bluuebunny/crossref_metadata_2025 huggingface dataset:
Inequivalence of equivalent bonds: Symmetry breaking in Co(CH3)2+
Authors: Perry Jason K. , Goddard William A. , Ohanessian Gilles
Published: July 2002

In a theoretical study of the gas phase insertion of transition-metal cations into the C–H and C–C bonds of simple alkanes, an unusual aspect of the metal-carbon bond was discovered. Using ab initio methods (generalized valence bond and configuration interaction), it was found that the two methyl groups in Co(CH3)2+ do not bond to equivalent sd hybrid orbitals as one might expect. Instead, using a single valence bond (VB) spin coupling, we found two distinctly different bonds: one to a Co 4s-like orbital and the other to a 3d-like orbital, leading to a distortion of the molecule from its symmetrical geometry. With the resonance of two valence bond configurations, the bond distances become equivalent and symmetry is restored, however, the bonding orbitals in each configuration remain quite inequivalent. Similar behavior was observed on the potential-energy surface of CoH2+ and this description was found to carry over to Co(H)(CH3)+, where one VB configuration dominates: the hydrogen bonds to the Co 4s orbital while the methyl group bonds to a 3d orbital. The behavior of the cobalt complexes contrasts dramatically with that of the isoelectronic complexes Rh(R1)(R2)+ and Ir(R1)(R2)+ (R1,R2=H,CH3), in which the ligands form bonds to sd hybrid orbitals. These orbitals are essentially the same in the case of R1=H and R2=CH3 and strictly equivalent in the case of R1=R2.
An electron–nuclear double resonance study of the lowest triplet state of pyrimidine
Authors: Donckers M. C. J. M. , Gorcester J. , Groenen E. J. J. , Schmidt J.
Published: July 2002

Optically-detected electron–nuclear double resonance was employed to measure the entire hyperfine and quadrupole tensors of the two nitrogen nuclei and the hyperfine tensors of the four hydrogen nuclei in the lowest triplet state T0 of pyrimidine-h4 as a guest in a single crystal of benzene-d6. The electron-spin-density distribution and the molecular geometry in the lowest triplet state have been determined as well as the orientation of the molecule in the benzene crystal. It is found that upon excitation into T0, pyrimidine remains a planar molecule of approximately C2v symmetry; the C–N–C angles, which are 115.5° in the ground state, and the angle between the directions of the nonbonding nitrogen orbitals both become >120°. Nearly 60% of the total electron-spin density is located at the two nitrogen atoms, the two nonbonding orbitals each carrying a spin density of 0.21, and the π orbitals a spin density of 0.08. We conclude that the lowest triplet state of pyrimidine is appropriately described in terms of an nπ* excitation; we find no evidence for vibronic coupling of this state with higher lying 3ππ* states.
The effect of excluded volume on aggregation kinetics
Authors: Poland Douglas
Published: July 2002

We consider the kinetics of the reversible aggregation of monomers to form roughly spherical clusters where the effect of excluded volume is taken into account by using Lebowitz’s exact solution of the Percus–Yevick equation for a mixture of hard spheres, assuming rapid spatial equilibration relative to the rate of aggregation. Using the radial distribution function evaluated at the contact distance between two spheres, we obtain a differential equation for the aggregation process that is a sum over cubic terms in the appropriate cluster densities. The differential equation is converted into a recursion relation for the coefficients in a series in powers of the time. The series can then be used to estimate the asymptotic relaxation time for the aggregation process.
Identification of the nsσ and ndλ Rydberg states of O2 for n=3–5
Authors: Yokelson R. J. , Lipert R. J. , Chupka W. A.
Published: July 2002

The 4s-3d and 5s-4d Rydberg complexes of diatomic oxygen have been studied by (2+1) resonance-enhanced multiphoton ionization of the X 3∑g− ground state of O2. We have located and identified at least two vibrational levels of each of the following states: Three of four expected 4sσ Π states; all four expected 5sσ Π states; 18 of 22 expected 3d states (with only the states of the 3dσ orbital remaining unobserved); and 5 of the 10 predicted 4dπ states. State assignments were assisted by the following: the results of rotational cooling and laser polarization experiments which facilitated the rotational analysis, band positions, band intensities, and parameterized calculations. The experimentally determined state locations are compared with the state locations obtained from ab initio calculations. We have carried out isotope experiments and rotational linewidth analysis to study in some detail the mixing between the Rydberg states and the repulsive valence states as well as the mixing between the Rydberg states themselves. We conclude that direct predissociation dominates indirect predissociation as a dissociative mechanism, but there is evidence of Δv≠0 interactions which perturb the rotational structure of the 3dπ∑ and Δ states. The relative intensities of the states detected are found to span a range in excess of 104 with the nsσ Π states being the weakest and the ndπ ∑ states being the strongest. Photoionization of the ndπ ∑ states appears to be most affected by the shape resonance in the continuum. Our measurements confirm the expectation that many of the properties of the Rydberg states in the same series scale as (n*)−3.
Dynamics of the N(4S
u) + NO(X 2Π) → N2(X 1Σg+) + O(3P
g) atmospheric reaction on the 3A″ ground potential energy surface. I. Analytical potential energy surface and preliminary quasiclassical trajectory calculations
Authors: Gilibert Miquel , Aguilar Antonio , González Miguel , Mota Fernando , Sayós R.
Published: July 2002

The N(4Su)+NO(X 2Π)→N2(X 1Σg+)+O(3Pg) reaction plays an important role in the upper atmosphere chemistry and as a calibration system for discharge flow systems. Surprisingly, very little theoretical and experimental work has been devoted to the characterization of the dynamical features of this system. In this work a Sorbie–Murrell expression for the lowest 3A″ potential energy surface (PES) connecting reactants in their ground electronic states based upon the fitting of an accurate ab initio CI grid of points has been derived. The PES fitted shows no barrier to reaction with respect to the reactants asymptote in accordance with experimental findings and becomes highly repulsive as the NNO angle is varied away from the saddle point geometry. The results of preliminary quasiclassical trajectory calculations on this surface reproduce very well the experimental energy disposal in products, even though the vibrational distribution derived from trajectories seems to be a bit cooler than the experimental data. Moreover, thermal rate constants derived from trajectories are in excellent accordance with experimental values.
Photodissociation dynamics of CH3I adsorbed on MgO(100): Theory and experiment
Authors: McCarthy M. I. , Gerber R. B. , Trentelman K. A. , Strupp Paul , Fairbrother D. Howard , Stair P. C. , Weitz Eric
Published: July 2002

Theoretical and experimental results are compared for the 257 nm photolysis of methyl iodide adsorbed on an MgO(100) crystal. Molecular-dynamics calculations treat CH3I as a pseudodiatomic molecule and describe the geometry and the vibrational and librational frequencies of ground state CH3I on the surface of a solid at 125 K. The simulations modeled the photodissociation dynamics of the adsorbed species. The photoexcitation of CH3I at 257 nm is to the 3Q0 state which is, in turn, coupled to the 1Q1 state. The electronic surface coupling allows for two dissociation pathways, producing either ground- or excited-state iodine atoms in concert with ground-state methyl radicals. The I*/I branching ratio and the velocity and angular distributions of both photofragments are predicted by the theory. A comparison is made between these predictions and experimental observation of the I*/I branching ratio, the velocity distribution of the methyl fragment, and the internal state distribution of the methyl. A substantial lowering of the I*/I ratio as compared to data from the gas-phase photodissociation studies is both predicted by theory and seen experimentally. Theoretical simulations attribute this change to efficient trapping of the I* photofragments by the surface. Further comparisons between the theoretical predictions and the experimental data support a model where the molecule is aligned perpendicular to the surface and the escape of iodine atoms from the surface following the photodissociation of adsorbed methyl iodide involves collisions with the surface.
Langevin dynamics of a linear rotor in a Maier–Saupe potential: Kramers turnover of the flipping rate
Authors: Pastor Richard W. , Szabo Attila
Published: July 2002

The frictional dependence of the flipping rate of a linear rotor in a Maier–Saupe potential is evaluated from Langevin dynamics simulations. By assuming that the azimuthal angular velocity is rapidly thermalized, an approximate analytic expression for the rate is obtained within the framework of the Mel’nikov–Meshkov treatment of the Kramers turnover problem for one-dimensional bistable potentials. The predictions of this expression are in good agreement with the accurate simulation results.
Collisional cooling between 5 and 20 K: Low-temperature helium pressure broadening of CH3F
Authors: Willey Daniel R. , Choong Vi-En , Goodelle Jason P. , Ross Kenneth A.
Published: July 2002

A recent adaptation of the collisional cooling technique which permits gas-phase spectroscopy from 5 K to ≥20 K is described. The new apparatus was used to measure pressure broadening parameters and cross sections for the the J=0–1, K=0 rotational transition of CH3F broadened by helium from 5 to 21 K. The cross sections show a general upward trend with decreasing temperature ranging from 55.9 Å2 at 20 K, to 114.0 Å2, at 5 K. This compares with a 295 K cross section of 48.8 Å2. While an accurate CH3F–He potential surface is not presently available for calculating theoretical cross sections, the rise in cross section at low temperature can be attributed to the dominance of resonant collisional processes at very low energy. Although resonant structure in many systems (e.g., CO–He, H(D)Cl–He) appears to be smoothed out by Boltzmann averaging, there is experimental evidence that at least one resonance may survive the thermal average in the CH3F–He system.
X-ray diffraction study of heneicosanol monolayers on the surface of water
Authors: Shih M. C. , Bohanon T. M. , Mikrut J. M. , Zschack P. , Dutta P.
Published: July 2002

We have studied Langmuir monolayers of CH3(CH2)20OH (heneicosanol) using x-ray diffraction. Over the temperature range 7–55 °C, the monolayer isotherms show only one discontinuity, which may be either a ‘‘flat section’’ or a ‘‘kink;’’ we find that these are transitions between tilted and untilted phases. There are several untilted phases, with hexagonal and distorted-hexagonal structures, as in monolayers of the corresponding (heneicosanoic) acid. However, in contrast to the acid, when the pressure is lowered the molecules tilt only towards a next-nearest neighbor. The dependence of the lattice spacings on pressure and temperature indicate that the monolayer does not form a ‘‘tilted hexagonal’’ phase, even at temperatures where the untilted phase is hexagonal (rotator-II).
A master equation approach to multiphoton dissociation of a Morse oscillator
Authors: Gangopadhyay Gautam , Ray Deb Shankar
Published: July 2002

Based on the previous work [Gangopadhyay and Ray, J. Chem. Phys. 96, 4693 (1992)] on the generalization of the dissipative master equation for nonlinear oscillators, a theory of multiphoton excitation and dissociation of a Morse oscillator in presence of dissipation has been formulated. Interplay of excitation and dissipation with the nonlinearity of the system has been illustrated in the calculation of dissociation probabilities and the mean first passage time for the escape problem.
Computer simulation of electron scavenging in multipair spurs in dielectric liquids
Authors: Wojcik Mariusz , Bartczak Witold M. , Hummel Andries
Published: July 2002

A Monte Carlo simulation was applied to model the electron scavenging processes in radiation-induced spurs in low-mobility liquid hydrocarbons. The scavenging probabilities were calculated for spurs containing from one to ten electron-cation pairs, for different initial cation–cation and cation–anion distances, in a range of the scavenger concentration from 10−4 mol/dm3 to 10−1 mol/dm3. It was found that the scavenging probability decreases as the number of pairs in the spur grows. For two-pair spurs the scavenging probability is lower by about 20%–30% as compared with the single-pair value, for small cation–cation distances, for five- and ten-pair spurs, a decrease of about 50%–60% and 60%–70%, respectively, is found. The Laplace transform technique, which converts the recombination kinetics in the absence of scavenger into the scavenging probability, is found to work reasonably well also in the multipair case. A simplified model of the fast electron track is applied to calculate the track-averaged values of the scavenging probability.
Time-dependent Monte Carlo studies of diffusion with surface steps
Authors: Bowler Ann M. , Hood Eric S.
Published: July 2002

Our initial time-dependent Monte Carlo investigations of diffusion on perfect surfaces indicated that a variety of diffusion mechanisms were operative depending on the adparticle interactions and other controlling parameters in the system. We have now extended our approach to examine diffusion in the presence of defects. We find that the diffusion coefficient and mechanism are intimately related to the concentration and nature of the defects. Surface traps have been shown to act as nucleation sites for island formation. Surface steps facilitate a number of diffusion phenomena. We observe step-mediated ‘‘corralling’’ or ‘‘herding’’ of adparticles which creates regimes of different adparticle concentrations on the lattice. Steps also directionally enhance the diffusion rate and act as nucleation sites. Our studies indicate that current mathematical interpretations of surface diffusion experiments are flawed by the inability to explicitly account for diffusion dynamics in the presence of step features. Surface steps and traps exert roughly the same influence on diffusion rates at low adparticle interactions. In the systems examined at higher adparticle interactions, defects play a less significant role in diffusion dynamics than adparticle interactions.
Calculation of the dispersion-dipole coefficients for interactions between H, He, and H2
Authors: Bishop David M. , Pipin Janusz
Published: July 2002

Collisions between atoms and molecules create an induced dipole moment which, at long range separations, stems, in part, from the van der Waals interactions between the colliding species. This contribution is known as the dispersion dipole moment and is of the order R−7, where R is the separation between particles. Although there have been several approximate calculations of the dispersion-dipole coefficients which govern this contribution, and are the counterparts to the van der Waals dispersion-energy coefficients, there have been few ab initio calculations. In this article we present highly accurate results, based on explicitly electron-correlated wave functions, for the dispersion-dipole coefficients pertaining to interactions between pairs chosen from H, He, and H2. We also obtain values with some of the currently used approximate formulas. A comparison shows that these values differ, in general, by a significant amount (∼20–∼40 %) from the accurate ones. We also tabulate values of the dipole–dipole-quadrupole polarizability tensor (B) for imaginary frequency (iω) for a range of frequencies appropriate to a 64-point Gauss–Legendre quadrature for H, He, and H2. These values were used in certain numerical integrations we made to verify our original results which had been obtained by analytic integration—they may, however, be useful in other contexts. For H–H2 and H2–H2, these are the only ab initio calculations of the dispersion-dipole coefficients of which we are aware.
The adsorption and thermal decomposition of digermane on Ge(111)
Authors: Lu Guangquan , Crowell John E.
Published: July 2002

We have used multiple internal reflection infrared spectroscopy to investigate the interaction of digermane with Ge(111) at temperatures between 104–600 K. Digermane predominantly adsorbs molecularly on the surface below 120 K, displaying a vibrational spectrum similar to that of condensed digermane. At temperatures between 120–150 K, digermane dissociates via Ge–Ge bond scission to form adsorbed GeH3. Chemisorbed germyl GeH3 has a distinct symmetric deformation vibration at ∼772 cm−1, compared to a value of 721 cm−1 for molecularly adsorbed Ge2H6. At 200 K, Ge2H6 adsorption produces surface GeH3, GeH2, and GeH species with stretching vibrations at 2063, 2023, and 1968 cm−1, respectively. The surface GeH2 species is also identified by a characteristic scissor mode at ∼830 cm−1. Adsorption at 300 and 400 K produces only GeH2 and GeH, with a much lower concentration of GeH2 at 400 K. The surface GeH2 and GeH species are also generated by the successive decomposition of GeH3 upon heating. All surface hydrogen desorbs at ∼600 K.
Spontaneous pinning of domain growth during spinodal decomposition of off-critical polymer mixtures. II. Scaling analysis
Authors: Takenaka Mikihito , Izumitani Tatsuo , Hashimoto Takeji
Published: July 2002

Later-stage spinoidal decomposition (SD) of off-critical mixtures of poly(styrene-ran-butadiene) SBR/polyisoprene or SBR/polybutadiene was investigated by time-resolved light scattering. Changes in the scaled structure factor F(x,t)≡qm(t)3I(x,t) with x≡q/qm(t) were determined as a function of the composition w of the mixture and of the phase separation temperature T, where qm(t) is the wave number q at the peak position of the scattering intensity profile I(q,t) at time t. It was found that the later-stage SD for the off-critical mixture is divided into four stages: intermediate, pseudolate, transition, and pinning. The change in the structure factor F(x,t) at large x implied that the dynamical percolation-to-cluster transition occurs before the pinning stage.
The reaction of O(3P) with cyclohexane clusters
Authors: Rudich Y. , Hurwitz Y. , Lifson S. , Naaman R.
Published: July 2002

The effect of cluster formation on the reactivity of cyclohexane was investigated. When single molecules react with O(3P) the products are the OH radical and cyclohexyl radical. In contrast, we found that when small clusters react with O(3P), the OH product is suppressed; furthermore, the ‘‘liquid’’-like product, cyclohexanol, is observed, although the yield is unknown. It is proposed that blocking of the abstraction reaction occurs in the reaction when clusters are involved. In addition, an efficient insertion process can take place. These results provide a new explanation for the process in the liquid phase.
Mobilities of metal cluster ions: Aluminum and the electronic shell model
Authors: Jarrold Martin F. , Bower J. Eric
Published: July 2002

The mobilities of size-selected aluminum cluster ions containing 5–73 atoms have been measured using injected ion drift tube techniques. The mobilities show an apparent correlation with the electronic shell model: Clusters close to the electronic shell closings have enhanced mobilities. A structural transition appears to be associated with the shell closing with 138 valence electrons. Structural isomers were identified for a few clusters at low temperatures. But with only one exception (Al45+) the annealing temperatures of these isomers are below room temperature.
Predissociation lifetimes of vibrational levels of the excited 1B1 (K
a′=0) electronic states of Cd⋅H2 and Cd⋅D2 complexes
Authors: Bililign Solomon , Morse Michael D. , Breckenridge W. H.
Published: July 2002

The experimental rates of predissociation of vibrational levels of the 1B1 (Ka′=0) excited states of the Cd⋅H2 and Cd⋅D2 complexes are shown to be consistent with both semiclassical and quantum-mechanical pseudodiatomic theoretical treatments of the process. The 1B1 pseudodiatomic potential was constructed by fitting an analytical function to the experimentally estimated Rydberg–Klein–Rees (RKR) inner and outer turning points. The potential of the repulsive 3A1 state was estimated by fitting an exponential function to the ab initio points of Boatz, Gutowski, and Simons, then adjusting the exponential parameter slightly to maximize overall agreement with the observed Cd⋅H2 and Cd⋅D2 predissociation lifetimes. The best-fit repulsive curves for both the semiclassical and quantum-mechanism calculations result in slightly ‘‘outer-wall’’ 1B1/3A1 crossings at only 84 and 76 cm−1, respectively, above the 1B1 potential minimum. The 1B1/3A1 coupling matrix elements derived from both treatments were ∼150–160 cm−1, much smaller than the 404 cm−1 expected if the spin–orbit interaction were unchanged from that of the asymptotic Cd(5s5p) states. It is suggested that the apparent reduction in the coupling strength could be due to the marked change in the nature of the Cd 5pσ orbital due to the strong repulsive interaction with H2 in the 3A1 state and to the neglect of the anisotropy of the 1B1/3A1 triatomic potential surface crossing.
A new method for the calculation of photodissociation cross sections
Authors: Seideman Tamar
Published: July 2002

A time-independent quantum mechanical approach to the calculation of photodissociation cross sections is developed. The method is based on the use of a discrete variable representation (DVR) and the application of absorbing boundary conditions (ABC). Both total photodissociation cross sections and partial cross sections (hence product state distributions) are obtained using the same basic technique, but the calculation is particularly efficient when only the former quantity is required. The method is applied to the photodissociation of HCl+ for which accurate potential energy curves and dipole moment functions are available, to the photodissociation of ClCN which is a direct process, involving a single excited electronic state, and to the photodissociation of ICN which involves several strongly coupled excited electronic states. The applicability of the suggested scheme to a variety of other field–matter interaction processes is pointed out.
Closed-form Mueller scattering matrix for a random ensemble of long, thin cylinders
Authors: Shi Yaoming , McClain W. M.
Published: July 2002

We present a closed-form solution to the problem of elastic light scattering by a randomly oriented ensemble of cylinders of finite length. All the Mueller scattering matrix elements are calculated, so the solution is complete in the sense that all possible polarization effects are treated. The cylinders are assumed to be made of an isotropic material, which may be transparent or absorbing. Cylinder radius a and length L must obey a≤λmed/2π≤L, where λmed is the wavelength in the supporting fluid medium. Our solution includes all multipole effects due to the length of the cylinder, as well as all internal reflections at the boundary between the cylinder and the supporting medium. Nonzero values of the retardation elements M34 and M43 may be caused either by retardative internal reflections, or by absorption, or both. These elements are easily observable in experiments on scattering by rod-like viruses, but are completely missing in theories that ignore retardative contributions to the scattered amplitude. The curve M34(θ) is affected strongly in distinct ways by rod length, by index of refraction, and by absorptivity.
Creation of discrete quasienergy resonance states in strong electromagnetic fields
Authors: Ben-Tal Nir , Moiseyev Nimrod , Kosloff Ronnie
Published: July 2002

The combined complex-coordinate-Floquet formalism, which has been described in detail in J. Chem. Phys. 94, 7311 (1991), is used for numerical studies of the creation of discrete long-lived quasienergy resonance states for two model Hamiltonians which describe the interaction of dissociative/ionizing systems with intense monocromatic field. Despite the fact that the two model Hamiltonians have the same energy spectrum in the absence of field, the number of resonance states which they support and the general behavior of the resonances in the presence of the field is entirely different. An analysis of the dependence of the quasienergy resonance spectrum on field intensity in terms of the dependence of the dressed potential spectrum in the field intensity was found adequate for medium intensity fields, but failed for strong fields.
Laser induced fluorescence and radiative lifetimes of the low-lying electronic states of gaseous AgF
Authors: Wang He , Gole James L.
Published: July 2002

At the fringes of the visible region, two low-lying 1 (Ω=1) electronic states A′Ω1 and aΩ1 of gaseous AgF located ∼4300 cm−1 below the previously known lowest excited A0+ state have been excited for the first time in a silver vapor–fluorine reaction system. The A′Ω1–X 1Σ+ and a Ω1–X 1Σ+ band systems (also observed in chemiluminescence) have been excited and studied using pulsed laser induced fluorescence spectroscopy. The band system associated with the A′Ω1–X 1Σ+ transition has been rotationally analyzed. The UV fluorescence of the A0+ and B0+–X 1Σ+ transitions has also been excited. The radiative lifetimes of these four low-lying electronic states have been measured as 7.1 μs (A′Ω1), 9.1 μs (aΩ1), 240 ns (A0+), and 21 ns (B0+), respectively, revealing that the two Ω=1 states are of triplet character, while the two 0+ states are of singlet character. The observed low-lying states of AgF appear to dissociate adiabatically to neutral atoms in contrast to the apparent dissociation of the low-lying electronic states in CuF to ion pairs. The observation of the low-lying 1 states of AgF also indicates the existence of similar stable 1 states for the remaining silver halides, all of which should absorb visible photons. Major molecular constants of the newly observed A′Ω1 state of 107AgF are Te=24 950.71(10) cm−1, ΔG1/2=506.74(8) cm−1, Be=0.281 32(15) cm−1, De=0.116(60)×10−6 cm−1, and re=1.927 Å.
Refinement of the OH A 2Σ+(v=0)+Ar intermolecular potential energy surface
Authors: Lester Marsha I. , Loomis Richard A. , Giancarlo Leanna C. , Berry Mary T. , Chakravarty Charusita , Clary David C.
Published: July 2002

Several intermolecular vibrational levels of the excited electronic state of OH–Ar correlating with OH A 2Σ+(v=0)+Ar have been characterized by laser-induced fluorescence and hole-burning experiments. The OH–Ar levels identified include the lowest intermolecular level, an intermolecular bending level with a lower degree of stretching excitation than previously observed, and intermolecular levels with two quanta of bending excitation. The intensities of electronic transitions to these levels from the lowest intermolecular level of the ground electronic state of OH–Ar (X 2Π) are significantly weaker than those of transitions previously reported. These data are used to refine a semiempirical potential for OH A 2Σ+(v=0)+Ar proposed by Bowman et al. [J. Phys. Chem. 94, 2226 (1990)]. The potential parameters have been adjusted to increase the potential anisotropy and the steepness of the radial potential in the O–H–Ar well region. The bound states supported by the adjusted potential have been calculated by taking into account the electron spin angular momentum of the OH radical. The calculated vibrational energies and rotor constants reproduce the rovibrational structure observed experimentally. A theoretical simulation of the OH–Ar electronic excitation spectrum based on the adjusted intermolecular potential yields an intensity pattern which is consistent with experimental results.
Does chlorine peroxide exhibit a strong ultraviolet absorption near 250 nm?
Authors: Stanton John F. , Bartlett Rodney J.
Published: July 2002

A theoretical investigation of the low-lying singlet electronic states of chlorine peroxide (ClOOCl) is presented. Calculations of excitation energies and oscillator strengths at the equation of motion coupled-cluster singles and doubles level predict that six excited states of ClOOCl are accessible at photon wavelengths above 200 nm. These states occur in three sets of pairs which can be approximately represented as symmetrized and antisymmetrized excitations of electrons from lone pair and Cl–O bonding orbitals to antibonding Cl–O orbitals. While small oscillator strengths are predicted for transitions to the lowest two ‘‘pairs’’ of states, sizable values are predicted for the X 1A→E 1A and X 1A→F 1B transitions (0.02 and 0.08, respectively). Furthermore, our estimation of the vertical excitation energy for these processes is ≊5.20 eV, in satisfactory agreement with the experimentally observed band maximum of ClOOCl at 5.04 eV. The results of this work lend strong support to the contention that photodissociation of ClOOCl plays an important role in the catalytic destruction of stratospheric ozone.
A theoretical study on the structure and stability of the PFn and PFn− series n=1–6
Authors: Gutsev G. L.
Published: July 2002

The electronic and geometrical structure of the phosphorus fluorides PFn and their singly charged anions PFn−, n=1–6 are studied by a density functional method. The ground and some low-lying excited states of both series are considered. The results of our calculations are used for an estimation of the adiabatic electronic affinity (E.A.ad) of the neutrals and fragmentation energies through different decay channels of both series. We have found that all the neutral species possess the positive E.A.ad, except for PF3 whose E.A.ad is near zero from the negative energy side, and all the members of both series are stable towards dissociation. Some excited states which are stable towards the loss of an extra electron and dissociation are found for all the anions except for PF3−.
Rotational spectrum and structure of the Ne–HCN dimer
Authors: Gutowsky H. S. , Keen J. D. , Germann T. C. , Emilsson T. , Augspurger J. D. , Dykstra C. E.
Published: July 2002

Microwave rotational transitions have been observed at low J (0–3) for several isotopic species of the Ne–HCN dimer using the Balle/Flygare Mark II Fourier transform spectrometer with a pulsed nozzle as the source. For 20Ne–HC 14N, the main K=0 transitions give rotational constants B̄, DJ, and HJ of 2772.816 and 1.280 MHz and 1.173 kHz. The 14N nuclear quadrupole constant increases linearly with J(J+1) at a slope Dχ of −12.7 kHz from a value for χa(14N) of −0.957 MHz at J=0. The pseudodiatomic approximation for B̄ and χa(14N) leads to a value of 3.89 Å for the Ne to HC 14N center-of-mass (c.m.) distance R, and to 46.8° for the ‘‘average’’ bend angle θ of HC 14N. Some of the K=0, J=1→2, and J=2→3 transitions exhibit one or two weak satellites ∼30 MHz away, usually below, but also both above and below. The J=1→2 low frequency satellites for 20Ne–HC 14N and 20Ne–HC 15N, nominally 111→212, are symmetrical doublets with splittings of 305 and 439 kHz, respectively. The 14N hyperfine structure (hfs) is identical for the two 20Ne–HC 14N components as is the Stark effect for 20Ne–HC 15N. The molecular mechanics for clusters (MMC) model was used to calculate potential energy surfaces for Rg–HCN dimers, giving stabilities of 21, 37, 85, and 108 cm−1 with He, Ne, Ar, and Kr as the rare gas. A qualitative comparison of the experimental properties for the dimers with Ne, Ar, and Kr as the rare gas is based on the surfaces. The extremely mobile internal dynamics of Ne–HCN are attributed to its potential surface, which is both very shallow and isotropic.
Fourier-transform microwave spectroscopy of the HCCN radical. Determination of the hyperfine coupling constants
Authors: Endo Yasuki , Ohshima Yasuhiro
Published: July 2002

Two low-J transitions, N(J)=1(0)−0(1) and 1(2)−0(1), of the HCCN radical in its 3Σ− ground state have been studied by a pulsed-discharge-nozzle Fourier-transform microwave spectrometer. Spectrum with well-resolved hyperfine splittings due to hydrogen and nitrogen nuclei was observed, which enabled us to determine accurate hyperfine coupling constants for both the nuclei for the first time in the present study, as well as a much improved spin–spin coupling constant. These hyperfine coupling constants suggest that the electronic structure of the HCCN radical can be described as the one with the allenic structure to be almost twice more important than the carbenic structure. The present results seems to support a conclusion that the molecular structure of the radical is almost linear with a possibility of a large amplitude bending motion.
Photoinduced charge-transfer dissociation in van der Waals complexes. II. Na2...ClCH3, Na2...(ClCH3)2, and Na...FPh
Authors: Liu Kun , Polanyi John C. , Yang Shihe
Published: July 2002

van der Waals complexes Nam...(XR)n (R=CH3 and Ph; X=Cl and F) have been generated by crossing a beam of Na with a supersonic jet of halide molecules XR seeded with Ar. The complexes were identified and measured by photoionization time-of-flight mass spectrometry. Tunable visible radiation (530–680 nm) was used to measure the photodissociation spectrum (an ‘‘action spectrum’’) of these clusters. The peak cross section for photodissociation of Na2...(ClCH3)n (n=1 and 2) was found to be large (∼6 Å2) with its maximum at 635 nm for n=1 and at 615 nm for n=2. These peak wavelengths suggest that the primary route for photodissociation is excitation of the Nam followed by charge-transfer dissociation: Na2...(ClCH3)1,2 + hυ → [Na*2...(ClCH3)1,2]‡ → [Na+2...(ClCH3)1,2−]‡→products (the double dagger indicates a transition state). By contrast, the photodissociation of Na2+...(ClCH3) appeared to follow a direct mechanism involving the dissociation of the [Na–Na]+. For Na...FPh, two excited states of the complex (Π and Σ) were thought to be responsible for the two broad peaks in the action spectrum, each with vibrational structure.
Double electron–electron resonance spin–echo modulation: Spectroscopic measurement of electron spin pair separations in orientationally disordered solids
Authors: Larsen Russell G. , Singel David J.
Published: July 2002

A DEER (double electron–electron resonance) spin–echo technique was applied to measure the electron–electron dipolar spectrum of a frozen toluene solution of the biradical, 2,6-bis[(((2,2,5, 5-tetramethyl-1-oxypyrrolin-3-yl)carbonyl)oxy)]-anthracene. Modulation of the DEER spin–echo envelope was observed and identified as originating from the dipolar coupling between the two nitroxide spins of the biradical. Fourier transformation of the modulated components of the echo envelope yielded a dipolar spectrum from which a spin–pair separation of 19.73±0.14 Å was calculated. Constraints on the relative orientation of the two nitroxide spin moieties were obtained by analysis of the effect of the microwave pulse orientational selectivity on the DEER modulation amplitudes. Molecular models of the studied compound exhibit structures that correspond well with the structural information deduced by DEER spectroscopy.
Predissociation linewidths in O2 B 3Σu− (v=0,2)
Authors: Cosby P. C. , Park H. , Copeland Richard A. , Slanger T. G.
Published: July 2002

Using laser-induced fluorescence techniques applied to Schumann–Runge absorption transitions from vibrationally excited O2(X3Σg−), we have measured the rotational and fine-structure level-specific linewidths in v=0 and v=2 of the B3Σu− state. These linewidths represent the first measurement of fine-structure level-specific predissociation rates in B3Σu−(v≤10), and they are found to vary considerably among the various rotational and fine-structure levels, encompassing a range of 0.09–0.34 cm−1 in v=0, N′≤36, and 0.4–1.4 cm−1 in v=2, N′≤24. Orbit-rotational coupling in the B3Σu−–3Πu interaction, in addition to spin-orbit coupling in the B3Σu−–1Πu, −3Πu, −5Πu interactions, is found to be crucial to explaining the relative predissociation rates among the fine-structure levels, even in low rotational levels. Measurements were made in the (v′,v″)=(0,9), (0,10), (0,21), (2,10), and (2,22) Schumann–Runge [B3Σu−(v′)←X3Σg−(v″)] bands without presumption as to the molecular constants in either the X or B states. The use of high ground state vibrational levels as a starting point for the photoexcitation measurements produces a spectral separation among the previously blended triplet components of the absorption lines. All lines in these bands are found to be broadened by predissociation, with those terminating in v′=0 and in the N′=J′−1 (F′1) levels exhibiting the smallest effect.
Theoretical search for small linear doubly charged anions
Authors: Boldyrev A. I. , Simons J.
Published: July 2002

The electronic stabilities of linear diatomic C22−, BN2−, BP2−, BeO2−, BeS2−, MgO2−, MgS2−, O22−, SO2−, and S22−; triatomic NCN2−, SBN2−, OBeO2−, SBeC2−, SBeO2−, OMgO2−, SMgS2−, PCP2−, and SCC2−; tetratomic OCCO2− and SCCS2−; and pentatomic Be2O32−, Be2S32−, Mg2O32−, and Mg2S32− dianions were studied at the Hartree–Fock level with Gaussian basis sets including diffuse and polarization functions, as well as at the second-order Mo/ller–Plesset level. Vertical first detachment energies of several dianions were also calculated at the fourth-order Mo/ller–Plesset level. All diatomic, triatomic, and tetratomic dianions were found to be unstable to electron loss. The pentatomic Mg2S32− dianion is predicted to be stable to electron loss with an electron detachment energy of 0.2 eV. This species is likely to be the smallest electronically stable linear dianion.
Density functional calculations of isotropic hyperfine coupling constants of radical cations
Authors: Eriksson Leif A. , Malkin Vladimir G. , Malkina Olga L. , Salahub Dennis R.
Published: July 2002

The hyperfine (hf) structures of radical cations are calculated using first principles density functional theory. Within the local spin density (LSD) approximation, the isotropic hyperfine coupling constants (Fermi contact terms) are computed for a set of linear (nonbranched) alkane and alkene radical cations with up to four carbon atoms. The results are as a whole in very good agreement with experimental data, obtained from low temperature matrix isolation electron-spin resonance (ESR) measurements, and with results from previous configuration interaction calculations.
Study of α-, β-, and γ-relaxation processes in some supercooled liquids and supercooled plastic crystals
Authors: Gangasharan  , Murthy S. S. N.
Published: July 2002

Using dielectric spectroscopy, we have studied different types of relaxation processes, namely, primary (α), secondary (β), and other sub-Tg processes, in the supercooled liquids of tritolylphosphate (TTP), 3-bromopentane (3BP), isopropylbenzene (IPB), glucose (GL), and also in the supercooled plastic crystals of cyclohexanol (CHOL) and camphor, over a wide frequency (10−3–106 Hz) and temperature (above 77 K) range. Asymmetric Cole–Cole plots are found at temperatures above Tg in all the systems except camphor where they are very symmetric. TTP and 3BP are found to have weak sub-Tg processes and the corresponding α process do not show significant change in the shape of Cole–Cole plots with temperature. TTP, 3BP, and IPB are found to possess at least two sub-Tg processes (designated as β and γ processes, respectively) and the evidence for any of them to be intermolecular in nature is not strong. The origin of these processes probably lies in a side group and/or segmental rotation which still survive in the glassy state. In both GL and CHOL, the sub-Tg process previously designated as the β process by earlier workers is found to be non-Arrhenius in character representing another glass transition below the main Tg. Calorimetric evidence is provided for the purpose. The origin of this process is probably due to the ‘‘free’’ molecules. The origin of the various sub-Tg processes has been discussed in greater detail.
Thermodynamic properties of planar square-well dumbbell fluids: Monte Carlo simulations and perturbation theory
Authors: Brańka A. C. , Heyes D. M.
Published: July 2002

We calculate for the first time the equation of state and other thermodynamic properties of the planar square-well dumbbell (SWD) fluid. The SWD molecule consists of two fused hard discs of diameter σ, separated by a reduced separation L*=L/σ with attractive annuli of outer radii, Kσ around each hard disc. Monte Carlo simulations were carried out over a range of densities and temperatures for two choices of potential width and several molecular elongations. In particular, we concentrated on states with the molecular elongation parameter, L*=0.3 and the potential-width parameter, K=1.5. In this article we concentrate on analytic representations of the thermodynamic behavior of the SWD fluid. The second virial coefficients of the SWD system are calculated by a part analytic/part numerical procedure. The free energy expansion coefficients are calculated up to third order. Convergence of the perturbation expansion based on the hard-dumbbell fluid is examined and shown to be very satisfactory. A generalization of the quasichemical (QCA) coordination number model for the SWD fluid is proposed. The equation of state and energy derived from this model, compared with the simulation results, show that the QCA model represents well the simulated properties of the SWD at low density but fails at high density.
The mutual diffusion coefficient for the van der Waals binary mixture of type I
Authors: Castillo R. , Garza C. , Dominguez H.
Published: July 2002

In the framework of the mean-field kinetic variational theory, the explicit dependence of the mutual diffusion coefficient of the van der Waals binary mixture with composition and interaction parameters is obtained. The different kinds of behavior shown by this coefficient can be classified according to the scheme of van Konynenburg and Scott devised to describe the global phase diagram of this model mixture. A numerical study to understand the concentration dependence of the mutual diffusion coefficients for mixtures of type I is presented here, in terms of molecular masses, sizes, and interaction parameters. Moreover, the behavior of the mutual diffusion coefficient of the van der Waals mixture is compared with that of a hard-sphere mixture. In addition, a comparison is made between our calculations and experimental data of binary systems classified as belonging to type I: H2O/D2O, hexane/heptane, toluene/hexane, and benzene/hexane. From the explicit model presented here, one can obtain semiquantitative explanations of the role played by the variables that determine the concentration dependence of the mutual diffusion coefficient.
Theoretical potential energy functions and rovibronic spectrum of electronically excited states of HCO+
Authors: Weis Bernhard , Yamashita Koichi
Published: July 2002

Three-dimensional potential energy functions (PEFs) and rovibronic spectra for several low-lying electronically excited singlet states of HCO+ have been investigated based on the ab initio complete active space self-consistent field (CASSCF) method. The calculated electronic transition moments show that the ultraviolet (UV) absorption spectrum of the ion should be dominated by the transition C 1Π–X 1Σ+. Analytical representations of the PEFs for the two components of the degenerated C 1Π state have been used in beyond Born–Oppenheimer calculations of the rovibronic energy levels of HCO+ and DCO+ by a Renner–Teller variational approach which accounts for anharmonicity, rotation–vibration, and electronic angular momentum coupling effects. Since the excited states of HCO+ are experimentally unknown up to now, the calculated spectroscopic constants, the vertical and adiabatic excitation energies, and the absorption spectra should provide valuable information for future experimental characterizations of the excited states.
A lower bound on the loss of graphite by atomic oxygen attack at asymptotic energy
Authors: Cohen L. Kenneth
Published: July 2002

A method is devised to find a lower bound on the loss function of graphite in the oxygen high energy range where that function is expected to assume constant value. It requires obtaining a small ensemble of classical trajectories of oxygen on graphite and making static measurements of an oxygen on ethylene potential energy surface. An oxygen atom is assumed to be bonded to the graphite surface by diabatic curve crossing from its initial repulsive potential. Subsequently, it is assumed to be ejected as part of a carbon monoxide molecule due to surface impact of another incident oxygen atom. A dynamical calculation is suggested to improve on the approximate method of this work. Once an excited state cross channel coupling constant is known, the ratio of the loss function of that excited state to the loss function of the ground state may be estimated at asymptotic oxygen energies.
The permanent electric dipole moment of NbN
Authors: Fletcher D. A. , Dai D. , Steimle T. C. , Balasubramanian K.
Published: July 2002

A high resolution optical Stark study of a niobium nitride (NbN) supersonic molecular beam sample has been performed. The F′=2.5–F″=3.5 hyperfine component of the R(1) branch feature of the (0,0) B 3F2–X 3D1 subband system was recorded as a function of static electric field strengths up to 2.7 kV/cm. The permanent electric dipole moment of 3.26(6) D and 4.42(9) D for the X 3D1 and B 3F2 states, respectively. The properties of the X 3D1 state were calculated ab initio using a complete active space multiconfiguration self-consistent-field approach followed by first-order configuration interactions plus multireference singles+doubles configuration interaction treatment. The calculated dipole moment for the X 3D1 state at the experimentally determined equilibrium bond distance is 3.65 D.
Hot carrier induced photodesorption dynamics of SO2 from Ag(111)
Authors: Sun Z.-J. , Gravelle S. , Mackay R. S. , Zhu X.-Y. , White J. M.
Published: July 2002

The photon-driven (2.5 to 6.4 eV) desorption of SO2 (coverages up to 2.7 monolayers) on Ag(111) has been studied by time-of-flight mass spectrometry in combination with temperature programmed desorption and Auger electron spectroscopy. The photodesorption yields per incident photon increase with photon energy. Consistent with substrate-mediated excitation, the mean translational energy 〈Etrans/2k〉 is constant (650 K) for photon energies between 3.5 and 6.4 eV, but decreases at longer wavelengths (480 K at 2.5 eV). The decrease is attributed to changes in the energy distribution of the hot substrate carriers responsible for desorption. The photodesorption yields vary with the initial coverage, the method of preparing the initial coverage, and the extent of photolysis. These variations are attributed, in part, to changes in the SO2 orientation on the surface. Even at 6.4 eV, where unimolecular photodissociation occurs in the gas phase and multilayer, there is no photodissociation in monolayers. This effect is attributed to strong substrate quenching.
An improved estimate of the heat of formation of HOOF
Authors: Francisco J. S.
Published: July 2002

The heat of formation of HOOF has been reexamined using an isodesmic scheme with energies and geometries calculated with quadratic configuration theory. The ΔH0f (HOOF, 0 K) is −9.3±2 kcal mol−1.
The equilibrium pair distribution function of a gas: Aspects associated with the presence of bound states
Authors: Rainwater James C. , Snider R. F.
Published: July 2002

At thermal equilibrium the momentum distribution of atoms in a gas is usually assumed to be Maxwellian, whether classically or quantally. However, if an atom is bound in a diatomic molecule, the atom’s momentum distribution is non-Maxwellian. This paper explores the consequent singlet and pair particle distribution functions in a gas having both unbound atoms and bound pairs of atoms. Comment is made on the range of behavior associated with whether the chemical equilibrium constant for diatom formation is small or large. Calculations of distribution functions and their moments for atoms which are members of dimers are presented for some specific model potentials.