bluuebunny/crossref_metadata_2025 huggingface dataset:
Variational transition state theory and tunneling for a heavy–light–heavy reaction using an ab initio potential energy surface. 37Cl+H(D) 35Cl→H(D) 37Cl+35Cl
Authors: Garrett Bruce C. , Truhlar Donald G. , Wagner Albert F. , Dunning Thom H.
Published: February 2003

Ab initio POL–CI calculations, augmented by a dispersion term, are used to predict the potential energy surface for the reaction Cl+HCl. The saddle point is found to be nonlinear. The surface is represented by a rotated-Morse-oscillator spline fit for collinear geometries plus an analytic bend potential. Variational transition state theory calculations, based on a linear reference path, are carried out, and they yield much smaller rate constants than conventional transition state theory, confirming that earlier similar results for this heavy–light–heavy mass combination were consequences of the small skew angle and were not artifacts of the more approximate potential energy surfaces used in those studies. Transmission coefficients are calculated using approximations valid for large-reaction-path curvature and the potential along the reference path is scaled so that the calculated rate constant agrees with experiment. The resulting surface is used to compute the H/D kinetic isotope effect which is in qualitative agreement with experiment.
Resonant multiphoton ionization of H2 via the B 1Σ+u, v=7, J=2 and 4 levels with photoelectron energy analysis
Authors: Pratt S. T. , Dehmer P. M. , Dehmer J. L.
Published: February 2003

We report measurements of photoelectron spectra of an electronically excited state of H2 (B 1Σ+u) in selected vibrational and rotational levels. The excited state was prepared by resonant three photon excitation of H2 X 1Σ+g, v=0,  J=3 and subsequently was ionized by a single additional photon. Partially resolved rotational structure is observed in the photoelectron spectra and is discussed in terms of the selection rules for direct photoionization and the partial wave composition of the ejected photoelectrons. An additional peak is observed in the spectra, which is tentatively interpreted as arising from photoionization of H*(n=2) formed by photodissociation of H2 in the B 1Σ+u state.
Systematics in two-photon rovibrational contours of 141 (1B2u←1A1g) in benzene derivatives
Authors: Thakur Surya N. , Goodman Lionel
Published: February 2003

Two-photon rotational contours of the principal vibronic band (1410) in the 1B2u←1A1g transition in benzene and its s-C6H2D4, o-C6H4D2, C6H5F, p-C6H4F2, o-C6H4F2 derivatives have been recorded by multiphoton ionization at room temperature with a laser excitation bandwidth of 0.1 cm−1. The main peaks in the rotational contour are shown to result from multiple coincidences of rovibronic transitions due to ‘‘oblate rotorlike’’ and ‘‘prolate rotorlike’’ behavior of the asymmetric rotor molecules. The observed separations between these features agree very well with the predicted ones obtained from simple formulas. These systematics in the two-photon rotational contours are found to hold over a range of the asymmetry parameter from the least κ=1 to κ=−0.9. The theory is used to obtain the rotationless origin for each of the molecules studied.
A quasiclassical trajectory study of the F+HH⇄FH+H reaction
Authors: Ron Shlomo , Baer Michael , Pollak Eli
Published: February 2003

A detailed forward and reverse quasiclassical trajectory computation for the FHH reaction is presented. An adiabatic analysis of the results shows that to a large extent the differences between HF(v=3) and HF(v=2) product distributions are due to the existence of an exit channel adiabatic barrier for the v=3 state. A sideways peak in the angular distribution for HF(v=2, j) is found in the reverse quasiclassical computation. Total cross sections computed from reverse quasiclassical trajectories are in good agreement with the quantal lin reactive infinite order sudden approximation. We conclude that many of the discrepancies between forward quasiclassical results and quantal computations are not due to quantal resonances but rather to the large boxing of vibrational states.
Scavenging probability of a geminate pair in the presence of an external electric field. Application of the method of matched perturbation solutions
Authors: Sano Hisatake
Published: February 2003

An analytical expression for the scavenging probability of a geminate pair interacting with a general potential in the presence of an external electric field is derived for both small scavenger concentrations and small field strengths. The derivation is based on the method of matched perturbation solutions of the differential equation satisfied by the recombination probability. The expression includes as an integral part the rate constant observed for reaction of the geminate partners in the bulk. Agreement of the expression with the exact ones is found when it is applied to ion recombination and recombination of the electron photodetached from an anion.
Rotational relaxation of HF in a free-jet expansion of dilute HF–He mixtures: Information content on state-to-state rate constants
Authors: Gough T. E. , Miller R. E.
Published: February 2003

An F-center laser–molecular beam spectrometer has been used to provide detailed information concerning the translational and rotational relaxation resulting from the free-jet expansion of dilute mixtures of hydrogen fluoride in helium. The observed non-Boltzmann rotational state distributions are modeled using an empirical kinetic approach and are shown to contain information concerning the individual state-to-state rate constants for HF–He.
Trajectory studies within the framework of the infinite order sudden approximation for the F+H2→HF+H reaction
Authors: Jellinek Julius , Baer Michael
Published: February 2003

A detailed classical mechanical study of the F+H2 reaction within the framework of the infinite order sudden approximation (IOSA) is presented. The main object of this research was to study the sensitivity of the state-to-state and total opacity functions and cross sections with respect to a parameter B which determines the border between the reagents and products channels. It was found that the results tend to vary when the value of B is changed but, within a particular range of B values, the IOSA results fit the exact results very nicely. Results of an approach where B is eliminated are also presented and compared with those obtained with the other treatments.
Adiabatic T matrix theory for three dimensional reactive scattering: Application to the (H, H2) system
Authors: Sun J. C. , Choi B. H. , Poe R. T. , Tang K. T.
Published: February 2003

We present an adiabatic transition matrix (T matrix) method of atom–molecule reactive scattering. In this method, the coupling between vibrational and rotational motions is taken into account for obtaining the adiabatic molecular wave functions. These wave functions are expanded in terms of the basis functions taken from the eigenfunctions of a double well potential. From the full potential surface in the linear configuration of three atoms, the double well potential is obtained. Convergence of the expansion is achieved for evaluating the adiabatic wave functions and two body atom–molecule interaction potential. Cross sections are computed with the T matrix method employing the converged adiabatic wave functions. Numerical results for the H+H2 reactive cross section on an ab initio potential surface are presented. Results on D+H2 reaction will be reported in a subsequent paper. The differential cross sections and final state distributions computed from the present adiabatic T matrix method are similar to ones obtained from the adiabatic distorted wave approximation which we published earlier. However, the absolute cross section from the present approach is considerably larger than the previous one. The present results on the reactive cross section are in very good agreement with those obtained from a close coupling calculation. The present cross sections are much bigger than the classical ones in the threshold region. Among the cross sections from various T matrix approaches, the present one is the largest and seems to be most accurate.
Polarization model for NH3–H–H2O proton transfer reaction
Authors: David Carl W.
Published: February 2003

The polarization model for the proton transfer reaction between ammonia and water is used to obtain potential energy surfaces. The results are compared with ab initio computations of Scheiner.(AIP)
The dipole moments of some chloro- and bromopentanes
Authors: Berberian John G.
Published: February 2003

The dipole moments of 3-chloropentane, 3-chloromethylpentane, 3-chloro-e-methlypentane, 3-bromopentane, 3-bromomethylpentane, and 3-bromo-3-methylpentane are reported in this note; the total polarizations of these comopounds aer also presented. The measurements of the total polarization on these comppounds were made as a function of solute concentration in benzene at 25 °C and extrapolated to zero solute concentration. (AIP)
The solution of the MSA for asymmetrical hard ions: The effects of the coupling parameter P
n on the shielding parameter Γ and the structure of the electrolyte solutions
Authors: Lee Lloyd L.
Published: February 2003

The new spherical approximation solution for the asymmetrical hard ions in a dielectric continuum is discussed. Numerical calculations are carried out for concentrated 1−1 electrolyte solutions. (AIP)
Exchange interaction in oxygen-bridged dimers of copper (II) with aromatic N-oxides. I. Method of calculation and application to di-μ-(pyridine N-oxide)-bis [dichlorocopper(II)]
Authors: Kettle Sidney F. A. , Tsaparlis Georgios R.
Published: February 2003

A method, based on the Dirac–Van Vleck–Serber permutation degeneracy model, for the estimation of the exchange integral J in oxygen-bridged dimers of copper (II) with aromatic N-oxides is demonstrated for the case of the dimer di-μ-(pyridine N-oxide)-bis (dichlorocopper II), [(C6H5NO)CuCl2]2. The calculations are limited to the planar three-center, four-electron system Cu(↑)–O(↑↓)–Cu(↑), with due account of the second bridging oxygen and all omitted atoms or electrons. The computational details and approximations are discussed. It is found that J is sensitive to the structural data, the atomic orbitals and functions used, the ionic state of copper, and configuration interaction (inclusion of excited, charge-transfer configurations). The exchange interaction is mediated by a σ superexchange pathway, all π pathways being excluded. Finally, both the oxygen 2s and 2p orbitals contribute to J.
Second and third derivatives of variational energy expressions: Application to multiconfigurational self-consistent field wave functions
Authors: Pulay Péter
Published: February 2003

General analytical expressions are given for the second and third derivatives of constrained variational energy expressions. It is pointed out that variational energy expressions and odd-order derivatives have a distinct advantage over nonvariational (e.g., perturbative) energy expressions and even-order derivatives. In particular, the first-order wave function suffices to determine the derivatives of the variational energy up to third order. The coupled-perturbed multiconfigurational SCF (MC-SCF) equations, obtained from the general results, are equivalent, with minor corrections, to the ones very recently presented by Osamura, Yamaguchi, and Schaefer. Explicit expressions are given for the second and third derivatives of the MC-SCF energy. Computational implementation is briefly discussed.
Sum and density of states for anharmonic polyatomic molecules. Effect of bend–stretch coupling
Authors: Bhuiyan L. B. , Hase William L.
Published: February 2003

A model anharmonic molecular Hamiltonian is used to study the effect of nonseparable bend–stretch coupling on sum and density of states. A bond-energy–bond-order (BEBO) type method is used to couple bending and stretching potential energy terms of a local mode Hamiltonian. The model Hamiltonian is applied to the hydrocarbon fragments H–C≡C, H–C=C, and H–C–C. The results show significant deviation from the independent oscillator model, especially when the stretches are taken to be the Morse type and the energy is close to the dissociation limit.
A 2H NMR study of the solid-phase behavior of nonadecane
Authors: Taylor Michael G. , Kelusky Eric C. , Smith Ian C. P. , Casal Hector L. , Cameron David G.
Published: February 2003

The 2H NMR spectra of perdeuterononadecane and several specifically deuterated nonadecanes, in solid phases I (orthorhombic) and II (rotator), are reported. In phase I, the central portion of the chain is static on the time scale of 10−5 s; however, the chain ends are subject to torsional motions. In phase II, the chains undergo hindered rotation. The phase II spectra of the methylene groups located in the center of the chain can be simulated with a model in which the rigid chain undergoes 82° jumps between equivalent sites. However, this model is inadequate for the methyl and methylene groups near the chain ends. The data suggest higher amplitudes of torsional motion in these positions.
Pulsed molecular beam study of state-to-state vibrational excitation in He+I2 collisions: Energy dependence of the v=0→1 cross section
Authors: Hall Gregory , Liu Kopin , McAuliffe Michael J. , Giese Clayton F. , Gentry W. Ronald
Published: February 2003

We report measurements of the kinetic energy dependence of the v=0→1 vibrational excitation cross section for He+12 collisions. These results were obtained with a new crossed, pulsed molecular beam apparatus having a variable molecular beam interaction angle and laser-induced fluorescence detection. The cross section shows a sharp onset at threshold, and rises nearly linearly with increasing energy to the highest experimental value of kinetic energy, about 13 times the vibrational quantum. At the higher energies, v=0→2 excitation is also observed. These data constitute the first measurements of the energy dependence of a microscopic state-to-state cross section for vibrational transitions in collisions of neutral species.
A theoretical analysis of very low energy collision induced vibrational relaxation in the system He–I2 (3Π0u+)
Authors: Cerjan Charles , Rice Stuart A.
Published: February 2003

Vibrational deexcitation in the low energy collisions of He with I2 (3Π0u+) is examined within the close-coupled scattering formulation using a sum of pairwise Morse interactions to describe the potential surface. Cross sections for zero total angular momentum are given as a function of energy for various rotational and vibrational transitions with the I2 molecule prepared in the n=25 vibrational state. Similar results for total angular momentum of 2, 1, and 0 are also reported for the n=1 initial state of I2. These calculations are in general agreement with the known experimental results and support the arguments advanced previously to explain the unexpectedly large observed relaxation cross section.
Density-functional theory for solid nitrogen and carbon dioxide at high pressure
Authors: LeSar R. , Gordon R. G.
Published: February 2003

Structural properties of solid N2 and CO2 under pressure have been studied with a recently reported theory. The model allows for calculation of the structure and dissociation energy of molecular crystals, using no empirical parameters where dispersion energy is included as a sum of the pair interactions. Comparison with available low temperature experimental results shows good agreement with average errors in lattice constants of about 1% and in lattice energies of about 6%. Calculations were performed on nitrogen in a number of structures, including the experimentally observed Pa3 (α), P42/mnm (γ), P63/mmc (β), and Pm3n structures, as well as a recently proposed structure with space group R3̄m. The R3̄m structure was found to be more stable than the Pa3 structure at about 10 kbar. Calculations were carried out on carbon dioxide in the Pa3 structure. Comparison of the calculated pressure-volume curves with room temperature experimental curves to 100 kbar shows very close agreement for both N2 and CO2.
Basis set effects on spectroscopic constants for C2 and Si2 and the symmetry dilemma in the Xα model
Authors: Dunlap B. I. , Mei W. N.
Published: February 2003

In an attempt to perform thorough and accurate linear combination of atomic orbitals (LCAO) Xα, calculations on C2 and Si2, the effects of basis set composition on the computed spectroscopic constants are investigated for the first time. The questions of how to tell if an adequate basis set has been found and what Xα state corresponds to the lowest state of 1Σ+g symmetry which involves strong electron correlation effects are addressed. With the larger bases, general agreement with experiment and previous density functional calculations are obtained. Remarkable agreement with the configuration interaction (CI) calculations of Bruna et al. on Si2 is obtained if the fractional occupation number method of Slater et al. is used.
The OH stretching spectrum of liquid water: A random network model interpretation
Authors: Belch Alan C. , Rice Stuart A.
Published: February 2003

We report the results of an analysis of the OH-stretching region of the vibrational spectrum of liquid water. The random network model is realized by time-averaged molecular dynamics configurations of ST-2 water. The hydrogen bond network is defined using a geometric definition of bonded pairs of molecules. Force constants are calculated using empirically derived formulas relating the shifts of the intramolecular frequency and anharmonic potential energy coefficients with a hydrogen bond strength parameter. The strength parameter is related to the separation in a molecular pair, hence, it has a distribution of values defined by the molecular configuration in the model liquid. Fermi resonance between the OH stretching mode and the bending overtone is shown to be an important second-order effect in the spectral band shape. The calculated spectra compare well with experimental results and support the contention that the spectral features are dominated by intermolecular interactions.
Microwave structure measurements on the cyclopropane–HCN complex
Authors: Kukolich S. G.
Published: February 2003

Microwave rotational transitions for the cyclopropane-HCN complex were observed using the Flygare–Balle pulsed beam Fourier transform spectrometer. The 111→212, 101→202, 110→211, 212→313, 202→303, 211→312, 313→414, 303→404, 312→413, 414→515, 404→505, and 413→514 transitions were observed. The rotational constants obtained are A=20 243±1344 MHz, B=1384.209±0.001 MHz, and C=1327.901±0.001 MHz. The distortion constants are DJ=1.53±0.03 kHz and DJK=50.9±1.1 kHz. The nitrogen quadrupole coupling strength is eQqaa=−4.38±0.01 MHz with eQqbb−eQqcc= 0.002±0.003 MHz. The cyclopropane carbon atoms and HCN lie in a plane. The most reasonable bond distance is obtained with the HCN hydrogen atom bonding to the midpoint between two of the cyclopropane carbon atoms. The separation of the centers of mass of HCN and cyclopropane is Rcm= 4.472±0.002 Å. The HCN carbon atom is RCC–C=3.476±0.002 Å from the cyclopropane carbon–carbon bond. The stretching force constant is ks=0.062 mdyn/Å and the approximate bonding energy is 860 cm−1.
NMR study of spectral densities over a large frequency range for intermolecular relaxation in liquids: Pair correlation effects
Authors: Albrand J. P. , Taieb M. C. , Fries P. H. , Belorizky E.
Published: February 2003

In order to study the dynamic effects of pair correlation functions on intermolecular translational relaxation in liquids, we have measured the relaxation times T1 of the protons in neopentane in the presence of di-t-butylnitroxide free radicals with concentrations varying up to 6.5×1020 paramagnetic centers × cm−3. Under these conditions, the dominant relaxation mechanism for the resonating protons arises from their interactions with the electronic spins of the free radicals. NMR experiments were performed between 2.2 and 250 MHz and these allowed us to obtain information about the behavior of the relevant spectral densities over a very wide range of frequencies 0<ωSτ≲50, where ωS is the electronic resonance frequency of the radical and where τ is the translational correlation time. It is shown that a model in which the effects of eccentricity of the spins are included and where the translational motion of the molecules is described by a Smoluchowski diffusion equation, (i.e., taking into account their impenetrability and the nonuniform relative distribution of the molecules calculated in the approximation of Lee and Levesque) gives good agreement with experiment. It is also emphasized that introducing pair correlation effects provides a model without any adjustable parameters which is an advantage compared to a theory of random jumps.
On relativistic contributions to the bonding in Cu2
Authors: Martin Richard L.
Published: February 2003

The influence of relativity on the spectroscopic parameters of Cu2 has been investigated by evaluating the mass-velocity and one electron Darwin terms of the Breit–Pauli Hamiltonian in the first order of perturbation theory. The relativistic corrections are of the order of 10% of the SCF and GVB results and result in a deeper (∼1.5 kcal), stiffer (∼15 cm−1) well, with the bond length contracted by about 0.1a0. (AIP)
On the conditions for a unidirectional D vector in a Dzyaloshinsky–Moriya magnetic chain
Authors: Zaspel C. E.
Published: February 2003

The nature of the d vector which occurs in the Dzialoshinsky–Moriya interaction is investigated fortwo simple chain systems. A single chain with inequivalent nearest neighbors has a D with alternating direction. A double chain system has a D with a constant component perpendicular to the chains. (AIP)
Electronic energy partitioning in reactions occurring on more than one potential energy surface: Metastable Mg(3P) atoms with halogen molecules
Authors: Kowalski Andrzej , Menzinger Michael
Published: February 2003

For the reactions metastable Mg(3P) atoms and dihalogen molecules F2, Cl2, and Br2 we have measured by a beam-gas experiment the absolute cross sections for all three electronic reaction channels: formation of MgX(X 2Σ+) ground state, MgX(A 2Π) chemiluminescent products and MgX+(1Σ+) chemi-ions. The chemiluminescence and chemi-ion yields both turn out to be of the order of 1% in all three systems. By qualitative MO theory, it is shown that the dynamical bottleneck in the adiabatic chemiluminescence channel is an early barrier and in the diabatic chemi-ionization process a spin flip occurring during the approach phase, following a long-range electron transfer. An information theoretic analysis reveals considerable dynamic bias associated with most of the CL and CI products [except for MgCl (A 2Π)], and the surprisal plots are nonlinear for all three reactions.
Time-independent energy-sudden transformationa)
Authors: Ding Shi-Liang , Wyatt Robert E.
Published: February 2003

The time-independent energy sudden (ES) representation is defined through application of the energy shift operator S=exp[−(h−ωn)∂/∂ε], where h is the internal (molecular) Hamiltonian. Our introduction of S follows from an earlier study by Chang, Eno, and Rabitz where exp[−iht], which ‘‘factors out’’ internal motion, was used to define the time-dependent ES representation. Exact integral equations for the scattering wave function within the ES representation are derived, the leading terms being the approximate ES wave function. Corrections to the ES wave function are nonsingular and involve the generalized potential increment V=S−1VS−V, where V is the interaction potential. Boundary conditions and transition amplitudes are discussed, as is the connection between wave functions in the ES and the original representations.
Electronic properties of sulfur containing conjugated polymers
Authors: Brédas J. L. , Elsenbaumer R. L. , Chance R. R. , Silbey Robert
Published: February 2003

Valence effective Hamiltonian (VEH) calculations are performed on a number of sulfur containing organic conjugated polymers of interest to the conducting polymers area. Theoretical results for parameters related to conductivity such as ionization potentials, bandwidths, and bandgaps are presented. Systems considered include various derivatives of poly (p-phenylene sulfide), polybenzothiophene, and polythiophene, as well as potentially interesting compounds such as polythieno [3,2-b] thiophene and polyvinylene sulfide. The electronic structure description afforded by the VEH method for sulfur containing polymers is demonstrated to be of the same quality as that presented previously for hydrocarbon polymers. In particular, for ionization potentials, good agreement with available experimental data on poly (p-phenylene sulfide) and polybenzothiophene is obtained, after scaling downward the VEH values by a 1.9 eV polarization correction. The comparison between the theoretical and experimental XPS spectra for polybenzothiophene is excellent with use of the same energy scaling factor previously employed for polyacetylene, poly(p-phenylene), and poly(p-phenylene sulfide). These results, in conjuction with previous results obtained on hydrocarbon polymers, lend confidence in the predictive capabilities of this purely theoretical technique. Calculations show that polyvinylene sulfide, as yet unsynthesized, should display very promising characteristics as a conducting polymer.
Cluster studies of CO adsorption. III. CO on small Cu clusters
Authors: Post D. , Baerends E. J.
Published: February 2003

Chemisorption of CO on (100) and (111) Cu surfaces has been investigated by calculations on a series of small clusters. The dependency of the results on both the adsorption site (top or hollow) and the size of the cluster has been analyzed using a decomposition of the adsorption energy in various terms (steric repulsion, σ bonding, π backbonding). Some properties, i.e., M–CO distance, M–CO vibration, C–O distance, can be obtained from small cluster calculations as they are determined by ‘‘local’’ bonding aspects, whereas, e.g., the adsorption energy is not reliable due to delocalized features of the bonding being not well represented by small clusters. The deficiency of small clusters in modeling the polarizability of a metal substrate is stressed.
Singlet and triplet energy surfaces of NiH2
Authors: Blomberg M. R. A. , Siegbahn P. E. M.
Published: February 2003

Contracted CI calculations have been performed for the concerted dissociation of NiH2 into Ni and H2. All low-lying states have been considered. The most important result of the calculations is that the 1A1 state is much lower in energy than the other states for bent geometries. This state even forms a slightly stable complex with an H–Ni–H angle of 49°. The binding is between Ni sd-hybridized orbitals and a weakened H2 molecule. The ground state of NiH2 is, however, found to be a linear 3Δg state, with the lowest singlet almost 1.5 eV higher in energy at this geometry. The forbidden dissociation reactions for the triplets and singlets with the lowest barriers proceed in steps with the occupation of each symmetry changing by only one unit in each step. The lowest triplet barrier is 1.8 eV, and for this state a concerted dissociation is slightly preferred over a stepwise loss of a hydrogen at a time. The mechanism for the concerted triplet dissociation may also be through spin-orbit coupling to the 1A1 state, since the states cross. The correlation effects on the potential energy surfaces of NiH2 are further shown to be of qualitative importance.
Interaction between two Co atoms. An all electron ab initio HF–CI investigation
Authors: Shim Irene , Gingerich Karl A.
Published: February 2003

In the present work we present all electron ab initio Hartree–Fock (HF) and configuration interaction (CI) calculations of the 84 low-lying electronic states resulting from the interaction between two Co atoms in the 4F(3d)8(4s)1 term. The basis set used was of double zeta quality in general, but the 3d orbital was represented by a triple zeta function. The calculations were performed to the same degree of accuracy for all 84 states. The predicted 5Σ+g ground state of the Co2 molecule is due to the natural orbital occupation (3dσg)2.00(3dπu)3.09(3dδg)3.02 (3dδu)2.98(3dπg)2.91(3dσu)2.00 (4sσg)1.94(4sσu)0.06. From our calculated spectroscopic data of the Co2 molecule, we have estimated the equilibrium distance to be 4.6 a.u. and the vibrational frequency as 240 cm−1. The chemical bond between the Co atoms is a single bond almost entirely due to the 4sσg molecular orbital. The 84 low-lying electronic states make a large contribution to the partition function, resulting in a lowering of the experimental dissociation energy of the Co2 molecule. Reevaluating the data by Kant and Strauss resulted in an experimental dissociation energy D00 of 22±6 kcal mol−1 or 93±25 kJ mol−1.
Corrected Debye–Hückel theory of electrolyte solutions
Authors: Nordholm Sture
Published: February 2003

A simple correction for the finite size of the ions in the atmosphere is introduced into the Debye–Hückel theory. The main effect is to decrease the repulsion among the counterions and thus increase κ. The internal energy, osmotic coefficient, and activity coefficient in the restricted primitive model are found to agree with simulation results for 1–1 and 2–2 electrolytes qualitatively and often to nearly quantitative accuracy.
Extra-atomic relaxation in HCl, ClF, and Cl2 from x-ray photoelectron spectroscopy
Authors: Bomben K. D. , Gimzewski J. K. , Thomas T. D.
Published: February 2003

Application of the Manne–Åberg sum rule to the full photoelectron spectrum of core-ionized HCl, ClF, and Cl2 gives relative extra-atomic relaxation energy for these molecules. The values obtained are in excellent agreement with those determined from the Auger parameter. These measurements together with the core-ionization energies confirm that ClF is Clδ+Fδ− rather than Clδ−Fδ+, as had been suggested by measurements of the molecular Zeeman effect.
Dissociation of benzylamine ions following infrared multiple photon absorption, electron impact ionization, and UV multiphoton ionizationa)
Authors: Catanzarite J. H. , Haas Y. , Reisler H. , Wittig C.
Published: February 2003

The dissociation of benzylamine ions following (i) electron impact (EI) ionization, (ii) multiphoton ionization (MPI) at 266 nm, and (iii) infrared multiple photon absorption (IRMPA) at 9.26 μm is reported. In the EI and MPI experiments, three competitive dissociation pathways are observed. In the IRMPA experiments, benzylamine ions prepared by MPI at low fluences are fragmented very efficiently following irradiation with the focused output from a pulsed CO2 laser. However, in contrast to the EI and MPI results, the IRMPD experiments reveal only a single, lowest energy, dissociation pathway and the fragmentation pattern is consistent with a sequential mechanism in which daughter ions continue to absorb the IR radiation and dissociate. The differences are explained by the different natures of the excitation processes: in IRMPA, the relatively slow up-pumping rate and the long rise time of the CO2 laser pulse restrict the levels of excitation in the dissociating parent ions and favor sequential processes along the lowest energy decomposition pathways.
Rotational energy transfer in HF(v=2): Double resonance measurements and fitting law analysis
Authors: Copeland R. A. , Crim F. F.
Published: February 2003

Data from overtone vibration-laser double resonance experiments show extremely rapid rotational relaxation in HF (v=2) for rotational states up to J=6 and serve to test energy based fitting laws for rotational energy transfer rate constants. The observed rotational states equilibrate in less than a gas kinetic collision with some individual rates being more than twice the hard sphere collision rate. Energy based fitting laws reproduce the observed evolution of the state populations to within 30% but fail systematically for higher (J=5 and 6) rotational levels. Fitting laws formulated for collisions between an atom and a molecule are virtually as successful as those for collisions between two molecules. The extracted rate constants show that multiple rotational quantum energy transfer is an important aspect of relaxation dynamics.
The enhancement of Raman scattering, resonance Raman scattering, and fluorescence from molecules adsorbed on a rough silver surface
Authors: Weitz D. A. , Garoff S. , Gersten J. I. , Nitzan Abraham
Published: February 2003

The enhancements of normal Raman scattering, resonance Raman scattering, and fluorescence from molecules adsorbed on identical, well-characterized, silver-island films are reported. The enhancement arises from the electromagnetic interaction between the molecules and the electronic plasma resonance of the silver islands. A hierarchy of enhancement ratios is found, with typical values of 105 for RS, 103 for RRS and 10−1 to 10 for fluorescence, depending on the quantum yield of the molecular fluorescence. A model, developed on heuristic grounds and substantiated using the density matrix formalism, describes the light scattering processes and the effects of the plasma resonance. This model presents a unified picture of the surface-induced enhancement effects and is consistent with the experimental values. The comparison of all the forms of optical scattering leads to a complete determination of the role of the plasma resonances in the various portions of the scattering process. The excitation of the electronic plasma resonance results in an increased local field at the molecules leading to an increased excitation or absorption rate. Similarly, the excitation of the plasma resonance by the molecular emission dipole results in an increase in the radiative decay rate. However, the electromagnetic coupling of the molecule to the plasma resonance also adds an additional damping channel which can result in a reduction of the absorption or excitation rate as well as the emission yield. The resultant balance of these processes leads to the hierarchy in the measured enhancements. The hierarchy of enhancements is also shown to have important spectroscopic consequences.
Infrared multiphoton decomposition and energy-dependent absorption cross sections of chloroethane-d, -2-d1, and -2,2,2-d3
Authors: Francisco J. S. , Qingshi Zhu , Steinfeld J. I.
Published: February 2003

The infrared photochemistry of chloroethane-d0, -2-d1, and -2, 2, 2-d3 have been investigated. In all cases, the multiple infrared photon induced dissociation is found to proceed by four-center elimination of HCl (or DCl). An energy-grained master equation is used to interpret the data; in this way, the energy-dependent photon absorption cross section σ(E) is determined for all three species. The functional form of σ(E) approaches that for a ‘‘large’’ (i.e., high density of states) molecule with increasing deuteration. Ensemble properties, such as vibrational energy distributions and average energy deposition, can then be determined; but these observables do not by themselves suffice to determine a unique form for σ(E). A form for σ(E) can, however, be specified within certain limits by applying constraints imposed by intramolecular vibrational coupling models.
UV absorption spectra of vibrationally highly excited toluene molecules
Authors: Hippler H. , Troe J. , Wendelken H. J.
Published: February 2003

Vibrationally highly excited toluene molecules with an energy of 52 000 cm−1 have been prepared by UV laser excitation and subsequent isomerization of the isomer cycloheptatriene. The UV absorption spectrum of the toluene, whose vibrational energy would correspond to the average energy of thermal molecules at 2800 K, has been measured over the range 210–270 nm. It is compared with the UV spectrum of thermal molecules heated in shock waves up to 1800 K. All spectra, corresponding either to microcanonical or canonical energy distributions, can be represented by a modified Sulzer–Wieland equation. It is concluded that the absorption coefficients can be expressed to a first approximation by the average energy of the excited molecules independent of their energy distribution.
Approximate energy relationships for molecules
Authors: Thakkar Ajit J.
Published: February 2003

The semiempirical formula for the total energy of a molecule is evaluated for several values of the constant A. E=AΣiνiE; Ei being the orbital energy and ν; the orbital occupation numbers. (AIP)
Reply to Comments of Lehmann, Scherer, and Klemperer
Authors: Henry Bryan R. , Mohammadi M. Ali , Tarr Allan W.
Published: February 2003

The comments of Lehmann Scherer and Klemperer on a recent paper by the authors on the overtone spectrum of acetylene are answered. (AIP)
Rotational-vibration analysis of the n=0, nν6+ν1−nν6 subband in the hydrogen-bonded system 16O 12C ⋅⋅⋅ 1H 19F
Authors: Kyrö E. K. , Shoja-Chaghervand P. , McMillan K. , Eliades M. , Danzeiser D. , Bevan J. W.
Published: February 2003

Thirty-three P(J) branch and 15 R(J) branch transitions associated with the n=0, nν6+ν1−nν6 vibration in 16O 12C ⋅⋅⋅ 1H 19F have been assigned. Rotational constants B, centrifugal distortion constants DJ, rotational-vibrational interaction constant α1, and the frequency of the band origin ν0, have been determined as: B″=0.102 148(14)cm−1; B′=0.104 196(14) cm−1; D″J=3.6(1.8)×10−7 cm−1; D″J=3.8 (1.8)×10−7 cm−1; α1=−61.4(5) MHz; ν0=3844.0294 (50) cm−1. The spectrum is consistent with a linear complex having a hydrogen bond ν6 bending frequency of 75±12 cm−1 and excited state r(C⋅⋅⋅F) distance of 3.012 Å. A lower limit to the excited state lifetime is set at ≥2.8×10−10 s.
Ring puckering of p. dioxane: A three dimensional approach
Authors: Graveron-Demilly D.
Published: February 2003

The three dimensional ring puckering Hamiltonian of p. dioxane has been diagonalized by the perturbation method. The three out-of-plane ring puckering modes and their combination levels are found, showing the couplings that occur. The far IR intensities have been calculated and bands of several types have been obtained; in particular, the difference bands previously observed by Pickett and Strauss are found and assigned.
On the valence excited states of conjugated polyenes
Authors: McDiarmid Ruth
Published: February 2003

In this investigation, the theoretical and experimental evidence concerning the energies and symmetries of the lower excited valence states of conjugated polyenes is reevaluated.
Quantum yields of I(2P1/2) for CF3I, C2F5I, i-C3F7I, n-C3F7I, n-C6F13I, and 1,2-C2F4I2 at 308 and 248 nm
Authors: Gerck Edgardo
Published: February 2003

A new method is applied to measure the wavelength-resolved quantum yields of I(2P1/2) for six molecules relevant to the iodine I(2P1/2) atoms and accounts for the strong exciplex emission, also at 1.3 μm, from the I(2P1/2)–iodide molecules. In addition, digital processing is used to correct for noise, quenching, and the detector rise time. These features avoided the shortcomings of previous measurements. The following molecules were investigated: CF3I, C2F5I, i-C3F7I, n-C3F7I, n-C6F13I, and 1,2-C2F4I2. All six iodides present high yields of I(2P1/2) at the pumping wavelengths of 308 and 248 nm used. The high I(2P1/2) yields for the molecules n-C6F13I and 1,2-C2F4I2 are reported for the first time.
An experimental and theoretical investigation of the far-infrared spectra of HCl and HBr crystals under pressure at 4.2 K
Authors: Obriot J. , Fondère F. , Marteau Ph. , Allavena M.
Published: February 2003

By slowly cooling a gaseous mixture of Ar and HX hydrogen halides (X=Cl, Br) a rare gas solid containing crystallites of either HCl or HBr is formed at 4.2 K. The far infrared spectrum of this solid is recorded under a pressure range of 0 to 5 kbar. Pressure frequency shifts of external modes are observed. Dynamical calculations of external modes are performed using an intermolecular potential including multipolar expansion up to quadrupolar term as well as short and long range contributions represented by a Buckingham type potential. The pressure frequency shifts are evaluated using the model of Pawley and Mika which provides a simple method of estimating the effects of pressure constraint on the intermolecular potential. The P–V relation is numerically derived and the Grüneisen parameters are calculated.
First-order wetting transition at a liquid–vapor interface
Authors: Schmidt J. W. , Moldover M. R.
Published: February 2003

In certain binary solutions the lower of the two liquid phases forms a layer which intrudes between the upper liquid phase and the vapor. We find that such intruding layers form above binary solutions of a fluorocarbon (C7F14) and an alcohol (i-C3H7OH). As the temperature of C7F14–i-C3H7OH solutions is increased, the intruding layer abruptly appears at a characteristic wetting temperature TW=311 K. This temperature is well below the consolute temperature (363 K). At temperatures slightly above Tw the intruding layer’s thickness (measured by ellipsometry) is several hundred angstroms and its variation with temperature is extremely weak. Below Tw, the layer’s thickness may be zero and is no greater than 20 Å when a naive slab model is used to interpret the data. Below Tw, three-phase contact can occur between the vapor and both the upper and the lower liquid phases. Our measurements show that one of the angles (θ) which characterizes this three-phase contact has a very simple temperature dependence: cos θ=1−0.8 (Tw−T)/Tw. Thus, both the temperature dependence of the layer’s thickness and the temperature dependence of the three-phase contact are consistent with a first-order wetting transition at Tw. The thickness of the intruding layer has been monitored as the solutions approach equilibrium. Below Tw the behavior is complex; however, a feature does appear which suggests a nucleation-like event is required for the layer to vanish. Phenomenological theories of the first-order wetting transition predict that a discontinuity in the temperature derivative of σ occurs at Tw where σ is the macroscopic surface tension between the upper liquid phase and the vapor phase. From our data we estimate that this discontinuity is very small: −0.009 dyn cm−1 K−1. A first-order wetting transition is predicted by approximate theories which use long-ranged interatomic potentials (e.g., inverse sixth power attractions), provided that the range of the potential between unlike species is longer than the range of the potentials between like species. We find that the thickness of the intruding layer d depends quite weakly on L in the vicinity of a first-order wetting transition. (L is the height spanned by the upper liquid phase.) This feature is in contrast with earlier ellipsometric measurements of the thickness of intruding layers which showed that d∝L−1/3 in different binary mixtures away from wetting transitions.
Study of hydration of the Na+ ion using a polarizable water model
Authors: Perez P. , Lee W. K. , Prohofsky E. W.
Published: February 2003

We report some results of applying a polarizable water model (the PE model) to the molecular dynamics simulation of hydrated Na+ ion microclusters Na+(H2O)n with n=1,...,6. We found that the PE model with just two adjustable parameters reproduces the experimental enthalpies of formation of the ion–water microclusters better than a number of other methods. We also found that for n=6 at 0 K the water molecules do not form a regular octahedron. This predicted change in structure appears to be in agreement with experimental observations.
Variationally exact rovibrational spectra of nonrigid triatomics: The HeHF van der Waals molecule
Authors: Tennyson Jonathan , Sutcliffe Brian T.
Published: February 2003

A variationally exact method of obtaining the rovibrational levels of triatomic potentials is presented. This LC-RAMP method derives from the close-coupling approach in body-fixed coordinates, but uses optimized Morse oscillator functions to give radial basis sets for both the diatomic and complex stretching coordinates. The method is suited to the nuclear dynamics of nonrigid molecules and atom–diatom van der Waals complexes. A recent full interaction surface for HeHF is fitted and used for dynamical calculations. The binding energy of the complex increases slowly with HF vibrational excitation. Correlation between the HF and complex vibrational modes is found to be negligible.
Single-photon induced conductivity of solutes in nonpolar solvents
Authors: Holroyd R. A. , Preses J. M. , Zevos N.
Published: February 2003

Synchrotron radiation was used for determining single-photon photoconductivity thresholds for various solutes in nonpolar solvents. For anthracene as a solute the thresholds (Eth) are 6.14, 6.07, and 5.87 eV in 2,2,4-trimethylpentane, 2,2,4,4-tetramethylpentane, and tetramethylsilane, respectively. The threshold decreases as the conduction band energy (V0) decreases. The polarization energy (P+) of the anthracene cation, as derived from the data for 2,2,4-trimethylpentane, is one-third larger in magnitude than that reported for the polarization energy of the anthracene anion in this solvent, which suggests that the cation is smaller than the anion. Thresholds were also measured for 1,2-benzanthracene, azulene, perylene, triphenylamine, and diazabicyclo-octane. The thresholds in solution and solvation terms are related quantitatively by the expression Eth=I.P.+V0+P+.
Nonradiative relaxation of electronically excited N2 on Al(111). Comparison with nonlocal optical theory
Authors: Avouris Ph. , Schmeisser D. , Demuth J. E.
Published: February 2003

The contribution of surface electron-hole pair creation processes to the nonradiative relaxation of the C 3Πu electronically excited state of N2 on Al(111) is investigated using electron energy loss spectroscopy. The homogeneous broadening of the spectral lines is found to be ∼140 meV, corresponding to a lifetime of ∼5×10−15 s. We compute the contributions of surface and bulk electron-hole pair generation processes to the lifetime broadening, utilizing a theory for the nonlocal dielectric response of the metal based on the Lang–Kohn model for the surface potential. The theory predicts correctly the observed overall decay rate and shows that surface processes make an order of magnitude higher contribution to that rate than the bulk processes. The physical conditions which determine the relative importance of surface and bulk processes are discussed.
The vibrationally averaged, temperature-dependent structure of polyatomic molecules. I. CO2
Authors: Mawhorter R. J. , Fink M. , Archer B. T.
Published: February 2003

A new high-temperature nozzle has been added to our counting electron diffraction apparatus, enabling precise studies of the vibrationally averaged structure of molecules at temperatures between 300 and 1000 K. Our results for CO2 are compared with the harmonic theory and the recent variational wave function calculation of Kohl and Hilderbrandt. The new nozzle system is described, and differences between the data and the theories are discussed.
The infrared photodissociation spectroscopy of OCS-alkane van der Waals clusters in pulsed molecular beams
Authors: Hoffbauer Mark A. , Giese Clayton F. , Gentry W. Ronald
Published: February 2003

In order to investigate the dependence of van der Waals (vdW) cluster infrared photodissociation spectra on the size and complexity of the cluster, we carried out a series of pulsed molecular beam experiments on vdW clusters of OCS with methane (d4), ethane, butane, and hexane. A CO2 laser was used to excite the first overtone of the OCS ν2 bending mode at frequencies around 1045 cm−1, and absorption followed by dissociation was monitored by observing the attenuation of the mass spectrometer signal due to the cluster. All four clusters display Lorentzian absorption profiles with linewidths between 4.0 and 5.7 cm−1, a remarkably small range considering the variation in the number of vibrational degrees of freedom within this series. It appears that the OCS excited-state decay which is responsible for the photodissociation linewidth does not occur by coupling of the OCS vibration to vibrational degrees of freedom in the other molecule, but by coupling to rotational or librational coordinates which express the relative orientations of the cluster constituents. The absolute absorption intensities of OCS ⋅ ethane and OCS ⋅ hexane are about the same as that of isolated OCS, but those of OCS ⋅ CD4 and OCS ⋅ butane are about a factor of 2 smaller.