bluuebunny/crossref_metadata_2025 huggingface dataset:
High-pressure studies on the excited-state isomerization of 2-vinylanthracene: Experimental investigation of Kramers turnover
Authors: Hara Kimihiko , Kiyotani Hideyuki , Kajimoto Okitsugu
Published: July 2002

Isomerization of 2-vinylanthracene in the first excited singlet state has been studied over wide solvent-viscosity ranges. Steady-state and time-resolved fluorescence spectroscopy is used to determine the isomerization rate in compressed liquid solutions and in supercritical fluids. The ‘‘Kramers turnover,’’ which indicates the transition of kinetics from ‘‘energy diffusion control’’ regime to ‘‘spatial diffusion control’’ regime, is clearly observed. The turnover behavior can be expressed by a single curve which bridges the two extremes by an additive form. The activation energy shift between the two extremes is 1.6 kJ/mol. Activation volumes at large viscosity in compressed liquid solutions are also discussed.
Path integral Monte Carlo simulations: Study of the efficiency of energy estimators
Authors: Fernandes Pedro Alexandrino , Carvalho Alfredo Palace , Ramalho J. P. Prates
Published: July 2002

This article presents a study of the relative efficiency of two commonly used energy estimators for path integral Monte Carlo (MC) simulations; the Barker estimator and the virial estimator. Two different path integral MC algorithms are considered; the simple algorithm augmented with whole chain moves and the normal-mode algorithm. The behavior of the two estimators is analyzed in some model systems and the suitability of its application is discussed, considering the general characteristics of the systems to be simulated. It is concluded that there is no obvious choice when it comes to decide which estimator to use. Instead, each one may be the most appropriate depending on the specific conditions of the simulation. Identical conclusions are drawn for both algorithms considered here. For low temperature systems or with high gradient potentials, where quantum effects are more significant, the Barker estimator is to be preferred as the variance of the mean energy is smaller. On the other hand, for high temperature systems or with low gradient potentials, where quantum effects are less significant, the virial estimator has a smaller variance of the mean energy.
Laser spectroscopy of molecular ions: The Ã–X̃ transition of the acetylene radical cation
Authors: Cha Ch. , Weinkauf R. , Boesl U.
Published: July 2002

Resonance-enhanced multiphoton-dissociation spectroscopy has been used to measure the first highly resolved UV spectrum of the acetylene radical cation. The bent structure of the ionic Ã state could be proved. In the Ã–X̃-spectrum progressions of all three bending modes of bent acetylene appear. Their frequencies, anharmonicities and estimated potential thresholds are given. Furthermore, for the origin and the 5a20 band (out-of-plane cis-bending mode) rotationally resolved spectra have been obtained and rotational constants A, B, and DK have been determined as well as a directly measured value of the X̃-state spin–orbit splitting. From rotational linewidths or envelopes information about single vibrational lifetimes has been deduced; a mode-specific behavior of these lifetimes has been found. Several nonradiative processes involving isomerization to vinylidene seem to be important in the energy region of our spectrum.
Computerized simulation and fitting of singlet–triplet spectra of orthorhombic asymmetric tops: Theory and extensions to molecules with large multiplet splittings
Authors: Judge R. H. , Korale A. A. , York J. J. , Joo Duck-Lae , Clouthier Dennis J. , Moule D. C.
Published: July 2002

Motivated by our recent finding that the singlet–triplet bands of selenoformaldehyde involve an upper state with large zero field splittings, we have extended the theory and written a program for predicting and fitting such rotationally resolved spectra. Triplet state matrix elements for a case (A) basis have been developed, including corrections for centrifugal and spin–centrifugal distortion. The full Hamiltonian matrix has been symmetry adapted, simplifying the problem to four individual matrices of approximately equal size for molecules of orthorhombic symmetry. Diagonalization of these matrices yields triplet state energies that are in agreement with previous treatments using a basis in which the spin splittings are small relative to the rotational intervals. Methods have been developed for sorting the eigenvalues and assigning quantum labels regardless of the magnitude of the spin splittings. The calculation of the relative intensities of the rotational lines within a band has been programmed using transition moment matrix elements from the literature. The selection rules for various upper state symmetries have been developed in a form useful for the analysis of spectra. Band contour predictions of spectra for various coupling cases have been presented.
Avoided resonance overlapping beyond the energy independent formalism. I. Vibrational predissociation
Authors: Desouter-Lecomte M. , Liévin J. , Brems V.
Published: July 2002

The interaction of overlapping resonances is studied in the case of a vibrational predissociation in a linear molecule. The energy independent approach rests on the neglect of the variation of the coupling elements with the energy in the continuum. It predicts the formation of trapped and short-lived decay modes when resonances overlap. The relevance of this prediction is discussed on the basis of results obtained from two different methods going beyond this energy independent formalism. The first one consists in searching the solutions of the implicit equation derived from the Feshbach partitioning method, taking into account the full energy dependence of the local effective Hamiltonian Heff(E). The second one is the variational complex coordinate method (CCM), implemented in a discrete variable representation (DVR). The results demonstrate that the energy dependency enhances the trapping effect. The distribution of the resonance energies in the complex plane is different from the prediction of the energy independent formalism. The fast modes are found to be slower than expected. Their position is also completely shifted. The mean width is not constrained by a sum rule which is characteristic of the energy independent approximation.
The pressure of a fluid confined in a disordered porous material
Authors: Kierlik E. , Rosinberg M. L. , Tarjus G. , Monson P.
Published: July 2002

Using the Green–Bogoliubov method, we derive an expression for the thermodynamic pressure or grand potential density for a molecular model of a fluid confined in a disordered porous matrix. The expression is equivalent to the virial equation derived previously via the replica method [Rosinberg et al., J. Chem. Phys. 100, 5172 (1994)] but differs from an expression recently derived from the condition of mechanical equilibrium. It is concluded that the condition of mechanical equilibrium does not yield information about the thermodynamics of these systems. The thermodynamic pressure can be obtained from experiment indirectly by integrating the Gibbs adsorption isotherm and, in principle, by direct measurement.
State-resolved inelastic collisions of single rotational, fine-structure, and Λ doublet levels of NH(A 3Π) with helium: A combined experimental and theoretical study
Authors: Neitsch L. , Stuhl F. , Dagdigian Paul J. , Alexander Millard H.
Published: July 2002

A comprehensive set of single-collision state-to-state rate constants for the relaxation of NH(A 3Π, v=0,N,Fi,e/f ) levels in collisions with helium are presented. These rate constants were extracted from measured fluorescence intensities of isolated A→X rotational lines in the presence of He subsequent to laser excitation of individual levels in the A state. There is no indication of the collisional propensities predicted for a Hund’s case (a) 3Π state, most likely because NH(A) represents an intermediate coupling case. With increasing N, NH(A) rapidly approaches the Hund’s case (b) limit. For high initial N, rotational relaxation proceeds predominantly through ΔN=−1 fine-structure conserving transitions to Λ doublet levels with Π(A′) reflection symmetry. In addition, a propensity to conserve the Π(A′)/Π(A″) symmetry of the initial level is found for ΔN=0 fine-structure changing transitions. The observed propensities have been interpreted by comparison with full quantum close-coupling and coupled-states calculations of cross sections based on ab initio potential energy surfaces (PES’s). The propensity for forming Π(A′)Λ doublet levels in ΔN=−1 collisions of high N initial levels is found to be facilitated by approach in a ‘‘helicopterlike’’ orientation on the more attractive HeNH(A)A′ PES, followed by curve crossing to the more repulsive A″ PES which correlates adiabatically to the next lower rotational manifold. In addition, thermal rate constants derived from the calculated cross sections agree extremely well with those obtained from a deconvolution of the experimental spectra.
Avoided resonance overlapping beyond the energy independent formalism. II. Electronic predissociation
Authors: Brems V. , Desouter-Lecomte M. , Liévin J.
Published: July 2002

The transition between the diabatic and the adiabatic zero order representation when the electronic potential coupling increases, is studied in the partitioning method (PM) and in the complex coordinate method (CCM) implemented in a discrete variable representation (DVR). The model is a C+ type predissociation in a diatomic system with a crossing between a Morse potential energy curve and an exponentially repulsive curve. In the weak coupling regime (isolated diabatic metastable states), both methods confirm the linear variation of the resonance widths with the strength of the electronic interaction, as expected in a perturbative treatment. When the coupling is large in the diabatic representation (strong overlap regime), the formation of narrow resonances supported by the upper adiabatic potential can be related, in PM, to the process of avoided resonance overlapping among interfering states. The complete change of representation can be described in the initial diabatic basis set in PM. However, the full energy dependence of the discrete-continuous matrix elements must then be taken into account. The deformation and the final splitting of a diffuse spectral line with increasing coupling is re-examined in terms of these energy dependent eigenvalues of the effective Hamiltonian. The use of the appropriate representation, either diabatic or adiabatic, according to the strength of the electronic coupling, has been found decisive in CCM so as to observe the correct migration of the resonance positions towards the zero order adiabatic states, and the decrease of their widths. No relevant results have been obtained for the intermediate strength of the coupling in CCM. Analytical expressions for the derivative coupling matrix elements (∂/∂R) in the fixed node DVR (corresponding to the particle-in-a-box wave functions) have been established.
Resonance enhanced multiphoton ionization spectroscopy of carbon disulphide
Authors: Morgan Ross A. , Baldwin Michael A. , Orr-Ewing Andrew J. , Ashfold Michael N. R. , Buma Wybren Jan , Milan Jolanda B. , de Lange Cornelis A.
Published: July 2002

Rydberg excited states of the CS2 molecule in the energy range 56 000–81 000 cm−1 have been further investigated via the two and three photon resonance enhancements they provide in the mass resolved multiphoton ionization (MPI) spectrum of a jet-cooled sample of the parent molecule. Spectral interpretation has been aided by parallel measurements of the kinetic energies of the photoelectrons that accompany the various MPI resonances. Thus we have been able to extend, and clarify, previous analyses of the tangled spin–orbit split vibronic structure associated with the 3Πu and 1Πu states derived from the configuration [2Πg]4pσu and the 3Δu, 1Δu, and 1Σ+u states resulting from the configuration [2Πg]4pπu, and to deduce an approximate wave number for the origin of the hitherto unidentified 3Σ+u state derived from this same configuration. Moving to higher energies we are able to locate, unambiguously, the origins of the next (n=5) members of four of these [2Πg]np Rydberg series, and to identify extensive series based on the presumed Rydberg configurations [2Πg]nsσg and [2Πg]nfλu with, in both cases, n≤10. We also identify MPI resonances attributable to CS(a 3Π) fragments, to ground state C atoms, and to S atoms in both their ground (3P) and excited (1S) electronic states. Analysis of the former resonances indicates that the CS(a 3Π) fragments resulting from two photon dissociation of CS2 at excitation wavelengths around 300 nm are formed with substantial rovibrational excitation.
Surface diffusion of carbon monoxide and potassium coadsorbed on Ru(001): Confirmation of a 1:1 CO:K trapping interaction
Authors: Westre E. D. , Brown D. E. , Kutzner J. , George S. M.
Published: July 2002

The surface mobilities of both CO and K coadsorbed on Ru(001) were studied using laser-induced thermal desorption (LITD) techniques. The LITD measurements revealed that CO was essentially immobile on the potassium-promoted Ru(001) surface when the CO surface coverage, ΘCO, was less than the potassium coverage, ΘK. The CO surface mobility increased dramatically when ΘCO≳ΘK. At ΘK=0.10 ML and 315 K, the CO diffusion coefficient was DCO<4×10−10 cm2/s for ΘCO<0.10 ML and increased to DCO=2.5×10−7 cm2/s for ΘCO≳0.10 ML. At ΘK=0.25 ML and 315 K, the CO surface diffusion coefficient was DCO<4×10−10 cm2/s for ΘCO<0.25 ML and increased to DCO=5×10−9 cm2/s for ΘCO≳0.25 ML. The potassium surface mobility also decreased dramatically as a function of CO coverage at both ΘK=0.10 ML and ΘK=0.25 ML. These diffusion results are consistent with a mutually stabilizing and trapping interaction between coadsorbed CO and K with a 1:1 CO:K stoichiometry. The thermal desorption spectra of both CO and K were also studied using temperature programmed desorption (TPD) techniques. At ΘK=0.10 ML and CO coverages ranging from ΘCO=0.06 ML to ΘCO=0.42 ML, CO TPD peak temperatures were consistently higher than CO peak temperatures obtained on clean Ru(001). These TPD results indicate that CO is stabilized by K on Ru(001). In addition, the potassium TPD spectra at various CO coverages with ΘK=0.10 ML and ΘK=0.25 ML revealed that K was stabilized by coadsorbed CO. The experimental results for CO and K coadsorbed on Ru(001) argue for a mutually stabilizing, short-ranged trapping CO–K interaction with a 1:1 CO:K stoichiometry. Monte Carlo simulations based on this model for the CO:K interaction were also consistent with the coadsorbate surface diffusion results.
Vibronic effects in the photon energy-dependent photoelectron spectra of the CH3CN− dipole-bound anion
Authors: Bailey Christopher G. , Dessent Caroline E. H. , Johnson Mark A. , Bowen Kit H.
Published: July 2002

Photoelectron spectra are reported for the ‘‘dipole-bound’’ CH3CN− negative ion at three photodetachment energies (1.165, 2.331, and 3.496 eV), where the anion is prepared by photodissociation of the I−⋅CH3CN ion–molecule complex. While all three spectra are dominated by a single feature centered near zero electron binding energy, as expected for a dipole-bound anion, vibrational structure is also observed and found to depend strongly on the photodetachment energy. This observation indicates that the vibrational excitation is not exclusively due to distortion between the ion and neutral, but also involves non-‘‘Franck–Condon’’ effects. The origin of the energy dependence is traced to excitation of the πCN* shape resonance corresponding to the valence or ‘‘chemical’’ anion. The vibrational envelope of the nonresonant spectrum is surprisingly similar to the infrared spectrum of neutral acetonitrile, suggesting that even this excitation may not result from intramolecular distortions. We develop a simple model to illustrate that vibrational excitation can occur upon photodetachment of a dipole-bound electron due to the perturbation of the weakly bound electron by the fluctuating dipole moment of the vibrating neutral molecule. We treat this effect in a Herzberg–Teller interaction picture where the dipole-bound state is mixed with the low lying electron continuum through a dipolar interaction with the neutral molecule.
A coupled cluster study of the spectroscopic properties and electric dipole moment functions of nitrous sulfide
Authors: Pak Youngshang , Woods R. Claude , Peterson Kirk A.
Published: July 2002

Three-dimensional near-equilibrium potential energy surfaces and dipole moment functions have been calculated for the ground state of nitrous sulfide (NNS), using a large basis set and the coupled cluster method with single and double substitutions, augmented by a perturbative estimate of triple excitations [CCSD(T)]. The CCSD(T) equilibrium bond lengths with a correlation consistent polarized valence quadruple zeta (cc-pVQZ) basis set are re(NN)=1.1284 Å and Re(NS)=1.5904 Å, which have been corrected to 1.126 and 1.581 Å, respectively, based on the results of the corresponding calculations on the NN and NS diatomics. Rotational–vibrational energy levels and the corresponding infrared intensities for NNS have been determined using variational methods with the CCSD(T)/cc-pVQZ potential energy and dipole moment functions. The calculated band origins (cm−1) ν1, ν2, and ν3 and their intensities (km/mol) at the CCSD(T)/cc-pVQZ level are 740.7/38.6, 463.1/0.01, and 2061.4/385.8, respectively. A complete set of second-order spectroscopic constants have been obtained from the ab initio potential energy surface using both the standard perturbation theory formulas and the variationally determined rovibrational energies. Comparison of the theoretical vibration–rotation interaction constants (αi) with those obtained from the published high resolution Fourier transform infrared (FTIR) spectra clearly demonstrate that the rotational quantum number (J) assignments must be revised in all the observed hot bands. A new set of spectroscopic constants for NNS, derived from a reanalysis of the published FTIR frequencies, is presented. These are in excellent agreement with our CCSD(T) predictions. Values of the quadrupole coupling constants at each nucleus are predicted using multireference configuration interaction (MRCI) with the same cc-pVQZ basis.
Evidence for thermalization of surface-desorbed molecules at heating rates of 108 K/s
Authors: Maechling C. R. , Clemett S. J. , Engelke F. , Zare R. N.
Published: July 2002

Laser desorption of aniline-d7 from a single-crystal surface (0001) of sapphire (Al2O3) at a heating rate on the order of 108 K/s was studied using pulsed infrared laser radiation for desorption and resonance enhanced multiphoton ionization (REMPI) for detection of the desorbed aniline molecules. On the basis of single-vibronic-level fluorescence (SVLF) spectra we unambigiously assign the 10b transition. REMPI spectroscopy provides vibrational temperatures and therefore describes the internal energy distribution, whereas the time-of-flight (TOF) profiles provide translational temperatures. All results are consistent with a thermal mechanism for desorption, i.e., pulsed heating of the sapphire surface on the nanosecond time scale leads to thermal desorption and rapid thermalization of the escaping molecules.
Wetting and prewetting transition in metallic fluid K–KCl solutions studied by second harmonic generation
Authors: Tostmann H. , Nattland D. , Freyland W.
Published: July 2002

We report a detailed optical investigation of the wetting phenomena in fluid metalrich KxKCl1−x solutions for temperatures up to 820 °C and x≥0.86. To this end the fluid sample–sapphire interface has been probed by second harmonic generation (SHG) with continuous variation of temperature T at various x. At coexistence a wetting transition near Tw≊500 °C is signaled by a clear change in the temperature dependence of the SH intensity. This observation is in agreement with previous ellipsometric measurements. The precise location of the transition in these systems is strongly influenced by the presence of oxygen impurities segregating at the interface. This is manifested in a strong enhancement of the SH signal in the vicinity of 450 °C. In the homogeneous metal-rich phase the SH intensity shows a pronounced maximum as a function of temperature for T≳Tw. The occurrence of these maxima is explained consistently by assuming a salt-rich microscopic wetting film in between the metallic bulk fluid and the sapphire window whose thickness changes from thick to thin with increasing temperature. The line connecting these maxima positions in the x–T plane exhibits all the characteristics of a prewetting line. In particular, it is curved away from the coexistence curve as has been predicted theoretically for systems with screened Coulombic interactions, but is found here for the first time experimentally.
Geometry changes in molecular photoionization: The NH+3 (X̃ 2A2″)←NH3 (X 1A1) transition
Authors: Niu Baohua , White Michael G.
Published: July 2002

Single-photon threshold photoionization has been used to explore the ionization dynamics of the NH+3 (X̃ 2A2″)←NH3 (X 1A1) transition which involves a pyramidal-to-planar geometry change. Rotationally resolved threshold photoelectron spectra are presented for four vibrational levels of the X̃ 2A2″ cation state of NH+3 corresponding to excitation of even and odd quanta of the inversion mode, ν+2. The rotational state distributions exhibit strong symmetry effects imposed by the nuclear spin statistics and vibronic parity of the neutral and cation out-of-plane bend levels. The observed rotational structure is consistent with dominant ΔK=0 transitions and parity assignments for specific transitions show that both l=even and l=odd photoelectron final states are excited. These observations indicate that the photoelectron experiences a non-planar (pyramidal) cation potential. These results are discussed in light of previous rotationally resolved measurements on NH3 using multiphoton ionization and other systems which undergo bend excitation following ionization.
Analytical energy derivatives for a realistic continuum model of solvation: Application to the analysis of solvent effects on reaction paths
Authors: Dillet Valérie , Rinaldi Daniel , Bertrán Juan , Rivail Jean-Louis
Published: July 2002

Analytical expressions for the first and second derivatives of the Hartree–Fock energy have been derived in case of a solvated system simulated by a multipolar charge distribution embedded in a cavity of arbitrary shape and a solvent represented by a dielectric continuum. A computer code has been written on these bases. It allows geometry optimizations and more generally the determination of the critical points of the potential energy surface for a molecular system interacting with a solvent as easily as in the case of an isolated molecule. The use of this code is illustrated by the computation of the main features of the reaction path of a Menshutkin-type reaction in various solvents. The results compare pretty well with those obtained by a full Monte Carlo simulation of the solvent by Gao. This agreement supports the idea that solvents, including water, can be safely modeled by a continuum. The advantage of such models rests in the fact that they allow refined computations on the solute at a minimum computational expense.
The frequency detuning and band-average approximations in a far-wing line shape theory satisfying detailed balance
Authors: Ma Q. , Tipping R. H. , Boulet C.
Published: July 2002

We develop the basic formalism of a far-wing line shape theory that satisfies the detailed balance principle. For molecular systems of interest, e.g., CO2–Ar at room temperature or higher, there are many individual vibration–rotational lines in a given band and many bands in the spectrum. In such cases, one must make additional approximations in order to carry out accurate calculations of the absorption coefficient using a reasonable amount of computer time. In the present paper, we discuss two such simplifications: the frequency detuning approximation of the line-coupling functions and the band-average approximation. We then apply the theory to a calculation of the far-wing absorption of the ν3 band of CO2 perturbed by Ar, successively including the effects of more lines in the calculations by increasing Jmax from 40 to 108. From the results of this work, we find that the frequency detuning approximation is good only for frequencies of interest far from the band center. In addition, we find that contrary to previous assertions of the adequacy of the first-order band-average approximation, the higher-order terms are significant. To a good approximation these can be incorporated by introducing a frequency shift in the first-order results so that extensive additional calculations are not required.
Laser cooling of molecules: A sequential scheme for rotation, translation, and vibration
Authors: Bahns J. T. , Stwalley W. C. , Gould P. L.
Published: July 2002

A novel scheme is proposed for sequential cooling of rotation, translation, and vibration of molecules. More generally, this scheme manipulates and controls the states and energies of molecules. The scheme, while somewhat complex, is simpler and more feasible than simply providing a large number of synchronously but independently tunable lasers. The key component is a multiple single frequency laser (MSFL) in which a single narrow band pump laser generates an ensemble of resonant ‘‘stimulated Raman’’ (RSR) sidebands (subsequently amplified and selected) in a sample of the molecules to be cooled. Starting with a relatively cold molecular sample (e.g., a supersonic beam of Cs2), the rotation of molecules is cooled by sequential application of P branch electronic transition frequencies transverse to the molecular beam beginning at higher rotational angular momentum J. Then translation of molecules is cooled by application of multiple low J, P, and R branch transition frequencies which counterpropagate with the molecular beam and are synchronously chirped over their Doppler profiles. Finally, vibration of molecules is cooled by blocking the R(0) line of the 0–0 band. Only this specific order of rotation–translation–vibration appears feasible (using molecules produced by photoassociation of ultracold atoms avoids the requirement for translational cooling). Each step employs true dissipative cooling (i.e., reduction of system entropy in three degrees of freedom) by spontaneous emission and should yield a large translationally cold sample of molecules in the lowest (v=0, J=0) level of the ground electronic state, suitable for studies such as molecule trapping, ‘‘molecule optics,’’ or long range intermolecular states.
Many-body potentials and dynamics based on diatomics-in-molecules: Vibrational frequency shifts in ArnHF (n=1–12,62) clusters
Authors: Grigorenko B. L. , Nemukhin A. V. , Apkarian V. A.
Published: July 2002

The conjecture that limited basis diatomics-in-molecules type potentials may serve as an accurate representation of many-body interactions is explored through molecular dynamics simulations of ArnHF (n=1–12,62). The important ingredient in the constructed potentials is the inclusion of ionic configurations of HF. Once the admixture between ionic and covalent configurations is calibrated by reference to an ab initio surface of the ArHF dimer, a single three-body potential energy surface is defined, and used in subsequent simulations of larger clusters. The vibrational frequencies of HF, which are computed from velocity–velocity autocorrelation functions, quantitatively reproduce the cluster size dependent redshifts.
Electronically adiabatic reaction field approach to solvation. II. Solvent effects on electronic spectra
Authors: Kim Hyung J.
Published: July 2002

The theoretical formulation developed in the preceding article [H. J. Kim, J. Chem. Phys. 105, 6818 (1996)] is analyzed via a second-order perturbation method and applied to the static electronic spectra of polarizable solutes in solution. In the Born–Oppenheimer (BO) framework of the solvent electronic polarization P⃗el, the solute electronic wave functions, together with their (free) energy levels and associated Franck–Condon (FC) energies, are examined in the presence of a spherical cavity of arbitrary size and a nonequilibrium solvent orientational polarization configuration P⃗or. It is found that the solute electronic structure and its free energetics vary strongly with both P⃗or and the cavity size. The solute dipole enhancement due to solvation decreases with increasing cavity size. Comparison with the self-consistent (SC) reaction field theory predictions shows that classical P⃗el is more effective in polarizing the solute than quantum P⃗el couched in the BO description. This is due to the dispersion stabilization mechanism present in the latter. The static electronic spectroscopy is studied to linear order in the solute polarizability and in the cavity size difference between the lower and upper electronic states involved in the FC transition. In the case of the vanishing cavity size difference, our analytic results for the solvent spectral and Stokes shifts are compared with various existing theories and the sources of the discrepancies are briefly discussed. The effects of the cavity size variation on the electronic spectra are illustrated by using a simple two-state model description for the solute. It is found that even in a nonpolar solvent, there can be a significant Stokes shift arising from the cavity size relaxation subsequent to the FC transition. Also the cavity size fluctuations can make a non-negligible contribution to the spectral line broadening.
Symmetry induced kinetic isotope effects in the formation of Ar⋅CO+2
Authors: Yoo R. K. , Gellene Gregory I.
Published: July 2002

Symmetry induced kinetic isotope effects (SIKIE) have been observed mass spectrometrically in the termolecular association reaction of Ar and CO+2 to produce Ar⋅CO+2 making this the first example of SIKIE in a system where the reactants have no atoms in common. The 18O SIKIE shows a pronounced dependence on whether the CO+2 are produced directly by electron ionization or indirectly by charge transfer from Ar+. The results can be understood generally in terms of reaction restrictions based on a symmetry correlation scheme recently developed by one of the authors. This scheme indicates that CO+2 ions can have very different clustering reactivity with Ar depending on whether the ion is in an e (restricted) or an f (allowed) parity label state. Kinetic modeling of the 18O SIKIE indicates that CO+2(e)/CO+2(f )≊60 for CO+2 produced by 21 eV electron ionization.
Electron attachment to ClONO2 at 300 K
Authors: Van Doren Jane M. , McClellan Joseph , Miller Thomas M. , Paulson John F. , Viggiano A. A.
Published: July 2002

A flowing–afterglow Langmuir probe apparatus with mass spectral analysis has been used to measure the rate constant for electron attachment to ClONO2 at 300 K. Electron attachment is efficient with a rate constant of 1.1 (±50%)×10−7 cm3 s−1 and proceeds principally through dissociative channels to produce the major product ions NO−2 (∼50%), NO−3 (∼30%), and ClO− (∼20%). The parent ion ClONO−2 and Cl− are also observed in the mass spectra but are at most minor products in the attachment process, ≤2% and ≤6%, respectively. A description of the secondary ion–molecule chemistry that takes place following electron attachment is given.
Computer simulation of secondary structure folding of random and ‘‘edited’’ RNA chains
Authors: Galzitskaya Oxana V. , Finkelstein Alexei V.
Published: July 2002

Computer simulation of secondary structure folding of RNA chains demonstrates that these chains under some temperature and solvent conditions find their native conformations much faster than by exhaustive sorting of all the possible chain conformations. This is true both for the random and the edited chains. However, for a random chain the native structure is thermodynamically stable in one region of temperature and solvent quality, while fast folding (and even faster unfolding) of the random chain occurs in another region. Some editing of a chain is necessary to make its native structure thermodynamically stable at the optimal for a fast folding range of conditions. Only for the edited chains can the thermodynamically stable native secondary structures fold fast. The dependence of folding time on the chain length is not an exponential one but much less steep.
Phase asymmetry in thermocapillary motion
Authors: Bhagavatula Ravi , Jasnow David
Published: July 2002

It is shown that bulk thermodynamic effects, in addition to surface tension effects, can play a crucial role in thermocapillary phenomena (e.g., bubble or droplet thermomigration and phase separation in multi-phase systems subjected to an imposed temperature gradient) due to asymmetric thermodynamic properties of the underlying coexisting phases. The resultant new phenomena are elucidated in a representative model of diffusive systems with a conserved order parameter.
Ionization probabilities of A 2Σ+(v′=0,1,2) and B 2Π(v′=0,2) states of NO
Authors: Zacharias H. , de Rougemont F. , Heinz T. F. , Loy M. M. T.
Published: July 2002

Ionization probabilities of NO molecules electronically excited in the A 2Σ+ and B 2Π states have been determined by (1+1) resonance-enhanced, two-photon ionization. Various vibrational levels within these states have been excited prior to ionization. Measurements of the unsaturated ionization signal yields accurate values for the relative detection probabilities of NO of 1:(0.70±0.07): (0.67±0.11) for excitation via the γ(0−0), γ(1−1), and γ(2−2) bands, respectively, and (3.7±0.36)×10−7 and (5.8±0.65)×10−4 for ionization through β(0−0) and β(2−1) bands, respectively. Applying published data for the γ- and β-band transition probabilities allows the deduction of the ionization cross section of A 2Σ+ and B 2Π vibrational states. The respective ionization cross sections are (7.0±0.9)×10−19 cm2, (8.5±0.8)×10−19 cm2, (6.0±1.0)×10−19 cm2 for A 2Σ+(v′=0, 1, and 2) and (5.0±0.5)×10−21 cm2 and (1.7±0.2)×10−20 cm2 for B 2Π(v′=0 and 2). These values are based on the experimentally determined cross section for A 2Σ+(v′=0). Using a larger theoretical cross section for this state the other cross sections scale accordingly, within the experimental uncertainties.
Simulation of activation free energies in molecular systems
Authors: Neria Eyal , Fischer Stefan , Karplus Martin
Published: July 2002

A method is presented for determining activation free energies in complex molecular systems. The method relies on knowledge of the minimum energy path and bases the activation free energy calculation on moving along this path from a minimum to a saddle point. Use is made of a local reaction coordinate which describes the advance of the reaction in each segment of the minimum energy path. The activation free energy is formulated as a sum of two terms. The first is due to the change in the local reaction coordinate between the endpoints of each segment of the path. The second is due to the change in direction of the minimum energy path between consecutive segments. Both contributions can be obtained by molecular dynamics simulations with a constraint on the local reaction coordinate. The method is illustrated by applying it to a model potential and to the C7eq to C7ax transition in the alanine dipeptide. It is found that the term due to the change of direction in the reaction path can make a substantial contribution to the activation free energy.
Prediction of liquid–liquid phase equilibria of He+H2 mixtures by NpT molecular dynamics simulations
Authors: Liu Anping , Beck Thomas L.
Published: July 2002

Thermodynamic quantities of the classical He+H2 mixtures were simulated in the isothermal–isobaric ensemble at T=300 K, P=54 kbar, and several concentrations. The chemical potentials were extrapolated in three directions (T,P,x) with their derivatives calculated by the energy difference method in combination with a modified Kirkwood–Buff fluctuation theory. The phase equilibria at different pressures and temperatures were successfully determined.
Resonance coherent anti-Stokes Raman spectroscopy of iodine in solution
Authors: Schenk M. , Kiefer W.
Published: July 2002

Coherent anti-Stokes–Raman scattering (CARS)-excitation profiles (CEPs) of the fundamental vibrational mode of I2 dissolved in cyclohexane and dibromomethane are determined with excitation wave numbers ranging from 17 000 to 23 000 cm−1. The experiments were performed at room temperature and with laser beams polarized parallel to each other. Calculations were performed for both the absorption spectrum and the CEPs of iodine in both solvents and compared with experimental results. Good qualitative agreement of the major features of the CEPs was found using gas phase potentials for the excited states 1Π1u and 3Π0+u and the ground state 1Σ+g. The excited state potential functions were slightly shifted to match the measured absorption spectra.
Hydride exchange via two-dimensional vibrational tunneling
Authors: Close J. D.
Published: July 2002

We obtain analytic expressions for the zero temperature and finite temperature exchange interaction between two hard sphere fermions bound in a symmetric double-well potential in 1, 2, and 3 dimensions and discuss the effect of dimensionality on the model. The three-dimensional model (due to Zilm/Landesmann) and the two-dimensional model presented here are of relevance to the strongly temperature dependent exchange interaction that has been observed between hydrides bound in metal polyhydride complexes.
Pulse propagation and oscillatory behavior in the NO+H2 reaction on a Rh(110) surface
Authors: Mertens F. , Imbihl R.
Published: July 2002

Target patterns, rotating spiral waves and solitary pulses have been found in the NO+H2 reaction under nonoscillatory conditions, i.e., when the system was an excitable medium. Using photoelectron emission microscopy (PEEM) as spatially resolving method the parameter dependence of the front velocities, the width of the pulses and the rotational period of the spiral waves were studied for fixed pNO=1.8×10−6 mbar in a T- range 520–620 K. The front velocities were strongly anisotropic with the degree of anisotropy depending on the pH2, T parameters. Under reaction conditions close to the high pH2 boundary for pattern formation, gas-phase coupling becomes efficient, thus, oscillations in the N2 production rate can be observed.
Theoretical study of the trans-stilbene isomerization reaction in ethane
Authors: Gershinsky Gidon , Pollak Eli
Published: July 2002

A theoretical investigation of the experimental measurements of the isomerization rate of trans-stilbene in liquids is presented. Monte Carlo and molecular dynamics simulations of the reaction indicate that the predominant solvent effect is in raising the isomerization barrier in the potential of mean force as the solvent density is increased. Dynamic friction effects are small. Good agreement is obtained between the numerical and experimental rates.
Transition state observation of excited harpoon reactions, within Ca-HX van der Waals complexes
Authors: Keller A. , Lawruszczuk R. , Soep B. , Visticot J. P.
Published: July 2002

The observation of reactions through the excitation of van der Waals precursors allows for the spectroscopic investigation of the reaction surface of the excited state adducts. This method has been applied to the reaction of calcium 1D and 1P with the various hydrogen halide molecules, the reaction being endothermic with ground state calcium by less than 1 eV. The complexes were produced by supersonic expansion following laser ablation of the calcium atom. The chemiluminescent channel producing the CaX (A 2Π, B 2Σ) molecules is found to be an important channel and was investigated through action spectra. These spectra reveal the local electronic excitation of the calcium atom within the complex with the local orbital symmetry A′ or A″ of the Cs molecular complex. The band structure has been analyzed in terms of the intermolecular modes, the bending mode being prominent in the A″ state correlating with Ca 1P1. This state can be characterized by a single generic two dimensional reaction surface (bending+H-X reaction coordinate) common to the HX molecules. This surface is characterized by an identical entrance valley in the bending and reaction coordinates opened to the products through a transition state characterized by a channel of variable size depending upon the halogens. The experimental results have been successfully simulated on this model surface by performing one- and two-dimensional wave packet propagation calculations. The position of the bands in the spectrum are related to the bending modes and their bandwidth to the opening in the channel. These excited state reactions appear through their various characteristic as a generic model for excited state harpoon type reactions and display resonances despite the high cross sections for the reactions observed in collisions.
Geometries and energy separations of 28 electronic states of Ge5
Authors: Dai Dingguo , Balasubramanian K.
Published: July 2002

Geometries and energy separations of 28 low-lying electronic states of Ge5 with different structures (trigonal bipyramid, D3h; edge-capped tetrahedron, C2v; tetragonal pyramid, C4v; planar square, D4h; planar pentagon, D5h; linear, D∞h; and tetrahedron, Td) are investigated in this study. We employ the complete active space multiconfiguration self-consistent-field method (CASSCF) followed by large scale multireference singles+doubles configuration interaction (MRSDCI) computations that included up to 3.86 million configurations. Atomization and dissociation energies of Ge5 are computed and compared with smaller clusters.
Electron spin resonance rare gas matrix studies of 12CO2−, 13CO2−, and C17O−2: Comparison with ab initio calculations
Authors: Knight Lon B. , Hill Devon , Berry Kenneth , Babb Robert , Feller David
Published: July 2002

The 12C16O2−, 13C16O2−, 12C17O2−, and 12C16,17O2− radical anions have been generated by four independent methods and isolated in neon and argon matrices for detailed ESR (electron spin resonance) investigations. Included with these experimental measurements for the various magnetic parameters of CO−2 are high level ab initio calculations (MR SD-CI and others) of the 13C and 17O hyperfine A tensors. Some of the calculations included the effects of a 42-atom neon cage on the electronic structure of CO−2. Previous ESR studies of CO−2 have been conducted in more perturbing environments, such as ionic crystals, where the close proximity of the counter cation can alter the anion’s properties. A comparison of the earlier measurements in more interactive materials with these theoretical and rare gas matrix results reveals a significantly different distribution of the spin density. The neon magnetic parameters (MHz) for CO−2 are gx=2.0018, gy=1.9964, gz=2.0010; for 13C, Ax=320.4, Ay=296.1, Az=394.5; for 17O, Ax=−81.6, Ay=−74.9 and Az=−151.8. The argon results are similar to these neon values; isotropic spectra in argon were also observed at elevated temperatures that yielded giso and Aiso parameters consistent with the low temperature (4 K) anisotropic spectra.
Dynamics of proton attachment to water cluster: Proton transfer, evaporation, and relaxation
Authors: Kobayashi Chigusa , Iwahashi Kensuke , Saito Shinji , Ohmine Iwao
Published: July 2002

A proton attachment dynamics to a water cluster is investigated by using a classical molecular dynamics calculation. It is found that three dynamical stages are involved: (1) ultrafast (∼10−14 s) proton attachment to a water molecule of the cluster which followed by (2) the fast (∼10−13 s) sequential proton transfer over several water molecules on the cluster surface and then, (3) the gradual (∼10−11 s) proton penetration to the cluster core. In the first two stages, the large kinetic energy of the order of hundreds kcal/mol is released to the system, which results in the evaporation of a few water molecules from the cluster. The water molecules evaporating in these early stages have large vibrational and translational energies. The mechanism of the energy relaxation and the proton transfer in each process are investigated. The large amplitude vibrational motion promotes sequential concerted proton exchange transfers in the earlier stages (1) and (2). The precise configurational matching of the hydrogen bond coordination and the O–O distance fluctuation are the most important factors to determine the direction and the rate of the proton transfer in the stage (3).
Kinetic oscillations and hysteresis phenomena in the NO+H2 reaction on Rh(111) and Rh(533): Experiments and mathematical modeling
Authors: Makeev A. G. , Slinko M. M. , Janssen N. M. H. , Cobden P. D. , Nieuwenhuys B. E.
Published: July 2002

Interesting kinetic phenomena, such as multiple steady states and kinetic oscillations recently found in the NO+H2 reaction over Rh(533) and Rh(111) single crystal surfaces in the 10−6 mbar pressure range have been studied by means of experiments and computer modeling. A mathematical model, consisting of five ordinary differential equations and taking into account the lateral interactions in the adlayer, has been developed for simulating the NO+H2/Rh(533) and NO+H2/Rh(111) reactions. The simulation results make it possible to explain in detail the underlying reasons for the experimentally observed complex dynamic behavior. In particular, the kinetic oscillations and their properties have been reproduced. It was found that accumulation of NHads species, which serves as an intermediate in the pathway of NH3 production, is an important step in the oscillatory mechanism. In addition, the same mathematical model is able to successfully reproduce the experimental data concerning temperature programmed desorption (TPD) spectra, hysteresis phenomena, and the dependence of selectivity upon temperature and reactant partial pressures. Lateral interactions in the adlayer are shown to play a crucial role in the adequate simulation of the experimental observations.
Geometric structure of Ar⋅NO+: Revisited. A failure of density functional theory
Authors: Wright Timothy G.
Published: July 2002

High level ab initio calculations are performed on the Ar⋅NO+ complex to ascertain its geometric structure. The calculations show that the complex is bent, with an Ar–N–O angle of ∼105°, i.e. the argon atom is on the nitrogen side of the complex cation. Calculations are performed with the cc-pVDZ and cc-pVTZ basis sets at the MP2, MP4(SDQ), QCISD, CISD, CCSD(T), and QCISD(T) levels of theory; in addition the hybrid density functionals BLYP, B3LYP, and BPW91 are used. Harmonic vibrational frequencies are also calculated for all except the two highest levels of theory, but it is recognized that the surface is anharmonic and these are not good estimates of the vibrational separations; however, by comparison with the ab initio results, they demonstrate that these density functional methods are not trustworthy for this complex.