bluuebunny/crossref_metadata_2025 huggingface dataset:
Ab initio study of nonadiabatic interactions in the photodissociation of ketene
Authors: Cui Qiang , Morokuma Keiji
Published: July 2002

Ab initio calculations have been carried out on potential-energy surfaces for the photodissociation of ketene. S0 and S1 state cross extensively around the Franck–Condon (F–C) region upon C–C–O bending, and the S1→S0 internal conversion is expected to be very efficient. S1 and T1 stay close in energy in the F–C region, but do not couple strongly due to the small spin–orbit coupling, and direct S1→T1 intersystem crossing is unlikely. The triplet state, which produces the ground-state products is likely to be formed via the process S1→S0→Tn. S0 crosses with the lowest triplet state (T1 or T2) at rather low energy near the triplet minimum. The S0/Tn crossing persists all along the C–C dissociation pathway. As C–C is stretched, the energy of the crossing increases and the crossing structure deviates substantially from the reaction path. These results suggest that, if intersystem crossing at higher potential energy is favored, the rate of reaction may reflect the dynamics of intersystem crossing and that on the triplet surface.
Radiation damping transfer in nuclear magnetic resonance experiments via chemical exchange
Authors: Chen Jin-Hong , Mao Xi-An
Published: July 2002

In nuclear magnetic resonance (NMR) experiments on samples containing biomolecules (proteins, polypeptides, and nucleic acids), chemical exchange between the biomolecular NH protons and water influences NMR measurements in many aspects, and it is necessary to know the exchange rate constants. However, because the exchanging system usually consists of weak NH signals and a strong water resonance that is undergoing radiation damping, it is difficult to quantitatively study the exchange dynamics using established methods. We suggest a new approach, called radiation damping transfer method, for studying the exchange dynamics, which takes advantage of radiation damping of water. When the radiation damping effect is transferred from water to the mobile NH protons through chemical exchange, the exchange rate constants can be quantitatively evaluated. In this paper, the Bloch equations modified by both radiation damping and chemical exchange are solved. Radiation damping transfer effects are discussed. The new method is demonstrated by an example through analysis of the inversion-recovery data that are strongly influenced by radiation damping transfer.
ZEKE spectroscopy with coherent vacuum ultraviolet radiation: The X 2∑g+ and A 2∏u states of N2+ in the 15.5 eV to 17.7 eV photon energy range
Authors: Hepburn J. W.
Published: July 2002

The zero kinetic energy (ZEKE) photoelectron spectrum of nitrogen has been recorded using coherent vacuum ultraviolet light for single-photon excitation to the various ionization thresholds. Ionization thresholds from the lowest (15.58 eV) to 17.7 eV were studied. In this energy range, both Franck–Condon allowed (v+=0 and 1) and Franck–Condon forbidden (v+=4, 5, 6, and 8) levels of the N2+ X 2∑g+ ground electronic state were studied. The rotational intensities of these bands were studied, and a comparison made with previous ab initio work on the photoionization of N2. The role of complex resonances in the alteration of rotational linestrengths is discussed with reference to a high resolution photoionization spectrum recorded in the region of the v+=1 threshold. It was also possible to compare the relative intensities of all of these vibrational bands, to investigate the mechanism for observation of Franck–Condon forbidden vibrational bands in ZEKE spectroscopy. These results are discussed in terms of resonant autoionization, which is commonly observed in threshold photoelectron spectroscopy. The v+=0, 1, 2, and 4 levels of the A 2∏u excited state of N2+ were also studied, and the rotational structure was compared with ab initio and experimental results for the same state in the isoelectronic system, CO+. The band intensities were also measured, and compared with the Franck–Condon factors for excitation to the A state vibrational levels for the v=0 state of N2. A comparison with the v+=0 band of the X state is used to obtain an estimate of the relative signal yield for these excited state thresholds. For most of the bands observed in the present study, there are previous lower resolution threshold photoelectron spectra recorded using synchrotron radiation, and a comparison is made with these previous results.
Ab initio study and millimeter-wave spectroscopy of P2O
Authors: Bailleux Stéphane , Bogey Marcel , Demuynck Claire , Destombes Jean-Luc , Liu Yuyan , Császár Attila G.
Published: July 2002

The millimeter-wave spectrum of P2O produced by reacting P4 vapor with atomic or molecular oxygen directly inside the absorption cell has been observed in 13 vibrational states. Reliable identification of the various vibrationally excited states has been achieved with the help of high-level ab initio calculations resulting in accurate geometric data and an anharmonic force field. Careful analysis of the vibrational levels observed resulted in 13 sets of rotational and centrifugal distortion parameters, which have been used, in turn, to determine the equilibrium rotational, quartic centrifugal distortion, and rovibrational interaction constants of P2O. The 2ν1−ν3 Fermi resonance has been analyzed.
Rovibrational states of a tetrahedral molecule in a hexagonal close-packed crystal
Authors: Momose Takamasa
Published: July 2002

The vibration-rotation states of a tetrahedral molecule in a field of D3h symmetry are studied by an extended group theory. Symmetry adapted rotational wave functions for the ground and the triply degenerate excited vibrational states are constructed. The crystal field potential is derived explicitly for a tetrahedral molecule in a hexagonal close-packed crystal. As an example, the rovibrational energy states of methane in solid parahydrogen are computed. Optical selection rules are also derived from the theory.
Silacyanogen
Authors: Ignatyev Igor S. , Schaefer Henry F.
Published: July 2002

Structures of all possible linear isomers of the Si-substituted cyanogen were optimized at the SCF, B3LYP, MP2, and CCSD levels of theory. At all levels of theory the most stable isomer was found to be SiNCN, while for cyanogen the most stable structure is well known to be NCCN. The theoretical ordering of the stability of the acyclic structures is SiNCN>SiNNO>NSiCN>NSiNC. The difference in the order of stability of silacyanogen and cyanogen isomers is rationalized, taking into account the position of the unpaired electron on carbon in the CN radical and that on nitrogen in the SiN radical.
A structural model for associated liquid ethanol developed from transient spectroscopy
Authors: Laenen R. , Rauscher C.
Published: July 2002

We propose a detailed structural model for ethanol oligomers assuming it to form open chains. An inhomogeneous broadening with a width of about 100 cm−1 of the OH-stretching mode of the hydrogen-bonded species is considered, a parabolic increase of the absorption strength and a cubic dependency of the frequency of this vibration with position within the associate. With these asumptions we are able to account for the measured conventional IR absorption spectra of ethanol at different dilutions in CCl4 with the most probable length of the associates ranging between 2 and 13 molecules, respectively. Application of our model to time-resolved spectroscopy on a 0.17 M sample with excitation performed at 3340 cm−1 yields a length of the excited oligomers of 4 to 5 and of broken species of 2 to 3, respectively.
Effect of temperature and hydration on protein fluctuations: Molecular dynamics simulation of Cu, Zn superoxide dismutase at six different temperatures. Comparison with neutron scattering data
Authors: Melchionna S. , Falconi M. , Desideri A.
Published: July 2002

The equilibrium properties of solvated Cu,Zn superoxide dismutase have been sampled in the isothermal-isobaric ensemble at six different temperatures, namely at T=80, 130, 200, 225, 250, and 300 K. Molecular dynamics simulations covering the same range of temperatures have been also carried out for the protein in vacuo. Evaluation of the Debye–Waller factors as a function of temperature indicates that the “glasslike” transition, experimentally observed at approximately 200 K by neutron scattering experiments, is well reproduced when the protein is simulated in the presence of water solvent whereas the simulations without solvent are not able to reproduce the experimental results. Analysis of anharmonicity and anisotropy of the atomic motions indicates that these parameters are good indicators for the occurrence of the transition. Analysis of the atomic fluctuations of different protein shells, having different degrees of exposure to the solvent, shows that the transition is driven by the protein atoms belonging to the external shell, whose Debye–Waller factors are found to be larger in vacuo than in the solvated system.
Vibration–torsion–rotation analysis of the ν12 band of CH3CD3
Authors: Moazzen-Ahmadi N. , Ozier I. , Mukhopadhyay I. , McKellar A. R. W.
Published: July 2002

A Fourier transform spectrum of the lowest frequency degenerate fundamental of CH3CD3 (v12=1←0) near 680 cm−1 has been measured in order to investigate the vibration–torsion–rotation effects in a symmetric top. The spectrum was recorded at an instrumental resolution of 0.0024 cm−1 using a modified Bomem spectrometer. The temperature and pressure of the sample were 130 K and 0.4 Torr, respectively. Although the intrinsic tunneling splittings in each (J′←J″) multiplet are the order of 0.002 cm−1 or smaller, the splittings typically observed for an intermediate J″ of 10 are two to three orders of magnitude larger. These splittings are caused primarily by the Coriolis interaction between the torsional stack of levels v6=0,1,2,… for v12=1 and the corresponding stack for v12=0. The shift of each upper level (v12=1, l; J, v6=0, K, σ) is seen to be a sensitive function of (l;K,σ), where σ labels the torsional sublevels. Because of near degeneracies between the upper level and its interaction partner with (v12=0, v6=3), four different (l;K,σ) torsion–rotation series are resonantly perturbed. For three of these cases, perturbation-allowed (v6=3←0) torsional transitions have been identified. Over 2000 transitions belonging to the ν12 and 3ν6 bands have been assigned. The measurements from the present experiment and frequencies from previously reported studies in the ground vibrational state were fitted to within experimental uncertainty using an effective Hamiltonian discussed earlier in connection with CH3SiH3 [Moazzen-Ahmadi et al., J. Mol. Spectrosc. 137, 166 (1989)]. Several Coriolis parameters were determined. In particular, the experimental value of ζ̃12z=0.248 657(50) is in good agreement with the calculated value of 0.24, whereas the experimental value of ζ̃6,12x=0.2245(45) is in clear disagreement with the calculated value of 0.56.
Vibronic spectra in condensed matter: A comparison of exact quantum mechanical and various semiclassical treatments for harmonic baths
Authors: Egorov S. A. , Rabani Eran , Berne B. J.
Published: July 2002

We consider the problem of calculating the vibronic absorption spectrum of a diatomic molecule coupled to a condensed phase environment, where all nuclear degrees of freedom are taken in the quadratic approximation, and where the two electronic states couple differently to the solvent. This simple model is used to examine several commonly used semiclassical approximations. The method of Kubo–Toyozawa is adapted to enable exact calculation of the real-time dipole autocorrelation function for the quantum mechanical treatment. Alternatively, we derive an expression for this correlation function in terms of a path-integral influence functional, which is not limited to a finite number of bath modes and could be applied to treat anharmonic solutes in condensed matter. We then obtain an analytical solution for the classical treatment of nuclear dynamics, and develop a mixed quantum-classical approach, where the dynamics of the diatomic vibrational mode is treated quantum mechanically and the bath is treated classically. It is shown that the mixed quantum-classical treatment provides better agreement with the exact quantum treatment than the other approximations for a wide range of parameters. Exact analytical results similar to the pure dephasing theory of Skinner and Hsu are obtained for the asymptotic long time behavior of the dipole autocorrelation functions.
Dynamics of self-assembled surfactant systems
Authors: von Gottberg Friedrich K. , Smith Kenneth A. , Hatton T. Alan
Published: July 2002

The dynamics of self-assembling systems were investigated for the model amphiphile A2B2 using stochastic dynamic simulations. Temperature jump computer “experiments” were performed and the evolution of the system to its new equilibrium state monitored. The results were interpreted based on the Aniansson–Wall theory of micellar kinetics. The transient behavior predicted using the Aniansson–Wall theory agrees well with the simulated data, particularly at short times. At long times, deviations are observed which may be ascribed to errors in estimating the dissociation rate and number density of aggregates in the all important micelle-depleted zone. The micellar dissociation constant was calculated from independent tagging simulations. The amphiphile exit rate constant was calculated at different temperatures from which an activation energy associated with the removal of a surfactant chain from a micelle was found to be of order 10–15 kT and to be independent of the friction coefficient. Finally the Helmholtz free energy profile associated with the extraction of a surfactant chain from a micelle was determined. A free energy barrier height of order 5 kT was obtained. Kramers’ rate theory was employed to determine the corresponding exit rate constant which was found to be in excellent agreement with the results obtained from tagging runs. The barrier height associated with the insertion of a surfactant chain was order 1 kT, suggesting that the association process is diffusion controlled.
Contracted distributed approximating functions: Derivation of non-oscillatory free particle and harmonic propagators for Feynman path integration in real time
Authors: Szalay Viktor
Published: July 2002

Contracted continuous distributed approximating functions (CCDAFs) have been developed. In particular, it has been shown that, continuous distributed approximating functions (CDAFs) based on standard orthogonal polynomials can be contracted to functions formed as the product of a weight function and the sinc function or a Bessel function of the first kind. The CCDAFs of Hermite type have been applied to derive new expressions for the coordinate representation of the free particle evolution operator and that of the evolution operator of harmonic oscillator. These new expressions of free particle and harmonic propagators have as compact mathematical form as Makri’s effective free propagator [N. Makri, Chem. Phys. Lett. 159, 489 (1989)] and Gaussian decay identical to that of the CDAF class free and harmonic propagators due to Kouri et al. [D. J. Kouri, W. Zhu, X. Ma, B. M. Pettitt, and D. K. Hoffman, J. Phys. Chem. 96, 9622 (1992)] and Marchioro et al. [T. L. Marchioro II, M. Arnold, D. K. Hoffman, W. Zhu, Y. Huang, and D. J. Kouri, Phys. Rev. E50, 2320 (1994)], respectively. The Gaussian decay of a CCDAF Hermite free propagator has been shown to be the result of including momentum eigenstates in the propagator which have momenta larger than the momentum of the wave packet of largest momentum that still can be well approximated by the CCDAF considered.
Reactive scattering of highly vibrationally excited oxygen molecules: Ozone formation?
Authors: Lauvergnat David , Clary David C.
Published: July 2002

A new ab initio potential energy surface based on an internally contracted multireference configuration–interaction wave function is constructed for the O2(X 3Σg−,υ)+O2(X 3Σg−,υ=0)→O3(X 1A1)+O(3P) reaction with υ>20. The vibrational state-to-state reaction probabilities are calculated with a time independent reactive scattering method. The state selected reactive rate constants calculated with 2D reduced dimensionality theory are very small, suggesting that the reaction of ozone formation is not significant in the O2(X 3Σg−,υ)+O2(X 3Σg−,υ=0) collision.
Vibrational energy relaxation, nonpolar solvation dynamics and instantaneous normal modes: Role of binary interaction in the ultrafast response of a dense liquid
Authors: Biswas Ranjit , Bhattacharyya Sarika , Bagchi Biman
Published: July 2002

Recently instantaneous normal mode analysis has revealed an interesting similarity of the solvent dynamical influence on two rather different phenomena, namely vibrational energy relaxation (VER) and nonpolar solvation dynamics (NPSD). In this work we show that this similarity can be rationalized from a mode coupling theoretic analysis of the dynamic response of a dense liquid. The present analysis demonstrates that VER and the initial NPSD are coupled primarily to the binary part of the frequency dependent frictional response of the liquid. It is found that for strong solute–solvent interaction, the initial decay of nonpolar solvation dynamics can proceed with time constant less than 100 fs. In addition, a very good agreement between the calculated and the simulated VER rates have been obtained for a large range of frequency.
Semiclassical theory of vibrational energy relaxation
Authors: Karrlein Robert , Grabert Hermann
Published: July 2002

A theory of vibrational energy relaxation based on a semiclassical treatment of the quantum master equation is presented. Using new results on the semiclassical expansion of dipole matrix elements, we show that in the classical limit the master equation reduces to the Zwanzig energy diffusion equation. The leading quantum corrections are determined and discussed for the harmonic and Morse potentials.
Theoretical investigations on the reactions NH+HO2 and NH2+O2: Electronic structure calculations and kinetic analysis
Authors: Sumathi R. , Peyerimhoff S. D.
Published: July 2002

Electronic structure calculations at the MP2, B3LYP-DFT, and quadratic configuration interaction singles and doubles levels of theory, with 6-311++G** and 6-311++G(2df,2pd) basis sets, are reported for the stationary points on the NH+HO2 doublet potential energy surface. Also the transition state on the quartet surface for the direct hydrogen abstraction reaction has been identified. Two minima viz., HNOOH and NH2O2, of almost equal stability and with a very high interconversion barrier have been found. Preferential dissociation of HNOOH to HNO and OH is reported due to its high isomerization barrier. The favorable dissociation channels of the NH2O2 adduct are those leading to NH2+O2 and NH2O+O products. Detailed kinetic analyses have been performed on the calculated DFT-B3LYP potential energy surfaces using quantum statistical Rice–Ramsperger–Kassel theory. The calculated total rate constant for NH+HO2 reaction at 300 K and 1 atm is 1.52×1010 cm3 mol−1 s−1 and the predominant contribution to the disappearance of the HNOOH adduct is the HNO+OH dissociation channel, K31. The NH2+O2 reaction is found to be a slow reaction and the calculated rate coefficient is in good agreement with the upper limit predicted by the experimentalists.
A photoelectron spectroscopic study of monovanadium oxide anions (VOx−, x=1–4)
Authors: Wu Hongbin , Wang Lai-Sheng
Published: July 2002

We report on a photoelectron spectroscopic study of monovanadium oxides, VOx− (x=1–4), at four photon energies: 532, 355, 266, and 193 nm. Vibrationally resolved spectra are obtained for VO− at 532 and 355 nm detachment photon energies. Two new low-lying excited states are observed for VO at 5630 and 14 920 cm−1 above the ground state. These states are assigned to two doublet states, Σ−2 and Φ,2 respectively. The 532 and 355 nm spectra of VO2− reveal a single vibrational progression for the ground state with a frequency of 970 cm−1 (ν1). Three electronic excited states are observed for VO2 in the 193 nm spectrum. For VO3−, three surprisingly sharp detachment transitions are observed at 193 nm. The two excited states of VO3 are measured to be 0.59 and 0.79 eV above the ground state. The spectra of VO2− and VO3− are interpreted using the molecular-orbital schemes obtained in a recent ab initio theoretical study [Knight, Jr. et al., J. Chem. Phys. 105, 10237 (1996)], which predicts that both VO2 and VO3 neutrals are of C2v symmetry with a doublet ground state. The spectrum of VO4− is obtained at 193 nm, showing features similar to that of VO3−, but much more broadened. The adiabatic electron affinities of VO, VO2, VO3, and VO4 are measured to be 1.229 (8), 2.03 (1), 4.36 (5), and 4.0 (1) eV, respectively, with a significant increase from VO2 to VO3. The electronic and geometrical structures of the series of monovanadium oxide species are discussed based on the current observation and previous spectroscopic and theoretical results.
A stochastic technique for solving the Lorentz–Boltzmann equation for hard spheres: Application to the kinetics of gas absorption
Authors: Lowe C. P. , Masters A. J.
Published: July 2002

The Lorentz–Boltzmann equation for tagged particle motion in a hard sphere fluid may be interpreted as describing the motion of a particle propagating via a series of binary uncorrelated collisions in a structureless bath of fluid particles with a Maxwellian distribution of velocities. We describe a very general stochastic technique for solving the equation. The method can also be extended to the Enskog level, valid up to somewhat higher densities, by a simple scaling of the time. Having reproduced several known results for the Lorentz–Boltzmann equation we extend the method to a simple reaction process where there is no analytic result—the kinetics of gas absorption for a gas confined between two plates. For this process there are two simple analytic limits—the Knudsen limit (in which there are no collisions between absorbing particles) and the diffusive limit (where there are a large number of collisions between absorbing particles). We show that regardless of the Knudsen number, Kn, the Knudsen limit describes the very short time kinetics and the diffusive limit describes the long time kinetics. However, at moderate values of the Knudsen number the rate constant characterizing the long time kinetics differs from the diffusive value. This discrepancy scales away slowly (as 1/Kn) with increasing Knudsen number.
On the ground electronic states of copper silicide and its ions
Authors: Boldyrev Alexander I. , Simons Jack , Scherer J. J. , Paul J. B. , Collier C. P. , Saykally R. J.
Published: July 2002

The low-lying electronic states of SiCu, SiCu+, and SiCu− have been studied using a variety of high-level ab initio techniques. As expected on the basis of simple orbital occupancy and bond forming for Si(s2p2)+Cu(s1) species, 2Πr, 1Σ+, and 3Σ− states were found to be the ground electronic states for SiCu, SiCu+, and SiCu−, respectively; the 2Πr state is not that suggested in most recent experimental studies. All of these molecules were found to be quite strongly bound although the bond lengths, bond energies, and harmonic frequencies vary slightly among them, as a result of the nonbonding character of the 2π-MO (molecular orbital) [composed almost entirely of the Si 3p-AO (atomic orbital)], the occupation of which varies from 0 to 2 within the 1Σ+, 2Πr, and 3Σ− series. The neutral SiCu is found to have bound excited electronic states of 4Σ−, 2Δ, 2Σ+, and 2Πi symmetry lying 0.5, 1.2, 1.8, and 3.2 eV above the 2Πr ground state. It is possible but not yet certain that the 2Πi state is, in fact, the “B state” observed in the recent experimental studies by Scherer, Paul, Collier, and Saykally.
Remarks on the information entropy maximization method and extended thermodynamics
Authors: Eu Byung Chan
Published: July 2002

The information entropy maximization method was applied by Jou et al. [J. Phys. A 17, 2799 (1984)] to heat conduction in the past. Advancing this method one more step, Nettleton [J. Chem. Phys. 106, 10311 (1997)] combined the method with a projection operator technique to derive a set of evolution equations for macroscopic variables from the Liouville equation for a simple liquid, and a claim was made that the method provides a statistical mechanical theory basis of irreversible processes and, in particular, of extended thermodynamics which is consistent with the laws of thermodynamics. This line of information entropy maximization method is analyzed from the viewpoint of the laws of thermodynamics in this paper.
Reorientational tunneling of partially deuterated methyl groups: A single-crystal deuteron NMR study of aspirin-CH2D
Authors: Detken A. , Zimmermann H.
Published: July 2002

Partially deuterated methyl groups in single crystals of aspirin are investigated by deuteron NMR at temperatures between 8 K and room temperature. The CH2D groups perform reorientations which are governed by a rotational potential with three wells, two of which are almost equally deep whereas the third is significantly deeper. At temperatures below 20 K, a so far unobserved type of incoherent tunneling process is identified. This process consists in reorientations between the two upper potential wells which are fast on the time scale of the quadrupolar interaction, whereas transitions into the deeper well are slow on this time scale. At temperatures above 35 K, the methyl groups perform thermally activated stochastic reorientations between all three potential wells. By determining the relative populations of the three wells as a function of temperature, the energy difference between the lower and the two upper wells is found to be 3.3 meV. This amounts to almost 8% of the average barrier height, which is determined from the temperature dependence of the spin-lattice relaxation time to be 43 meV.
Time-dependent self-consistent-field dynamics based on a reaction path Hamiltonian. I. Theory
Authors: Fang Jian-Yun , Hammes-Schiffer Sharon
Published: July 2002

A method that combines the time-dependent self-consistent-field (TDSCF) method with the reaction path Hamiltonian (RPH) derived by Miller, Handy, and Adams [J. Chem. Phys. 72, 99 (1980)] is proposed. This TDSCF-RPH method allows the calculation of the real-time quantum dynamics of chemical reactions involving polyatomic molecules. When both the coupling between the normal modes and the curvature are zero, the dynamics of an F-dimensional system is shown to reduce to a one-dimensional numerical time propagation. When the reaction path curvature is zero and the coupling between the normal modes is non-zero, the dynamics is shown to still reduce to a one-dimensional problem for a specific choice of initial wavepacket (which can have an arbitrary component for the reaction coordinate), but F coupled one-dimensional equations of motion must be propagated for a general initial wavepacket (unless the RPH is transformed to the diabatic representation). When the coupling between the normal modes is zero and the reaction path curvature is non-zero but small, the dynamics is shown to reduce to a one-dimensional numerical time propagation for an arbitrary initial wavepacket. The derivations of the equations of motion for these cases are presented in this paper, and numerical tests are presented in a separate paper.
Ground-state properties of doped He3 clusters
Authors: Garcias F. , Serra Ll. , Casas M. , Barranco M.
Published: July 2002

In the framework of finite-range density functional (FRDF) theory, we have investigated the ground-state properties of HeN3 drops doped with inert gas atoms and SF6 molecules. Compared with the HeN4 case, the solvation energies turn out to be smaller, but still negative. Due to the attraction exerted by the impurity on the surrounding He3 atoms, doped drops of any size are bound. The number of He3 atoms in the first solvation shell is calculated, and it is compared with that of doped HeN4 drops, for which FRDF results are also available.
Network forming fluids: Integral equations and Monte Carlo simulations
Authors: Duda Yurko , Segura Chad J. , Vakarin Eduard , Holovko Myroslav F. , Chapman Walter G.
Published: July 2002

A network forming four-site model associative fluid (with freely located sites) is investigated by means of associative Ornstein–Zernike integral equation theory supplemented by a Percus–Yevick-like closure relation. Since the model exhibits critical behavior, the structure relevant to the gaseous and to the liquid phases are discussed. The properties of network forming systems with different strengths of the site-site attraction are analyzed. This allows us to describe topologically asymmetrical network clusters and branching polymers. It is determined that the critical temperature as well as the critical density become lower with an increasing degree of asymmetry. NVT Monte Carlo simulations for the same model, but with a fixed location of sites, are presented. Theoretical predictions are compared to the simulation results. It is shown that the theory agrees well with the simulations, except for low densities and temperatures, where the simulations predict a well developed waterlike structure with a tetrahedral arrangement. This disagreement is shown to be caused by the difference in the site location imposed by the model potentials.
Vibrational relaxation of T1 pyrazine: Results from the refined competitive radiationless decay method
Authors: McDowell Derek R. , Wu Fei , Weisman R. Bruce
Published: July 2002

Gas phase collisional vibrational relaxation of pyrazine in its lowest triplet electronic state has been further investigated using a refined version of the competitive radiationless decay (CRD) method. Improvements to the experimental apparatus now provide primary kinetic data of much greater accuracy and precision, allowing the use of sample pressures low enough to ensure negligible self-relaxation. A major refinement of the data handling procedure permits the sample’s energy-dependent triplet–triplet molar absorptivity to be determined through an iterative analysis. To validate the refined CRD method, average energy loss per collision has been deduced as a function of donor energy under conditions of varying sample pressure, relaxer pressure, excitation beam energy, and excitation wavelength. Consistent results have been obtained for all of these variations, suggesting the absence of major systematic errors. The two main findings of earlier pyrazine triplet relaxation studies are confirmed by the new measurements: the presence of a threshold donor energy above which relaxation becomes much more efficient, and relatively large values of average energy loss per collision. Comparing to results from a recent ground state study [L. A. Miller and J. R. Barker, J. Chem. Phys. 105, 1383 (1996)], it is found that pyrazine containing 5000 cm−1 of vibrational energy is relaxed by a variety of monatomic, diatomic, and polyatomic gases approximately seven times more efficiently when the donor is in its triplet rather than its ground electronic state. The order of relaxer efficiencies toward triplet pyrazine is found to be He<H2<Ne<D2<N2<Kr<Ar<Xe<CH4<CO<CO2<CH3F<H2O. Energy loss efficiencies correlate rather well with relaxer boiling points.
Time-reversal of cross-polarization in nuclear magnetic resonance
Authors: Ernst Matthias , Meier Beat H. , Tomaselli Marco , Pines Alexander
Published: July 2002

It is demonstrated that the time evolution of the heteronuclear polarization-transfer process in a dipolar-coupled nuclear spin system can be reversed, leading to the observation of cross-polarization echoes. The cross-polarization echoes are induced by consecutive application of two pulse trains that produce effective Hamiltonians that differ only in sign. Cross-polarization echoes have been recorded for a powder sample of alanine. The time evolution of the spin system is consistent with both unitary quantum dynamics and with spin thermodynamics of two systems approaching a common spin temperature.
Reactions of oxygen atoms with van der Waals complexes: The effect of complex formation on the internal energy distribution in the products
Authors: McCoy A. B. , Lufaso M. W. , Veneziani M. , Atrill S. , Naaman R.
Published: July 2002

Reactions of atomic oxygen with complexes containing HCl are investigated and the OH product state distributions are compared to those observed for the corresponding reactions of HCl monomers. In previous studies of reactions of O(3P) with HCl and hydrocarbon complexes, rotationally colder OH product state distributions were observed, when compared to the corresponding reactions of monomers. In contrast, we find that reactions of O(1D) with HCl clusters yield OH rotational distributions that are unaffected by the incorporation of HCl into a van der Waals complex. Quasiclassical trajectories are run on collisions of oxygen with HCl and Ar⋯HCl at 1 eV collision energies to investigate the differences in the dynamics of the O(1D) and O(3P) reactions. It is found that when the van der Waals complex is longer lived than the collision complex, rotational and vibrational cooling are observed. In contrast, when the dissociation of the van der Waals complex is prompt, compared to the collision complex lifetime, the effects of complex formation on the internal energy of the OH product become negligible.
Self-assembly of neutral and ionic surfactants: An off-lattice Monte Carlo approach
Authors: Bhattacharya Aniket , Mahanti S. D. , Chakrabarti Amitabha
Published: July 2002

We study self-assembly of surfactants in two dimensions using off-lattice Monte Carlo moves. Here the Monte Carlo moves consist of slithering snake reptation motion of the surfactant chains and kink-jump of the individual monomers. Unlike many previous studies an important feature of our model is that the solution degrees of freedom are kept implicit in the model by appropriate choice of the phenomenological interaction parameters for the surfactants. This enables us to investigate rather large systems with less number of parameters. The method is powerful enough to study multimicellar systems with regular and inverted micelles for both neutral and ionic surfactants. As a function of several parameters of the model, we study self-assembly of neutral surfactants into micelles of various forms and sizes and compute appropriate cluster-size distributions. Ionic surfactants exhibit, apart from micellization, additional intermicellar ordering. We further study the role of host particles to mimic recent experiments on surfactant-silicate cooperative self-assembly, and demonstrate the possibility of generalized pathways leading to host encased micellization.
Pure bending dynamics in the acetylene X̃ 1Σg+ state up to 15 000 cm−1 of internal energy
Authors: Jacobson Matthew P. , O’Brien Jonathan P. , Silbey Robert J. , Field Robert W.
Published: July 2002

We investigate the large-amplitude bending dynamics of acetylene, in its ground electronic state, using an effective Hamiltonian model that reproduces all relevant experimental data, up to 15 000 cm−1 in internal energy, with 1.4 cm−1 accuracy (1σ). The experimental data which make this analysis possible are derived from the dispersed fluorescence (DF) data set that we recently reported [J. P. O’Brien et al., J. Chem. Phys. 108, 7100 (1998)] for the acetylene Ã 1Au→X̃ 1Σg+ system, which includes DF spectra recorded from five different vibrational levels of the Ã 1Au state. A numerical pattern recognition technique has permitted the assignment of polyad quantum numbers to observed transitions in these spectra, with up to 15 000 cm−1 in internal energy. Here we analyze a special subset of the identified polyads, those which involve excitation exclusively in the trans and cis bending modes: the pure bending polyads. The bending dynamics that is encoded in these polyads is analyzed using both frequency and time-domain formalisms. Among the conclusions of this analysis is that, in many ways, the observed bending dynamics is somewhat simpler at 15 000 than it is at 10 000 cm−1; this rather surprising result is explained in terms of qualitative changes in the structures of the pure bending polyads as a function of increasing internal energy.
Theory for magnetic linear dichroism of electronic transitions between twofold-degenerate molecular spin levels
Authors: Bominaar Emile L. , Achim Catalina , Peterson Jim
Published: July 2002

Magnetic linear dichroism (MLD) spectroscopy is a relatively new technique which previously has been almost exclusively applied to atoms. These investigations have revealed that the study of MLD, in conjunction with electronic absorption and magnetic circular dichroism (MCD) spectroscopies, provides significant additional information concerning the electronic structure of atoms. More recent measurements have indicated that MLD is also observable from transition ions in inorganic compounds and metalloproteins. While the theory for atomic MLD has been worked out in considerable detail during the last two decades, an MLD theory of practical utility for the analysis of the spectra derived from the majority of paramagnetic molecules is not available. In the present contribution, the MLD of an electric-dipole-allowed transition between twofold-degenerate molecular spin levels is analyzed, assuming nonsaturating conditions. As for atomic systems, it is found that the MLD of a single molecule is dominated by the term G0. However, this term vanishes in the powder average evaluated for a randomly oriented ensemble of molecules, leading to a drastic reduction of the MLD differential absorption for systems with spin S=1/2 compared to that observed for systems with higher ground-state spin. It is found that MLD and MCD spectroscopies on solution samples have complementary spin-state specific sensitivities which suggest that the two methods can be used to selectively probe the individual metal sites in multicenter metalloprotein assemblies.
Theory for the ultrafast melting and fragmentation dynamics of small clusters after femtosecond ionization
Authors: Garcia M. E. , Reichardt D. , Bennemann K. H.
Published: July 2002

The ultrafast relaxation of small clusters immediately after ultrashort single ionization is studied. We use an electronic model and molecular dynamics simulations to analyze the ionization induced fragmentation dynamics of small Hgn clusters. Our results show that fragmentation consists mainly in emission of single atoms. We also find a remarkable difference between this nonequilibrium unimolecular dissociation and that described by the Rice–Ramsperger–Kassel–Marcus theory. For fixed cluster size, we determine the time-dependent fragmentation probability of a cluster ensemble as a function of the temperature before ionization. A dramatic change of the fragmentation behavior occurs when the temperature before ionization reaches the “melting temperature” of the neutral clusters. For all cluster sizes studied, the temperature dependence of the average fragmentation times after ionization shows a remarkable correlation with the root-mean-square bond length fluctuations δ(T) of the clusters before ionization. This new effect reflects sensitivity of the ultrashort-ionization induced fragmentation dynamics to the melting dynamics of the neutral clusters.
Classical Monte Carlo study of phase transitions in rare-gas clusters adsorbed on model surfaces
Authors: Matos Nidia Y. , López Gustavo E.
Published: July 2002

Heat capacities, as a function of temperature, are calculated for Ar7, Ne7, and Kr7 clusters adsorbed on the basal plane of graphite using the J-walking Monte Carlo technique. The graphite surface is modeled using the Steele potential, with and without corrugation. By applying simulated annealing techniques, the minimum-energy configuration at zero kelvin is obtained. The minimum-energy configuration in the flat surface is a two-dimensional structure at a distance above the surface, identical to that determined for the rare-gas atom forming the cluster. On the other hand, the corrugated surface yields incommensurate clusters, but which are centered in the hexagonal adsorption sites of the basal plane of graphite. The large number of high-energy isomers in the corrugated surface leads to an increase in the coexistence region when compared to the flat surface. Calculation of heat capacity as a function of temperature clearly shows this increment.
Energy dependence of the isomerization of HCN+ to HNC+ via ion molecule reactions
Authors: Hansel A. , Glantschnig M. , Scheiring Ch. , Lindinger W. , Ferguson E. E.
Published: July 2002

The mechanism for isomerization of HCN+ ions to the more stable HNC+ in collision with CO2 at thermal energy has been confirmed to be a double proton transfer in the collision complex. First a proton is transferred from C in HCN+, then, following CN rotation in the complex, the proton is transferred to the N atom, both proton transfers being exothermic. The mechanism has been established by measuring the reaction of HCN+ with CO2 as a function of energy from thermal up to 0.7 eV relative kinetic energy in a selected ion flow drift tube. The isomers are distinguished by the use of monitors that react differently with the two isomers. A similar HCN+ to HNC+ isomerization occurs in thermal collisions with CO.
Rainbow scattering of methane from LiF(100): Probing the corrugation and anisotropy of the gas-surface potential
Authors: Wight A. C. , Miller R. E.
Published: July 2002

We have used high-resolution infrared laser spectroscopy and optothermal detection to investigate the scattering of methane from LiF(100). The angular, translational, and rotational distributions of the scattered molecules are reported as a function of incident energy, surface temperature, and incident angle. The results show structured angular distributions that can be understood in terms of rainbow scattering, providing information on the surface corrugation appropriate to methane, as well as insights into the energy transfer between the methane and the surface. In addition, the energy dependence of the rainbow scattering, together with the rotational distributions, provide insights into the molecular anisotropy at both long and short ranges. The experimental results are compared with those of the washboard model for rainbow scattering [J. C. Tully, J. Chem. Phys. 92, 680 (1990)].
Nonresonant intermolecular spectroscopy beyond the Placzek approximation. I. Third-order spectroscopy
Authors: Murry Robert L. , Fourkas John T. , Keyes T.
Published: July 2002

We present an instantaneous normal mode analysis of the intermolecular Raman-weighted density of states of CS2, including the effects of the second-derivative tensor of the many-body polarizability. Based on the results of these calculations, we suggest a set of minimal elements that are necessary for collective-coordinate models of polarizability dynamics in liquids. The effects of different scattering mechanisms, the tensor nature of the polarizability, and the complex relationship between derivatives of the polarizability for intermolecular modes are shown to be essential elements of such a model.
Rotationally resolved photoelectron spectroscopy of autoionizing states of water
Authors: Glab W. L. , Child M. S. , Pratt S. T.
Published: July 2002

Rotationally resolved photoelectron spectra are reported for rovibronically state-selected autoionizing levels of water. These photoelectron spectra are helpful for the spectroscopic assignment of the autoionizing levels and provide considerable dynamical information on the mechanisms for the transfer of energy and angular momentum between the ion core and the Rydberg electron. As a result of angular momentum restrictions, photoelectron spectra for J=0 autoionizing levels provide a direct partial wave analysis for the ejected photoelectrons.
The wavelength dependence of photoinduced hot electron dissociative attachment to methyl bromide adsorbed on gallium arsenide (110)
Authors: Camillone N. , Khan K. A. , Lasky P. J. , Wu L. , Moryl J. E. , Osgood R. M.
Published: July 2002

The wavelength dependence of photoinduced dissociation of CH3Br via dissociative electron attachment (DEA) of “hot” electrons for one monolayer CH3Br adsorbed on GaAs(110) has been measured. The cross section for dissociation is found to decrease monotonically by two orders of magnitude as the incident wavelength is varied from 308 to 550 nm. There is an apparent threshold near 490 nm (2.5 eV), well below the gas phase photodissociation threshold near 250 nm (5.0 eV), but in good agreement with a simple estimate based on expected values for the decrease in the photoemission threshold and the lowering of the molecular affinity level upon adsorption of CH3Br on a semiconductor surface. The observed threshold is found to move to higher energy as dissociation of the monolayer proceeds. Based on the work of Hasselbrink and co-workers [F. Weik, A. de Meijere, and E. Hasselbrink, J. Chem. Phys. 99, 682 (1993)], a simple theoretical model is developed which considers the tunneling of hot electrons through the interfacial barrier between the physisorbed CH3Br and the GaAs. The results of our theoretical model in conjunction with those of earlier ab initio calculations [S. Black, R. Friesner, P. H. Lu, and R. M. Osgood, Jr., Surf. Sci. 382, 154 (1997)] suggest that the adsorbate affinity level is centered at ∼0.6 eV above the (adsorbate- modified) vacuum level of the substrate. This value corresponds to a ∼1.8 eV stabilization of the negative ion resonance upon adsorption.
Vibrational states in the electronic ground state of the OH+–He and OH+–Ne complexes
Authors: Meuwly Markus , Maier John P. , Rosmus Pavel
Published: July 2002

Two-dimensional potential energy functions for the electronic ground state of OH+–He and OH+–Ne have been generated by electronic structure calculations. The potential energy functions have been modified according to an adiabatical correction scheme and used in variational calculations of vibrational energies. Dissociation energies D0 have been calculated to be 360 (OH+–He) and 810 cm−1 (OH+–Ne) for OH+ in its vibrational ground state and 416 and 974 cm−1 in the vibrationally excited state, respectively. The large amplitude bending vibration agrees to within 10 cm−1 with the experimental value. Predictions are made for further bound vibrational states.
Approximate one-electron density matrix functionals for the electron–electron repulsion energy from the hypervirial theorem
Authors: Cioslowski Jerzy , Lopez-Boada Roberto
Published: July 2002

Application of the hypervirial theorem to a certain class of two-electron operators yields the electron–electron repulsion energy as an approximate functional of the Hartree–Fock one-electron reduced density matrix. This functional is entirely parameterized by a single screening function that, upon the assumption of transferability, can be readily retrieved from the known correlation energy of the homogeneous electron gas. Invoking the properties of the actual electronic Hamiltonian, the new formalism explicitly incorporates dispersion effects that are not accounted for by the conventional density functional approaches. Even more importantly, it affords analogous functionals for other variational methods such as Monte Carlo self-consistent field (MC SCF). Since such functionals are bi- and trilinear in the occupied orbitals, their fully analytical computation should be feasible with a carefully chosen approximate representation of the screening function.