bluuebunny/crossref_metadata_2025 huggingface dataset:
Multiphoton metastable ion spectra and ion dissociation kinetics: Analysis of the decay channels of the aniline cation with a reflectron time-of-flight instrument
Authors: Kühlewind H. , Neusser H. J. , Schlag E. W.
Published: July 2002

Multiphoton mass spectrometry when carried out in conjunction with a reflecting field time-of-flight mass spectrometer is shown to yield rich new metastable ion spectra of polyatomic molecules, here aniline. The reflecting field acting also as an energy analyzer is shown to be particularly suitable for sensitive metastable ion detection without interference from stable fragment ions. With this technique several new metastable ion decay channels of the aniline cation have been observed. On the basis of the measured metastable ion spectrum a reasonable fragmentation pathway for the aniline cation C6H7N+ after multiphoton excitation is presented. The prominent intensity of the metastable ion peak C5H+6 points to a narrow energy distribution within the aniline cation typical for a ladder switching multiphoton excitation mechanism.
Near resonant charge transfer in the reaction F++CO→F+CO+
Authors: Kusunoki Isao , Ishikawa Toshiaki
Published: July 2002

Charge transfer reactions in the F++CO system were investigated using a F+ ion beam in the energy range 10–300 eVlab. The electronically excited product CO+ (A 2Πi) was observed by the emission from the A→X transitions. At low collisional energy the dominant product is in the vibrational level v′=5. The reaction cross section σ(5) is about 1 Å2 at 12 eVc.m. and decreases with increasing collision energy. The large cross section at v′=5 can be interpreted by near resonant charge transfer reactions. The rotational temperature of the product is about 300 K, which is the temperature of the reactant CO gas. For the resonant charge transfer, the translational energy is not effective, but the electronic and vibrational energy couple with each other strongly.
Energy surfaces of low-lying states of O3 and SO2
Authors: Jones R. O.
Published: July 2002

Density functional calculations have been performed for the energy surfaces of the ground states (x 1A1) and low-lying excited states (1 3B1, 1 3B2, 1 1B1, 1 1B2) of O3 and SO2. Vertical excitation energies for the 1 3A2 and 1 1A2 states have also been calculated. The agreement with available experimental information is qualitatively better than for Hartree–Fock calculations and comparable to the predictions of configuration interaction calculations. Plots of the geometry dependence of the one-electron eigenvalues (Walsh diagrams) give insight into the similarities and differences between the bonds in O3 and SO2. Calculations have also been performed for the ground state of SO. Dissociation energies in O2, O3, SO, and SO2 are overestimated by ∼2 eV, a consequence of the local density approximation used for the exchange-correlation energy.
Intramolecular distributions. III. N coupled modes
Authors: Kupka H. , Olbrich G.
Published: July 2002

A previous theory of intramolecular distributions (ID) is extended to N (Duschinsky) mixed vibrational modes. For this case a multidimensional ID is derived which provides a comprehensive, complete, and unified description of vibronically assisted electronic transitions in polyatomic molecules. The multidimensional ID is introduced with the aid of a multivariable generating function. The latter quantity incorporates both the transformation of normal mode coordinates between the initial and final electronic states and its frequency changes. This normal coordinate transformation leads to the generation of additional sets of geometrical displacements which cannot be uniquely assigned to normal mode displacements as in the parallel mode approximation. It is shown that normal coordinate rotation causes the final state vibrational distribution or the spectral line shape to deviate considerably from that observed at parallel modes. A further derivable property of the multidimensional ID is its selectivity which at certain values of the rotation angles leads to specific mode selections. Similarly, the nonradiative decay probability is subject also to variations and exhibits considerable increase under certain circumstances.
Spectra and structure of organophosphorus compounds. XXIII. Microwave spectra, electric dipole moment, and molecular structure of two conformers of ethyldifluorophosphine
Authors: Groner P. , Church J. S. , Li Y. S. , Durig J. R.
Published: July 2002

The microwave spectrum of ethyldifluorophosphine, CH3CH2PF2, has been investigated in the region from 18.0 to 39.5 GHz. The spectra of two conformers have been identified. The ground state and seven vibrationally excited states have been assigned for the more stable gauche conformer (phosphorus lone pair with respect to CH3 group), whereas the ground state and one excited state have been assigned for the trans form. Rotational and most quartic centrifugal distortion constants have been determined with A=6784.501(3), B=2930.156(2), C=2352.576(2) MHz for gauche and A=5444.910(3), B=3290.133(3), C=2871.915(3) MHz for trans, both in the ground state. Electric dipole moments were obtained from ground state Stark effect measurements: ‖ μa‖=1.990(4), ‖ μb‖=0.855(4), ‖ μc‖=0.091(7), ‖ μt‖=2.168(2) D for gauche and ‖ μa‖=1.780(10), ‖ μb‖=0, ‖ μc‖=1.077(10), ‖ μt‖=2.081(10) D for trans. Using a new computer program, a diagnostic least-squares fit of the structural parameters suggests significant structural differences between the conformers, notably the C–C–P angle which changes from 110.7° in the gauche form to 114.9° in the trans form. From a temperature study of the microwave spectrum, it was found that the gauche conformer is at least 80 cm−1 (229 cal/mol) more stable than the trans conformer. These results are compared to similar quantities in some corresponding molecules.
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o study of the methyl internal rotation of acetaldehyde in the S1 (n ,π*) state
Authors: Baba Masaaki , Nagashima Umpei , Hanazaki Ichiro
Published: July 2002

A b initio SCF calculations have been performed on the methyl internal rotation of acetaldehyde in the S1 (n, π*) state as well as in the ground state. The calculated barrier heights to methyl rotation (481 and 732 cm−1 for the ground and S1 states, respectively) are in good agreement with experimental values. The Mulliken population analysis has revealed that the ‘‘group’’ overlap population between the methyl and formyl groups correlates very well with the potential curve for methyl rotation, suggesting that the intergroup bonding governs the potential curve. It is also shown that the intergroup π bonding (hyperconjugation) is important in determining the barrier. The change of the stable conformation from the H eclipsing O type in the ground state to the ‘‘H antieclipsing O’’ conformation in the S1 state is shown to be due to the presence of an antibonding π electron in the latter.
Extended Brownian dynamics of diffusion controlled reactions
Authors: Allison S. A. , Northrup S. H. , McCammon J. A.
Published: July 2002

The Brownian dynamics simulation method of Northrup et al. is extended so that dynamical trajectories can be initiated with the reactants in close proximity to one another. A more general analysis is presented which shows that this procedure is exact in cases where the first-time encounter flux to the more proximal starting surface is isotropic, such as cases where interparticle forces are centrosymmetric, but is approximate otherwise. Diffusion controlled rate constants for three model systems obtained by this procedure are compared with analytic results or with exact rate constants derived from simulations following the original Northrup procedure. Agreement is good to excellent in all cases considered. The extended method is expected to be of considerable practical importance in systems with highly anisotropic reactivity where it is computationally inefficient to obtain rate constants by the original method.
Ionicity and electron molecular vibration interaction in mixed stack CT systems: M2P–TCNQ and M2P–TCNQF4
Authors: Meneghetti Moreno , Girlando Alberto , Pecile Cesare
Published: July 2002

The powder Raman spectra and the polarized infrared absorption spectra of a polycrystalline oriented film of (1:1) CT complex of 5,10-dihydro-5,10-dimethylphenazine (M2P) with 7,7,8,8-tetracyanoquinodimethane (TCNQ) are reported together with the polarized reflectance infrared spectra of a mosaic of single crystals [(010) crystal face] and the conductivity spectra obtained from a Kramers–Krönig analysis of the reflectance data. The powder Raman and infrared spectra of M2P–TCNQF4 and their temperature dependence (300–15 K) are also presented. The M2P–TCNQ structure is known to be made up of donor–acceptor dimerized mixed stacks. The spectral features related to electron–molecular vibration (e–mv) interaction are successfully interpreted in terms of the dimer model, whose applicability is further validated after its first use in the case of the low temperature phase of tetrathiafulvalene–chloranil complex. The success is testified by the good

fit obtained between experimental and calculated vibronic features of the near and midinfrared spectra. The extracted values of the e–mv coupling constants of TCNQ moiety compare well with those obtained from other TCNQ complexes, whereas those of M2P are a first informative evaluation. The value of the degree of ionicity of M2P–TCNQ ( ρ=0.5±0.1) is obtained by completing the assignment of the spectra in terms of fundamental vibrational modes normally active under the molecular symmetry of the component molecules and by using carefully chosen diagnostic infrared frequencies. Results show that M2P–TCNQ is definitely one of the still rare cases of mixed stack CT complexes with intermediate ionicity. By using analogous criteria, the interpretation of room temperature and low temperature (15 K) spectra of M2P–TCNQF4 leads to the conclusion that for this complex ρ∼1.0. Its temperature dependent phase transition (T=120 K) between the room temperature regular mixed stack and the low temperature dimerized one is well characterized. The vibronic features appearing in the low-temperature infrared spectrum agree with predictions by the dimer model for a distorted stack. Consequently, it is likely that the phase transition is attributable to a dimerization of the stack due to a Spin–Peierls instability, in agreement with the previous indication given by magnetic susceptibility measurements.
Extensions of the Kramers–Kronig transformation that cover a wide range of practical spectroscopic applications
Authors: Bardwell Jennifer A. , Dignam Michael J.
Published: July 2002

The Kramers–Kronig (KK) transformation has been widely used for the determination of the spectra of the refractive and absorption indices from the normal-incidence reflectivity spectrum of solids. This paper considers the extension of this technique to s-polarized light at nonnormal incidence with a transparent incident medium of arbitrary, constant refractive index. While the KK transformation is known to be inapplicable under certain circumstances, we show that in all cases satisfactory corrections can be applied, and readily applied in most cases. As an example, for the experimentally important case of attenuated total internal reflection from an organic liquid in the infrared, only one additional piece of information is required, namely, the refractive index at a frequency where the absorption is low. The procedures are demonstrated using model systems consisting of sets of damped harmonic oscillators. All the KK transformations were done using the FT software of a commercial Fourier transform infrared spectrometer in order to demonstrate the considerable potential of this approach to the routine determination of optical constants.
Absolute photodissociation cross sections of gas phase sodium chloride at room temperature
Authors: Silver J. A. , Worsnop D. R. , Freedman A. , Kolb C. E.
Published: July 2002

Absolute photodissociation cross sections for gas phase NaCl are measured over the wavelength range of 189.7–359.8 nm at 300 K. Two well-resolved peaks are observed at 235 and 260 nm. The cross section also rises below 210 nm. These results are in good qualitative agreement with previous high temperature measurements and with cross sections computed from theoretically calculated potential surfaces. However, there are two significant differences, the spectral width and the absolute magnitude of the cross sections. The importance of these measurements in understanding the photodissociative processes of ionic compounds and the implications for the role of NaCl in the stratospheric chemistry of chlorine compounds are discussed.
Anion ordering in alkali cyanide crystals
Authors: Ferrario Mauro , McDonald Ian R. , Klein Michael L.
Published: July 2002

Further molecular dynamics calculations have been carried out on sodium, potassium, and rubidium cyanides in their orientationally disordered cubic (Fm3m) phase. Simple, computationally efficient, rigid ion potentials are found to account for the overall range of static and dynamic properties. However, minor but systematic variations are required in the anion charge distribution. These changes mean that the charge distribution becomes more spherical as the size of the cation increases.
Molecular mobility and phase transitions in thermotropic liquid crystals: A spectroscopic study of dodecylcyano biphenyl
Authors: Galbiati Elisabetta , Zerbi Giuseppe
Published: July 2002

The solid-smectic A and smectic A-liquid phase transitions in dodecylcyanobiphenyl (12-CB) have been followed by infrared and Raman spectroscopy. Clear evidence is found that at the solid to smectic A transition the alkyl residue goes from the all trans to a conformationally disordered structure. These findings require a modification of the generally accepted model proposed for the structure of these materials in the smectic phase. Moreover, information has been obtained on the intermolecular interactions which exist in the various phases.
Brownian coagulation in a field of force
Authors: Sceats Mark G.
Published: July 2002

The Fokker–Planck equation for two particles, when reduced to a one-dimensional problem in the radial coordinate, yields a transition state in the effective potential V(R)−2kT ln(R/R′) for potentials exhibiting capture characteristics. Application of Kramers solution to barrier crossing yields an analytic formula for the capture rate constant which applies for all particle sizes relative to the bath mean free path, giving the exact result for large particles for diffusion on V(R) and the kinetic theory result for small particles. The influence of the potential is accounted for by two stability factors W(0) and W(∞). For hard spheres the results are in good agreement with extant theory. Extension to include hydrodynamic interaction and relaxation is indicated.
Potential energy curves for the a 3Σu+ and c 3Σ+g states of He2 consistent with differential scattering, a
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Authors: Jordan R. M. , Siddiqui H. R. , Siska P. E.
Published: July 2002

Crossed beam scattering experiments on He*(2 3S)+He at energies of 0.94, 1.51, and 2.24 kcal/mol have been combined with ab initio theory and kinetic data on metastability exchange rates to produce new estimates of the title potential energy curves. The long-range potential barrier in the a state is found to be 1.43±0.05 kcal/mol at a separation of 2.717±0.04 A. Combining these results with those for the corresponding singlet states from an earlier paper [J. Chem. Phys. 80, 5027 (1984)], we discuss the long-range behavior of the curves, particularly with respect to curve crossing between a given pair of g and u states. These crossings, which conform to theoretical predictions, appear to be essential for a consistent description of all available data.
Calculated ground state potential surface and excitation energies for the copper trimer
Authors: Walch Stephen P. , Laskowski Bernard C.
Published: July 2002

The results of an SCF/SDCI treatment are presented for selected portions of the ground state potential energy surface for the Cu3 molecule. For equilateral triangle geometries (D3h) the lowest state is 2E′ arising from 4sa′214se′1. The 2E′ state exhibits strong Jahn–Teller distortion, leading to 2A1 (acute angle) and 2B2 (obtuse angle) minima in C2v symmetry. Here the 2B2 minimum is a true minimum on the surface while the 2A1 minimum is a saddle point or very shallow secondary minimum connecting adjacent 2B2 minima. This strong Jahn–Teller distortion is consistent with the observed ground state vibrational levels in the gas phase spectroscopic studies of Morse, Hopkins, Langridge-Smith, and Smalley and in the matrix Raman studies of Moskovits. The 2B2 minimum is also consistent with the observed ESR spectrum of Cu3 in a matrix, which has been interpreted as an obtuse angle structure with most of the spin density on the end Cu atoms. A linear 2Σ+u state is found to be 0.26 eV higher. Two possible candidates have been found for the upper state in the spectrum of Morse et al. (i) a 3s Rydberg state of 2A1 symmetry and (ii) a 3d→4s state of 2E″ symmetry. Both of these states are consistent with the observed selection rules. The 2E″ state would be expected to be weakly Jahn–Teller distorted in agreement with the fit to the upper state levels by Morse et al. and by Thompson, Truhlar, and Mead.
Some physical consequences of large shear rates on simple liquids
Authors: Heyes D. M.
Published: July 2002

The structural and dynamical changes that take place in simple liquids and glasses during extreme shear thinning are investigated by nonequilibrium molecular dynamics (NEMD). Simulations performed on the Lennard-Jones liquid at states close to the triple point reveal long range order accompanying extreme shear thinning which is characterized by linear lines of molecules spontaneously forming along the shear flow lines. These ‘‘strings’’ of molecules pack into a triangular lattice. The density of strings increases with shear rate, as does the difference in density in various directions with respect to the flow field. Local structural asymmetry with lifetimes comparable to the simulation are observed at the highest shear rates considered, leading eventually to structural disorder via dislocations in the triangular lattice.
A modified Tang–Toennies model for ion–atom potential curves
Authors: Siska P. E.
Published: July 2002

A modification of the Tang–Toennies potential model suitable for representing heteronuclear ion–atom interactions is proposed. It has the form V(r) =A exp(−br) −B exp(−br/2) −∑3n=2  f2n(r) C2n  r−2n with f2n(r) =1−[∑2nk=0  (br)k/k!]  exp(−br) and displays realistic behavior over a wide range of r. It provides an accurate representation of both spectroscopic data and potentials derived from mobility. Parameters are tabulated for HeNe+, HeAr+, HeLi+, and XeCl−.
Study of molecular motions in liquids by electron spin relaxation: Halogenated p-semiquinone anions in alcohols
Authors: Prabhananda B. S. , Hyde James S.
Published: July 2002

The p-benzosemiquinone radical anion has served as a model system in numerous electron spin resonance liquid-phase relaxation studies. An inconsistency exists of more than an order of magnitude between measured spin-lattice relaxation times T1e and calculated values based on the spin-rotational mechanism if the rotational correlation time τθ that is required in the calculation is obtained from linewidth measurements. This paper presents a number of experiments designed to contribute to the solution of this problem. The rigid-limit g values of tetrahalogenated benzosemiquinone anions have recently become available, permitting the study of the effect of Zeeman anisotropy in a relatively homologous series of compounds. 13C labeled tetrachlorobenzosemiquinone anion (TSCQ−) has additionally been prepared in order to determine the dependence of linewidth on the 13C nuclear quantum number. Linewidth and continuous wave (cw) saturation measurements have been made from 1.0 to 35 GHz. Saturation-recovery pulse measurements of T1e have been made at 9 GHz. Solvent viscosity and temperature have been varied. It is established that an expression of the form [T−11e=AT/η+B exp(−ΔE/RT)] is required to fit the data in hydrogen-bonding solvents with both terms arising from spin-rotation. Values of τθ were determined in five different ways for TCSQ−. The variation is fivefold in ethanol and tenfold in n-butanol. The discrepancy that motivated the research has not been fully resolved, but the data suggests the existence of an additional line broadening mechanism arising from the superposition of signals from radicals in a distribution of relatively long-lived local environments.
Doppler spectroscopy of OH in the photodissociation of hydrogen peroxide
Authors: Klee Stefan , Gericke Karl-Heinz , Comes Franz Josef
Published: July 2002

The photofragmentation of hydrogen peroxide has been investigated at an excitation wavelength of 266 nm using polarized laser beams. Doppler spectroscopy in connection with the laser induced fluorescence technique was applied to determine the nascent translational distribution of OH products. The hydroxyl radicals are formed in their electronic ground state, X 2Π3/2,1/2. The released energy is transformed almost exclusively in translation of these photoproducts. The Q1(4) transition in OH(X 2Π3/2→A 2Σ+) was used to study the anisotropic angular distribution of recoiling OH photofragments and analyzed to obtain information about the lifetime and symmetry of the excited dissociative state. The Doppler profiles for different directions of the electric field vectors of dissociation and probe laser relative to the detector axis have been determined for two different geometries. The angular distribution measured by Doppler spectroscopy, peaks in the direction perpendicular to the electric vector of the dissociating laser light, indicating that the predominant electronic excited state in H2O2 must be of 1A symmetry. An upper limit for the lifetime τ of the excited H2O2 parent molecule before dissociation is τ≤6×10−14 s.
Intensity parameters for the Raman trace scattering of ethane
Authors: Gough K. M. , Murphy W. F.
Published: July 2002

Absolute Raman trace scattering cross sections have been measured for gaseous ethane-h6 and -d6 and the two rotational conformers of 1,1,2,2-ethane-d4. Sets of ∂ᾱ/∂Sj intensity parameters were determined for the C2H6/C2D6 pair and for the two CHD2CHD2 isomers; these sets were equal to within the experimental accuracy. The concept of an isotopically independent set of Raman intensity parameters for describing the Raman spectrum of various isotopic species of a molecule is found to be as valid as the concept of an isotopically independent harmonic force field for describing vibrational frequencies.
Dynamics of kinematically constrained bimolecular reactions having constant product recoil energy
Authors: Noda Chifuru , Zare Richard N.
Published: July 2002

A model is presented for kinematically constrained reactions in which the product recoil energy is assumed constant (CPR approximation). It is further assumed that the reaction probability is independent of both the impact parameter and the collision energy for all collisions that lead to products. This model predicts that (1) the product vibrational distribution is bell-shaped, peaking at the vibrational level with an energy equal to the reaction exoergicity minus the product recoil energy, (2) small values of the impact parameters produce high vibrational excitation while large values produced low vibrational excitation, the specific opacity function for the most populated vibrational level being sharply peaked at the impact parameter equal to the equilibrium internuclear distance of the product diatomic, (3) the product rotational distribution for each vibrational level differs but has the form of a sharp leading edge for some J value followed by a falloff whose shape depends on the form of the collision energy distribution, and (4) the product average rotational energy associated with each vibrational level decreases linearly with increasing v to a value of v corresponding to the maximum in the vibrational distribution followed by a more slowly changing behavior. Comparisons are made of these predictions with some experiments on kinematically constrained bimolecular reactions. Some further extensions of the CPR model are suggested.
A structural analysis of the vitreous silica surface via a molecular dynamics computer simulation
Authors: Levine S. M. , Garofalini S. H.
Published: July 2002

Molecular dynamics computer simulations were used to study surfaces of pure silica glass. The potentials used here were those previously established to model bulk silica and have been extended to study surface relaxation in a perfect vacuum. A large number of surfaces were made using different starting configurations; system sizes, and cooling procedures. Following ‘‘fracture,’’ many broken bonds rearranged in response to the changes in the net forces in the surface region. After this reconstruction, the simulations showed the expected general features observed experimentally, such as a prevalence of oxygen atoms at the outermost surface, nonbridging oxygens, and strained siloxane bonds. Threefold silicons (similar to e′ centers) were initially present in the ‘‘fractured’’ surfaces but most often were incorporated into the network tetrahedrally after reconstruction. Other defects produced during the reconstruction were five coordinated silicons and more importantly, edge sharing tetrahedra, forming the strained siloxane bonds. Bond angles and bond lengths for each defect were determined, showing good agreement with previously published results as well as providing new information. Finally, estimations for silanol concentrations were made which compare well with experimentally determined coverages. The computer simulation technique used here adequately reproduces many of the structural and dynamic characteristics of silica glass surfaces.
Coverage-dependent diffusion of CO on Ni(100)
Authors: Roop B. , Costello S. A. , Mullins D. R. , White J. M.
Published: July 2002

The coverage dependent diffusion rate of CO on Ni(100) has been studied by laser-induced desorption methods. The CO diffusion rate varies with the morphology of the adlayer. At low coverages, where the CO is disordered, the rate is relatively low but constant. At high coverages where a c(5√2×√2)R45 structure is known to prevail, the rate is significantly faster but again constant with coverage. Between these two regions (0.3<θCO <0.5), where a c(2×2) structure develops, the rate is intermediate and tends to increase with coverage. The activation energy Ed decreases but the preexponential term D0 also decreases when the rate increases. This compensation effect between Ed and D0 is used to anticipate surface phase changes.
Quenching of NH(a 1Δ)
Authors: Bower R. D. , Jacoby M. T. , Blauer J. A.
Published: July 2002

The rate coefficients for quenching of metastable NH(a 1Δ) by N2, H2, HF, F2, HNCO, NH3, N2H4, and He were measured. The 193 nm ArF pulsed laser photolysis of HNCO, NH3, and N2H4 produced NH(a 1Δ). Its lifetime was measured as a function of quenching species concentration by an LIF technique which made use of the a 1Δ→c 1π transition near 325 nm.
Two-photon spectra of the S1–S transition in glyoxal
Authors: Bickel G. A. , Innes K. K.
Published: July 2002

The two-photon excitation spectrum of the 1Au(S1)–1Ag(S0) transition in trans-glyoxal is observed in the region of 21 800–25 400 cm−1. The excitation source is the H2 Raman shifted output of a pulsed dye laser. The source yields tunable output from 0.7 to 1.4 μm and enables both one- and two-color two-photon spectra to be observed. These spectra are complementary to the well known one-photon spectra for this transition. However, the two-photon selection rules cause much of the vibrational structure to be simplified. Many of the observed bands belong to the torsional mode, ν7, revealing vibrational quanta up to 12 in the excited state and 8 in the ground state. The two-photon rotational contours for a number of transitions are calculated for comparison with the observed spectra.
The gas phase infrared spectrum of ν1 and ν1−ν4 HCN---HF
Authors: Bender D. , Eliades M. , Danzeiser D. A. , Jackson M. W. , Bevan J. W.
Published: July 2002

A continuously tunable single frequency color center laser has been used to investigate the rovibrational static gas phase infrared spectra of the ν1 (H–F stretching vibration) and its hot bands ν1+ν17 −ν17 and ν1+ν4−ν4 in the linear dimer HCN---HF. Observed perturbations in the ν1 and ν1+ν17 −ν17 subbands results from Coriolis interactions of the excited vibrational states ν1 and ν1+ν17 with ν2+2ν4+ν17 and ν2+2ν4+2ν07 , respectively. The influence of Coriolis interactions and vibrationally predissociating excited state lifetimes of 1.06(10)×10−10 s are considered in simulation of the observed band profiles. Molecular and anharmonic cross term parameters associated with investigated vibrational states are also presented. Anharmonic crossterms X○14 , X○17 , and X○47 are evaluated as 8.0252(73), 4.2162(53), and 1.000(49) cm−1, from the available data, including the analysis from the unresolved spectra of ν1−ν4 and ν1−ν4+ν17 −ν17 . The differences of the band origins in ν1 and ν1−ν4 give a value of ν4=168.344(21) cm−1 for the local spacing of the low frequency hydrogen-bond stretching vibration.
Geometries, energies, and electrostatic properties of nonadditively optimized small water clusters
Authors: Belford David , Campbell Edwin S.
Published: July 2002

Several local energy minima have been determined for small water clusters, (H2O)n, of n=3, 4, and 6 molecules. Their geometries were optimized with the nonadditive potential of Campbell and Mezei. For n=6, the inclusion of the nonadditive component of the potential altered the order of the energies of the local minima on the energy surface. Whereas the additive approximation (pairwise energy sum) favored a nearly planar ring, the nonadditive energy preferred a staggered hexagon, as found in ice Ih. It has been shown that the nonadditive component—and even the energy contribution from the cooperative reinforcement of the induced dipole fields—are larger than the energy differences between different ice forms with their very different orientations. When both nonadditive and dispersion energy contributions were included, the equilibrium oxygen–oxygen distance for (H2O)6 was reduced from the optimal dimer distance to the range of vibrationally averaged oxygen–oxygen distances in condensed phases. Electric fields and substantially enhanced molecular dipole vectors have been calculated. The molecular dipole moments in the clusters are substantially larger than the isolated molecule moment.
Calculation of free energy changes in ion–water clusters using nonadditive potentials and the Monte Carlo method
Authors: Cieplak Piotr , Lybrand Terry P. , Kollman Peter A.
Published: July 2002

A thermodynamic perturbation method, using a Monte Carlo procedure, has been applied to the calculation of the free energy differences between ion–water clusters containing a given number of water molecules (n=1,...,6) and different monovalent ions, i.e., Li+, Na+, K+, F−, and Cl−. To our knowledge, this is the first application of the free energy perturbation approach with a nonadditive intermolecular potential. The results confirm the usefulness of the method and yield free energy differences of hydration which are in generally very good agreement with experiment. Our calculations have also revealed some structural changes which take place within the first hydration shell when one ion is replaced by another in the hexahydrate clusters. For Li+ one obtains a well defined 4+2 mean structure, whereas for a larger ion such as K+ the solvation structure is closer to a 5+1 type. For anions, this tendency is reversed, i.e., for F− one finds a well defined four coordinate structure for a cluster with four water molecules, but for the larger anion Cl− the cluser tends to be a 3+1 structure. In addition, favorable water–water interactions are apparent in anion–water clusters, but not in cation–water clusters.
Molecular fluorescence and near resonance Raman yield as a probe for solvation dynamics
Authors: Yan Yi Jing , Mukamel Shaul
Published: July 2002

A microscopic theory for spontaneous emission line shapes of polyatomic molecules in condensed phases is developed. The distinction between the Raman and the fluorescence component is discussed and a rigorous transform relation between the total Raman excitation profile and the absorption line shape is obtained. The effects of solvation are introduced via generalized solvent line shape functions corresponding to the solvent dynamics before and after the reorganization process which follows the electronic excitation. The Raman yield and its frequency dependence are shown to provide a sensitive probe for the solvent dynamics. Detailed calculations for model systems and for the retinal chromophore in bacteriorhodopsin are presented.
A quasiclassical trajectory study of the heavy–light–heavy ClHCl and IHI reactions: Do three-dimensional partial cross sections oscillate as a function of energy?
Authors: Last I. , Baer M.
Published: July 2002

In this work we examined the possibility that characteristic heavy–light–heavy collinear oscillatory behavior of the reactive probability functions will also be apparent in three-dimensional partial cross sections. The study was carried out using the quasiclassical trajectory method for ClHCl and IHI. For this study the DIM-3C potential energy surfaces were employed. It was found that whereas in general the collinear features were reproduced in three dimensions, the prospects for them to be detected experimentally exist for only part of them.
Nearly monodisperse fluids. I. Monte Carlo simulations of Lennard-Jones particles in a semigrand ensemble
Authors: Kofke David A. , Glandt Eduardo D.
Published: July 2002

A hybrid, semigrand ensemble is applied to the simulation of mixtures of very many components. The independent variables are temperature, density, and the distribution of activity ratios. The system samples different compositions, with particles changing species identities during the simulation. Thus, the method is particularly well suited for applications to polydisperse mixtures. Results are presented for the simulation of mixtures of Lennard-Jones particles continuously varying in size and energy parameters. Four system sizes are used: 108, 216, 343, and 512 particles. The effect of degree of polydispersity is investigated by varying the width of the imposed distribution of activity ratios, which is Gaussian. It is found that very large systems are needed to describe wide distributions, due to the presence of ever-larger particles. A limit on sizes eventually arises from the competition of energetic and entropic effects. Methods for determining phase equilibrium for these mixtures are also discussed.
Kinetic theory of polymer melts. IX. Comparisons with experimental data
Authors: Schieber J. D.
Published: July 2002

The modification to the Curtiss–Bird model to account for polydispersity is used to compare predictions of several material functions to experimental data. The data were taken from samples of polymer melts and concentrated solutions with two types of molecular weight distributions, log-normal and Flory–Schulz. The Curtiss–Bird model is also compared to predictions of the Doi–Edwards model for viscosity of polydisperse samples. Experimental data for first normal stress coefficient, elongational stress growth viscosity, and steady elongational viscosity were also compared to the predictions of the Curtiss–Bird model. In all cases, the best fit to the experimental data were found for nonzero values of the link tension coefficient ε. It is found that HDPE η̄ data (and η̄+ data) can be fit by ε≂0.05, a value of ε that is low enough to predict rod dipping for this material.
Vibrational relaxation of ammonia trapped in rare gas matrices
Authors: Lakhlifi A. , Girardet C.
Published: July 2002

A theory of the vibrational energy relaxation of a symmetric top molecule trapped in a rare gas matrix is presented. The direct relaxation mechanisms of the energy on the molecular orientation and on the lattice vibrations (including the local modes) are described within the framework of the nonadiabatic coupling between the internal vibrational mode of the molecule and the low-frequency external modes. The three types of relaxation constants are analyzed. The transfer to the orientational modes of the molecule is shown to be the more efficient since the corresponding relaxation time ranges between 1 μs and 50 ns, according to the nature of the rare gas matrix and to the temperature. The multiphonon relaxation constant is calculated for two specific deexcitation channels. The phonon multimode process provides relaxation times which range between 1 ms and 10 μs. Such a process is a characteristic of the nonadiabatic treatment of the total Hamiltonian. In contrast, the multiphonon monomode process, where the vibrational energy is transferred to the vibration of the center of mass of the molecule, gives larger relaxation times around 1 ms. This process is connected to the high orders in the interaction potential anharmonicity. The third species of relaxation constant which mix the orientational and translational processes is also analyzed for various typical relaxation channels. The corresponding relaxation times are one order of magnitude longer than those obtained from the multiphonon mechanisms. The calculated relaxation times are close to the experimental measurements and exhibit the same trends with rare gas changes and temperature rises.
K-shell shape resonances and intramolecular bond lengths. Comments on ‘‘The relationship between shape resonances and bond lengths’’
Authors: Hitchcock A. P. , Stöhr J.
Published: July 2002

A rebuttal is given to the critique by Piancastelli et al. [J. Chem. Phys. 86, 2765 (1987)] of the relationship between 1s→σ* excitation energies and bond lengths proposed by Sette et al. [J. Chem. Phys. 81, 4906 (1984)]. The anomalous character of σ* resonances in the C1s and O1s spectra of CO2 is explained in terms of dipole selection rules and a large energy splitting between the σ*g and σ*u orbitals.
The harmonic force field of propane
Authors: Gough K. M. , Murphy W. F. , Raghavachari Krishnan
Published: July 2002

The quadratic vibrational force field of propane has been obtained by scaling calculated ab initio force fields to fit well determined vibrational frequencies from the Raman and infrared spectra of gaseous propane-h8, propane-d8, 1,1,1,2,3,3,3-propane-d7, and the two rotational isomers of 1,1,2,2,3,3,3-propane-d7. Two different ab initio force fields were investigated, one using Hartree–Fock theory (HF/6-31G*) and the other including electron correlation effects by means of second-order Mo/ller–Plesset perturbation theory (MP2/6-31G*). The scaled MP2/6-31G* force field gives a significantly better fit of the experimental frequencies, especially for those involving contributions from the CC stretching coordinate.
Ozone visible photodissociation dynamics
Authors: Levene Harold B. , Nieh Jong-Chen , Valentini James J.
Published: July 2002

The photolysis of ozone at 300 K in the Chappuis band has been investigated through collision-free coherent anti-Stokes Raman scattering (CARS) spectroscopy of the molecular oxygen photofragment. We have obtained the nascent electronic, vibrational, rotational, and translational energy distributions for nine photolysis wavelengths over the range 560–638 nm. The O2 photofragments are always formed in the ground (3Σ−g) state, there is no evidence for production of the excited (1Δg) state. Only a narrow range of high rotational levels are populated. The rotational distribution shifts to higher rotational states as the available energy increases. The vibrational distribution, however, is independent of photolysis wavelength. Only states v=0 to v=4 are populated, and there is a population inversion between v=2 and v=3. The average partitioning of energy among the vibrational, rotational, and translational degrees of freedom is 10%, 24%, and 66%, respectively. These results are interpreted to imply vibrationally adiabatic but rotationally impulsive dissociation dynamics. The O2 photofragment vibrational distribution is explained by Franck–Condon vibrational overlaps between O2 and the ground 1A1 electronic state of O3. Detailed consideration of the angular momentum disposal in the photodissociation indicates a strong correlation between the direction of the O2 angular momentum produced by the dissociative energy release, and the direction of the angular momentum of the O3 in-plane rotation. The correlation is such that of two energy degenerate final states, the one of higher orbital angular momentum is not produced.
Simple perturbation method for convex-molecule fluids
Authors: Boublík Tomáš
Published: July 2002

The second-order perturbation theory of convex-molecule fluids with pair interactions of the generalized Kihara type is considered. The double Yukawa potential function and the Laplace transform of the hard-sphere correlation function in the PY approximation are used to evaluate a part of the perturbation integral. The perturbation theory was applied to model systems of the LJ molecules and Kihara rod-like particles; calculated thermodynamic functions agree well with the corresponding Monte Carlo data and results from the Weeks–Chandler–Andersen theory. Characteristic parameters of several fluids with spherical and rod-like molecules were evaluated from the saturated liquid properties. Comparison with the corresponding parameters of the multicenter LJ potential is given.
Kinetic and internal energy distributions via velocity-aligned Doppler spectroscopy: The 193 nm photodissociation of H2S and HBr
Authors: Xu Z. , Koplitz B. , Wittig C.
Published: July 2002

We report center-of-mass kinetic energy distributions for the 193 nm photodissociation of H2S and HBr using the method of velocity–aligned Doppler spectroscopy. Nascent H atoms are detected by sequential two-photon photoionization via Lyman-α (121.6 nm + 364.7 nm), and internal SH(X 2Π) and Br excitations are observed directly in the H-atom kinetic energy distributions. The kinetic energy resolution is much better than in ‘‘conventional’’ sub-Doppler resolution spectroscopy and results from detecting spatially selected species whose velocities are aligned with the wave vector of the probe radiation, kprobe, thereby providing a kinetic energy distribution for a specific laboratory direction. This improved resolution is achieved in the present experiments by using pulsed, collimated, and overlapped photolysis and probe beams, but the vital aspect of the technique involves increasing the delay between the two lasers in order to discriminate against species having velocity components perpendicular to kprobe. In the case of HBr, we identify the Br(2P3/2) and Br(2P1/2) contributions and find that the Br(2P1/2) channel accounts for approximately 14% of the fragmentation associated with perpendicular electronic transitions. Concerning H2S, SH(X 2Π) vibrational structure is clearly evident in the H-atom kinetic energy distribution, and the SH vibrational distribution shows oscillations, with [v″=0]>[v″=1], [v″=1]<[v″=2], [v″=2]>[v″=3], [v″=3]<[v″=4], and [v″=4]>[v″=5]. Such oscillatory behavior was predicted theoretically by Kulander. A simulation of our data places 32% of the SH in v″>0 (〈Evib(SH)〉∼2700 cm−1, which is approximately 14% of the available energy, hν-D0), while the general features of our H2S data are in accord with the TOF study of van Veen et al. Presently, our measurements appear to be limited by the dye laser resolution (∼0.06 cm−1 at 364.7 nm), but a significant improvement of the laser bandwidth is possible using commercially available sources. The velocity-aligned Doppler spectroscopy technique is not limited to detecting atoms, and species can be monitored using ionization, LIF, and absorption spectroscopy. As a result, this method should find applications in many areas of molecular physics.
Direct measurement of rotational energy transfer rate constants for H35Cl (v=1)
Authors: Rohlfing Eric A. , Chandler David W. , Parker David H.
Published: July 2002

We investigate the rotational energy transfer of H35Cl (v=1) in collisions with thermal HCl at 298 K. Rotationally state-selective excitation of v=1 over J=1–6 is achieved by stimulated Raman pumping, and the rotational relaxation from the initially pumped level is monitored via 2+1 resonantly enhanced multiphoton ionization (REMPI) through the E 1Σ+–X 1Σ+0–1 band. The ions are detected in a time-of-flight mass spectrometer in order to ensure that only relaxation of H35Cl is observed. We present empirical correction factors for determining relative rotational populations from the REMPI spectral line intensities and extract the rate constants for rotational energy transfer from the time-dependent populations using numerical techniques. The excellent sensitivity of the REMPI technique makes it possible to monitor the relaxation on very short collisional time scales (<0.1 hard sphere collision) and thereby enables us to determine the rate constants for both single quantum and multiquanta rotational transitions directly, without the use of a simplifying model for the matrix of rate constants. The measured rate constants illustrate the importance of multiquanta transitions for HCl; such transitions account for ∼30% of the total population loss from a given rotational level. We compare our rate constants to those obtained in previous measurements on HCl (v=1) and HF (v=2). We find that the rate constants are not adequately described by models based solely on the rotational energy defect of the vibrationally excited molecule.
Quantitative characterization of the viscosity of a microemulsion
Authors: Berg Robert F. , Moldover Michael R. , Huang John S.
Published: July 2002

We have measured the viscosity of the three-component microemulsion water/decane/AOT as a function of temperature and droplet volume fraction. At temperatures well below the phase-separation temperature the viscosity is described by treating the droplets as hard spheres suspended in decane. Upon approaching the two-phase region from low temperature, there is a large (as much as a factor of 4) smooth increase of the viscosity which may be related to the percolation-like transition observed in the electrical conductivity. This increase in viscosity is not completely consistent with either a naive electroviscous model or a simple clustering model. The divergence of the viscosity near the critical point (39 °C) is superimposed upon the smooth increase. The magnitude and temperature dependence of the critical divergence are similar to that seen near the critical points of binary liquid mixtures.
Time dependent quantum mechanical theory of gas–surface energy transfer
Authors: Jackson Bret
Published: July 2002

A fully quantum mechanical theory for phonon inelastic gas–surface scattering is presented. Both surface and bulk phonons are coupled to the molecule via perturbation theory. The reduced density matrix of the molecule is written in terms of three wave-function-like objects, which obey Schrodinger-like equations of motion. These highly coupled equations of motion are evolved simultaneously in time, using standard techniques. The method can be applied to any of several recent quantum and semiclassical time dependent scattering theories, and allows one to compute the thermally averaged properties of the system at all times. The total computational effort is only about three times that of the zero temperature calculation. Results are presented for the inelastic scattering of He and Ne beams from Cu(111), Cu(100), and Ni(111).