bluuebunny/crossref_metadata_2025 huggingface dataset:
One-electron model for photodissociation dynamics of diatomic anion
Authors: Ka Jaejin , Shin Seokmin
Published: July 2002

A one-electron model for the photodissociation and recombination dynamics of a diatomic anion has been proposed. The main purpose of the study is to provide a simple model to better understand the effect of strong coupling between the solvent polarization and the extra charge in a system like I2−. The model diatomic anion consists of two identical nuclei and an extra electron whose dynamics are treated explicitly. The effect of solvent polarization is modeled by introducing an effective solvent field representing a nonequilibrium solvent configuration. Nonadiabatic theoretical calculations, in which the electronic and the nuclear dynamics are treated simultaneously, can reveal the importance of nonadiabatic effects by including intrinsically all the electronic states involved. It is found that a purely dissociative excited state can support recombination due to coupling of the anomalous charge distribution with the solvent polarization. The charge switching and the subsequent charge separation for the dissociating fragments are strongly coupled with the fluctuating solvent polarization, as represented by the time-dependent solvent field in the present model. The results of the calculations with varying time scales for the solvent response have demonstrated the possibility of numerous diverse phenomena resulting from nonadiabatic transitions. In particular, we found charge transfer induced by changing solvent polarization. The general model presented in the study provides a reasonable interpretation, at least on a qualitative level, for the interesting features obtained from recent experiments and nonadiabatic molecular dynamics studies on the photodissociation of I2− in molecular clusters.
Collision-induced vibration–rotation fluorescence spectra and rovibrational symmetry changes in acetylene
Authors: Saarinen Maria , Permogorov Dmitri , Halonen Lauri
Published: July 2002

The laser-induced fluorescence method has been used to investigate collision-induced processes in the hydrogen-stretching vibrational overtone region of the ground electronic state of acetylene. The fluorescence signal has been dispersed by a high-resolution interferometer. Collision-induced rovibrational symmetry changes have been observed in the spectra.
The ultraviolet photodissociation dynamics of hydrogen bromide
Authors: Regan Paul M. , Langford Stephen R. , Orr-Ewing Andrew J. , Ashfold Michael N. R.
Published: July 2002

The technique of H Rydberg atom photofragment translational spectroscopy has been applied to investigate the ultraviolet photodissociation dynamics of hydrogen bromide. Branching fractions between the channels forming ground Br(2P3/2) and spin-orbit excited Br(2P1/2) atoms have been determined at 15 independent wavelengths in the range 201–253 nm, and photofragment recoil anisotropies for these two channels have been characterized at six different wavelengths within the same wavelength range. The channel forming ground state products, H+Br(2P3/2), is observed to arise solely from a perpendicular (i.e., ΔΩ=1) transition at all excitation energies, whereas the channel to formation of excited state products, H+Br(2P1/2), has a marked wavelength dependence: at long wavelengths (λ=243 nm), the photofragments are produced by a parallel (i.e., ΔΩ=0) photodissociation mechanism, which becomes more perpendicular in character as the photolysis energy is increased. Within the wavelength range studied, the branching fractions indicate that Br(2P3/2) products are formed in preference to Br(2P1/2) products, with propensities that are relatively invariant to excitation wavelength, although a small, yet pronounced, cusp appears at λ∼235 nm. The observations are discussed with reference to the known behavior of the other hydrogen halides and highlight the influence of spin-orbit interactions in the photofragmentation dynamics of this series of molecules.
Accurately solving the electronic Schrödinger equation of atoms and molecules using explicitly correlated (r12-) multireference configuration interaction. III. Electron affinities of first-row atoms
Authors: Gdanitz Robert J.
Published: July 2002

The computation of electron affinities of atoms and molecules is one of the most demanding tasks in quantum chemistry. This is because the electronic structures of neutral systems compared to their respective anions are qualitatively different and thus errors in the computed correlation energies, in general, do not cancel. Correlation energies obtained from traditional configuration interaction (CI) expansions, however, are known to converge notoriously slowly due to the presence of interelectronic cusps in the exact wave function. We compute the electron affinities of the first-row atoms using the recently proposed (explicitly correlated) r12-[multireference configuration interaction (single double) MR-CI(SD)] and r12-MR-ACPF (averaged coupled-pair functional) methods which take care of the interelectronic cusps by means of terms being linear in the interelectronic distances (r12). The reference spaces and basis sets (which are further augmented with diffuse functions) are taken from our former study on neutral atoms and their respective positive ions [J. Chem. Phys. 109, 9795 (1998)]. The performance of MR-ACPF is validated by comparison with full CI. The computed electron affinities (corrected for relativistic effects and nuclear motion) deviate from experiment by: −0.4 (H), +0.3 (Li), +5 (B, within experimental uncertainty), −0.6 (C), −15 (O), and −16 meV (F). Without relying on fortuitous error compensations, the electron affinities of B, C, O, and F can presently not be obtained in such an accuracy with traditional CI methods without extrapolation to the basis set limit.
Conformational energy and dynamics of 9-ethylfluorene
Authors: Pitts Jonathan D. , Knee J. L. , Wategaonkar S.
Published: July 2002

The S1 excited state and cation ground state of jet cooled 9-ethylfluorene have been studied experimentally using resonant enhanced multiphoton ionization and zero electron kinetic energy (ZEKE) photoelectron spectroscopy. The spectroscopy has identified two conformations of the ethyl chain which are labeled symmetric and unsymmetric both of which exist in the supersonic expansion. Density functional quantum chemical calculations are used to calculate the ground state and cation energies of each conformer as well as the barrier to conformer interconversion via a bond rotation. Dynamics on the S1 surface are measured using picosecond and nanosecond ZEKE photoelectron spectroscopy. Fast irreversible vibrational redistribution is measured at energies ⩾990 cm−1 and the ZEKE spectra are shown to have a unique signature for each of the two isomers. Picosecond and nanosecond ZEKE spectroscopy are used to search for conformer interconversion but even at the highest energy probed (2648 cm−1) no evidence is seen for a dynamic barrier crossing. Statistical density of states calculations are used to predict the relative populations of each conformer expected as a function of excess energy as well as related Rice–Ramsperger–Kassel–Marcus calculations to predict the expected isomerization rates.
Long time behavior of reversible diffusion-influenced reaction perturbed by photolysis: Brownian dynamics simulation
Authors: Kim Hyojoon , Yang Mino , Shin Kook Joe
Published: July 2002

New power law behavior (t−3/2) in the asymptotic relaxation of pseudo-first-order reversible diffusion-influenced reaction, A+B↔AB perturbed by photolysis is confirmed by a Brownian dynamics simulation in one dimension when the photolytic dissociation distance is equal to the thermal dissociation distance as predicted by Yang et al. [Phys. Rev. Lett. 79, 3783 (1997)]. For larger photolytic dissociation distances, however, our simulation curves follow the thermal relaxation curve at the beginning and deviate from the latter to show t−3/2 power law behavior at longer times. The effect of photolytic perturbation on various nonequilibrium initial states is also examined by Brownian dynamics simulations.
Does N2− exist? A coupled-cluster study
Authors: Gutsev Gennady L. , Rozyczko Piotr B. , Bartlett Rodney J. , Weatherford Charles A.
Published: July 2002

Potential energy curves of the ground-state N2 molecule and its doublet N2− anion are calculated at the coupled-cluster level with single and double excitations and with noniterative triples [CCSD(T)] as well as with the multireference averaged-quadratic coupled-cluster (MR-AQCC) method. The N2− anion is shown to be temporary and decays to its neutral parent plus a free electron at bond lengths shorter than ≈1.4 and larger than ≈2.5 Å. Thus, the N2− anion exists within the 1.4⩽R(N–N)⩽2.5 Å range at the Born–Oppenheimer approximation.
Monte Carlo simulations and self-consistent integral equation theory for polyelectrolyte solutions
Authors: Shew Chwen-Yang , Yethiraj Arun
Published: July 2002

The static structure and conformational properties of salt-free polyelectrolyte solutions are studied using many-chain Monte Carlo simulations and integral equations. The polymer molecules are modeled as freely jointed tangent sphere chains with a hard sphere plus screened Coulomb potential between the beads. Monte Carlo simulations are for the most part in agreement with previous molecular dynamics simulations of a related model of polymers but the present work facilitates an unambiguous comparison between theory and simulation. An integral equation theory is implemented where the properties of the polymer are obtained from a single chain simulation where the beads interact via the bare potential plus a self-consistently determined solvation potential. The theory overestimates the degree of liquidlike order and underestimates the size of the chains (when compared to many chain simulations) especially for long chains and high densities. A more approximate theory that employs a thread model for the polymers is actually more accurate.
Resolved high Rydberg spectroscopy of benzene⋅rare gas van der Waals clusters: Enhancement of spin–orbit coupling in the radical cation by an external heavy atom
Authors: Siglow Klaus , Neuhauser Robert , Neusser Hans Jürgen
Published: July 2002

Individual high n Rydberg states in van der Waals dimers of benzene and noble gas atoms are resolved after double resonance excitation with two Fourier-transform limited narrow band UV laser pulses. For a selected rovibronic intermediate state several Rydberg series appear converging to different rotational states of the cation. Their position is determined by an automated cross correlation (CRIES) of the experimental with a theoretical Rydberg spectrum with an accuracy of 0.02 cm−1. Analysis of the resolved rotational states of the cationic clusters in their vibrational ground state yields precise information on the ionization energies, the structure and average van der Waals distances. Rotational analysis provides clear spectroscopic evidence for an enhancement of spin–orbit coupling between the orbital angular momentum and the spin of the remaining unpaired electron in the ionic benzene by the external heavy atoms Ar and Kr. The resulting effect of the spin–orbit coupling on the rotational energy levels is fully resolved in these cases and we present accurate values for the spin–orbit coupling constants of the benzene+⋅Ar (a=0.51 cm−1) and benzene+⋅84Kr (a=2.89 cm−1) cluster cations.
Benzene trimer and benzene tetramer: Structures and properties determined by the nonempirical model (NEMO) potential calibrated from the CCSD(T) benzene dimer energies
Authors: Engkvist O. , Hobza P. , Selzle H. L. , Schlag E. W.
Published: July 2002

The energetics and structure of the benzene trimer and tetramer are investigated with the nonempirical model (NEMO) potential calibrated to high precision by comparison with CCSD(T) benzene dimer energies. From the obtained potential energy surface, possible configurations could be determined and the experimental observed structures could be identified. This potential also reproduces the binding energies and allows for the determination of all intermolecular modes. It could be shown that this potential is therefore well suited and important to predict the structure and thermodynamic data for larger clusters, which cannot be accurately computed by ab initio quantum chemical methods.
Molecular dynamics study of isobaric and isochoric glass transitions in a model amorphous polymer
Authors: Yang Liu , Srolovitz David J. , Yee Albert F.
Published: July 2002

We perform molecular dynamics simulations of the glass transition through isobaric and isochoric cooling of a model polymeric material. In general, excellent agreement between the simulation results and the existing experimental trends is observed. The glass transition temperature (Tg) is found to be a function of pressure under isobaric conditions and specific volume under isochoric conditions. Under both isobaric and isochoric conditions, the trans-state fraction and the torsional contributions to the energy undergo abrupt changes at the glass transition temperature. We analyze these data to show that the glass transition is primarily associated with the freezing of the torsional degrees of the polymer chains which is strongly coupled to the degree of freedom associated with the nonbonded Lennard-Jones potential. We attribute the greater strength of the glass transition under constant pressure conditions to the fact that the nonbonded Lennard-Jones potential is sensitive to the specific volume, which does not change during cooling under isochoric conditions. Comparison of the isochoric and isobaric data demonstrate that the thermodynamic state is independent of cooling path above Tg, while path-dependent below Tg. The simulation data show that the free volume at the isobaric glass transition temperature is pressure dependent. We also find that a glass transition occurs under isochoric conditions, even though the free volume actually increases with decreasing temperature.
The weakly bound complex CH4-H2: Observation and analysis of infrared spectra in the 350 and 1311 cm−1 regions
Authors: McKellar A. R. W. , Roth D. A. , Pak I. , Winnewisser G.
Published: July 2002

Infrared spectra of the weakly bound van der Waals complex CH4-paraH2 have been observed and analyzed for the first time. Measurements were made using a long-path (160–180 m) low-temperature (61–92 K) absorption cell which was probed with a Fourier transform spectrometer and a tunable diode laser. The partly resolved spectrum accompanying the S0(0) pure rotational transition of H2 around 350 cm−1 was analyzed in terms of an approximate model to obtain the rotational and centrifugal distortion constants of the complex in its ground state. The high resolution spectrum accompanying the R(0) transition of the ν4 fundamental band of CH4 near 1311 cm−1 was assigned in detail and analyzed using the Coriolis model used previously for analogous spectra of methane–rare gas complexes. The effective intermolecular separation and dissociation energy of the complex in its ground state were determined to be about 4.12 Å and 28 cm−1, respectively.
Molecular dynamics simulation study of water near critical conditions. I. Structure and solvation free energetics
Authors: Bursulaya Badry D. , Kim Hyung J.
Published: July 2002

The structure and solvation free energetics of water under ambient and near critical conditions are studied with the Molecular Dynamics (MD) computer simulation method. By employing the truncated adiabatic basis-set description with 10 basis functions (TAB/10D), the water electronic structure variation both in and out of its molecular plane and associated polarizability effects are accounted for. It is found that the average dipole moment μ̄s of a water molecule varies considerably with the thermodynamic conditions; while the TAB/10D model yields μ̄s=2.65 D under an ambient condition, it reduces to 2.24 D in supercritical water at temperature 673 K and density 0.66 g cm−3. The MD results on the radial distribution functions and static and optical dielectric constants are in reasonable agreement with experiments. The fluctuations of the local electric field are found to be significant, so that at elevated temperatures, it can sometimes be in the opposite direction of the local water dipole vector. Also, the free energy curves associated with solvation of water molecules become tighter and more asymmetric with increasing temperature and decreasing density. The dynamic and spectroscopic properties of supercritical water are studied in the following paper [B. D. Bursulaya and H. J. Kim, J. Chem. Phys. 110, 9656 (1999)].
A quantum chemical study of negatively charged methanol clusters
Authors: Turi László
Published: July 2002

We performed high-level quantum chemical density functional theory calculations on negatively charged methanol clusters containing up to six monomers. The calculations suggest that there exist stable methanol cluster anions and that these anions are more stable than similar cluster anions of water. Linear hydrogen bonded methanol chains are observed to bind the excess electron on dipole bound states. The orientation and the size of the excess electron were characterized by the position of the center of mass and the radius of gyration of the highest occupied molecular orbital (HOMO). The electron occupies a large diffuse orbital concentrated outside the molecular frame in the molecular dipole direction. The tendencies of the dipole moments, the vertical electron detachment energies, and the size of the HOMOs all fit in the same cooperative trend, suggesting stronger interactions in larger anions. We also located stable cluster anions which can serve as model systems for the solvated electron in liquid methanol. Multiple O–H⋯e− interactions with dominantly bond-oriented arrangement toward the solvated electron are probably strongly favored in the liquid phase for energetic reasons. Although the size of the excess electron is still significantly larger than expected from quantum molecular dynamics simulations, the general decreasing trend of the radius of gyration with increasing cluster size is reassuring. Similarly to the O–H⋯e− interactions, we located C–H⋯e− interactions between appropriately oriented methyl hydrogens and the excess electron in a large anion of six methanol molecules. We propose the interactions of both the hydroxyl hydrogens and the methyl hydrogens with the excess electron to be considered hydrogen bonds.
Tracing coexistence lines in multicomponent fluid mixtures by molecular simulation
Authors: Escobedo Fernando A.
Published: July 2002

Novel formulations of Gibbs-Duhem integrations are proposed to trace phase diagrams for multicomponent fluid mixtures, including generalized bubble-dew lines, whereby the composition of one of the phases is specified, but may not be constant along the integration path. The algorithms presented in this work constitute an extension of a recently proposed pseudo-ensemble framework. Different types of phase equilibrium calculations with open ensembles are cast into a unique formulation, which leads to a simple and flexible integration scheme for stepping along a coexistence line. This approach is applied to trace phase diagrams for systems described by realistic force fields, including hydrocarbon mixtures. A variant of a Gibbs ensemble that permits simulation of bubble-dew point pressures is also implemented.
A molecular-dynamics simulation study of water on NaCl(100) using a polarizable water model
Authors: Stöckelmann Elmar , Hentschke Reinhard
Published: July 2002

We carry out molecular-dynamics simulations of the water–NaCl(100) interface. The study includes the bulk interface at T=298 K and thin physisorbed films for coverages ranging from 0 to 1.5 at T=140 K. We use an efficient SPC/E based fluctuating charge water model to account for polarization effects. The water model is tested calculating cluster, gas, and liquid phase properties of neat water as well as structural and dynamic properties of solutions containing Na+-, Cl−-, and Ca2+-ions. For the bulk water–NaCl(100) system we analyze the surface induced hydration shell structure as well as residence times and the dipole orientation autocorrelation near the surface. At the low temperature we calculate the coverage dependence of the isosteric heat of adsorption, which is compared to available experimental data, including the coverage dependence of the adsorbate structure and dynamics. We note that our simulations support the formation of the (1×1) superstructure seen in helium scattering experiments.
Photodissociation of CBrCl3 at 234 and 265 nm: Evidence of the curve crossing
Authors: Jung Young-Jae , Park Moon Soo , Kim Yong Shin , Jung Kyung-Hoon , Volpp Hans-Robert
Published: July 2002

The photodissociation dynamics of CBrCl3 was studied near 234 and 265 nm using a two-dimensional photofragment ion imaging technique. Bromine fragments monitored in this study were produced via direct dissociation of CBrCl3, represented by CBrCl3→CCl3+Br(2P1/2)/Br(2P3/2). The branching ratio of Br(2P1/2) (denoted Br*)/Br(2P3/2) (denoted Br) showed strong excitation energy dependence. The product quantum yields at two different excitation wavelengths were Φ 234 nm(Br*)=0.31±0.01 and Φ 265 nm(Br*)=0.68±0.02, respectively. The speed and angular distributions of Br* and Br fragments were determined. Similar values of β(234 nm)=−0.44 and β(265 nm)=−0.47 for Br were observed, while β values for Br* were found to be markedly different, β(234 nm)=−0.34 and β(265 nm)=1.43. The strong curve crossing, 1Q1→3Q0, and the angular distribution of Br* suggesting a typical perpendicular transition, were observed at 234 nm photodissociation.
The He–LiH potential energy surface revisited. I. An interpolated rigid rotor surface
Authors: Taylor Brian K. , Hinde Robert J.
Published: July 2002

We reconsider the potential energy surface of the He–LiH system recently examined by Gianturco and co-workers [F. A. Gianturco et al., Chem. Phys. 215, 227 (1997)]. We compute the He–LiH interaction energy at the CCSD(T) level using large correlation consistent atomic basis sets supplemented with bond functions. To capture the severe anisotropy of the He–LiH potential, we interpolate our ab initio points in the angular direction with cubic splines, then expand the splines in terms of Legendre polynomials. The resulting smooth potential surface differs substantially from that of Gianturco et al.; in particular, our attractive He–LiH well is more than twice as deep as that of Gianturco et al., with a He–LiH binding energy of De=176.7 cm−1.
Hyper-Rayleigh scattering of azulenic donor–acceptor molecules at 1064 and 1907 nm
Authors: Woodford J. N. , Wang C. H. , Asato A. E. , Liu R. S. H.
Published: July 2002

Hyper-Rayleigh scattering (HRS) has been carried out for new azulenic donor–acceptor chromophores using near-infrared laser excitation at wavelengths of 1064 and 1907 nm. Hyperpolarizabilities obtained with the 1064 nm incident excitation are large compared with those obtained using the 1907 nm excitation, due to the effects of two-photon resonance enhancements in the former case. The HRS experiment using the 1907 nm excitation, while challenging to carry it out, gives the hyperpolarizability value closer to the intrinsic molecular hyperpolarizability. The popular two-level model that is used to predict the first hyperpolarizability of donor–acceptor charge transfer chromophores is employed to compare the measurements made on the same chromophores with 1064 and 1907 nm excitations. The model is found not to be able to correlate the results obtained at the two wavelengths. The first hyperpolarizability data obtained by using the 1907 nm excitation are consistent with electric field induced second harmonic generation data obtained by using the same wavelength.
Brownian dynamics simulations of linear polymers under shear flow
Authors: Lyulin Alexey V. , Adolf David B. , Davies Geoffrey R.
Published: July 2002

Brownian dynamics simulations of a polymer chain described by three different models under the influence of a shear flow have been performed. Model A is a freely jointed Kramers chain consisting of beads connected by rigid rods. Model B is a freely jointed chain consisting of finitely extensible nonlinear elastic (FENE) springs. Excluded volume and hydrodynamic interactions are not taken into account in either of these two models. Model C is a chain with rigid bonds, valence, and torsional angle potentials, excluded volume and hydrodynamic interactions. Asymptotic dependencies [η]∼γ̇−1/3 and [η]∼γ̇−2/3 for the intrinsic viscosity [η] at large shear rates γ̇ for models A and B, correspondingly, have been obtained. Asymptotic dependencies for the first normal stress coefficient Ψ1∼γ̇−4/3 do not depend on the particular choice of model. At intermediate shear rates [η]∼γ̇−1/2 is followed for all models. Scaling dependencies of rheological properties on molecular weight have been studied. Results of the simulations show that chains are not fully stretched even at extremely high shear rates but form rather compact anisotropic objects. Correlation functions of the chain end-to-end vector relax quicker with increasing shear rate and reveal evidence of the end-to-end vector flipping between orientations parallel and antiparallel to the flow direction.
Photodissociation of HBr molecules and clusters: Anisotropy parameters, branching ratios, and kinetic energy distributions
Authors: Baumfalk Reinhard , Buck Udo , Frischkorn Christian , Nahler Nils Hendrik , Hüwel Lutz
Published: July 2002

The ultraviolet photolysis of HBr molecules and (HBr)n clusters with average size around n̄=9 is studied at three different wavelengths of 243, 205, and 193 nm. Applying polarized laser light, the kinetic energy distribution of the hydrogen photofragment is measured with a time-of-flight mass spectrometer with low extraction fields. In the case of HBr monomers and at 243.1 nm, an almost pure perpendicular character (β=−0.96±0.05) of the transitions is observed leading to the spin–orbit state Br(2P3/2). The dissociation channel associated with the excited state Br*(2P1/2) is populated by a parallel transition (β*=1.96±0.05) with a branching ratio of R=0.20±0.03. At the wavelength of 193 nm, about the same value of R=0.18±0.03 is found, but both channels show a mainly perpendicular character with β=−0.90±0.10 for Br and β*=0.00±0.10 for Br*. The results for 205 nm are in between these two cases. For the clusters at 243 nm, essentially three different groups appear which can be classified according to their kinetic energy: (i) A fast one with a very similar behavior as the monomers, (ii) a faster one which is caused by vibrationally and rotationally excited HBr molecules within the cluster, and (iii) a slower one with a shoulder close to the fast peak which gradually decreases and ends with a peak at zero velocity. The zero energy fragments are attributed to completely caged H atoms. The angular dependence of the group (iii) is isotropic, while that of the other two is anisotropic similar to the monomers. At 193 nm only the fast and the slow part is observed without the peak at zero energy. Apparently the kinetic energy is too large to be completely dissipated in the cluster.
Direct observation of nonadiabatic transitions in Na+rare-gas differential optical collisions
Authors: Grosser J. , Hoffmann O. , Wischeler F. Schulze , Rebentrost F.
Published: July 2002

We study the dynamics in optical collisions of Na with Ne, Ar, Kr, and Xe in a differential scattering experiment. We report the observation of nonadiabatic transitions in the excited collisional quasimolecule based on measurements of the population ratio of the Na(3p)2P1/2 and 2P3/2 fine-structure levels. Comparison with theoretical results shows a generally very good agreement over the range of collision energies (0.01–0.3 eV) scanned in our experiment, using the best available potentials. For the heavier rare-gas systems a strong influence of the BΣ–AΠ crossing on the population ratios is observed. We further extract a universal function for the nonadiabatic transition probability for these systems. In the thermal energy range, our results are in good qualitative agreement with data from gas phase optical collision experiments.
Predicting intense field laser ionization probabilities: The application to C2Hn species
Authors: Prall Bradley S. , DeWitt Merrick J. , Levis Robert J.
Published: July 2002

A structure-based tunneling mechanism developed to predict the ionization of molecules subjected to intense, ultrafast irradiation is tested on the series of C2 hydrocarbons: acetylene, ethylene, and ethane. Relative ionization probabilities (1, 4.1, and 8.7 for ethane, acetylene, and ethylene, respectively) are measured upon excitation with 780 nm, 125 fs pulses of 6×1013 W cm−2 and compared to predictions of the model (1, 4.1, and 7.9 for ethane, acetylene, and ethylene, respectively). Ionization probabilities determined via the ADK (Ammosov, Delone, and Krainov) model for atomic ionization (1, 2.7, and 13.1 for ethane, acetylene, and ethylene, respectively) are shown to be near those of the structure-based model.
Fourier transform spectroscopy and cross-section measurements of the Herzberg II bands of O2 at 295 K
Authors: Yoshino K. , Esmond J. R. , Parkinson W. H. , Thorne A. P. , Learner R. C. M. , Cox G.
Published: July 2002

Fourier transform spectroscopic measurements of the absorption bands of the Herzberg II (c 1Σu−–X 3Σg−) of O2 at 295 K have been made with a resolution of 0.06 cm−1 in the wavelength region 240–275 nm. Rotational line positions are determined with an accuracy of 0.005 cm−1, and rotational term values are presented for the vibrational levels, v′=6–16. Precise band oscillator strengths of the (7,0)–(16,0) bands are obtained for the first time by direct measurement by summing the cross sections of individual rotational lines of the bands. The rotational line strengths and the branching ratios are also presented for the same bands.
Regularity of the vibrational spectrum of the CS2 in the Σg+ state: Our previous results revisited
Authors: Michaille L. , Rasbach U. , Pique J. P.
Published: July 2002

In previous experiments, a nearly complete vibrational spectrum of the CS2 molecule in the Σg+ state has been obtained using a laser induced fluorescence method. The statistical tests of the random matrix theory, applied on this spectrum, suggest a transition to quantum chaos. The aim of this letter is to show that these statistical results are biased because of a nonsingle excitation of the molecule. This is experimentally confirmed by high-resolution new experiments in supersonic jet, using the powerful laser chain of the french project for the isotopic separation of uranium by laser (SILVA project).
Two-dimensional nuclear magnetic resonance correlation spectroscopy at zero field
Authors: Liao Ming-Yuan , Harbison Gerard S.
Published: July 2002

Three methods for two-dimensional correlation nuclear magnetic resonance spectroscopy at zero field are discussed. All three involve coherence transfer via longitudinal polarization, double quantum coherence, or both in parallel. The double quantum pulse sequences exploit the spinor property of spin states. These sequences have been applied to connected Δm=1 transitions, as well as for the indirect detection of forbidden or nearly forbidden Δm>1 transitions.
Electron spin resonance and theoretical studies of the 14N⋅⋅⋅⋅14N and 15N⋅⋅⋅⋅15N spin-pair radicals in neon matrices: The effects of mixing among the 1Σg+, 3Σu+, 5Σg+, and 7Σu+ electronic states
Authors: Knight Lon B. , Bell Benjamin A. , Cobranchi Daryl P. , Davidson Ernest R.
Published: July 2002

The first experimental and theoretical study of the N⋅⋅⋅⋅N spin-pair radical is reported. Its high-resolution ESR (electron spin resonance) spectrum has been observed in neon matrices and interpreted on the basis of weakly interacting atoms using a model recently developed for the H⋅⋅⋅⋅H spin-pair. To fully interpret the N⋅⋅⋅⋅N radical results it was necessary to include electronic state mixing effects among all possible spin states, namely the 1Σg+, 3Σu+, 5Σg+, and 7Σu+ states. Several different trapping sites were observed which indicated the interaction of N atoms at distinctly different separation distances in the neon lattice. Calculated J values at the complete active space self-consistent field (CASSCF) level (TZP basis set) were compared with the experimental results for the various trapping site distances. The 15N⋅⋅⋅⋅15N radical in the dominant trapping site had magnetic parameters of g=2.0016(2), A(15N)=15.9(1) MHz, D=−178(1)MHz and J=468(2) MHz. Using the point dipole approximation this corresponds to a N⋅⋅⋅⋅N separation distance of 6.41 Å. A most unusual type of magnetic dipole transition was observed that involves a transition between electronic states of formally different S values where S is the total spin quantum number for a given electronic state.
Time–temperature and time–density superposition in the simulation of rheological properties of polymers
Authors: Rouault Yannick , Kremer Kurt
Published: July 2002

Applying a periodical elongational strain in a computer simulation of polymer melts and networks, we are able for the first time to employ a time–temperature and time–density superposition in a numerical experiment for the study of the dynamic moduli. The simulation results can be analyzed and understood within the semiempirical free volume concept.
Structure and spectroscopy of NenSH (Ã 2Σ+) complexes using adiabatic diffusion Monte Carlo (ADMC)
Authors: Lee Hee-Seung , Herbert John M. , McCoy Anne B.
Published: July 2002

Adiabatic rigid body diffusion Monte Carlo techniques are used to investigate the structure and spectroscopy of complexes of one to four neon atoms with SH (Ã 2Σ+). While these systems contain multiple low-lying minima, the ground state wave functions are well described by a neon cluster weakly interacting with SH. This structure persists in low-lying excited states, in the case of complexes of two neon atoms with SH or SD.
Benzene-argon S1 intermolecular potential energy surface
Authors: Fernández Berta , Koch Henrik , Makarewicz Jan
Published: July 2002

The benzene-argon S1 intermolecular potential energy surface is evaluated using coupled-cluster linear response methods and an augmented correlation-consistent polarized valence double-zeta basis set extended with midbond functions. As a result of the S1←S0 excitation, the well depth of the potential increases and the benzene-argon equilibrium distance is decreased by 0.065 Å. Full three-dimensional vibrational calculations of the van der Waals vibrational energy levels, using the ab initio potential, are in good agreement with available experimental data.
Rotational spectrum, ring-puckering vibration and ab initio calculations on 1,1-difluorocyclobutane
Authors: Lesarri Alberto , López Juan C. , Alonso José L. , Wlodarczak Georges , Demaison Jean
Published: July 2002

The rotational spectra of the ground and first five ring-puckering excited states of 1,1-difluorocyclobutane have been investigated in the centimeter- (8–40 GHz) and millimeter-wave regions (96–106 and 144–159 GHz). Analysis of the spectra for the pairs of coupled vibrational states vp=0/1 and vp=2/3 has been carried out using two-state Hamiltonians, yielding accurate rotational and centrifugal distortion constants, vibration-rotation coupling parameters, and the energy spacings ΔE01 and ΔE23. The spectrum for the vp=4 to vp=6 ring-puckering states was satisfactorily accounted for in terms of effective semi-rigid Hamiltonians for each vibrational state. A double minimum ring-puckering potential function with a barrier to ring inversion of 231(4) cm−1 has been obtained from the analysis of the vibrational dependence of the rotational constants and inversion splittings ΔE01 and ΔE23. The calculated variation of the quartic centrifugal distortion constants with the ring-puckering quantum number reproduces satisfactorily the experimental trends, confirming the validity of the derived potential function. Ab initio calculations with HF, MP2, and B3LYP density functional hybrid methods have been carried out for this molecule using different basis sets. The experimental and ab initio potential functions, coupling terms, and ring-puckering dynamical parameters are compared. Finally, an ab initio near-equilibrium structure is presented.
Substituent effects and the noncrossing rule: The importance of reduced symmetry subspaces. I. The quenching of OH(A 2Σ+) by H2
Authors: Yarkony David R.
Published: July 2002

The effects of substituent substitution on the locus of a seam of conical intersection and the importance of conical intersections in the associated low symmetry subspaces are considered. For molecules with more than three atoms and with some symmetry the seam of conical intersection may well include an accidental symmetry-allowed portion involving two states of different symmetry. However, in regions of reduced point group symmetry, conical intersections involving two states of the same symmetry may exist. This later class of conical intersections is rarely considered although it could significantly alter the predicted outcome of a nonadiabatic process. The efficient quenching of OH(A 2Σ+)by H2, a consequence of OH–H2 conical intersections, is particularly compelling in this regard. Previous analyses have considered only the C2v2A1–2B2 accidental symmetry-allowed portion of the seam of conical intersection. It is demonstrated that when intersections of states of the same symmetry are considered conical intersections with Cs symmetry are found that are likely to play an important role in the quenching process.
Brownian dynamics simulation of bead–rod chains under shear with hydrodynamic interaction
Authors: Petera D. , Muthukumar M.
Published: July 2002

We have performed Brownian dynamics simulations of bead–rod chains under shear using the algorithm introduced by Öttinger. We have investigated the effect of excluded volume and hydrodynamic interactions on material functions and configurational quantities. The model exhibits shear thinning and a negative second normal stress coefficient. We find that excluded volume interactions increase the magnitude of rheological quantities. Hydrodynamic interactions lower the viscosity for small shear rates and long chains and do not contribute significantly to the viscosity for high shear rates. The shear thinning behavior of a chain in theta solvent with hydrodynamic interactions ends at lower shear rates in comparison with other combinations of interactions. We find that chains in theta solvent expand more relative to their quiescent state than chains in good solvent for small and intermediate shear rates. Their expansion, however, is much smaller than predicted by the Rouse or Zimm model, in agreement with recent experiments. The size of chains in theta solvent with hydrodynamic interactions shrinks for very high shear rates. Chains in good solvent orient more into the shear flow than chains in theta solvent if plotted versus the absolute shear rate.
A Fourier transform infrared study of collision induced desorption of N2 on Ni(100) surface
Authors: Takaoka T. , Terahara M. , Sakai M. , Kusunoki I.
Published: July 2002

By collision with hyperthermal atoms, desorption of surface species can be induced. It is called collision induced desorption (CID). We studied a CID process of N2 adsorbed on a Ni(100) surface in an apparatus equipped with a molecular beam apparatus and an Fourier transform infrared spectrometer. The CID cross section was estimated, and the mechanism was discussed.
Penning detachment from atomic clusters
Authors: Martı́n F. , Madjet M. E. , Hervieux P. A. , Hanssen J. , Politis M. F. , Berry R. S.
Published: July 2002

Theoretical cross-sections are presented for Penning detachment of negatively-charged sodium clusters, receiving their detachment energy from Na*(3p0,2P). Two clusters are examined: Na7− and Na19−. Classical trajectories describe the relative motion of the colliding species. Kohn–Sham density functional theory in local approximation, with exchange, correlation, and self-interaction corrections, and a spherical jellium potential, describe the electrons involved in the transition. In the range of collision energies from 0.1 to 10 eV/amu, the cross-sections for Penning detachment are approximately 10−13 cm2. This implies that Penning detachment may be an effective means to prepare neutral clusters from size-selected negative-ion clusters in the laboratory.
Structure of electric double layers: A simple weighted density functional approach
Authors: Patra Chandra N.
Published: July 2002

A simple weighted density functional approach is developed for inhomogeneous ionic fluids and applied to the structure of the electric double layer using the restricted primitive model where the ions are considered to be charged hard spheres of equal diameter. The formalism is nonperturbative with both hard-sphere and electrical contributions to the one-particle correlation function evaluated through a suitably averaged weighted density, the only input being the second-order direct correlation functions of the corresponding uniform system. The approach is designed in such a way, that the calculation of the weight function is decoupled from the weighted density. Numerical results on the ionic density profile and the mean electrostatic potential near a hard wall at several surface charge densities are shown to compare well with available simulation results. The corresponding results for the nonprimitive molecular solvent model provide insight into the layering effect and the charge inversion phenomena.
An empirical potential energy surface for the Ne–OH/D complexes
Authors: Lee Hee-Seung , McCoy Anne B. , Harding Lawrence B. , Carter Christopher C. , Miller Terry A.
Published: July 2002

An empirical potential energy surface has been developed for the Ne–OH/D complexes from the experimentally observed vibrational and rotational energy levels with the results of ab initio calculations being used to provide initial estimates of the values of the parameters in the empirical potential. To determine the final surface a reassignment of the Ã 2Σ+–X̃ 2Π bend–stretch combination bands, to fundamentals of the bend, was made, and the experimental data for both the Ne–OH and Ne–OD complexes were fit simultaneously due to the small number of available values. The vibrational energies and rotational constants resulting from the fit surface are within 0.2 and 0.0013 cm−1, respectively, of the experimental values. The details of the potential energy surface are discussed in relationship to those of other inert gas complexes containing OH and SH, as well as HCl and HF.
Infrared spectra of hydrogen-bonded ion–radical complexes: I−⋅HCH2 and Br−⋅HCHBr
Authors: Nielsen Steen B. , Ayotte Patrick , Kelley Jude A. , Weddle Gary H. , Johnson Mark A.
Published: July 2002

We report the preparation and infrared spectra of the CH3I− and CH2Br2− anions formed by argon cluster-mediated electron attachment to the neutral molecular precursors. Infrared predissociation spectra were acquired for both the bare and argon-solvated species in the C–H stretching region. Partial rotational structure was recovered in the CH3I− system, consistent with the hydrogen-bonded, C2v structure suggested in an earlier analysis of its photoelectron spectrum [J. Kim et al., J. Am. Soc. Mass Spectrom. 10, 810 (1999)]. The spectrum and photofragmentation pattern confirm that this species is trapped in a very weakly bound ion–methyl radical form (I−⋅HCH2) involving a single ionic H bond. The CH2Br2− anion displays a similar spectrum, where one CH stretch is significantly redshifted, again signaling the single H-bonding motif.
Femtosecond photoelectron spectroscopy of I2−(Ar)n clusters (n=6,9,12,16,20)
Authors: Greenblatt B. Jefferys , Zanni Martin T. , Neumark Daniel M.
Published: July 2002

The photodissociation of I2− embedded in mass-selected I2−(Ar)n clusters (n=6–20) was studied using femtosecond photoelectron spectroscopy. The Ã′←X̃ transition in the I2− chromophore was excited using a femtosecond pump pulse, and the subsequent dynamics were followed by photodetachment with a femtosecond probe pulse and measurement of the resulting photoelectron spectrum. In all clusters, dissociation of the I2− is complete by 300 fs. From 300 fs to 1 ps, the spectra yield the number of Ar atoms interacting with the I− fragment. At later times, recombination of I2− occurs in I2−(Ar)n⩾12 on both the X̃ and Ã states. Analysis of the spectra yields the time scale for X̃ state vibrational relaxation and solvent evaporation. In I2−(Ar)20, energy transfer from I2− to Ar atoms through vibrational relaxation is slightly faster than energy loss from the cluster through Ar evaporation, indicating the temporary storage of energy within Ar cluster modes.
Vibrationally resolved photoelectron spectra of CuCN− and AgCN− and ab initio studies of the structure and bonding in CuCN
Authors: Boldyrev Alexander I. , Li Xi , Wang Lai-Sheng
Published: July 2002

Vibrationally resolved photoelectron spectroscopy is combined with ab initio calculations to investigate the structure and chemical bonding in CuCN, CuCN−, AgCN, and AgCN−. The photoelectron spectra were measured at two photon energies, 532 and 355 nm and only detachment to the ground state of the neutral was observed at both detachment energies. The adiabatic electron affinity and metal-C vibrational frequency were obtained to be 1.466 (0.010) eV and 480(30) cm−1, 1.588 (0.010) eV and 390(30) cm−1 for CuCN and AgCN, respectively. In the ab initio calculations, both CuCN and CuCN− were found to have linear C∞V structures. Isocyanide CuNC and CuNC− were found to be 10.7 and 6.5 kcal/mol [at the CCSD(T)/6-311+G(3d)//CCSD(T)/6-311+G* level of theory] higher in energy. Cyclic structures were found to be transition states for the cyanide–isocyanide isomerization. The calculated electron binding energies and vibrational frequency are in good agreement with the experimental measurements. The combined experimental and theoretical efforts allow us to elucidate the structures of CuCN and CuCN−, and the nature of their chemical bonding.
Vibrational energy relaxation of polyatomic solutes in simple liquids and supercritical fluids
Authors: Egorov S. A. , Skinner J. L.
Published: July 2002

A microscopic statistical mechanical theory of vibrational energy relaxation rates for polyatomic solutes in simple solvents is presented. The theory is based on a model of a spherical solute present at infinite dilution in a fluid of spherical solvent particles, and the solute–solvent interaction potential depends on the vibrational coordinates of the solute. The theory is applied to study the experimentally observed anomalous density and temperature dependence of vibrational relaxation rates in supercritical fluids in the vicinity of the critical point. A quantitative comparison of the theory with experiment is presented, and the agreement is satisfactory.