bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo study of a microscopic lattice model for microemulsions
Authors: Stockfisch Thomas P. , Wheeler John C.
Published: July 2002

A microscopic lattice model of microemulsion-forming ternary solutions has been studied by Monte Carlo simulation. Compelling evidence for three-phase equilibrium among oil-rich, water-rich, and bicontinuous microemulsion phases is reported. The simple two-surfactant bending energy term used in our earlier mean-field study of this model was not found to produce three-phase equilibrium in our simulations. Additional three-surfactant terms were found to be necessary, indicating that the bending energy term in a lattice model must be chosen with care. These new bending energies have a simple physical motivation. The results reported here are obtained for a particularly simple set of interaction energies, involving only a single nonzero bending energy. A method is presented for calculating the surface excess densities and interfacial tension from the simulation data. It yields a plausible excess surface density of surfactant and surface tensions among the coexisting phases in three-phase equilibrium that fall into the ‘‘ultralow’’ regime, about 1000 times smaller than typical oil–water surface tensions.
Theoretical studies of hydrogen-abstraction reactions from diamond and diamond-like surfaces
Authors: Chang Xiao Yan , Perry Martin , Peploski James , Thompson Donald L. , Raff Lionel M.
Published: July 2002

Reaction probabilities, cross sections, rate coefficients, frequency factors, and activation energies for hydrogen-atom abstraction from a hydrogen-covered C(111) surface have been computed using quantum wave packet and classical trajectory methods on the empirical hydrocarbon ♯1 potential hypersurface developed by Brenner. Upper bounds for the abstraction rates, activation energies, and frequency factors have been obtained for six different chemisorbed moieties on a C(111) diamond surface using a classical variational transition-state method. For the hydrogen-covered surface, the results of the wave packet/trajectory calculations give k(T)=1.67×1014 exp(−0.46 eV/kbT) cm3/mol s, which is about a factor of 2.9 less than the gas-phase abstraction rate from tertiary carbon atoms at 1200 K. The variational calculations show that the activation energies for hydrogen-atom abstraction vary from 0.0 to 1.063 eV. Some sp2-bonded hydrogen atoms can be removed in a barrierless process if adjacent to a carbon radical. In contrast, abstractions that produce a methylene carbon are associated with much larger activation energies in the range 0.49–0.82 eV. Abstraction from nonradical chemisorbed ethylene structures of the type that might be formed by the chemisorption of acetylene at two lattice sites is a particularly slow process with a 1.063 eV activation energy. Hydrogen abstraction from sp3 carbon atoms have activation energies ∼0.4 eV. The results suggest that phenomenological growth models which assume either an equilibrium distribution between surface hydrogen/H2 or a common abstraction rate for surface hydrogen atoms are unlikely to be accurate.
The electric dipole moment of NO A 2Σ+ v′=0 measured using Stark quantum-beat spectroscopy
Authors: Gray J. A. , Farrow R. L. , Durant J. L. , Thorne L. R.
Published: July 2002

We report measurements and analysis of Stark quantum beats observed in the fluorescence of nitric oxide (NO) from which we determine the electric dipole moment of the A 2Σ+ v′=0 state. A pulse-amplified cw dye laser was used to excite the A–X (0,0) Q1(1) transition of 14N16O in electric fields up to 22.5 kV/cm. Fourier analysis of the time-resolved laser-induced fluorescence signals yielded Stark tunings for each of the six ‖MF‖ hyperfine sublevels in the N=1, J=3/2 spin-rotational level. The measurements were fit to a model Hamiltonian including fine, hyperfine, and Stark matrix elements. The resulting dipole moment was then corrected for polarizability effects to yield a value for the A 2Σ+ v′=0 state of μA=1.08±0.04 D. This result compares favorably to a previous measurement of μA in v′=3 and to our quantum theoretical calculations of the A 2Σ+ v′=0 state reported here.
Resonant excitation of low frequency modes in electron scattering by physisorbed O2 on Pt(111)
Authors: S̆iller L. , Palmer R. E. , Wendelken J. F.
Published: July 2002

The lineshape of the v=0–1 vibrational mode in the high resolution electron energy loss spectrum of physisorbed O2 on Pt(111) shows discrete loss peaks attributed to a low frequency molecule-surface vibration and its overtones. The energy and angular dependence of these vibrations is consistent with the molecular negative ion resonance mechanism predicted by Gadzuk.
The spectroscopy and dynamics of the n=3,4 Rydberg states in O+2
Authors: van der Kamp A. Barbara , Hiemstra Rob S. , van der Zande Wim J. , Fink R. , Jungen M.
Published: July 2002

An experimental and theoretical study is presented on the (4s) 2Σ+g, (3p) 2Σ+u, 2Πu, and (3d) 2Σ+g, 2Πg, 2Δg Rydberg states in O+2. Their spectroscopic properties (T0,ωe,ωexe) have been determined using translational spectroscopy. The Rydberg molecules are formed in collisions of fast O2+2 ions with atomic Cs, K, and Na. The potential energy curves of the Rydberg states have been calculated using a self-consistent field/averaged coupled pair functional (SCF/ACPF) method and agree well with the experimental results. The (3p) 2Σ+u state is strongly perturbed by the lowest valence state of the same symmetry. The Rydberg-valence interactions leading to the predissociative decay are discussed.
Response to ‘‘Comment on ‘State-specific unimolecular reaction of NO2 just above the dissociation threshold’ ’’ [J. Chem. Phys. 100, 4714 (1994)]
Authors: Miyawaki Jun , Yamanouchi Kaoru , Tsuchiya Soji
Published: July 2002

In this comment, the authors have given a response to the foregoing comment by Wittig and Ionov on our previous paper [J. Chem. Phys. 99, 254 (1993)] entitled ‘‘State-specific unimolecular reaction of NO2 just above the dissociation threshold.’’ Discussion was made by referring to recent theoretical papers on the NO2 photodissociation to harmonize the results obtained from the time-domain experiment with our observations by the frequency-domain experiment and to clarify the problem to be solved for further understanding dynamics of the unimolecular reaction dynamics.
Why quasiclassical cross sections can be rotationally and vibrationally hot
Authors: Mandy M. E. , Martin P. G. , Keogh W. J.
Published: July 2002

State-to-state cross sections for the collision-induced transitions of H+H2 (and isotopic analogs) have often been examined using quasiclassical trajectory calculations. The assignment of quasiclassical trajectory results to final rotational and vibrational states by histogram methods can lead to distributions for endoergic (upward) transitions that are rotationally or vibrationally hot when compared with quantum calculations or with experimental results. This effect is most pronounced near threshold for the transition of interest. Various histogram methods are examined to determine why they can lead to distributions that are hot. The application of detailed balance (microscopic reversibility) to cross sections calculated for the inverse of the endoergic transition results in distributions that are neither rotationally nor vibrationally hot; only those effects that can be attributed to the classical approximation of a quantum process remain.
A near-wing correction to the quasistatic far-wing line shape theory
Authors: Ma Q. , Tipping R. H.
Published: July 2002

A new representation is introduced in which the rapidly varying time-dependent part of the time displacement operator can be factored out and the remaining part, which varies with time more slowly, can be expanded in the usual perturbational fashion. The lowest order approximation leads to the far-wing quasistatic line shape theory developed previously, whereas the next order approximation, related to the noncommutation of the Liouville operators describing the unperturbed absorber and bath molecules and the interaction between them, leads to a near-wing correction. Explicit expressions are derived for both the corrections to the spectral density and the statistical band-average line shape function assuming an anisotropic dipole–dipole interaction. Detailed computations for the case of self-broadened H2O are carried out for the line-shapes and the corresponding absorption coefficients for several temperatures and for frequencies to 10 000 cm−1. From these results, we conclude that the near-wing corrections generally increase the line shape function between 10 and 200 cm−1, and that this increase is more important for lower temperatures than for higher ones. This in turn leads to increased absorption nearer the band centers, especially for lower temperatures, and thus to improved agreement between theory and experiment.
About the nature of intermolecular three-body forces in ionic systems: The case of protonated hydrates
Authors: Kelterbaum R. , Turki N. , Rahmouni A. , Kochanski E.
Published: July 2002

The three-body intermolecular energy is computed for the water/oxonium ion/water system from ab initio self-consistent-field (SCF) calculations and 170 geometrical configurations. It is decomposed into two parts: ΔE1stnonadd is obtained at the first iteration of the calculation and ΔEdelocnonadd is due to the following iterations till convergency. The nonadditive three-body energy, Eindnonadd, is computed from the perturbation theory. It is shown that all these energy components can be either attractive or repulsive according to the geometry. Edelocnonadd is often seriously different of Eindnonadd, a nonadditive ‘‘induction–exchange’’ term being sometimes as large as Eindnonadd. In most cases, this ‘‘induction–exchange’’ term is much larger than E1stnonadd, with the same sign. This suggests that these two terms can be approximated by a same expression, but because of the sign, more than one exponential term should be used. An approximation of Eindnonadd is discussed with a particular attention to the range of the validity according to the intermolecular distances.
Electronic structure and potential energy surfaces of the ethylene radical cation at and in the vicinity of the 90°-twisted form
Authors: Takahashi Ohgi , Kikuchi Osamu
Published: July 2002

The potential energy surfaces of the ethylene radical cation near the 90°-twisted form were clarified by an appropriate configuration interaction calculation. A characteristic conical intersection between the ground doublet and the first excited doublet states at a D2d 90°-twisted geometry was revealed. It was found that spin-charge separation occurs suddenly near the twist angle of 90°. The effect of substitution on the shape of the potential energy surfaces is also discussed.
Direct measurement of thermal rate constants for state-to-state rotational energy transfer in collisions of CN(X 2Σ+, v=2, N) with He
Authors: Fei R. , Lambert H. M. , Carrington Tucker , Filseth S. V. , Sadowski C. M. , Dugan C. H.
Published: July 2002

Stimulated emission pumping state preparation and laser induced fluorescence state detection in the CN(B 2Σ+–X 2Σ+) violet system have been employed to study relaxation of single rotational states of CN(X 2Σ+, v=2) in collisions with He at 295 K. Approximately 2/3 of the value of the total removal rate constant for CN(X 2Σ+, v=2, N=2, 11, or 14) corresponds to changes in the rotational quantum number of ‖ΔN‖≤3 and a strong propensity is evident which favors even changes in N for ‖ΔN‖≤4. The measured rate constants are consistent with detailed balance, can be described with statistical power-gap and exponential energy-gap fitting functions and, where compared, are in accord with an IOS-based scaling function. No significant difference is found between a sum of state-to-state rate constants and separately measured total removal rate constants for N=2, 11, and 14. The total removal rate constants decrease monotonically between N=0 and N=41 to about 1/3 of their value at N=0.
On the two reaction pathways of planar HCN isomerization
Authors: Ling Song
Published: July 2002

Although by symmetry the two reaction pathways of the planar HCN isomerization is equivalent for J=0, for nonzero J whether the two pathways remain equivalent is largely a dynamical, rather than a topological, question. This Note presents both theoretical analysis for the linearized dynamics near the fixed points, i.e., the saddles, of the effective potential surface for J not equal to zero (symmetry feature) and computational results of the actions of the periodic orbits near the saddles with energies high above the range of the validity of the linearized dynamics (dynamical feature).
Deexcitation of He(2 1P) in a collision with Ne
Authors: Kitajima Masashi , Hidaka Koji , Kusumori Hisashi , Ukai Masatoshi , Kouchi Noriyuki , Hatano Yoshihiko
Published: July 2002

Collisional deexcitation cross sections of He(2 1P) by Ne have been measured in a region of the mean collisional energy between 17 and 38 meV using a pulse-radiolysis method. Measured cross sections are in a range of 20–25 Å2 and increase with increasing collisional energy. The experimental cross sections are compared with the calculated ones for total and partial excitation transfer into highly excited Ne atoms. Fairly good agreement between calculated and experimental results indicates that in the deexcitation of He(2 1P) the excitation transfer into Ne*(6p3, 5s2, or 6s4) is important in the measured mean collisional energy region. It is also shown that a dipole–dipole interaction plays an important role in the excitation transfer from He(2 1P) to Ne.
Fluids in micropores. IV. The behavior of molecularly thin confined films in the grand isostress ensemble
Authors: Schoen Martin , Diestler Dennis J. , Cushman John H.
Published: July 2002

The behavior of molecularly thin prototypical confined films [Lennard-Jones (12,6) fluid constrained between two plane-parallel walls consisting of like atoms fixed in the fcc (100) configuration] is studied by Monte Carlo in a new (grand isostress) ensemble whose parameters are the thermodynamic state variables [temperature T, chemical potential μ, and normal stress (load) applied to the walls Tzz] controlled in the surface forces apparatus used to study lubrication experimentally on a molecular scale. Additional parameters of the ensemble not generally controlled in this experiment are the film–wall interfacial area A and the crystallographic alignment (registry, or shear strain α) of the walls. A multiplicity of phases is found to comport with a given choice of the parameters. The thermodynamically stable one minimizes the grand isostress potential (free energy). By means of thermodynamic integration the stable phase of the film is determined as a function of α at fixed T, μ, Tzz, and A. Solid films comprising integral numbers of layers of atoms parallel with the walls are stable when the walls are appropriately aligned. When such films are sheared (α is varied), they undergo drainage (imbibition) transitions at critical strains, whereupon the film loses (gains) whole layers. Depending on the conditions, the solid film may melt just prior to the transition.
Vibrationally assisted electronic desorption: Femtosecond surface chemistry of O2/Pd(111)
Authors: Misewich J. A. , Kalamarides A. , Heinz T. F. , Höfer U. , Loy M. M. T.
Published: July 2002

The process of desorption for the system of O2/Pd(111) under excitation by 100 fs pulses of visible light has been examined. Molecular desorption is found to occur with high efficiency and a nonlinear dependence on laser fluence. Direct time-domain measurements using a two-pulse correlation scheme reveal a dominant subpicosecond response together with a weaker, but significant correlation persisting for tens of picoseconds. These results imply a desorption process driven by the high electron temperatures produced by the femtosecond laser radiation. The slower component of the correlation response is interpreted as an enhancement of the desorption rate by adsorbate vibrational excitation.
On the field ionization spectrum of high Rydberg states
Authors: Baranov L. Ya. , Kris R. , Levine R. D. , Even U.
Published: July 2002

We discuss the spectrum of very high Rydberg states as detected via ionization in weak external electric fields. For the conditions of interest, namely, states just below the ionization continuum and weak fields, the classical barrier to dissociation is extremely far out from the core. About the saddle point the potential is very shallow. It is concluded that ionization by tunneling is far too slow. Only electrons whose energy is above the classical barrier can be detected via ionization. However, not all electrons which energetically can ionize will necessarily do so. Electrons may fail to ionize if the fraction of their energy which is in the direction perpendicular to the field is high. The computed fraction of electrons which fails to ionize does depend, in a sensitive way, on the diabatic vs adiabatic switching on of the external field. More experiments and theoretical work is needed on this point. A classical procedure based on the adiabatic invariance of the volume in phase space is developed for the computation of the fraction of electrons that can surmount the classical barrier for a given field. Analytically exact results are obtained for adiabatic switching and for the sudden limit where the rise time of the field is shorter than the period of the orbit. For the case of diabatic switching (which is appropriate for very high n values), the exact classical computations on the yield of ionization show that the onset of ionization is at an energy of 4.25 F1/2 cm−1 below the ionization potential and the 50% point it as 3.7 F1/2 cm−1 for a field F in V/cm.
A study of the singlet–triplet perturbations in the Ã 1A
u state of acetylene by high resolution ultraviolet spectroscopy
Authors: Drabbels Marcel , Heinze Johannes , Meerts W. Leo
Published: July 2002

Laser-induced fluorescence spectra of the 330 K10 and 340 K10 vibronic bands of the Ã 1Au←X̃ 1Σ+g transition in acetylene have been recorded with a resolution of 18 MHz. Each rotational transition consists of a group of lines due to coupling of the electronically excited singlet state with isoenergetic triplet states. Using the standard deconvolution procedure the singlet–triplet coupling elements and the density of coupled triplet states are derived for rotational levels up to J=4 in both bands. From the density of coupled triplet states it is concluded that the Ã 1Au state is perturbed by the T1 3B2 state. Magnetic field measurements have shown that the predissociation of acetylene in the 4ν3′ vibrational level of the Ã state is caused by a coupling via the T1 3B2 state with predissociating vibrational levels of the electronic ground state.
Micelle formation, relaxation time, and three-phase coexistence in a microemulsion model
Authors: Stauffer Dietrich , Jan Naeem , He Yan , Pandey Ras B. , Marangoni D. Gerrard , Smith-Palmer Truis
Published: July 2002

Our Larson-type microemulsion model for surfactant chains in oil–water solvents leads to long relaxation times as well as, for essential modifications, to a stable peak in the chain-cluster size distribution. Transfer energies for surfactant chains moving to the oil–water interface, and characteristic micelle concentrations (CMC) as a function of chain length are compared with experiment.
Experimental study on three chemical oscillators coupled with time delay
Authors: Nishiyama Nobuaki , Eto Kaori
Published: July 2002

A new experimental system of coupled chemical oscillators is presented. Three cation-exchange beads loaded with ferroin are spatially distributed in triangular forms and immersed in the Belousov–Zabotinsky reaction mixture. The cation-exchange beads were in no contact with each other. The cation-exchange bead was used as a chemical oscillator. Spontaneous switching between two out-of-phase oscillation was observed.
Photodissociation of acetyl chloride: Cl and CH3 quantum yields and energy distributions
Authors: Deshmukh Subhash , Hess Wayne P.
Published: July 2002

Jet cooled acetyl chloride is dissociated at 236 nm via excitation of the 1[n, π* (C=O)] transition. Chlorine atom photofragments Cl(2P3/2) and Cl*(2P1/2) are detected using (2+1) multiphoton ionization. The relative Cl* yield φ*=[Cl*]/([Cl]+[Cl*]) is measured to be 0.4±0.02, and the mean translational energy of the Cl atoms is 0.28±0.02 eV. Methyl fragments are also detected following primary dissociation and the internal state distributions are probed using (2+1) multiphoton ionization through the 4pz Rydberg state at 286 nm and the 3pz Rydberg state at 334 nm. The quantum yield for CH3 formation is estimated to be 28%, relative to the CH3 yield from methyl iodide photolysis. A small fraction of the available energy is channeled into methyl fragment angular momentum as the rotational state distribution extends only to N″=5. Our results indicate that CH3CO, generated as a primary photoproduct in the dissociation of acetyl chloride, subsequently decomposes to produce CH3 and CO. Energetic constraints in the mechanism of acetyl chloride dissociation are discussed.
Spectra and relaxation of Hg atoms and molecules in low temperature matrices. II. H2O/Ar and NH3/Ar matrices
Authors: Crépin C. , Tramer A.
Published: July 2002

The absorption, excitation and emission spectra are recorded for Hg⋅NH3 and Hg⋅H2O/D2O complexes and Hg(NH3)n and Hg(H2O)n/(D2O)n clusters in argon and krypton matrices. The excited states correlated to the 3P1 state of Hg were selectively pumped. The 3P1■3P0 nonradiative relaxation of HgM (HgMn) systems is so efficient that the 3P1→1S0 emission is completely quenched. The excited systems decay by the 3P0→1S0 emission, this radiative transition being weakly allowed. As evidenced by the broad band exciplex emission, these systems are strongly bound in their excited states correlated to the 3P0 state of Hg, while the interaction of ground-state Hg atoms with water and ammonia is weak. The existence of specific Hg...(H2O)n sites with the emission spectra involving a coupling between electronic transition in Hg and intramolecular modes of the H2O (D2O) molecule is reported.
Multiphoton ionization of uranium hexafluoride
Authors: Armstrong D. P. , Harkins D. A. , Compton R. N. , Ding D.
Published: July 2002

Multiphoton ionization (MPI) time-of-flight mass spectroscopy (TOFMS) and photoelectron spectroscopy (PES) studies of UF6 are reported using focused light from the Nd:YAG laser fundamental (λ=1064 nm) and its harmonics (λ=532, 355, or 266 nm), as well as other wavelengths provided by a tunable dye laser. The MPI mass spectra are dominated by the singly and multiply charged uranium ions rather than by the UF+x fragment ions, even at the lowest laser power densities at which signal could be detected. In general, the doubly charged uranium ion (U2+) intensity is much greater than that of the singly charged uranium ion (U+). For the case of the tunable dye laser experiments, the Un+ (n=1–4) wavelength dependence is relatively unstructured and does not show observable resonance enhancement at known atomic uranium excitation wavelengths. The MPI-PES studies reveal only very slow electrons (≤0.5 eV) for all wavelengths investigated. The dominance of the U2+ ion, the absence or very small intensities of UF+x (x=1–3) fragments, the unstructured wavelength dependence, and the preponderance of slow electrons all indicate that mechanisms may exist other than ionization of bare U atoms following the stepwise photodissociation of F atoms from the parent molecule. The data also argue against stepwise photodissociation of UF+x (x=5,6) ions. Neither of the traditional MPI mechanisms (‘‘neutral ladder’’ or the ‘‘ionic ladder’’) are believed to adequately describe the ionization phenomena observed. We propose that the multiphoton excitation of UF6 under these experimental conditions results in a highly excited molecule, superexcited UF6**. The excitation of highly excited UF6** is proposed to be facilitated by the well known ‘‘giant resonance,’’ whose energy level lies in the range of 12–14 eV above that of ground state UF6. The highly excited molecule then primarily dissociates, via multiple channels, into Un+, UF+x, fluorine atoms, and ‘‘slow’’ electrons, although dissociation into F− ions is not ruled out.
Reassignment of the vibrational structure of the Huggins absorption band of ozone
Authors: Joens Jeffrey A.
Published: July 2002

A new assignment of the vibrational structure of the Huggins band of ozone is given. The assignment is consistent with the symmetry requirements on changes in the asymmetric stretching (v3) quantum number and with data on the differences in the location of vibronic bands between 16O3 and 18O3. Based on the reassignment of the spectrum, new values for the vibrational constants for 16O3 and 18O3 in the excited electronic state responsible for the Huggins system are calculated. The reassignment of the Huggins system provides a simple explanation for several otherwise puzzling observations concerning the spectrum, and suggests that the Huggins and Hartley band systems arise from transitions to different electronic states, in accord with previous molecular orbital calculations.
Laser ionization mass spectrometry of homogeneous and binary molecular clusters of nitric oxide
Authors: Desai Sunil R. , Feigerle C. S. , Miller John C.
Published: July 2002

A potpourri of binary heteroclusters of the form (NO)m(Y)n has been investigated by coexpansion of the two components (along with argon) in a supersonic nozzle. They are subsequently ionized by picosecond, nonresonant, two-photon absorption and characterized by mass spectrometry. Neat (NO)+m clusters exhibit an odd–even intensity alternation which is attributed to electron pairing effects. A varied set of partners for the binary nitric oxide clusters includes N2O, NO2, H2O, CO2, CS2, SO2, and SF6. The observed cluster distributions are discussed in terms of cluster formation and decay mechanisms and relative bond strengths. Possible structures for some especially stable species are discussed. Of special interest are the observations of especially stable (NO)+3(Y)n compounds where Y represents the isovalent N2O, CO2, or CS2 partners. SF6 and its clusters also form notably stable clusters with the nitric oxide trimer ion.
Ultrafast infrared pump–probe spectroscopy of water: A theoretical description
Authors: Bratos S. , Leicknam J-Cl.
Published: July 2002

A theory is developed to describe a recent infrared pump–probe experiment in water. This theory is a statistical theory, similar to those elaborated earlier to interpret ultraviolet and visible spectra. Nonlinear optical processes involved are analyzed in terms of four-time correlation functions of the electric dipole moment of the system and of the incident electric fields, respectively. An analytical expression for the transient infrared signal is presented. The observed spectral characteristics are attributed to a gradual thermalization of the initial, pump-prepared state. The substructure of the observed bands is interpreted. This experiment probes the kinetics of conversion of H-bonds of different lengths into each other.
Quantum flux analysis of the scattering of oriented NO molecules from surfaces
Authors: Lemoine Didier
Published: July 2002

We present a time-resolved quantum flux analysis of oriented NO collisions with flat Ag,Pt surfaces. We adopt both the molecular and the adiabatic viewpoints. This enables us to understand the basic mechanisms that allows one to correlate the final steric effects with the initial molecular orientation. We find that rotational excitation is always enhanced (diminished) when the O (N) end of the molecule is initially directed towards the surface, in qualitative agreement with the experimental observations of Kleyn and co-workers. Our simulations illustrate the two limiting regimes of rotational excitation and stereospecificity. One is sudden and implies that the initial orientation is conserved as the molecule strongly interacts with the surface. The other one is adiabatic and includes reorientation effects. The adiabatic description proves to be invaluable in complementing a molecular analysis in surface scattering.
Ultraviolet photodissociation dynamics of methyl iodide at 333 nm
Authors: Fairbrother D. Howard , Briggman K. A. , Weitz Eric , Stair P. C.
Published: July 2002

The ultraviolet (UV) photodissociation dynamics of methyl iodide at 333.45 nm has been studied using resonantly enhanced multiphoton ionization in conjunction with a time-of-flight mass spectrometer (REMPI-TOFMS). At this wavelength methyl iodide photolysis involves a transition with significant parallel and perpendicular character. Both ground (I) and spin–orbit (I*) excited iodine result from a parallel transition to the 3Q0 state. In contrast, the perpendicular transition, which most likely involves initial excitation to the 3Q1 state, leads exclusively to production of I. Based on data from this study, an upper limit for the conical intersection between the 3Q0 and 1Q1 potential surfaces is ∼30 500 cm−1.
Gas-phase solvation of NO+, O+2, N2O+, N2OH+, and H3O+ with N2O
Authors: Hiraoka Kenzo , Fujimaki Susumu , Aruga Kazuo , Sato Tetsuya , Yamabe Shinichi
Published: July 2002

Gas-phase clustering reactions of NO+, O+2, N2O+, N2OH+, and H3O+ with N2O were measured with a pulsed electron-beam high pressure mass spectrometer. The bond in NO+(N2O)n is found to be electrostatic, while those in O+2(N2O)1, N2O+(N2O)1, and N2OH+(N2O)1 have covalent character. The observed n dependence of −ΔH0n−1,n for the clustering reactions suggests that the cluster ions have the structures of the core ion plus ligand molecules, i.e., NO+(N2O)3(N2O)n−3, O+2N2O(N2O)n−1, (N2O)+2(N2O)n−1, H+N2O(N2O)1(N2O)n−1, and H3O+(N2O)3(N2O)n−3. The N2O molecule forms stronger bonds with NO+, O+2, and H3O+ ions than the isoelectronic CO2 molecule, indicating that N2O is a stronger nucleophilic reagent than CO2.
Large amplitude skeletal isomerization as a promoter of intramolecular vibrational relaxation in CH stretch excited hydrocarbons
Authors: McIlroy Andrew , Nesbitt David J.
Published: July 2002

The high resolution, slit jet cooled infrared v=1←0 methyl asymmetric stretch spectra of trans-2-butene and 1-butene are reported. Both of these molecules are singly unsaturated butene chains, have 30 vibrational degrees of freedom, and yield nearly equivalent vibrational state densities (ρvib≊200 states/cm−1) at CH stretch levels of excitation. The key difference between these two molecules is the presence of a large amplitude C–C–C skeletal torsional coordinate in 1-butene corresponding to a low barrier, internal isomerization pathway which is completely absent in trans-2-butene. The trans-2-butene asymmetric CH stretch (ν16) spectrum is fully discrete at 0.002 cm−1 resolution, and the coarse structure readily assigned to zero order rovibrational transitions (J′K′aK′c ← J″K″aK″c) in an asymmetric top. Fragmentation of these zero order transitions into spectral ‘‘clumps’’ of fine structure provides direct evidence for coupling of the CH stretch to vibrational bath states, but no evidence for loss of Ka′ and Kc′ as good quantum labels in the spectrum.

The average density of coupled states is found directly from the spectrum to be 114 states/cm−1, i.e., on the order of 0.5 ρvib. In contrast to the behavior in trans-2-butene, the 1-butene v=1←0 methyl asymmetric stretch spectrum exhibits an essentially continuous absorption contour even at Trot=6 K and 0.002 cm−1 resolution. On closer inspection, the 1-butene spectral envelope exhibits reproducible, intramolecular vibrational relaxation (IVR) induced fine structure limited by apparatus resolution and characteristic of highly congested IVR coupling. Analysis of this fine structure indicates a density of coupled states on the order of 1 000–10 000 states/cm−1, i.e., 20–30-fold in excess of ρvib, and 1–2 orders of magnitude larger than observed in trans-2-butene. In order to model the degree of fine structure observed in the spectrum, this level of spectral congestion essentially requires complete mixing of all ρvib⋅(2J′+1) rovibrational states consistent with conservation of total energy and angular momentum. The qualitatively dramatic differences between 1-butene and trans-2-butene behavior support a simple model for strong vibration-rotation (V-R) coupling in the bath states due to large amplitude skeletal motion in the C–C–C torsional mode which greatly enhances the available state density for IVR. Hence, the presence of a low barrier, skeletal isomerization coordinate may prove to be a general, moiety specific promoter for IVR processes in CH stretch excited hydrocarbons.
Activated rotational diffusion in solids
Authors: Georgievskii Yu. , Burshtein A. I.
Published: July 2002

Rotation of a molecule in a solid is strongly suppressed due to the corrugated potential created by the crystal and is reduced to small librations around quasistable positions with infrequent jumps between them. We study how the interaction with the crystal vibrations influences the rotational relaxation of the molecule. A simple model, in which the molecular rotation is described by a 1-D Langevin equation, is considered. The analytic expressions for the orientational and angular momentum correlation times are obtained. The Hubbard relation, which was established for dense gases and liquids, is shown to be valid for solids in the intermediate-to-high damping regime and to be violated at sufficiently low damping.
Proton transfer in mesoscopic, molecular clusters
Authors: Consta Styliani , Kapral Raymond
Published: July 2002

Proton transfer within a proton–ion complex embedded in a liquid cluster composed of dipolar molecules is studied. The clusters under investigation are large with dimensions in the nanometer regime and contain tens of solvent molecules. Such mesoscopic clusters possess properties that differ from those of either bulk liquids or small clusters containing a few solvent molecules. The free energy along the reaction coordinate for a quantum proton in a classical cluster solvent is computed using path integral techniques and the results are compared with fully classical treatments of the system. The mechanism of proton transfer as reflected in the solvent structure for given values of the reaction coordinate is examined. The solvent structure that exists in the cluster as a result of the surface forces and finite size gives rise to a coupling between the solvent and the proton–ion complex that differs from that in the bulk. This leads to a mechanism of the transfer process that involves gross solvent density shifts and orientational changes distinct from those seen in bulk polar liquids.
Competitive reversible binding: A theoretical study of density effects on the long-time relaxation
Authors: Naumann Wolfgang
Published: July 2002

Competitive reversible binding is studied for a one-dimensional continuum model. Taking the reaction as a stochastic two-state process (free or occupied binding site), from many-body equations by superposition a closed system of three coupled reactive equations is derived. Linearized versions of these equations are used to get low density approximations for the long-time kinetics. Starting point is an approximation (a) from which a Szabo–Zwanzig type t−1/2 long-time law can be followed. On the basis of an approximate relation between the state-specific distribution functions obtained in (a), a higher order in density approximation (b) is derived which prescribes a concentration effect on the long-time kinetics. According to (b) the t−1/2 law is also asymptotically valid for t→∞, but with a different concentration-dependent amplitude. For time windows in an intermediate long-time range the relaxation to equilibrium appears as satisfying a modified power law (∝ t−α with α≠1/2). These analytic results, which are interesting with respect to deviations from a predicted t−3/2 decay law observed for photoexcited proton transfer in water [D. Huppert et al., Phys. Rev. Lett. 68, 3932 (1992)], are related to recent Brownian simulations of the pseudounimolecular reaction.
Nonadiabatic transitions in the photodissociation of HCl
Authors: Gersonde I. H. , Hennig S. , Gabriel H.
Published: July 2002

Nonadiabatic dynamics accompanying the photodissociation of HCl via excitation X 1Σ+→A 1Π have been investigated. The energy-dependent branching ratio 2P1/2/2P3/2 of the Cl fragment resulting from the ‘‘exact’’ numerical solution of the time-dependent nuclear Schrödinger equation is compared to the branching ratio gained from a classical-path ansatz. Different sets of potential energy functions taken from previous investigations are used. It is shown that discrepancies between the results of two recent papers are due to substantial differences concerning the nonadiabatic transition moments between the corresponding potential functions.
A density functional study of small alkali halide systems
Authors: Modisette J. , Lou L. , Nordlander P.
Published: July 2002

The electronic structures of lithium-, sodium-, and potassium-containing alkali halide monomers and dimers are calculated using the local density method. In particular, for lithium and sodium halides, the calculated equilibrium bond lengths and vibrational frequencies are in excellent agreement with experimental gas-phase results. For potassium halides, we find small but systematic deviations between calculated properties and experiment. Inclusion of nonlocal exchange corrections improves the agreement between theory and experiment for these systems. The investigation shows that the density functional approach provides an efficient and accurate method for the calculation of the electronic structure of alkali halides.
Structure and intermolecular interactions in fluid ammonia: An investigation by neutron diffraction at high pressure, statistical-mechanical calculations and computer simulations
Authors: Bausenwein T. , Bertagnolli H. , David A. , Goller K. , Zweier H. , Tödheide K. , Chieux P.
Published: July 2002

Neutron scattering experiments on deuterated ammonia were performed in the overcritical region at high pressure. The structure factors and the radial distribution functions were determined at a temperature of 449 K at four densities extending from 0.318 to 0.700 g cm−3. With increasing density the intramolecular structure changes as the covalent bond becomes slightly longer and the angle between D–N–D is getting larger. At the two highest densities a shoulder at about 2.5 Å appears in the radial distribution function which cannot be assigned to a specific atom pair. The experimental radial distribution functions were compared with theoretical results obtained from the site–site-Ornstein–Zernike-(SSOZ) equation in combination with the HNC closure (XRISM). A series of potentials were tested and a site–site Lennard-Jones potential was fitted to the experimental results. The reverse Monte Carlo technique was used to calculate the atom pair correlation and angular distribution functions by minimizing the deviation of the radial distribution function of a configuration from the experimentally determined function. The agreement between SSOZ calculations and simulations are quite good in the overcritical region. The angular distribution functions show no significant features, but with increasing density a parallel orientation of the molecules become more unlikely.
A model classical study of nonlinear resonance and torsional isomerization
Authors: Schranz Harold W. , Collins Michael A.
Published: July 2002

A simple two degrees of freedom classical model of the nonlinear resonance interaction between symmetric bond bending and torsional motion in linearly bonded tetra-atomic molecules is investigated. The rate and mechanism of intramolecular vibrational energy redistribution at different relative vibrational frequencies is evaluated, and comparison is made with quantum calculations. The rate of torsional isomerization is compared to the prediction of transition state theory, and related to the observed intramolecular vibrational energy redistribution.
Effects of X3 frame vibrations on the CH chromophore in CHX3 type molecules: Harmonic coupling in curvilinear internal coordinate approach
Authors: Kauppi Esa
Published: July 2002

Effects of harmonic coupling between X3 frame vibrations and the CH stretching and bending vibration chromophore in CHX3 type molecules have been investigated using a curvilinear internal coordinate model. The CH chromophore Hamiltonian used consists of a Morse oscillator Hamiltonian for the CH stretching vibration, two-dimensional isotropic harmonic oscillator Hamiltonian for the CH bending vibration, anharmonic bending terms, and Fermi resonance interaction terms between the stretching and bending vibrations. Van Vleck perturbation theory is used to take the harmonic coupling terms between the CH chromophore and the X3 frame vibrations into account. Analytical matrix elements for the perturbation theory contributions have been derived. The model has been applied to vibrational energy level and infrared absorption intensity data for CHD3. Potential energy parameters for the CH chromophore have been optimized by a nonlinear least-squares method. The model reproduces the experimental data well. Only small changes in the potential parameters are observed when the coupling terms due to D atom motions are introduced into the model. The overall conclusion is that it is a good first approximation to ignore the effects of harmonic coupling between the CH chromophore and the X3 frame vibrations in the curvilinear internal coordinate model for the CH chromophore in CHX3 type molecules.
Interaction of ammonia molecules with the MgO(100) surface: Application to the measure of the effective ionic surface charge
Authors: Sidoumou M. , Panella V. , Suzanne J.
Published: July 2002

The interaction of ammonia molecules with the (100) surface of MgO has been investigated on uniform powders and single crystal surfaces using volumetric isotherm and low energy electron diffraction measurements in the temperature ranges of 170–205 and 90–120 K, respectively. The kinetics of condensation and the isosteric heat of adsorption have been measured. The results show that there is a strong repulsive interaction between NH3 molecules within the adsorbed monolayer. No ordered structure has been found by low energy electron diffraction up to monolayer coverage. The comparison of our experimental value of the isosteric heat of adsorption at low coverage and at monolayer completion with the calculated value using semiempirical potentials found in the literature leads to an estimated value of the effective surface ionic charge q/e=1.25.
Frequency shifts in infrared spectra of ethylene clusters
Authors: Buck Udo , Schmidt Burkhard
Published: July 2002

Based on structure calculations of small ethylene (C2H4)n clusters published previously [R. Alrichs et al., Z. Phys. D 15, 341 (1990)], shifts and splittings of the fundamental excitation frequency of the ν7 mode are calculated for the dimer, trimer, and tetramer. Using a first order perturbation approach, we find blue shifts in the order of 1–3 cm−1 which compare well with experimental findings. It is shown that the shifts are approximately independent of the cluster size and of the isomeric structure (ringlike or chainlike) of the ethylene complexes.
An efficient cluster elongation method in density functional theory and its application to poly-hydrogen-bonding molecules
Authors: Aoki Yuriko , Suhai Sándor , Imamura Akira
Published: July 2002

The elongation method to synthesize the electronic states of polymers is developed at the level of the density functional theory using the linear combination of Gaussian-type orbitals local spin density method. In this treatment, the interactions between the localized molecular orbitals of a cluster and the canonical molecular orbitals of an attacking monomer are successively included, where the Kohn–Sham equation is self-consistently solved instead of the Hartree–Fock equation in the conventional ab initio method. In the process of the cluster extension, an efficient treatment is implemented to calculate the matrix elements of Coulomb integral and exchange-correlation potential. The reliability and the efficiency of this method are examined via applications to hydrogen molecule cluster, linear water cluster (H2O)n and formamide cluster (CHONHH2)n. It was shown that the present method saves significantly the computational time and disk storage in the large cluster calculations, and provides good agreements with the results by the conventional density functional treatment for the whole system.
High-resolution analysis of the fundamental bending vibrations in the Ã 2Π and X̃ 2Σ+ states of CaOH and CaOD: Deperturbation of Renner–Teller, spin–orbit and K-type resonance interactions
Authors: Li Mingguang , Coxon John A.
Published: July 2002

The v2=1 bending vibrational levels of the Ã 2Π and X̃ 2Σ+ states of both CaOH and CaOD have been rotationally analyzed via laser excitation of the Ã(010) κ 2Σ, 2Δ, μ 2Σ←X̃(010)2Π, (000)2Σ+ bands. The estimated measurement accuracy for rotational transitions is 0.0035 cm−1. K-type resonances and level crossings have been identified between the Ã(010) 2Σ and 2Δ vibronic components, and account for the observation of dramatic intensity anomalies from quantum mechanical interference. An effective Hamiltonian has been constructed to model the Renner–Teller, spin–orbit, and K-type resonance interactions in the Ã(010) vibronic manifold, and to fit all the observed bands simultaneously for each isotopomer. The fundamental bending frequencies and Renner–Teller parameters have been determined: ν2(X̃)=352.9259(9), ν2(Ã)=363.1736(8) and εω2=−36.2634(6) cm−1 for CaOH; ν2(X̃)=266.8398(10), ν2(Ã)=274.6475(5), and εω2=−26.9601(8) cm−1 for CaOD. The isotope relations between the fitted molecular constants of CaOH and CaOD are examined. The spin-rotation constant γ and the centrifugal distortion constant AD of the spin–orbit coupling have been separated in the Ã 2Π(010) level owing to the Renner–Teller effect. The equilibrium bond lengths and force constants in the X̃ 2Σ+ state have been derived based on the data of the two isotopomers. The Coriolis coupling constants have been derived from the harmonic force field, and yield calculated l-type doubling constants that are in excellent agreement with the experimental values for both CaOH and CaOD.
Layering, dewetting, and first-order wetting in ternary amphiphilic systems
Authors: Gompper G. , Hennes M.
Published: July 2002

A Ginzburg–Landau theory with a single scalar order parameter is used to study the wetting behavior in ternary mixtures of oil, water, and amphiphile. In particular, interfacial tensions and the contact angle of the middle phase between oil-rich and water-rich phases are examined, when the system approaches a Lifshitz critical point or an ordinary critical point. We find a variety of unexpected phenomena near these points, such as layering transitions and an extremely nonmonotonic behavior of the contact angle. Our results can be understood with the help of an effective interface potential for the oil/water interface, which we calculate exactly as a function of the width l of the middle phase.
Detection of phase transitions in liquid crystals on a water surface by a Maxwell displacement current-measuring technique
Authors: Iwamoto Mitsumasa , Kubota Tohru , Muhamad Muhamad Rasat
Published: July 2002

The phase transitions of Langmuir films of liquid-crystal 4-cyano-4′-5-alkyl-biphenyl (5CB), 4-cyano-4′-7-alkyl-biphenyl (7CB), 4-cyano-4′-8-alkyl-biphenyl (8CB), and 4-cyano-4′-10- alkyl-biphenyl (10CB) were investigated by a Maxwell displacement current (MDC)-measuring technique. A weak first-order phase transition of the orientation of LC molecules from the isotropic phase to the polar one was clearly detected by compressing the film in the range of immeasurably low surface pressure by means of the technique. For further compression, a first-order phase transition between a stable monolayer phase and a stable three-layer phase of the liquid crystal film was detected. The result shows that the MDC-measuring technique is useful in the detection of phase transitions over the entire range of molecule areas.