bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic structure of metal–rare gas dimers with sp configuration: Application to strong spin–orbit interaction in HgAr
Authors: Onda Ken , Yamanouchi Kaoru
Published: July 2002

In order to describe the electronic states of metal (M)–rare gas (Rg) van der Waals dimers having an sp configuration with a strong spin–orbit interaction, we derived an e/f parity adapted molecular Hamiltonian matrix by adopting a symmetry-adapted atomic orbital approach. The molecular Hamiltonian was constructed by introducing (i) the interaction between the p electron and the attached rare gas atom, VRg, (ii) the exchange interaction between the s and p orbitals, e2/rsp, and (iii) the spin–orbit interaction for the p electron. As a basis set, twelve molecular electronic wave functions were derived by taking into account their e/f parities. We applied the derived molecular Hamiltonian matrix to the first excited 6s6p configuration of HgAr by performing a least-squares fit to the spectroscopically determined term values for the v=0 levels of the a 3Π0−, A 3Π0+, B 3Π1, b 3Π2, and C 1Π1 states. From the results of the least-squares fit, we clarified how the above interactions (i)–(iii) split twelve degenerate molecular wave functions into the eight electronic eigenstates; i.e., a 3Π0−, A 3Π0+, B 3Π1, b 3Π2, c 3Σ+1, d 3Σ0−+, C 1Π1, and D 1Σ+0. On the basis of (i) a critical comparison between the atomic Hamiltonian matrix for Hg and the determined molecular Hamiltonian matrix and (ii) an examination of the mixing among the symmetry-adapted molecular wave functions, characteristic features of the electronic structure arising from the formation of a van der Waals bond, were extracted.
Bridge-mediated electronic interactions: Differences between Hamiltonian and Green function partitioning in a non-orthogonal basis
Authors: Priyadarshy Satyam , Skourtis Spiros S. , Risser Steven M. , Beratan David N.
Published: July 2002

An analysis of the partitioning (projection) technique is given with emphasis on non-orthogonal basis sets. The general expression for the effective Hamiltonian obtained via Löwdin partitioning of the Schrödinger equation is discussed in the context of semi-empirical theories and electron transfer matrix elements. Numerous pitfalls in calculations of matrix elements are pointed out. More importantly, it is shown that contrary to the case of an orthogonal basis, for a non-orthogonal basis Löwdin partitioning of the Schrödinger equation and partitioning of the Green function equation are not equivalent. The latter method provides a more general prescription for deriving effective Hamiltonians. Such Hamiltonians reproduce the full propagation in the partitioned subspace.
Spectroscopy and dynamics of rare gas–spherical top complexes. II. The infrared spectrum of the ν3 band of Ne–SiH4 ( j=1←0 and j=0←1 transitions)
Authors: Brookes Matthew D. , Hughes Daniel J. , Howard Brian J.
Published: July 2002

The infrared spectrum of the rare gas–spherical top complex Ne–SiH4 has been recorded in a supersonic jet in the region of the SiH4 ν3 triply degenerate stretching vibration at ∼2189 cm−1. In contrast to the previously measured Ar–SiH4 spectrum which showed almost equal rotational spacings within each band (corresponding to transitions between different internal rotor states of SiH4 within the complex), the Ne–SiH4 spectrum is complex with no obvious regular band structure. However, by analogy with the Ar–SiH4 spectrum, four bands of the Ne–SiH4 have been assigned and analyzed in terms of Hamiltonians incorporating Coriolis interaction between the angular momentum of the SiH4 monomer unit and the overall end over end rotation of the complex. These bands correlate with the SiH4 R(0) (K=0←0, K=1←0) and P(1) (K=0←0, K=0←1) transitions. Derived rotational constants demonstrate that the neon–silane separation (∼4.13 Å in the ground vibrational state) is larger than expected by analogy with Ar–SiH4, indicative of nearly free internal rotation by the silane monomer unit in Ne–SiH4. The smaller anisotropy of Ne–SiH4 compared with Ar–SiH4 results in a new angular momentum coupling scheme. Transitions arising from 22Ne–SiH4 correlating to SiH4 R(0) have also been observed and fitted; the higher than anticipated intensities demonstrate a novel isotope enrichment effect in the supersonic jet which is discussed.
Potential energy surface and quasiclassical trajectory studies of the CN+H2 reaction
Authors: ter Horst Marc A. , Schatz George C. , Harding Lawrence B.
Published: July 2002

We present dynamical studies of the CN+H2 reaction based on an empirical potential energy surface that is derived from high quality ab initio calculations. The ab initio calculations, which use a multireference configuration interaction method with large correlation consistent basis sets, indicate that the linear HHCN barrier is about 4.3 kcal/mol above CN+H2, and that there is no reaction path which connects CN+H2 to the stable intermediate H2CN, although there is a path for dissociation of H2CN to H+HCN. The empirical surface is written as a sum of two-, three-, and four-body terms, with the two- and three-body terms for HCN based on an accurate global surface that describes both the HCN and HNC force fields. The four-body terms are developed so as to describe the HHCN linear saddle point and the H2CN minimum accurately, as well as dissociation of H2CN into HCN+H, and the ridge which separates the abstraction and H2CN dissociation pathways. Other features of the potential surface, such as the HCNH cis and trans minima, and the pathways leading to the formation of HNC+H are also described, though less accurately. Three different choices for the HHCN saddle point properties are considered. We find that the surface which matches the ab initio barrier energy most accurately gives rate constants that are too low. Much better agreement is obtained using a 3.2 kcal/mol barrier. The trajectory results show typical dependence of the CN+H2 reactive cross sections on initial translational energy and initial vibration/rotation state, with CN behaving as a spectator and H2 playing an active role in the reaction dynamics. Analysis of the H+HCN products indicates that both the C–H stretch and bend modes are significantly excited, with bend excitation showing strong sensitivity to the saddle point properties and to reagent translational energy. At translational energies below 20 kcal/mol, direct H abstraction is strongly favored over addition elimination.
A quantitative theory of linear chain polymer dynamics in the melt. II. Comparison with simulation data
Authors: Herman Michael F. , Panajotova Biliana , Lorenz K. Thomas
Published: July 2002

Estimates are evaluated for all constants in the theory of polymer dynamics developed in the preceding paper. These theoretical estimates are employed in calculations on the many chain systems simulated previously by Kremer and Grest and by Paul and co-workers. The agreement between the theoretical calculations and the simulations of Kremer and Grest is found to be very good for the bead mean squared displacement. The theoretically evaluated center of mass mean squared displacement is in good qualitative agreement with the Kremer–Grest simulations, although there are quantitative differences. Good qualitative agreement with the simulations of Paul and co-workers is also obtained. It is shown that quantitative agreement can be obtained with modest changes in the theoretical estimates of a few of the constants in the theory.
The far-infrared vibration–rotation–tunneling spectrum of the water tetramer-d8
Authors: Cruzan J. D. , Brown M. G. , Liu K. , Braly L. B. , Saykally R. J.
Published: July 2002

The far-infrared vibration–rotation–tunneling spectrum of (D2O)4 has been measured in the spectral region near 2.04 THz. Observation of additional transition doublets with a constant 5.6 MHz spacing in a parallel (c-type) spectrum extends the first detailed study of this cluster [Science 271, 59 (1996)]. Three possibilities are explored for the origin of this small splitting: tunneling between degenerate equilibrium structures via facile torsional motions analogous to those observed in the water trimer, tunneling between nondegenerate structural frameworks, and tunneling made feasible only through excitation of a specific vibrational coordinate. The degenerate tunneling scheme best accounts for the spectral features, although the precise dynamics responsible for the observed spectral features cannot be uniquely established from the present data. A further doubling of spectral features, observed only in the K=2 manifold of transitions for J≥3, is symmetric about the unperturbed symmetric top energy levels and shows an approximate J2 dependence of the spacing. The origin of these additional splittings, which are shown to be present in both upper and lower vibrational states of the spectrum, is likely to be due to an interaction between overall rotational angular momentum and the type of internal motion (‘‘pseudorotation’’) that gives rise to a manifold of low energy states in water trimer. The measured interoxygen separations in (D2O)n (n=2,3,4,5) are analyzed in order to quantify the contribution of many-body forces in bulk water. An exponential contraction of this property toward the value found in ordered ice is observed.
Photofragmentation of I2−⋅Arn clusters: Observation of metastable isomeric ionic fragments
Authors: Vorsa Vasil , Campagnola Paul J. , Nandi Sreela , Larsson Mats , Lineberger W. C.
Published: July 2002

We report the 790 nm photofragmentation of mass-selected I−2⋅Arn clusters, n=1 to 27. We determine the I−+I caging efficiency as a function of the number of solvent Ar atoms and compare these results with I−2 in CO2 clusters. Caging is much less effective with Ar. In addition to ‘‘normal’’ caged photoproducts (I−2⋅Arm, where m<n), the evaporation process following photoexcitation produces ‘‘solvent-separated’’ (I−...I)⋅Arm photofragments, where the I−2 bond has not reformed. These metastable species comprise ∼55% of the photofragment yield for precursor clusters for n≥14 and have lifetimes ≳5 μs. This unusual photofragment exists either as a trapped excited electronic state or as a solvent-separated pair at an internuclear separation of ∼5.5 Å. The photofragmentation data also exhibit the existence of two distinct isomeric forms of the precursor I−2⋅Arn, for n≤14. These forms are evaporatively distinct in that one isomer displays highly nonstatistical fragmentation, probably arising from a cluster in which the I−2 resides on the surface, rather than in the interior. The photofragmentation distribution of the other form exhibits statistical behavior, consistent with the evaporation of an I−2 solvated inside the cluster.
Quantum mechanically exact analytical solutions of a two-state exponential model
Authors: Osherov Vladimir I. , Nakamura Hiroki
Published: July 2002

A certain two-state exponential potential model is solved quantum mechanically exactly. Compact expressions for nonadibatic transition matrices are obtained. Interesting quantum mechanical threshold effects are found. Simple very accurate expressions are found from a semiclassical viewpoint for the nonadiabatic transition probabilities, indicating that the exponential model may present a third important basic model in addition to the Landau–Zener–Stueckelberg and the Rosen–Zener–Demkov models. Extension to general cases is also briefly discussed.
Relaxation of collective excitations in LJ-13 cluster
Authors: Salian Umesh A. , Behera S. N. , Ramamurthy V. S.
Published: July 2002

We have performed classical molecular dynamics simulation of Ar13 cluster to study the behavior of collective excitations. In the solid ‘‘phase’’ of the cluster, the collective oscillation of the monopole mode can be well fitted to a damped harmonic oscillator. The parameters of the equivalent damped harmonic oscillator—the damping coefficient, spring constant, time period of oscillation and the mass of the oscillator—all show a sharp change in behavior at a kinetic temperature of about 7.0 °K. This marks yet another characteristic temperature of the system, a temperature Ts below which collective excitations are very stable, and at higher temperatures the single particle excitations cause the damping of the collective oscillations. We argue that so long as the cluster remains confined within the global potential energy minimum the collective excitations do not decay; and once the cluster comes out of this well, the local potential energy minima pockets act as single particle excitation channels in destroying the collective motion. The effect is manifest in almost all the physical observables of the cluster.
Classical trajectory and adiabatic channel study of the transition from adiabatic to sudden capture dynamics. I. Ion–dipole capture
Authors: Maergoiz A. I. , Nikitin E. E. , Troe J. , Ushakov V. G.
Published: July 2002

Classical trajectory calculations of ion–permanent+induced dipole capture processes are performed over very wide ranges of conditions. The results are represented in a simple, two-parametric analytical form of high precision. The transition from adiabatic to nonadiabatic dynamics is expressed in terms of the Massey parameter. In the adiabatic range, perfect agreement (better than 0.4%) of the derived thermal capture rate constants from classical trajectories and results from accurate statistical adiabatic channel (SACM) calculations is obtained.
The influence of multiple scattering processes on the electron mobility in low density methanol gas
Authors: Asaad A. N. , Klahn Th. , Krebs P.
Published: July 2002

The electron drift mobility μ has been measured in CH3OH gas in the temperature range 303≤T≤363 K and in the number density range 2.4×1017≤n≤2.1×1019 cm−3. It is usually assumed—although there is no foundation for it—that at such low gas densities the ‘‘zero-field’’ density-normalized mobility (μn) does not depend on n, i.e. it can be described by the so-called Lorentz single collision approximation [see, e.g., N. Gee and G. R. Freeman, Can. J. Chem. 61, 1664 (1983)]. We observed, however, a density dependence of (μn) which can be explained approximately in terms of coherent and incoherent multiple scattering corrections where the coherent contribution due to correlations between the scatterers predominates at lower temperatures.
Response to ‘‘Comment on ‘The effect of charged impurities on a glass transition in a polar medium’ ’’ [J. Chem. Phys. 105, 8979 (1996)]
Authors: Dakhnovskii Yuri , Lubchenko Vassiliy
Published: July 2002

The theory presented in [J. Chem. Phys. 104, 669 (1996)] deals with the effect of separated charged impurities on the glass transition temperature Tg. To explain experimental data on the effect of salt on the glass transition temperature, it is necessary to include mechanisms of aggregation and dissociation of salt molecules. Due to aggregation and dissociation, the concentration of charges can be different than the salt concentration. It is pointed out that this might explain the discrepancy between some experimental data and the theory.
On the interrelation between nuclear dynamics and spectral line shapes in clusters
Authors: Heidenreich Andreas , Jortner Joshua
Published: July 2002

We analyze spectral absorption line shapes simulated using the molecular dynamics spectral density method. We explore three classes of line shapes: (1) the region of the 0–0 S0→S1(ππ*) transition of perylene⋅ArN clusters, (2) the Xe1S0→3P1 transition of XeArN clusters, and (3) the photoelectron spectrum of the Li4F4 cluster in the valence region. These spectra represent examples for weak, unresolved, and extensive vibrational progressions, which have been analyzed and assigned. Employing a simplified model for the energy gap autocorrelation function allows for an understanding of the different behaviors and for a classification of the interrelation between nuclear dynamics and spectral line shapes. With decreasing the characteristic decay time of the transition dipole autocorrelation function, the line shape passes the limiting cases of the model in the order fast modulation limit→vibrational progression limit→slow modulation limit, with the vibrational progression limit extending the limiting cases of the Kubo stochastic model of line shapes. Some simple qualitative rules have been extracted to predict the overall character of a line shape.
Vibrational frequency shift of HF in helium clusters: Quantum simulation and experiment
Authors: Blume Dörte , Lewerenz Marius , Huisken Friedrich , Kaloudis Michael
Published: July 2002

We report accurate variational and diffusion quantum Monte Carlo calculations for the size dependence of the vibrational frequency shift of HF molecules embedded in helium clusters with up to n=198 helium atoms. The frequency shift exhibits a strong initial size dependence and saturates at a redshift of about 2.7 ± 0.1 cm−1 for clusters with over 100 atoms. This value is in good agreement with our experimental redshift of 2.65 ± 0.15 cm−1 for clusters with over 1000 atoms. The helium cluster is found to undergo significant structural changes upon embedding of HF. The density in the nearest neighbor shell exceeds the bulk helium density by a factor of two. A second nearest neighbor density maximum and a peripheral density plateau very close to the bulk helium value is found. In spite of the anisotropic interaction between HF and helium all clusters have almost perfectly spherical helium density profiles and indicate close to free rotor behavior of HF inside the cluster. The cluster size dependence of the redshift can be qualitatively described by an induced dipole model.
Competitive photodissociation channels in jet-cooled HNCO: Thermochemistry and near-threshold predissociation
Authors: Zyrianov M. , Droz-Georget Th. , Sanov A. , Reisler H.
Published: July 2002

The photoinitiated unimolecular decomposition of jet-cooled HNCO has been studied following S1(1A″)←S0(1A′) excitation near the thresholds of the spin-allowed dissociation channels: (1) H(2S)+NCO(X2Π) and (2) NH(a1Δ)+CO(X1Σ+), which are separated by 4470 cm−1. Photofragment yield spectra of NCO(X2Π) and NH (a1Δ) were obtained in selected regions in the 260–220 nm photolysis range. The NCO(X2Π)yield rises abruptly at 38 380 cm−1 and the spectrum exhibits structures as narrow as 0.8 cm−1 near the threshold. The linewidths increase only slowly with photolysis energy. The jet-cooled absorption spectrum near the channel (1) threshold [D0(H+NCO)] was obtained using two-photon excitation via the S1 state, terminating in a fluorescent product. The absorption spectrum is similar to the NCO yield spectrum, and its intensity does not diminish noticeably above D0(H+NCO), indicating that dissociation near threshold is slow. The NCO product near threshold is cold, as is typical of a barrierless reaction. NH (a1Δ) products appear first at 42 840 cm−1, but their yield is initially very small, as evidenced also by the insignificant decrease in the NCO yield in the threshold region of channel (2). The NH (a1Δ) yield increases faster at higher photolysis energies and the linewidths increase as well. At the channel (2) threshold, the NH (a1Δ) product is generated only in the lowest rotational level, J=2, and rotational excitation increases with photolysis energy. We propose that in the range 260–230 nm, HNCO (S1) undergoes radiationless decay terminating in S0/T1 followed by unimolecular reaction. Decompositions via channels (1) and (2) proceed without significant exit channel barriers. At wavelengths shorter than 230 nm, the participation of an additional, direct pathway cannot be ruled out. The jet-cooled photofragment yield spectra allow the determination, with good accuracy, of thermochemical values relevant to HNCO decomposition. The following heats of formation are recommended: ΔH0f(HNCO)=−27.8±0.4 kcal/mol, and ΔH0f(NCO)=30.3±0.4 kcal/mol. These results are in excellent agreement with recent determinations using different experimental techniques.
Theoretical studies of the structure and dynamics of metal/hydrogen systems: Diffusion and path integral Monte Carlo investigations of nickel and palladium clusters
Authors: Chen B. , Gomez M. A. , Sehl M. , Doll J. D. , Freeman David L.
Published: July 2002

Using both classical and quantum mechanical Monte Carlo methods, a number of properties are investigated for a single hydrogen atom adsorbed on palladium and nickel clusters. In particular, the geometries, the preferred binding sites, site specific hydrogen normal mode frequencies, and finite temperature effects in clusters from two to ten metal atoms are examined. Our studies indicate that hydrogen is localized in the present systems. The preferred hydrogen binding sites are found to be tetrahedral in clusters with five or fewer metal atoms and octahedral for clusters of six to ten atoms. The exceptions to this rule are Ni9H and Pd9H for which the outside, threefold hollow and the inside tetrahedral sites are preferred, respectively. Hydrogen induced ‘‘reconstruction’’ of bare cluster geometries is seen in seven and ten-atom clusters.
Solvation dynamics study of 3-aminophthalimide in n-butanol solution at different temperatures
Authors: Laitinen Eira , Salonen Keijo , Harju Timo
Published: July 2002

We have studied the solvation dynamics of 3-aminophthalimide (3-AP) in n-butanol. The fluorescence lifetimes and the rotational correlation times of the molecule were measured at several wavelengths over the emission spectrum at six temperatures. Solvation time correlation functions were evaluated from wavelength dependent fluorescence lifetime distributions at four temperatures for 3-AP in n-butanol solution. The solvation dynamics of 3-AP was strongly temperature dependent. The solvation times of 3-AP were compared to the dielectric relaxation times (τL and τD) of n-butanol and to the solvation times of 4-amino-N-methyl-phthalimide (4-ANMP) studied earlier in n-butanol and n-decanol solutions. The solvation time of 3-AP molecule in n-butanol at room temperature was much closer to the longitudinal dielectric relaxation time than to the Debye time. At low temperatures the solvation times of 3-AP and 4-ANMP approach the Debye times of n-butanol. The solvation dynamics of 3-AP occurs on a time scale roughly given by the longitudinal relaxation time as predicted by the simple continuum theory.
Kinetics in a globally connected, correlated random energy model
Authors: Wang Jin , Saven Jeffery G. , Wolynes Peter G.
Published: July 2002

We study the dynamics on a biased globally connected energy landscape with correlations, using the generalized random energy model. Though the kinetic model has many applications, we discuss the model in the context of protein folding kinetics. Restricting the hierarchy to just two tiers, we find that the time scale of the overall folding coincides with that predicted by the random energy model at temperatures above the transition to partial freezing. At intermediate temperatures below the partial freezing transition, the search time is reduced due to the correlation effects. The dispersion of the relaxation time from sample to sample is also discussed.
Absence of quantum-mechanical effects on the mobility of argon ions in helium gas at 4.35 K
Authors: Viehland Larry A. , Hurly John J.
Published: July 2002

Quantum-mechanical transport cross sections are accurately computed from recent spectroscopic potentials for the three lowest energy levels of the molecular ion HeAr+. Statistical and ground state combinations of the cross sections are used to compute the transport coefficients describing the motion of Ar+ ions through He gas at 4.35 K. The calculated mobilities do not show the rapid decrease of the experimental values as the ratio of the electric field strength to the gas number density becomes very small. The experimental observations therefore are not due to the effects of orbiting resonance.
Comment on ‘‘Infrared spectrum and theoretical study of the dinitrogen pentoxide molecule (N2O5) in solid argon’’ [J. Chem. Phys. 104, 7836 (1996)]
Authors: Zhun Ilya , Zhou Xuefeng , Liu Ruifeng
Published: July 2002

Density functional theory using B3LYP functional predicts the structural and IR spectral features of N2O5 accurately. The disagreement between DFT and observed IR frequencies reported in a recent paper is due to incorrect vibrational assignment. Agreement between the calculated observed isotope shifts indicates that the major product of the reaction between NO2 and 18O3 is more likely NO2–O–NO18O instead of NO2–18O–NO2.
Reaction dynamics calculations for the CN+H2→HCN+H reaction: Applications of the rotating-bond approximation
Authors: Takayanagi Toshiyuki , Schatz George C.
Published: July 2002

We present quantum dynamics studies of the CN+H2→HCN+H reaction. An extended version of the rotating bond approximation (RBA), in which nonreactive CN stretch motion is taken into account in the dynamics explicitly, has been employed, and we have used a potential energy surface that was recently developed by ter Horst, Schatz, and Harding [J. Chem. Phys. 105, 558 (1996)]. This surface describes the HCN force field quite accurately and has significant coupling between CH and CN stretch vibrations in HCN. We find that neither CN vibration nor rotation affect the reaction cross section for the forward reaction significantly. This result is consistent with the fact that the forward reaction has an early barrier. On the other hand, for the reverse reaction HCN+H, HCN vibrational excitation significantly lowers the reactive threshold and enhances the cross section at energies above threshold. We find for the reverse reaction that all three modes of HCN (CH stretch, CN stretch and bend) reduce the threshold by an amount which equals the energy put into reagent vibration, but the enhancement in the cross section at a fixed energy above threshold is largest for energy put into the CH stretch. We also find that the HCN vibrational state distributions for the CN+H2 reaction show significant population in both CH and CN stretching normal modes. These results indicate that the nonreactive CN bond in the CN+H2 reaction, as well as its reverse, cannot be described totally by a simple spectator model, and that coupling between CH and CN stretch vibrations plays an important role in the dynamics.
Dynamics of collision-induced desorption: Ar–Xe/Pt(111)
Authors: Kulginov D. , Persson M. , Rettner C. T.
Published: July 2002

We have performed a detailed study of collision-induced desorption (CID) of Xe from Pt(111) by hyperthermal Ar atoms striking the surface. The study combines extensive molecular beam experiments with detailed molecular dynamics calculations in an effort to quantify fully and to understand in detail the dynamics of this model system. The calculations employ accurate semi-empirical potential energy functions for all relevant interatomic interactions. We have characterized the CID process with respect to incidence energy, Ei, angle, θi, and substrate surface temperature, Ts, for both the isolated adsorbate and the Xe/Pt(111) monolayer. The generally good agreement of the calculated results such as the desorption yield and desorbate energy and angular distributions with the experimental measurements demonstrates that the calculations capture the most important aspects of the CID processes.
Structure of inhomogeneous dipolar fluids: A density functional approach
Authors: Patra Chandra N. , Ghosh Swapan K.
Published: July 2002

A density functional approach is developed for inhomogeneous dipolar fluids consisting of dipolar hard spheres in presence of external electric fields. The theory is applied to two systems, viz. a confined fluid between two planar charged walls where the field is uniform, and also a fluid where the dipoles are subjected to a radial field due to a uniformly charged hard sphere ion at the centre. A nonperturbative weighted density approximation is employed to incorporate the effect of short range hard sphere-like correlations while the long-range effects are obtained perturbatively. The nonuniform density is expanded in terms of spherical harmonics and the correlation function used as input corresponds to the mean spherical approximation. In the case of planar geometry, the calculated density and polarization profiles are quite comparable with the available simulation and other results. In the presence of the radial field, the polarization exhibits oscillations showing a clear variation of the effective dielectric constant within the interface. This theory also gives an approximate estimate of the static solvation energy of an ion in a dipolar solvent.
Alteration of the lifetimes of autoionizing Rydberg states by a circularly polarized microwave field
Authors: Jones R. R. , Fu P. , Gallagher T. F.
Published: July 2002

Autoionizing Rydberg states in zero field exhibit lifetimes scaling as the cube of the principal quantum number, n3, but in a linearly polarized static or microwave field the lifetimes scale as n4 due to l mixing. To mimic the effects of collisions with charged particles we have examined the effect of a circularly polarized field on the decay rate and find longer lifetimes than in a linearly polarized field, but not an n5 dependence.
The thermal conductivity of defective crystals
Authors: Paolini G. V. , Lindan P. J. D. , Harding J. H.
Published: July 2002

We have calculated the thermal conductivity of pure KCl and KCl doped with RbCl using the Green–Kubo theory and classical molecular dynamics. Both shell-model and rigid-ion potentials are employed to describe the ionic interactions, and we find that the inclusion of ionic polarisability significantly changes the calculated conductivity at a given temperature. We find fairly good agreement between the simulation results and the available experimental data for both pure and doped systems. The shell model provided a slightly better description than the rigid-ion potential, although neither proved to be entirely satisfactory over the temperature range investigated (300–900 K). The relative merits of the potentials are discussed in the light of these results, as are the technical issues raised by the calculations. These studies demonstrate the capacity of the approach to yield quantitative predictions of the transport properties of ionic materials using shell-model potentials. This will be of use in treating systems for which rigid-ion models are inadequate.
Photodissociation dynamics in the UV laser photolysis of DNCO: Comparison with HNCO
Authors: Brownsword R. A. , Hillenkamp M. , Laurent T. , Vatsa R. K. , Volpp H.-R.
Published: July 2002

Using the laser photolysis/laser-induced fluorescence (LP/LIF) pump–probe technique, the gas phase photodissociation dynamics of room temperature DNCO were studied at photolysis wavelengths of 193 and 248 nm. D atoms produced via DNCO (X̃ 1A)+hν→D(2S)+NCO(X̃ 2Π) were detected by (2p 2P←1s 2S)-LIF using tunable narrow-band Lyman-α laser radiation (λLα≈121.5 nm) generated by resonant third-order sum-difference frequency conversion of pulsed dye laser radiation. By means of a photolytic calibration method absolute cross sections for the direct photochemical D atom formation were measured to be σD(193 nm)=(5.5±0.5)×10−20 cm2  molecule−1 and σD(248 nm)=(1.2±0.2)×10−21 cm2  molecule−1. For the wavelength of 193 nm, the measurement of the optical absorption cross section σDNCO(193 nm)=(2.6±0.2)×10−19 cm2  molecule−1 allowed the direct determination of the D atom product quantum yield of φD(193 nm)=(0.21±0.03). At the wavelength of 248 nm, where the optical absorption cross section was too small to be measured with reasonable accuracy, a kinetic calibration method was used to measure product quantum yields for D atom, φD(248 nm)=(0.77±0.10), and for “spin-forbidden” ND(X 3Σ−) formation, φND(248 nm)=(0.23±0.10). From the D atom Doppler profiles, measured under collision-free conditions, the fraction of the available energy released as product translational energy was determined to be fT(D+NCO)=(0.44±0.04) for 193 nm and fT(D+NCO)=(0.64±0.17) for 248 nm photolysis wavelengths. In addition, using polarized photodissociation laser light, measurements were carried out in order to determine the anisotropy of the D atom product angular distribution. It was found that the value of the anisotropy parameter β decreases from −(0.45±0.09) for a photolysis wavelength of 193 nm to −(0.01±0.10) at 248 nm, suggesting a moderately anisotropic distribution at the shorter wavelength and an effectively isotropic one at the longer wavelength. Comparisons are made with results from recent photodissociation dynamics studies of the isotopomer HNCO.
Density dependence of the collisional deactivation of highly vibrationally excited cycloheptatriene in compressed gases, supercritical fluids, and liquids
Authors: Benzler J. , Linkersdörfer S. , Luther K.
Published: July 2002

The collisional deactivation of highly vibrationally excited 1,3,5-cycloheptatriene (CHT) in compressed gases, supercritical fluids, and in liquids was studied. Different bath gases and solvents (alkanes, alcohols, CO2, CHF3, nitrogen) and pressures up to 200 bars were used. This way, measurements covered the complete gas–liquid transition regime. CHT with a vibrational energy of 41 000 cm−1 was generated by laser excitation into the S1 state and internal conversion to the S0 ground state. From picosecond UV absorption measurements, energy-loss profiles were obtained taking into account the density dependent absorption spectra, which were measured separately. A monoexponential decrease of the internal energy in CHT was observed under all studied conditions. In gases at pressures up to 40 bars, the relaxation rates increase linearly with the density and the energies transferred per collision, 〈ΔE〉, agree with those values obtained in earlier low pressure gas phase experiments. At higher densities, the relaxation rates deviate markedly from predictions based on simple scaling assumptions with appropriate collision frequencies. The deactivation in liquids is slower by a factor of 3–5 than expected from such isolated binary collision (IBC) models. These results are rationalized using a model that takes into account the finite lifetime of collision complexes.
Laser probing of velocity-subgroup dependent rotational alignment of N2+ drifted in He
Authors: Anthony Eric B. , Schade Wolfgang , Bastian Michael J. , Bierbaum Veronica M. , Leone Stephen R.
Published: July 2002

Results are presented for Doppler-resolved laser-induced fluorescence measurements of collision-induced rotational alignment of N2+(v″=0) drifted in He in a drift-tube apparatus. A single-frequency ring dye laser is used to probe the R1(N′′=15) rotational line of the B 2Σu+–Χ 2Σg+ system both parallel and perpendicular to the drift field at three different field strengths and at several different Doppler-selected velocities. A strong correlation is found between the degree of rotational alignment and the velocity subgroup probed along the field direction. For field strengths of 8 and 16 Td and laser probe parallel to the drift field, there is a monotonic increase in the quadrupole alignment parameter A0(2) with higher velocity subgroup, up to a maximum value of A0(2)=−0.150 (6) for 16 Td at the high-velocity tail. There is evidence that the correlation between alignment and velocity increases with increasing field strength. The mechanisms of the alignment are discussed and these results are attributed primarily to the change in anisotropy of the relative velocity vector distribution of the N2+–He pair with field strength.
Inhomogeneous and single molecule line broadening of terrylene in a series of crystalline n-alkanes
Authors: Vacha Martin , Liu Yi , Nakatsuka Hiroki , Tani Toshiro
Published: July 2002

We present a study of single molecule linewidth broadening of terrylene in dodecane, tetradecane, and hexadecane matrices. The Shpolskii bulk absorption spectra exhibit increase of the bandwidths and site complexity with increasing n-alkane chain length. Single molecule lines are broadened above the lifetime limit even at 1.6 K in all three solvents. The linewidth distributions suggest the existence of relaxing two-level systems coupled to the molecular transitions. Spectral diffusion was observed in the forms of spontaneous and photoinduced frequency jumps. Temperature dependencies of the single molecule linewidths show a wide range of power-law dependencies below 3 K and, in some cases, exponentially activated behavior above 3 K. Possible origins of the line broadening are discussed.
Long range moves for high density polymer simulations
Authors: Deutsch J. M.
Published: July 2002

Monte Carlo simulations of proteins are hindered by the system’s high density which often makes local moves ineffective. Here we devise and test a set of long range moves that work well even when all sites in a lattice simulation are filled. We demonstrate that for a 27-mer cube, the ground state of random heteropolymers can quickly be reached. We discuss results for 48-mer systems where the ground state is known exactly. For ten sequences that were examined, the introduction of long range moves speeds up the search for the ground state by about one order of magnitude. The method is compared to a fast folding chain growth algorithm that had previously been used with much success. The new algorithm here appears to be more efficient. The point is illustrated by the folding of an 80-mer four-helix bundle considered previously.
Matrix isolation spectroscopy of laser ablated carbon species in Ne, D2, and H2 matrices
Authors: Tam Simon , Macler Michel , Fajardo Mario E.
Published: July 2002

The results of matrix isolation experiments on carbon atoms and clusters generated by pulsed laser ablation of graphite are presented. C3 and other Cn clusters are observed by their infrared absorptions in Ne, ortho-D2, and para-H2 matrices. CH4 molecules and CH3 radicals are observed in H2 matrices, as are CD4 molecules in D2 matrices. The CH4:C3 concentration ratio in the H2 matrices is seen to increase dramatically with increasing ablation laser intensity. These results confirm and illuminate previous work by Miki, Wakabayashi, Momose, and Shida [J. Phys. Chem. 100, 12135 (1996)] on Cn doped cryogenic para-H2 samples produced by combined laser ablation and gas condensation in an enclosed cell. Their nonobservation of any hydrocarbon reaction products (particularly CH4) is explained by the lower intrinsic absorption strengths of the CH4 vs C3 vibrational transitions, and by attributing a low C1:C3 production ratio to the ablation process under their laser intensity and wavelength conditions.
Controlled surface photochemistry: Bond- and isotope-selective photodesorption of neutrals by adsorbate vibrational preparation with infrared laser pulses
Authors: Saalfrank Peter , Paramonov Guennaddi K.
Published: July 2002

The possibility of controlling surface photochemistry by the selective vibrational preparation of adsorbates with infrared (ir) laser pulses is investigated theoretically. In particular, the selective ir plus ultraviolet (uv) light-induced desorption of different isotopomeric neutral adsorbates from metal surfaces is studied with the help of nuclear density matrix theory. As a concrete example the system NH3/ND3/Cu(111) is chosen. In a first step of the “vibrationally mediated chemistry” advocated here, based on computed two-mode dipole functions and model potentials, optimal infrared laser pulses are designed to selectively excite the umbrella mode ν2 of either adsorbed NH3 or ND3. In a second step, an uv/visible photon enforces an electronic transition, leading, after ultrafast quenching, to desorption induced by electronic transitions (DIET). It is argued that despite strong dissipation, the proper vibrational preparation not only increases desorption yields substantially, but also allows for an almost complete separation of both isotopomers.
Spectroscopic characterization of the metastable 4pπ 2Π− valence states and the 5s 3Σ+ Rydberg states of the CaAr, CaKr, and CaXe van der Waals molecules
Authors: Kaup John G. , Breckenridge W. H.
Published: July 2002

The metastable Ca(4s4p 3PJ)⋅RG(3Π0−) states and the lowest-energy Ca(4s5s 3S1)⋅RG(3Σ+) Rydberg states (RG=Ar, Kr, Xe) have been characterized by means of resonance-enhanced two-photon ionization spectroscopy (R2PI). All of the Ca⋅RG states have smaller bond energies D0 and greater bond lengths R0 than their Mg⋅RG analogs. This is rationalized as being due to repulsive forces setting in at greater internuclear distances R for the larger Ca atom. Similar to the analogous MgRG states studied earlier, the CaRG(4s5s 3Σ+) states are almost as strongly bound as the CaRG+ “core” ions, indicating quite efficient penetration of the diffuse Ca(5s) Rydberg orbital by the RG atoms. The vibrational frequencies ωe are actually higher for all the less strongly bound CaRG(3s4s 3Σ+) states than for their analogous CaRG+ ions, consistent with “narrower” potential curves due to possible small maxima in the Rydberg state potentials at large R because of the difficulty of the RG atoms penetrating the outer lobe of the Ca(4s) orbital.
Prediction of re-entrant swelling behavior of poly(N-isopropyl acrylamide) gel in a mixture of ethanol–water using lattice fluid hydrogen bond theory
Authors: Lele A. K. , Karode S. K. , Badiger M. V. , Mashelkar R. A.
Published: July 2002

The re-entrant volume phase transition of poly(N-isopropyl acrylamide) gel in ethanol–water mixtures has been predicted by using the extended lattice-fluid-hydrogen bond (LFHB) theory. In our calculations we do not make any arbitrary assumptions for the polymer–solvent interaction parameters. Instead, we determine the interaction parameters by fitting the LFHB theory to the swelling data of the gel in each of the solvents. In addition to predicting the re-entrant transition, the theory predicts selective absorption of ethanol over water, particularly by the collapsed gel. Simultaneously, the hydrogen bonding between water and ethanol is predicted to be enhanced in the presence of the gel. The interpolymer hydrogen bonds increase during the gel collapse region. The polymer–ethanol hydrogen bonds increase and the polymer–water hydrogen bonds decrease continuously with increasing ethanol composition in the outside phase. These predictions are in qualitative agreement with experimental observations and overcome the empiricism in previous theoretical work. A variety of qualitatively different swelling behaviors of gels in mixed solvents is also predicted for varying hydrophilic–hydrophobic balance in the chemical structure of the gels.
Deuterium quadrupole coupling tensors in methyl halides: Ab initio effective core potential and liquid crystal nuclear magnetic resonance study
Authors: Vaara Juha , Hiltunen Yrjö
Published: July 2002

The deuterium quadrupole coupling tensors were calculated for the methyl halides CD3X (X=F, Cl, Br, and I) using ab initio effective core potential (ECP) methods up to the QCISD level. The effects of basis set quality and electron correlation were investigated, and a comparison with all-electron calculations was made. We also determined a new experimental value for methyl chloride by the liquid crystal nuclear magnetic resonance (LC NMR) method. Locally dense basis sets were used for the deuterium atoms and ECPs for the halogens. The Br and I ECPs were scalar relativistic. Computations at the equilibrium (re) geometry result in slightly too negative tensor elements along the molecular C3 symmetry axis for all X, as compared to the experimental LC NMR and microwave spectroscopic results. Values corresponding to the rα(300 K) geometry are in a very good agreement with experiment. As the tensors are sensitive to the length of the associated CD bond, we applied approximate vibrational corrections to the theoretical results. The current corrected values for the deuterium quadrupole coupling constants and asymmetry parameters are likely to be the most reliable available for the present set of molecules, despite that residual computational errors still remain. The common approximation of a cylindrically symmetric electric field gradient tensor at the deuterium site is shown to lead to an underestimation of the corresponding quadrupole coupling constant in methyl halides.
High resolution electronic spectroscopy of the R⋅SH complexes (R= Ne, Ar, Kr)
Authors: Carter Christopher C. , Miller Terry A.
Published: July 2002

The high resolution laser induced fluorescence spectra of the Ã 2Π3/2 electronic transition of the R⋅SH/D (R= Kr, Ar, Ne) van der Waals (vdW) complexes are reported. Analysis of these bands requires the inclusion of rotation, fine and hyperfine structure, spin-rotation interactions, and parity splittings. A number of molecular parameters are determined, along with internuclear bond distances between the R and the SH moiety. Comparison of the present results for R⋅SH/D is made with the analogous R⋅OH/D species where applicable. In addition, the detailed “rotational” structure and the highly precise determinations of the band origins of the different heavy atom isotopomers are critical for absolute vibrational quantum number assignment in the Ã state.
Effect of potential of mean force on short-time dynamics of a diffusion-controlled reaction in a hard-sphere fluid
Authors: Ibuki Kazuyasu , Ueno Masakatsu
Published: July 2002

We calculated the time-dependent rate constant of a diffusion-controlled reaction between hard-spheres in a hard-sphere fluid at short times starting from the Fokker–Planck–Kramers equation combined with the approximation of half-range Maxwellian velocity distributions. For the potential function, we employed the potential of mean force (PMF) obtained from the equilibrium radial distribution function. The rate constant at short times was much larger than that neglecting the PMF effect, though the steady state rate constant did not sensitively depend on the PMF effect. This indicates that the effect of the initial distribution of the reactants is important in determining the rate constant at short times. The results were compared with a computer simulation. The dependences of the survival probability of a target on the time, the transmission coefficient, and the reactant concentration were examined, and satisfactory agreements between the calculation and the simulation were obtained at a relatively low density. At a high density, the non-Markovian effect should be taken into account to explain the simulation result.
The dynamics of proton transfer in H5O2+
Authors: Cheng Hai-Ping , Krause Jeffrey L.
Published: July 2002

We perform high-level, quantum molecular dynamics simulations of proton transfer in the protonated water dimer, H5O2+. The electronic structure of the system is calculated concurrently with the nuclear motion using Born–Oppenheimer molecular dynamics plus density functional theory. Performing the calculations at finite (thermal) temperatures allows us to observe and quantify such effects as the broadening of the electronic density of states, the thermal splitting of degenerate states, the shift of the highest occupied molecular orbital, the thermal expansion of the dipole moment, and the thermal shift, coupling and broadening of the vibrational density of states. At two of the temperatures considered (225 K and 360 K), we find that H5O2+ exists in a dynamical equilibrium state in which the proton oscillates between two water molecules. The characteristic frequencies of the proton motion are very sensitive to temperature. At 40 K and 225 K, strong peaks are identified in the vibrational spectrum corresponding to the motion of the proton between the two oxygen atoms. At 360 K, the frequencies of this motion are distributed among a series of peaks between 1100 and 1800 cm−1. At all temperatures investigated, the proton motion is coupled strongly to other degrees of freedom in the water molecules and the dimer. Statistically, the proton is localized near one of the two H2O molecules to form H3O+ with a probability of 0.09 at 225 K, and 0.18 at 360 K. At low temperature (40 K), the proton remains localized near the midpoint of the two oxygens, and has almost zero probability to exist as H3O+.