bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical–optical double resonance polarization spectroscopy of the B 1Π state of 39K85Rb
Authors: Okada Naoko , Kasahara Shunji , Ebi Tsuyoshi , Baba Masaaki , Katô Hajime
Published: July 2002

High resolution spectra of the B 1Π←X 1Σ+ transition of the 39K85Rb molecule have been measured with the technique of Doppler-free optical–optical double resonance polarization spectroscopy. Molecular constants of the B 1Π(v′=0−12) levels are determined, and the RKR potential is calculated. A number of irregularities in the energies of B 1Π state levels and intensity anomalies are observed. The energy shifts of the e and f levels of the B 1Π state are studied, and the perturbations between the B 1Π and 3Σ+ states and between the B 1Π and 3Π1 states are identified. Several transitions to the 3Π1 and 3Σ+ states are also observed, and the molecular constants of the 3Π1 and 3Σ+ states are evaluated. The single rovibrational level B 1Π(v′=2, J′=41) is selectively excited by a single frequency laser chopped by an electro-optical modulator, and the decay time of the resulting fluorescence is measured at various pressures. From the Stern–Volmer plot, the radiative lifetime and the collisional cross section of the fluorescence quenching are determined to be 11.6 ns and 29 Å2, respectively. The intensity distribution of the dispersed fluorescence of the B 1Π(v′=2, J′=41)→X 1Σ+(v″=0−11, J″=42) transitions is in very good agreement with the calculated values of ν4|〈v′=2|v″〉|2. The magnitude of the transition dipole moment is determined to be 9.2 D in the range of 4.08 Å<R<4.73 Å. These results are compared with the results of recent ab initio calculations.
Fully complex implementation of the Robert–Bonamy formalism: Half widths and line shifts of H2O broadened by N2
Authors: Lynch R. , Gamache R. R. , Neshyba S. P.
Published: July 2002

The complex semiclassical formalism of Robert and Bonamy is used to calculate both half widths and line shifts for water vapor in a bath of nitrogen. The assumed intermolecular potential is a combination of electrostatic, Lennard-Jones 6-12 atom–atom, induction, and dispersion terms. The complex valued resonance functions have been previously evaluated when the assumed potential was electrostatic only. In this work these functions are evaluated when the potential is extended to include the atom–atom terms. Calculations made in the 3ν1+ν3 vibrational band of H2O are in good agreement with experimental results for both the half width and line shifts. It is shown that the imaginary parts are important for both the line shift and half width calculations.
Absolute state-selected total cross sections for the ion–molecule reactions O+(4S,2D,2P)+H2(D2)
Authors: Li X. , Huang Y.-L. , Flesch G. D. , Ng C. Y.
Published: July 2002

Absolute total cross sections for the state-selected reactions of O+(4S,2D,2P)+H2 (D2) have been measured in the center-of-mass collision energy (Ec.m.) range of 0.02–12 eV. The cross sections for OH+ (OD+) from O+(2D)+H2 (D2) are slightly higher than those from O+(4S)+H2 (D2), whereas the OH+ (OD+) cross sections from O+ (2P)+H2 (D2) are ≈40% lower than those from O+(4S)+H2 (D2) and O+ (2D)+H2 (D2). At Ec.m.<0.5 eV, the total cross sections for OH+ (OD+) from O+ (4S)+H2 (D2) and O+(2D)+H2 (D2) are in accord with those predicted by the Langevin–Gioumousis–Stevenson model. Significantly higher cross sections are observed for H+ (D+) and H2+ (D2+) from O+(2D)+H2 (D2) and O+(2P)+H2 (D2), as compared to those from O+(4S)+H2 (D2). The exothermic nature of the O+(2D,2P)+H2 (D2) charge transfer collisions accounts for the high cross sections observed for H2+ (D2+). While the H+ (D+) ions observed in the O+(4S)+H2 (D2) reaction are identified with the H+ (D+)+O+H channel, the H+ (D+) ions from the reactions involving O+(2D) and O+(2P) are associated mostly with the H+ (D+)+OH (OD) channel, the formation of which obeys the spin-conservation rule. The comparison of the sum (σT) of cross sections for OH+ (OD+), H2+ (D2+), and H+ (D+) from O+(4S)+H2 (D2) to those from O+(2D)+H2 (D2) and O+(2P)+H2 (D2) shows that the σTs for O+(4S)+H2 (D2), O+(2D)+H2 (D2), and O+(2P)+H2 (D2) at Ec.m.<0.5 eV are comparable. At Ec.m.>0.5 eV, the σTs for O+(2P)+H2 (D2) are greater than those for O+(2D)+H2 (D2), which in turn are greater than those for O+(4S)+H2 (D2). This observation is attributed to the increase in the number of accessible product channels for reactions involving the excited O+(2D) and O+(2P) reactant ions.
Crossed-beam reaction of carbon atoms with hydrocarbon molecules. IV. Chemical dynamics of methylpropargyl radical formation, C4H5, from reaction of C(3Pj) with propylene, C3H6 (X 1A)
Authors: Kaiser R. I. , Stranges D. , Bevsek H. M. , Lee Y. T. , Suits A. G.
Published: July 2002

The reaction between ground state carbon atoms and propylene, C3H6, was studied at average collision energies of 23.3 and 45.0 kJ mol−1 using the crossed molecular beam technique. Product angular distributions and time-of-flight spectra of C4H5 at m/e=53 were recorded. Forward-convolution fitting of the data yields a maximum energy release as well as angular distributions consistent with the formation of methylpropargyl radicals. Reaction dynamics inferred from the experimental results suggest that the reaction proceeds on the lowest A3 surface via an initial addition of the carbon atom to the π-orbital to form a triplet methylcyclopropylidene collision complex followed by ring opening to triplet 1,2-butadiene. Within 0.3–0.6 ps, 1,2-butadiene decomposes through carbon–hydrogen bond rupture to atomic hydrogen and methylpropargyl radicals. The explicit identification of C4H5 under single collision conditions represents a further example of a carbon–hydrogen exchange in reactions of ground state carbon with unsaturated hydrocarbons. This versatile machine represents an alternative pathway to build up unsaturated hydrocarbon chains in combustion processes, chemical vapor deposition, and in the interstellar medium.
Fluorescence excitation and depletion spectroscopy of the BAr complex: Electronic states correlating with the excited valence B(2s2p2 2D) asymptote
Authors: Yang Xin , Dagdigian Paul J.
Published: July 2002

Laser fluorescence excitation and depletion spectra of the BAr van der Waals complex, in the spectral region around the B atomic 2s2p2 2D←2s22p 2P transition at 208.9 nm, are reported. The fluorescence excitation spectrum displays a series of structured bands, whose excited levels are weakly bound (⩽60 cm−1), and an unstructured continuum to the blue. The onset of the latter allows determination of the ground state dissociation energy D0″=102.4±0.3 cm−1. The fluorescence depletion spectrum reveals, in addition, bands at lower wave numbers to a strongly bound, predissociating state, assigned as C2Δ. The bands observed by fluorescence excitation are assigned to the D2Π, E2Σ+–X2Π transitions. The derived spectroscopic data are used to construct potential energy curves for the C2Δ and D2Π states. The predissociation of the C2Δ state is due to spin–orbit coupling to a repulsive 4Π state correlating with the B(2s2p2 4P)+Ar asymptote, as shown in the accompanying paper [K. Sohlberg and D. R. Yarkony, J. Chem. Phys. 106, 6607 (1997)]. A weak nonradiative decay process is also observed for the D2Π state.
Photoionization efficiency spectrum and ionization energy of HSO studied by discharge flow-photoionization mass spectrometry
Authors: Cheng Bing-Ming , Eberhard Jürg , Chen Wei-Chen , Yu Chin-hui
Published: July 2002

The photoionization efficiency (PIE) spectrum of HSO was measured in the spectral range (107–130) nm by means of a discharge flow and a photoionization mass spectrometer coupled to a synchrotron as the radiation source. HSO radicals were generated by reacting O atoms with various organothiol compounds, C2H5SH, 2-C3H7SH, or HSC2H4SH, in the flow tube. The ionization energy of HSO was determined for the first time and found to be (9.918±0.016) eV. GAUSSIAN-2 calculations predict 9.897 eV for ionization to HSO+, the singlet ground state of the molecular ion, in satisfactory agreement with the experimental result. The onset to triplet HSO+ may occur at (11.15±0.04) eV. A vibrational frequency of HSO+ of (1150±160) cm−1 was derived from the separation of steps in the PIE spectrum. The heat of formation of HSO+ was also derived and calculated to be ΔfH298∘(HSO+)=(228±5) kcal mol−1.
Shear behavior of squalane and tetracosane under extreme confinement. II. Confined film structure
Authors: Gupta S. A. , Cochran H. D. , Cummings P. T.
Published: July 2002

This paper focuses on the structural characteristics of confined squalane and tetracosane under shear flow conditions. Nonequilibrium molecular dynamics simulation is used to explore the rheology of these model lubricants. A preceding paper describes the molecular model and the simulation method, and examines interfacial slip. The lubricants are confined between model walls that have short chains tethered to them, thus screening the wall details. In this paper we examine the density profiles and chain conformations of the alkanes under shear flow conditions. Our results indicate a profound influence of the walls on the fluid structure. In particular, when the wall spacing is close to an integral multiple of the molecular diameter, tetracosane shows the formation of distinct layers with the molecules being in a fully extended state. This behavior is not observed for squalane. Under shear flow conditions the molecules tend to orient parallel to the walls, as would be expected, with a greater degree of orientation (a) close to the walls, (b) at the positions of local density maxima, and (c) at higher strain rates.
Vibrational self-consistent field method for many-mode systems: A new approach and application to the vibrations of CO adsorbed on Cu(100)
Authors: Carter Stuart , Culik Susan J. , Bowman Joel M.
Published: July 2002

We report calculations of the vibrational energies of CO–Cu(100) using a new code to perform vibrational self-consistent field (VSCF) and state-mixing calculations for many-mode systems. The major new feature of the code is the representation of the potential. Unlike recent implementations of the VSCF method, the potential is not expanded in terms of normal coordinates as a multinomial series about a minimum. The full potential, in normal coordinates, is used in the Watson Hamiltonian. This approach, while rigorous, can lead to prohibitively large numerical quadratures, and so we suggest a novel representation of the potential as an expansion in all two-mode, or all three-mode, or all four-mode coupling terms. The new code is tested against previous exact calculations of vibrational states of HCO, and also against previous VSCF calculations that used a fourth-order, normal coordinate force field representation of the global HCO potential. The new code is applied to calculations of the vibrations of CO adsorbed to Cu(100). We explicitly treat nine modes corresponding to the motion of the C and O atoms and the Cu atom that is bonded to C. The potential used is a semi-empirical one developed by Tully and co-workers [J. C. Tully, M. Gomez, and M. Head-Gordon, J. Vac. Sci. Technol. A 11, 1914 (1993)], and is used fully, i.e., without recourse to multinomial expansion in displacement coordinates. We test the convergence of the results with respect to the number of modes coupled and find that the errors in the two-mode coupling representation vary from 0.6 to 6 cm−1 for the fundamentals but grow to 30 cm−1 for overtone and combination states. The errors in the three-mode representation of the potential are less than 0.2 cm−1 for the fundamentals and no larger than 2.5 cm−1 for high overtone/combination states with as much as 9 quanta of excitation. We calculate the thermally broadened spectra of the CO-stretch fundamental, the CO–Cu frustrated rotation and the CO–Cu frustrated translation over the temperature range 50–350 K. We compare the temperature dependence of the average frequency and standard deviation of these modes with experiment, and find semiquantitative agreement.
Calculating the hopping rate for self-diffusion on rough potential energy surfaces: Cage correlations
Authors: Rabani Eran , Gezelter J. Daniel , Berne B. J.
Published: July 2002

We present a method for estimating the hopping rate for Zwanzig’s model of self-diffusion in liquids [R. Zwanzig, J. Chem. Phys. 79, 4507 (1983)]. To obtain this estimate, we introduce the cage correlation function which measures the rate of change of atomic surroundings, and associate the long-time decay of this function with the basin hopping rate for diffusion. Results from a set of simulations on Lennard-Jones particles are presented. A simple analytic model for the diffusion constant in supercooled and normal liquids that is based on estimates of the activation energy obtained via the cage correlation function is derived. We discuss the breakdown of Zwanzig’s hopping mechanism for mass transport as well as the low temperature behavior of the self-diffusion constant on rough potential energy surfaces.
Effect of length and geometry on the highest occupied molecular orbital-lowest unoccupied molecular orbital gap of conjugated oligomers: An analytical Hückel model approach
Authors: Onipko Alexander , Klymenko Yuriy , Malysheva Lyuba
Published: July 2002

It is shown that the asymptotic behavior of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap of conjugated oligomers of types M−(M)N−2−M and M− (M)N−2−M1 with M = M1−M2, where M, M1, and M2 are alternant but otherwise arbitrary monomers described by the Hückel Hamiltonian, is ruled by the law ΔHL(N)=ΔHL(∞)+const⋅N−2. On this basis we suggest an approximate expression for the HOMO-LUMO gap as a function of oligomer length, that is exact for minimal- and infinite-length oligomers. Two parameters of this function determine the dependence of ΔHL(N) on the oligomer geometry. By comparing the proposed approximation with the exact model results for oligomers of polyene, polyparaphenylene (PPP), and polyparaphenylenevinylene (PPV) (some experimental data and results of more elaborate calculations have been also used for this purpose) the proposed approximation is proven to give a useful estimate of the conjugation length and geometry effect on the HOMO-LUMO gap of the molecules under consideration. Applying our approach to PPP and PPV oligomers, we rederive the geometry effects on the PPP band gap reported previously (however, an important point is taking end effects into account) and predict that the HOMO-LUMO gap of PPV decreases with the increase of the quinoid character of the backbone geometry much more strongly, as compared with PPP. The band gap closing in the infinite chain limit as well as the problem of the existence of discrete in-gap states were also examined, and this analysis has resulted in the formulation of general conditions of the occurrence of the above mentioned situations. Applied to the polymers (infinite oligomers), these conditions allow one to decide whether the gap closing or the existence of in-gap states is possible under the given π electronic structure of monomer. Since the conditions obtained are expressed in terms of the monomer Green function only, they provide a simple and efficient tool with which to search for new polymer materials with the band gaps desired.
Extended cross correlation: A technique for spectroscopic pattern recognition
Authors: Jacobson Matthew P. , Coy Stephen L. , Field Robert W.
Published: July 2002

Recent improvements in spectrum excitation, recording, and processing capabilities have led to enormous enhancement in the quality and quantity of spectroscopic data sets. We describe here a pattern recognition technique, extended cross correlation (XCC), that is well suited to take advantage of large, high quality data sets. In particular, spectra are used to decode each other without any knowledge of or assumptions about the patterns that are sought. This paper describes the motivation for and construction of the XCC, and illustrates one of its simplest applications: To identify, in spectra of mixtures of chemical species, which peaks correspond to which chemical species. This application of the XCC is illustrated with both synthetic data and experimental data on mixtures of ammonia isotopic species.
Decay of single and double quantum coherences in deuterated glassy ortho-terphenyl
Authors: Kircher Oliver , Diezemann Gregor , Böhmer Roland , Hinze Gerald , Schug Kai Uwe , Sillescu Hans , Zimmermann Herbert
Published: July 2002

The decay of single and double quantum coherences in amorphous ortho-terphenyl is studied by means of deuteron NMR echo experiments near and below the calorimetric glass transition temperature Tg. In perdeuterated samples a Gaussian decay for these coherences is found at low temperatures, in accord with theoretical predictions for random local dipolar fields. A method is proposed to extract information about slow motional processes from ordinary solid echo experiments below Tg. Partly deuterated ortho-terphenyl does not show a Gaussian decay. Simple model calculations show that this observation indicates the importance of correlations of the local dipolar fields in the presence of protons.
Observation of acoustic quantum beats in nanometer sized Au particles
Authors: Hodak José H. , Martini Ignacio , Hartland Gregory V.
Published: July 2002

Quantum beats due to coherently prepared acoustic vibrational modes have been observed for 14 to 17 nm diameter Au particles. The beat frequency is inversely proportional to the particle size. The dephasing time is ca. 15 ps, which is limited by the broad size distribution (±2 nm) of the samples.
Model of laser-driven mass transport in thin films of dye-functionalized polymers
Authors: Barrett Christopher J. , Rochon Paul L. , Natansohn Almeria L.
Published: July 2002

Based on Newtonian fluid dynamic relations, a model is constructed to describe laser-induced mass transport in thin films of polymers containing isomerizable azobenzene chromophores, in which surface profile diffraction gratings can be inscribed with an interference pattern of coherent light. The Navier–Stokes equations for laminar flow of a viscous fluid are developed to relate velocity components in the film to pressure gradients in the polymer film, by definition of boundary layer conditions. This general laminar flow model is applicable to the formation of surface gratings through a variety of mechanisms. Considering the mechanism of an isomerization-driven free volume expansion to produce internal pressure gradients, a specific model is developed to describe polymer flow resulting from laser-induced isomerization of the bulky chromophores. This yields an expression relating the time evolution of the surface gratings to properties which could be varied experimentally, such as those of the irradiating light, inscription geometry, and bulk polymer, which incorporates no arbitrary fitting parameters or integration constants. In the general model, the rate of grating inscription is predicted to vary directly with the intensity of the inscription laser, to vary inversely with the molecular weight of the polymer below the limit of entanglement, and to scale with the third power of the initial thickness of the film. Considering an isomerization pressure mechanism, the model predicts the rate of grating inscription to further vary with the free volume requirements of the induced geometric transformation of the dye molecules, and the polarization state of the inscription laser. Predictions from the model were tested against the results of experiments to vary these parameters, and are shown to be in good agreement.
Dissociation dynamics of acetylene Rydberg states as a function of excited state lifetime
Authors: Löffler P. , Wrede E. , Schnieder L. , Halpern J. B. , Jackson W. M. , Welge K. H.
Published: July 2002

The state selective photodissociation of acetylene, C2H2/C2D2, was studied in the wavelength range 121.2–132.2 nm by high resolution Rydberg atom time-of-flight measurements on the atomic fragment, H/D. In the wavelength region studied members of all four Rydberg series and the highly excited Ẽ valence state were state selectively excited using tunable vacuum-ultraviolet laser radiation. The lifetime of the excited states which were studied varied from 58 fs to more than 2 ps. Formation of the ethynyl radical in its X̃ electronic ground state and its first electronically excited Ã state is observed with practically no indication of B̃ state fragments. Two decay channels with different dissociation dynamics were also observed. In both channels the observed decay dynamics depended strongly on the excited state of the parent molecule. Further there are major differences between these two dissociation pathways with respect to the measured internal energy and angular distributions. In one channel the dissociation is dominated by dynamical effects and the C2H fragments are formed with a high degree of vibrational excitation. In contrast to this in the second channel a smooth internal energy distribution is observed indicating that the fragment quantum state distribution is spread over a considerable range of the available phase space. Moreover, this second channel can be fit with a phase space model constrained only by conservation of energy and angular momentum. This is further evidence for the randomization of internal energy during the dissociation process.
Application of spin-restricted open-shell Kohn–Sham method to atomic and molecular multiplet states
Authors: Filatov Michael , Shaik Sason
Published: July 2002

A recently proposed spin-restricted open-shell Kohn–Sham (ROKS) method is applied to investigate various atomic and molecular multiplet states. A wide range of multiplets is considered: multiplet terms for which the spin-restricted open-shell theory of Roothaan applies, as well as state situations which cannot be described by Roothaan’s theory (e.g., states of square cyclobutadiene, etc.). Problems associated with the use of approximate density functionals and possible perspectives of the ROKS method are discussed.
Evidence for a thermal contribution to emission intermittency in single CdSe/CdS core/shell nanocrystals
Authors: Banin U. , Bruchez M. , Alivisatos A. P. , Ha T. , Weiss S. , Chemla D. S.
Published: July 2002

The on–off intermittent behavior of emission from single CdSe/CdS core/shell nanocrystals was investigated as a function of temperature and excitation intensity. Off times were found to be independent of excitation power and the temperature dependence reveals substantial reduction in the number of on–off cycles prior to final particle darkening at low temperatures. On times are found to vary linearly with excitation intensity over a broad range and the turn off rate shows activated Arrhenius behavior down to T=50 K. These observations are consistent with a darkening mechanism that is a combination of Auger photoionization and thermal trapping of charge. The inhomogeneity of various possible trap sites is discussed. A thermally activated neutralization process is required for the particle to return to the on state. The influence of shell composition on intermittency is compared for CdS and ZnS [M. Nirmal et al., Nature 383, 802 (1996)].
Quantum mechanical transition state theory and tunneling corrections
Authors: Thompson Ward H.
Published: July 2002

An efficient implementation of the quantum mechanical transition state theory recently proposed by Hansen and Andersen [J. Chem. Phys. 101, 6032 (1994); J. Phys. Chem. 100, 1137 (1996)] is presented. Their method approximates the flux–flux autocorrelation function by using short-time information to fit an assumed functional form (with physically correct properties). The approach described here exploits the low rank of the half-Boltzmannized flux operator, thereby facilitating application to reactions involving many degrees of freedom. In addition, we show how the quantum transition state theory can be used to obtain tunneling corrections within the framework of more traditional transition state theory approaches, i.e., those making an assumption of separability. Directions for possible improvements of the theory are discussed.
Benchmark enthalpies of formation and binding energies of proton-bound pairs between HCN and HCN, NH3, H2O, and HF
Authors: Mayer Paul M.
Published: July 2002

The relative thermochemical properties of cluster ions (solvation enthalpies, entropies, and free energies) can be obtained from experimental techniques such as high pressure mass spectrometry and selected-ion flow tube mass spectrometry. Theory can play an important role in these studies by providing both accurate binding energies of the smaller members of the cluster families and insight into the structure and bonding in the cluster ions. This study assesses the performance of a variety of levels of ab initio and density functional theories for predicting the structures and energies of one family of cluster ions, the proton-bound dimers between HCN and HCN, NH3, H2O, and HF. The theoretical procedures were assessed based on their performance relative to high-level treatments such as QCISD(T) correlation, the 6-311+G(2df,p) basis set, and G2 energy calculations. The results of the assessment indicate that MP2/6-31G(d) optimized geometries are sufficient for the calculation of binding energies and heats of formation with advanced methods such as G2. Further increases in basis set size and electron correlation improve the geometries of the dimers, but these geometric changes have little impact on the final high-level energy calculations. The heats of formation and binding energies of the clusters are best described by G2 theory, but modified versions of G2 such as G2(MP2) and G2(MP2,SVP) also provide reliable values. Calculated binding energies of these four proton-bound dimers are compared to available experimental values from the literature, and the effect of basis set superposition error is examined
Effect of molecular shape and electrostatic interactions on the water layer around polar and apolar groups in solution
Authors: Sidhu K. S. , Goodfellow J. M. , Turner J. Z.
Published: July 2002

We have carried out neutron diffraction using hydrogen/deuterium isotope substitution and molecular dynamics simulations in order to investigate intermolecular water structure in aqueous solutions of phenol, ethanol, propanoic acid, and glucose. Partial radial distribution functions (rdfs) calculated from both experiment and simulation are compared. These show good overall agreement although the simulation gives more structure than the experimental data in the intramolecular region. From the simulation, the relative number of water molecules coordinated to different chemical groups are calculated and the three-dimensional water density around the different chemical groups on the solute molecule is obtained. For both the experimental and simulated rdfs the intermolecular region is relatively featureless but the simulated water structure shows significant differences between the water coordination around polar and apolar groups. The water is strongly coordinated to the polar groups (OH and COO−), although the hydroxyl groups have fewer close waters than would be possible if all hydrogen bonds were satisfied. There is little evidence for stable water networks around apolar groups. There is also some evidence that the polarized regions of the solutes influence the water coordination around neighboring apolar groups. Apart from the relatively strong polar interactions, the overall shape of the solute molecule is the most important determinant of the shape of the hydration shell. In the case of glucose, the OH groups take part in a well-defined hydrogen-bonded network around the glucose molecule which incorporates the solute molecule in a similar way to water molecules.
Coupled cluster response theory for solvated molecules in equilibrium and nonequilibrium solvation
Authors: Christiansen Ove , Mikkelsen Kurt V.
Published: July 2002

It is shown how response functions can be derived as derivatives of a time-averaged quasienergy for molecules surrounded by a polarizable dielectric continuum. The vacuum quasienergy is modified with additional solvent terms which depend on the chosen solvent configuration. We consider equilibrium and nonequilibrium solvent models. Coupled cluster response functions are derived by introducing a corresponding coupled cluster quasienergy variational Lagrangian. Computational tractable expressions are derived for linear, quadratic, and cubic response functions. Sample calculations are reported for solvation effects on excitation energies, oscillator strengths, dipole moments, and frequency-dependent polarizabilities of H2S and furan.
Oscillatory dynamics in low-dimensional supports: A lattice Lotka–Volterra model
Authors: Provata A. , Nicolis G. , Baras F.
Published: July 2002

The effects of low-dimensional supports (one and two dimensions) on the steady state and the dynamics of open reactive systems capable of giving rise to oscillatory behavior are studied. A lattice Lotka–Volterra model involving reaction, adsorption, and desorption mechanisms is developed for which mean-field behavior predicts a continuum of closed trajectories around a center. It is shown that the spatial constraints of the support radically change this behavior. Specifically, while in one dimension, oscillations are suppressed, in two dimensions, the system selects a preferred oscillation frequency depending on the intrinsic parameters and the lattice geometry.
Reactions of laser-ablated iron atoms and cations with carbon monoxide: Infrared spectra of FeCO+, Fe(CO)2+, Fe(CO)x, and Fe(CO)x− (x=1–4) in solid neon
Authors: Zhou Mingfei , Andrews Lester
Published: July 2002

Laser-ablated iron atoms, cations, and electrons have been reacted with CO molecules during condensation in excess neon. The FeCO molecule is observed at 1933.7 cm−1 in solid neon. Based on isotopic shifts and density functional calculations, the FeCO molecule has the same Σ−3 ground state in solid neon that has been observed at 1946.5 cm−1 in a recent high resolution gas phase investigation [Tanaka et al., J. Chem. Phys. 106, 2118 (1997)]. The C–O stretching vibration of the Fe(CO)2 molecule is observed at 1917.1 cm−1 in solid neon, which is in excellent agreement with the 1928.2 cm−1 gas phase value for the linear molecule. Anions and cations are also produced and trapped, absorptions at 1782.0, 1732.9, 1794.5, and 1859.7 cm−1 are assigned to the linear FeCO−, Fe(CO)2−, trigonal planar Fe(CO)3−, and C3v Fe(CO)4− anions, respectively, and 2123.0, 2134.0 cm−1 absorptions to the linear FeCO+ and Fe(CO)2+ cations. Doping these experiments with CCl4 virtually eliminates the anion absorptions and markedly increases the cation absorptions, which confirms the charge identifications. Higher iron carbonyl Fe(CO)3, Fe(CO)4, and Fe(CO)5 absorptions are produced on photolysis.
Density matrix theory and calculations of nonlinear yields of CO photodesorbed from Cu(001) by light pulses
Authors: Yi Zhigang , Micha David A. , Sund James
Published: July 2002

The photodissociation of CO from Cu metal surfaces due to absorption of visible and ultraviolet (UV) light pulses is described within a density matrix approach, including the nonlinear optical response of the substrate to pulses of large fluence. We introduce a self-consistent coupling of adsorbate and substrate regions, and treat the substrate as a stochastic medium to account for dissipative effects following its electronic excitation. Our model is based on potential energy surfaces, couplings, and transition dipoles parametrized from electronic structure calculations for CO/Cu. The dynamics of photodesorption is obtained propagating wave packets with a nonperturbative treatment which includes the time dependence of the light pulse. Results have been obtained for the time evolution of state populations, and for yields of CO versus pulses fluence, with a range of values of the pulse width and light wavelength and of the dissipation time constant. Our numerical results for the desorption yields and desorption times are consistent with results of femtosecond photodesorption experiments at both low and high fluence values.
Integrated absorption intensity and Einstein coefficients for the O2 a 1Δg–X 3Σg− (0,0) transition: A comparison of cavity ringdown and high resolution Fourier transform spectroscopy with a long-path absorption cell
Authors: Newman Stuart M. , Lane Ian C. , Orr-Ewing Andrew J. , Newnham David A. , Ballard John
Published: July 2002

The two experimental techniques of cavity ringdown spectroscopy and high-resolution, long-path Fourier transform spectroscopy have been used to measure quantitative absorption spectra and determine the integrated absorption intensity (Sint,B) for the O2 a 1Δg–X 3Σg− (0,0) band. Einstein A-factors and radiative lifetimes for the O2 a 1Δg v=0 state have been derived from the Sint,B values. The two methods give values for the integrated absorption intensity that agree to within 2%. The value recommended from the results of this study is Sint,B=3.10±0.10×10−24 cm molecule−1, corresponding to an Einstein-A coefficient of A=2.19±0.07×10−4 s−1 and a radiative lifetime of τrad=76 min. The measurements are in excellent agreement with the recent absorption study of Lafferty et al. [Appl. Opt. 37, 2264 (1998)] and greatly reduce the uncertainty in these parameters, for which accurate values are required for determination of upper stratospheric and mesospheric ozone concentrations.
Structural transitions in nitrogen molecular clusters: Experiment and simulation
Authors: Calvo F. , Torchet G. , de Feraudy M.-F.
Published: July 2002

The structure of (N2)n clusters is investigated up to the size of several hundred molecules with means of electron diffraction experiments and numerical simulations. The comparison relies on the calculated diffraction patterns. The structures extracted numerically are obtained with a global optimization procedure. Two transformations are observed. The first one is the transition from polyicosahedral to multilayer icosahedral. It is seen from simulations to occur near the crossover size n*∼40 molecules, but is hardly seen in the experiment. The second one is the transition from icosahedral to cubic. In this case, both approaches indicate a crystalline crossover size nc located near 150–200 molecules. However, while the structures resulting from global optimization clearly show the enhanced stability of perfect cubic structures at zero temperature above nc, the information obtained from experimental diffraction functions at finite temperature and pressure exhibits a more progressive appearance of the crystalline features. This suggests the presence of significant defects in the real structure of nitrogen molecular clusters as they undergo their transformation toward the bulk face-centered-cubic geometry.
Correlation effect in spectral line shape from the Doppler to the collision regime
Authors: Lance B. , Robert D.
Published: July 2002

The spectral line shape model recently proposed by the present authors [J. Chem. Phys. 109, 8283 (1998)] is extended in order to account for correlation between velocity- and phase-changing collisions. The possible additional source of asymmetry due to the finite duration of collision is also included. In this implemented form, the model is consistent with previous exhaustive experimental studies in the collision regime for H2–rare gases and H2–nitrogen mixtures and has been shown to also accurately describe the observed spectral features at lower pressure for H2–Ar. It should be pertinent for various other molecular systems in a wide range of pressure and temperature, in particular for optical diagnostics.
Adsorption of methane molecules on neutral titanium Met-Cars
Authors: Sakurai H. , Castleman A. W.
Published: July 2002

Titanium metallocarbohedrenes (Met-Cars) are observed to adsorb methane molecules at low temperatures. The observed formation of the methane-Ti8C12 cluster complexes provides further direct evidence establishing that stable neutral titanium Met-Cars are present in copious amounts in the cluster beam. At sufficiently low temperatures, a dominant peak is observed for Ti8C12(CH4)4 along with less intense peaks of other methane-Ti8C12 clusters, which suggests the geometry of titanium Met-Cars may be the tetrahedral cage structure with Td symmetry. By studying the photoionization efficiency of the methane-Ti8C12 complexes near ionization threshold, their ionization potentials are found to be lower than that of Ti8C12.
Ultrafast internal conversion and photodissociation of molecules excited by femtosecond 155 nm laser pulses
Authors: Farmanara P. , Steinkellner O. , Wick M. T. , Wittmann M. , Korn G. , Stert V. , Radloff W.
Published: July 2002

The dynamics of several prototypical molecular systems after excitation with femtosecond laser pulses at 155 nm has been studied in pump–probe experiments. The vacuum ultraviolet (VUV) pump pulses with a pulse width of 350–450 fs were generated by near-resonant four-wave difference frequency mixing in argon. The careful analysis of the time-dependent ion signals has allowed us to determine the lifetime of the excited molecular states down to about 30 fs. The extremely short lifetime of water molecules excited to the repulsive Ã state has been directly observed for the first time: τD⩽20 fs. For molecular oxygen highly excited in the Schumann–Runge band, a decay time of 40±20 fs was obtained. The lifetimes of ethylene and chloroethylenes as well as of benzene and toluene reaching from 40 up to 180 fs are primarily caused by internal conversion. The decay times τD=(1.9±0.1) and τD=(90±20) ps obtained for carbon disulfide and nitric oxide, respectively, are due to predissociation of the VUV excited states.
Pressure and temperature variation of the electrical conductivity of poly(propylene glycol) containing LiCF3SO3
Authors: Fontanella J. J.
Published: July 2002

Complex impedance and differential scanning calorimetry (DSC) studies have been carried out on poly(propylene glycol) with average molecular weight 1025 (PPG) and PPG containing LiCF3SO3. The impedance studies were made at frequencies from about 1 mHz to 100 MHz at pressures up to 0.3 GPa (3 kbar) over the temperature range 215–365 K. Both the impedance and DSC studies were carried out in vacuum or at atmospheric pressure over a temperature range of about 100–375 K. As a consequence, the impedance studies overlap the DSC glass transition temperature. The inadequacy of the widely used Vogel–Tammann–Fulcher or Williams–Landel–Ferry equations to describe the temperature variation of the vacuum electrical conductivity data is discussed. It is shown that the Bendler–Shlesinger formalism is a better representation of the data, particularly in the region close to the glass transition. The first pressure derivative of the electrical conductivity, and hence apparent activation volume, decreases strongly as temperature increases. In addition, the activation volumes are larger for the low molecular weight liquids than the values reported previously for related, high molecular weight rubbery electrolytes. Next, there appears to be an exponential relationship between the activation volume and the electrical conductivity. Finally, the pressure variation of the electrical conductance exhibits negative curvature which shows that the activation volume increases as pressure increases. A qualitative explanation of each of these results concerning the relationship between conductivity, temperature, and pressure is given in terms of free volume. However, whether free volume can provide a quantitative explanation remains to be determined.
Ab initio classical trajectories on the Born–Oppenheimer surface: Hessian-based integrators using fifth-order polynomial and rational function fits
Authors: Millam John M. , Bakken Vebjo/rn , Chen Wei , Hase William L. , Schlegel H. Bernhard
Published: July 2002

Classical trajectories can be computed directly from electronic structure calculations without constructing a global potential-energy surface. When the potential energy and its derivatives are needed during the integration of the classical equations of motion, they are calculated by electronic structure methods. In the Born–Oppenheimer approach the wave function is converged rather than propagated to generate a more accurate potential-energy surface. If analytic second derivatives (Hessians) can be computed, steps of moderate size can be taken by integrating the equations of motion on a local quadratic approximation to the surface (a second-order algorithm). A more accurate integration method is described that uses a second-order predictor step on a local quadratic surface, followed by a corrector step on a better local surface fitted to the energies, gradients, and Hessians computed at the beginning and end points of the predictor step. The electronic structure work per step is the same as the second-order Hessian based integrator, since the energy, gradient and Hessian at the end of the step are used for the local quadratic surface for the next predictor step. A fifth-order polynomial fit performs somewhat better than a rational function fit. For both methods the step size can be a factor of 10 larger than for the second order approach without loss of accuracy.
Quantum scattering calculations for H2S–He between 1–600 K in comparison with pressure broadening, shift, and time resolved double resonance experiments
Authors: Ball Christopher D. , Mengel Markus , De Lucia Frank C. , Woon David E.
Published: July 2002

We have performed quantum scattering calculations to predict pressure broadening, pressure shift, and inelastic depopulation cross-sections for the rotational transitions 11,0←10,1 and 22,0←21,1 of the H2S molecule in collision with helium atoms over a temperature range from 1 to 600 K. The calculated cross-sections are compared with experimental values obtained by millimeter wave spectroscopic techniques and the collisional cooling method. We observe good agreement between theory and experiment over the temperature region from 20 to 600 K, but increasing differences below 20 K. Possible reasons for the deviations at lower temperatures are discussed. The calculations also illustrate the contribution of elastic collisions to the pressure broadening cross-sections.
The density matrix of H2O–N2 in the coordinate representation: A Monte Carlo calculation of the far-wing line shape
Authors: Ma Q. , Tipping R. H.
Published: July 2002

The far-wing line shape theory within the binary collision and quasistatic framework has been developed using the coordinate representation. Within this formalism, the main computational task is the evaluation of multidimensional integrals whose variables are the orientational angles needed to specify the initial and final positions of the system during transition processes. Using standard methods, one is able to evaluate the seven-dimensional integrations required for linear molecular systems, or the seven-dimensional integrations for more complicated asymmetric-top (or symmetric-top) molecular systems whose interaction potential contains cyclic coordinates. In order to obviate this latter restriction on the form of the interaction potential, a Monte Carlo method is used to evaluate the nine-dimensional integrations required for systems consisting of one asymmetric-top (or symmetric-top) and one linear molecule, such as H2O–N2. Combined with techniques developed previously to deal with sophisticated potential models, one is able to implement realistic potentials for these systems and derive accurate, converged results for the far-wing line shapes and the corresponding absorption coefficients. Conversely, comparison of the far-wing absorption with experimental data can serve as a sensitive diagnostic tool in order to obtain detailed information on the short-range anisotropic dependence of interaction potentials.
Collisional effects on spectral line shape from the Doppler to the collisional regime: A rigorous test of a unified model
Authors: Chaussard F. , Michaut X. , Saint-Loup R. , Berger H. , Joubert P. , Lance B. , Bonamy J. , Robert D.
Published: July 2002

The paper presents high resolution Raman investigations of the Q(1) line of H2 in Ar mixture from low density (Doppler regime) to high density (collisional regime) analyzed with a unique line shape profile. Measurements are performed by stimulated Raman gain spectroscopy between 300 and 1000 K in a wide density range (from 0.2 to 11 amagat). All the observed spectral features are accurately described by a unified model recently proposed by two of the authors. This model accounts for a velocity-memory process, not restricted to the usual hard and soft limits. It also includes correlation between velocity- and phase-changing collisions. An exhaustive analysis of various possible mechanisms on the line shape is achieved. These mechanisms are the Dicke narrowing, the radiator speed dependence of the collisional broadening and shifting parameters, the collisionally induced speed-class exchange and the nonimpact effect. The present test shows the high consistency of the unified model, since it allows one to get a remarkable agreement with all the data by using a unique set of parameters. This model should be useful for optical diagnostics at moderate density, as required for combusting media or atmospheric work.
Direct dissociative chemisorption of alkanes on Pt(111): Influence of molecular complexity
Authors: Weaver Jason F. , Krzyzowski Michael A. , Madix Robert J.
Published: July 2002

The direct dissociative chemisorption of ethane, propane, n-butane, isobutane, and neopentane on Pt(111) was investigated as a function of the initial translational energy, ET, polar angle of incidence, θi, initial vibrational temperature, and surface temperature using supersonic molecular beam techniques. For each alkane, the initial probability for direct dissociative chemisorption scales with the initial normal energy of the alkanes, En=ET cos2 θi, and is independent of both the surface temperature and initial vibrational energy of the alkanes under the experimental conditions employed. Above initial normal energies of approximately 125 kJ/mol, at constant En, the dissociation probability decreases with increasing chain length of the C2–C4 linear alkanes; however, the dissociation probability of neopentane is greater than that of isobutane, and both isobutane and neopentane are more reactive than n-butane. By assuming that cleavage of primary C–H bonds is the dominant reaction pathway for all of the alkanes investigated here, the trends in reactivity are best explained by considering the differences in the steric factors for primary C–H bond cleavage for these alkanes. Secondary C–H bond cleavage does appear to contribute to the reactivity of propane and n-butane but only at the highest energies examined. Additionally, the reaction probabilities of each of these alkanes were estimated using a statistical model recently proposed by Ukrainstev and Harrison [J. Chem. Phys. 101, 1564 (1994)]. Assuming cleavage of only primary C–H bonds, the trends in reactivity for ethane, propane, n-butane, and isobutane were qualitatively reproduced by the statistical model; however, except for ethane, which was used to obtain the necessary parameters for the theory, there was poor quantitative agreement, and the predictions for neopentane were significantly lower than the measured values. The model also predicts that the dissociation probability is enhanced by increasing the energy in all vibrational modes, which is inconsistent with the experimental results. Thus, we believe that direct alkane dissociation would be better described using a dynamical rather than statistical approach.