bluuebunny/crossref_metadata_2025 huggingface dataset:
Random sequential multilayer deposition of different-sized k mers on a one dimensional infinite substrate
Authors: Bhattacharjee Biplab , Rangarajan S. K.
Published: July 2002

Kinetics of random sequential, irreversible multilayer deposition of macromolecules of two different sizes on a one dimensional infinite lattice is analyzed at the mean field level. A formal solution for the corresponding rate equation is obtained. The Jamming limits and the distribution of gaps of exact sizes are discussed. In the absence of screening, the jamming limits are shown to be the same for all the layers. A detailed analysis for the components differing by one monomer unit is presented. The small and large time behaviors and the dependence of the individual jamming limits of the k mers and (k−1) mers on k and the rate parameters are analyzed.
Density-functional theory of nonuniform classical liquids: An extended modified weighted-density approximation
Authors: Likos C. N. , Ashcroft N. W.
Published: July 2002

An extension of the modified weighted-density approximation (MWDA) is presented which retains the key features of the original MWDA in that it continues to describe the nonuniform system through the use of low-order correlation functions of the uniform counterpart. However, the approximate free energy functional is now exact up to third order in the functional expansion of the free energy, and therefore requires as input both the second- and third-order direct correlation functions of the uniform liquid, as well as its excess free energy per particle. The theory has been applied previously to the problem of isochoric freezing of the classical one-component plasma, and is here applied to the well-known problem of isobaric freezing of hard spheres. We use two different approaches to describe the less well-known third-order direct correlation function of the uniform liquid. The first approach is representative of a class of models for this function that are derived through three functional density differentiations of an approximate free energy model. The second is a factorization ansatz based on liquid-state diagrammatic expansions. The results are quite sensitive to these choices: The first leads to an improvement over the already satisfactory results of the original MWDA for the hard-sphere system, whereas the second does not lead to freezing at all. These differences are traced to the ways in which the approximations treat long-range and short-range potentials.
Treatment of zero-point motions in cluster dynamics: Semiclassical time-dependent self-consistent-field simulation of (Ne)N
Authors: Li Zhiming , Gerber R. Benny
Published: July 2002

A semiclassical time-dependent self-consistent-field (TDSCF) method is developed for dealing with the difficulties caused by the nonpreservation of zero-point energy in classical molecular dynamics simulation. The method is applied to a collinear model of a (Ne)12 cluster at very low temperatures. Classically, this system dissociates rapidly due to its zero-point energy. We show that the system remains stable when treated by the new method. The normal mode dynamics of the anharmonic cluster are calculated and discussed. Interesting results are obtained on the lifetimes of single-mode states and energies due to the coupling between the modes. Some of the single-mode states have subpicosecond lifetimes, while others are stable for at least 60 ps. The results illustrate the power of semiclassical TDSCF as a tool for studying the vibrational dynamics of anharmonic cluster at very low temperatures.
Free-jet infrared absorption spectroscopy of the C2H2–Ar complex in the doubly degenerate monomer C–H bending region
Authors: Ohshima Yasuhiro , Matsumoto Yoshiyasu , Takami Michio , Kuchitsu Kozo
Published: July 2002

Vibration–rotation transitions of the C2H2–Ar complex have been observed in the doubly degenerate monomer C–H bending (ν5) region via direct IR absorption of tunable diode laser radiation in a pulsed supersonic free jet. The spectrum is composed of in-plane and out-of-plane C–H bending bands, which are coupled with each other through a strong Coriolis interaction. Anomalous Coriolis coupling and A rotational constants indicates the failure in a semirigid-molecule description of this highly nonrigid system in the doubly degenerate bending manifold. A dynamical model calculation, where the large-amplitude van der Waals bending motion is treated as an internal rotation of the C2H2 monomer hindered by an anisotropic intermolecular potential, has revealed that the characteristic energy-level structure originates from its specific angular momentum coupling scheme involving an additional angular momentum for the intramolecular degenerate bending. The difference potential providing the parity doubling in the C–H bending manifold is estimated to be only about −0.1 cm−1.
Bond properties of Be3–7 clusters
Authors: Sudhakar Pamidighantam V. , Lammertsma Koop
Published: July 2002

Geometries, binding energies, and bonding properties have been investigated for the Be3–7 clusters with ab initio molecular orbital theory using double (6-31G*) and triple [6-311(+)G*] ζ-basis sets with inclusion of the effects of electron correlation estimated with the Mo/ller–Plesset perturbation (up to full MP4) and coupled cluster (QCI) theories. The geometries and binding energies suggest that the single-reference approach gives already reasonable results for Be5. For the larger clusters Bader’s electron density analyses reveals small concentrations of charge within groups of three Be atoms.
Correlation and pairwise exchange in the metal trihydrides
Authors: Hiller Eric M. , Harris Robert A.
Published: July 2002

The model of Hiller and Harris [J. Chem. Phys. 98, 2077 (1993)] is modified by a Jastrow correlation function to incorporate short-range correlations. The resulting model describes exchange in the metal trihydrides at low temperature. Direct and virtual exchange processes are compared for upper and lower bounds to the repulsive proton–proton potential, eight different locally isotropic double-well frequencies, and four well separations. Exchange through the virtual double-occupancy state dominates the coupling, although direct exchange significantly renormalizes the tunneling matrix element. It is shown that a simpler two-state Jastrow–Gaussian Heitler–London basis used in other calculations suffers in this strongly correlated system.
Electronic structure of the hollow-cage M8X12 clusters
Authors: Lou L. , Guo T. , Nordlander P. , Smalley R. E.
Published: July 2002

The electronic structure of recently discovered hollow-cage molecule Ti8C12 has been calculated and compared to similarly shaped hypothetical molecules M8X12 (M=metal and X=B, C, or N). The cohesion of the cage is related to the σ-bonding between M–X arising from d–sp hybridization. It is also shown that the cage can become electronically closed shell with all the valence electrons paired when the carbon is replaced by boron or nitrogen, e.g., in the formula of Sc8B12.
A (3+1) resonance enhanced multiphoton ionization study of the C 1Σ+ and E 1Π states of CO: Polarization dependence used to probe electronic excitation routes and electronic character
Authors: Koeckhoven S. M. , Buma W. J. , de Lange C. A.
Published: July 2002

The three-photon excitations of the C 1Σ+ and E 1Π states have been investigated with resonance enhanced multiphoton ionization spectroscopy as a function of the polarization of the excitation light. From the observed polarization dependence the contributions of the various excitation routes have been determined, and the electronic character of these states has been analyzed. The results indicate that the v′=0 and 1 vibrational levels of the E 1Π state have slightly different electronic characters.
The static dielectric constant of SPC and TIP4P water by perturbation theory
Authors: Goldman Saul , Joslin Chris
Published: July 2002

A second-order perturbation theory that was previously applied to hard-sphere-dipolar and Stockmayer fluids is extended to molecules with spherical cores and discrete point charge distribution of arbitrary symmetry. We use a recently published improvement on the Kirkwood superposition approximation for the reference state triplet distribution function, and we apply the theory to obtain the static dielectric constant (ε) of SPC and TIP4P water at ∼1 gm cm−3 and a range of temperatures. By comparison with the results of simulations on these models we find that the theory works up to y≂3 (where y is the dipolar strength function) but overestimates ε beyond y≂3. y≲3 is also the range of validity of the theory when applied to a Stockmayer fluid, but for the latter the theory underestimates ε for y≳3. We physically interpret our results for the angular correlation parameter (G) by reference to structures that are believed to be important in real water and in dense dipolar fluids.
Photodissociation of ICN in solid and in liquid Ar: Dynamics of the cage effect and of excited-state isomerization
Authors: Krylov A. I. , Gerber R. B.
Published: July 2002

Photodissociation of ICN by UV excitation in solid and liquid Ar is studied by molecular dynamics simulations. The focus is on the differences between the cage effects on the CN photoproduct in the two phases, and on the excited state isomerization ICN*→INC* dynamics in the solid matrix. Nonadiabatic transitions are neglected in this first study. The main results are: (1) No cage exit of the CN product is found in solid Ar, even in simulations at temperatures close to melting and for large excess energies. The result is in accord with recent experiments by Fraenkel and Haas. This should be contrasted with the large cage-exit probabilities found in many systems for atomic photofragments. The result is interpreted in terms of geometric and energy transfer considerations. It is predicted that complete caging of diatomic and larger photofragments will be typically the case for photodissociation in rare-gas matrices. (2) Almost 100% cage-exit probability for the CN product is found for ICN photolysis on the 1Π1 potential surface in liquid Ar. On the other hand, photolysis on 3Π0+ potential surface does not lead to cage exit on a time scale of 15 ps. The large differences between the reaction in the solid and in the liquid, and between the behavior of the process on the 3Π0+ and the 1Π1 potentials, respectively in the liquid, are interpreted. (3) CN rotational dynamics and subsequent relaxation leads to isomerization in the excited electronic states. On the 3Π0+ potential surface one finds after t≳0.5 ps roughly equal amounts of the ICN and INC isomers. On the 1Π1 surfaces only INC is found after t≳3.5 ps. This is explained in terms of the barriers for CN rotation in the two excited states, and in terms of the time scales for rotational relaxation. The results throw light on the differences between cage effects for photochemical reactions in solid and in liquid solution, and on cage-induced isomerization dynamics in solid matrices.
Electron stimulated desorption of NO from step sites on Pt(112): The role of chemisorption site geometry on the cross section
Authors: Heiz U. , Xu J. , Yates J. T.
Published: July 2002

The electronic excitation of adsorbed NO on Pt(112) has been observed to cause mainly desorption. Species-specific total desorption cross sections of terminally bound and bridged-bound NO present exclusively on the step sites of Pt(112) have been measured using Fourier transform infrared reflection absorption spectroscopy (FT-IRAS) as a sensitive detector of the surface coverage of various chemisorbed NO species. Other electron stimulated processes which might influence the measurement of these cross sections, such as electron stimulated fragmentation, site-exchange, and occupied site density effects have been found to be unimportant. It was found for 275 eV electrons that terminally bound NO desorbs with a larger cross section (2.3×10−18 cm2) than bridged-bound NO, which has a desorption cross section of 8.7×10−19 cm2. This is the expected relationship between the total desorption cross sections for these two species, with the smaller cross section being observed for the bridged NO.
Comment on ‘‘The weakest bond: Experimental observation of helium dimer’’ [J. Chem. Phys. 9
8, 3564 (1993)]
Authors: Meyer Eric S. , Mester John C. , Silvera Isaac F.
Published: July 2002

Recently, Luo et al. reported indirect evidence for the formation of dimers in a supersonic expansion of helium. They interpreted their data as confirmation of the existence of a bound state of 4He–4He with a binding energy of order 1 mK. We show that their data are actually consistent with the formation of helium trimers and not dimers. The two control experiments that they performed do not provide any evidence for discarding this alternative hypothesis.
On the static dielectric function of the sticky electrolyte model
Authors: Pizio Orest , Quintana Jacqueline
Published: July 2002

The static dielectric function of the sticky electrolyte model of electrolyte solutions is studied. We consider a model with bonding distance L=1/2. It contains steric saturation for association at dimer level. At increasing polarity of the diatomic molecules the ordering in the system changes and is manifested in the shape of the static dielectric function ε(k). The short ranged order is characterized by the presence of negative branch of ε(k).
A quantum state vector phase space representation
Authors: Harriman John E.
Published: July 2002

A phase space representation is defined for quantum mechanical state functions. General requirements of quantum mechanics and some additional, very reasonable restrictions lead to a representation that is uniquely determined apart from a scaling. With one choice of this scaling, the representation is equivalent to that of Bargmann, although different aspects are emphasized here. Equivalence between position and momentum representations leads to a phase space representation that is invariant to, or at most rescaled by, Fourier transform. Phase space equivalents of some common functions are presented including Cartesian Gaussian basis functions and the hydrogen atom ground state function.
The anisotropic potential energy surfaces of H2, N2, and Ar with C2H2 from total differential scattering experiments
Authors: Yang Moonbong , Watts Robert O.
Published: July 2002

The total differential scattering cross sections for H2, N2, and Ar with C2H2 were measured on an in-plane crossed molecular beam apparatus at collision energies of 1051, 1056, and 929 K, respectively. Well resolved rainbow oscillations for N2 and Ar, and diffraction oscillations for H2, showed two interference effects. Effective spherical and anisotropic potentials were obtained from analysis using single channel and infinite-order-sudden (IOS) methods, based on the semiclassical Jeffreys–Wentzel–Kramers–Brillouin (JWKB) approximation. The damping of oscillations showed the importance of anisotropy in the pair potentials. Total differential scattering cross sections calculated using the IOS method were in excellent agreement with the experimental data.
A variational method for the calculation of rovibrational energy levels of triatomic molecules using a Hamiltonian in hyperspherical coordinates: Applications to H+3 and Na+3
Authors: Carter Stuart , Meyer Wilfried
Published: July 2002

A full variational procedure is presented for the calculation of rovibrational (J≳0) energy levels which is particularly suited for triatomic potentials that support large amplitude motions and that may be of high permutational symmetry. It is based on a kinetic energy operator expressed in hyperspherical coordinates (ρ,Θ,Φ). Particular attention is paid to the singularities of this operator in the derivation of the primitive basis functions, which should exactly cancel all singularities, and in their subsequent contractions. The method is applied to the D3h molecules H+3 and Na+3, for which converged rovibrational energies are calculated for J=0, 1, 2 to 25 000 cm−1 for H+3 and to 1250 cm−1 for Na+3, respectively. A spectral analysis of these energy levels is undertaken. For the lowest ten vibrational levels of H+3, converged rovibrational energies up to J=10 are also calculated. These energies, which extend previous calculations, should prove useful in the interpretation of the observed spectra. Our J=1 results for H+3 compare well with recently published values. The results for Na+3 constitute new data which supplement our previous J=0 calculations. It is shown that the spectrum of this molecule can be expressed very well by an appropriate effective Hamiltonian, quite in contrast to H+3. In order to verify that the method is general, J≳0 calculations are performed for model potentials of H2O+ (C2v) and HLiH− (D∞h). The results are in full agreement with those from a proven variational method in valence coordinates.
Transient, collision-induced dipoles in pairs of centrosymmetric, linear molecules at long range: Results from spherical-tensor analysis
Authors: Li X. , Hunt K. L. C.
Published: July 2002

In dense gases or liquids, interactions between linear, centrosymmetric molecules induce transient dipole moments, and thus give rise to far-infrared absorption. For line shape analyses of collision-induced rototranslational absorption spectra, the net dipole of a pair of D∞h molecules is needed in a symmetry-adapted form, in terms of spherical harmonics of the orientation angles for the interacting molecules and the intermolecular vector R, and induced-dipole coefficients DλAλBdλL(R). In this work, we have used spherical-tensor analysis to obtain the contributions to the coefficients DλAλBλL(R) due to multipolar induction, local field nonuniformity, back induction, and dispersion through order R−7. Our analysis explains interrelations among coefficients with different values of λA, λB, λ, and L, for each of the long-range polarization mechanisms; in addition, it provides compact equations for DλAλBλL(R) that are useful for line shape computations.
Nonoptical excited state spectroscopy of CHF2Cl: Characterization of nondipole n→σ* valence transitions by angle-resolved electron energy loss spectroscopy
Authors: Ying J. F. , Leung K. T.
Published: July 2002

A nondipole low-lying excitation feature at 8.0 eV in difluorochloromethane (CHF2Cl) has been observed in angle-resolved electron energy loss spectra obtained at 2.5 keV impact energy. Based on the results of a single-excitation configuration interaction calculation, we assign this feature predominantly as electronic transitions from Cl nonbonding 14a′ and 7a″ orbitals [the highest occupied molecular orbital (HOMO) and second HOMO, respectively] to an antibonding σC–Cl* 15a′ orbital (the lowest unoccupied molecular orbital), the so-called n→σ* type transitions. The generalized oscillator strength (GOS) profile of this transition has been determined and found to be similar in shape and magnitude to that of a recently reported nCl → σC–Cl* (7e→11a1) transition in CF3Cl [Ying et al., Chem. Phys. Lett. 212, 289 (1993)]. Both GOS profiles are found to have a shape characteristic of a quadrupole transition, with a maximum at momentum transfer of ∼1 a.u. The similarity in the nature of these nCl → σC–Cl* transitions in CHF2Cl and CF3Cl to that of a p-to-p transition in a chlorine atom can be demonstrated qualitatively by using contour maps of the density functions of the dominant initial-state and final-state orbitals generated from ab initio self-consistent field wave functions. The homology of transition-related properties (including the GOS profile) among the nCl → σC–Cl* transitions in ‘‘simple’’ polyatomic molecules that contain a single highly localized C–Cl bond is also discussed. Furthermore, the calculated potential energy diagram for CHF2Cl along the C–Cl bond direction suggests that electronic transitions from the 14a′ and 7a″ orbitals to the 15a′ orbital may lead to dissociation of the C–Cl bond. Such predissociation may represent a possible common consequence of the nCl → σC–Cl* transitions in these monochloro-substituted chlorofluorocarbons and related derivatives.
Collisional excitation of very high rotational levels of HF in small angle scattering with He+ at Elab=25–50 eV
Authors: Ruhaltinger T. , Sathyamurthy N. , Toennies J. P. , Wang R. G.
Published: July 2002

Energy loss distributions of He+ ions after collisions with HF molecules in a secondary beam at laboratory collision energies in the range 25–50 eV show a regular structure indicating rotational excitation up to ΔJ=35 with no clear evidence for vibrational excitation. The observed propensity for large ΔJ transitions decreases with increasing collision energy but increases with increase in scattering angle. Three dimensional quasiclassical trajectory calculations using a potential energy surface based on the He+–Ne interaction at short range and a damped long range ion–dipole (quadrupole) interaction reproduce all the essential features of the experimental results.
Ab initio relativistic configuration interaction calculations of the spectrum of bismuth oxide: Potential curves and transition probabilities
Authors: Alekseyev Aleksey B. , Liebermann Heinz-Peter , Buenker Robert J. , Hirsch Gerhard , Li Yan
Published: July 2002

A series of configuration interaction calculations employing relativistic effective core potentials including the spin–orbit interaction is reported for the X1 2Π1/2 ground and numerous low-lying excited states of the bismuth oxide molecule up to 30 000 cm−1. Special difficulties connected with the treatment of open-shell systems and double-group irreducible representations are discussed and a feasible computation scheme is developed for dealing with them. The spin–orbit interaction is found to cause a high level of mixing between a variety of low-lying λ–s states, producing a number of avoided crossings which play a key role in determining the character of the BiO spectrum. A comparison with existing experimental data for both the energy locations and intensities of a large number of band systems indicates that the present calculations are capable of predicting Te values to an accuracy of 0.1–0.2 eV. Corresponding radiative lifetime results generally agree within a factor of 2, with the best experience occurring for relatively strong transitions. The state which was originally assigned as A 2Π1/2 actually turns out to be dominated by the 4Π λ–s state. The corresponding state with Ω=3/2 has recently been discovered by Fink and Shestakov and is found to undergo a strong nonadiabatic interaction with the X2 2Π3/2 state. Two other related states A3 4Π5/2 and A4 4Π1/2 are predicted by the present calculations, but have not yet been verified experimentally. Similarly, the L1 2Φ7/2 and L2 2Φ5/2 states found in the present work have also not yet been observed.
Solvated alkali atoms and electron-transfer paramagnetic ion pairs: Some common trends from computer simulation studies
Authors: Fois Ettore S. , Gamba Aldo
Published: July 2002

Quantum molecular dynamics simulations have been performed for an alkali atom (sodium) diluted in a model dipolar fluid, at various solvent densities, and for the sodium/pyrazine ion pair in the same liquid. The excitonic state proposed by Logan, i.e., an hybrid s/p dipolar atom, is found to be the ground state of the solvated sodium. The Na atomic dipole increases with solvent density. The dipole moment of the Na/pyrazine pair increases with solvent density as well: In this case the Na self-excitation is replaced by electron transfer from Na to pyrazine resulting in a larger dipole moment stabilized by the solvent. Our results compare favorably with available experimental data.
Millimeter- and submillimeter-wave spectroscopy of dibridged Si2H2 isotopomers: Experimental and theoretical structure
Authors: Bogey M. , Bolvin H. , Cordonnier M. , Demuynck C. , Destombes J. L. , Császár A. G.
Published: July 2002

Various isotopomers of the free dibridged disilyne molecule, Si(H2)Si, have been observed by millimeter- and submillimeter-wave spectroscopy in a silane–argon plasma produced by an abnormal electric discharge. In order to make measurement of the weak absorption lines possible a novel computer processing treatment has been developed. From the molecular constants measured for 28Si(H2)28Si, 29Si(H2)28Si, 30Si(H2)28Si, and 28Si(D2)28Si an accurate substitution structure has been deduced with rs(Si–Si)=2.2154 Å, rs(Si–H)=1.6680 Å, and ∠(HSiSiH)=104.22°. The spectrum analysis and the structure determination have been aided by correlated level ab initio calculations resulting in accurate estimates of the equilibrium geometry and rotational constants, the cubic force field, the quartic and sextic centrifugal distortion constants, and the inversion barrier height of dibridged disilyne. The barrier to inversion of the ‘‘butterfly-type’’ Si(H2)Si molecule must be relatively high as no splitting due to inversion could be experimentally observed.
Ultraviolet spectroscopy of the B 2A′ and C 2A″ states of FCO
Authors: Maricq M. Matti , Szente Joseph J. , Su Yi , Francisco Joseph S.
Published: July 2002

Gas phase uv spectra of FCO obtained from the flash photolysis of F2, or F2CO, in the presence of CO are presented. The spectrum consists of three prominent progressions assigned to the B and C states of FCO and a fourth, weaker, progression which may belong to the second excited 2A′ state. Analysis of the B 2A′ state progressions provides vibrational frequencies of ω1=1180±35 cm−1 and ω2=770±30 cm−1 and a zero point energy in the range 24 000<T0<27 000 cm−1. The spectrum of the C 2A″ state yields ω1=1458±6 cm−1 and ω2=651±5 cm−1 and a zero point energy of T0=28 500±25 cm−1. The electronic energies and vibrational frequencies are compared to recent ab initio calculations.
Molecular theory of nonpolar solvation dynamics
Authors: Bagchi Biman
Published: July 2002

Recently Berg and co-workers have reported interesting time resolved studies of ultrafast solvation dynamics in a completely nonpolar liquid. The relevant time correlation function was found to exhibit many fascinating properties. It was biphasic and highly nonexponential in the supercooled liquid near the glass transition temperature. In this article we present a general microscopic theory of such solvation phenomena. It is found that in contrast to solvation of charged species in polar solvents, the nonpolar solvation dynamics in controlled essentially by the dynamic structure factor of the liquid; the orientational relaxation is seen to play a less important role. As the dynamic structure factor shows anomalous behavior in the supercooled liquid, so does the solvation time correlation function. The theory suggests that nonpolar solvation dynamics can be an important scale dependent probe of microscopic dynamical processes in the supercooled liquid.
Perylene–rare-gas heteroclusters. I. Electronic spectroscopy
Authors: Bahatt Dar , Heidenreich Andreas , Ben-Horin Narda , Even Uzi , Jortner Joshua
Published: July 2002

In this paper we report on the electronic two-photon two-color near threshold spectroscopy of mass-resolved perylene⋅Arn (n=1–45), perylene⋅Krn (n=1–35), perylene⋅(N2)n (n=1–12), and perylene⋅(CH4)n (n=1–10) heteroclusters. The S0→S1 inhomogeneously broadened spectra of perylene⋅Arn (n=1–6) and perylene⋅Krn (n=1–4) exhibit resolved spectral features, which were assigned on the basis of experimental combination rules and polarizability relations to the electronic origins of distinct two-sided and one-sided structural isomers. Larger perylene⋅An (A=Ar, Kr; n=6–10) heteroclusters exhibit an ‘‘abnormal’’ specific size dependence of the red spectral shifts, which decrease with increasing n and reach a local minimum at n=8. Similar characteristics of the red spectral shifts are exhibited for perylene⋅(N2)n and perylene⋅(CH4)n (n=4–8) heteroclusters. This abnormal size dependence of the spectral shifts is attributed to the dominance of one-sided single-layered and double-layered structural isomers in this cluster size domain. On the basis of the comparison between the spectroscopic data and molecular dynamics simulations of the absorption line shapes we have obtained a quantitative description of isomer-specific structures for n=2–6, a semiquantitative description of the abnormal size domain for n=6–10 (due to the dominance of one-sided structures with the abundance of two-layered structures increasing at higher n), the prevalence of one-sided structures for n=16 and n=22, and the realization of two-sided multilayered structures at n=45.
TixZryC12 and TixHfyC12 (x+y=8): Binary metal metallocarbohedrenes
Authors: Cartier S. F. , May B. D. , Castleman A. W.
Published: July 2002

We report the production of binary metal metallocarbohedrenes. Direct laser vaporization of mixtures of zirconium or hafnium powders with titanium carbide yield the species TixZryC12 and TixHfyC12 (x+y=8). These results are discussed in the context of recent theoretical models addressing the stabilities and structures of the metallocarbohedrenes, their formation mechanism and possible implications for the synthesis of ‘‘doped’’ metallocarbohedrenes.
Quenching rate constants of excited halogen atoms in quartet states
Authors: Mizuta K. , Kuramasu T. , Ishikawa Y. , Arai S.
Published: July 2002

Excited halogen atoms in quartet spin states F*(2p43s, 4P5/2), Cl*(3p44s, 4P5/2), and Br*(4p45s, 4P5/2) were produced from helium sensitized radiation chemical decomposition of SF6, CF3Cl, CF3Br, and CF2Br2. Quenching rate constants of these excited halogen atoms by simple gas molecules such as O2, N2, H2, CO, CO2, NO, NO2, N2O, CH4, C2H6, and Xe including parent molecules were determined from absorption decay curves at 685.8 nm for F*, 837.5 nm for Cl*, and 827.4 nm for Br*. The optical densities were assumed to be proportional to (number of excited atoms per one cubic centimeter)0.9. The quenching rate constants obtained here were compared to those reported of metastable rare-gas atoms and an excited oxygen atom O*(2p33s, 5S2), and further discussed in terms of several theoretical kinetic models.
Derivation of Kramers’ formula for condensed phase reaction rates using the method of reactive flux
Authors: Tannor David J. , Kohen Daniela
Published: July 2002

Kramers’ formula for the rate of barrier crossing as a function of solvent friction is here rederived using the method of reactive flux. In the reactive flux formalism trajectories are started at the top of the barrier and propagated forward for a short time, to determine whether they are reactive or not. In isolated molecules it is customary to associate with each set of initial conditions a reactivity index (traditionally known as the characteristic function), which is 1 for a reactive trajectory and 0 for a nonreactive trajectory. In this paper we suggest that if the solvent interaction with the system is treated stochastically, it is appropriate to generalize the reactivity index to fractional values between 0 and 1, to take into account an ensemble average over different stochastic histories. We show how this fractional reactivity index can be calculated analytically, by using an analytic solution of the phase space Fokker–Planck equation. Starting with the distribution δ(x)δ(u−u0) that originates at the top of a parabolic barrier (x=0) at t=0, the fraction of the distribution function that is to the right of x=0, in the limit that t→∞, is the fractional reactivity index. The analytical expression for the fractional reactivity index leads immediately to Kramers’ expression for the rate constant. The derivation shows explicitly that the dynamical origin of Kramers’ prefactor is trajectories that recross the barrier. The evolution of the phase space distribution that originates at the top of the barrier highlights an interesting underlying phase space structure of this system, which may be considered as a paradigm for dissipative systems whose underlying dynamics is unstable.
Exponential transformation of molecular orbitals
Authors: Hutter Jürg , Parrinello Michele , Vogel Stefan
Published: July 2002

In this paper we present new formulas related to the parametrization of molecular orbitals by exponential transformations. We show that it is not necessary to use series expansions of the exponentials and that the first derivative of the energy with respect to independent parameters can be calculated at arbitrary positions in an efficient manner. These formulas can be implemented such that computational costs are of order N2M and storage requirements of order NM, where N is the number of occupied orbitals and M the size of the basis set. We discuss possible applications to Hartree–Fock calculations and ab initio molecular dynamics.
Role of the HNO3↔NOH Isomerization in reactions (i) NH(3Σ−)+O(3P) and (ii) N(4S)+OH(2Π): Ab initio calculations and quantum statistical Rice–Ramsperger–Kassel analysis of the potential energy surfaces
Authors: Sengupta Debasis , Chandra A. K.
Published: July 2002

The minimum energy potential energy surfaces for combination of NH(3Σ−)+O(3P) to form HNO(1A′,3A″) and N(4S)+OH(2Π) to form NOH(3A″), and the isomerization of HNO(1A′,3A″) to NOH(1A′,3A″), decomposition of NOH(1A′,3A″) to N(4S)+OH(2Π) as well as to H(2S)+NO(2Π) and HNO(3A″) to H(2S)+NO(2Π) have been obtained by the ab initio methods with geometry optimization at the 6–311G**/MP2=full level with corrections for electron correlation at the MP4SDTQ=full level. At all stationary points on the potential energy surfaces (PES), correction for the zero point vibrational energies are made. The addition reactions to form energized adducts have then been analyzed using a bimolecular version of the quantum statistical Rice–Ramsperger–Kassel (QRRK) theory at different temperatures and pressures. Our analysis predicts that at all temperatures isomerization of HNO(3A″) to NOH(3A″) and its reverse isomerization are important. Formation of NO(2Π) in the interstellar clouds can take place from decomposition of NOH(3A″) as well as HNO(3A″).
Line shape asymmetries in Ar-broadened HF(v=1–0) in the Dicke-narrowing regime
Authors: Pine A. S.
Published: July 2002

Collisional line shapes have been measured for the fundamental band transitions of HF in an Ar buffer gas at T=296 K using a tunable difference-frequency laser spectrometer. The broadening and shift coefficients are in excellent agreement with recent close-coupling scattering cross section calculations [Green and Hutson, J. Chem. Phys. 100, 891 (1994)] based on a realistic intermolecular potential surface determined by high-resolution infrared and microwave spectroscopy of the Ar–HF van der Waals complex. Below atmospheric pressure, the line shapes exhibit strong collisional (Dicke) narrowing of the Doppler distribution and a slight asymmetry which we model with hard collision (Rautian) or soft collision (Galatry) profiles modified for partial correlation between velocity- and state-changing collisions.
Turing patterns and solitary structures under global control
Authors: Pismen L. M.
Published: July 2002

Striped Turing patterns and solitary band and disk structures are constructed using a three-variable multiscale model with cubic nonlinearity and global control. The existence and stability conditions of regular structures are analyzed using the equation of motion of curved boundaries between alternative states of the short-range component. The combined picture of transitions between striped and spotted patterns with changing level of global control is in qualitative agreement with the results of the computer experiment by Middya and Luss [J. Chem. Phys. 100, 6386 (1994)].
Hierarchical structure in the 3ν1 band of propyne
Authors: Davis Michael J. , Bethardy G. A. , Lehmann Kevin K.
Published: July 2002

A hierarchical analysis of the 3ν2 band of propyne shows that there are distinct tiers of states and demonstrates that the major avenue for vibrational mixing is anharmonic coupling for K′=0 up to a time of approximately one ns.
Solvent-induced forces between solutes: A time- and space-resolved molecular dynamics study
Authors: Brugè F. , Fornili S. L. , Palma-Vittorelli M. B.
Published: July 2002

A molecular dynamics statistical mechanical simulation study of solvent-induced forces (SIFs) acting between two fixed model solutes in a bath of 727 water molecules is reported. Simulations were run up to 500 ps on a multiprocessor system, involving 60 T800-20 transputers. Results are discussed within the statistical mechanical approach of inherent structures. It is shown that SIFs are associated with the occurrence of nonpermanent, but longer-lived statistically relevant solvent configurations promoted by solutes. Their size and sign are shown to be critically dependent on the type of solute, solute–solute distance, and mutual orientation. These features give SIFs the specificity character crucially required for functional interactions and for functional conformation and dynamics of biomolecules.
2H electron spin echo envelope modulation spectroscopy of strong, α-hydrogen hyperfine coupling in randomly oriented paramagnetic systems
Authors: Warncke Kurt , McCracken John
Published: July 2002

Experimental characterization of strong, anisotropic 2H hyperfine interactions in randomly oriented organic radicals in the solid state by using electron spin echo envelope modulation (ESEEM) spectroscopic techniques has been examined systematically. The α-3,5-2H coupling in the tyrosine neutral radical in a low temperature aqueous glass was used as a model. Envelope modulation was obtained by integration of the stimulated-echo generated by a three-pulse, microwave pulse-swapping sequence. Division of envelope modulation from the 2H-substituted radical by that from the per-protonated radical remedies discontinuities introduced in the envelope by the eclipse of the second and third pulses. Envelope modulation data was collected for τ values from 214 to 1295 ns (9.132 GHz, 0.3265 T). The common, spectrometer dead-time-limited data collection start-point (140 ns) for the different τ values attenuates loss of anisotropic information and allows summation of time domain data. The Fourier transform of the envelope summation shows the line shape determined only by the orientation-dependencies of the modulation depth and the random distribution of spin systems. The systematic influence of τ-suppression on the line shapes is examined in the individual spectra. Intensity in the region of the ΔmI=±2, double quantum transition is revealed at values of τ sufficiently long to relieve τ-suppression of this feature. Algebraic expressions for the orientation-dependence of the modulation frequencies and amplitudes are given for the general case of rhombic hyperfine interaction in S=1/2, I=1 systems where hyperfine interactions dominate nuclear quadrupole coupling, and are used to describe the changes in the experimental line shapes wrought by variation of τ and magnetic field strength. Theoretical simulation of the spectra yields the complete 3,5-2H hyperfine tensor, (−3.0, −3.9, −1.1)±0.1 MHz. The results show that the sculpting of the ESEEM line shape by the suppression effect allows retrieval of information, obscured by the orientation dependence of the modulation depth, that is necessary to determine the rhombic hyperfine tensor.
Photoionization of gas-phase bromotrifluoromethane and its complexes with methanol: State dependence of intracluster reactions
Authors: Clay J. T. , Walters E. A. , Grover J. R. , Willcox M. V.
Published: July 2002

Weakly bound complexes of CF3Br molecules or of CF3Br and CH3OH were prepared in supersonic jet expansions and studied by photoionization mass spectrometry. Argon was present in the jet expansion to promote cooling of the complexes. Measured ionization potentials are CF3Br, 11.404±0.014 eV; (CF3Br)2, 11.10±0.02 eV; (CF3Br⋅CH3OH), 10.76±0.05 eV. The appearance potential of CF+3 from CF3Br was found to be 11.56±0.02 eV, and evidence is presented that this value is adiabatic. From this appearance potential, ΔH0f0(CF+3)=86.6±1.1 kcal mol−1. The magnitude of the spin–orbit splitting in X̃ 2E CF3Br+ into E1/2 and E3/2 states dominates any Jahn–Teller distortion. The effects of these phenomena on the stability of X̃ 2E CF3Br+ are discussed. The following dissociation channels were observed: CF+3 from X̃ 2E CF3Br+, CF2Br+ from Ã 2A1 CF3Br+, Br+ from B̃ 2A2 CF3Br+, and CF+2 from D̃ 2E CF3Br+. (CF3⋅CH3OH)+ is not produced from neutral dimers, but is created when a CF3Br moiety embedded in a trimer or larger cluster is ionized to Ã 2A1 CF3Br+. In contrast, (Br⋅CH3OH)+ does come from dimers as well as larger clusters. The cross sections for the production of (CF3Br)+2 and (CF3Br⋅CH3OH)+ from trimers or larger progenitors are enhanced when intracluster CF3Br is ionized to CF3Br+ in the B̃ or higher electronic states. Possible reasons for these state-specific reactions are given. Finally, a new method is presented for determining neutral cluster distributions in a molecular beam, and the method is shown to be in quantitative agreement with the near-threshold technique previously published by Grover et al. [J. Phys. Chem. 95, 6473 (1991)].
Automatic numerical integration techniques for polyatomic molecules. Backward trimming
Authors: Pérez-Jordá José M.
Published: July 2002

We present an improvement (the backward trimming procedure) over our algorithm for 3D numerical integration of molecular integrals [J. M. Pérez-Jordá, A. D. Becke, and E. San-Fabián, J. Chem. Phys. 100, 6520 (1994), the standard procedure]. Tests with 21 molecules indicate that the backward trimming procedure gives errors similar to those obtained with the standard procedure, but with a fivefold reduction in the number of grid points.
Deexcitation electron spectroscopy of core-excited NO as a function of excitation energy
Authors: Carroll T. X. , Coville M. , Morin P. , Thomas T. D.
Published: July 2002

Deexcitation electron spectra of core-excited NO have been measured at several excitation energies in the N 1s→2π and O 1s→2π resonances. The nitrogen spectra exhibit significant variation with excitation energy; the oxygen spectra vary only slightly. Sensitivity to excitation energy occurs because each resonance represents the overlap of three transitions to 2Σ+, 2Δ, and 2Σ− states, and each of these excited states decays to a unique set of levels in the final-state ion. We have analyzed all spectra by taking into account excitation energy, lifetime-vibrational interference, and the ordering and splitting of the core-excited levels. Good agreement between calculated line shapes and experiment occurs if it assumed that the level ordering is 2Δ, 2Σ−, 2Σ+ for core-excited nitrogen and 2Σ−, 2Δ, 2Σ+ for core-excited oxygen. Photoexcitation data for oxygen have been analyzed to determine the energies of these states 531.7, 532.7, and 533.7 eV. The deexcitation spectrum from the 2Δ state of nitrogen core-excited NO to the ground state of NO+ has been analyzed using the theory of lifetime-vibrational interference to give a lifetime width for the core-excited state of 146 meV. A similar analysis for the deexcitation of the oxygen core-excited state is less conclusive, but is consistent with a lifetime width of 180 meV.
The internal and translational energy dependence of molecular condensation coefficients: SF6 and CCl4
Authors: Sibener S. J. , Lee Y. T.
Published: July 2002

In this paper we describe a series of beam–surface scattering experiments which examine the internal and translational energy dependence of the molecular condensation probabilities for CCl4 or SF6 colliding with their respective condensed phases. It has been conclusively shown that thermal excitation of a polyatomic molecule’s rotational and vibrational degrees of freedom can inhibit its probability of sticking upon impact with a cryogenically cooled surface. This effect is most pronounced in the limit of low incident kinetic energy, and essentially vanishes at higher velocities. As part of these experiments, we have also obtained the angular and velocity distributions for reflected SF6 and Kr which have been used to examine the energy and momentum exchange of these gases with their respective condensed phases. Our findings suggest that heterogeneous laser isotope separation schemes based on precollision molecular excitation may warrant further investigation.
Aluminum–sulfur cluster ions: Formation and photolysis
Authors: Zhang Nan , Shi Yang , Gao Zhen , Kong Fanao , Zhu Qihe
Published: July 2002

Aluminum–sulfur cluster ions were obtained by direct laser ablation (248 nm, ≥10 MW/cm2) and analyzed with a tandem time-of-flight mass spectrometer. The regular peaks of cluster ions having composition of AlS+(Al2S3)n (n=1,2,3,...,31) were shown on the first time-of-flight mass spectrum (TOF-MS). The cluster ions of a certain mass number (m/e) could be selected and photolyzed by an ArF laser (193 nm, 1 MW/cm2). The product ions were analyzed by a second TOF-MS. It was found that the cluster ions lost (Al2S3)x (x=1, 3, or 4) and became smaller cluster ions of the same stable formula AlS+(Al2S3)n−x. Cagelike structures were proposed to be possible for these stable cluster ions. Semiempirical and ab initio quantum chemical calculations have also been performed for some of the clusters.
Reaction barrier for the methyldiazenyl radical decomposition (CH3N2→CH3+N2)
Authors: Hu Ching-Han , Schaefer Henry F.
Published: July 2002

The reaction of ground state (2A′) CH3N2→CH3+N2 was studied using the ab initio quantum mechanical techniques, including the self-consistent field (SCF), single and double excitation configuration interaction (CISD), single and double excitation coupled cluster (CCSD), and the single, double, and perturbative triple excitation coupled cluster [CCSD(T)]. The classical barrier for the methyldiazenyl radical decomposition was predicted at the highest level of theory to be 4.5 kcal/mol and was 2.3 kcal/mol when zero-point vibrational energy corrections are included. This result is pertinent to the apparently conflicting experimental results of the Rice and Berkeley groups. The very small theoretical reaction barrier agrees with the recent experimental observation that lifetime of CH3N2 is very short, at the picosecond range.
Torsional dependence of molecular electrical properties and long-range interactions: CH3OH–rare gas systems
Authors: Davis Stephen L. , Dennison Elizabeth C.
Published: July 2002

Self-consistent-field (SCF) values are reported for all Cartesian tensor components of the dipole, quadrupole, and octupole moments (μ,Θ,Ω) and polarizabilities α and A of the methanol molecule in its staggered, eclipsed, and three intermediate conformations. The methanol geometry was held rigid except for a single internal rotation angle γ, describing the relative orientation of the methyl rotor and the hydroxyl framework. Three different basis sets were used, including a 110 contracted Gaussian set based on the electrical properties (ELP) basis of Dykstra et al. [Adv. Chem. Phys. 75, 37 (1989), and references therein]. It was found that the tensor components Ωxxx, Ωxyy, Axxx, Axyy, and Ayxy vary as cos 3γ, while the components Ωyyy, Ωyxx, Ayyy, Ayxx, and Axyx vary as sin 3γ. All other components of Ω and A, as well as all components of μ, Θ, show little variation with γ. This dependence was explained using a simple model that treats each property as a sum of a constant, hydroxyl framework contribution and a conformation-dependent, methyl rotor contribution. Torsional averages of these properties were computed from torsional wave functions obtained by diagonalization of the internal axis method (IAM) Hamiltonian.

It was found that the large amplitude internal rotation in methanol gives rise to large vibrational effects on the Axxx, Axyy, and Ayxy polarizability components. The conformational dependence of the electrical properties was used to describe the conformational dependence of long-range interactions involving a near-symmetric, nonrigid molecule such as methanol. The leading γ-dependent interaction term was shown to vary as R−8 and R−7 for the induction and dispersion interactions (respectively) between a methanol molecule and a structureless atom. Cartesian tensor expressions are given for the long-range dispersion interaction within second-order perturbation theory, and the leading torsionally dependent interaction is shown to vary as sin3 θ cos 3(φ−γ), where θ,φ are the spherical coordinates of the atom.