bluuebunny/crossref_metadata_2025 huggingface dataset:
Collisions of excited alkali atoms with O2. II. Reactive scattering
Authors: Mestdagh J. M. , Paillard D. , Berlande J.
Published: July 2002

Inelastic and reactive channels in excited Na atom/ O2 molecule collisions have been included into a single model based on multiple crossings between curves of the Na(n,l)–O2, Na+–O−2 (X 2Πg), and Na+–O−2 (A 2Πu) systems. Additionally, estimation is provided of the excited ion pair surface Na+–O−2 (A2Πu) in collinear geometry. This paper is essentially concerned with the reaction of Na(4D) giving NaO as product, a reaction where experimental results are available, but where no reaction mechanism has been proposed so far. When the reaction proceeds via the excited ion pair Na+ –O−2 (A 2Πu) , the calculations indicate that the cross section has a threshold collision energy and that impact parameters smaller than 1 Å are involved. These features are consistent with the experimental data. This suggests that the excited ion pair Na+–O−2 (A2 Πu) is a likely intermediate in the reaction.
Near infrared spectroscopic observation of the linear and cyclic isomers of the hydrogen cyanide trimer
Authors: Jucks K. W. , Miller R. E.
Published: July 2002

Sub-Doppler resolution infrared spectra have been obtained for both the linear and cyclic conformers of the hydrogen cyanide trimer. In the case of the linear trimer, all three vibrational bands correlating with the C–H stretching fundamental of the hydrogen cyanide monomer (ν1) have been observed. The vibrational predissociation lifetime of the complex is found to be strongly mode specific. For the cyclic trimer, which has only one (doubly degenerate) infrared allowed band associated with the C–H stretch, the rotational structure is characteristic of an oblate planar symmetric top. Molecular constants are reported for both conformers. In addition, several other bands are observed in the spectrum which, although not rotationally resolved, are tentatively assigned to the tetramer.
(1+1) resonant enhanced multiphoton ionization via the A 2Σ+ state of NO: Ionic rotational branching ratios and their intensity dependence
Authors: Rudolph H. , Dixit S. N. , McKoy V. , Huo W. M.
Published: July 2002

Recent high resolution photoelectron spectroscopic studies of the (1+1) resonant enhanced multiphoton ionization (REMPI) of NO via the 0–0 transition of the A–X band (γ band) have shown a pronounced ΔN=0 signal (ΔN≡N+−Ni) and smaller, but measurable, ΔN=±2 peaks. The authors [K. S. Viswanathan et al., J. Phys. Chem. 90, 5078 (1986)] assign the excitation to be via an R(21.5) line, with no further specification. We have performed ab initio calculations of the rotational branching ratios for the four possible ‘‘R(21.5)’’ transitions, namely, the rotationally ‘‘clean’’ R21 and R22, and the ‘‘mixed’’ R12+Q22 and R11+Q21 branches. We find the mixed R12+Q22(21.5) branch to agree best with the observed photoelectron spectrum collected parallel to the polarization vector of the light. The discrepancy is larger for detection perpendicular to the polarization. To understand this difference, we have assessed the influence of laser intensity and polarization ‘‘contamination’’ on the branching ratios and photoelectron angular distributions.
Theoretical state-to-state inelastic cross sections for collisions of Ar+(2P3/2, 2P1/2) with N2
Authors: Parlant Gérard , Gislason Eric A.
Published: July 2002

State-to-state cross sections for fine-structure transitions and vibrational excitation have been calculated for collisions of Ar+(2P3/2, 2P1/2) with N2 over the relative collision energy range 1–4000 eV. The computations were done by means of a vibronic semiclassical method, and the potential energy surfaces of Archirel and Levy were used. The cross sections are in good agreement with the limited amount of experimental data now available. The cross sections for fine-structure transitions are surprisingly large at all energies.
Experimental determination of the direction of the electric dipole moment of CF3I by the oriented molecule technique
Authors: Gandhi Suketu R. , Bernstein Richard B.
Published: July 2002

The technique of polarized laser-induced photofragmentation of oriented molecule beams has now been applied to CF3I in order to ascertain the sign of its electric dipole moment: −F3CI+.
Ground-state and finite-temperature energetics and topologies of germanium microclusters
Authors: Antonio Giomal A. , Feuston Bradley P. , Kalia Rajiv K. , Vashishta P.
Published: July 2002

We have investigated the ground-state and finite-temperature properties of Ge microclusters (N=2 to 14) using molecular dynamics (MD) simulation along with the method of steepest-descent quench (SDQ). The interaction potential adopted is the three-body Stillinger–Weber potential as modified by Ding and Andersen for amorphous Ge. Our results indicate that the experimentally observed greater stability of certain cluster sizes can be explained by the topology and energetics of the clusters at finite temperature rather than by the binding energies of the ground-state structures.
Theoretical study of the nitrogen atom hyperfine coupling constant
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Partridge Harry , Chong Delano P.
Published: July 2002

The nitrogen atom isotropic hyperfine coupling constant Aiso is studied as a function of improvements in both the one-particle and n-particle basis sets. Our study underscores the importance of diffuse basis functions. For example, the (9s 5p) primitive set of Huzinaga augmented with an even-tempered diffuse s function yields values for Aiso that are virtually identical to an energy-optimized (23s 12p) even-tempered set. The Aiso constant is found to converge relatively quickly with increasing l quantum numbers: d, f, and g functions are estimated to contribute 2.5±0.2, 0.4±0.1, and 0.05±0.05 MHz, respectively. Full CI calibration calculations indicate that very high levels of correlation treatment are required for quantitative results. In addition, a strong coupling is observed between the one-particle and n-particle requirements. Our best result of 10.4 MHz is in excellent agreement with the accurate experimental value of 10.4509 MHz.
Theoretical calculations on electronic transitions for H3, including Rydberg and transition state spectra
Authors: Petsalakis I. D. , Theodorakopoulos G. , Wright J. S.
Published: July 2002

MRD-CI calculations have been carried out on the ground and excited electronic states of H3 for D3h, D∞h, C∞v, and C2v geometries. Dipole transition moments between the various electronic states have been also obtained at the different geometries calculated. The present work provides accurate theoretical information relevant to the transition state spectroscopy of H+H2 along a collinear path and also along a perpendicular path. In addition, the present work is the first all-electron configuration interaction treatment of the Rydberg states of H3, and the results are in excellent agreement with the observed spectra.
Electron binding energies of anionic alkali metal atoms from partial fourth order electron propagator theory calculations
Authors: Ortiz J. V.
Published: July 2002

Ionization energies of Li−, Na−, K−, Rb−, and Cs− are calculated with ab initio electron propagator theory. Effective core potentials are employed for K, Rb, and Cs. Third order diagonal self-energy corrections to Koopmans’s theorem are augmented with fourth order terms that are easily determined from by-products of a third order calculation. This partial fourth order procedure is derived using superoperator theory. The superiority of this method to an outer valence diagonal approximation is demonstrated for these systems. Covergence of results with respect to the order of electron interaction in the propagator self-energy and with respect to basis set improvement is studied. Agreement to within 0.1 eV of experiment is achieved. Remaining sources of error are analyzed.
Gas kinetic approach for electron mobility in dense media
Authors: Kaneko Kazuyo , Usami Yoshiyuki , Kitahara Kazuo
Published: July 2002

The motion of electrons in nonpolar dense media is studied in the framework of gas kinetic transport theory. The observed external electric field dependence of electron mobility is explained by means of the solution of Boltzmann-type transport equation. Numerical calculations of the mobility, mean energy, and energy distribution function of electrons in liquid Ar and in liquid CH4 are presented and a good agreement with experimental data is obtained. Also the cases of gaseous Ar are studied.
Molecular beam studies of weak interactions for open-shell systems: The ground and lowest excited states of ArF, KrF, and XeF
Authors: Aquilanti Vincenzo , Luzzatti Emilio , Pirani Fernando , Volpi Gian Gualberto
Published: July 2002

Absolute integral cross sections for scattering of ground state fluorine atoms by argon, krypton, and xenon have been measured in the thermal velocity range. Information has been obtained on the long range interaction and using a technique for magnetic analysis of substates of F atoms, a characterization is given for the bonding in the ground and the two lowest excited states of these rare gas fluorides. The potentials are represented as a spherical part and an anisotropic component, which have been obtained in an adiabatic decoupling treatment, including also information from other scattering data. Nonadiabatic coupling matrix elements and other general features of these interactions are also presented.
Resonance Raman scattering and optical absorption studies of CdSe microclusters at high pressure
Authors: Alivisatos A. P. , Harris T. D. , Brus L. E. , Jayaraman A.
Published: July 2002

The pressure dependence of the HOMO–LUMO transition energy and the frequency of the longest wavelength longitudinal optical vibration of 45 Å diameter CdSe clusters in methanol–ethanol solution have been measured up to 50 Kbar. The LO mode shifts to higher frequency at a rate of 0.43 cm−1/Kbar, which corresponds to a Grüneisen parameter of 1.1. The HOMO–LUMO transition shifts to higher energy at 4.5 meV/Kbar, yielding a deformation potential of 2.3 eV. The pressure dependence of these properties closely resemble those of the corresponding bulk solid, confirming the point of view that the lattice properties of these clusters resemble those of the bulk, even though the optical properties are quite distinct.
Reaction of aluminum cluster ions with oxygen and nitrous oxide: Energetics and dynamics of cluster oxidation
Authors: Ruatta Stephen A. , Anderson Scott L.
Published: July 2002

We report a study of the absolute cross sections, reaction threshold energies, and product branching ratios for interactions of aluminum cluster ions (Al+n, n=1–9) with O2 and N2O. Most of the reactions are observed to have collision energy thresholds which are attributed to activation barriers or bottlenecks. At collision energies above threshold, exoergic oxidation of the cluster ions occurs, and the resulting energy release causes extensive fragmentation to yield a variety of product ions. Comparison of threshold behavior for different product channels suggests that in addition to the dominant oxidation reactions, the clusters undergo Al abstraction, O2–Al+ exchange, and collision induced dissociation.
A molecular beam study of the potential anisotropy of laser vibrationally excited Li2(v=0,20) scattered from Kr
Authors: Rubahn H.-G. , Toennies J. P.
Published: July 2002

A beam of highly vibrationally excited (v≤20) Li2 molecules has been generated via Franck–Condon pumping using a cw ring dye laser. The linearly polarized light from a second cw dye laser is used to determine the polarization of the remaining unscattered molecules of the beam after it has passed through a scattering cell filled with krypton gas. The dependence of the state selective total integral cross section on orientation is obtained from the anisotropy of the angular momentum distribution of the beam molecules described by 1+a2P2(cos θ), where θ is the angle between the angular momentum vector and the velocity vector. The measured polarization increases from a2=−0.019±0.007 for Li2(v=0)–Kr collisions to a2=−0.074±0.016 for Li2(v=20)–Kr collisions. The measurements are interpreted in a vibrationally adiabatic approximation in terms of the potential anisotropy parameter q2 for an assumed simple anisotropic potential model V=±(Cs/Rs) [1+q2⋅P2(cos γ)], where γ is the angle between bond distance r and distance between center of mass R. For the v=20 vibrationally excited molecules we obtain q2=1.38±0.31 compared to q2=0.47±0.15 for the v=0 molecules. This large increase in anisotropy with vibrational excitation is compared with that obtained from a previously proposed model for Li2(v)–He [J. Chem. Phys. 85, 7062 (1986)].
Reactive scattering of N++H2 and deuterated analogs: Statistical calculation of cross sections and rate coefficients
Authors: Gerlich D.
Published: July 2002

A dynamically biased statistical theory is used to calculate state specific cross sections for the nearly thermoneutral ion–molecule reaction N+(3PI)+H2( j)→NH++H. Conservation of parity and nuclear spin, and the anisotropy of the long-range interaction potential are taken into account explicitly. The theory is used to calculate cross sections for all three 3PI states of the N+ ion and for rotationally excited hydrogen ( j=0–6). ln addition, the calculation has been performed for different electronic degeneracy factors (gs =1/3, 1/2, and 1), and for reaction endothermicities ΔH varying between 15 and 20 meV. The integral cross sections are used to determine numerically exact thermal rate coefficients. Comparison with published experimental results allows to determine gs and ΔH. The room-temperature data lead to gs =1/2, while rate coefficients measured below 50 K are very sensitive to the threshold behavior and are best fitted with ΔH=17 meV. Some new experimental results, measured with a liquid nitrogen cooled RF (radio frequency) ring electrode ion trap are also included. For H2 they are in good agreement with both theory and former experiments. Some discrepancies occurring for D2 as target will be discussed.
Electronic properties and fragmentation processes for singly and doubly charged indium clusters
Authors: Rayane D. , Melinon P. , Cabaud B. , Hoareau A. , Tribollet B. , Broyer M.
Published: July 2002

Indium clusters are produced by the inert gas condensation technique. The ionization potentials are found higher for small clusters than for the indium atom. This result is interpreted by the p character of the bonding as in aluminum. The fragmentation processes are studied and discussed. The behavior of the ionization potential explains why the evaporation of neutral monomers is an unlikely channel in the dissociation processes. Finally doubly charged indium clusters are detected for n≥15 and their ionization threshold is determined. Triply charged clusters are also observed.
Relaxation processes following excitation and ionization of SiF4 in the vicinity of the silicon 2p threshold. II. Dissociation of the molecular ions
Authors: Lablanquie P. , Souza A. C. A. , de Souza G. G. B. , Morin P. , Nenner I.
Published: July 2002

Partial ion yields from photoionization of SiF4 have been measured with time-of-flight mass spectrometry, photoion–photoion coincidence and triple photoion coincidence techniques using synchrotron radiation from 100 to 136 eV, in the region of the silicon 2p edge where strong resonances are found. From the photon energy dependence of positive ion pairs and of doubly charged fragment intensities, with a suitable normalization procedure, we have estimated the total double photoionization cross section. Below the Si2p edge, the double to single dissociative ionization branching ratio follows the resonance behavior, and is consistent with the photoelectron results of de Souza et al. (Paper I) and discussed in terms of multibond breaking dissociation pathways of residual excited ions (singly and doubly charged) produced by resonant Auger decay processes. Above the Si2p edge, the fragmentation is found to originate from dissociative double ionization occurring after normal Auger processes. The small additional contribution of triple dissociative ionization is consistent with a second order Auger process. In contrast, the underlying valence ionization continuum is responsible for all parent intensity and most of the SiF+3 fragment expected from the normal dissociative ionization channels. The measurement of appearance energies of fragment ion pairs by the photoion–photoion coincidence method, offers a direct measurement of the first direct double ionization onset in SiF4 and new values for other dissociative SiF2+4 states which complement those found by Auger spectroscopy.
Analytical gradient of the linear combination of Gaussian-type orbitals—local spin density energy
Authors: Fournier R. , Andzelm J. , Salahub D.R.
Published: July 2002

An expression has been derived for the analytical evaluation of the energy gradient within the linear combination of Gaussian-type orbitals—local spin density method. This expression is valid for any exchange-correlation energy functional which can be represented in a density gradient expansion. In practice, because the exchange-correlation terms are fitted with auxiliary functions, one has to introduce an approximation. Results are reported of tests on diatomics that show that it is possible to attain a typical accuracy of ±0.01 a.u. on equilibrium distances, relative to the energy minimum. The formulas for molecular integral derivatives that we implemented are based on the highly efficient recurrence formulas of Obara and Saika. We report here an additional formula for angular momentum transfer which is very useful for efficient programming of the gradient. In all cases studied, the time required to compute the gradient is a fraction of the time spent to solve the self-consistent-field Kohn–Sham equations.
Infinitely polydisperse fluids
Authors: Kofke David A. , Glandt Eduardo D.
Published: July 2002

A generalized semigrand formalism for polydisperse fluids is presented and is used to derive a thermodynamic consistency equation. In the infinitely polydisperse limit—corresponding to a flat distribution of chemical potential differences—a characteristic parameter is eliminated, and the description of the mixture is greatly simplified. In the case of infinitely polydisperse hard spheres, the absence of a characteristic diameter implies that all quantities must scale to the density, which provides the only length. This leads to an exact equation of state which, remarkably, is PV/NkBT=4/3 at all densities. The treatment is generalized, to show that there exists a whole family of stationary composition distributions which have invariant compressibility factors. Monte Carlo simulation is used to verify these results, and applications to other potentials are discussed. Infinitely polydisperse fluids provide a convenient starting point for new mixture theories.
Femtosecond resolved solvation dynamics in polar solvents
Authors: Kahlow Michael A. , Jarzȩba Wl/odzimierz , Kang Tai Jong , Barbara Paul F.
Published: July 2002

The transient solvation of a polar fluorescent probe has been studied by the time resolved Stokes shift technique with roughly five times shorter time resolution than previously reported. New shorter time components in the solvation relaxation function C(t) have been discovered for methanol, propionitrile, and propylene carbonate; the C(t) function for acetonitrile is singly exponential within the limitations of the instrument. The observed C(t) has been compared to theoretical calculations using the dielectric continuum (DC) model for each solvent, with non-Debye expressions for the solvent dielectric response. For methanol the DC model predictions agree closely with experiment. For the polar aprotic solvents propylene carbonate and propionitrile, the shape of the experimental decay is different from the DC predictions, but the average decay times 〈τs〉 are closer to the DC predictions than previously reported. The comparison of theory and experiment is clearly limited by the inconsistencies and limited frequency range of the dielectric relaxation data found in the literature. The dynamic solvation measurements have also been compared to predictions of the mean spherical approximation as applied to solvation dynamics, which appear to give slower solvation rates than are observed experimentally.
Models of rotational relaxation above the glass transition
Authors: Kivelson Daniel , Kivelson Steven A.
Published: July 2002

Below a temperature TB, the rotational relaxation of small molecules in supercooled liquids appears to bifurcate into slow α and faster β relaxations. The characteristic time for the former diverges in Vogel–Fulcher fashion at the glass transition Tg and that of the latter follows Arrhenius behavior from above TB to below Tg. We have examined models for α- and β-rotational relaxation. Above TB, it appears that only β relaxation occurs and that τ1/τ2≊3, where τl is the relaxation time for the lth order spherical harmonic, while below TB it seems that for α relaxation this ratio is closer to unity and for β relaxation it is unknown. The value of three for the ratio is characteristic of rotational diffusion, the smaller ratio of large angular jumps and of restricted diffusion. We have ascribed β relaxation to rapid angular diffusion within a long-lived torsional potential well and the α relaxation to random restructuring of the torsional potential itself. Thus, the appearance of the α relaxation signals the emergence of a longer length scale, which is consistent with the observation that the τ corresponding to the α relaxation appears to diverge at the glass transition.
Theory of activated rate processes for arbitrary frequency dependent friction: Solution of the turnover problem
Authors: Pollak Eli , Grabert Hermann , Hänggi Peter
Published: July 2002

An analytical theory is formulated for the thermal (classical mechanical) rate of escape from a metastable state coupled to a dissipative thermal environment. The working expressions are given solely in terms of the quantities entering the generalized Langevin equation for the particle dynamics. The theory covers the whole range of damping strength and is applicable to an arbitrary memory friction. This solves what is commonly known as the Kramers turnover problem. The basic idea underlying the approach is the observation that the escape dynamics is governed by the unstable normal mode coordinate—and not the particle system coordinate. An application to the case of a particle moving in a piecewise harmonic potential with an exponentially decaying memory-friction is presented. The comparison with the numerical simulation data of Straub, Borkovec, and Berne [J. Chem. Phys. 84, 1788 (1986)] exhibits good agreement between theory and simulation.
A
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o calculation of the hyperfine coupling constants in B2
Authors: Carmichael Ian
Published: July 2002

Hyperfine coupling constants for the ground electronic state of the B2 molecule, [3∑−g] , are computed using correlation procedures based on spin-unrestricted wave functions. Two distinct methods have been employed. Firstly, a variational configuration mixing approach with all configurations arising from single and double replacements in the reference wave function has been augmented by an approximate size-consistency correction. Secondly, a many-body perturbation theory procedure based on the coupled–cluster expansion with infinite-order doubles has been approximately corrected for single and triple excitations through fourth order. Both isotropic and dipolar coupling constants have been obtained as expectation values over the appropriate wave functions for the unrestricted Hartree–Fock (UHF) and the variational configuration-interaction (CI) methods. The anisotropic coupling is already satisfactorily accounted for at the UHF level, while even with the approximate correction for the effect of higher excitations, the variational CI approach still underestimates the isotropic splitting by a factor of 2. The origin of this deficiency is traced using the other correlation method in which the isotropic coupling is derived from the normalized spin density at the nucleus, obtained by finite-field perturbation theory. The effect of the inclusion of triple excitations from the UHF wave function is seen to contribute about one third of the observed value. Such excitations are not accounted for in the size-consistency corrected variational CI approach. This conclusion is confirmed by a similar finding in a full fourth-order Mo/ller–Plesset (MP) calculation.
Electron scattering cross sections and negative ion states of silane and halide derivatives of silane
Authors: Wan Hai-Xing , Moore John H. , Tossell John A.
Published: July 2002

The interaction of low-energy electrons with silane and its halogenated derivatives is a fundamental step in many plasma chemical processes. The total scattering cross-section for electrons in the 0.2–12 eV range are observed to range up to 10−14 cm2. Scattering resonances are assigned to temporary negative ion states on the basis of correlations with analogous inner shell excitation resonances.
Theoretical characterization of selected regions of the ground state potential surface of N2H2
Authors: Walch Stephen P.
Published: July 2002

The results of CASSCF/multireference contracted CI calculations with large atomic natural orbital (ANO) basis sets are presented for selected regions of the N2H2 potential surface. The calculations focused on the addition of a H atom to HN2. This process involves four channels. Approach of the H atom along the HN bond leads to N2+H2 with no barrier (H abstraction), while approach of the H atom to the other three positions leads to trans-HNNH, cis-HNNH, and NNH2, which are stable minima. These calculations provide the most accurate energetics for these three stable species obtained to date. Combining the results of these calculations with previous results for HN2 provides accurate thermochemical data on these important molecules.
Algebraic approach to molecular electronic spectra. I. Energy levels
Authors: Frank A. , Lemus R. , Iachello F.
Published: July 2002

An algebraic approach to molecular electronic spectra is presented. This approach is used to provide a simultaneous description of electronic, vibrational and rotational energy levels in the series of molecules LiH, BeH, BH, CH, NH, and OH.
Bimolecular annihilation reactions with immobile reactants
Authors: Schnörer H. , Kuzovkov V. , Blumen A.
Published: July 2002

We study the bimolecular annihilation reaction A+B→0 with immobile reactants in arbitrary dimension. For this we derive a closed set of integrodifferential equations by using Kirkwood’s superposition approximation to decouple the infinite hierarchy of equations for the many-center correlation functions. We find that for exchange-type interactions and equal numbers of A and B species the reactant concentration n decays as n∼ξ−d/2, where ξ is time-dependent, ξ=ln t, and may be interpreted to be an effective reaction radius. A comparison to numerical simulations shows very good agreement to the theoretical expressions, both for the temporal evolution of particle concentrations and also for the pair-correlation functions; this confirms the validity of the superposition approximation.
Photodissociation and continuum resonance Raman cross sections and general Franck–Condon intensities from S-matrix Kohn scattering calculations with application to the photoelectron spectrum of H2F−+hν→H2+F, HF+H + e−
Authors: Zhang John Z. H. , Miller William H.
Published: July 2002

It is shown how the S-matrix version of the Kohn variational method for quantum scattering can be readily adapted to compute matrix elements involving the scattering wave function and also matrix elements of the scattering Green’s function. The former of these quantities is what is involved in computing photodissociation cross sections, photodetachment intensities from a bound negative ion to a neutral scattering state, or the intensity of any Franck–Condon transition from a bound state to a scattering state. The latter quantity (i.e., a matrix element of the scattering Green’s function between two bound states) gives the resonance Raman cross section for the case that the intermediate state in the Raman process is a scattering state. Once the basic S-matrix Kohn scattering calculation has been performed, it is shown that little additional effort is required to determine these quantities. Application of this methodology is made to determine the electron energy distribution for photodetachment of H2F− to F+H2, HF+H. Resonance structure in the J=0 reaction probabilities is seen to appear in the electron energy distribution.
Structure and dynamics of small I2 . . . Hen van der Waals clusters (n=1–9)
Authors: Garcia-Vela A. , Villarreal P. , Delgado-Barrio G.
Published: July 2002

Energetics and dynamics of van der Waals (vdW) I2⋅⋅⋅Hen clusters are studied in an approximate way by using a model that considers 2n+1 degrees of freedom, that is, the I2 stretch and the 2n stretching and bending modes of the He atoms restricted to move along a plane perpendicular to the I2 axis. For n=2,3 a configuration-interaction treatment is carried out to obtain energy levels. For n=4–8, ground-level energies are estimated from those corresponding to n=2,3 and the geometric relationships among the n vdW bonds. A quasiclassical trajectory approach is used to study the dynamics of these clusters, and lifetimes and half-widths for vibrational predissociation have been calculated for n=1–9. A large increase in the half-width of n=9 with respect to the cases n=1–8 is observed, which would imply the existence of a first coordination shell for He about I2 containing eight atoms. Also, it is found that the mechanisms of dissociation for these clusters become statistical as the number of vdW bonds increases.
Triplet Ps annihilation parameters in ethanol/c
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s-2-pentene/platinum mixtures containing platinum or platinum on charcoal
Authors: Ciottone Judith L. , Lander Frits
Published: July 2002

Positron lifetimes and intensities were compared, with and without external mixing, for simulated hydrogenation reaction mixtures consisting of 2/1, 1/1, 0.25/2, 0.5/2, 0.75/2, 1/2, and 1.5/2 ethanol/cis-2-pentene molar ratios to which 0.6 mg platinum powder or 5% platinum on charcoal were added. For platinum mixtures marked quenching of all o-Ps lifetimes and intensities occurred with external mixing. For 1/2 and 1.5/2 platinum-on-charcoal mixtures, the combined o-Ps intensity increased from 66% to 76% and from 30% to 76%, respectively, with external mixing.
On the geometrical structure of the C+3 cation—an a
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Authors: Martin J. M. L. , François J. P. , Gijbels R.
Published: July 2002

The potential energy surface of the C+3 cation has been investigated using coupled cluster techniques and large basis sets. The results are particularly sensitive towards the level of electron correlation. Spin contamination even produces a ‘‘false stationary point’’ at the UHF/6-31G* level. C+3 has a cyclic 2B2 ground state with predicted geometry r=1.3242 Å, θ=73.06° (MP2/6-311G*, empirically corrected bond distance). At the highest level of theory considered, the linear structure (2Σ+u state) lies about 2 kcal/mol above the ground state: this might imply quasilinearity. There is also a low barrier towards degenerate isomerization: at high temperatures, C+3 will be extremely floppy. Harmonic frequencies (UHF/6-31G*) as well as double-harmonic IR and Raman intensities are given for various structures of C+3. Interesting analogies of C+3 with B3 and B2N are pointed out. The heat of formation at 298.15 K, vertical and adiabatic ionization potentials of C3 are predicted as 194.9±2 kcal/mol, 11.92±0.1 eV, and 11.84±0.1 eV, respectively.
Double-Rydberg anions: Ground-state electronic and geometric stabilities
Authors: Gutowski Maciej , Simons Jack
Published: July 2002

Each member of the class of Double-Rydberg (DR) molecular anions consists of an underlying closed-shell cation core around which a pair of highly correlated electrons move in diffuse orbitals. We have examined the geometric and electronic stabilities of the ground states of candidate DR anions resulting from the following cation cores: H+3 , NeH+, FH+2 , H3O+, NH+4 , and CH+5 . Near the equilibrium geometry of the cation, all of the DR anions, except H−3 , are electronically stable with respect to the corresponding Rydberg radicals. Results of our geometry optimizations indicate, however, that only NH−4 and H3O− are locally geometrically stable; the other DR anions undergo fragmentation. Vertical ionization potentials for the Td isomer of NH−4 and the C3v isomer of H3O− are found to be 0.45 and 0.46 eV, respectively.
Double continuum fragmentation in the vibrational predissociation X⋅⋅⋅BC(v)⋅⋅⋅Y→BC(v′<v)+X+Y of van der Waals complexes: A perturbative treatment
Authors: Villarreal P. , Miret-Artés S. , Roncero O. , Serna S. , Campos-Martínez J. , Delgado-Barrio G.
Published: July 2002

We present an approximate quantal model to study the double continuum problem arising in the complete fragmentation of X⋅⋅⋅BC(v)⋅⋅⋅Y van der Waals(vdW) complexes, where BC is a conventional diatomic molecule vibrationally excited and X and Y are rare gas atoms, through vibrational predissociation (VP). Assuming a near equilibrium geometry of the complex and using an adiabatic approximation for describing the oscillation in the angle formed by the BC⋅⋅⋅X and BC⋅⋅⋅Y weak bonds, the rates for complete fragmentation are expressed in the frame of Fermi’s ‘‘Golden Rule’’. Double continuum wave functions may be obtained by a perturbative treatment that allows one to take properly into account the symmetry of the problem in the particular and very frequent case X≡Y.
Reducing quasi-ergodic behavior in Monte Carlo simulations by J-walking: Applications to atomic clusters
Authors: Frantz D. D. , Freeman D. L. , Doll J. D.
Published: July 2002

A method is introduced that is easy to implement and greatly reduces the systematic error resulting from quasi-ergodicity, or incomplete sampling of configuration space, in Monte Carlo simulations of systems containing large potential energy barriers. The method makes possible the jumping over these barriers by coupling the usual Metropolis sampling to the Boltzmann distribution generated by another random walker at a higher temperature. The basic techniques are illustrated on some simple classical systems, beginning for heuristic purposes with a simple one-dimensional double well potential based on a quartic polynomial. The method’s suitability for typical multidimensional Monte Carlo systems is demonstrated by extending the double well potential to several dimensions, and then by applying the method to a multiparticle cluster system consisting of argon atoms bound by pairwise Lennard-Jones potentials. Remarkable improvements are demonstrated in the convergence rate for the cluster configuration energy, and especially for the heat capacity, at temperatures near the cluster melting transition region. Moreover, these improvements can be obtained even in the worst-case scenario where the clusters are initialized from random configurations.
Molecular dynamics simulation of diffusion of small molecules in polymers. II. Effect of free volume distribution
Authors: Takeuchi Hisao , Roe Ryong-Joon , Mark James E.
Published: July 2002

The influence of the free volume distribution on the diffusion of small molecules in polymers is studied by means of molecular dynamics simulations. A model is used which is composed of 10 O2 molecules, and 600 CH2 groups each belonging to an infinite length chain or its periodic images. A fully vibrational treatment is applied to the chains with a torsional potential and nonbonded interactions. Four model systems, each consisting of chains having the same molecular parameters except for the equilibrium skeletal bond angle, are compared at 300 K and at densities giving the same free volume fraction. The self-diffusion coefficient of oxygen strongly correlates with the average interchain separation; a tight chain packing reduces the diffusion even when the total free volume content is kept constant. The importance of the free volume distribution for diffusion was demonstrated by the good correlation obtainable in terms of the effective free volume available between the nearest neighbor chains.
Optical anisotropy in colloidal crystals
Authors: Monovoukas Yiannis , Fuller Gerald G. , Gast Alice P.
Published: July 2002

We use the theory of dynamical diffraction to interpret polarization-dependent light attenuation phenomena in thin colloidal crystals. We measure light attenuation anisotropies, resulting from coherent and incoherent scattering from a suspension of monodisperse aqueous polystyrene microspheres, with rotary polarization modulating polarimetry. Measured anisotropies are in excellent agreement with dynamical diffraction predictions. These measurements provide a new means to quantify crystal orientation in thin cells. We relate anisotropy to crystal color observed between crossed polarizers and to crystal structure, volume fraction and orientation.
The structure of the C4 cluster radical
Authors: Heath J. R. , Saykally R. J.
Published: July 2002

The first infrared spectrum of gas phase, jet-cooled C4 has been measured by high resolution diode laser absorption spectroscopy. Twelve rovibrational transitions are assigned to the ν3(σu) antisymmetric stretch of linear 3Σ−g C4. No evidence is observed for the bent structure of triplet C4 recently observed in a matrix study by Cheung and Graham [J. Chem. Phys. 91, 6664 (1989)]. Indeed, the measured band origin (1548.9368(21) cm−1) and effective ground state C–C bond length [1.304 31(21)A] are consistent with several ab initio predictions of a rigid, linear, cumulenic structure for this cluster radical.
Ultrafast infrared saturation spectroscopy of chloroform, bromoform, and iodoform
Authors: Bakker H. J. , Planken P. C. M. , Kuipers L. , Lagendijk A.
Published: July 2002

In this paper we present a study of the vibrational energy relaxation processes of chloroform, bromoform, and iodoform in solution after excitation of the C–H stretch vibration. The relaxation is studied with ultrafast infrared saturation spectroscopy using intense infrared pulses with a pulse duration of 19 ps. The experiments were performed in a polar and a nonpolar solvent in order to study the effects of the polarity of the solvent on the relaxation processes. We observe that in both types of solvent the relaxation takes place via two consecutive relaxation processes and that the relaxation leads to ultrafast changes of the absorption band of the C–H stretch vibrations. We discuss the differences in the time constants of the relaxation processes of the haloforms in terms of the energy differences between the vibrational levels and the interactions with the solvent.
Guided ion–beam studies of the ground-state O+2 (2Πg, v=0) reaction with CH4: Effect of internal versus translational energy
Authors: Fisher Ellen R. , Armentrout P. B.
Published: July 2002

Guided ion-beam mass spectrometry is used to study the reactions of methane with O+2 in its ground electronic and vibrational state. In addition to the three previously reported reaction products, CH2OOH+, CH+3, and CH+4, we also observe three other products, CH2O+2, H3O+, and CO+2 . Reactions of excited O+2 ions are also examined and are shown to be more efficient than those for ground-state ions. The thermochemistry and potential-energy surfaces for this reaction are discussed as well as the effects of vibrational, electronic, and translational energy on the reaction system. A heat of formation for CH2O+2 of 201.5±1.6 kcal/mol is measured and tentatively assigned to the methyne hydroperoxy ion structure, HC–O–OH+.
The torsional Raman spectra of partially deuterated ethane molecules. I. One conformer derivatives: CH3CD3, CH3CH2D, CH3CHD2, CH2DCD3, and CHD2CD3
Authors: Fernández-Sánchez J. M. , Montero S.
Published: July 2002

The torsional Raman spectra of one-conformer partially deuterated derivatives of ethane, CH3CD3, CH3CH2D, CH3CHD2, CH2DCD3, and CHD2CD3 have been experimentally investigated with an improved conventional Raman spectrometer. The recorded spectra have been interpreted with a rotational–torsional effective Hamiltonian and a simple model for the torsional dependence of the molecular polarizability. The effective threefold torsional potential parameters for CH3CD3 (V3=997.72 cm−1, V6=8.45 cm−1 ) have been taken from the recent literature, whilst the following parameters were obtained for the other derivatives: CH3CH2D, V3=1007 cm−1, V6=7.3 cm−1; CH3CHD2, V3=1003 cm−1, V6=8.5 cm−1; CH2DCD3, V3=993 cm−1, V6=8.2 cm−1; CHD2CD3, V3=995 cm−1, V6=6.7 cm−1. The estimated accuracy of these parameters is of the order of ±2 cm−1.
Calculation of electronic structure and photoabsorption spectra of monosilane molecules SiH4, SiF4, and SiCl4
Authors: Ishikawa Hideaki , Fujima Kazumi , Adachi Hirohiko , Miyauchi Eizo , Fujii Toshio
Published: July 2002

The electronic structure, ionization potentials, and photoabsorption spectra of monosilane molecules SiH4, SiF4, and SiCl4 were calculated using the discrete variational (DV) Xα method. Valence molecular orbitals (MOs) of SiH4 consist (from the lowest) of two occupied bonding MOs between Si and H, a1 and t2. Inner valence MOs of SiF4 and SiCl4 consist of the bonding MOs between Si and halogen, a1 and t2, and outer valence MOs consist of bonding MOs a1 and t2, and the MOs e, t2, and t1 localized on halogen. The lowest unoccupied MOs of SiH4 include two antibonding states t2 and a1, and two localized states, e and t2. The lowest unoccupied MOs of SiF4 and SiCl4 are antibonding states a1 and t2 between Si and halogen. Calculated ionization potentials agree well with measured photoelectron spectra.

Calculation of the photoabsorption spectrum for Si 2p core excitation for SiH4, SiF4, and SiCl4 shows that peak positions and intensities agree well with measured photoabsorption spectra in both gas and solid phases. The absorption bands of SiH4, measured near the edge and at about 125 eV, consist of transitions from core to antibonding states consistent with experiments. The four main absorption bands of SiF4 and SiCl4 measured between 105 and 140 eV are assigned to transitions from the core Si 2p level to antibonding MOs a1, t2, e, and t2. Calculated photoabsorption spectrum for valence excitation of SiH4 agrees well with measurements. The first and second absorption bands measured at about 138 and 128 nm correspond to the transition from bonding to antibonding states between Si and H. This is consistent with the facts that monosilane is photolyzed in Xe 147 nm ultraviolet light in a vacuum and that fluorescence has not been observed from 130 to 150 nm, because these are interpreted in terms of photodissociation by transition from bonding to antibonding states. Photoabsorption spectra for valence excitation of SiF4 and SiCl4 are also calculated. We found that the first absorption band consists of two transitions from localized states on halogen to antibonding states between Si and halogen (t2→a1 and t1→t2 ).
Rotational strengths and dissymmetry factors for molecular rotation of NHDT, PHDT, and the chiral deuterated oxiranes
Authors: Salzman W. R.
Published: July 2002

The known experimental molecular g tensors for NH3, PH3, and oxirane are translated and rotated to give g tensors in the new principal axis coordinate systems produced by isotopic substitution for hydrogen. The new g tensors are used to calculate rotational strengths and Kuhn’s dissymmetry factors for transitions among the eight lowest rotational energy levels of NHDT, PHDT, and the chiral deuterated oxiranes. Predictions are made of the rotational strengths and dissymmetry factors (including sign) for specified absolute spatial configurations of the model molecules. The dissymmetry factors range from 10−5 to 10−9 and some of the larger ones are associated with transition dipole moments greater than 0.02 Debye. Dissymmetry factors of some of the transitions are no more than an order of magnitude smaller than current vibrational circular dichroism capability.
Comprehensive theory of the Fourier transform ion cyclotron resonance signal for all ion trap geometries
Authors: Grosshans Peter B. , Shields Patrick J. , Marshall Alan G.
Published: July 2002

An analytic solution for the charge induced on each of the detection electrodes of a Fourier transform ion cyclotron resonance (FT/ICR) ion trap has been derived from basic electrostatics for both tetragonal and cylindrical traps of arbitrary aspect ratio, by use of a Green’s function formalism. Dunbar has shown that the result of that calculation is in general equivalent to that obtained from prior ‘‘reciprocity’’-based methods (see text). A primary advantage of the present treatment is its variety of functional forms arising from the various forms of the Green’s function, some of which may converge much more rapidly in numerical evaluation. (Moreover, because the Green’s function is the potential field of a unit point charge, the Green’s function must be employed in any treatment of ion–ion repulsions.)

The present results (a) exactly confirm prior analyses of the cubic and tetragonal traps; (b) provide the first complete analysis of the cylindrical trap; and (c) may be extended to any trap geometry for which the Green’s function is known. In the absence of an available Green’s function, the reciprocity-based treatment, either analytic or numerical, is the method of choice to solve for the induced charge for any ion trap geometry (e.g, unbroken or segmented hyperbolic). For circular orbits centered on the longitudinal axis of the trap, the presence of spectral components at odd multiples of the fundamental ICR orbital frequency is explained and a closed form solution for the relative magnitudes of these components is presented for tetragonal and cylindrical traps. The ratio of the spectral peak height at the third harmonic to that at the first (i.e., the ratio of the third to first Fourier coefficients) is a strong monotonic function of orbital radius; thus, measurement of that ratio provides a simple and direct means for determining the cyclotron orbital radius and hence its orbital translational energy.

The presence and location of magnetron and trapping sidebands of the fundamental peak are also predicted. In addition, we show that a cylindrical trap whose ring electrode is divided into equal quadrants is only slightly more sensitive than a tetragonal trap of the same aspect ratio. Finally, we develop a general circuit model which relates the charge induced on one or more detection electrodes to the detected voltage (i.e., the unamplified signal). Since the effect of trapped-ion motion on each detection electrode is modeled as a charge (or current) source relative to ground, the net signal from any given electrode arrangement and interconnection scheme can be accommodated simply by adding, subtracting, or grounding the signal from each detection electrode, e.g., single-electrode detection, Comisarow’s differential detection between two electrodes, or any of various multiple-electrode configurations. From the measured ICR signal and ICR orbital radius, the number of coherently orbiting ions may be determined.