bluuebunny/crossref_metadata_2025 huggingface dataset:
Ab initiocomputation of semiempirical π-electron methods. III. The benzene molecule, the zero-differential-overlap approximation, and the transferability of parameters
Authors: Martin Charles H. , Freed Karl F.
Published: July 2002

The correlated, size-extensive ab initio effective valence shell Hamiltonian (ℋν) method is used to compute the true π-electron Hamiltonian of benzene and to use it for testing various assumptions of the Pariser–Parr–Pople (PPP) semiempirical electronic structure method. On one hand, the ab initio ℋν method enables computing the low lying valencelike vertical excitation energies and ionization potentials. For example, the calculated ℋν excitation energies deviate from experiment on average by only 0.28 eV, which compares well with similar correlated ab initio calculations. More remarkably, however, the ℋν method can reproduce the vertical excitation energies on average to 0.35 eV simply by employing valence orbitals constructed from symmetry adapted linear combinations of carbon atom pπ orbitals. Thus the ℋν calculations demonstrate that accurate benzene excitation energies can be obtained using a common set of valence orbitals for every valencelike state and, in fact, for every conjugated hydrocarbon! In addition, the true, correlated benzene π-electron effective integrals are ab initio counterparts of semiempirical parameters and transfer extremely well to other small molecules such as ethylene and cyclo-butadiene. The degree of transferability is improved when increasing the size of the primitive ab initio basis set. An analysis of the ℋν effective integrals demonstrates that the true PPP model can indeed neglect a portion of one- and two-electron parameters which are assumed to be small by the zero differential overlap (ZDO) approximation, since the correlation contributions render some effective integrals negligible. On the other hand, some typically neglected parameters, such as two-electron, three-center interactions, are computed as too large to just neglect. Likewise, the exact π-electron ℋν contains three-electron many-body interactions which are large (greater than 0.1 eV) and thus cannot be explicitly neglected in PPP models.
Extreme-ultraviolet photodissociation of N2O in superexcited states
Authors: Ukai Masatoshi , Kameta Kosei , Machida Shuntaro , Kouchi Noriyuki , Hatano Yoshihiko , Tanaka Kenichiro
Published: July 2002

We demonstrate the observation of neutral dissociation, which provides the possibility of a spectroscopy of highly lying superexcited states. The yield spectrum of the undispersed fluorescence radiation of wavelength λf in the region 113≤λf≤180 nm from excited neutral fragments in the photodissociation of N2O is presented in the region of excitation photon wavelength λex in the region 30≤λex≤111 nm (photon energy region 41.3–11.2 eV). We show the evidence of selective or preferential neutral dissociation in the decay of superexcited N2O in competition with autoionization; in particular, the evidence for an important role of neutral dissociation of superexcited N2O followed by the production of ionic fragments. We also show the neutral dissociation of superexcited states with the character of double-holed doubly excited states located for 30≤λex≤60 nm. The aspect of superexcitation (bound channel of electron promotion) in the region of inner-valence excitation is discussed in relation with free (continuum) channels.
Accurate measurements of the chemical potential of polymeric systems by Monte Carlo simulation
Authors: Wilding N. B. , Müller M.
Published: July 2002

We present a new Monte Carlo method for estimating the chemical potential of model polymer systems. The method is based on the gradual insertion of a penetrable ‘‘ghost’’ polymer into the system and is effective for large chain lengths and at high densities. Insertion of the ghost chain is facilitated by use of an expanded ensemble, in which weighted transitions are permitted between states characterizing the strength of the excluded volume and thermal interactions experienced by the ghost chain. We discuss the implementation and optimization of the method within the framework of the bond fluctuation model and demonstrate its precision by a calculation of the finite-size corrections to the chemical potential.
Energy levels and structure of tetra-atomic van der Waals clusters
Authors: Villarreal Pablo , Roncero Octavio , Delgado-Barrio Gerardo
Published: July 2002

A variational treatment is presented to study bound and quasibound states of X1...BC...X2 van der Waals clusters, where X1 and X2 are rare gas atoms and BC is a conventional diatomic molecule. The Hamiltonian operator, including all the degrees of freedom, is expressed in terms of the B–C relative vector and bond coordinates which describe the position of each rare gas atom with respect to the BC center of mass. In a body-fixed reference system, with the Z axis parallel to the diatomic axis, all the matrix elements of the Hamiltonian are evaluated in a basis set of functions which takes into account the symmetries of the system. Numerical applications to the He2...Cl2 and Ne2...I2 complexes are presented and discussed.
A theoretical study on the first ionic state of benzene with analysis of vibrational structure of the photoelectron spectrum
Authors: Takeshita Kouichi
Published: July 2002

Ab initio calculations have been performed to study the vibrational levels of the first ionic state (2E1g) of benzene. The equilibrium molecular structure and vibrational modes of the state are presented. The theoretical ionization intensity curve including the vibrational structure is also presented and compared with the photoelectron spectrum. A number of new assignments of the photoelectron spectra are proposed.
Low-temperature proton transport in clathrates
Authors: Cappadonia M. , Kornyshev A. A. , Krause S. , Kuznetsov A. M. , Stimming U.
Published: July 2002

The impedance spectra are measured for protonated and deuterated clathrates, HClO4⋅5.5H2O and DClO4⋅5.5D2O, between 10 and 300 K. The conductance is investigated between 80 K and room temperature and the dielectric constant between 10 and 120 K. The data show deviation from the Arrhenius behavior of conductance in the low-temperature regime. A description of proton conductivity is developed on the basis of quantum theory of an elementary act of proton tunneling between donor–acceptor sites interacting with environmental fluctuations. Several models of the elementary act are considered. The mechanism, most consistent with the obtained data, incorporates—strong coupling of the proton with local vibrational modes of the closest environment and system diabatic transitions along these vibrational ‘‘coordinates’’—fluctuations of the tunneling barrier for the proton. At low temperatures the motion along the vibrational coordinates is no longer purely classical and the slow mode tunneling takes place. The latter gives rise to a curvature in the Arrhenius plots apprehended as a decrease of the apparent activation energy at lower temperatures. The observed isotope effect is in line with the lower deuteron tunneling probability due to doubling the mass, though other parameters may also be affected by deuteration.
Adiabatic and postadiabatic channel description of atom–diatom long-range half-collision dynamics: Interchannel radial coupling for P1 and P2 anisotropy
Authors: Nikitin E. E. , Troe J. , Ushakov V. G.
Published: July 2002

It is shown that the adiabatic channel states of an atom–diatom system with a low-rank interaction anisotropy (proportional to cos γ and cos2 γ) exhibit a nonlocalized nonadiabatic coupling which persists into the strong coupling region. This feature of adiabatic channel states restricts application of the statistical adiabatic channel model (SACM) for processes of complex decomposition and complex formation to low energies. The change of the representation from adiabatic into the postadiabatic (dynamic) one transforms the coupling to a localized form and makes it possible to find conditions for description of the half-collision dynamics in terms of uncoupled dynamic states. This result can be regarded as the extension of the statistical adiabatic channel model beyond its formal limits of applicability provided the adiabatic channel potentials are replaced by the postadiabatic (dynamic) potentials. The obtained results are exemplified by calculation of the capture cross section in the approximation of uncoupled channels.
Shape dependences in pulsed laser–matter interaction
Authors: Dietz K. , Jaeger L. , Porath R. , Pröbsting M.
Published: July 2002

It is shown that the shape dependence of final state probability distributions in pulsed laser–atom or molecule interactions has a simple explanation in the theory of quantum transport of Floquet states. More precisely, the predictions of a quasienergy resonance model—complex poles in the quasienergy plane produced by quantum transport and populated by state-mixing at near-degeneracies—are successfully compared with numerical solutions of the initial value problem of the Schrödinger equation for a Morse-potential model.
Solvation dynamics in dielectric solvents with restricted molecular rotations: Polyethers
Authors: Olender Roberto , Nitzan Abraham
Published: July 2002

Molecular dynamics simulations are used to study solvation and solvation dynamics of a classic charge in a series of ethers of increasing molecular weights, CH3(CH2OCH2)nH with n=1, 2, and 4. Equilibrium structures of the solvated species, ion mobility, linear response solvation functions, and nonequilibrium solvation are studied and compared with the corresponding results for a simple (Stockmayer) fluid. For a typical positive ion, Na+, solvation in these systems is found to belong to the nonlinear response regime; the nonlinear behavior is associated with the specific binding of the cation to the negative oxygen sites. Solvation dynamics in the timescale studied (t<0.5 ns) is found to be essentially bimodal, with a short component similar in duration and magnitude to that found in simpler solvents. However, except for the simplest system studied (ethyl methyl ether) the short time component is not Gaussian (i.e., its Gaussian part is limited to insignificantly short times) and cannot be interpreted as inertial free streaming of solvent molecules in the potential field of the solute. Instead we suggest that it originates from damped solvent vibrations about solvent inherent structures. The character of the solvent motions that drive the solvation process changes as the molecular size increases: From overall molecular rotations in the monoether, to intramolecular segmental motions in the larger solvents. It is suggested that solvation dynamics (studied, e.g., by laser induced fluorescence) can be used as a probe for the dynamics of such segmental motions in polymer electrolytes.
Statistical theory of time-dependent diffusion-controlled reactions in fluids and solids
Authors: Felderhof B. U. , Jones R. B.
Published: July 2002

A new theory of the time-dependence of diffusion-controlled reactions in fluids and solids near thermal equilibrium is presented. Quite general reaction schemes can be handled, but the treatment is limited to the case of particle-conserving unimolecular and bimolecular reactions. Mori’s projection operator formalism, as applied to a general master equation with detailed balance, and Ursell’s cluster expansion are used as tools. The theory is applied to the time-dependence of fluorescence in a simple model without hydrodynamic or potential interactions.
Ultrasonic pulses in a glass forming salt melt nearTg. A time and space resolved study using interferometry
Authors: Arachovitis G. , Balschun B. , Soltwisch M. , Quitmann D.
Published: July 2002

An optoacoustic experiment was set up to study generation, propagation, and damping of broadband acoustic pulses in the MHz region. The sound pulse front is detected interferometrically. The analysis is based on linear hydrodynamic theory and comprises diffraction of the sound pulse. To introduce relaxational effects, it was extended by splitting the specific heat into a ‘‘fast’’ and a ‘‘slow’’ part. Thus, a good description of the observed pulse shapes and speeds could be obtained. Results for the glass forming 1-methyl-3-ethyl-imidazoliumchloride-AlCl3 mixture at three concentrations and for temperatures between −80 °C and +20 °C are reported.
Infrared spectroscopy of the molecular hydrogen solvated carbonium ions, CH+5(H2)n (n=1–6)
Authors: Boo Doo Wan , Lee Yuan T.
Published: July 2002

The infrared spectra for the molecular hydrogen-solvated carbonium ions, CH+5(H2)n (n=1–6) in the frequency range of 2700–4200 cm−1 are presented. Spectroscopic evidence was found in support of the scrambling of CH+5 through the large amplitude motions such as the CH3 internal rotation and the in-plane wagging motion of three-center two-electron bond. More importantly, the scrambling motions of CH+5 cores were slowed down considerably by attaching the solvent H2 molecules to the core ion. The complete freezing of the scrambling motions was found when the first three H2 molecules were bound to the CH+5 core. A good agreement between the experimental and the theoretical predictions was found in the dynamics of CH+5.
Electronic polarizability of a fluoride ion estimated by refractive indexes and molar volumes of molten eutectic LiF–NaF–KF
Authors: Iwadate Yasuhiko , Fukushima Kazuko
Published: July 2002

The electronic polarizability of a fluoride ion was evaluated from the refractive indexes extrapolated to infinite wavelength and the molar volumes of molten eutectic LiF–NaF–KF, and compared with the data previously reported.
Vibrational spectroscopy of small (HF)n clusters (n=4–8) in size-selected molecular beams
Authors: Huisken Friedrich , Kaloudis Michael , Kulcke Axel , Laush Curtis , Lisy James M.
Published: July 2002

Molecular beam depletion and fragment spectroscopy has been employed to study the absorption behavior of small hydrogen fluoride clusters [(HF)n, n=4–8] in the spectral region between 3100 and 3700 cm−1. As tunable infrared radiation source we used an IR-seeded optical parametric oscillator (OPO) with a LiNbO3 crystal as nonlinear medium. Size-specific information has been obtained by scattering the cluster beam from a secondary rare gas beam. In this way all spectral features are assigned to specific cluster sizes. While the qualitative agreement with earlier experiments is good, the new assignments differ by one or more monomer units. In general, it is found that the absorption bands must be assigned to larger clusters. For some cluster sizes a doublet structure is observed. This observation is discussed in terms of combination bands involving intra- and intermolecular modes; but the possibility of isomeric structures is considered, too.
A surface-enhanced hyper-Raman and surface-enhanced Raman scattering study of trans-1,2-bis(4-pyridyl)ethylene adsorbed onto silver film over nanosphere electrodes. Vibrational assignments: Experiment and theory
Authors: Yang Wen-hui , Hulteen John , Schatz George C. , Van Duyne Richard P.
Published: July 2002

We present theoretical and experimental studies of the infrared (IR) spectroscopy, normal Raman spectroscopy (NRS), surface-enhanced Raman spectroscopy (SERS), and surface-enhanced hyper-Raman (SEHRS) spectroscopy of trans-1,2-bis(4-pyridyl)ethylene (BPE). This centrosymmetric molecule is expected to have no common Raman and hyper-Raman lines provided that it is not strongly perturbed by adsorption on the surface. The measured SERS spectrum, obtained under electrochemical conditions on Ag film over nanosphere (AgFON) electrodes, shows a well defined C=C stretch band that is not seen in the SEHRS spectrum, but many of the other bands overlap closely. We use ab initio calculations for isolated BPE to assign the spectra, and find excellent agreement between the calculated and measured IR and SERS spectra, and good agreement between the calculated and measured SEHRS spectrum. We find that the apparently overlapping IR, SERS, and SEHRS bands are in fact due to modes that have similar vibrational characteristics but different symmetry. Our results indicate that SEHRS spectra are consistent with the expected (three photon) selection rules and intensities. This rules out an alternative mechanism in which the observed spectrum arises from surface second harmonic generation (SHG) followed by SERS excited at the second harmonic frequency.
Modification of Ni(111) reactivity toward CH4, CO, and D2 by two-dimensional alloying
Authors: Holmblad P. M. , Larsen J. Hvolbæk , Chorkendorff I.
Published: July 2002

Alloying Au into the outermost atomic layer of Ni(111) significantly alters the physical and chemical properties of the surface. The reactivity is investigated by the use of seeded supersonic molecular beams of CH4 and it is found that the nobleness of Au reduces the overall reactivity toward CH4. This is accounted for in an ensemble model resolving the sticking probability on Ni atoms having different nearest neighbor surroundings. Although a mean field description of site distributions is found to be a very good approximation it is improved by using experimentally determined ensemble statistics from STM images. The strong influence of the vibrational temperature on the sticking coefficients of CH4 vs translational energy on the pure Ni(111) is also demonstrated. Desorption energies of CO and D2 is observed to decrease approximately 25–30 kJ/mole as the coverage of Au is increased from 0.0 to 0.7 ML. In TPD spectra of deuterium saturated surface alloys a new clearly resolved desorption state is observed at 180–220 K with maximum intensity around θAu=0.3–0.4 ML. This state is clearly related to chemisorption sites involving both Au and Ni. A site model based on mean field statistics adequately accounts for the appearance of this state. The effect of Au is also evident in the TPD spectra of CO saturated Au/Ni(111) surface alloys where the saturation coverage decreases and new structure develops.
Molecules interacting with short, intense laser pulses: The dynamical role of quasistationary states
Authors: Dietz K. , Pröbsting M. , Jaeger L.
Published: July 2002

A theoretical framework for the description of the pulse-induced reaction dynamics of molecules interacting with short, intense laser pulses is derived. Born–Oppenheimer adiabaticity is formulated in terms of quasistationary Floquet states; its violation and the structure of the potential mixing electronic levels, and thus effecting this violation, is discussed; it is shown that, on the basis of quasistationary states, this potential is (to a very good approximation) nonvanishing only around near degeneracies in the quasienergy spectrum where it has a simple two-level structure. Ab initio calculations showing the effectivity of this scheme are performed for the H2 molecule. Particular emphasis is put on bond-softening reactions; a new mechanism is described.
Sol–gel phase transitions in thermoreversible gels: Onset of gelation and melting
Authors: Liu Y. , Pandey R. B.
Published: July 2002

A Monte Carlo simulation model is proposed to study the phase transition and the structural evolution of thermoreversible gels with the coexistence of phase separation and gelation processes. Our model includes the mobility of all the species and the reversibility of bonds of the clusters formed due to cross-linking reactions. These features provide a more realistic description of a polymer–solvent system. We attempt to elucidate the effects of interactions, solvents, polymer fraction, etc. on the phase behaviors of the thermoreversible gel. Sol-to-gel transition is studied in detail as a function of temperature, and the related critical exponents are evaluated. Two different energy parameters are used to describe the gelation and the melting processes. The collective structure factors are calculated, and their dynamic behaviors are analyzed. The competing effects of the phase separation and the network formation on the structural evolutions of the gel are discussed.
Study of electronically excited states of ozone by electron-energy-loss spectroscopy
Authors: Allan Michael , Mason Nigel J. , Davies Julia A.
Published: July 2002

Electronically excited states of ozone have been studied by electron-energy-loss spectroscopy. Two broad bands without visible vibrational structure are observed at 1.8±0.2 eV and 2.05±0.05 eV under scattering conditions favoring singlet excitation, that is a scattering angle of ϑ=10° and residual energy Er=20 eV. The lower is assigned as 1A2, the higher as 1B1 (Chappuis band). Bands with rich vibrational structure are observed under scattering conditions favoring triplet excitation, ϑ=30°–135° and residual energy Er=1–3 eV. At least two vibrational progressions can be discerned. The first has an origin at 1.30 eV, the origin of the second cannot be determined unambiguously, it is either at 1.53 or 1.45 eV. The well-known Hartley band and a number of other singlet and triplet excited states are observed at higher energy losses. Excitation functions and angular distributions of the triplet band at 1.30 eV and of the Hartley band are presented. The absolute value of the differential cross section for excitation of the Hartley band is given.
Computer simulations of dense hard-sphere systems
Authors: Rintoul M. D. , Torquato S.
Published: July 2002

We present comprehensive results of large-scale molecular dynamics and Monte Carlo simulations of systems of dense hard spheres at volume fraction φ along the disordered, metastable branch of the phase diagram from the freezing-point φf to random close packing volume φc. It is shown that many previous simulations contained deficiencies caused by crystallization and finite-size effects. We quantify the degree of local crystallization through an order parameter and study it as a function of time and initial conditions to determine the necessary conditions to obtain truly random systems. This ordering criterion is used to show that previous methods employed to ascertain the degree of randomness are inadequate. A careful study of the pressure is also carried out along the entire metastable branch. In the vicinity of the random-close packing fraction, we show that the pressure scales as (φc−φ)−γ, where γ=1 and φc=0.644±0.005. Contrary to previous studies, we find no evidence of a thermodynamic glass transition.
Thermodynamics of binary liquid mixtures of partially deuterated methanes with CH4 or CD4
Authors: Lopes J. N. C. , Calado J. C. G. , Rebelo L. P. N. , Nunes da Ponte M. , Staveley L. A. K.
Published: July 2002

The vapor pressure isotope effect of samples of isotopically substituted methane and their mixtures was measured as a function of temperature and mixture composition: The differential vapor pressure between CHiD4−i (with i=0 or 4) and CHjD4−j (with j=1, 2, or 3), the differential vapor pressure between mixtures of (CHiD4−i+CHjD4−j) and CH4 (if i=4) or CHjD4−j (if i=0) and the absolute vapor pressure of CH4 (if i=4) or CHjD4−j (if i=0), were measured simultaneously between 96 and 121 K for mixtures of nominal composition 0.25, 0.50, and 0.75 mole fraction in the reference methane species. The p(x,T) data were used to calculate the excess molar Gibbs energy function, GE(x,T) and the excess molar enthalpy HE(x), assuming that this last function is independent of temperature in the experimental range. The deviations from ideal behavior are very small, GE being only some tenths of J/mol for equimolar mixtures. The experimental GE values compare well with estimated results based on a modified version of the statistical theory of isotope effects in condensed phases. Comparisons with reported values of the liquid–vapor isotope fractionation factor for the CD3H–CH4 system are also made.
On a modeling of voltage-application to metallic electrodes using density functional theory
Authors: Bureau Christophe , Lécayon Gérard
Published: July 2002

With the ultimate purpose of designing molecule/surface interaction potentials for the dynamical study of electropolymerization reactions, a theoretical framework, based on density functional theory (DFT), is proposed to obtain in a self-consistent manner the true electronic density of a metallic surface set under an applied voltage. A direct link is made between the DFT chemical potential μ and the (experimental) electrode potential drop Δε imposed in electrochemical experiments. An emphasis is made on the possibility of using cluster models to describe polarized surfaces. It is found that within “experimental” applied voltages, the induced surface charges may be lower that previously expected. This suggests that the electrostatic component in molecule/polarized-surface interaction potentials may not be overwhelmingly important towards other terms such as polarization, dispersion or even quantum contributions. In this respect, the μ=f(Δε) equation suggests that the Lewis acidic or basic character of a metallic surface can be monitored continuously by simply tuning the electrode potential drop. Possible experimental verifications of this proposal are examined.
The bending vibration of FeD2 at 226 cm−1, detected by far-infrared laser magnetic resonance spectroscopy
Authors: Körsgen Helga , Evenson Kenneth M. , Brown John M.
Published: July 2002

We report the first gas phase observation of a bending vibration in an open shell molecule by far-infrared laser magnetic resonance. The spectrum is due to FeD2; its observation shows that rovibrational transitions can be detected with such a spectrometer. The molecule was produced in the reaction between deuterium atoms and iron pentacarbonyl. FeD2 has a linear 5Δ ground state and shows the effects of Renner–Teller coupling in the excited vibrational (010) level.
Liquid water in the domain of cubic crystalline ice Ic
Authors: Jenniskens P. , Banham S. F. , Blake D. F. , McCoustra M. R. S.
Published: July 2002

Vapor-deposited amorphous water ice, when warmed above the glass transition temperature (120–140 K), is a viscous liquid which exhibits a viscosity vs temperature relationship different from that of liquid water at room temperature. New studies of thin water ice films now demonstrate that viscous liquid water persists in the temperature range 140–210 K, where it coexists with cubic crystalline ice. The liquid character of amorphous water above the glass transition is demonstrated by (1) changes in the morphology of water ice films on a nonwetting surface observed in transmission electron microscopy (TEM) at around 175 K during slow warming, (2) changes in the binding energy of water molecules measured in temperature programmed desorption (TPD) studies, and (3) changes in the shape of the 3.07 μm absorption band observed in grazing angle reflection-absorption infrared spectroscopy (RAIRS) during annealing at high temperature, whereby the decreased roughness of the water surface is thought to cause changes in the selection rules for the excitation of O–H stretch vibrations. Because it is present over such a wide range of temperatures, we propose that this form of liquid water is a common material in nature, where it is expected to exist in the subsurface layers of comets and on the surfaces of some planets and satellites.
Angle-resolved ultraviolet photoelectron spectroscopy of thin films of bis(1,2,5-thiadiazolo)-p-quinobis (1,3-dithiole) on the MoS2 surface
Authors: Ueno Nobuo , Kitamura Akinori , Okudaira Koji K. , Miyamae Takayuki , Harada Yoshiya , Hasegawa Sinji , Ishii Hisao , Inokuchi Hiroo , Fujikawa Takashi , Miyazaki Takafumi , Seki Kazuhiko
Published: July 2002

In this paper we report on the angle-resolved ultraviolet photoelectron spectroscopy on ultrathin films of bis(1,2,5-thiadiazolo)-p-quinobis (1,3-dithiole) (BTQBT) deposited on a MoS2 surface with synchrotron radiation, and describe the quantitative analysis of the angular distribution of photoelectron from the highest occupied state. The observed angular distributions were better explained by those calculated with the single-scattering approximation combined with molecular orbital calculation considering intramolecularly scattered waves than the previously used independent-atomic-center approximation combined with molecular orbital calculation. Further, the low-energy-electron diffraction measurements were performed on the film. From the low-energy-electron diffraction, the two-dimensional lattice of the ultrathin films on the MoS2 was found to be MoS2(0001)−(13×13, R=±13.9°)-BTQBT, and from the analysis of the photoelectron angular distributions, it was found that at the lattice points the molecules lie flat with azimuthal orientations of 19° and 47° with respect to each surface crystal axis of MoS2. On the basis of these results, the full structure of the thin film, the two-dimensional lattice, and full molecular orientations at the lattice points, is proposed.
Polarization dependent cavity ring down spectroscopy
Authors: Engeln Richard , Berden Giel , van den Berg Esther , Meijer Gerard
Published: July 2002

We here theoretically outline and experimentally demonstrate that polarization spectroscopy can be combined with cavity ring down (CRD) spectroscopy, thereby retaining the specific advantages of both techniques. The b1Σg+(v′=2)←X3Σg−(v′′=0) transition of molecular oxygen around 628 nm is used to demonstrate the possibility to selectively measure either the polarization-dependent absorption or the resonant magneto-optical rotation of gas-phase molecules in the appropriate setup. Just as in CRD absorption spectroscopy, where the rate of absorption is measured, in the here presented polarization-dependent CRD (PDCRD) detection scheme the rate of polarization rotation is measured, which enables the polarization rotation to be quantitatively determined. Apart from studying electro-optic and magneto-optic phenomena on gas-phase species, the PDCRD detection scheme is demonstrated to be applicable to the study of magneto-optical rotation in transparent solid samples as well.
Comparison of high order perturbative convergence of multireference perturbation methods: Application to singlet states of CH2
Authors: Chaudhuri Rajat K. , Freed Karl F.
Published: July 2002

High order perturbative computations for the lowest lying singlet states of the CH2 molecule are used to analyze the efficacy of various multireference perturbation methods (MRPTs). Whereas traditional Möller–Plesset MRPT calculations produce divergent perturbation expansions, the effective Hamiltonian Hv and intermediate Hamiltonian Hint approaches produce well behaved expansions for well-chosen reference spaces. The three methods are compared to assess their convergence properties, the sources of divergence when appropriate, their accuracy when truncated at low orders, and their behavior when applied in conjunction with large reference spaces. The analysis of the sources of divergent or slowly convergent perturbation expansions provides insights into necessary ingredients for useful MRPT methods as well as into possible approaches for further improving these methods. Calculations are also presented for a simple problem whose divergent traditional MRPT perturbation expansion mimics that commonly encountered when these methods are applied in transition state or bond breaking regions of potential surfaces.
Thermal rate constants of the O2+N→NO+O reaction based on the A2′and A4′ potential-energy surfaces
Authors: Bose Deepak , Candler Graham V.
Published: July 2002

A detailed quasiclassical trajectory study of the O2+N→NO+O reaction is performed based on ab initio potential-energy surfaces of the 2A′ and 4A′ states. The study is aimed at generating a database of thermally averaged and O2 state-specific rate constants needed for accurate simulations of NO kinetics in high-temperature flow processes. The rate constants obtained show good agreement with the available experimental data and with other quasiclassical trajectory calculations. It is found that the reactant internal energy of the O2+N→NO+O reaction is less effective in enhancing the rate than in the N2+O→NO+N reaction. An analysis of the product vibrational energy shows that NO formed by the O2+N→NO+O reaction has a non-Boltzmann distribution. It is also found that the most populated NO vibrational level is determined by the reactant vibrational energy, while the terminal slope of the NO vibrational distribution is a strong function of the reactant translational temperature.
Identifying 2- and 3-coordinated H2O in protonated ion–water clusters by vibrational pre-dissociation spectroscopy and ab initio calculations
Authors: Wang Y.-S. , Jiang J. C. , Cheng C.-L. , Lin S. H. , Lee Y. T. , Chang H.-C.
Published: July 2002

Clustering of water on protonated molecular ions has been investigated by vibrational predissociation spectroscopy. Systematic measurements at different cluster sizes reveal a close resemblance of the OH stretch spectra between NH4+(H2O)n, CH3NH3+(H2O)n, and H3O+(H2O)n. Particularly at n⩾6, a sharp feature, identical to that found on ice and water surfaces, emerges at 3690 cm−1 for free-OH stretching. The feature is distinguished from the other free-OH absorption, commonly observed for small- and medium-sized (H2O)n clusters at 3715 cm−1. The results, in conjunction with ab initio calculations, provide compelling evidence for 2- and 3-coordinated H2O in the protonated ion–water clusters.
An examination of intrinsic errors in electronic structure methods using the Environmental Molecular Sciences Laboratory computational results database and the Gaussian-2 set
Authors: Feller David , Peterson Kirk A.
Published: July 2002

The Gaussian-2 (G2) collection of atoms and molecules has been studied with Hartree–Fock and correlated levels of theory, ranging from second-order perturbation theory to coupled cluster theory with noniterative inclusion of triple excitations. By exploiting the systematic convergence properties of the correlation consistent family of basis sets, complete basis set limits were estimated for a large number of the G2 energetic properties. Deviations with respect to experimentally derived energy differences corresponding to rigid molecules were obtained for 15 basis set/method combinations, as well as the estimated complete basis set limit. The latter values are necessary for establishing the intrinsic error for each method. In order to perform this analysis, the information generated in the present study was combined with the results of many previous benchmark studies in an electronic database, where it is available for use by other software tools. Such tools can assist users of electronic structure codes in making appropriate basis set and method choices that will increase the likelihood of achieving their accuracy goals without wasteful expenditures of computer resources.
Density functional solvation model based on CM2 atomic charges
Authors: Zhu Tianhai , Li Jiabo , Hawkins Gregory D. , Cramer Christopher J. , Truhlar Donald G.
Published: July 2002

We extend the SM5 solvation model for calculating solvation free energies of a variety of organic solutes in both aqueous and organic solvents so that it can be employed in conjunction with high-level electronic structure calculations. The extension is illustrated by presenting three implementations based on density-functional theory (DFT). The three implementations are called SM5.42R/BPW91/MIDI!6D, SM5.42R/BPW91/DZVP, and SM5.42R/BPW91/6-31G*. They have the following features: (1) They utilize gradient-corrected DFT with polarized double zeta basis sets to describe the electronic structure of a solute. The particular exchange-correlation functional adopted is Becke’s exchange with the Perdew–Wang 1991 correlation functional, usually called BPW91. The MIDI!6D, DZVP, and 6-31G* basis sets are used. (2) They employ fixed solute geometries in solvation calculations. The model is designed to predict solvation free energies based on any reasonably accurate gas-phase solute geometry. (3) The electric polarization in the solute-solvent system is described by the generalized Born approximation with self-consistent reaction-field solute partial atomic charges obtained from the CM2 class IV charge model. (4) The solvation effects within the first solvation shell are included in the form of SM5-type atomic surface tensions. Both DFT parameterizations are developed using 275 neutral solutes and 49 ions with gas-phase Hartree–Fock/MIDI! geometries. These solutes contain a wide variety of organic functional groups which include H, C, N, O, F, P, S, Cl, Br, and I atoms. For 2135 free energies of solvation of the neutral molecules in water and 90 organic solvents, SM5.42R/BPW91/MIDI!6D, SM5.42R/BPW91/DZVP, and SM5.42R/BPW91/6-31G* yield mean unsigned errors in solvation free energies of 0.45 kcal/mol, 0.44 kcal/mol, and 0.43 kcal/mol, respectively. For 49 ions in water, SM5.42R/BPW91/MIDI!6D produces a mean unsigned error of 3.9 kcal/mol, while SM5.42R/BPW91/DZVP and SM5.42R/BPW91/6-31G* give 3.6 kcal/mol and 3.9 kcal/mol, respectively.
Second molecular hyperpolarizability of 2,2′-diamino-7,7′-dinitro-9,9′- spirobifluorene: An experimental study on third-order nonlinear optical properties of a spiroconjugated dimer
Authors: Kim So Yeon , Lee Minyung , Boo Bong Hyun
Published: July 2002

Nonlinear optical properties of a fixed molecular dimer, 2,2′-diamino-7,7′-dinitro9,9′-spirobifluorene (ANSF), have been investigated in solution by the degenerate four-wave mixing technique. It has been observed that the orientationally averaged second molecular hyperpolarizability of ANSF in THF is enhanced ten times in comparison with its monomer unit, 2-amino-7-nitrofluorene. Since electronic coupling between monomer units in ANSF is negligible owing to symmetry, we attribute the large enhancement to the orientational effect of the two constituting dipoles with possible involvement of vibronic coupling and spiroconjugation.
The magnetic-field influence on the inelastic electron tunnel current mediated by a molecular wire
Authors: Petrov E. G. , Tolokh I. S. , May V.
Published: July 2002

The magnetic-field influence on the inelastic interelectrode tunnel current mediated by a molecular wire is studied theoretically for the case in which the wire includes paramagnetic ions with frozen angular momenta. The vibrational and spin relaxation within the terminal sites of the wire (adjacent to the electrodes) are assumed to be much faster than the related jump of the tunneling electron. For a wire containing a pair of paramagnetic ions, a low-temperature blocking of the current is demonstrated for the cases of ions both coupled by an exchange interaction and largely separated in space. The blocking is shown to appear if the paramagnetic ion reduces its electronic ground-state spin S to S−12 during the formation of the intermediate bound state with the transferred electron. At low temperatures the current formation is predominantly originated thus by tunnel channels for which the magnetic energy of paramagnetic ions has a minimal value. In the presence of an exchange spin–spin interaction or a single-ion anisotropy, just these predominant channels are manifested to form a steplike behavior of the current before the current itself is completely blocked by the increasing magnetic field.
The Kramers problem in the energy-diffusion limited regime
Authors: Sancho José M. , Romero Aldo H. , Lindenberg Katja
Published: July 2002

The Kramers problem in the energy-diffusion limited regime of very low friction is difficult to deal with analytically because of the repeated recrossings of the barrier that typically occur before an asymptotic rate constant is achieved. Thus, the transmission coefficient of particles over the potential barrier undergoes oscillatory behavior in time before settling into a steady state. Recently, Kohen and Tannor [D. Kohen and D. J. Tannor, J. Chem. Phys. 103, 6013 (1995)] developed a method based on the phase space distribution function to calculate the transmission coefficient as a function of time in the high-friction regime. Here we formulate a parallel method for the low-friction regime. We find analytic results for the full time and temperature dependence of the transmission coefficient in this regime. Our low-friction result at long times reproduces the equilibrium result of Kramers at very low friction and extends it to higher friction and lower temperatures below the turn-over region. Our results indicate that the single most important quantity in determining the entire time evolution of the transmission coefficient is the rate of energy loss of a particle that starts above the barrier. We test our results, as well as those of Kohen and Tannor for the Kramers problem, against detailed numerical simulations.
What is the best semiclassical method for photochemical dynamics of systems with conical intersections?
Authors: Topaler Maria S. , Allison Thomas C. , Schwenke David W. , Truhlar Donald G.
Published: July 2002

We present a systematic test of four general semiclassical procedures for the theoretical treatment of multistate molecular processes such as electronically nonadiabatic photochemical reactions. The methods are tested by comparing their predictions to accurate quantal results for three two-state model reactions involving conical intersections. The four methods tested are Tully’s fewest-switches version of trajectory surface hopping (1990), the Blais–Truhlar trajectory surface hopping method (1983), the Ehrenfest scheme (1975–1979), and the Meyer–Miller method (1979). We test the ability of the classical path methods to predict both electronic probabilities and product rovibrational distributions. For each of the four basic approaches we test six options for extracting final-state information from the calculated dynamics. We find that, although in most cases there is qualitative agreement between average quantum mechanical and trajectory results, the overall average error is about 50% for Tully’s fewest-switches method, the Ehrenfest method, and the Meyer–Miller method, and even higher, about 60%, for the Blais–Truhlar method. These values do not include additional errors in the below-threshold regions, which are especially large for the Meyer–Miller method because of the electronic zero-point energy in the Meyer–Miller classical analog Hamiltonian.
Deterministic identifiability of two-state excited-state models with transients: Recovery of deactivation rate constants
Authors: Molski Andrzej , Boens Noël
Published: July 2002

A deterministic identifiability analysis of kinetic models of two-state excited-state processes in the presence of transient effects is performed to establish the conditions for the unique determination of the excited-state species deactivation rate constants with no assumption regarding the kinetics of interconversion in the excited state. It is necessary that decays of at least one excited-state species can be monitored separately to uniquely determine the excited-state species deactivation rate constants. This conclusion holds for reversible and irreversible intermolecular as well as intramolecular excited-state processes in the presence of transient effects. Sufficient conditions for the recovery of the deactivation rate constants are established for several cases of practical relevance.
Definitive evidence for the existence of a long-lived vinylidene radical cation, H2C=C+.
Authors: Hayakawa Shigeo , Takahashi Mitsuhito , Arakawa Kazuo , Morishita Norio
Published: July 2002

Charge inversion mass spectra of C2H2+ ions produced by electron impact from HC≡CH and CH2=CCl2 were measured using K and Cs targets. Clear differences in the charge inversion spectra between HC≡CH and CH2=CCl2 indicate that the C2H2+ ion formed from CH2CCl2 is H2C=C+⋅. The lifetime of H2C=C+⋅ is found to be longer than 8.5 μs, and the state is proposed to be the 2B1 ground-state rather than the 4A2 state.
A quantum mechanical study of the role of out-of-plane vibrations in benzene in the intramolecular redistribution of vibrational energy
Authors: Rashev S.
Published: July 2002

In this work we have carried out a theoretical investigation on the role of out-of-plane vibrational modes in vibrational level mixing and intramolecular vibrational energy redistribution (IVR) in benzene. A fully symmetrized vibrational basis set, based on a combined local mode (LM)+normal modes (NM) formalism was employed in the study. The Hamiltonian formalism for description of out-of-plane vibrations has been developed in due detail. Model calculations on the absorption spectrum and IVR dynamics at the second overtone state CH(v=3) at ∼8800 cm−1 have been carried out, supplementing previous studies and demonstrating the importance of out-of-plane modes in benzene.
Measurements of ground-state OH rotational energy-transfer rates
Authors: Kliner Dahv A. V. , Farrow Roger L.
Published: July 2002

We have studied rotational energy transfer (RET) in collisions of OH with the bath gases Ar, N2, O2, and H2O at 293 K. Rotationally hot OH(X 2Π3/2, v″=0, N″=1–12) was generated by photolysis of H2O2 at 266 nm, and collisional relaxation of the nascent rotational distribution was monitored by laser-induced fluorescence. The data are remarkably well described by an exponential-gap model for the matrix of state-to-state RET rate constants. For Ar, N2, and O2, RET rates are significantly faster at low N″ than high N″; for H2O, RET is approximately an order of magnitude faster than for the other bath gases, and the rate is not as strongly dependent on N″. The rates of rotationally inelastic energy transfer are similar in the X and A states, but the X-state depopulation rate constants (including nearly elastic, Λ-doublet-changing collisions) are faster than the A-state values. By comparing the depopulation rates derived from the present experiment with previous linewidth measurements, we conclude that RET is the dominant source of pressure broadening for OH microwave transitions and makes a significant contribution for ultraviolet A–X transitions. While generally good agreement is found between the present results and previous OH RET studies for both the ground and excited electronic states, some significant discrepancies are noted.
Multidimensional femtosecond correlation spectroscopies of electronic and vibrational excitons
Authors: Zhang Wei Min , Chernyak Vladimir , Mukamel Shaul
Published: July 2002

Four two-dimensional (2D) four-wave-mixing techniques that can be used to extract information about structure and coupling patterns of interacting chromophores are proposed. These techniques have close conceptual similarities with multiple-pulse NMR spectroscopies. Closed expressions for the signals are derived by solving the nonlinear exciton equations (NEE) which describe the dynamics of multiple excitations using the one-exciton Green function and the exciton–exciton scattering matrix. Possible applications include electronic spectroscopy of aggregates, e.g., photosynthetic antenna complexes, and infrared spectroscopy of localized vibrations (e.g., amid bands in polypeptides). Model calculations are presented for three-chromophore aggregates.
Charge migration and control of site selective reactivity: The role of covalent and ionic states
Authors: Remacle F. , Levine R. D.
Published: July 2002

A many-electron description of charge migration along a molecular backbone is discussed. Reference is made to site selective reactivity and the recent experiments of Weinkauf and Schlag on the dissociation of peptide ions following a localized ionization. The use of many-electron states allows a classification of the charge migration pathways through either covalent or ionic states. Electron correlation is introduced via Coulomb repulsion of electrons of opposite spins à-la Hubbard. Complete configuration interaction is implemented using the unitary group basis of Paldus. The primary factor determining charge migration is found to be the local ionization potential. It is shown that, at lower levels of excitation, the majority of possible initial states which describe localized ionization at one end of the chain lead to a preferential dissociation at the other end of the chain.