bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational distributions from photodissociations. IV. The bent triatomic molecule
Authors: Morse M. D. , Freed Karl F.
Published: February 2003

The generalized Franck–Condon theory for the photodissociations of bent triatomic molecules is formulated, including bending and rotational degrees of freedom. The nonseparable multidimensional Franck–Condon amplitudes are reduced to a rapidly convergent series of products of one-dimensional integrals. The Franck–Condon factors provide the photofragment quantum state distributions when inelastic scattering on the unbound surface is negligible. When the scattering is important, they provide the nascent amplitudes which are the driving terms in the full scattering treatment. Calculations of the expected photofragment rotational distributions are given under dipole and scalar couplings which are, respectively, appropriate to direct photodissociation and predissociations. Semiquantitative closed form WKB approximations to the Franck–Condon amplitudes are derived to exhibit the essential physical features of these amplitudes. The results are readily interpreted in terms of the Heisenberg uncertainty principle, the orientational constraints imposed by the bond angle of the bent triatomic molecule, and the conservation of angular momentum. A simple classical approximation is also used and results are in qualitative agreement to the quantal calculations. Comparisons are also provided with recent rotational distributions observed in H2S photodissociation and illustrative calculations are presented for predissociation of HCN from the C̃ state and direct photodissociation of HOCl from its ground state.
Modeling of ion–molecule reactions at high pressures
Authors: Collins C. B. , Lee F. W. , Tepfenhart W. M. , Stevefelt J.
Published: February 2003

Reported here is a model of ion–molecule reactions that includes a variety of constituent steps which might be appropriate in an environment at atmospheric pressures. A fairly general structure has been assumed for the type of reaction complex that might be assembled from the reacting partners and both coherent and incoherent processes of excitation of the complex have been included in the model. The transition to the product state was modeled as a simple predissociation. From these considerations a comprehensive reaction rate matrix was constructed that described the transition rates between each pair of levels of the complex for a particular set of experimental parameters. The smallest eigenvalue of such a rate matrix was found to generally correspond to an eigenvector of populations that reasonably described the decay of the original reacting species. Since the reaction of He+2 with N2 at atmospheric pressures was the best characterized of the systems appearing in the literature, it was chosen as a vehicle for the first application of this model. The results were found to be in complete agreement with experiment for reasonable values of model parameters.
The dynamics of the reactions F+IX→IF+X (X=Cl, Br, I): A laser-induced fluorescence study
Authors: Trickl T. , Wanner J.
Published: February 2003

The internal product state distributions of IF formed in the homologous exoergic reactions F+IX→IF+X (X=Cl, Br, I) were measured by the laser-induced fluorescence technique. The study was carried out with crossed molecular beams at collision energies of ∼3.2 kJ mol−1. The observed bimodal IF vibrational product state distributions indicate microscopic branching. The average fraction of the total energy available channeled into vibration is modest in the case of X=Br, Cl (〈f′V〉=0.37, 0.27) and very low for X=I (〈f′V〉=0.07). For the reaction F+ICl, rotational population analysis could be carried out. The average fraction of rotational product excitation was found to be 〈f′R〉=0.16. The amount of translational excitation 〈f′T〉=0.15 was determined by Wong and Lee for the reaction F+I2. Since this value is also low, a substantial amount of the exoergicity must be channeled into electronic excitation of the 2P1/2 state of the product halogen atoms. The bimodal vibrational product state distributions are interpreted as resulting from two microscopic pathways: (1) a direct channel leading to the formation of highly vibrationally excited IF and (2) a complex interaction which results in monotonically decreasing IF distributions at low vibrational levels and halogen product atoms in the 2P1/2 state. The branching ratio k1/k2 for the two channels was determined to be k1/k2=0.90, 1.44, 0.12 for X=Cl, Br, I, respectively.
Reduction of derivatives and simple applications of the Legendre transformed density functional theory
Authors: Nalewajski Roman F.
Published: February 2003

A reduction of derivatives in the Legendre transformed density functional theory (DFT) of the single-component (atomic or fixed-nuclei molecular) systems is described. It is shown that a given derivative can be expressed by an identity in terms of a set of basic derivatives. The basic partial functional derivatives for the general case of N electrons in the external field v(r) are: α(r)=[∂ρ(r)/∂N]/ρ(r), β(r, r′)=−[∂ρ(r)/∂v(r′)]N/ρ(r), and γ(r, r′)=−[∂ρ(r)/∂v(r′)]μ/ρ(r); ρ is the density and μ denotes the chemical potential. For atoms, an additional alternative set of four integral derivatives, involving only global (referring to a system as a whole) parameters, is introduced: α̃=−[∂vne/∂N]Z/vne, β̃=−[∂vne /∂Z]N/vne, γ̃=[∂N/∂Z]μ/N, and χ̃=−[∂vne/∂Z]μ/vne; Z is the nuclear atomic number and vne is the electron-nuclear attraction energy per unit nuclear charge. The reduction procedure makes free use of the DFT ‘‘thermodynamic’’ diagram equations, Maxwell relations, and mathematical manipulations on functional derivatives. Several representative applications are indicated dealing with the system responses to slight changes in one of the system parameters subject to different constraints.
Valence effective Hamiltonian technique for nitrogen containing polymers: Electronic structure of polypyrrole, pyrolized polyacrylonitrile, paracyanogen, polymethineimine, and derivatives
Authors: Brédas J. L. , Thémans B. , André J. M.
Published: February 2003

We present the parametrization of nitrogen for use in the valence effective Hamiltonian (VEH) technique. We describe five different parametrization schemes and discuss the quality of the results each of them affords. The VEH method is then applied to the study of the electronic structure of nitrogen containing polymers of interest with regard to the conducting polymers area. These polymers include polypyrrole, poly-β-dimethylpyrrole, poly-N-methylpyrrole, pyrolized polyacrylonitrile, paracyanogen (polypyrazinopyrazine), and polymethineimine [CH=N]x. Parameters related to the conductivity properties upon doping such as ionization potentials, bandgaps, and bandwidths, are discussed in detail.
ABBOT: Atomic billiard ball orbital theory
Authors: Underhill P. R.
Published: February 2003

A new semiempirical molecular orbital method is proposed which draws a formal link with Xα atomic calculations. The method permits use of the Slater transition state and also calculation of total energies.
Unusual proton transfer patterns in crystalline ammonia hemihydrate (2NH3⋅H2O)
Authors: Bertie John E. , Devlin J. Paul
Published: February 2003

The crystalline hemihydrate of ammonia (2NH3⋅H2O) resembles water–ice in having an H-bonded network structure. However, the H bonds are all hetero in nature with the result that the hemihydrate offers a marked contrast to ice in its proton transport properties. At low temperatures ice is characterized by a very few highly mobile ion defects with the mobility believed to depend on H3O+ tunneling involving O—H—O linkages. By contrast, the hemihydrate is presumed to have an abundance of ion-pair defects (NH+4, OH−) which, the present study suggests, are lacking in mobility. As a result proton transport in water–ice at 140 K is comparatively very rapid. The ability to matrix-isolate D2O intact in crystalline ice by a codeposition at T≊125 K, was basic to the 140 K water–ice proton-transport study. Similarly, all compositions of the hemihydrate embodied in the formula xNH3⋅(2−x)ND3⋅yH2O⋅(1−y) D2O are now accessible by codeposition methods, and thermally induced proton exchange has been monitored for a number of such crystalline samples. Though little, if any, exchange occurs for the pure hemihydrate at T≊140 K, proton transfer within the ammonia subsystem can be induced by either ammonia or water doping of the hemihydrate. Since ion defects are abundant in the pure hemihydrate, the role of the dopant molecules is considered to be the mobilization of the NH+4 ion defects rather than a modification of the ion-defect concentrations, the normal dopant role in ice proton transfer. Despite the induced proton transfer within the ammonia subsystem, the water subsystem invariably remained blocked to proton transfer such that, despite reaction times of several hours at 135 K, much less than 5% of the water molecules normally experienced isotopic exchange.
Temperature dependence of the mutual diffusion coefficients of He–Ar, Ne–Ar, and Xe–Ar from 350 to 1300 K
Authors: Taylor W. L. , Cain D.
Published: February 2003

Mutual diffusion coefficients of the He–Ar, Ne–Ar, and Xe–Ar systems were measured in the temperature range from approximately 350 to 1300 K. These results agree within the mutual uncertainty with the Marrero and Mason correlations derived from diffusion and molecular beam data. Viscosity and thermal diffusion derived data were found to be mutually consistent with diffusion to within ∼5% at high temperatures and probably better than ∼2 1/2% at the lower temperatures. A comparison with theoretical diffusion coefficients calculated from recently proposed intermolecular potentials is made.
Bonding and structure of Nd3+ in BeF2 glass by XANES and EXAFS spectroscopy
Authors: Rao K. J. , Wong J. , Weber M. J.
Published: February 2003

The LIII, LII, and LI XANES and EXAFS of Nd3+ have been measured using synchrotron radiation from SPEAR at Stanford Synchrotron Radiation Laboratory to probe the bonding and structure of Nd3+ in a BeF2 glass containing 4 mol % NdF3. Crystalline NdF3 and Nd2O3 were used as reference compounds to model the chemical environment of Nd3+ in the glass. It is found that in the glass there is a substantial increase in the intensity of the Nd LIII and LII white lines compared with those of crystalline NdF3. EXAFS analysis showed that there is a shrinkage of the innermost Nd–F bond distance and a reduction of nearest neighbor fluorine coordination to ∼7 in the glass compared with nine in pure NdF3. The sevenfold coordination is in agreement with recent molecular dynamics calculations. Using Slater’s atomic shielding constants, the observed increase in white line intensities in the glass was attributed to a covalency effect due to back donation of valence electron from the ligand to the partially filled 4f orbitals of the Nd3+ ions. This study illustrates ability of the combined XANES and EXAFS technique in elucidating the chemical bonding and local structure of a given atomic constituent in glassy solids.
Magnetic susceptibility measurements on a frozen solution of FeCl−4 anions in isopropanol in the 0.07–4.2 K temperature range
Authors: Rodríguez F. , Moreno M. , Gómez-Sal J. C. , Tholence J. L.
Published: February 2003

Magnetic susceptibility measurements in the range 0.07–4.2 K have been performed on a 0.32 M solution of [FeCl4]− anions in isopropanol. At 0.1 K the experimental susceptibility is more than an order of magnitude smaller than that corresponding to a single [FeCl4]− monomer, a fact which clearly suggests the formation of dimers of [FeCl4]− with antiferromagnetic coupling. A careful analysis of the susceptibility curve has been performed allowing the existence of dimers as well as of monomers in the solution. Moreover, zero-field splitting terms of the form D[s2z−1/3 s(s+1)] have also been introduced in the theoretical description of both monomers and dimers. The experimental susceptibility can be satisfactorily explained along these lines and is consistent with the following conclusions: (1) Around 90% of the [FeCl4]− units present in the solution are in dimer form while the remainder are present as monomers; (2) The effective exchange interaction between the two ferric ions involved in a dimer is J=(0.15±0.02) cm−1. This value is consistent with a Fe–Cl–S–Cl–Fe superexchange path where S could be a solvent molecule; and (3) The [FeCl4]− units do not display a right Td symmetry but they are distorted by the interaction with the solvent giving rise to a value D=−(0.60±0.10) cm−1. The present results point out that magnetic susceptibility measurements in that range of temperature can provide us with useful and detailed information on the solute–solute and solute–solvent interactions in liquid solutions involving a paramagnetic complex.
Two-fluid theory of coupled kinetic equations for a test particle and a background fluid
Authors: Sung Wokyung , Dahler John S.
Published: February 2003

The generalized Langevin equation formalism of Mori and Zwanzig and Akcasu and Dudenstadt is used to derive coupled kinetic equations for the distribution functions of a tagged test particle and of the untagged members of the surrounding background fluid. The interaction between any two of these particles is taken to be the sum of a long-ranged continuous potential and a short-ranged, hard-core contribution. The distribution function of the background fluid is chosen to depend on the location of a solvent particle measured relative to the instantaneous position of the test particle. Because of this choice, the frequency matrix associated with the pair of distribution functions incorporates much of the coupling between the tagged and untagged particles which otherwise would be imbedded within their separate memory functions. Consequently, the mean-field approximation obtained by neglecting the memory function of this two-fluid theory is expected to be quite accurate. Explicit forms are derived for the coupled mean-field kinetic equations gotten in this way. In regions remote from the test particle, the kinetic equation for a member of the solvent reduces to previously derived kinetic equation of the type called revised or generalized Enskog equations. Finally, it is shown that the kinetic equations of this two-fluid, mean-field theory also can be gotten by truncating the appropriate BBGKY hierarchy of equations, using a plausible, easily interpretable connection between pair-space distribution functions and the singlet distribution functions specific to the tagged and untagged particles.
Electron paramagnetic resonance study of Mn2+ doped ammonium iodide single crystals
Authors: Chand Prem , Upreti G. C.
Published: February 2003

ERP of Mn2+ doped in ammonium iodide single crystals has been studied at room temperature (∼300 K) and at X band (∼9.3 GHz). An axial complex with the following spin-Hamiltonian parameters has been observed: g∥=2.0014±0.0005, g∥=2.0028±0.0005, b02=−1611±2 G, b04=6±1 G, ‖A‖=‖B‖=90±1 G. Some unusual features observed in the spectra at X band due to the large value of b02 have been discussed.
Laser light scattering studies of the dynamics of molecular reorientation of a viscoelastic liquid: α-phenyl o-cresol
Authors: Wang C. H. , Ma R. J. , Fytas G. , Dorfmüller Th.
Published: February 2003

The reorientational motion of α-phenyl o-cresol molecules in the normal liquid and supercooled liquid state in the temperature range between 97 and −49.3 °C has been investigated using the dynamic light scattering technique. Above 5 °C, Fabry–Perot interferometry is used, but the photon correlation spectroscopic technique is employed below −28.9 °C. With the combined use of these two light scattering spectroscopic techniques, the orientational relaxation time over ten decades of change is measured. The shape of the VH time correlation function is nonexponential. The bimodal and the Williams–Watts distribution functions are used to fit the shape of the VH correlation functions at various temperatures. The Williams–Watts function gives the best fit to the observed shape. The correlation time and the shape of the VV time correlation function are found to be similar to that of the VH, suggesting that the reorientational process is coupled to the localized translational motion for molecules in the supercooled state. Angular dependent studies of the VH and VV time correlation functions indicate that in the supercooled state, the shear wave (a collective translational motion) influences the reorientational motion only negligibly. Various theoretical predictions about the profile of the central component of the VH spectrum based on the coupling of the reorientation to the shear wave are not supported by the experiment result. The temperature dependence of the orientational relaxation time does not follow the Arrhenius equation, but it follows the Antoine-WLF equation reasonably well. The Debye–Stokes–Einstein equation is found to be adequate to describe the temperature and viscosity dependence of the orientational relaxation time over ten decades of change.
Preliminary rovibrational analysis of the nν6+ν1−nν6 vibration in HCN⋅⋅⋅HF
Authors: Kyrö E. , Warren R. , McMillan K. , Eliades M. , Danzeiser D. , Shoja-Chaghervand P. , Lieb S. G. , Bevan J. W.
Published: February 2003

A preliminary rotation-vibration analysis of the n=0 and n=1 subbands associated with the nν6+ν1−nν6 hydrogen-bonded vibration in HCN⋅⋅⋅HF has been completed. The following excited state rotational constants B′ and band origin frequencies ν0 have been determined for the complex. The results are consistent with a rotation-vibration interaction constant α1=−68.3±1 MHz which correlates with an excited state r(N⋅⋅⋅F) internuclear distance of 2.762 Å, a decrease of 0.034 Å relative to the ground state. Excited state lifetimes associated with assigned transitions are demonstrated to be ≥1.8×10−10s while the x16 anharmonic constant is evaluated to be 4.01±0.03 cm−1.
Vibrational energy transfer in CO2–C2H2n+2 (n=0, 1, 2) mixtures from 220 to 300 K
Authors: Manzanares I. Carlos , Pinzon Hernando , Fumero Juan , Gonzalez Carlos , Sanchez Eugenio
Published: February 2003

Time resolved laser induced infrared fluorescence studies have been performed to obtain the rate constants of energy transfer of CO2 (ν3) in mixtures with C2H2, C2H4, and C2H6 at temperatures between 220 and 300 K. A pulsed 10.6 μm CO2 laser light has been used to directly excite the CO2 molecules at low temperatures. The CO2 self-relaxation was measured and the results are in agreement with previous measurements. The rate constants for the transfer of energy in the CO2–C2H2 system show normal temperature behavior, (rate constant increases as the temperature increases) associated with large energy defect and caused by short-range repulsive forces. The rate constants for the systems CO2–C2H4 and CO2–C2H6 show an inverse temperature effect, (rate constant increases as the temperature decreases) associated with near resonant energy exchange and long-range attractive forces between the molecules. The observed behavior with temperature for the three systems is explained using a Morse potential which includes repulsive and attractive terms to describe the interaction between the molecules and a probability expression formulated by H. K. Shin, to calculate the energy transfer probabilities of some important reactions considered for each system.
Dynamics of the chemiluminescent oxidation of zinc atoms by nitrous oxide
Authors: Wicke Brian G.
Published: February 2003

The dynamics of the chemiluminescent oxidation of zinc atoms by nitrous oxide to form ZnO* have been studied utilizing a laser vaporization pulsed beam source to produce the zinc reactant. The kinetic energy dependence of the chemiluminescent reaction cross section has a threshold at 1.6×10−19 J/molecule (1.0 eV) in the center of mass; this threshold value has been used to estimate the dissociation energy of ZnO as D≥4.48×10−19±0.32×10−19 J [2.8(±0.2) eV]. The reaction cross section increases with increasing kinetic energy in accord with the Arrhenius model from threshold to 3.2×10−19 J (2.0 eV), but then increases more rapidly. Above threshold, small amounts of thermal energy added to the N2O reactant increase the reaction cross section 70 times more than the equivalent energy supplied as relative kinetic energy. This energy selectivity is interpreted in terms of an electron jump reaction mechanism which is significantly enhanced by the ν2 bending vibrational mode of N2O.
Atomic charges and charge flows from infrared intensities: C–H bonds
Authors: Gussoni M. , Jona P. , Zerbi Giuseppe
Published: February 2003

Use is made of experimental infrared absorption intensities for calculating atomic charges and charge flows for a class of simple organic molecules. Various methods are discussed. Correlations of the calculated quantities with other independent structural parameters shed some new light on the peculiarities of the electrical properties of CH bonds.
Ab initio calculation of the X 1Σ+ state of CsH
Authors: Laskowski Bernard C. , Walch Stephen P. , Christiansen Phillip A.
Published: February 2003

Relativistic effective core potential (RECP) and nonrelativistic all electron (NRAE) calculations are reported for the X 1Σ+ state of CsH. The calculations are based on GVB plus single and double excitation CI wave functions with the Cs(5s, 5p) as well as the two valence electrons correlated. The bonding in CsH is found to involve a Cs(6s)–H(1s) bond but with a significant ionic (Cs+H−) component. Correlating the Cs(5s, 5p) electrons leads to a significant bond shortening effect (∼0.15a0). This effect is attributed to a contraction of the Cs(6s) orbital and a shift toward higher energy of the Cs+H− asymptote upon correlating the 5s, 5p shell. Bond shortening due to relativistic contraction of the Cs(6s) is only ∼0.05a0 in agreement with the small amount of Cs(6s) character in the ionic bond orbital. The NRAE and RECP potential curves are very similar except for the shorter Re for the RECP case (due to relativistic bond contraction).
Stochastic transport in disordered systems
Authors: Sahimi Muhammad , Hughes Barry D. , Scriven L. E. , Davis H. T.
Published: February 2003

We develop a theory of stochastic transport in disordered media, starting from a linear master equation with random transition rates. A Green function formalism is employed to reduce the basic equation to a form suitable for the construction of a class of effective medium approximations (EMAs). The lowest order EMA, developed in detail here, corresponds to recent approximations proposed by Odagaki and Lax [Phys. Rev. B 24, 5284 (1981], Summerfield [Solid State Commun. 39, 401 (1981)], and Webman [Phys. Rev. Lett. 47, 1496 (1981)]. It yields an effective transition rate Wm which can be identified as the memory kernel of a generalized master equation, and used to define an associated continuous-time random walk on a uniform lattice. The long-time behavior of the mean-squared displacement arising from an initially localized state can be found from Wm, as can diffusion constants in any case where the long-time behavior of the system is diffusive. Detailed calculations are presented for seven lattice systems in one, two, and three dimensions, and for a variety of probability density functions f(w) for the transitions rates. For percolation-type densities, i.e., those with only a fraction p<1 of the bonds transmitting, the EMA predicts three distinct kinds of behavior: localization, ‘‘fractal’’ transport with slower than linear growth of the mean-squared displacement, and diffusion in the cases p<pc, p=pc, p≳pc, respectively, where pc is the bond percolation threshold of the lattice. Depending on the form of f(w) near w=0, critical exponents may take values independent of f(w) (‘‘universality’’) or heavily dependent on f(w) (‘‘nonuniversality’’).
Experimental measurements of solvation forces in nonpolar liquids
Authors: Christenson Hugo K.
Published: February 2003

Forces between molecularly smooth mica surfaces immersed in tetrachloromethane, benzene, and 2,2,4-trimethylpentane have been measured. In tetrachloromethane and benzene the force law is an oscillatory function of the separation between the mica surfaces, with a periodicity equal to the mean molecular diameter and a measurable range of appoximately ten periods, or about 5 nm. Beyond this separation, the oscillations merge into a purely attractive ‘‘tail’’. In 2,2,4-trimethylpentane the force law shows only short-range oscillatory behavior, below 2 nm; at larger separations the force is everywhere attractive. The results are compared with measurements on cyclohexane, octamethylcyclotetrasiloxane and n-octane, including some repeat measurements. When the distance scale is normalized by the mean periodicity of the force curve for each liquid, the results for the four fairly rigid molecules benzene, tetrachloromethane, cyclohexane, and octamethylcyclotetrasiloxane are very similar; minor differences possibly being due to varying trace amounts of water in the liquids. In the above systems the force laws are oscillatory functions of the surface separation, with a periodicity roughly equal to the molecular diameter and a measurable range of about ten periods. The peak-to-peak amplitude shows an essentially exponential decay. By contrast, the existence of free intramolecular rotation in the two flexible molecules 2,2,4-trimethylpentane and n-octane is sufficient to reduce the range of the solvation forces to 2 nm or less. The results indicate that the amount of liquid structuring for inert liquids depends mainly on the internal rigidity of the molecules, while the effect of molecular shape, size, and temeperature is of secondary importance.
Photophysical anomalies of acridine in n-hexane
Authors: Struve W. S. , Hedstrom J. H. , Morgante C. G. , Jameson A. K. , Lim E. C.
Published: February 2003

The picosecond risetime of the 430 nm Tn←T1 transient absorption and the T1 formation quantum yield for acridine in n-hexane are temperature independent within experimental error between −51 and 25°C, in sharp contrast to the large temperature dependence of the fluorescing state of acridine in n-hexane is not a kinetic precursor to T1. The absence of marked tempertaure dependence in the T1 formation yield presents and exception to the general observation that fluorescence and T1 yields both decrease with increasing temperature in N-heterocycles and aromatic carbonyls with closely spaced lowest--energy n, π* and π, π* singlet states. (AIP)
Elastic scattering of electrons and positrons by CH4 at 25–800 eV
Authors: Jain Ashok
Published: February 2003

The differential and integral cross sections for the e±-CH4 elastic scattering are reported in the energy range 25–800 eV. A simple model is used: assuming methane molecules to be spherical, simple analytic form of the static potential is derived by employing spherical wave function; this static potential, along with approximate polarization potential and a local exchange potential, is treated exactly to calculate cross sections via partial-wave analysis. The integrated cross sections for electrons and positrons start approaching each other above 500 eV. The positron-methane cross sections are found to be smaller everywhere and without any forward peak than the corresponding electron cross sections. The differential cross sections for the electron scattering are in very good agreement with the available experimental data.
The dependence of the rate constant of chemical reaction on density derived from the generalized Enskog equation for dense gases
Authors: Cukrowski A. S. , Popielawski J.
Published: February 2003

For reaction A+A=B+C it is shown (after application of formalism similar to that used by Prigogine and Xhrouet for analysis of generalized Boltzmann equation) that the ratio k/k0 (where k and k0 are the values of rate constant in dense and dilute gas, respectively) is equal to the radial distribution function χ taken at the point of contact of reacting spheres. Comparison of theoretical results derived in this paper and experimental results due to Kistiakowsky on decomposition of hydrogen iodide is given and good agreement between those results is obtained.
Pulsed laser photolysis study of the reaction between O(3P) and HO2
Authors: Ravishankara A. R. , Wine P. H. , Nicovich J. M.
Published: February 2003

The kinetics of the reaction O(3P)+HO2 → k1OH+O2 (1) has been investigated at 298 K as a function of N2 diluent gas pressure from 10 to 500 Torr. O(3P) was produced via 248.5 nm KrF laser photolysis of O3 followed by the rapid quenching of the O(1D) to O(3P) by N2 and HO2 was produced via simultaneous photolysis of H2O2 (by the same laser) to create OH radicals which in turn reacted with H2O2 to yield HO2. The O(3P) temporal profile was monitored via the time resolved resonance fluorescence technique. [HO2] was calculated based on known and experimentally measured parameters. The measured value of k1 was independent of N2 pressure with a mean value of (6.2±1.1)×10−11 cm3 molecule−1 s−1, where the error represents 2 σ and includes estimates of systematic errors in the determination of [HO2]. This work is the first study where reaction (1) has been investigated under pseudo-first order conditions at pressures greater than a few Torr. Also, these experiments represent the first time the described techniques have been employed for studying radical–radical reactions.
Effect of electronic transition dynamics on iodine atom recombination in liquids
Authors: Ali Domenic P. , Miller W. H.
Published: February 2003

The Langevin stochastic trajectory model used by Hynes, Kapral, and Torrie [J. Chem. Phys. 72, 177 (1980)] to describe secondary recombination of iodine atoms in solution has been generalized to allow for electronically inelastic transitions between the ten I–I potential curves that dissociate to ground state iodine atoms. The electronic transitions are treated via the Miller–George version of the Tully–Preston surface hopping model. The main qualitative result is that electronically inelastic processes substantially slow down the rate (and ultimate probability) of recombination. It is also seen that the electronic inelasticity changes from being strong at large r, where an essentially Boltzmann distribution over the various electronic states is maintained, to being weak at small r, where the distribution is far from Boltzmann.
Dynamics of O(1D2) reactions with bifunctional substrates: Alcohols and thiols
Authors: Goldstein Neil , Wiesenfeld J. R.
Published: February 2003

The OH and OD product ratios and vibrational distributions following the reactions of O(1D2) with isotopically labeled alcohols and several thiols were characterized using laser induced fluorescence. The primary site of O(1D2) attack upon the alcohols is the O–H bond, attack upon the C–H bond also being observed. Both reactions occur via an insertion/elimination mechanism. Product vibrational distributions for each pathway are similar to those observed following reaction with the corresponding monofunctional substrate.
M dependence in rotationally inelastic collisions in cell experiments: Implications of an irreducible tensor expansion for molecules in 1Σ electronic states
Authors: Alexander Millard H. , Davis Stephen L.
Published: February 2003

The tensorial coupling of initial and final angular momenta, commonly used in atomic collisions and attributed to Grawert, is here applied to M dependent collisions of molecules in 1Σ electronic states under conditions where the relative velocity vectors of the collision partners are uniformly distributed with respect to a laboratory fixed z axis. The integral inelastic cross sections are given by sums over tensor opacities, weighted by squares of vector coupling coefficients. The resulting expressions differ from those obtained within the tensorial treatment usually applied to molecular collisions. One can rigorously show that strict M conservation will not occur and, furthermore, that the complete matrix of M → M′ cross sections can be related to the smaller set of tensor opacities. In the energy sudden limit the JM → J′M′ cross sections can be related to the degeneracy-averaged J″ → 0 cross sections. Accurate close-coupling cross sections for the He–CO system due to Green are used to obtain the relevant tensor opacities and to demonstrate how degeneracy averaged J″ → 0 cross sections can be extracted from the fully resolved JM → J′M′ cross sections. Finally, the sudden scaling relation is used to analyze rotational relaxation in the Na2(A 1Σ+u)+He system. We show that θ conservation, where θ=cos−1(J ⋅ ẑ), is a more appropriate dynamical model than M conservation.
Vibrational excitation of the allene–fluorine reaction in cryogenic matrices: Possible mode selectivity
Authors: Knudsen Arne K. , Pimentel George C.
Published: February 2003

The infrared induced reaction of allene (C3H4) and fluorine in N2, Ar, Kr, and Xe matrices at 12 K has been studied. The reaction is promoted by the selective vibrational excitation of C3H4 ⋅ F2 reactive pairs and the course of the reaction followed by infrared spectroscopy. Four products are detected, consistent with the concerted addition of F2 across the C–C double bond to form vibrationally excited cis- and gauche-CH2CFCH2F which may then eliminate HF to yield either CH≡CCH2F ⋅ HF or CH2=C=CHF ⋅ HF pairs. The relative abundance of each product depends markedly upon the matrix. Selective vibrational photoisomerization of CH2=CFCH2F was used to distinguish and assign spectral features due to the cis- and gauche-rotameters as well as to provide evidence about the torsional barrier in the matrix. In a particular matrix, the allene–fluorine reaction product distribution is independent of the exciting frequency, with measurements at 1679, 1953, 1999, and 3076 cm−1. However, the relative quantum yield at 3076 cm−1 is three orders of magnitude higher than observed at the lower frequencies. Furthermore, the quantum yields at 1680, 1953, and 1999 cm−1 show differences that implicate mode selectivity in the allene–fluorine reaction. The CH2 rocking overtone 2ν9 at 1999 cm−1 and perhaps as well the CH2 bending overtone 2ν10 at 1679 cm−1 show a higher reaction propensity than that of the asymmetric skeletal stretching vibration ν6 at 1953 cm−1.
Time resolved polarization spectroscopy: Level kinetics and rotational diffusion
Authors: Cross Albert J. , Waldeck David H. , Fleming Graham R.
Published: February 2003

Time resolved polarization spectroscopy is a powerful and sensitive technique for the study of dynamics in the liquid phase. Using this technique, it is possible to obtain information about various relaxation processes in solution, including the motions of molecules in different electronic states, and the transitions between these states. Since the experiment typically measures both electronic relaxation and molecular reorientation, the observed signal can have a complicated form which represents a coupling of these two effects in a way that depends on the particular scheme of decay processes which are present in a given system. We present a general algorithm for deriving the form of the experimental signal for an arbitrary scheme for systems exhibiting these phenomena, assuming that the molecular motion is described by asymmetric rotational diffusion. Several examples are presented and experimental results interpreted using the derived formulas, including cases where (1) the excited state diffusion tensor differs from that of the ground state, (2) decay of the excited state occurs to levels that do not return to the ground state, (3) the principal contribution to the signal results from birefringence, (4) there are several intermediate excited states which undergo rapid relaxation, and (5) two overlapping absorption bands create an initial condition with more than one excited state populated. Application of this formalism to fluorescence depolarization experiments is also discussed.
Beam-maser spectroscopy on cyanoacetylene-D
Authors: Tack L. M. , Kukolich S. G.
Published: February 2003

The J=1→0 transition frequencies were measured for NCCCD using a beam-maser spectrometer. Hyperfine structure from 14N and D was completely resolved. Analysis of the data directly gives the 14N and deuterium quadrupole coupling constant along the internuclear axis eqQ(14N)=−4318.0±0.1 kHz, eqQ(D)=203.5±1.5 kHz. From the data the spin-rotation constant on 14N was also measured: C(14N)=1.1±0.2 kHz. The spin-rotation data were used to calculate paramagnetic chemical shift tensor components σPxx= σPyy=−593±40 ppm. These experimentally determined tensor components were combined with calculated diamagnetic tensor components to give the isotropic shift σav=18±27 ppm.
Absorption spectra of cold dilute solid solutions
Authors: Holland R. F. , Maier William B. , Freund S. , Beattie Willard H.
Published: February 2003

Infrared absorption spectra have been obtained for some compounds trapped in crystalline solids by freezing liquid Xe, Kr, Ar, or CH4 solutions. The optical quality of the solid solutions is good, and they have been cooled to ∼80 K in 1.35 cm sample thicknesses to study the absorption in fundamental vibrational bands of the solutes. In the cases discussed, the bands are narrow, with observed full widths at half-maximum absorbance 0.05–0.30 cm−1 greater than the instrumental resolution (0.18–0.29 cm−1). The spectra appear to be free of ‘‘multiple site’’ and solute aggregate absorptions. Spectra displaying isotropic splitting in bands of natural BCl3, SeF6, OsO4, TiCl4, and MoF6 are presented, and band frequencies are compared with some results obtained in evaporative matrices, in the gas phase, and in liquid solutions. For this comparison we have obtained some spectra of SeF6 and BCl3 gas.
Photoelectron spectroscopy of HNO− and DNO−
Authors: Ellis H. Benton , Ellison G. Barney
Published: February 2003

We have obtained the photoelectron spectra of HNO− and DNO− using a recently constructed negative ion photoelectron spectrometer. We observe detachment to three different electronic states of HNO and DNO, namely the X̃ 1A′, ã 3A′′, and Ã 1A′′ states. The electron affinity of HNO is determined to be 0.338±0.015 eV while that of DNO is 0.330±0.015 eV. The ã 3A′′ state, which has not been observed directly before, is found to lie 0.778±0.020 eV above the X̃ 1A′ state in HNO and 0.785±0.020 eV in DNO. We see vibrational excitation up to ν′2=3 of the ν2 (N–O stretch) mode in the HNO and DNO X̃ 1A′ state. In the ã 3A′′ and Ã 1 A′′ states, we see excitation of both ν2 and ν3 (H–N–O bend) modes. The fundamental frequencies for the ã 3A′′ state are ν2 =1468±140 cm−1 and ν3=992±150 cm−1 for HNO and ν2= 1452±140 cm−1 and ν3=750±140 cm−1 for DNO. Analysis of transitions from vibrationally excited ions gives a ν2 fundamental frequency of 1153±170 cm−1 in HNO− and 1113±170 cm−1 in DNO−. A study of the angular distribution of the photodetached electrons yields the values of β, the anisotropy parameter, for the various transitions in the spectrum. A Franck–Condon factor analysis of the HNO X̃ 1A′ band results in an estimate of the N–O bond length in the negative ion of 1.33±0.02 Å.
A matrix isolation study of the benzene iodine atom complex
Authors: Engdahl Anders , Nelander Bengt
Published: February 2003

The IR and visible spectra of the benzene iodine atom complex in argon and nitrogen matrices at 17 K, have been obtained. The IR data indicate an axial complex of about the same strength as the molecular iodine complex. In argon, the visible spectrum of the benzene iodine atom complex has a charge transfer band with a well-developed fine structure. The dominating progression has a 280 cm−1 spacing which can be interpreted as evidence for a strongly bound excited state.
Frequency dependent polarizabilities for the ground state of H2, HD, and D2
Authors: Rychlewski J.
Published: February 2003

A variation-perturbation method has been employed to calculate the dynamic dipole polarizability for the ground state of the hydrogen molecule. The explicit correlated electronic wave functions were used. The averaged values of α(ω) and γ(ω) for several vibration-rotation states of HD and D2 are presented. Similar values for H2 have also been calculated and were used to test the efficiency of the method and the validity of the assumption applied in the present calculation. The agreement of the present theoretical results with the existing experimental data is found to be satisfactory.
An adiabatic study of the A 1Σ+ state of LiH
Authors: Bishop David M. , Cheung Lap M.
Published: February 2003

Using a 250-term configurational-interaction wave function with a 33-term elliptical orbital basis, the potential curve for the first excited state (A 1Σ+) of lithium hydride has been determined. With the corresponding wave functions, the adiabatic (diagonal nuclear motion) corrections have been calculated. These are found to be, in part, in qualitative agreement with the values extracted from an extensive analysis of the spectroscopic data for the A 1Σ+–X 1Σ+ system for the four LiH isotopes.
Molecular dynamics study of homogeneous nucleation for liquid rubidium
Authors: Mountain Raymond D. , Basu P. K.
Published: February 2003

Molecular dynamics has been used to study the dependence of homogeneous nucleation of the crystal on the rate with which the liquid is quenched, the initial state of the liquid and on the depth of the quench. A total of 54 simulations of quenching for a liquid rubidium system were performed and the dependence on these quantities of the time required for the onset of nucleation, the rate with which nucleation proceeds and the symmetry of the resulting crystal were determined. With the exception of the onset times for slow quenches, the results show an element of randomness in that the growth rates and the development of bcc or fcc structures do not correlate with quench rates, etc. For slow quench rates, the onset times show the C-shaped profile found experimentally in time-temperature trajectory studies of nucleation in strongly supercooled liquids.
Anomalous components of supercooled water expansivity, compressibility, and heat capacity (C
p and C
v) from binary formamide+water solution studies
Authors: Oguni M. , Angell C. A.
Published: February 2003

Recently reported heat capacity studies of N2H4+H2O and H2O2+H2O solutions, from which an anomalous component of the pure water behavior could be extracted by extrapolation, have been extended to a system NH2CHO+H2O which has the chemical stability needed to permit expansivity and compressibility measurements as well. Data accurate to ±2% for each of these properties as well as for the heat capacity are reported. The expansivity data support almost quantitatively an earlier speculative separation of the bulk and supercooled water expansivity into a ‘‘normal’’ (or ‘‘background’’) part and an ‘‘anomalous’’ part, the latter part fitting a critical law αanom=A(T/Ts−1)−γ with exponent γ=1.0. According to the present analysis, the anomalous part of the expansivity which is always negative, yields Ts in the range 225–228, γ in the range 1.28–1.0, depending on the choice of background extrapolation function. The normal contribution to the heat capacity obtained from the present work is intermediate in character to that from the previous two systems and leads to similar equation parameters. The normal contribution to the compressibility on the other hand is very different from that speculated earlier by Kanno and Angell and approximately verified by Conde et al. for ethanol–water solutions. The background component from the present analysis is ∼50% larger, with the result that the anomalous component, at least when values above 0 °C are included in the analysis, cannot be sensibly fitted to the critical point equation. The possible origin and interest content of these differences is discussed. Combination of the new thermodynamic data permit estimation of Cv values for the solution, and by extrapolation, a normal Cv component for water. The anomalous component of Cv for pure water obtained by difference has the form of a Shottky anomaly in contrast with the corresponding Cp component which diverges.
Order parameters of some nematic p, p′ substituted tolanes as determined by polarized Raman scattering
Authors: Dalmolen L. G. P. , de Jeu W. H.
Published: February 2003

The order parameters 〈P2〉 and 〈P4〉 of three nematic compounds have been determined by polarized Raman scattering. Results obtained with p-pentyl-p′-cyanobiphenyl (5CB) agree well with published data. Next the influence of a strongly polar cyano end group on the order parameters was investigated. This was done by comparing two nematic liquid crystals of the tolane class, namely p-heptyl-p′-cyanotolane and p-heptyl-p′-methoxytolane, which have an almost identical geometrical shape. In spite of the difference in polarity hardly any difference in the order parameters 〈P2〉 and 〈P4〉 of both compounds could be observed. 〈P2〉 and 〈P4〉 of both tolanes show approximately mean field behavior in contrast to the low 〈P4〉 of 5CB.
A generalization of the Ewald–Oseen extinction theorem: Relation to surface polariton modes
Authors: Mohanty Udayan , Rice Stuart A.
Published: February 2003

We derive an exact equation which describes how an electromagnetic field incident on a medium is extinguished inside the medium and there replaced by a different electromagnetic field propagating with a different velocity. Our result, which does not depend on use of the dipole limit or on the form of the dielectric function, generalizes the classical Ewald–Oseen extinction theorem and its recent extension to the x-ray frequency region by Oxtoby, Novak, and Rice. We also demonstrate that the generalized extinction theorem we derive gives the dispersion relations for the modes which the system sustains due to the presence of a surface across which there is a change in the dielectric function.
Evidence for ‘‘subsurface’’ hydrogen on Pd(110): An intermediate between chemisorbed and dissolved species
Authors: Behm R. J. , Penka V. , Cattania M.-G. , Christmann K. , Ertl G.
Published: February 2003

We report experimental evidence for the formation of a ‘‘subsurface’’ hydrogen species at 130 K on a Pd(110) single crystal surface. This is well separated by distinct activation energy barriers from species which are chemisorbed or which are dissolved in the Pd bulk, thereby confirming a model of H absorption in Pd in which a subsurface state acts as a reaction intermediate.
Photoelectron angular distributions of the N2O outer valence orbitals in the 19–31 eV photon energy range
Authors: Truesdale C. M. , Southworth S. , Kobrin P. H. , Lindle D. W. , Shirley D. A.
Published: February 2003

Photoelectron asymmetry parameters, partial cross sections, and branching ratios for the X, A, B, and C states of N2O+ were measured using synchrotron radiation in the photon energy range 19–31 eV. Vibrationally averaged data are reported for all four states, as well as vibrationally resolved data for the A and C states. The data are compared with a multiple scattering calculation, (e, 2e) dipole measurements, and similar data on CO2. The N2O and CO2 results show remarkable state-by-state similarity in their asymmetry parameters.
Multiphoton ionization of nitrogen dioxide: Four photon spectroscopy of the n
pσu Rydberg series
Authors: Rockney Bennett H. , Hall Gregory , Grant Edward R.
Published: February 2003

Mass resolved multiphoton ionization (MPI) of NO2 reveals resonance-enhanced production of NO+2 in the region from 490 to 540 nm. From an analysis of the MPI spectrum, we assign these resonant features to four photon transitions to high-lying members of the npσu Rydberg series. A high resolution scan of one of these features shows clear rotational structure, the assignment of which yields a rotational constant for the linear upper state (B′=0.440±0.005 cm−1).