bluuebunny/crossref_metadata_2025 huggingface dataset:
Sticking and thermal desorption of O2 on Ag(001)
Authors: Buatier de Mongeot F. , Rocca M. , Cupolillo A. , Valbusa U. , Kreuzer H. J. , Payne S. H.
Published: July 2002

The coverage dependence of the sticking coefficient and temperature programmed desorption kinetics for molecular adsorption of oxygen on Ag(001) are investigated both experimentally and theoretically. The nearly exponential decrease of the sticking coefficient as a function of coverage by about five orders of magnitude is explained within a kinetic lattice gas model to be the result of strong nearest- and next-nearest-neighbor repulsions. The latter also shape the desorption spectra which are calculated within the same model under the assumption that fast surface diffusion maintains the adsorbate in quasiequilibrium during desorption. Because the desorption rate is proportional to the sticking coefficient under these conditions, we find a massive decrease of the pre-exponential factor in the Arrhenius parametrization of the rate as a function of coverage which explains the peculiar shape of the desorption spectra.
Ab initio calculations of the interaction of He with the B 3Π0u+ state of Cl2 as a function of the Cl2 internuclear separation
Authors: Rohrbacher Andreas , Williams Jason , Janda Kenneth C. , Cybulski Sl/awomir M. , Burcl Rudolf , Szczȩśniak M. M. , Chal/asiński Grzegorz , Halberstadt Nadine
Published: July 2002

Ab initio calculations using unrestricted Mo/ller–Plesset perturbation theory to fourth order (UMP-4) were carried out for the interaction of He with the B 3Π0u+ state of Cl2. Also, more reliable unrestricted coupled cluster singles, doubles, and noniterative triples (UCCSD(t)) calculations were performed for several points on the B electronic state surface and were used to scale the UMP-4 points. Exp-6 type two center potential energy functions were fitted to the modified UMP-4 points (B state) to construct an analytical three-dimensional potential energy surface. An r (Cl–Cl separation) dependence was incorporated in the B state potential energy surface to allow the calculation of HeCl2 properties in different vibrational states so that vibrational predissociation rates could be calculated. Excitation spectra, predissociation lifetimes, and rotational product distributions were calculated and compared to the available experimental data. It was found that the calculated B←X, 8←0 spectrum is in good agreement with the experimental one, and the calculated blueshifts for ν=8, 10, 12 show the right trend when compared to the experimental findings, i.e., the blueshifts get larger with increasing ν. The blueshift values are generally too small which suggests that the ab initio calculations underestimate the van der Waals interactions in the B state less than they do in the X state. The calculated vibrational predissociation lifetimes τ are in good agreement with the experiment, as are the rotational product distributions for ν=8, 10, and 12.
Predictions of thermoreversible volume phase transitions in copolymer gels by lattice-fluid-hydrogen-bond theory
Authors: Lele A. K. , Devotta I. , Mashelkar R. A.
Published: July 2002

Certain copolymeric hydrogels such as poly(N-ispropylacrylamide)/poly(acrylic acid) show first-order volume phase transition on heating. By using the lattice-fluid-hydrogen-bond theory we have shown earlier that both hydrogen bonding interactions and hydrophobic interactions are responsible for this behavior in homopolymer gels. In this paper we demonstrate that the transition temperature of copolymer gels can be varied by changing the ratio of the two monomer concentrations in the copolymerization and also by slightly varying the hydrophobic nature of the gels. Thus it might be possible to “tailor-make” gels with desired transition temperatures. Such gels can find novel applications in biotechnology, soft robotics, separations, and sensors.
Separating connectivity and expansion effects in polymer single chain dynamics
Authors: Kranbuehl David E. , Verdier Peter H.
Published: July 2002

The effects of chain volume and connectivity upon the motions of flexible polymers in dilute solution have been studied by computer simulation of simple off-lattice bead-flip models of from 9 to 99 beads. Long internal relaxation times are given for free-draining chains with bead diameters from zero to 0.93 times the stick lengths. Moves are forbidden which would result either in bead overlap (excluded volume) or in one stick passing through another (chain connectivity). In the extreme case of zero bead diameter, where there is no expansion of the chains by excluded volume, the long relaxation time varies as about the 2.1 power of chain length, as opposed to the 2.0 power for similar chains without connectivity constraints. As bead diameter is increased until it equals stick length, the exponent increases to the value of 2.48 established by previous work. Over the range of bead diameters employed, the chain-length dependence of long relaxation times and translational diffusion constants can be described by the sum of two terms, the first due to chain swelling by excluded volume and consistent with the predictions of scaling theory, the second due only to chain connectivity.
The intermolecular potential energy surface of the Ar⋅NO+ cationic complex
Authors: Bush Andrew M. , Wright Timothy G. , Špirko Vladimı́r , Juřek Michael
Published: July 2002

The intermolecular potential energy surface of the Ar⋅NO+ cationic complex has been calculated using ab initio methods: RHF, MP2, MP3, MP4, CCSD, and CCSD(T), using the cc-pVDZ and cc-pVTZ basis sets. An additional surface was calculated at the MP2/cc-pVQZ level. All calculations were performed with a fixed NO+ bond length, but in one set of calculations the effect of variation of the NO+ bond length was studied. Finally, the MP2/cc-pVQZ intermolecular potential energy surface was recalculated by performing a point-by-point correction for basis set superposition error. All of these surfaces were used to extract anharmonic intermolecular vibrational frequencies, in order to compare to previous calculations, and experimental results. Rotational energy levels were also calculated.
Vibrational state dependence of D2 dissociation on Ag(111)
Authors: Cottrell C. , Carter R. N. , Nesbitt A. , Samson P. , Hodgson A.
Published: July 2002

Initial sticking probabilities for D2 dissociative chemisorption at a Ag(111) surface have been measured for translational energies in the range Ei=220–500 meV, as a function of incident angle θi and gas temperature, using seeded molecular beams. Sticking probabilities are dependent on the D2 internal state distribution and scale with the normal component of the translational energy. The data has been fit by assuming that dissociation is independent of molecular rotation, being the sum of contributions from molecules in different vibrational states v with a sticking function S0(Ei,θi,v)=A/2{1+tanh[Ei cos2 θi−E0(v)]/w(v)}, in a manner similar to the behavior on copper. Sticking parameters E0, the translational energy required for S0 to reach half of its maximum value, are determined with good precision (±25 meV) for levels v=3 (328 meV) and v=4 (170 meV) with width parameters w=54 and 63 meV, respectively, while the barriers for levels v=1 and 2 are close to the upper limit of the sticking data and E0 is estimated as 700±100 and 510±70 meV, respectively. Parameters for the vibrational ground state (v=0) could not be obtained, since sticking of this state is negligible at translational energies less than 500 meV. No dissociation could be observed at thermal energies Ei⩽70 meV either on a flat or a defective surface.
Fourier transform infrared emission study of the mechanism and dynamics of HOI formed in the reaction of alkyl iodides with O(3P)
Authors: Loomis Richard A. , Klaassen Jody J. , Lindner Jörg , Christopher Patty G. , Leone Stephen R.
Published: July 2002

Vibrationally excited hypoiodous acid (HOI) is observed as a product in the reaction of alkyl iodides with O(3P). Fourier transform infrared emission techniques are used to detect the excited ν1, OH, stretch of the HOI product, to determine the mechanism of HOI production, and to measure the vibrational product state distributions. The HOI product is formed by O atom reaction with two-carbon and larger straight or branched chain alkyl iodides and cyclic alkyl iodides, e.g., C2H5I, n-C3H7I, i-C3H7I, (CH3)3CI, n-C6H13I, and c-C6H11I, but not with CH3I. Experiments with selectively deuterated ethyl iodides provide direct evidence that HOI is formed in a beta-elimination mechanism involving a five-membered ring transition state. The O atom attacks the iodine and then abstracts a hydrogen from the beta carbon during the lifetime of the complex. Time-resolved experiments allow the extraction of nascent vibrational state distributions for the ν1 stretch of HOI (v=1:v=2:v=3) using different alkyl iodides and assuming the radiative rates are given by Aν→ν−1=vA1→0: C2H5I, 0.53(4):0.39(3):0.08(3); n-C3H7I, 0.61(6):0.34(5):0.05(2); and i-C3H7I, 0.54(6):0.38(4):0.08(3). These distributions are nonstatistical with the v=2 states having only slightly less population than those with v=1. For product molecules with up to three quanta of ν1 excitation, more than 50% of the reaction exothermicity is deposited into the OH stretch.
Photoabsorption and fluorescence cross sections of SiCl4 in the region of 6.2–31 eV
Authors: Ibuki Toshio , Kono Mituhiko , Asari Yukiko , Hiraya Atsunari , Shobatake Kosuke
Published: July 2002

Absolute photoabsorption and fluorescence cross sections of gaseous SiCl4 have been measured in the energy region 6.2–31 eV using synchrotron radiation as the light source. Higher order light from the 1-m Seya monochromator was suppressed by use of an Ar gas filter in the energy range 11.3–15.5 eV (110–80 nm) and a LiF window at hν<11.8 eV (105 nm). Emissions have been observed for the C̃ 2T2→Ã 2T2 and C̃ 2T2→X̃ 2T1 processes of the SiCl4+ molecular ion and the Ã 1B1→X̃ 1A1 and B13→X̃ 1A1 of the SiCl2 radical. The total fluorescence cross section has been determined to be 22.4±1.0 Mb at 21.22 eV. By comparing with the partial cross section for formation of the C̃ 2T2 ionic state [Carlson et al., J. Chem. Phys. 84, 641 (1986)], it has been concluded that the SiCl4+(C̃ 2T2) formed decays via radiative processes with quantum yield φ≈1. The experimental results provide information on the breakdown pathways of the SiCl4+(C̃ 2T2) ion state. The bands observed in photoabsorption and fluorescence excitation spectra have been assigned as the (4s, np, and np)←3t2, (4s and np)←1e, (4s and np)←1e, (4s and 4p)←2t2, and (np and nd)←2a1 Rydberg series. The ionization energies for the (2t2)−1 and (2a1)−1 processes have been found to be 15.04±0.03 and 18.17±0.03 eV, respectively.
The vibrational deactivation of CO(v=1) by inelastic collisions with H2 and D2
Authors: Reid J. P. , Simpson C. J. S. M. , Quiney H. M.
Published: July 2002

Calculations of the relaxation rate constants, kCO–H2, for the vibrational deactivation of CO(v=1) by pH2 and oH2 are reported in the temperature range 30 K<T<300 K. The CO rotation is treated using the infinite-order sudden (IOS) approximation, while the rotation of H2 is included using the coupled states (CS) approximation. A near-resonant energy transfer process, in which the H2 molecule is rotationally excited from J=2 to J=6 on relaxation of CO(v=1), is found to account for the experimental observation that kCO–pH2/kCO–oH2>1 for this system at temperatures above 80 K. Evidence is presented to suggest that below this temperature, which represents the current lower limit of existing experimental data for the CO(v=1)-pH2 system, thermal depopulation of the J=2 rotational state in pH2 reduces the importance of the near-resonant energy transfer process in the determination of kCO–pH2. For T≪80 K the ratio kCO–pH2/kCO–oH2<1 is predicted on the basis of these calculations. At impact energies less than 60 cm−1, the relaxation cross sections increase at a rate which is insufficient to account for the observed upturn in the experimentally determined deactivation rate constants for the CO–nH2 system below 60 K. Rate constants for the deactivation of CO(v=1) by oD2 and pD2 have also been calculated and compared with experimental data.
Hysteresis effects at the tilted to nontilted transition in octadecanol monolayers as observed with Brewster angle autocorrelation spectroscopy
Authors: Lautz C. , Fischer Th. M. , Kildea J.
Published: July 2002

With Brewster angle autocorrelation spectroscopy involving a combination of Brewster angle microscopy and autocorrelation technique we present quantitative measurements of the tilt angle in octadecanol carried out at the triple point of the next nearest neighbor tilted L2′, the nontilted destorted hexagonal LS(Rot I), and the hexagonal LS(Rot II) phases. We show that the transition from the tilted phase to the nontilted phases, which changes from first order (L2′/LS(Rot I)) to second order (L2′/LS(Rot II)), is associated with strong hysteresis effects in the L2′ phase, leading to an ambiguity of the tilt angle in the vicinty of the triple point. The behavior gives indications for a hindered first-order phase transition within the L2′ phase.
The H+D2 reaction in the vicinity of the conical intersection
Authors: Wrede E. , Schnieder L. , Welge K. H. , Aoiz F. J. , Bañares L. , Herrero V. J. , Martínez-Haya B. , Sáez Rábanos V.
Published: July 2002

Scattering measurements performed at an energy slightly higher than that of the lowest crossing between the ground and the first electronically excited state of the H3 system are reported. The essentials of the reactive dynamics are describable in terms of a classical motion of the nuclei on the lowest adiabatic electronic potential.
Bounds on spin dynamics and the design of multiple-pulse NMR experiments
Authors: Hughes Colan E. , Wimperis Stephen
Published: July 2002

The “universal bound on spin dynamics’’ proposed by So/rensen is examined in detail and shown to be of great assistance in the design of novel multiple-pulse NMR experiments. The efficiency of coherence transfer between all possible states of a spin system, including populations, single-quantum coherences, and multiple-quantum coherences, is investigated. Examples are drawn from coherence transfer processes in quadrupolar coupled spin I=1 and 3/2 nuclei and weakly J coupled systems of two and three spin I=1/2 nuclei. It is found that many of the most commonly used NMR pulse sequences fail to achieve the maximum coherence transfer efficiency when applied to spin I=3/2 or to three spin I=1/2 nuclei. However, it is shown that, with knowledge of the universal bound, novel multiple-pulse NMR experiments that achieve optimal efficiency can be easily derived using computer optimization. The application of the universal bound to two-step coherence transfer experiments presents a number of conceptual difficulties. In particular, examples are presented where the product of the universal bounds on the two individual coherence transfer coefficients is larger than the universal bound on the overall transfer from the initial to the final state. These difficulties are resolved and explained in terms of the presence of a “residue’’ that is created together with the intermediate state. The universal bound is used to examine the conditions under which the effect of this residue can be suppressed and the constraints that this places on the design of optimal multi-step coherence transfer NMR experiments.
Phase transitions in dipolar fluids: An integral equation study
Authors: Klapp Sabine , Forstmann Frank
Published: July 2002

We consider several dipolar fluids in their isotropic phase by integral equations in the reference hypernetted chain approximation. The systems differ in the form of the nondipolar potential. At low temperatures all of these fluids exhibit a spinodal line. Approaching this line from above, several fluctuations strongly increase. We interpret these fluctuations as indications for the low temperature states of the systems. In agreement with simulation results we find that the fluctuations at low densities strongly depend on the form of the nondipolar potential. In the dipolar hard sphere fluid we see a strong tendency that the particles associate into chainlike structures. Considering the same system with an additional Lennard-Jones attraction (Stockmayer fluid), we find that fluctuations of the number density which point to a usual condensation, clearly dominate. By switching off this isotropic attraction gradually, there is a smooth changeover from condensation to dipole clustering. At higher densities the behavior of the fluctuations becomes independent of the nondipolar potential: now the fluctuations indicate a transition into a fluid state with long-ranged ferroelectric order. By minimizing a density functional which has direct correlation functions of the isotropic phase as an input, we find the corresponding coexistence lines. Only at very high densities do the fluctuations point to crystallization.
Double many-body expansion potential energy surface for ground-state HCN based on realistic long range forces and accurate ab initio calculations
Authors: Varandas A. J. C. , Rodrigues S. P. J.
Published: July 2002

A single-valued double many-body expansion potential energy surface is reported for ground-state HCN based on a realistic representation of the long-range forces, and a fit to accurate ab initio calculations for the HCN↔CNH regions [J. M. Bowman, B. Gazdy, J. A. Bentley, T. J. Lee, and C. E. Dateo, J. Chem. Phys. 99, 308 (1993)] and novel full valence complete active space energies for CHN geometries. The various topographical features of the novel global potential energy surface are examined, and vibrational calculations are presented to characterize the minima associated to the HCN and CNH isomers. The quality of the new fit is compared with that of the seminumerical representation of Bowman et al. in terms of root-mean-squared deviations for stratified energy regimes.
Monte Carlo simulations of the γ–β, α–γ, and α–β phase transitions of nitrogen
Authors: Mulder A. , Michels J. P. J. , Schouten J. A.
Published: July 2002

We have performed Monte Carlo simulations in order to determine the γ–β, α–γ, and the α–β phase transition lines of nitrogen with a recently developed nitrogen–nitrogen potential, and to examine the driving forces for these transitions. We have shown that it is possible to obtain the α–γ phase transition line by starting, at higher temperature, with the hexagonal representation of the disordered fcc structure. The transition line was found about 0.4 GPa higher than the experimental line. The orientational order–disorder behavior of the γ–β and the α–β phase transitions could be observed, but the structural changes, fcc–hcp, did not occur, probably due to a potential barrier. It is also possible that the potential model causes the fcc structure to be stable with respect to the hcp structure. The orientational entropy of the various phases has been compared qualitatively by introducing the orientational order parameters. It is suggested that a small difference in translational entropy, due to a difference in the c/a ratio, stabilizes the hcp structure (β phase) with respect to the fcc structure at zero pressure. In contrast with previous work, our simulations reveal that not all the layers of the ordered hcp structure at low temperature have the same orientational order. This might be the reason that the ordered fcc structure is stable at low temperature.
The N2H+–He intermolecular potential energy surface: A vibrational adiabatic correction
Authors: Meuwly M. , Bemish R. J.
Published: July 2002

This paper presents a method for constructing computationally cheap adiabatically corrected ab initio potential energy surfaces (PES) for intermolecular vibrational states. The approach reasonably reproduces previously published experimental data for the N2H+–He complex in the ground and excited intramolecular vibrational states. A comparison made between a set of intermolecular PES’s with the N2H+ core frozen into the equilibrium geometry and a set where the N–H+ stretch is averaged demonstrates the importance of including this motion. This is also reflected in a considerable improvement in the agreement between the experimental and the calculated intermolecular bending and stretching frequencies and the origin red shift (νb,calc=117.9 cm−1, νs,calc=165.2 cm−1, Δνcalc=−93.0 cm−1). A comparison is also made between the Born–Oppenheimer angular radial separation (BOARS) angular average of the adiabatically corrected PES and the previously published rotational Rydberg–Klein–Rees (RRKR) PES. The results indicate that the two-dimensional νNH=1 PES has a qualitatively correct well depth and dissociation energy (De=684.7 cm−1; D0=433.6 cm−1).
Comment on “Time-resolved laser induced harpoon reactions’’
Authors: Apkarian V. A.
Published: July 2002

Time resolved studies of laser induced harpoon reactions in clusters and in liquid phase have succeeded in separating coherent and sequential two-photon channels, and have provided direct observations of bimolecular chemistry through control of products. The same cannot be said about the gas phase experiments on these systems.
Comment on: “Successive capture and coagulation of atoms and molecules to small clusters in large liquid helium clusters’’ [J. Chem. Phys. 102, 8191 (1995)]
Authors: Macler Michel , Bae Young K.
Published: July 2002

We derive a capture cross section which takes into accounts the effect of evaporative losses. We suggest that the coagulation cross sections measured for van der Waals impurities reflect the mass and energetic stability of the guest cluster, rather than incomplete coagulation of the impurity molecules.
Macromolecular separation through a porous surface
Authors: Deutsch J. M. , Yoon Hyoungsoo
Published: July 2002

A new technique for the separation of macromolecules is proposed and investigated. A thin mesh with pores comparable to the radius of gyration of a free chain is used to filter chains according to their length. Without a field it has previously been shown that the permeability decays as a power law with chain length. However by applying particular configurations of pulsed fields, it is possible to have a permeability that decays as an exponential. This faster decay gives much higher resolution of separation. We also propose a modified screen containing an array of holes with barblike protrusions running parallel to the surface. When static friction is present between the macromolecule and the protrusion, some of the chains get trapped for long durations of time. By using this and a periodic modulation of an applied electric field, high resolution can be attained.
Application of the effective valence shell Hamiltonian method to accurate estimation of valence and Rydberg states oscillator strengths and excitation energies for π electron systems
Authors: Chaudhuri Rajat K. , Mudholkar Anil , Freed Karl F. , Martin Charles H. , Sun Hosung
Published: July 2002

The ab initio effective valence shell Hamiltonian (Hv) is used to compute the low lying vertical excitation energies and oscillator strengths for ethylene, trans-butadiene, benzene and cyclobutadiene. Calculated excitation energies and oscillator strengths of ethylene, trans-butadiene and benzene to various valence and Rydberg states are in good agreement with experiment and with values from other highly correlated computations. The present work further investigates the dependence of Hv computations on the nature and choice of the molecular orbitals and provides a comprehensive study of the convergence with respect to the enlargement of the valence space. Minimal valence space Hv computations yield very accurate estimates of the excitation energies for the low lying excited triplet states and are slightly poorer (a deviation of ⩽0.5 eV from experiment) for low lying excited singlet states. More accurate low lying singlet state excitation energies are achieved by slightly enlarging the valence space to include Rydberg functions. The computed oscillator strengths from the Hv method are in excellent agreement with experiment and compare favorably with the best theoretical calculations. A very quick estimation of the transition dipoles and oscillator strengths may be obtained from second order Hv computations. The accuracy of these calculations is almost as good as those from the more expensive third order Hv computations and far superior to those from other quick methods such as the configuration interactions singles technique. Although no experimental data are available for the excitation energies and oscillator strengths of cyclobutadiene, our predicted values should be quite accurate and should aid in observing its π→π* transitions. We also provide the first correlated computations of oscillator strengths for excited→excited singlet and triplet transitions.
An ab initio study of the reaction mechanism of Co++NH3
Authors: Taketsugu Tetsuya , Gordon Mark S.
Published: July 2002

To investigate the mechanism for N–H bond activation by a transition metal, the reactions of Co+(3F,5F) with NH3 have been studied with complete active space self-consistent field (CASSCF), multireference configuration interaction (MR-SDCI), and multireference many body perturbation theory (MRMP) wave functions, using both effective core potential and all-electron methods. Upon their initial approach, the reactants yield an ion–molecule complex, CoNH3+(3E,5A2,5A1), with retention of C3ν symmetry. The Co+=NH3 binding energies are estimated to be 49 (triplet) and 45 (quintet) kcal/mol. Subsequently, the N–H bond is activated, leading to an intermediate complex H–Co–NH2+ (C2ν symmetry), through a three-center transition state with an energy barrier of 56–60 (triplet) and 70–73 (quintet) kcal/mol. The energy of H–Co–NH2+, relative to that of CoNH3+, is estimated to be 60 to 61 (triplet) and 44 (quintet) kcal/mol. However, the highest levels of theory employed here (including dynamic correlation corrections) suggest that the triplet intermediate HCoNH2+ may not exist as a minimum on the potential energy surface. Following Co–N or H–Co bond cleavage, the complex H–Co–NH2+ leads to HCo++NH2 or H+CoNH2+. Both channels (triplet and quintet) are found to be endothermic by 54–64 kcal/mol.
Cooperativity in binding of proteins to DNA
Authors: Ben-Naim A.
Published: July 2002

Three different, linear, three-site binding models are presented for the binding of proteins, such as a repressor, to DNA. The first model assumes that the DNA is rigid and all correlations between the ligands are due to direct ligand–ligand interactions. In the second and third models conformational changes in the DNA are induced by the binding process. In these models we find both direct and indirect correlations between the ligands. Examination of the indirect correlation reveals two characteristic features that, in general, are negligible in the direct correlation. (1) Long range correlation, between nonadjacent sites. (2) Nonadditivity of the triplet correlations. It is shown that these two features cannot be neglected in the indirect correlations. The arguments for explaining these features are similar to those which explain long range and nonadditivies of correlations in the liquid state.
The influence of surface motion on the direct subsurface absorption of H2 on Pd(111)
Authors: Olsen R. A. , Kroes G. J. , Lo/vvik O. M. , Baerends E. J.
Published: July 2002

Based on density functional theory (DFT) within the generalized gradient approximation (GGA) we have calculated a three-dimensional (3D) potential energy surface (PES) for H2+Pd(111), depending on two hydrogen molecular degrees of freedom and one palladium surface degree of freedom. The PES is then used in 3D quantum mechanical wave packet calculations to investigate the effect of the surface motion on the direct subsurface absorption. We also compare the full 3D calculations to calculations where the surface motion is included through the sudden approximation. The calculations show a large downward shift of the onset energy for direct subsurface absorption upon inclusion of palladium surface motion (from 0.74 eV to 0.40 eV for H2). The use of the sudden approximation works well at collision energies greater than 1.3 eV (for H2), but leads to a significant underestimation of the direct subsurface absorption probability at lower energies.
Dissociative scattering of polyatomic ions from a liquid surface: CF3+ on a perfluoropolyether film
Authors: Koppers Wim R. , Beijersbergen Jaap H. M. , Weeding Tina L. , Kistemaker Piet G. , Kleyn Aart W.
Published: July 2002

The dissociative scattering of CF3+ ions from a liquid insulating perfluorinated polyether surface was explored in the incident energy range 100–500 eV. We have measured the angular and energy distributions of the positive and negative fragment ions. The impulsive energy transfer to the liquid molecules and to internal energy of the scattered ions has been determined as a function of incoming energy and total scattering angle. The energy transfer in the ion/surface collision indicated scattering off the –CF3 groups of the liquid molecules. The dissociation of the CF3+ ions was due to a single impulsive collision with these groups. The parent ion was observed as well as CF2+, CF+, and C+ ions. The degree of dissociation depends only on the total scattering angle and the incoming energy, as in gas phase scattering. Remarkably, negatively charged F− fragment ions were also observed. These were formed in a process referred to as collisionally-induced “ion-pair” formation from neutrals leaving the surface. The important parameter which determined the internal energy uptake of the scattered ions in the collision was the translational energy loss ΔE the parent ion suffered. Finally, a comparison has been made between scattering from a liquid perfluorinated polyether surface and a metal Ag(111) surface.
Cooperative adsorption of network forming fluids onto crystalline surfaces: Structure and connectivity of the interface
Authors: Vakarin Eduard , Duda Yurij , Holovko Myroslav
Published: July 2002

Density profiles and first layer adsorption isotherms of a network forming fluid near a crystalline surface are investigated using the Percus–Yevick approximation for the associative Henderson–Abraham–Barker (HAB) equation. The fluid molecule is represented as a hard sphere with four independent attractive sites. The surface is described as a hard wall with a lattice of adsorbing sites. It is shown that the association induces a new type of cooperativity related to a bridging between the adsorbing sites due to tree-like clusters adsorption. The density profiles calculated exhibit reasonably good agreement with the Monte Carlo computer simulation data. The connectivity properties of the interface are studied and an interfacial percolation threshold is predicted to exist. The mean thickness τ of the adsorbed layer and the connectedness wall-particle correlation functions are calculated via the connectivity analogue of the HAB equation. It is shown that the adlayer may become infinitely thick with the mean cluster size S in the bulk being finite.
Field-cycling nuclear magnetic resonance relaxometry of thermoreversible polybutadiene networks
Authors: Kimmich R. , Gille K. , Fatkullin N. , Seitter R. , Hafner S. , Müller M.
Published: July 2002

Chain dynamics in thermoreversible polybutadiene networks were studied in comparison to linear polybutadiene using field-cycling NMR (nuclear magnetic resonance) relaxometry. The effect of rapidly fluctuating cross links on the chain mode relaxation time is shown to result in dynamics rescaled according to a new effective segmental friction coefficient. The frequency dependence of the spin-lattice relaxation time consists of a sequence of three regions characterized by power laws with exponents 0.5±0.05, 0.25±0.05, and 0.44±0.05 from high to low frequencies (and low to high temperatures). Thermoreversible cross-linking shifts the crossover frequencies toward lower values. In our previous work on linear polymers these frequency dependences were shown to be a consequence of the once (or twice) renormalized Rouse theory. The same formalism can be used for the thermoreversible polybutadiene networks by rescaling the segmental relaxation time.
An instantaneous normal mode theory of condensed phase absorption: The collision-induced absorption spectra of liquid CO2
Authors: Moore Preston , Space Brian
Published: July 2002

An instantaneous normal mode (INM) theory of condensed phase absorption spectra is presented. The present theory approximately includes contributions to the spectra from the imaginary frequency modes. The line-shapes of the absorptions are analyzed by way a motional narrowing parameter, Γ=Δτc. It is found that the entire intramolecular vibrational spectrum is motionally narrowed, and line-widths determined in this manner are in good agreement with intramolecular spectra obtained from time correlation function results. The low frequency, intermolecular vibrational spectrum is found to be in the motionally narrowed limit only below 50 cm−1. The frequency region from 50–300 cm−1 is intermediate between the slow and fast modulation limit. The far infrared (IR), collision-induced spectrum of dense liquid CO2 is calculated using the present theory and found to be in near quantitative agreement with experimental results.
Experiments on gas–liquid nucleation of sulfuric acid and water
Authors: Viisanen Yrjö , Kulmala Markku , Laaksonen Ari
Published: July 2002

We present measurements of binary homogeneous nucleation of sulfuric acid and water vapors at 298 K at two different relative humidities (38.2% and 52.3%), H2SO4 vapor concentrations ranging from about 1⋅1010 to 3⋅1010 molecules/cc. The experimental setup consisted of a turbulent mixing unit combined with a flow chamber where the nucleation took place. Particular attention was paid to the generation of sulfuric acid vapor. The measured nucleation rates vary between about 2 and 3000 cm−3 s−1. The results are compared with other measurements found in the literature. Our experimental rates are in agreement with predictions of the revised classical nucleation theory incorporating the hydration effect.
A systematic and feasible method for computing nuclear contributions to electrical properties of polyatomic molecules
Authors: Luis Josep M. , Duran Miquel , Andrés José L.
Published: July 2002

An analytic method to evaluate nuclear contributions to electrical properties of polyatomic molecules is presented. Such contributions control changes induced by an electric field on equilibrium geometry (nuclear relaxation contribution) and vibrational motion (vibrational contribution) of a molecular system. Expressions to compute the nuclear contributions have been derived from a power series expansion of the potential energy. These contributions to the electrical properties are given in terms of energy derivatives with respect to normal coordinates, electric field intensity or both. Only one calculation of such derivatives at the field-free equilibrium geometry is required. To show the useful efficiency of the analytical evaluation of electrical properties (the so-called AEEP method), results for calculations on water and pyridine at the SCF/TZ2P and the MP2/TZ2P levels of theory are reported. The results obtained are compared with previous theoretical calculations and with experimental values.
Prediction of bond dissociation energies and transition state barriers by a modified complete basis set model chemistry
Authors: Jungkamp Tim P. W. , Seinfeld John H.
Published: July 2002

The complete basis set model chemistries CBS-4 and CBS-q were modified using density functional theory for the geometry optimization step of these methods. The accuracy of predicted bond dissociation energies and transition state barrier heights was investigated based on geometry optimizations using the B3LYP functional with basis set sizes ranging from 3-21G(d,p) to 6-311G(d,p). Transition state barrier heights can be obtained at CBS-q with B3LYP/6-31G(d,p) geometries with rms error of 1.7 kcal/mol within a test set of ten transition state species. The method should be applicable to molecules with up to eight or more heavy atoms. Use of B3LYP/6-311G(d,p) for geometry optimizations leads to further improvement of CBS-q barrier heights with a rms error of 1.4 kcal/mol. For reference, the CBS-QCI/APNO model chemistry was evaluated and is shown to provide very reliable predictions of barrier heights (rms error=1.0 kcal/mol).
Product state(s)-resolved differential cross section of the reaction O(1D)+HD→OH(v,j)+D
Authors: Hsu Yen-Tsung , Liu Kopin
Published: July 2002

The Doppler-selected time-of-flight method was applied to map out directly the differential cross sections of the title reaction. These new results supersede those reported earlier from this laboratory using the conventional Doppler-shift technique. More significantly, clear structures are now revealed, which correspond to a series of subgroups of partially resolved states of OH(v,j). They offer considerable promise in unraveling the complicated dynamical attributes associated with the two microscopic pathways, insertion and abstraction, which play in this reaction.
Laser induced vibration–rotation fluorescence and infrared forbidden transitions in acetylene
Authors: Jungner Peter , Halonen Lauri
Published: July 2002

Laser induced fluorescence method has been used to study highly excited vibrational overtones in acetylene. A sample cell has been placed inside a Ti:Sapphire ring laser cavity and the total fluorescence collected by a parabolic mirror has been dispersed by a high-resolution FTIR spectrometer. The laser has pumped specific rotational states of the CH stretching vibrational overtone state ν1+3ν3(∑u+) and transitions to the symmetric state ν1+2ν3(∑g+) have been observed. The observations allow determining both the vibrational term value and the rotational constant of the symmetric state, which is not accessible from the ground vibrational state by one-photon absorption. The parameters obtained are in excellent agreement with simple local mode predictions.
A general methodology for quantum modeling of free-energy profile of reactions in solution: An application to the Menshutkin NH3+CH3Cl reaction in water
Authors: Truong Thanh N. , Truong Thanh-Thai T. , Stefanovich Eugene V.
Published: July 2002

We present a general methodology for calculating free-energy profile of reaction in solution using quantum mechanical methods coupled with the dielectric continuum solvation approach. Particularly, the generalized conductorlike screening model (GCOSMO) was employed in this study, though any continuum model with existing free-energy derivatives could also be used. Free-energy profile is defined as the steepest descent path from the transition state to the reactant and product channels on the liquid-phase free-energy surface. Application of this methodology to calculate the free-energy profile of the Menshutkin NH3+CH3Cl reaction in water is discussed. The efficiency of the GCOSMO method allows characterization of stationary points and determination of reaction paths to be carried out at less than 20% additional computational cost compared to gas-phase calculations. Excellent agreement between the present results and previous Monte Carlo simulations using a combined quantum mechanical/molecular mechanics (QM/MM) potential confirms the accuracy and usefulness of the GCOSMO model.
Ground-state intermolecular spectroscopy and pendular states in benzene–argon
Authors: Kim Wousik , Felker Peter M.
Published: July 2002

The results of nonlinear Raman spectroscopy at 0.03 cm−1 resolution on the intermolecular transitions of the perprotonated and perdeuterated isotopomers of benzene–Ar are reported. Five bands were observed for each species. For each, band-contour measurements were performed. These, when compared to simulated contours, allow for the unambiguous assignment of the bands in terms of the two intermolecular fundamentals, the two components of the first bending overtone, and the bend–stretch combination band. Further, it is shown that the measured contours are “pendular” contours arising from species in optical-field-induced pendular states. The results are discussed in terms of (a) prior experiments on and calculations of the intermolecular level structure of benzene–Ar, (b) the mechanisms giving rise to the Raman intensities of intermolecular transitions, and (c) their implications regarding the prevalence of pendular behavior in nonlinear Raman experiments on jet-cooled species.
Relaxation dynamics of H-bonded liquids confined in porous silica gels by Rayleigh wing spectroscopy
Authors: Carini G. , Crupi V. , D’Angelo G. , Majolino D. , Migliardo P. , Mel’nichenko Yu. B.
Published: July 2002

Reorientational dynamics of neat liquid propylene glycol molecules is compared with that of confined molecules in 25 and 75 Å pores of a sol-gel porous glass by means of depolarized light-scattering spectroscopy. The experimental spectra, performed as a function of temperature, show unambiguously the slowing down of the collective reorientational processes triggered by the confinement effects. Such results confirm the main role played by the strong dipolar interactions on the dynamics of hydrogen-bonded liquids diffusing within nanoscopic spherical pores.
Mass selected anion-zero kinetic energy photoelectron spectroscopy (anion-ZEKE): Ground and low excited states of FeO
Authors: Drechsler G. , Boesl U. , Bä\Smann C. , Schlag E. W.
Published: July 2002

Photodetachment-photoelectron (PD-PES) and anion-zero kinetic energy photoelectron (anion-ZEKE) spectra of FeO have been measured. The vibrational progression bands of the X 5Δi(FeO)←X 5Δ7/2(FeO−) transition in the PD-PES spectrum exhibit substructure which could not be resolved in earlier PD-PES spectra. A comparison with the high resolution anion-ZEKE spectrum clearly shows the existence of a second low energetic electronic state which could be the a 7Σ+ of neutral FeO proposed by several authors. In addition, for the A 5Σ+ state of FeO an excess energy of 4050 cm−1 was found. Vibrational frequencies for the X 5Δ, a 7Σ+, and A 5Σ+ states have been determined as 882, 887, and 800 cm−1. All spin orbit splittings of the neutral and anionic ground states could be measured directly or deduced from spin orbit combination transitions. We succeeded in resolving the rotational envelope of the vibrational origin of the neutral–anion ground states transition with indicated single rotational lines of the ΔJ =+3/2 branch.
Laser spectroscopy of the Ã 2Π←X̃ 2Σ+ transition of ytterbium monoacetylide
Authors: Loock Hans-Peter , Bérces Attila , Simard Benoit , Linton Colan
Published: July 2002

The first spectroscopic identification and characterization of ytterbium monoacetylide (YbCCH) is reported. By combining resonance-enhanced two photon ionization (R2PI), laser-induced fluorescence (LIF), and photoionization efficiency spectroscopy (PIE) with density functional calculations the X̃ 2Σ+ and the Ã 2Π1/2,3/2 states of YbCCH as well as the X̃ 1Σ+ state of YbCCH+ have been characterized. The Ã 2Π1/2,3/2–X̃ 2Σ+ system whose 0-0 band for the Ã 2Π1/2 component lies at around 16 848 cm−1 for YbCCH has been studied at 0.3 cm−1 resolution. The excitation spectra, both R2PI and LIF are characterized by progressions involving the YbCC bending mode (ν5) whose wave number has been determined to be 96 and 103 cm−1 for the X̃ 2Σ+ and Ã 2Π1/2,3/2 state, respectively. The dispersed fluorescence spectra show a progression in the Yb-C stretching vibration with a wave number of ω(ν3)=328 cm−1. Density functional calculations confirmed the vibrational assignment and yielded a linear geometry for both the X̃ and Ã state of YbCCH as well as for the X̃ 1Σ+ state of the cation. Photoionization efficiency spectroscopy yielded an adiabatic ionization potential of 47 165(10) cm−1 [5.8477(12) eV]. Rydberg series converging to the 51 and 52 level of YbCCH+ were observed and combined with the appearance potentials led to ω(ν5)=97 cm−1 for the YbCC bending mode of YbCCH+.
Superexcited states of OCS probed by using photoelectron spectroscopy for autoionizing atomic sulfur
Authors: Hikosaka Yasumasa , Hattori Hideo , Hikida Takumi , Mitsuke Koichiro
Published: July 2002

Neutral dissociation of superexcited states of OCS has been studied by two-dimensional photoelectron spectroscopy using synchrotron radiation in the photon energy range of 14.2–16.8 eV. A two-dimensional spectrum exhibits noticeable features which are assigned as resulting from autoionizing transitions of excited atomic sulfur, S*, from Rydberg states converging to S+(2Do) to S+(4So). The precursor molecular states leading to S*+CO are considered to be multiple-electron-excited Rydberg states, OCS*(Dis), converging to OCS+ with 2Σ− and/or 2Δ symmetry. The electron signal counts due to autoionization of S* show enhancement at excitation photon energies for the Rydberg states, OCS*(RB), converging to OCS+(B̃ 2Σ+). These results support a predissociation mechanism for the formation of S*: conversion from OCS*(RB) to OCS*(Dis). The quantum yield for the predissociation is evaluated to be ∼1% at the photon energy corresponding to the 5sσ state of OCS*(RB).
Crossover Leung–Griffiths model and the phase behavior of dilute aqueous ionic solutions
Authors: Belyakov M. Yu. , Kiselev S. B. , Rainwater J. C.
Published: July 2002

A new parametric crossover model for the phase behavior of a binary mixture is presented that corresponds to the Leung–Griffiths model in the critical region and is transformed into the regular classical expansion far away from the critical point. The model is optimized to, and leads to excellent agreement with, isothermal vapor–liquid equilibrium data for dilute aqueous solutions of sodium chloride by Bischoff and co-workers. It then accurately predicts constant-composition phase equilibrium loci as measured by independent workers. This crossover model is therefore capable of representing the thermodynamic surface of ionic solutions in a large range of temperatures and densities around the critical points of vapor–liquid equilibrium.
Perturbation theory in the kinetics of first-order phase transitions
Authors: Kukushkin Sergey A. , Osipov Andrew V.
Published: July 2002

Perturbation theory is applied to the description of the kinetics of a first-order phase transition illustrated by thin film growth. A small parameter was chosen as the quantity 1/ic, where ic is the number of particles in a critical nucleus under maximum supersaturation in the course of the phase transition. Uniform convergence of the expansions obtained is reached through time renormalization. The cases of single- and multi-component condensation have been studied.
Vibrational eigenvalues and eigenfunctions for planar acetylene by wave-packet propagation, and its mode-selective infrared excitation
Authors: Liu Li , Muckerman James T.
Published: July 2002

Vibrational eigenvalues with estimated errors <5×10−2 cm−1 and their corresponding eigenfunctions for J=0 5D (planar) acetylene modeled by the Halonen–Child–Carter potential-energy surface are obtained using an energy-shifted, imaginary-time Lanczos propagation of symmetry-adapted wave packets. A lower resolution (∼4 cm−1) vibrational eigenspectrum of the system is also calculated by the Fourier transform of the autocorrelation of an appropriate wave packet. The eigenvalues from both approaches are in excellent agreement. The wave function of the molecule is represented in a direct-product discrete variable representation (DVR) with nearly 300 000 grid points. Our results are compared with the previously reported theoretical and experimental values. We use our 69 computed eigenstates as a basis to perform an optimal control simulation of selective two-photon excitation of the symmetric CH-stretch mode with an infrared, linearly polarized, transform-limited, and subpicosecond–picosecond laser pulse. The resulting optimal laser pulses, which are then tested on the full DVR grid, fall within the capabilities of current powerful, subpicosecond, and tunable light sources.