bluuebunny/crossref_metadata_2025 huggingface dataset:
Ionization and dissociation of diatomic molecules in intense infrared laser fields
Authors: Dietrich Peter , Corkum Paul B.
Published: July 2002

We report the first observation of infrared multiphoton dissociation of a strongly bound diatomic molecule, HCl+. The dissociation is explained by a charge–resonance coupling of electronic states of the molecular ion HCl+. This coupling results in Stark shifts which depend on the internuclear separation thereby changing the molecular bonding. Using a barrier suppression model, we obtain good agreement with the observed dissociation threshold. We show the close relationship between barrier suppression and chaotic dissociation. We also report the first quantitative theoretical and experimental study of infrared multiphoton ionization of a small molecule, HCl. Based on tunnel ionization, we develop a molecular ionization model that incorporates both the large Stark shifts of the molecular ion and the associated large induced dipole moments. The model agrees with the experiment for the multiphoton ionization of HCl. It allows us to derive a general expression for the maximum intensity which can be applied to a neutral molecule without ionizing it.
Spectroscopy of AlAr and AlKr from 31 000 cm−1 to the ionization limit
Authors: Heidecke Scott A. , Fu Zhenwen , Colt John R. , Morse Michael D.
Published: July 2002

Spectra of jet-cooled AlAr and AlKr have been collected over the range from 31 000 cm−1 to the ionization limit, using the technique of resonant two-photon ionization spectroscopy with mass spectrometric detection. Adiabatic ionization potentials have been accurately measured for both molecules, and have been estimated for AlAr2 and AlAr3. The spin–orbit splittings in the X 2Π ground state have been determined for both AlAr and AlKr, and ground-state bond strengths have been accurately measured as D0[AlAr, X1 2Π1/2(3p)] =122.4±4 cm−1 and D0[27Al 84Kr, X1 2Π1/2(3p)] =194.7±0.8 cm−1. These values have also been combined with the measured ionization potentials to derive D0(Al+–Ar)=982.3± 5 cm−1 and D0(Al+–Kr)=1528.5±2 cm−1.
Nature of lithium trapping sites in the quantum solids para-hydrogen and ortho-deuterium
Authors: Scharf Daphna , Martyna Glenn J. , Li Daohui , Voth Gregory A. , Klein Michael L.
Published: July 2002

Quantum mechanical studies of a lithium impurity in solid para-hydrogen and ortho-deuterium have been performed using the path integral formulation of statistical mechanics. Since an isolated lithium atom is much larger than the host molecules, trapping sites consisting of from one to six vacancies have been investigated. Interestingly, all of the sites are comparable in energy. This is due to the large compressibility of para-hydrogen and ortho-deuterium solids, which permits the lattice to relax to comfortably accommodate the impurity. The inhomogeneously broadened dipole spectrum of the lithium impurity in the various sites was calculated using the radial fast Fourier transform Lanczos method and compared to experiments by Fajardo [J. Chem. Phys. 98, 110 (1993)]. Based on the present calculations, lithium atoms appear to occupy preferentially a three-vacancy trapping site in para-hydrogen while in ortho-deuterium a four-vacancy trapping site seems to be favored. Complementary variational Einstein model calculations predict that the four-vacancy trapping site is favorable in both para-hydrogen and in ortho-deuterium.
State-to-state cross sections for rotational excitation of OH by collisions with He and Ar
Authors: Schreel K. , Schleipen J. , Eppink A. , Meulen J. J. ter
Published: July 2002

Parity resolved state-to-state cross sections for rotational excitation of OH(X 2Π) colliding with He and Ar, have been obtained in a crossed molecular beam experiment. The OH radicals were produced in a pulsed dc discharge in a H2O/Ar gas mixture. Adiabatic expansion into vacuum resulted in an effective rotational cooling yielding a 94% population of the lowest Λ-doublet (X 2Π3/2, J=3/2). Further state preparation could be achieved via electrostatic state selection in a hexapole electric field, resulting in a 93.5% population of the upper Λ-doublet component (f,+). Experiments were performed both with and without the state selector to provide detailed information about the rotational excitation from both Λ-doublet states. The OH rotational state distribution was probed, before and after the collision event, by means of laser-induced fluorescence (LIF) spectroscopy of the A←X electronic band at 308 nm. The OH–Ar and OH–He scattering behave very similar with the exception of the J=3/2 Λ-doublet transition which is induced much weaker by the He collisions. For both collision systems the experimental results are in agreement with theoretical cross sections obtained from quantum scattering calculations of Werner et al. [J. Chem. Phys. (submitted)].
Static and dynamic properties of multiple light scattering
Authors: Štěpánek Petr
Published: July 2002

We have examined the onset and evolution of multiple scattering of light on a series of latex dispersions as a function of increasing volume concentration φ of particles. We have shown that using vertically polarized incident light, the static scattered intensity becomes progressively depolarized, with increasing φ. The polarization of scattered light is completely random in the limit of strong multiple scattering. The spectra of decay times of dynamic light scattering display a region of oligo scattering at intermediate φ where both the single and multiple scattering components can be dynamically identified. For φ≳0.03 the limit of diffusive transport of light is attained. The obtained results confirm that our earlier measurements of dynamic light scattering on systems exhibiting critical opalescence are not influenced by multiple light scattering.
Vibrational and dielectric properties of C60 from density-functional perturbation theory
Authors: Giannozzi Paolo , Baroni Stefano
Published: July 2002

The vibrational frequencies and electric polarizability of the C60 molecule, both in the gaseous and in the solid phases, are calculated from first principles using density-functional perturbation theory. This method also allows us to obtain the infrared and Raman activities which had never been calculated before. Our results are in excellent agreement with existing experimental data, and they provide accurate predictions for those quantities (such as silent-mode frequencies and vibrational eigenvectors) which are not easily accessible to experiments.
Rotationally inelastic and hyperfine resolved cross sections for OH–H2 collisions. Calculations using a new ab initio potential surface
Authors: Offer Alison R. , Hemert Marc C. van , Dishoeck Ewine F. van
Published: July 2002

Cross sections and rate constants are presented for the rotational excitation of OH in collision with ortho and para-H2, using a new ab initio interaction potential [Offer and Van Hemert, J. Chem. Phys. 99, 3836 (1993)]. The cross sections are given at a number of energies and are compared with those calculated using an earlier potential energy surface, and with the available experimental results. A strong oscillatory behavior is found in the cross sections for collisions with ground state para-H2 which was not apparent in earlier calculations. The oscillatory behavior is very much reduced in collisions with ortho-H2. Rate constants obtained by averaging the cross sections over a Maxwell–Boltzmann velocity distribution are given at a temperature of 300 K. Expressions for calculating the hyperfine resolved cross sections by transforming the S-matrices are discussed for the case where H2 is no longer constrained to its rotational ground state, and cross sections for transitions between the hyperfine resolved levels are given for collisions with both para and ortho-H2.
Quantum theory of planar four-atom reactions
Authors: Echave Julián , Clary David C.
Published: July 2002

An exact quantum mechanical theory is developed to treat four-atom reactions of the type AB+CD↔(BCD+A, ACD+B), where the atoms are constrained to move in a plane. The theory makes use of an unbiased set of hyperspherical coordinates. A method is proposed for implementing the theory that exploits the potential optimized discrete variable representation. Application is made to the calculation of rovibrational state-to-state reaction probabilities for the reaction H2+OH↔H2O+H, in which the length of the OH spectator bond is held fixed. The results show that a rotating bond approximation, in which the H2 molecule is not allowed to rotate, gives good results for vibrationally selected reaction probabilities. The effect of reactant rotation and vibration on the reactivity and product distributions is discussed for the reactions H2+OH→H2O+H and H2O+H→H2+OH.
Efficient calculation of highly excited vibrational energy levels of floppy molecules: The band origins of H+3 up to 35 000 cm−1
Authors: Bramley Matthew J. , Tromp John W. , Carrington Tucker , Corey Gregory C.
Published: July 2002

Recent testing of a discrete variable representation (DVR) Lanczos product-basis method to calculate polyatomic vibrational energy levels [M. J. Bramley and T. Carrington, J. Chem. Phys. 99, 8519 (1993)] suggested that, for increasingly floppy molecules, its efficiency will be increasingly competitive with that of contracted-basis explicit-diagonalization methods if one can overcome the problem of poor Lanczos convergence caused by kinetic energy singularities. This may be accomplished through the realization that nondirect product finite basis representations (FBRs) (and the related DVRs) can be used efficiently in dynamics calculations for which the rate-determining step is the evaluation of Hamiltonian matrix–vector products, as is the case with Lanczos recursion [J. W. Tromp and G. C. Corey, J. Chem. Phys. (to be submitted); D. Lemoine and G. C. Corey, J. Chem. Phys. (to be published)]. A synthesis of these two procedures provides a near-optimally efficient variational vibrational method for molecules for which good basis contraction schemes cannot be designed, and for which the inevitable coordinate singularities require ideally a nondirect product basis. To substantiate this claim, we have performed hybrid DVR/FBR Lanczos calculations of vibrational energies of the classic floppy triatomic molecule H+3 up to near dissociation with unprecedently good convergence and unprecedently low computational cost.
Calculations of the Mueller scattering matrix for a DNA plectonemic helix
Authors: Shapiro D. B. , Hull P. G. , Hunt A. J. , Hearst J. E.
Published: July 2002

The polarization state of a beam of light is fully described by the four elements of the Stokes vector. How the Stokes vector is transformed upon scattering is described by the sixteen element Mueller scattering matrix. Polarized light scattering provides a useful tool to study macromolecular structure. In particular, measurements of Mueller matrix elements have been used to study chromosome structure; changes in these measured quantities can be used to monitor structure as a function of cell cycle. Such measurements done on helical structures can be compared to theoretical computations to determine the geometrical parameters describing the helix. The matrix elements that are most sensitive to the chiral parameters of the helix are largest when the wavelength of light is of the order of the pitch of the helix. Therefore, polarized light scattering measurements made on DNA plectonemic helices would provide the most information in the far ultraviolet (UV) and x-ray region. The Mueller matrix elements are calculated using the coupled dipole approximation in the orientation average at wavelengths in the visible, ultraviolet, and x-ray regions. Each base-pair of the idealized plectonemic helix is represented by a single dipole. A complex polarizability tensor is assigned to each dipole. Calculations are sensitive to the writhe and polarizability of the DNA molecule.
On the measurement of superpositions of chiral amplitudes by polarized light scattering
Authors: Harris Robert A. , Shi Yaoming , Cina Jeffrey A.
Published: July 2002

We show that no experiment involving parity can measure the left–right coherence of a state ‖ψ〉=aL‖L〉+eiφaR‖R〉 if sin φ is zero and ‖L〉 and ‖R〉 are real mirror images of one another. We then exhibit a number of experiments in elastic linear light scattering which provide all-or-none measurements of left–right coherence.
Hard fluid model for molecular solvation free energies
Authors: de Souza Luís E. S. , Ben-Amotz Dor
Published: July 2002

The hard fluid model, which approximates packing forces in molecular liquids using hard sphere reference fluids, is applied to the prediction of excess solvation free energies of hard spheres and cavity size distributions in water, carbon tetrachloride, chloroform, n-hexane, n-dodecane, and n-undecyl alcohol. These are found to compare favorably with computer simulation measurements in these liquids, as well as experimental solubilities of rare gases in water, n-hexane, and n-dodecane (extrapolated to zero solute polarizability). The results are used to determine repulsive contributions to solvation free energies of atomic and molecular solutes in water and n-hexane. Attractive solvation free energies, determined from the difference between experimental and repulsive contributions, are found to correlate with solute polarizability, and are compared with dispersion energy estimates. The success of the hard fluid model in describing aqueous solvation suggests that the small size of water molecules, rather than their unique hydrogen bonding structure, plays an important role in hydrophobic hydration.
Calculation of triatomic vibrational eigenstates: Product or contracted basis sets, Lanczos or conventional eigensolvers? What is the most efficient combination?
Authors: Bramley Matthew J. , Carrington Tucker
Published: July 2002

Numerous practical methods have been described for exact quantum calculations of vibrational eigenstates (energy levels and wave functions) for three- and four-atom molecules. Many descriptions are accompanied by bold claims of efficiency. Such claims are, unfortunately, difficult to test in the absence of fair comparisons on a single computer. The efficiency of these calculations depends above all (once the most appropriate coordinate system has been chosen) on clever choices of (i) the multidimensional basis set, and (ii) the Hamiltonian matrix eigensolver. In the first category come techniques such as the discrete variable representation (DVR) and basis contraction (also known as sequential adiabatic reduction or diagonalization truncation). In the second category, the Lanczos recursion is being increasingly applied. In a recent study taking the HCN/HNC molecule as a test case [R. A. Friesner, J. A. Bentley, M. Menou, and C. Leforestier, J. Chem. Phys. 99, 324 (1993)], reductions in computational effort of one to three orders of magnitude were found for a method combining basis contraction and Lanczos recursion, compared to one widely considered to be state of the art in which the Hamiltonian matrix is diagonalized conventionally [Z. Bačić and J. C. Light, J. Chem. Phys. 86, 3065 (1987)]. We have investigated this finding by developing a computer program which permits choosing both between direct product and two kinds of contracted basis (all derived from DVRs), and between Lanczos and conventional eigensolvers. It has been applied to the calculation of vibrational frequencies both of HCN/HCN up to 12 000 cm−1 and of H2O up to 22 000 cm−1, with a strict convergence criterion of 1 cm−1 in each case. We find the conclusions of Friesner et al. to be exaggerated: while a contracted/Lanczos method is consistently most efficient, other combinations, even the rather simple direct-product Lanczos [M. J. Bramley and T. Carrington, J. Chem. Phys. 99, 8519 (1993)], are never as much as a factor of 5 more costly.
Overtone resonance Raman scattering beyond the Condon approximation: Transform theory and vibronic properties
Authors: Albrecht A. C. , Clark Robin J. H. , Oprescu Dan , Owens Suzanne J. R. , Svendsen Christian
Published: July 2002

The time correlator formalism is used to develop the expression for nth order (overtone) resonance Raman scattering (RRS) to include both Raman frequency shifts upon electronic excitation as well as non-Condon vibronic coupling. In particular the compact operator formalism recently introduced by Hizhnyakov and Tehver [J. Raman Spectrosc. 19, 383 (1988)] to obtain several RRS correlators (including overtone scattering with frequency shift, but in the Condon approximation) is used to extend the theory. At the same time a formal advantage is achieved by the limited introduction of the Born–Oppenheimer approximation. Also transform relationships including non-Condon effects are given that link the Raman excitation profile of nth order scattering to the absorption spectrum. Finally, it is emphasized how all three vibronic parameters—potential energy surface displacement, Raman mode frequency changes, and the linear non-Condon coupling parameter—can be quantitatively determined without the need for absolute Raman cross-section measurements. The relative scattering intensity of the fundamental and three (or more) overtones suffices to fix the three. By way of application, the vibronic parameters are determined from published single wavelength overtone RRS in six molecules.
The ethylenedione anion: Elucidation of the intricate potential energy hypersurface
Authors: Thomas J. Russell , DeLeeuw Bradley J. , O’Leary Paul , Schaefer Henry F. , Duke Brian J. , O’Leary Brian
Published: July 2002

Ab initio molecular orbital theory has been used to study the controversial potential energy surface of the ethylenedione anion C2O−2. Seven different basis sets, the largest being triple zeta plus two polarization functions and one set of higher angular momentum functions (TZ2Pf) in quality, were utilized in conjunction with five correlated methods, the highest-level being coupled-cluster theory including single, double, and perturbative triple excitations [CCSD(T)]. Equilibrium geometries and harmonic vibrational frequencies of the predicted 2Au trans-bent ground state are presented. The Renner–Teller potential energy surface resulting from the splitting of the doubly degenerate linear 2Πu transition state into the nondegenerate bent 2Au and linear 2Bu surfaces is also characterized by means of energy predictions for these three states. Several recent peak assignments in the experimental spectrum, as well as the isotopic shifts associated with them, are supported by theory. A correct description of the potential energy hypersurface is obtained only by application of large basis sets in conjunction with methods including high-level treatment of electron correlation effects. The TZP+/CCSD(T) methodology predicts the OCC bond angle to be 146.5°.
A model of low-frequency Raman scattering in glasses: Comparison of Brillouin and Raman data
Authors: Novikov V. N. , Duval E. , Kisliuk A. , Sokolov A. P.
Published: July 2002

A model of the low-frequency Raman scattering intensity in glasses is proposed, which describes the spectrum in the region of the boson peak, typical of glasses. The model considers the boson peak as a result of first-order light scattering by harmonic quasilocalized vibrations. The amplitude, frequency, and polarization dependence of the light on the vibrational coupling coefficient C(ω) are estimated. The model predicts a linear frequency dependence of C(ω) in the region of the boson peak and the depolarization ratio as a function of the relative contribution of the longitudinal and transverse-type motions which comprise these quasilocal vibrational modes. Low-frequency Raman and Brillouin measurements have been made on SiO2 glass in order to check the predictions of the model regarding the amplitude and integrated intensity of the boson peak. The estimated ratio of these values to those of the Brillouin lines are in good agreement with experiment. Comparison of the theoretical and experimental values of the depolarization ratio for the boson peak shows a predominance of the transverse-type atomic motions in the low-frequency quasilocalized vibrational modes in glasses.
Structure of CAl12
Authors: Seitsonen Ari P. , Laasonen Kari , Nieminen R. M. , Klein Michael L.
Published: July 2002

The structures of an isolated CAl12 cluster and a solid composed of CAl12 clusters have been studied using the Car–Parrinello method, based on the density functional theory and the local density approximation. We have compared the results of using the ultrasoft Vanderbilt pseudopotential with those of both a traditional pseudopotential and a linear combination of atomic orbitals method. We have confirmed the high stability of the cluster in its icosahedral structure. However, we show that the cluster-assembled solid is unstable against melting of the clusters, as previously found for SiAl12.
Effective core potential studies of lanthanide complexes
Authors: Cundari Thomas R. , Sommerer Shaun O. , Strohecker Lynn A. , Tippett Lynn
Published: July 2002

A recently developed compact effective core potential (ECP) scheme for lanthanides [T. R. Cundari and W. J. Stevens, J. Chem. Phys. 98, 5555 (1993)] is applied to molecular species—the lanthanide trihalides (LnX3, Ln=Ce to Lu; X=F, Cl, Br, I). This research is the first ECP study of lanthanide trihalides in which 4f electrons are explicitly included in the lanthanide valence space. The high-spin 4fn configuration of Ln(III) is described using multiconfiguration (MC) and unrestricted Hartree-Fock (UHF) wave functions. Both methods give excellent agreement with experiment for Ln–X bond lengths, although UHF is faster with no evidence of extensive spin contamination. There is no loss of accuracy from early to late members of the lanthanide series and from the lightest to heaviest halogen for the MC calculations. The root-mean-square difference between MC/ECP Ln–X bond lengths and those estimated from gas-phase experiments is only 0.05 Å (≊2%). The UHF calculations on LnF3 yield only three nonplanar minima (CeF3, PrF3, and PmF3). For the three complexes with pyramidal minima, the planar transition states are very close in energy (ΔEUHF<100 cm−1), suggesting these complexes will probably be fluxional at ambient conditions.
Low-lying stationary points and torsional interconversions of cyclic (H2O)4: An ab initio study
Authors: Schütz Martin , Klopper Wim , Lüthi Hans-Peter , Leutwyler Samuel
Published: July 2002

The global and local minima, stationary points, and torsional rearrangement processes of cyclic homodromic (H2O)4 were studied on its four-dimensional torsional intermolecular potential energy surface. Eight different energetically low-lying torsional stationary point structures were found by ab initio theory, and fully structure-optimized at the second-order Mo/ller–Plesset level, using large basis sets. Second-order energies close to the one-particle basis set limit were obtained at these geometries using the explicitly correlated Mo/ller–Plesset method. The effects of higher-order correlation energy terms were investigated by coupled cluster theory, and terms beyond second order were found to cancel in good approximation. The S4 symmetric global minimum has a square and almost planar O...O...O...O arrangement with free O–H bonds alternating ‘‘up’’ and ‘‘down’’ relative to this plane, with two isometric versions of this structure. Another torsional conformer with two neighboring up O–H bonds followed by two neighboring down O–H bonds is a local minimum, 0.93 kcal/mol above the global minimum. The four versions of this structure are connected to the global minima via two distinct but almost degenerate first-order torsional saddle points, which occur as two sets of eight isometric versions each, both about 1.24 kcal/mol above the global minimum. Yet another set of eight second-order saddle points lies at 1.38 kcal/mol. The structure with three O–H bonds up and one down is not a stationary point, while the structure with all four O–H bonds on the same side of the plane is a first-order saddle point.

The fully planar C4h symmetric structure is a fourth-order stationary point 2.8 kcal/mol above the minimum. The torsional interconversion paths between this multitude of points are complex, and are discussed in three-dimensional spaces of symmetry-adapted torsional coordinates, and also in a network representation. The torsional normal-mode eigenvectors point fairly directly along the torsional interconversion pathways, but the harmonic frequencies are well below the corresponding barriers. Tunneling interconversion between torsional conformers is, hence, less important than for the water trimer.
Na+–Cl− ion pair association in supercritical water
Authors: Chialvo A. A. , Cummings P. T. , Cochran H. D. , Simonson J. M. , Mesmer R. E.
Published: July 2002

Molecular dynamics simulations of supercritical electrolyte solutions with three different ion–water models are performed to study the anion–cation potential of mean force of an infinitely dilute aqueous NaCl solution in the vicinity of the solvent’s critical point. The association constant for the ion pair Na+/Cl− and the constant of equilibrium between the solvent-separated and the contact ion pairs are determined for three models at the solvent critical density and 5% above its critical temperature. The realism of the aqueous electrolyte models is assessed by comparing the association constants obtained by simulation with those based on high temperature conductance measurements. Some remarks are given concerning the calculation of the mean-force potential from simulation and the impact of the assumptions involved.
On the role of conical intersections in photodissociation. IV. Conical intersections and the geometric phase in the 2 3A″ and 3 3A″ states of CH2
Authors: Yarkony David R.
Published: July 2002

The 2 3A″–3 3A″(1 3A2–2 3B1) conical intersection seam in CH2 is characterized directly, that is without prior determination of the potential energy surfaces. Nuclear configurations in the vicinity of the seam are characterized in terms of a set of natural polar coordinates defined directly from the seam mapping procedure. Using these coordinates the geometric, or Berry phase, effect is demonstrated and the topology and interstate couplings in the vicinity of the seam are analyzed. The techniques used to study this surface of intersection are readily applied to larger polyatomic systems.
Theoretical calculations of zero-temperature absorption spectra of Li in solid H2
Authors: Cheng E. , Whaley K. Birgitta
Published: July 2002

We employ variational and diffusion Monte Carlo methods to calculate the ground state properties of both pure and Li doped solid H2. The absorption spectra of Li in the H2 solid are calculated from the sampled ground state configurations and a pseudothermal semiclassical spectral line theory, based on the Franck–Condon principle for a condensed phase system. The T=0 numerical results of various thermodynamic properties of the pure and doped solid agree well with those from previous theoretical calculations. However, the Li spectra differ significantly from results of a finite temperature path integral Monte Carlo calculation [D. Scharf et al., J. Chem. Phys. 99, 9013 (1993)], which employed a different Li*–H2 potential. The differences in two calculations are extensively discussed here, and comparisons are also made with available experimental data. We demonstrate that accurate measurements of the Li absorption spectrum provide a powerful means to probe the local structures of the Li trapping sites.
Energetics using the single point IMOMO (integrated molecular orbital+molecular orbital) calculations: Choices of computational levels and model system
Authors: Svensson Mats , Humbel Stéphane , Morokuma Keiji
Published: July 2002

The integrated MO+MO (IMOMO) method, recently proposed for geometry optimization, is tested for accurate single point calculations. The principle idea of the IMOMO method is to reproduce results of a high level MO calculation for a large ‘‘real’’ system by dividing it into a small ‘‘model’’ system and the rest and applying different levels of MO theory for the two parts. Test examples are the activation barrier of the SN2 reaction of Cl−+alkyl chlorides, the C=C double bond dissociation of olefins and the energy of reaction for epoxidation of benzene. The effects of basis set and method in the lower level calculation as well as the effects of the choice of model system are investigated in detail. The IMOMO method gives an approximation to the high level MO energetics on the real system, in most cases with very small errors, with a small additional cost over the low level calculation. For instance, when the MP2 (Mo/ller–Plesset second-order perturbation) method is used as the lower level method, the IMOMO method reproduces the results of very high level MO method within 2 kcal/mol, with less than 50% of additional computer time, for the first two test examples. When the HF (Hartree–Fock) method is used as the lower level method, it is less accurate and depends more on the choice of model system, though the improvement over the HF energy is still very significant. Thus the IMOMO single point calculation provides a method for obtaining reliable local energetics such as bond energies and activation barriers for a large molecular system.
A theoretical study of the valence- and dipole-bound states of the nitromethane anion
Authors: Gutsev Gennady L. , Bartlett Rodney J.
Published: July 2002

The valence- and dipole-bound states of CH3NO−2 are studied at the CCSD(T), HFDFT (B3LYP), and EA-EOMCC levels of theory. At both CCSD(T) and HFDFT levels, we have found a positive valence EA in nice agreement with the experimental data. The binding energy of the dipole-bound electron is about 13 meV according to the EA-EOMCC calculations. Interaction of the valence- and dipole-bound states (DBS) of CH3NO−2 is complicated, since the dipole-bound state exists at the equilibrium geometry of the anion and corresponds to an excited state of the valence-bound anion. Hence, excitations of the valence anionic state could lead to both the detachment of an electron or formation of a DBS, whose geometry is similar to the geometry of the neutral parent. At the equilibrium geometry of the anion, the energies of the dipole-bound and valence states are close to each other. Since typical lifetimes of rovibrational excited states of a DBS are two orders of magnitude higher than the lifetimes of ordinary vibrationally excited states, it might be possible to transform the DBS into the valence ground state under certain experimental conditions.
Thermodynamic consistency of the hard-sphere solid distribution function
Authors: Rascón C. , Mederos L. , Navascués G.
Published: July 2002

A discussion about the role of the two-particle density function in the fundamental equation of the compressibility is presented. The thermodynamic inconsistency found by different authors between the compressibility obtained from the equation of state and that from the compressibility equation, in both cases using simulation data, is clarified. A new parameterization for the solid radial distribution function, which includes properly the compressibility effects, is proposed.
Infrared characterization of amorphous and polycrystalline D2O ice on controlled wettability self-assembled alkanethiolate monolayers
Authors: Engquist Isak , Lundström Ingemar , Liedberg Bo , Parikh Atul N. , Allara David L.
Published: July 2002

Infrared reflection–absorption spectroscopy has been used to characterize thin overlayers (1–200 Å) of D2O ice deposited in UHV onto a set of self-assembled alkanethiolate monolayers (SAMs) of controlled wettabilities on gold. The SAMs were prepared from a series of controlled composition, mixed solutions of HS(CH2)15CH3 and HS(CH2)16OH, making it possible to investigate the whole wettability range from θ≈0° to θ=112°, where θ is the static contact angle with water. Dosing of D2O and infrared measurements were carried out at selected sample temperatures between 82 and 150 K. Experimental spectra of ice overlayers recorded below 100 K on all SAM substrates are in good agreement with simulated reflection–absorption spectra, derived from the optical constants of amorphous ice. This agreement allows accurate film thickness determination. In contrast, lack of correspondence in spectral signature is noted between the spectra of annealed films and simulated polycrystalline (or amorphous) ice spectra. We interpret this discrepancy to suggest that significant substrate-induced differences between thin overlayers and bulk ice persist in the latter case. Spectral indications of ice–substrate interaction are also seen for amorphous ice, and are especially prominent in the case of highly hydrophobic (pure CH3-terminated, θ=112°) substrates. In this case the substrate effect extends up to an average film thickness (150–200 Å) corresponding to ∼50 ice monolayers, in contrast to highly hydrophilic OH-terminated substrates where the substrate effects appear to vanish beyond ∼5 monolayers (15–20 Å average thickness). Annealing of thin ice overlayers (2–3 monolayers) clearly demonstrates a strong correlation between the onset as well as progression of the transition from amorphous to polycrystalline ice and the exact substrate wettability or chemical composition. The data further suggest the existence of metastable intermediate forms, that are neither purely amorphous nor polycrystalline. We discuss these observations in terms of substrate–overlayer interaction. A tentative “phase diagram” summarizing these results is presented.
General theoretical formalism for describing the high-order effects of the dipolar coupling between spin-1/2 and quadrupole nuclei
Authors: Ding Shangwu , McDowell Charles A.
Published: July 2002

A general theoretical description of the effect of the second-order dipolar coupling between spin-1/2 and quadrupole nuclei is presented based on the density operator formalism rather than the Shrödinger equation as in previous publications. The main task of this formalism is to diagonalize the evolution operator which can be performed analytically when the quadrupole nucleus is spin-1 or spin-3/2; no approximations such as the adiabatic assumption, perturbation expansion and the average Hamiltonian treatment are assumed. Therefore, it is general and can be used for all nuclear spins and any value of quadrupole coupling constant and in the case of magic-angle spinning (MAS) for any rotor spinning speed; it can also include the case when more than one species of spin-1/2 nuclei is coupled to the quadrupole nucleus. The effects of indirect dipolar coupling, chemical shift anisotropy and sample spinning speeds on Nuclear Magnetic Resonance (NMR) lineshapes can be uniformly incorporated in the formalism. Lineshape simulations based on this formalism can yield structural and electronic parameters of compounds and materials with high accuracy. Experimental results for several typical compounds of different complexities are demonstrated and are shown to be in good agreement with the theoretical spectral simulations.
Predissociation of the Na2 3 3Πg and other triplet states
Authors: Liu Y. , Li J. , Gao H. , Chen D. , Li Li , Field R. W. , Lyyra A. M.
Published: July 2002

The Na2 3 3Πg state adiabatically dissociates to the 3s+4p atomic limit. The 3 3Πg levels below the 3s+3d atomic limit have been observed by perturbation facilitated optical–optical double resonance (PFOODR) fluorescence excitation spectroscopy. Energy levels above the 3s+3d atomic limit cannot be observed by detecting molecular fluorescence but are observable by detecting atomic 3d→3p fluorescence. The 3 3Πg state interacts with the 3s+3d 2 3Πg state. At lower energy this interaction is responsible for many perturbations between bound vibrational levels of these two states. Above the 3s+3d limit, as a result of this interaction, the 3 3Πg state is strongly predissociated and the linewidths of levels close to the 3s+3d limit are about 22 cm−1. The predissociation interaction is due to an avoided crossing between the adiabatic 2 3Πg and 3 3Πg states at R≈2.85 Å. Weak predissociation of the 4 3Σg+ and 2 3Δg states are also observed and discussed.
Relation between cooperative effects in cyclic water, methanol/water, and methanol trimers and hydrogen bonds in methanol/water, ethanol/water, and dimethylether/water heterodimers
Authors: Masella Michel , Flament Jean Pierre
Published: July 2002

Ab initio calculations at the MP2 level have been performed on water, methanol/water, ethanol/water, and dimethylether/water dimers and on water, methanol/water, and methanol cyclic trimers. Several properties of their hydrogen bonds have been investigated, such as interoxygen distances, O–H bond lengths, binding energies, electronic densities at hydrogen bond (HB) critical points and OH stretch vibrational frequencies. Results exhibit HB enhancements for dimers where the acceptor molecule corresponds to water (HDA dimers) as compared to dimers where the donor is water (HDD dimers). In particular, HB reinforcement depends on the number of alkyl groups bonded to the donor oxygen. For trimers, a comparison among their HB properties and those of dimers shows that HB reinforcements (as compared to isolated dimers) occurring in trimers correlate with HB reinforcements observed in (HDA dimers (as compared to (HDDs). In particular, HB properties of the cyclic water trimer are close to those of alcohol/water HDA dimers, and for the methanol cyclic trimer to that of the dimethylether/water HDA dimer. All of these results agree with an orbital interpretation of hydrogen bonding in terms of charge transfer from donor lone pairs to acceptor antibond σOH*, even if all of the HB properties in cyclic trimers may not be explained from this approach.
Dynamics of solvated ion in polar liquids: An interaction-site-model description
Authors: Chong Song-Ho , Hirata Fumio
Published: July 2002

We present a molecular theory for the friction coefficient of an ion based on the interaction-site model for molecular liquids and on the mode-coupling theory. The ionic friction is described in terms of the response of collective excitations in solvent to a solute displacement, and the resultant formula expresses the friction as an amount of the energy dissipated during the relaxational processes after the solute perturbation. Utilizing this viewpoint, the ionic friction is shown to be naturally decomposed into hydrodynamic and dielectric contributions and their coupling. It is demonstrated from theoretical calculations of the ionic frictions that both classical pictures, the solventberg and dielectric friction pictures, are responsible for large frictions of small ions, and that the coupling term, which has been disregarded in theoretical considerations so far, should also be properly taken into account in dealing with small ions.
Scattering of aligned molecules. The potential energy surfaces for the Kr-O2 and Xe-O2 systems
Authors: Aquilanti Vincenzo , Ascenzi Daniela , Cappelletti David , de Castro Miguel , Pirani Fernando
Published: July 2002

Total integral cross sections for scattering of oxygen molecules on krypton and xenon atoms were measured in the thermal energy range, as a function of the collision energy and under a controlled alignment of the rotational angular momentum of the molecules [Aquilanti et al., Nature, 371, 399 (1994)]. Data obtained with a “hot” effusive molecular beam, which contains fast rotating and randomly oriented O2 molecules, mainly probe the spherical component of the potential energy surfaces. Experiments with supersonic seeded beams, where the oxygen molecules are cooled at the K=1 rotational level and selectively aligned [Aquilanti et al., Phys. Rev. Lett. 74, 2929 (1995)], probe the anisotropy of the potential energy surfaces. The analysis of the experimental results, based upon close-coupling exact quantum mechanical calculations of the cross sections, provides an accurate characterization of the interactions at intermediate and large intermolecular distances for the Kr-O2 and Xe-O2 systems. It is found that the most stable configuration of the two complexes is for perpendicular approach of the rare gas atom, with energies 15.84 for Kr and 17.87 meV for Xe, at intermolecular distances of 3.72 and 3.87 Å, respectively. An adiabatic approximation and a semiclassical description shed light on some general features of the collision dynamics of aligned molecules, in particular on the observed effects of the interaction anisotropy on the glory interference phenomenon.
Theory of surface light scattering from a fluid–fluid interface with adsorbed polymeric surfactants
Authors: Buzza D. M. A. , Jones J. L. , McLeish T. C. B. , Richards R. W.
Published: July 2002

We present a microscopic theory for the interfacial rheology of a fluid–fluid interface with adsorbed surfactant and calculate the effect of this on surface light scattering from the interface. We model the head and tail groups of the surfactant as polymer chains, a description that becomes increasingly accurate for large molecular weight surfactants, i.e., polymeric surfactants. Assuming high surface concentrations so that we have a double-sided polymer brush monolayer, we derive microscopic scaling expressions for the surface viscoelastic constants using the Alexander–deGennes model. Our results for the surface elastic constants agree with those in the literature, while the results for the viscous constants are new. We find that four elastic constants, i.e., γ (surface tension), ε (dilational elasticity), κ (bending modulus), λ (coupling constant), and three viscous constants, i.e., ε′,κ′,λ′ (the viscous counterparts of ε, κ, and λ, respectively) are required for a general description of interfacial viscoelasticity (neglecting in-plane shear). In contrast to current phenomenological models, we find (1) there is no viscous counterpart to γ, i.e., γ′≡0; (2) there are two additional complex surface constants (i.e., λ+iωλ′ and κ+iωκ′) due to the finite thickness of the monolayer. Excellent agreement is found comparing our microscopic theory with measurements on diblock copolymer monolayers. We further derive the dispersion relation governing surface hydrodynamic modes and the power spectrum for surface quasielastic light scattering (SQELS) for a general interface parameterized by all the surface viscoelastic constants. Limiting results are presented for (1) liquid–air interfaces; (2) liquid–liquid interfaces with ultralow γ. The significant contribution of κ in the latter case opens up the possibility for a direct measurement of κ using SQELS for polymeric surfactant monolayers. Finally, we show that the coupling constant λ can lead to apparent negative values of ε′, as observed in many experimental systems. This is the first time that this result has been explained for insoluble monolayers using a physically realistic model. Based on our theory, we speculate the existence of a thick sublayer (on the length scale of a micron) in recent experiments on insoluble copolymer systems where negative surface viscosities have been found. We also suggest methods to detect the presence of such a sublayer if it does exist.
Rotational spectrum and structure of the OCS–(CO2)2 trimer
Authors: Peebles Sean A. , Kuczkowski Robert L.
Published: July 2002

The rotational spectra of nine isotopes of the mixed trimer, OCS–(CO2)2, have been assigned using pulsed nozzle FTMW spectroscopy techniques. The structure resembles a distorted triangular cylinder. It can be thought of as the slipped (CO2)2 dimer with the OCS above the dimer and crossed about 23° to the axis of each CO2. The distance between the carbon atoms on the CO2 is 3.68(5) Å. The distance between the carbon on each CO2 and the carbon on the OCS is 3.59(5) and 3.66(5) Å, respectively. The axes of the linear molecules are tilted 30°–35° from perpendicular relative to the edges of the C–C–C plane. The dipole moment components for the trimer are μa=0.63(2) D, μb=0.16(10) D and μc=0.21(2) D. The structure and dipole moment components are consistent with an interaction model, which includes a distributed multipole moment electrostatic analysis and atom–atom terms describing dispersion and repulsion. The structure is compared to other related dimers and trimers containing CO2, OCS, and N2O.
A semiclassical approach to intense-field above-threshold dissociation in the long wavelength limit. II. Conservation principles and coherence in surface hopping
Authors: Thachuk Mark , Ivanov M. Yu. , Wardlaw David M.
Published: July 2002

This paper is a companion to our recently published semiclassical formalism for treating time-dependent Hamiltonians [J. Chem. Phys. 105, 4094 (1996)], which was applied to study the dissociation of diatomic ions in intense laser fields. Here two fundamental issues concerning this formalism are discussed in depth: conservation principles and coherence. For time-dependent Hamiltonians, the conservation principle to apply during a trajectory hop depends upon the physical origin of the electronic transition, with total energy conservation and nuclear momentum conservation representing the two limiting cases. It is shown that applying an inappropriate scheme leads to unphysical features in the kinetic energy of the dissociation products. A method is introduced that smoothly bridges the two limiting cases and applies the physically justified conservation scheme at all times. It is also shown that the semiclassical formalism can predict erroneous results if the electronic amplitudes for well-separated hops are added coherently. This is a fundamental problem with the formalism which leads to unphysical results if left unattended. Alternative schemes are introduced for dealing with this problem and their accuracies are assessed. Generalization of the well-known Landau-Zener formula to the time-dependent Hamiltonian case is derived, which allows one to significantly decrease the computational overhead involved with the numerical implementation of the semiclassical method. Finally, we show that in strong-field molecular dissociation a trajectory can “surf” a moving avoided crossing. In this case the hopping probability is a sensitive function of the interference between two closely spaced avoided crossing regions.
Vibrational predissociation of the I2⋯Ne2 cluster: A molecular dynamics with quantum transitions study
Authors: Bastida A. , Zuñiga J. , Requena A. , Halberstadt N. , Beswick J. A.
Published: July 2002

The MDQT (molecular dynamics with quantum transitions) method of Tully is applied to the vibrational predissociation of a Van der Waals cluster containing a diatomic molecule and two rare gas atoms, I2⋯Ne2. The vibrational degree of freedom of the diatomic is treated quantum mechanically using DVR (discrete variable representation) while all the other degrees of freedom are treated classically. The results are in very good agreement with the experimentally measured lifetimes and product state distributions. In particular, the final vibrational state distribution of I2, which could not be satisfactorily reproduced in quasiclassical studies, is well described. Based on these results a different kinetic scheme for interpreting the vibrational predissociation in this system is proposed. In addition, this work shows that the method is very promising for the study of clusters containing more rare gas atoms.
The pair of soliton-like distortions in organic ferromagnetic conjugated polymers
Authors: Li Z. J. , Lin H. Q. , An Z. , Yao K. L.
Published: July 2002

We study a soliton-like distortion in a quasi-one-dimensional conjugated carbon chain with a single side radical, which contains an unpaired electron, by taking into account the electron–phonon interaction, an extended Hubbard term due to on-site electron–electron interactions and intersite e–e interactions, and the ferromagnetic correlation between itinerant π-electrons and the unpaired radical electron. It is shown that a critical value of hopping integral exists between the side radical and the main chain, at which a transition from a single soliton-like distorted structure to a pair of soliton-like distortions occurs along the main chain. In addition, the spin density at the dangling site is transferred to the main chain due to the hopping interaction between the main chain and the side radical, and the spin density at the side radical is decreased to zero when the hopping integral is larger than the critical value.