experiment), accurate radiative transition probabilities among all vibrational levels (v=0–19 known, v=20 predicted) of the ground state of HF have been calculated for selected rotational quantum numbers. Comparisons of Einstein A spontaneous emission coefficients, dipole moment absorption matrix elements, and Herman–Wallis factors for absorption bands are presented.
System size dependence and time convergence in molecular dynamics simulations of monolayer films
Authors: Bishop Marvin , Clarke Julian H. R.
Published: July 2002

The sample size dependence and time convergence of property values of simple hydrocarbon monolayers has been investigated by performing molecular dynamics simulations at four surface coverages for systems ranging in size from 16 to 100 chains and for trajectories as long as 200 ps. Detailed studies of the tilt angle indicate that systems with 64 chains simulated for 40 ps (in addition to a 40 ps equilibration stage) are large enough to obtain statistically meaningful results. The equilibrium tilt angle is slightly sensitive to the boundary conditions.
Quantum mechanical study of the reaction He+H+2→HeH++H with hyperspherical coordinates
Authors: Lepetit B. , Launay J. M.
Published: July 2002

Integral cross sections for the reaction He+H+2→HeH++H have been computed with the hyperspherical method in the threshold energy range. The ab initio potential of McLaughlin and Thompson, fitted by Joseph and Sathyamurthy, has been used. A lifetime matrix analysis of the J=0 reaction probabilities shows the existence of many long-lived resonances, with several having lifetimes as long as 3.1 ps. The magnitude of the total reaction cross section is in reasonably good agreement with the experimental result of Chupka (1968).
Isomeric structures and van der Waals vibrational frequencies of the glyoxal⋅Ar complexes. I. Fluorescence excitation spectroscopy
Authors: Lapierre Luc , Frye Donald , Dai Hai-Lung
Published: July 2002

From the band shape and the concentration dependence of the peak intensity in the fluorescence excitation spectrum observed in a supersonic expansion of a He/Ar/glyoxal mixture near the 000Ã←X̃ band of glyoxal, it was found that the spectral features can be assigned to the following van der Waals complexes; (glyoxal)2⋅Ar, glyoxal⋅Ar2, (glyoxal)2, and three different isomers of glyoxal⋅Ar. The approximate structures of the three glyoxal⋅Ar isomers have been determined from rotational band contour analyses. In the electronic ground state, the Ar atom is either on the C2 axis 3.6 Å above the glyoxal plane (the top isomer), in the HCCO gulf of the glyoxal plane 4.3 Å away from the glyoxal center of mass (the side isomer), or in the HCO gulf of the glyoxal plane 4.8 Å away from the glyoxal center of mass (the front isomer). As the glyoxal molecule is electronically excited to the Ã 1A1 state, the Ar atom moves away from oxygen by more that 1 Å for all three isomers. Excited van der Waals vibrational levels associated with the 0̄0, Ã state of glyoxal were observed and assigned for all three glyoxal⋅Ar isomers.
From butadiene to polyacetylene: An a
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o study on the vibrational spectra of polyenes
Authors: Kofranek Manfred , Lischka Hans , Karpfen Alfred
Published: July 2002

We report the results of a systematic ab initio self-consistent field (SCF) study on the structure and vibrational spectra of all-trans polyenes ranging from C4H6 to C18H20. We include the limiting case of the infinite all-trans polyacetylene. Trends in the evolution of the vibrational frequencies upon increasing the size of the polyenes and the convergence of edge effects and of bulk properties are monitored. We point out the inherent limitations of any SCF-type approach for the description of the vibrational spectra of polyenes and discuss difficulties encountered in the extrapolation to infinite chain length. On the basis of a suite of calculations on the smaller members of the polyene series trans-1,3-butadiene and all-trans-1,3,5-hexatriene carried out with a high-level electron correlation method, a new scaled force field has been suggested recently by us [M. Kofranek, A. Karpfen, and H. Lischka, Int. J. Quantum Chem. Symp. 24, 721 (1990)] which reproduces most of the experimentally known trends in these oligoenes, in particular, the vibrational frequencies in the region of the carbon–carbon double and single bond stretches. The performance of this scaled force field for the prediction of the vibrational spectra of larger all-trans polyenes is investigated in detail. We also apply it to compute the harmonic phonon dispersion curves of polyacetylene and compare with available experimental data.
Monte Carlo study of titration of linear polyelectrolytes
Authors: Reed Christopher E. , Reed Wayne F.
Published: July 2002

An off-lattice Metropolis Monte Carlo algorithm with reptation is used to find the average fractional ionization ᾱ as a function of pH for a generic ionizable linear polyelectrolyte in a salt solution. The polyelectrolyte is treated as a threefold rotational isomeric state model polymer; each unit can bear a negative charge or not with intrinsic ionization constant pKa. Debye–Hückel screening is assumed between the charges. For computational convenience, the dielectric constant of the polymer is taken to be that of the solvent. The number of units N was either 50 or 100. Monte Carlo results were collected for various Debye screening lengths at six combinations of number of chain units N, bond angle θ, and Manning parameter when fully charged, ξ0. For four of the combinations, ξ0 was 1 to take partial account of counterion condensation. These runs had N and θ of 50 and 1°, 50 and 70°, 100 and 1°, and 100 and 70°. The fifth combination had N=50, θ=70°, and ξ0=2.85. The sixth had N=50, θ=27.34°, and ξ0=0.72, for comparison with data for hyaluronate. The Monte Carlo results are compared to third nearest-neighbor linear Ising type calculations and to simple mean field theories in α. Mean field theory in α worked very well in the (nearly rodlike) θ=1° cases using the known distance between units. Mean field theory in α using an estimate for the distance between units based on the ideas of electrostatic persistence length and excluded volume worked equally well for the θ=1° cases and moderately well for the θ=70° cases. The free energy and entropy per simulated chain were calculated by thermodynamic integration of the Monte Carlo results for ᾱ as a function of pH.
Prediction of anomalous redshift in semiconductor clusters
Authors: Rama Krishna M. V. , Friesner R. A.
Published: July 2002

Nonlocal and energy dependent pseudopotentials are employed to investigate the effect of size on spectroscopic transitions in silicon clusters. The symmetry allowed Franck–Condon electronic transition in clusters is to the red of the corresponding transition in bulk silicon, a feature opposite to that observed in other semiconductor clusters. This redshift increases with decreasing cluster size and it is sufficiently pronounced to be experimentally observable. Furthermore, this unusual quantum confinement effect on the excited electronic states is displayed by large clusters whose ground state structures have converged to the bulk limit. The symmetry forbidden non-Franck–Condon electronic transitions do not exhibit such an anomalous redshift. Finally, this study reveals that three-dimensional confinement does not make silicon, which is an indirect gap semiconductor, into a direct gap material.
Chaos in small clusters of inert gas atoms
Authors: Hinde Robert J. , Berry R. Stephen , Wales David J.
Published: July 2002

We have calculated the Kolmogorov entropy for three- and seven-particle clusters bound by Lennard-Jones potentials and for three-particle clusters bound by Morse potentials of various ranges. We have used two quite different methods, one of which is new, which give consistent results. We find that all of these systems are classically chaotic over a wide range of energies surrounding the estimated quantum-mechanical zero point energies. Furthermore, for the three-particle clusters, we can rationalize the variation in the degree of chaos with total energy in terms of the local structure of the clusters’ potential energy surface.
Assignment of the diabatic and adiabatic atomic asymptotic limits of K2 Rydberg states
Authors: Wang He , Stwalley William C. , Lyyra A. Marjatta
Published: July 2002

In order to make an unambiguous assignment of the atomic asymptotic limits for the previously observed Rydberg states of K2, two more low-lying Rydberg states (one 1Δg state and one 1Πg state), were observed with sub-Doppler resolution using cw optical–optical double resonance spectroscopy. The correlation of the molecular Rydberg states with the diabatic and adiabatic atomic limits, the quantum defects, and the diabatic and adiabatic dissociation energies are discussed and the principal quantum number assignment of Broyer et al. reinforced.
A quasiclassical trajectory study of OH rotational excitation in OH+CO collisions using a
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o potential surfaces
Authors: Kudla Kathleen , Koures Antonios G. , Harding Lawrence B. , Schatz George C.
Published: July 2002

We have performed large basis set configuration interaction calculations to characterize the two potential surfaces (2A′ and 2A″ ) which correlate to the ground state of OH+CO. Only planar geometries of the four atoms are considered, and the calculations restrict the OH and CO bond distances to their isolated diatomic values. Global representations of these potential surfaces have been developed and used in quasiclassical trajectory studies of rotational excitation in low energy (1–6 kcal/mol) collisions of OH and CO in their respective rovibrational ground states. We find that the collisional excitation cross sections are about equal for the two surfaces, and there is a monotonic increase in each cross section with translational energy. For OH rotational quantum numbers N between 2 and 6 there is approximately a factor of 2–3 decrease in the cross section for each unit increase in N. The energy and N dependence of these cross sections are generally in excellent agreement with recent experiments. We have also explored the sensitivity of these cross sections to the nature of the potential energy surface, and we have used a surface that describes the formation of the intermediate complex HOCO to determine sensitivity of the rotationally inelastic cross sections to complex formation. In agreement with the experiments, we find that the low energy, high N cross sections are appreciably perturbed by complex formation.
Real-time dynamics of clusters. I. I2Xn (n=1)
Authors: Willberg D. M. , Gutmann M. , Breen J. J. , Zewail A. H.
Published: July 2002

Vibrational predissociation of I2Xn (X=Ne, Ar) van der Waals clusters are studied in real-time using picosecond pump–probe (LIF) and molecular beam techniques. The state-to-state rates of vibrational predissociation are measured for specific vibrational levels v′i by monitoring the rise of nascent I2. Here, we report our study of I2X(B,v′i)k(v′i,v′f) →I2(B,v′f)+X. For I2Ne, the values of τ=k−1(vi,v′f) decrease nonlinearly from 216 ps for vi=13 to 53 ps for v′i=23. For I2Ar (B,vi), which undergo electronic and vibrational predissociation, the risetime of the nascent I2 is found to be 70 ps when v′i=18 and 77 ps when vi=21. A number of theoretical models for vibrational predissociation (the energy-gap law, the momentum-gap law, quantum and classical calculations) are compared with our experimental results in an attempt to understand the key features of the dynamics and the potential energy surface.
Vibrational analysis of a Hylleraas-configuration interaction potential for H+3
Authors: Lie G. C. , Frye D.
Published: July 2002

The potential surface for H+3, obtained from Hylleraas configuration interaction calculations by Frye et al. [J. Chem. Phys. 92, 4948 (1990)], has been fitted to a seventh-degree polynomial in Morse–Dunham coordinates to study the low-lying vibrational states of H+3, D+3, H2D+, D2H+, H2T+, and T2H+. Vibrational levels of up to two quanta excitations are reported and compared with experiments and other theoretical calculations. There are indications that adiabatic corrections may be important in correcting the small errors in the calculated vibrational quanta. The exothermicity of the proton-transfer reaction between H2 and H+2 predicted is 1.685 eV, to be compared with the experimental value of 1.7 eV. The calculated dissociation energy for H+3 is 4.337±0.002 eV, 355 cm−1 smaller than the experimental value of 4.381±0.021 eV. No source of discrepancy from theoretical side can be identified.
On the determination of cluster properties by ionization techniques
Authors: Booze Jon A. , Baer Tomas
Published: July 2002

Experimental differentiation between similar cluster ions of Acetylene produced by the chemical reactions is reported. (AIP)
Predissociation of the NH/ND(c 1Π,v′,J′) states
Authors: Bohn B. , Stuhl F. , Parlant G. , Dagdigian P. J. , Yarkony D. R.
Published: July 2002

The predissociation of the electronically excited NH/ND(c 1Π) state was studied by accurate lifetime measurements. The pulsed generation of single rotational states of different vibrational levels was performed by dye laser excitation in the NH/ND c 1Π–a 1Δ band system. The lifetime of ND(c,v′=0,J′=1) is 500±10 ns. This value is close to the purely radiative lifetime of the rotationless state. The lifetime increases steadily up to J′=15. The lifetime of ND(c,v′=1) is 235 ns for very low J′ and decreases for higher J′. These shorter lifetimes are due to predissociation by the repulsive 1 5Σ− state. Similarly, the NH(c,v′=0) state is predissociated for all J′, having an average lifetime of 463 ns for J′=1–7. For J′>7, the lifetime decreases steadily. The lifetime of NH(c,v′=1) is dominated by predissociation, with values near 65 ns for J′=1–6. These experimental results are found to be in very good agreement with an ab initio electronic structure calculation of the radiative and predissociative decay rates.
The electronic structure and vibrational frequencies of CNN and SiNN from local density functional methods
Authors: Dixon David A. , DeKock Roger L.
Published: July 2002

The geometry and vibrational frequencies of CNN and SiNN in the 3Σ− state have been calculated by local density functional theory. The geometry of CNN has an NN bond (1.202 Å) that is shorter than the CN bond (1.252 Å). For SiNN, the NN bond is shorter (1.164 Å) than that for CNN and the SiN bond (1.749 Å) is like an Si–N single bond. The calculated frequencies for CNN (1597, 1273, and 377 cm−1) can account for the lower two observed transitions but can not account for the observed transition at 2847 cm−1 as a fundamental. The infrared intensities are all predicted to be small for CNN. The calculated frequencies for SiNN (1821, 569, and 321 cm−1) can account for the observed spectrum and we note that ω1 is predicted to have a large infrared intensity. The local density functional results for SiNN and CNN are quite different from previously reported molecular orbital predictions which could not satisfactorily account for the experimental results.
Semiclassical theory of the photon echo: Application to polar fluids
Authors: Shemetulskis Norah E. , Loring Roger F.
Published: July 2002

The photon echo experiment probes the dynamical processes that cause the decay of a coherent macroscopic electric polarization. Application of the technique to liquid solutions permits the study of the molecular motions that are strongly coupled to an electronic transition. We develop a semiclassical approximation to the photon echo observable, based on the use of Wigner equivalents of quantum operators, that we demonstrate to be accurate for all values of the delay time between the two pulses of the photon echo. In this approximation, the echo arises from fluctuations in the spectroscopic transition frequency, as the fluid evolves on a potential surface that is the arithmetic mean of the ground and excited state surfaces. The echo may then be calculated from nonequilibrium molecular dynamics simulations. We use this method to calculate the photon echo for a fluid of dipolar soft spheres. The results are compared to calculations of the absorption spectrum for this model, and to previous calculations of the effects of solvent dynamics on optical spectroscopic observables.
A Bayesian analysis of Green’s function Monte Carlo correlation functions
Authors: Caffarel M. , Ceperley D. M.
Published: July 2002

Imaginary-time correlation functions calculated by quantum Monte Carlo (QMC) are analyzed using the maximum entropy method (MaxEnt) to determine the ground-state energy and spectral overlap function. In contrast to earlier applications of MaxEnt, the data is obtained from importanced-sampled zero-temperature quantum Monte Carlo simulations. The analysis includes two steps. First, that spectral overlap function and ground state energy which maximizes the entropy and agrees with the QMC correlation functions is obtained. Then the errors in the energy are evaluated by averaging over all the possible images (average MaxEnt method), the multidimensional integrals being computed using the Metropolis algorithm. The central feature of this approach is that all the information present in the correlation functions is used in the only way consistent with fundamental probabilistic hypotheses. This allows us to fully exploit the information contained in the correlation functions at small imaginary times, thus avoiding large statistical fluctuations associated with large imaginary times. In addition, the computed errors include both the statistical errors and systematic extrapolation errors. The method is illustrated with a harmonic oscillator and the four-electron LiH molecule.
Tilt transitions in Langmuir monolayers of long-chain molecules
Authors: Karaborni S. , Toxvaerd S.
Published: July 2002

Molecular dynamics simulations have been used to investigate tilt transitions in a monolayer model of amphiphilic molecules at an air–water interface. Eight simulations were performed at 300 K on monolayers in the density range of 18.5–25 Å2/molecule. The model amphiphilic molecules contained 19 pseudoatoms, each representing a methyl or a methylene group, and a head group representing a carboxylate group. Amphiphile–amphiphile interactions were modeled using a new anisotropic united atom model that accounts implicitly for the presence of hydrogen atoms in alkanes; water–amphiphile interactions were modeled using two external potentials that do not constrain the head groups to the interface, allow methylene segments to enter the water, and provide a finite size interface of the same order of magnitude as the size of the experimental water–air interface. The tilt behavior of the monolayer was monitored as a function of molecular area. Tilt angle results and structure factor analysis point to the occurrence of a transition between 20 and 21 Å2/molecule from an almost upright to a tilted monolayer. At 21 and 22 Å2/molecule, the monolayers do not have a well-defined tilt order. Upon monolayer expansion to 23 Å2/molecule, the molecules become tilted over their nearest neighbors.
The effects of confinement and surface interactions on coexistence in a binary polymer mixture
Authors: Budkowski Andrzej , Steiner Ullrich , Klein Jacob
Published: July 2002

We have measured the composition-distance profile across a film consisting of two thin layers (200–600 nm) of a model binary isotopic mixture of deuterated polystyrene (dPS) and protonated polystyrene (hPS), coexisting with each other near their equilibrium compositions below the critical temperature for phase demixing for this pair. Profiles were determined normal to the silicon wafer on which the bilayer is mounted using nuclear reaction analysis, both for an uncoated silicon surface and for one coated with a gold layer. Measurements reveal that when both layers are thick relative to the characteristic width w (∼100 nm) of the interfacial region between them, the coexisting compositions about the interface are close to their bulk values as determined earlier for this system. When the dimensions of the layers are made comparable with w, however, interactions with the confining surfaces may significantly modify the composition profile of the coexisting layers about the interface. This effect is marked at the polymer/silicon interface as a result of its interactions with one of the components (dPS), but is absent for a gold-coated surface in an identical geometry due to the much weaker influence of the surface. Our results are discussed in detail in terms of mean-field models of mixing in polymeric mixtures, and enable quantitative determination (using a Cahn construction approach) of the interaction parameters both at the polymer–air and polymer–silicon interfaces. Though we are not able to calculate in a completely a priori fashion the coexistence profiles as a function of the film thickness, we propose an approximate approach which provides good agreement of calculated composition profiles with those determined experimentally over the range of parameters in our experiments.
Generation of metal–carbon and metal–nitrogen clusters with a laser induced plasma technique
Authors: Guo B. C. , Wei S. , Chen Z. , Kerns K. P. , Purnell J. , Buzza S. , Castleman A. W.
Published: July 2002

During the course of investigating dehydrogenation reactions induced by transition metals, we find that using a carrier gas containing hydrocarbons and ammonia instead of pure helium, in conjunction with a laser vaporization device, enables the facile production of metal–carbon and metal–nitrogen clusters in both the neutral and ionic forms. With only a change in the nature of the carrier gas, a variety of new classes of clusters can be produced.
On the use of Chandrasekhar’s basis for helium and its isoelectronic series
Authors: Fontenelle Márcia T. , Gallas Jason A. C.
Published: July 2002

We report a systematic investigation of the ability of Chandrasekhar’s basis to produce energy eigenvalues for the helium isoelectronic sequence and compare it with the standard Hylleraas’ basis. The basis is found to be extremely useful to obtain fast and accurate energy approximations for H−. In addition, we also investigate the dependence of energy eigenvalues on the interelectronic u coordinate and find it to be more important for Z≥2.
Complex electron affinity processes and ionization in the clusters Si3–Si10
Authors: von Niessen W. , Zakrzewski V. G.
Published: July 2002

Vertical and adiabatic electron affinities and vertical ionization energies are presented for the clusters Sin, n=3,10 obtained from ab initio calculations with large basis sets. Electron correlation effects are taken into account by configuration interaction (CI) and Green function techniques. The clusters exhibit a complex behavior upon electron attachment. For each cluster there are several affinity states which result from capture of an electron into different orbitals. In some cases shake-up affinities are calculated which are positive. Si3 C2v is found to have at least three, Si4 D2h four, Si5 D3h two, Si5 C4v one, Si6 C2v three, Si7 D5h two, Si8 C2h five, Si9 Cs four, and Si10 C3v two affinity states (vertical processes, Sin+e−→Sin−+hν). The effects of electron correlation on the electron affinities are very large. In several cases the differences between the adiabatic and vertical electron affinities are exceedingly large amounting up to 1.5 eV.
A random matrix approach to rotation–vibration mixing in H2CO and D2CO
Authors: Burleigh Darin C. , Sibert Edwin L.
Published: July 2002

In a previous study of highly excited states of H2CO, we found that rotation–vibration mixing increases rapidly as a function of J for those states whose vibrational energy is greater than 6000 cm−1 [McCoy, Burleigh, and Sibert, J. Chem. Phys. 95, 7449 (1991)]. The present work elucidates the onset of this mixing in both H2CO and D2CO. In addition to Coriolis and centrifugal coupling, vibrational mixing between the normal modes is shown to have a strong influence on the breakdown of the separation between rotation and vibration. The role of this latter effect is accurately modeled using a random matrix approach. An ensemble of vibrational Hamiltonian matrices are defined whose matrix elements are chosen from distribution functions whose functional forms were determined by examining the matrix form of the original model Hamiltonian. In adopting such an approach, the goal is not to attempt to predict rotation–vibration mixing for a specific zero order state, but rather to model the distribution of rotation–vibration mixing for all the rotation–vibration states of a given J and within a total energy range E±ΔE. The trends observed for these distributions as J and E are increased are quantitatively reproduced using the random matrix approach. Furthermore, the random matrix approach allows one to accurately reproduce the mixing with distribution functions defined with only a few parameters.
Studies of the optical-like high frequency dispersion mode in liquid water
Authors: Resat Haluk , Raineri Fernando O. , Friedman Harold L.
Published: July 2002

An ‘‘optical’’-like collective mode (OM) above 500 cm−1, or ω=100 ps−1, is known on the basis of molecular dynamics (MD) simulation of the TIP4P model for liquid water at T=293 K. Recently we proposed a dielectric theory of the OM for the same water model by applying the reference memory function approximation (RMFA), a theory of the computationally convenient type. It enables us to extend the MD results for the dielectric permittivity εω into the region of finite wave vector k. Thus we calculate the longitudinal dielectric function εL(k,ω) and the corresponding correlation functions for collective fluctuations in local polarization charge densities and currents required to characterize the OM. In the work reported here, we find that the calculated OM and its dispersion relation are relatively insensitive to whether the RMFA is implemented with collective or single particle reference dynamics, the principal difference being a more structured OM in the latter case. This finding provides the justification for applying the RMFA with reference dynamics based on the MD results of Frattini et al. for the single particle dipole reorientation time correlation functions of the same water model at various temperatures. We find that the OM has a definite temperature dependence, but the effect is weak even in the temperature range of real supercooled water.
Observation of electronically excited states of tetraoxygen
Authors: Helm H. , Walter C. W.
Published: July 2002

We have investigated electron transfer to O4+ in reactions with O2, NO, and Cs. We observe formation of O4 molecules that decay by predissociation and by direct dissociation. The kinetic energy release in dissociation as well as the nature of the dissociation products are determined. Evidence for three short-lived electronic states of tetraoxygen at energies near 2 eV above O2+O2 is obtained in electron transfer from O2 and NO. In the experiment with cesium we find evidence for longer-lived electronic states (10−13≤τ≤10−7 s) at 9.4 and 10.5 eV. The energy release and symmetry of the dissociation pattern suggests that the high lying states are symmetric molecular configurations at extended bond lengths (∼2.4 Å).
Improved ground state rotational constants for the argon–hydrogen fluoride dimer
Authors: Stockman Paul A. , Blake Geoffrey A.
Published: July 2002

Ten new rotational transitions, J(4←3) to J(13←12), of the argon–hydrogen fluoride dimer (Ar–HF) have been measured in the ground vibrational state. The spectra were obtained in the microwave spectral region from a slit supersonic expansion. In a combination fit with previous microwave measurements [A. Dixon, C. H. Joyner, F. A. Baiocchi, and W. Klemperer, J. Chem. Phys. 74, 6539 (1981)] and high-J infrared spectra of the HF stretch (1000)←(0000) [G. T. Fraser and A. S. Pine, J. Chem. Phys. 85, 4890 (1986)], the values for the ground rotational constants have been improved considerably. The improved values for the constants allow predictions of ground state energies within 1 MHz up to J=20 and within 30 MHz for J values up to 40.
b-dipole transitions in trans-HOCO observed by far infrared laser magnetic resonance
Authors: Sears Trevor J. , Radford H. E. , Moore Mary Ann
Published: July 2002

Far infrared laser magnetic resonance spectroscopy is used to measure components of 12 rotational transitions in the ground state of the HOCO radical. The transitions are all b-dipole in character in contrast to the a-dipole rotational spectrum previously reported [Radford, Wei, and Sears, J. Chem. Phys. 97, 3989 (1992)]. The new data determine the A rotational constant to high precision and allow the determination of several centrifugal distortion constants for the first time. The hyperfine coupling in the radical leads to observable splittings in several of the observed transitions and these are used to estimate two of the four expected nonzero hyperfine parameters in the radical.
Molecular theory of ultrafast solvation in liquid acetonitrile
Authors: Roy Srabani , Komath Snehasudha , Bagchi Biman
Published: July 2002

A microscopic calculation of solvation dynamics of an immobile solute ion in liquid acetonitrile is presented. The dynamical information necessary for this calculation is obtained from the Kerr and dielectric relaxation of the neat liquid. The calculated solvation time correlation function is in excellent agreement with both the experimental and the simulated results.
Diffusion-influenced reaction kinetics on fractal structures
Authors: Barzykin A. V. , Tachiya M.
Published: July 2002

A theory of diffusion-influenced geminate and pseudo-first-order bulk reactions on fractal structures is presented within the Smoluchowski framework. The formalism is based on the solution of the backward equation for the pair survival probability, generalized to noninteger dimensionality using the effective potential approximation. Possible anomalous diffusion is taken into account by assuming a new time variable, ensuring linear asymptotics of the mean-squared displacement. The results for both absorbing and radiation inner boundary conditions are derived for noninteracting reactants and compared with those well known for integer dimensions. The asymptotic analysis for arbitrary potential is also carried out.
The spin–orbit effect on potential surfaces of NO2 photodissociation
Authors: Katagiri Hideki , Kato Shigeki
Published: July 2002

Potential energy surfaces for photodissociation reaction NO2→NO(2Π)+O(3P) have been studied by ab initio calculations. The effect of spin–orbit interaction on the potential surface features was studied near the product region. All the 12 potential surfaces asymptotically correlated to the NO(2Π)+O(3P) limit were obtained by the state-averaged complete-active-space-self-consistent-field (CASSCF) method. The adiabatic potential surfaces including the spin–orbit interaction were constructed using the full Breit–Pauli Hamiltonian. It was found that the lowest two states are attractive, while all the other states are repulsive. Assuming that NO2 undergoes the photodissociation on the ground state surface, we obtained the bending-rotation energy levels along the dissociation coordinate, and the transition state for each bending level was determined. The potential barriers for the vibrationally adiabatic energy curves were consistent with the recent experiments. Using a simplified model based on the infinite order sudden approximation (IOSA) and the Franck–Condon approximation, the product fine structure distribution was estimated, which is in good agreement with the experimental results.
Quantum dynamics studies of adsorption and desorption of hydrogen at a Cu(111) surface
Authors: Sheng Jia , Zhang John Z. H.
Published: July 2002

The quantum dynamics of dissociative adsorption and associative desorption of hydrogen on Cu(111) surface over an atop site has been studied in detail using the S-matrix Kohn variation method for reactive scattering. We employed an empirical London–Erying–Polanyi–Sato (LEPS) type potential energy surface (PES) with parameters fitted to the available experimental adsorption data and to theoretical cluster calculations. The dissociation probability of hydrogen, as a function of normal kinetic energy, is calculated for individual rovibrational states with the v=1 translational energy threshold being lower than that of v=0 by about 0.317 eV. Our calculation shows that dissociative adsorption of H2 on Cu(111) at relatively low kinetic energies (<0.4 eV) is dominated by the component of vibrationally excited H2(v=1), whereas ground H2(v=0) plays the dominate role at higher kinetic energies. In addition to vibrational enhancement of hydrogen dissociation, the role of hydrogen rotation in dissociative adsorption has also been examined. In particular, in-plane rotation of H2(m=j) is found to be more favorable for dissociation than out-of-plane rotation (m=0), similar to the finding from a previous study on H2/Ni(111) system. The present study also examined internal state distributions of H2 desorbed from Cu(111). The vibrational population ratio Pv=1/Pv=0 in desorption is much larger than the thermal distribution at surface temperatures. The relation between the vibrational population ratio in desorption and the vibrational enhancement in adsorption is discussed and analyzed. Our theoretical results are compared to the recent experimental results for both adsorption and desorption of H2 on Cu.
Numerical simulations of solvation dynamics in electrolyte solutions
Authors: Neria Eyal , Nitzan Abraham
Published: July 2002

Recent experimental studies of solvation dynamics in electrolyte solutions indicate the existence of a slow dynamical component associated with the salt ions. This contribution cannot be accounted for by the Debye–Falkenhagen theory of ionic atmosphere response. Molecular dynamics simulations of solvation dynamics in a simple model (Stockmayer solvent containing spherical ions) of electrolyte solution are presented. The simulations confirm the interpretation that the slow dynamical component is primarily an outcome of ion exchange between the first solvation shell about the solute and the solution bulk. The simulations also indicate the highly correlated motion between the salt ions and the solvent molecules.
Quantum scattering studies of long-lived resonances for the reaction Ne+H+2→NeH++H
Authors: Kress Joel D. , Walker Robert B. , Hayes Edward F. , Pendergast Phil
Published: July 2002

The bending-corrected rotating linear model (BCRLM) is used to investigate the reaction of neon with H+2 (v=0–3) using three different fits to the abinitio potential-energy surface computed by Urban, Jaquet, and Staemmler. Numerous long-lived scattering resonances are found for each surface. The number and position of these scattering resonances are found to be sensitive to the relatively small differences among these three surfaces. These BCRLM results demonstrate how the rich resonance structure that appears in the partial cross sections is washed out in the total cross section. The integrated rates for reactivity from v=0 and 1 are nearly identical for all three potential-energy surfaces over a wide range of temperatures. However, the integrated rates from v=2 and 3 exhibit significant differences among the potential-energy surfaces. A vibrationally adiabatic hyperspherical model of the trapped resonance states provides insight into the nature and contribution of these resonances to reactive scattering. The more accurate of the three fits to the abinitio potential-energy surface (obtained using the functional form of Aguado and Paniagua) is also used to obtain converged results for total angular momentum J=0 employing the adiabatically adjusting, principal axis, hyperspherical (APH) formulation of Pack and Parker for quantum reactive scattering in three dimensions (3D). An eigenlifetime analysis of these 3D scattering results reveals numerous resonances with lifetimes of 1 ps or more. While this resonance structure is sensitive to the details of the potential energy surface, with appropriate Gaussian averaging over the total scattering energy, the cumulative reaction probabilities (CRPs) are not very sensitive to changes in the potential energy surface. Moreover, these quantum CRPs agree rather well with CRPs predicted using variational Rice–Ramsperger–Kassel–Marcus (RRKM) calculations.
Mechanistic aspects of fullerene coalescence upon ultraviolet laser desorption from thin films
Authors: Beck Rainer D. , Weis Patrick , Bräuchle Götz , Kappes Manfred M.
Published: July 2002

Positively and negatively charged coalescence products of C60 and C70 were characterized following pulsed laser desorption from fullerene thin films. Coalescence occurs by gas-phase reactions. Positive ions are generated by thermoionization of hot neutrals, while negative ions likely form by electron attachment to neutral species in the outermost region of the desorption plume. Thermoionization rate and surface-induced dissociation (SID) measurements are consistent with fullerene-like structures for coalescence products detected as positive ions.
Decay of nuclear magnetization by bounded diffusion in a constant field gradient
Authors: de Swiet Thomas M. , Sen Pabitra N.
Published: July 2002

Transverse magnetization of spins diffusing in a bounded region in the presence of a constant field gradient is studied. We investigate the breakdown at short times of the much used formula for the Hahn echo amplitude in a constant gradient in unbounded space: ℳ(2τ)/ℳ(0)=exp(−2D0g2τ3/3). Here D0 is the diffusion constant in unbounded space and g is the field gradient multiplied by the gyromagnetic ratio. We find that this formula is replaced by ℳ(2τ)/ℳ(0)=exp[−2Deffg2τ3/3 +𝒪(D5/20g4τ13/2S/V)] with an effective diffusion coefficient Deff(2τ) =D0[1−α√D0τ(S/V) +...], where α is a constant and S/V is the surface to volume ratio of the bounded region. Breakdown is complex but we find that the interplay between a natural length scale lc=(g/D0)−1/3 and the geometry of the region governs the problem. The long-time behavior of the free induction decay and echo amplitude are then considered where pure exp[−const t] decay is expected. We consider some simple geometries and find in addition to the well-known result, ln‖M(z,t)‖∼−D0g2R4pt, valid for Rp≪lc (where Rp is the size of the confining space) that in the regime Rp≫lc the decay becomes ln‖M(z,t)‖∼−g2/3D1/30t. We then argue that this latter result should apply to more general geometries. We discuss implications for realistic experimental echo measurements and conclude that the g2/3D1/30 decay regime is hard to measure. Implications for the effect of edge enhancement in NMR microscopy are also discussed.
Monomer–excimer kinetics in solution. I. Stochastic many-particle approach
Authors: Naumann Wolfgang , Molski Andrzej
Published: July 2002

A stochastic many-particle approach is applied to study the kinetics of reversible excimer formation in solution. Coupled dynamic equations for the macroscopic concentrations and for the radial distribution function are derived, and applied to analyze (i) time resolved kinetics after a short pulse, and (ii) steady-state kinetics. Renormalization of the phenomenological excimer dissociation rate coefficient due to nonequilibrium effects is discussed. A relation is demonstrated between steady-state, reversible monomer–excimer kinetics and irreversible fluorescence kinetics. Explicit results are given for the excimer fluorescence yield, assuming the Smoluchowski–Collins–Kimball reactivity model.
Analysis of long range dispersion and exchange interactions between two Na atoms
Authors: Zemke Warren T. , Stwalley William C.
Published: July 2002

This paper critically surveys the best available data for the two lowest states (X 1Σ+g and a 3Σ+u) of Na2. Since both states are known up to near dissociation, they can be used to determine the separate Coulomb and exchange contributions to the intermediate and long range interaction potentials. The multipolar expansion representation of the Coulomb (dispersion) energy at long range (−∑nCn/Rn) and the exponential representation of the exchange energy at long range (Ae−aR) are tested.
Excess-electron alkali halide clusters Kn+1Cln and Lin+1Fn: A theoretical study
Authors: Ochsenfeld Christian , Ahlrichs Reinhart
Published: July 2002

Ab initio results are reported for the excess-electron systems M2X, M3X2, M14X13, and M18X17. In addition to self-consistent-field (SCF) calculations, electron correlation effects are taken into account for the smaller clusters by second-order Mo/ller–Plesset perturbation theory (MP2) and the single- and double-excitation coupled cluster method with a perturbational estimate of triple excitations CCSD(T). For the smaller systems stationary geometries are characterized as local minima by SCF and MP2 force field calculations. The excess electron is either localized in an anion defect location or at a weakly bound alkali atom in all cases.
Anion photoelectron spectroscopy of small indium phosphide clusters (InxP−y; x,y=1–4)
Authors: Xu Cangshan , de Beer Esther , Arnold Don W. , Arnold Caroline C. , Neumark Daniel M.
Published: July 2002

Small indium phosphide clusters having 2–8 atoms are studied using anion photoelectron spectroscopy of InxP−y (x,y=1–4). From these spectra, the electron affinities are determined. Both ground and low-lying excited electronic states of the neutral clusters are observed. An electronic gap is shown in the even cluster anion spectra.
Determination of the temperature and concentration dependence of the refractive index of a liquid mixture
Authors: Li W. B. , Segrè P. N. , Gammon R. W. , Sengers J. V. , Lamvik M.
Published: July 2002

We have developed a sensitive interferometric technique for measuring the temperature derivative of the refractive index of liquids with an accuracy of 0.5% as illustrated by measurements with mixtures of liquid n-hexane and toluene. In addition the concentration dependence of the refractive index of these mixtures has been measured with refractometry. The experimentally observed temperature dependence of the refractive index appears to be consistent with a modification of the Lorentz–Lorenz relation proposed by Looyenga.
Rearrangements of 55-atom Lennard-Jones and (C60)55 clusters
Authors: Wales David J.
Published: July 2002

Roughly 3000 rearrangement pathways have been calculated for each of two clusters bound by simple empirical potentials. The first system is the 55-atom complex described by a pairwise Lennard-Jones potential. The second consists of 55 C60 molecules with an intermolecular potential obtained by spherically averaging atom–atom Lennard-Jones terms, as previously employed in bulk simulations. Various properties of the pathways are calculated, such as a cooperativity index and the integrated path length, and the probability distributions are subjected to statistical analysis. Systematic differences between the atomic Lennard-Jones clusters and the C60 molecular clusters are explained in terms of the effective pair potential; e.g., rearrangements of (C60)55 are generally localized because the potential is relatively short ranged. A pseudo-third-derivative correction is developed to improve the convergence of the eigenvector-following method employed to optimize the geometries and follow the pathways.
Observation and characterization of the ArBH(X 1Σ+,A 1Π) van der Waals complex through fluorescence excitation spectroscopy
Authors: Hwang Eunsook , Dagdigian Paul J.
Published: July 2002

The laser fluorescence excitation spectrum of the ArBH van der Waals complex, in the vicinity of the A 1Π−X 1Σ+ (0,0) band of diatomic BH, is reported. This species was prepared in a pulsed free jet by 193 nm multiphoton dissociation of diborane seeded in Ar/He. Both rotationally resolved and diffuse bands of the ArBH complex were observed. Rotational analyses were carried out for most of the sharp bands; both perpendicular [P′=1←P″=0] and parallel [P′=0←P″=0] transitions were found. The assignment of the bending and van der Waals stretch vibrational quantum numbers were carried out with the help of the accompanying paper [M. H. Alexander, S. Gregurick, and P. J. Dagdigian, preceding paper, J. Chem. Phys. 101, 2887 (1994)], wherein ArBH(A,X) ab initio potential energy surfaces, and subsequently vibrational energies, are calculated. The pattern of ArBH(A) vibrational energies was found to be complicated, mainly because of the large difference in the Ar–BH(A) interaction energy when the unpaired π electron is in or perpendicular to the triatomic plane.
A new approach to the molecular biexciton theory
Authors: Vektaris G.
Published: July 2002

The paper deals with the energy and the optical spectra of Frenkel biexciton (bound two-exciton state, excitonic molecule) in molecular crystal. A very simple system of equations describing the biexciton states is obtained. The number of equations in this system is equal to the number of the nonzero matrix elements of dynamical exciton interaction. In contrast to the other papers, the kinematical interaction of excitons is included into the Green’s functions, which are equivalent to the Green’s functions for one exciton states in a crystal with a vacancy. The obtained system of equations is solved for one-, two-, and three-dimensional crystals taking into account only the isotropic nearest-neighbor interaction. It is shown that biexciton has anisotropic character in two- and three-dimensional crystals for certain quasimomentum directions, i.e., it can be stretched along one axis or plane. In the isotropic case the wave function of a biexciton is analogous to the s or d function of the hydrogen atom. This system of equations can be easily solved for more complex models as well. The transition rates from the ground state to the biexciton band (two-photon absorption) and from the one-exciton band (one-photon absorption) to the same states is investigated, too. It is shown, that transition is allowed only into the s biexciton state, and it has a resonance character analogous to the Rashba effect for the localized one-exciton state in impurity crystal.
Far-infrared spectra and two-dimensional potential energy surfaces for the out-of-plane ring vibrations of cyclopentanone in its S and S1(n,π*) electronic states
Authors: Choo Jaebum , Laane Jaan
Published: July 2002

The far-infrared spectra of cyclopentanone have been reexamined. In addition to the ring-bending series previously observed in the 80–100 cm−1 region, ring-twisting bands (200–240 cm−1), bend–twist sum bands (305–335 cm−1), and bend–twist difference bands (120–145 cm−1) were also detected for the first time. From these data a two-dimensional potential energy surface with a barrier to planarity of 1408 cm−1 and a barrier to pseudorotation of 1358 cm−1 was determined. The surface has energy minima at twist angles of ±29°. This surface yields calculated frequencies in good agreement for not only the undeuterated cyclopentanone, but also for four isotopomers. We also utilized our previously reported fluorescence excitation spectra to determine the corresponding potential energy surface for the S1(n,π*) electronic excited state. The barrier to planarity in the S1 state is 1445 cm−1 while the barrier to pseudorotation has been reduced substantially to 596 cm−1. The twist angles are ±30°.