bluuebunny/crossref_metadata_2025 huggingface dataset:
Excimer laser multiphoton dissociation of Cr(CO)6: Evidence for two distinct dissociation processes
Authors: Tyndall George W. , Jackson Robert L.
Published: July 2002

The excimer laser multiphoton dissociation of Cr(CO)6 has been investigated in the gas phase using emission spectroscopy to detect excited state photoproducts. Following laser irradiation at 193 nm (ArF*), 248 nm (KrF*), and 351 nm (XeF*) well-resolved Cr(I) emission was detected. The photodissociation mechanism was studied by determining the laser fluence dependence, buffer gas pressure dependence, and temporal profiles of the emission intensity for the various Cr(I) excited states. The data suggest that dissociation occurs via two distinct processes, sequential and direct. The sequential process is found to be extremely sensitive to buffer gas pressure, while the direct mechanism is pressure invariant. The Cr(I) excited state distributions formed in the direct process, following irradiation at the three laser wavelengths used, appear to be statistical.
Mercury 6 3P
J relaxation and intramultiplet mixing under collisions with molecular hydrogen
Authors: Bras N. , Butaux J. , El Afghani A. , Jeannet J. C. , Perrin D.
Published: July 2002

Absolute rate constants for quenching and intramultiplet mixing of Hg(6 3P0,1,2) by collisions with H2 molecules have been determined. Some of these results are original. The other ones are compared with previous determinations. In this study the 6 3P0,1,2 levels of mercury were populated directly via pulsed laser excitation and the time evolutions of the populations were analyzed by time-resolved laser induced fluorescence.
Deexcitation of metastable Ba+
Authors: Walker T. G. , Bonin K. D.
Published: July 2002

This is a study of the deexcitation mechanisms of the metastable 5 2D3/2 state of Ba+ in about 1 atm He buffer gas in the temperature range of 500–650 °C. The ions were produced by three-photon ionization with light tuned to a resonance between two excited states of Ba, enabling the electron density to be varied independently of the temperature and pressure. The deexcitation is dominated by superelastic scattering of electrons with a cross section of σse =45±20 Å2 (corresponding to an electron-impact excitation cross section of 11 Å2) and quenching by Ba atoms with a cross section of σBa =130±25 Å2. No dependence of the deexcitation rate on He density was detected, and we infer an upper limit on the cross section of σHe ≤9×10−22 cm2.
Sensitivity of intramolecular vibrational energy relaxation to stretch–bend potential energy coupling and stability of periodic orbits
Authors: García-Ayllón A. , Santamaría J. , Ezra G. S.
Published: July 2002

Stretch–bend coupling via 2:1 Fermi resonance is an important mechanism for rapid energy flow from overtone excited CH local mode states. To elucidate the role of potential energy coupling, we have studied the classical dynamics of a two-mode stretch–bend Hamiltonian for the benzene fragment C3 H. The effects of attenuation of the CCH bend force constant by stretching of the CH bond on the short time (up to 0.12 ps) probability decay dynamics of the model system are in good qualitative accord with trends found previously in full scale classical trajectory simulations on planar benzene by Lu, Hase, and Wolf. Surfaces of section are used to study the classical phase space structure of the stretch–bend Hamiltonian. A close correlation between instability of the CH periodic orbit and exponential decay of probability is found, and relaxation rates can be estimated to good accuracy by linear stability analysis of the periodic orbit. Increasing the strength of the potential coupling stabilizes the CH periodic orbit, thereby suppressing overtone relaxation. There is therefore an effective cancellation of kinetic and potential stretch–bend coupling terms.
Self-consistent-field iterative transfer perturbation method and its application to the interaction between a polymer and a small molecule
Authors: Aoki Yuriko , Imamura Akira , Morokuma Keiji
Published: July 2002

An iterative transfer perturbation method is proposed to treat the interaction between a polymer and a small molecule at the level of the ab initio method. The validity of our method is examined by applying it to a simple model system and comparing the results with those from the conventional tight-binding SCF crystal orbital method. The interaction energies and charge distributions obtained are in excellent agreement between the two methods. The present perturbational approach is promising for application to the more complicated interaction between a polymer and impurities.
An efficient reformulation of the closed-shell coupled cluster single and double excitation (CCSD) equations
Authors: Scuseria Gustavo E. , Janssen Curtis L. , Schaefer Henry F.
Published: July 2002

The closed-shell CCSD equations are reformulated in order to achieve superior computational efficiency. Using a spin adaptation scheme based on the unitary group approach (UGA), we have obtained a new set of equations that greatly improves our previous formulation. Based on this scheme we have also derived equations for the closed-shell configuration interaction including all single and double excitations (CISD) case. Both methods have been implemented and tested. For a range of test cases the new CCSD method is more efficient than the earlier CCSD method. The new closed-shell CISD procedure is faster than the shape-driven (SD)GUGA algorithm and the new CCSD scheme is less than two times more computation intensive than SDGUGA CISD per iteration.
Nascent rovibrational distribution of O+2(A 2Πu) produced by He(2 3S) Penning ionization of O2
Authors: Tsuji Masaharu , Obase Hiroshi , Endoh Minoru , Yamaguchi Sumio , Yamaguchi Kazuo , Kobarai Kazunari , Nishimura Yukio
Published: July 2002

The He(2 3S) Penning ionization of O2 to give O+2(A 2Πu) has been studied by observing the O+2(A 2Πu–X 2Πg) emission in beam and flowing afterglow apparatus. A comparison of beam and flowing afterglow data indicates that the nascent rovibrational distribution is lost in the flowing afterglow due to collisional relaxation. The nascent vibrational distribution of O+2(A 2Πu) shifts to lower vibrational levels in comparison with the Franck–Condon factors for vertical O2(X)→O+2(A) ionization. The rotational temperature decreases from 4200 K for v′=0 to 400 K for v′=13. Vibrational relaxation of O+2(A) accompanied by a significant rotational excitation is explained as a result of a short-range repulsive interaction [He–O+2(A)→He+O+2(A)] in the exit channel.
Is there resonance enhancement of the cross section for vibrational relaxation induced by very low energy collisions? The I2–He system revisited
Authors: Rock Andrew B. , Van Zoeren Carol M. , Kable Scott H. , Edwards Geoffrey B. , Knight Alan E. W.
Published: July 2002

Rate coefficients for state-to-field vibrational relaxation of I2 (B 3Π0u+,  υ′=16) induced by collisions with He at translational temperatures of 2–12 K are measured using state-selected time-resolved dispersed fluorescence spectroscopy in a supersonic free jet expansion. These vibrational relaxation rate coefficients in the 2–12 K regime are found to be an order of magnitude smaller than comparable rate coefficients at 300 K. The rate data are compared with calculated classical I2–He encounter rates and with rates computed using quantum mechanical cross sections for I2–He calculated by Schwenke and Truhlar. The role of scattering resonances in determining vibrational relaxation rates at low translational temperatures is discussed.
Microwave spectrum, structure, and electric dipole moment of the Ar–formamide van der Waals complex
Authors: Suenram R. D. , Fraser G. T. , Lovas F. J. , Gillies C. W. , Zozom J.
Published: July 2002

The microwave spectrum of the Ar–formamide van der Waals complex has been obtained using a pulsed-nozzle Fourier-transform microwave spectrometer. The rotational constants of the complex are: A=10 725.7524(48) MHz, B=1771.0738(22) MHz, and C=1548.9974 (16) MHz. The complex is shown to be nonplanar with an inertial defect of −6.21 u Å2. The Ar atom is located at 3.62 Å from the center of mass of the formamide unit at Ar–O, Ar–N, and Ar–C distances of 3.55, 3.79, and 3.93 Å, respectively. The shortest Ar–H distance is 3.25 Å which is similar to that observed for Ar–vinyl cyanide (3.21 Å). Stark effect and hyperfine analyses yield the following values for the electric dipole moment components and 14N quadrupole coupling constants for the complex: μa=0.922(1) D, μb=3.407(5) D, χaa=−1.164(7) MHz, χbb=1.906(5) MHz, and χcc=−0.742(6) MHz.
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u vibrational spectroscopy of specifically adsorbed azide on silver electrodes
Authors: Samant M. G. , Viswanathan R. , Seki H. , Bagus P. S. , Nelin C. J. , Philpott M. R.
Published: July 2002

I n situ infrared vibrational spectra of azide ions specifically adsorbed on polycrystalline silver electrodes immersed in aqueous electrolytes of composition 1 to 30 mM azide and 0.1 M perchlorate are reported for the potential range 0.25 to 0 V vs Ag/AgCl (3 M KCl). Adsorbed azide was not detected spectroscopically between 0 and −0.9 V, a range known from differential capacitance measurements to allow adsorption onto the silver electrodes. These experimental observations have been interpreted with the help of ab initio calculations of the vibrational frequencies of [AgN3]0 and [AgN3]− for two configurations with C∞v and C2v symmetries representing simple models for azide adsorbed with its configuration axis perpendicular and parallel to the electrode surface, respectively. Besides the charge states specified, calculations are also reported for static applied fields along the symmetry axis. Taken together, the experiments and calculations support a model in which azide adsorbs oriented perpendicular to the surface at the higher surface concentrations induced by potentials more positive than OV, and is adsorbed with long axis parallel to the surface at negative potentials.
Lifetime-vibrational interference in the autoionization of core-excited O2
Authors: Carroll T. X. , Thomas T. D.
Published: July 2002

The electron–electron coincidence technique has been used to measure the spectrum of autoionizing (Auger) electrons that are emitted following excitation of a core (1s) electron in O2 to the half-filled 1πg orbital. The highest kinetic energy peak in the deexcitation spectrum corresponds to a transition to the ground state of O+2(X,2Πg). The energy, width, and shape of the observed peak cannot be described by pure Franck–Condon vibrational calculations. Because the lifetime for deexcitation is comparable to a vibrational period, interference between vibrational levels in the core-excited state must be taken into account in calculating the transition profile. When this is done the agreement between observed and calculated line shapes is excellent.
Depolarization ratio of the ν1 Raman band of liquid carbon tetrachloride in the range 0–1.1 kbar
Authors: Ikawa S. , Whalley Edward
Published: July 2002

The depolarization of the totally symmetric (ν1) Raman band of liquid carbon tetrachloride has been measured at 22 °C and pressures up to the freezing pressure of 1.1 kbar using the scattering geometry Z(YZ)Y, which causes little depolarization by the windows. The depolarization decreases linearly from 6.5×10−3 at zero pressure to 5.6×10−3 at 1.15 kbar. The change is explained as the resultant of a decrease of both the vibrational amplitude and the mean intermolecular distance under pressure.
Nonequilibrium contributions to the rate of chemical reaction in the Lorentz gas: A comparison of perturbation and numerical solutions of the Boltzmann equation
Authors: Cukrowski A. S. , Popielawski J. , Schmidt R. , Stiller W.
Published: July 2002

The corrections to the equilibrium rate of the chemical reaction A+B→products are analyzed for the Lorentz gas composed of light particles A reacting with heavy particles B in the presence of heavy particles of a carrier gas C. Particles B and C are treated as a heat bath. The reaction is analyzed under conditions in which the products can be neglected. The model of reacting hard spheres (line-of-centers model) introduced by Present is used. Analysis is performed in two ways: (1) By obtaining analytical expressions following from the Chapman–Enskog perturbation solution of the Boltzmann equation generalized for gases reacting chemically. (2) By obtaining numerical results which follow from a numerical solution of a partial differential equation which can be derived from the Boltzmann equation for the reactive Lorentz gas. It is shown that for a specified region of system parameters both types of solution of the Boltzmann equation coincide. The comparison of both methods permits us to find out under which conditions the perturbation method of solution may be used.
Transport theory for the Lennard-Jones dense fluid
Authors: Karkheck John , Stell George , Xu Jiasai
Published: July 2002

A kinetic theory for a fluid of particles interacting via a pair potential with hard-core plus truncated tail is described and used to derive a transport theory for the Lennard-Jones fluid as well as the square-well fluid. Numerical results for shear viscosity, thermal conductivity, and the self-diffusion coefficient are given for the Lennard-Jones fluid and compared with simulation and experimental results. Our Lennard-Jones theory proves quantitatively useful over a wide range of states.
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o quantum chemical study of the molecular and spectroscopic (infrared and Raman) properties of sulfur dioxide: Comparison with ozone
Authors: Bacskay George B. , Rendell Alistair P. L. , Hush Noel S.
Published: July 2002

Using ab initio SCF, singles and doubles configuration interaction (CI-SD), and coupled pair functional (CPF) techniques with basis sets of triple zeta plus two polarization functions quality, the following properties have been computed for sulfur dioxide at its equilibrium geometry: dipole and quadrupole moments, electric field gradients at the nuclei, static and frequency dependent dipole polarizability (at λ=5154 Å and λ=6328 Å), dipole and polarizability derivatives, infrared absorption intensities, and Raman scattering activities. The properties, when calculated at the correlated level of theory have been evaluated by the energy derivative (ED) as well as the expectation value (EV) formalism. The results of the CPF(ED) calculations are in good agreement with the available experimental data and on the basis of additional two-configurational MCSCF studies the use of single reference CI-SD and CPF wave functions is judged to be justified. The analogous calculations on ozone serve to highlight the considerable differences between the two molecules, inasmuch as due to the considerably larger degree of near-degeneracy in O3, properties that involve mixing of the lowest 1B2 excited state with the 1A1 ground state are predicted poorly at the SCF level as well as at the single reference CI-SD and CPF levels of theory.
Invariant expansion. IV. The exponentials of tensorial expressions
Authors: Blum L. , Torruella A. J.
Published: July 2002

The expansion of the exponential of a tensorial expression such as the interaction or pair correlation function between two nonspherical molecules 1, 2 is of the form ∑mnl λmnlΦmnl(12), where Φmnl(12) are invariant tensorial expressions that depend only on the orientation of 1 and 2. The generating function e−∑mnl λmnlΦmnl =∑pqt ipqt(λ) Φpqt defines a generalized Bessel function (GBF). We discuss integral representations and recurrence relations for the GBF. The first GBFs for dipolar and linear quadrupolar exponents, which are of interest in the theory of ionic solutions are computed explicitly.
Laser probing of ion velocity distributions in drift fields: Parallel and perpendicular temperatures and mobility for Ba+ in He
Authors: Dressler Rainer A. , Beijers Johannes P. M. , Meyer Henning , Penn Stephen M. , Bierbaum Veronica M. , Leone Stephen R.
Published: July 2002

Measurements of ion velocity distributions are presented for Ba+ drifted in helium under well characterized conditions using single-frequency laser-induced fluorescence probing. We present the reduced mobilities and the Doppler profiles parallel and perpendicular to the electric field vector as a function of the ratio of the field strength (E) to the buffer gas density (N) up to 33.5 Td. The reduced mobility decreases monotonically with increasing E/N from the zero-field value of 16.7±0.4 cm2  V−1  s−1 at 313 K. The parallel and perpendicular ion temperatures are in very good agreement with both a repulsive Maxwell model and a parametrized version of the three-temperature theory of Lin et al. The parallel temperature is always higher than the perpendicular one. Effects of optical pumping on the Doppler profiles are also presented.
Acetylene in nematic liquid crystals: A vibrational analysis of the observed dipolar couplings
Authors: van der Est A. J. , Burnell E. E. , Barnhoorn J. B. S. , de Lange C. A. , Snijders J. G.
Published: July 2002

Experimental dipole couplings for several isotopically substituted modifications of acetylene dissolved in the nematic liquid crystals 1132 (Merck ZLI 1132), EBBA [N-(4-ethoxybenzylidene)-2, 6-dideutero-4-n-butylaniline] and a 55 wt. % 1132/EBBA mixture are obtained from NMR measurements. The dipolar couplings are calculated using a model for the solvent–solute interaction. The interaction is taken to be of second rank tensorial form and the effects of the coupling between the vibrations and rotations are taken into account. The calculated values are in good agreement with the experimental results although some discrepancies exist. These discrepancies are discussed in terms of a possible contribution to the observed carbon–carbon coupling from the anisotropy in the indirect coupling. The results show that the interaction between the vibrations and rotations of the solute plays an important role in determining the observed couplings and that specific interactions with the liquid crystal need not be invoked to explain the results. Previous studies have shown that molecular hydrogen experiences an external electric field gradient due to its liquid crystal environment and that in 55 wt. % 1132/EBBA the value of this field gradient is zero at 301.4 K. It has also been shown that the interaction between this field gradient and the solute molecular quadrupole moment dominates the ordering of molecular hydrogen in liquid crystals, such as 1132 and EBBA, where the field gradient is large. A qualitative analysis indicates that this interaction is important for the ordering of acetylene and that other mechanisms also play a role.
Identification of intramolecular energy transfer pathways in a reactive triatomic system
Authors: Smith R. Scott , Shirts Randall B.
Published: July 2002

A model of the HNC/HCN isomerization reaction with three vibrational degrees of freedom is studied. The semiclassical technique of adiabatic switching is used to obtain quantizing trajectories in the metastable HNC potential well. Adjustment of these eigenvalues to account for the degenerate bend mode not included in our model gives agreement to within 20 cm−1 of the quantum results. Isomerization data show that this system has a high degree of mode specificity. Fourier transform analysis of the vibrational motion identifies two pathways for energy transfer. One pathway occurs at high bending quantum numbers and is believed to involve potential features and chaotic motion at high bending angle. The second pathway is a classical resonance in which the sum of the CN stretching frequency and twice the bending frequency is equal to the H atom stretching frequency. The resonance acts to transfer energy into the bending mode which allows the trajectory to reach the high bending angle region of the potential where the first mechanism can cause isomerization. It is determined that the role of the CN mode is to allow for enhanced stretch–bend coupling through a three mode resonance. This study concludes that bending degrees of freedom play a major role in intramolecular energy transfer dynamics in agreement with the results of previous studies.
Electron degradation and yields of initial products. III. Dissociative attachment in carbon dioxide
Authors: Pagnamenta Antonio , Kimura Mineo , Inokuti Mitio , Kowari Ken-ichi
Published: July 2002

We demonstrate the importance of subexcitation electrons in CO2 (with energies below 6 eV) by studying the yield of negative-ion formation in the dissociative attachment process e−+CO2→CO+O−. We evaluate the electron degradation spectrum and the time dependence of the degradation process within the continuous-slowing-down approximation. Slowing down by vibrational and other excitation collisions and the O− production are competing processes. This explains why the O− yield is larger for subexcitation electrons with energies above 3.8 eV, which avoid the large energy loss by electronic excitation and can still pass through the resonance at about 4 eV. The attachment at 8 eV with a much larger resonance-like cross section contributes only about 30% to the total O− yield in the degradation process.
Product branching in predissociation of the e 1Πu, e′ 1Σ+u, and b′ 1Σ+u states of N2
Authors: Helm Hanspeter , Cosby Philip C.
Published: July 2002

A measurement of the N(2D0) and N(2P0) yields in predissociation of the e 1Πu, e′ 1Σ+u, and b′ 1Σ+u states of molecular nitrogen is reported using photofragment spectroscopy. These states are prepared by laser excitation from the long-lived a″ 1Σ+g (v=0) level in a fast molecular beam. A position- and time-sensitive detector is used to monitor the two correlated fragments produced by single molecular dissociation events. The translational energy imparted in the fragments establishes the electronic energy of the resulting nitrogen atoms and hence the branching among the available dissociation limits of nitrogen. Extremely pronounced rotational dependence in the branching yields is observed for the e and e′ members. The dependence mimics, in part, known molecular perturbation of the Rydberg states, and it reveals a complex competition among continuum states for the population in the Rydbergs.
Hyperpolarizabilities of 4-amino-4′-nitrodiphenyl sulfide and all of its chalcogen analogues
Authors: Robinson D. W. , Abdel-Halim H. , Inoue S. , Kimura M. , Cowan D. O.
Published: July 2002

The quadratic hyperpolarizability tensor of 4-amino-4′-nitrodiphenyl sulfide has been sampled by measurement of the z component of its vector part, when z is the permanent dipole direction. This was also accomplished for the oxygen, selenium, and tellurium analogues. It was found that large differences in their frequency-doubling efficiency as powdered solids are not reflected in this molecular property. The sulfur, selenium and tellurium ethers have about the same hyperpolarizabilities while that of the oxygen member is somewhat lower.
Intramolecular vibrational energy flow induced by potential and Coriolis couplings
Authors: Sahm D. K. , Uzer T.
Published: July 2002

The classical dynamics of intramolecular energy flow caused by potential and Coriolis couplings is studied in various representations. The mapping of the problem onto a perturbed asymmetric rotor is found particularly useful. This representation shows the origin of localization/delocalization of energy as well as factors that determine periodic energy exchange. Good agreement is found between the predictions and numerical calculations.
Cluster expansion for flexible polymeric fluids in which bonding constraints are treated as perturbations
Authors: Freed Karl F.
Published: July 2002

The equilibrium statistical mechanics of a dense fluid composed of flexible polymers is studied with a novel cluster expansion in which the bonding constraints are treated as perturbations. The cluster expansion is shown to be in the same spirit as the widely used Flory–Huggins approximation for the thermodynamics of polymeric fluids. The appropriate continuum analog of the Flory–Huggins approximation is defined in terms of the properties of the zeroth order reference monomeric fluid. A systematic method is provided for computing corrections to this mean field approximation as arising from correlations between the polymer bonds. The results strongly parallel those of our cluster expansion for lattice model polymers. The leading approximation for hard sphere monomers illustrates several facets of the general theory.
Laser induced photodissociation and desorption. II. CH2I2 adsorbed on Ag
Authors: Domen K. , Chuang T. J.
Published: July 2002

The adsorbate- and the substrate-mediated chemical processes induced by ultraviolet (UV) laser pulses have been identified for the CH2I2/Ag system. In contrast to CH2I2 on Al2O3, little photodissociation or desorption is observed for the molecules adsorbed on an Ag surface at submonolayer coverages. Photochemical effect is evident at coverages higher than a monolayer. Explosive desorption dominates at high surface coverages and high laser fluences. The explosive behavior is elucidated and its implications relevant to material ablation by lasers is discussed.
Barrier tunneling in the He2 c 3Σ+g state
Authors: Lorents Donald C. , Keiding So/ren , Bjerre Nis
Published: July 2002

High resolution fast beam photofragment spectroscopy on the a 3Σ+u has been used to measure level positions and widths of the c 3Σ+g in the positive energy region near the top of its barrier. Tunneling through the barrier produces energetic neutral fragments that diverge from the beam and are detected. The information is used to determine the shape and height of the barrier to spectroscopic accuracy.
Fractionation effects in muonium–molecular hydrogen reactions
Authors: Tennyson Jonathan , Miller Steven
Published: July 2002

The neutral–neutral and ion–neutral reactions of Mu, Mu+, HMu, and HMu+ with H, H2, H+2, and H+3 are considered. Vibrational zero-point energy effects make the Mu–H exchange reactions endothermic and suggest that little molecule formation occurs in room temperature Mu–H2 mixtures.
Adiabatic time evolution of atoms and molecules in intense radiation fields
Authors: Nguyen-Dang T. Tung
Published: July 2002

We derive the condition for a time dependent quantum system to exhibit an exact or higher order adiabatic time evolution. To this end, the concept of adiabaticity is first analyzed in terms of the transformation properties of the time-dependent Schrödinger equation under a general unitary transformation Û(t). The system will follow an adiabatic time evolution, if the transformed Hamiltonian, K̂(t)=Û°ĤÛ−iℏÛ°Û, is divisible into an effective Hamiltonian ĥ(t), defining adiabatic quasistationary states, and an interaction term Ω̂(t), whose effect on the adiabatic states exactly cancels the nonadiabatic couplings arising from the adiabatic states’ parametric dependence on the time. This decoupling condition, which ensures adiabaticity in the system’s dynamics, can be expressed in a state independent manner, and governs the choice of the unitary operator Û(t), as well as the construction of the effective Hamiltonian ĥ(t). Using a restricted class of unitary transformations, the formalism is applied to the time evolution of an atomic or molecular system in interaction with a spatially uniform electromagnetic field, and gives an adiabatic approximation of higher order to the solutions of the semiclassical Schrödinger equation for this system. The adiabatic approximation so obtained exhibits two properties that make it suitable for the studies of intense field molecular dynamics: It is valid for any temporal profile of the field, and improves further as the field intensity increases, as reflected in the weakening of the associated residual nonadiabatic couplings with increasing field strength.
Quenching of N2(a 1Πg, v′=0) by N2, O2, CO, CO2, CH4, H2, and Ar
Authors: Marinelli William J. , Kessler William J. , Green Byron David , Blumberg William A. M.
Published: July 2002

We have determined quenching rate coefficients for N2(a 1Πg, v′=0) by N2, O2, H2, CO2, Ar, CH4, and CO using two-photon laser excitation. Quenching by N2 appears to proceed via collisional coupling to N2(a′ 1Σ−u, v=0) with a rate coefficient of 2.2±0.2×10−11 cm3 molecule−1 s−1. Quenching by Ar is nearly as efficient. Gas-kinetic rate coefficients are obtained for quenching by CO, O2, H2, CO2, and CH4. Collisional energy transfer from N2(a,0) to CO(A) is observed in these experiments. However, quenching by O2, H2, CO2, and CH4 is thought to be reactive.
On Liapunov functions for single-variable reacting systems displaced from equilibrium
Authors: Hunt Katharine L. C. , Hunt Paul M. , Ross John
Published: July 2002

We prove that Liapunov functions for a single reactive intermediate evolving toward a nonequilibrium steady state can be obtained from a global potential φ. We consider reactions occurring in a chamber containing a reactant, the intermediate, and a product. Reservoirs connected to the chamber serve to hold the reactant and product concentrations constant, in nonequilibrium proportions. The Liapunov property of φ is significant because of the role it plays in the thermodynamic and stochastic analysis of nonequilibrium systems: φ is defined in terms of the reactive flux to produce the intermediate and the flux to remove the intermediate. The derivative of φ with respect to the concentration of the intermediate yields an effective chemical driving force that is specific to the intermediate, and its time derivative yields a species-specific component of the dissipation that is minimized at steady states. These results hold both near to equilibrium and far from equilibrium for systems with one intermediate, independent of the number of steady states. Local Liapunov functions are also provided by the ‘‘excess dissipation,’’ the second variation in the entropy or in the Helmholtz free energy for the reaction chamber, and quadratic functions introduced in Keizer’s fluctuation–dissipation theory. Linearization of the force and flux expansions for nonequilibrium systems yields an idealized model in which the dissipation decreases monotonically in time and thus provides a Liapunov function for evolution to steady states. This result does not hold for a chemical system approaching a steady state with an arbitrarily small, but macroscopic displacement from equilibrium, even though the series expansions of the reactive fluxes and conjugate thermodynamic forces are closely approximated by truncation at the linear terms. There are always small regions in the immediate vicinity of nonequilibrium steady states where the dissipation increases in time while the system relaxes.
Comment on the role of reaction path curvature in diffusional barrier crossing processes
Authors: Larson Richard S.
Published: July 2002

Recently, some doubt has arisen regarding the importance of reaction path curvature in affecting the rate of diffusional barrier crossing processes. In particular, it has been argued that correction terms previously attributed to curvature can instead be traced to other features of the potential energy surface. It is shown here that, while curvature is not the only possible cause of deviations from the conventional rate expression, its effect is nonetheless very real.
Comment on: X-ray diffraction study of the effects of pressure on bilayer to nonbilayer lipid membrane phase transitions
Authors: Shyamsunder E. , Botos P. , Gruner S. M.
Published: July 2002

X-ray diffraction data of aqueous dispersions of dioleoylphosphatidylethanolamine (DOPE) have been obtained as a function of pressure and temperature. The well-known lamellar (Lα) to inverted hexagonal (HII) thermotropic phase transition that occurs in DOPE at about 6 °C at ambient pressure can be reversed at 20 °C when the pressure is increased from 1 to 500 bar. X-ray data in which the temperature is varied at fixed pressures shows that the increase in dP/dTBH is about 20 bar/K.
The Hall mobility of excess electrons in 2,2-dimethylbutane, 2,2,4-trimethylpentane, and 2,2,4,4,-tetramethylpentane
Authors: Itoh Kengo , Muñoz Raul C. , Holroyd Richard A.
Published: July 2002

The Hall mobility (μH) of excess electrons was measured in liquid 2,2-dimethylbutane (22DMB), 2,2,4-trimethylpentane (224TMP), and 2,2,4,4-tetramethylpentane (2244TMP) at temperatures from 20 to 160 °C. Drift mobilities μD were also measured over this temperature range. At 20 °C the observed values of μH for 22DMB, 224TMP, and 2244TMP are 12, 22, and 32 cm2/V s, respectively. In all three liquids μH increases with temperature. The comparison of μH and μD indicates that electron traps are absent in 22DMB and 2244TMP. In contrast, in 224TMP μH is about 3.5 times μD at 18 °C. Thus, in this liquid, traps are present and the concentration of traps is about one per thousand solvent molecules. The Hall mobility results are in reasonable accord with the expectations of the deformation potential model and support the concept of scattering of electrons in the conduction band due to density fluctuations.
On the performance of high-level correlated single-reference methods: The energy, dipole moment, and polarizability functions of BH
Authors: Diercksen Geerd H. F. , Sadlej Andrzej J.
Published: July 2002

The performance of the fourth-order many-body perturbation theory (MBPT) and different coupled cluster (CC) methods in calculations of the potential energy and electric property functions has been investigated for the X 1Σ+ state of the BH molecule. In spite of the single reference approximation most of these methods are capable of correctly predicting the shape of the energy and property functions. Both the qualitative features of these functions and their numerical values are considerably improved on passing from the MBPT to the CC models. The stability with respect to the near degeneracy of the reference state has been found to increase in the same way. Excellent results are obtained from the CC calculations for the potential energy and electric properties of the BH molecule in the vicinity of the equilibrium bond distance.
Application of hard sphere perturbation theory to a high-temperature binary mixture
Authors: Saumon D. , Chabrier G. , Weis J. J.
Published: July 2002

We calculate the thermodynamics of a high-temperature binary mixture (kT/ε>50) using a WCA fluid perturbation theory and hard sphere reference system. The problems of the softness of the repulsive core and the nonadditivity of the hard sphere diameters can be successfully circumvented by using appropriate separations between the reference and the perturbation potentials, as shown by comparing the results with Monte Carlo simulations.
Free-jet infrared absorption spectroscopy of rare gas–11BF3 complexes in the 7 μm region
Authors: Matsumoto Yoshiyasu , Ohshima Yasuhiro , Takami Michio , Kuchitsu Kozo
Published: July 2002

High-resolution infrared absorption spectra of the van der Waals complexes of BF3 with a rare gas atom (Ne, Ar, and Kr) are obtained near the ν3 band of BF3 monomer in a supersonic free jet. Each spectrum shows a characteristic perpendicular band of a symmetric-top molecule with C3v symmetry. The bands are shifted toward the red with respect to the monomer band by 0.3933(4), 1.7609(1), and 2.4059(4) cm−1 for NeBF3, ArBF3, and KrBF3, respectively. The Coriolis coupling constants of the complexes are almost identical to that of the monomer. These results show that complexing with a rare gas atom does not strongly influence the ν3 vibrational motion in BF3. The observed red shifts correlate well with the polarizabilities of the rare gas atoms. This finding is explained in terms of the instantaneous dipole–induced dipole interaction. The observed full widths of the Doppler-limited spectral lines, typically 70 MHz, indicate that the lower limit of the vibrational predissociation lifetime is 2 ns.
Hessian-biased force fields from combining theory and experiment
Authors: Dasgupta Siddharth , Goddard William A.
Published: July 2002

We describe a new approach of combining experimental and theoretical information to develop accurate valence force fields. We combine the Hessian from ab initio calculations with the structural and spectroscopic data from experiment to generate a new Hessian that is used for extracting force fields. Emphasis is placed on obtaining accurate geometries and accurate frequencies. This technique is illustrated by calculations on formaldehyde, formate anion, thioformaldehyde, carbonyl chloride, and carbonyl fluoride.
The frequency-dependent polarizability of O2 and the dispersion interaction in dimers containing O2 from a single, double, triple configuration interaction perturbation approach
Authors: Rijks W. , van Heeringen M. , Wormer P. E. S.
Published: July 2002

The frequency-dependent dipole polarizability tensor of the O2 molecule is calculated from effective spectra, that were obtained from a CI-perturbation method. The zeroth-order wave function is a 3Σ−g CI wave function consisting of all single excitations and a selection of double excitations from a reference space. The first-order perturbed wave function is expressed in a basis of all singly excited states and several different sets of doubly and triply excited states selected with different thresholds. We extrapolated the frequency-dependent polarizabilities to zero threshold and we used these polarizabilities to compute the C6 dispersion coefficients and γ6 anisotropy factors for the dimers of O2 with He, Ne, H2, N2, O2, HF, CO, NH3, and H2O.
Theoretical study of low-lying 1Σ− and 1Δ states of CO
Authors: Rosenkrantz M. E. , Kirby K.
Published: July 2002

A b initio calculations of electronic wave functions and potential energy curves for each of two low-lying states of 1Σ− and 1Δ symmetry for CO are reported. The second state of each symmetry has Rydberg character. Computed spectroscopic constants for the metastable I 1Σ− and D 1Δ states appear to be in good agreement with experimental data. The radiative lifetimes of vibrational levels within the I 1Σ− and D 1Δ states have been computed considering two types of decay: cascade within each electronic state, and transitions to the nearby A 1Π. It is shown that the v=0 levels of each metastable electronic state have very long lifetimes. The observation of these metastable states in electron impact experiments is discussed in light of our results.
Relaxation in supercooled liquids: A generalized hydrodynamic description
Authors: MacPhail Richard A. , Kivelson Daniel
Published: July 2002

We focus on the frequency dependence of the viscosity η(ω), or more specifically, of the retardational compliance Jr(ω), for supercooled liquids. We pursue a phenomenological approach which is related to extended hydrodynamics and is of a form compatible with a mode–mode coupling theory. While many of the transport coefficients entering the analysis are interrelated so as to give the correct temperature and frequency dependences of the observed quantities, an important consequence of the model is the prediction of nonArrhenius behavior for the Maxwell relaxation time τm. Our phenomenological approach suggests a mode coupling picture in which the bilinear modes are ‘‘semiconserved’’; in turn, the mode coupling analysis gives molecular insights into the empirical transport or relaxation coefficients.
Laser control of unimolecular decay yields in the presence of collisions
Authors: Shapiro Moshe , Brumer Paul
Published: July 2002

We show that coherent light can be used to control the relative product yield in unimolecular photodissociation, even in the presence of collisions. That is, coherence contributions to the photodissociation, which are responsible for control over the reaction, are sufficiently robust to survive inhomogeneous broadening effects over a substantial temperature range. A number of experimental implementations for control in the presence of collisions are proposed.
Numerical implementation of reactive scattering theory
Authors: Linderberg Jan , Padkjær So/ren B. , Öhrn Yngve , Vessal Behnam
Published: July 2002

Algorithms for the effective calculation of reactive scattering probabilities are developed and tested on the hydrogenic atom–diatom system described by the Siegbahn–Liu–Truhlar–Horowitz potential energy surface. A three-dimensional finite element procedure is designed from a description in terms of hyperspherical coordinates. The Wigner–Eisenbud R-matrix theory is used for a recursive procedure which admits control with limits on the hyperradial propagation inward from an asymptotic region and for a symmetry preserving transformation to arrangement channel Jacobi coordinates.
Optically enhanced electron attachment to thiophenol
Authors: Pinnaduwage Lal A. , Christophorou Loucas G. , Hunter Scott R.
Published: July 2002

Enhanced electron attachment to KrF excimer laser irradiated thiophenol (C6H5SH) molecules has been studied in a electron swarm experiment using nitrogen as the buffer gas. Two distinct electron attachment processes were found to be responsible for the observed large enhancement in electron attachment under different experimental conditions. One enhanced electron attachment process occurred immediately (within a few μs) after laser irradiation and is shown to be due to dissociative electron attachment to electronically excited thiophenol molecules in their first excited triplet state produced indirectly via excited singlet states reached by excimer laser irradiation. At low mean electron energies (∼0.1 eV), up to 5 orders of magnitude enhancement in electron attachment has been observed for the triplet state compared to the ground electronic state. This enhanced electron attachment decreased with (i) increasing nitrogen pressure due to quenching of the first excited singlet state of thiophenol (precursor of the triplet state) by nitrogen, and (ii) increasing time delay between laser irradiation and subsequent electron attachment to the laser-irradiated molecules. This latter observation is shown to be due to the self-(triplet–triplet) quenching of the electron attaching triplet state molecules. The second observed enhanced electron attachment process occurred at longer times (>100 μs) after laser irradiation and is attributed to the electron attachment to diphenyl disulfide (C6H5SSC6H5) produced by the interaction of thiophenoxy radicals (C6H5S ̇) formed directly or indirectly via laser irradiation.
Desorption kinetics on an alkali metal-precovered surface: CO and K on Pt(111)
Authors: Whitman L. J. , Ho W.
Published: July 2002

The desorption kinetics of CO initially coadsorbed with K on Pt(111) at 100 K has been measured as a function of both CO and K coverage (θ) using temperature programmed desorption (TPD). CO and K desorb separately when the initial θCO is low, supporting the theory that CO and K interact via a strong Coulombic attraction and do not form an oxocarbon (salt-like) compound. Although precoverage with ≥0.15 ML K does induce a few percent of the desorbing CO to undergo atomic exchange, the effect is very small in comparison to that observed on other surfaces. The desorption is found to be first order in θCO on both clean and K-precovered surfaces. With low initial θCO the activation energy (E) and preexponential factor (ν) for desorption from the K-free surface are found to be 28.8±0.7 kcal mol−1 and 1013.3±0.2 s−1, respectively, in excellent agreement with previously reported results. As observed with most alkali metal-precovered surfaces, the low coverage CO desorption peak temperature increases monotonically with K coverage. We find that the higher desorption temperatures result from the combined effects of increasing E and simultaneously decreasing ν, with the change in ν causing most of the shift; when low θCO are initially coadsorbed with 0.15 ML K E=31.3±0.6 kcal mol−1 and ν=1011.4±0.2 s−1. For all surfaces studied both E and ν decreased approximately linearly with increasing θCO>0.1 ML. The θCO and θK dependence of the kinetics is generally consistent with the proposed structural model for this surface based on the formation of CO+K islands. Possible mechanisms for the K-induced change in the kinetics are discussed and further investigations are proposed.
Two-dimensional electron–electron double resonance and electron spin–echo study of solute dynamics in smectics
Authors: Gorcester Jeff , Rananavare Shankar B. , Freed Jack H.
Published: July 2002

Electron spin–echo (ESE) and two-dimensional electron–electron double resonance (2D ELDOR) experiments have been performed as a function of director orientation and temperature in the smectic A phase of the liquid crystal S2 for the spin–probe PD-tempone(2×10−3 M). Over the entire temperature range studied (288–323 K) we observe significant 2D ELDOR cross peaks only for ΔMI =±1 indicative of 14N spin–relaxation and negligible Heisenberg exchange. From the angular dependent 14N spin–relaxation rates we obtain the dipolar spectral densities at the hyperfine (hf) frequency, whereas from a combination of ESE and 2D ELDOR we obtain the dipolar and Zeeman-dipolar spectral densities at zero frequency. The angular dependent spectral densities were successfully decomposed into their basic components in accordance with theory. The angular dependent spectral densities at the hf frequency are not predicted by a model of anisotropic rotational diffusion in a nematic orienting potential, but are consistent with predictions of a model due to Moro and Nordio of solute rototranslational diffusion in a McMillan-type potential. The angular dependence also indicates that order director fluctuations in the smectic phase are suppressed at frequencies on the order of 10 MHz. An additional contribution to solute reorientation due to cooperative hydrocarbon chain fluctuations is suggested to account for the behavior of the observed spectral densities at zero frequency. An evaluation of the relevance of several other dynamical models to this experimental work is also presented.
Electronic spectroscopy of neutral indium phosphide clusters
Authors: Kolenbrander K. D. , Mandich M. L.
Published: July 2002

Electronic predissociation and two-color resonant two-photon ionization (R2PI) spectroscopic techniques have been successfully applied for the first time to the investigation of a broad distribution of small neutral clusters of the important III–V semiconductor, InP. Both techniques provide intense signals which display marked wavelength dependence with cluster composition. These techniques promise to provide a variety of critical spectroscopic results for the entire series of small InxPy clusters entrained in the molecular beam, and can be extended to permit the study of many other types of clusters.
Hund’s rule and singlet–triplet energy differences for the lowest nπ* states of formaldehyde, H2CO
Authors: Darvesh Katherine Valenta , Boyd Russell J.
Published: July 2002

Multireference single and double substitution configuration interaction calculations have been carried out for the 1A″ and 3A″ states arising from the lowest n→π* excitation of formaldehyde. In order to provide a fundamental basis for the interpretation of Hund’s rule in a polyatomic molecule, vertical and adiabatic differences of the total energy and its components have been computed from the wave functions. For a vertical comparison of the two states near their equilibrium geometries, it is shown that electron–electron repulsion is less in the triplet state than in the singlet state, in contrast with the corresponding results for the 1B1 and 3B1 states of methylene. The nπ* states of formaldehyde provide the first example of a molecular system in which a vertical comparison reveals greater electron–electron repulsion in the singlet than in the triplet state. For the adiabatic comparison, the opposite occurs. The greater electron–electron repulsion in the triplet state is a consequence of its shorter equilibrium bonds lengths.