bluuebunny/crossref_metadata_2025 huggingface dataset:
Unimolecular decay paths of electronically excited species. II. The C2H+4 ion
Authors: Sannen C. , Raşeev G. , Galloy C. , Fauville G. , Lorquet J. C.
Published: February 2003

The reaction paths of dissociation and the mechanisms of electronic relaxation of the ethylene cation have been calculated ab initio. Internal rotation is shown to bring about radiationless transition of states ? 2A and ? 2B2 to the ground state. Two competing channels are available for the first excited ? 2B3: it can either undergo internal conversion to the ground state ? 2B3 or lose a hydrogen atom by simple bond cleavage and give C2H+3+H fragments. The competition is governed by nonadiabatic interaction around a conical intersection. The coupling matrix element 〈?‖∂/∂q‖?〉 has been calculated and is shown to obey the linear model of nonadiabatic interaction quite well. The ground state ? 2B3 cannot eliminate directly a hydrogen molecule. It must first undergo rearrangement to a 2E state of a CH3CH+ structure via a bridged structure. Jahn–Teller interaction takes place at the doubly degenerate 2E state. Dissociation of CH3CH+ via 1,1-hydrogen elimination is then allowed and leads to C2H+2+H2. There is a small (possibly inexistent) energy barrier along this path. The complicated nature of this multistep reaction path brings about energy redistribution among many vibrational degrees of freedom and provides a plausible explanation for the success of statistical theories of unimolecular reactions (RRKM and QET).
Tunneling in asymmetric double-well potentials
Authors: Weiner J. H. , Tse Simon T.
Published: February 2003

Methods previously developed for the case of a symmetric double-well potential are here extended to study tunneling in an asymmetric, piecewise harmonic, double-well potential. The approach involves the decomposition of the real stationary states into complex right- and left-moving states and the determination of exact transmission coefficients for the latter. Numerical solutions for a selection of representative parameter values are obtained for the time evolution of states which correspond initially to a Gaussian wave packet localized in the upper well. Both resonant and nonresonant cases are studied and the significance of the transmission coefficient is established for each.
Contribution of nonhydrodynamic interactions to the concentration dependence of the friction factor of the mutual diffusion coefficient
Authors: Phillies George D. J.
Published: February 2003

The mutual diffusion coefficient Dm of a Brownian particle may be expressed as the ratio of a thermodynamic factor K and a friction factor fm, K and fm both being dependent on the macroparticle concentration. The contribution to fm of direct (e.g., hard sphere, electrostatic) interactions is here estimated to first order in the macroparticle concentration. In contrast to our earlier calculation [J. Chem. Phys. 67, 4690 (1977)] in which the direct interaction contribution Δfm to the Stokes’ Law drag coefficient was obtained from a fluctuation–dissipation equation. Δfm is here obtained directly from a mechanical argument based on the equations of motion of the macroparticles in solution. ΔfD is explicitly evaluated for the case of particles which interact through a weak Gaussian potential.
Two photon spectroscopy of UF6 in the near ultraviolet
Authors: Bernstein E. R. , Kennedy P. M.
Published: February 2003

The two photon excited fluorescence excitation spectrum of UF6 has been observed in the region 410 to 315 nm. The spectrum is virtually indistinguishable from the one photon absorption in this region. No vibronic structure was observed in absorption or dispersed emission. These data indicate a high density of u and g states for UF6 in this energy range and facile photochemical decompostion. Emission intensity is found to be proportional to laser power to the 3.0 to 3.6 power.
Electron impact ionization of Ba(OH)2(g)
Authors: Farber Milton , Srivastava R. D.
Published: February 2003

An electron impact ionization study of the Ba(OH)2(g) molecule has been completed. The observed intensities of the Ba(OH)2+ ion reach a maximum at 22 eV, with rapid decline thereafter, and become negligible at 70 eV. The observed phenomenon at the high electron energies appears to be the result of bond fragmentation to produce the hydroxide monomer, and not a displacement of the potential minima. This phenomenon is similar to that found in the periodic Group II dihalogen compounds. The ionization potential of Ba(OH)2 was found to be 8±1 eV, with a value of 6±1 eV for the monohydroxide.
A 100 μsec, reliable, 10 Hz pulsed supersonic molecular beam source
Authors: Byer R. L. , Duncan M. D.
Published: February 2003

We describe a 10 Hz repetition rate, 100 μsec duration, reliable pulsed supersonic molecular beam source. Mechanical and electrical design of the pulsed valve are given in detail. Characteristics of the supersonic expansion obtained using coherent anti-Stokes Raman spectroscopy (CARS) in acetylene are presented. They include pulse shape, gas rotational and translational cooling as a function of distance from the nozzle, clustering effects, and shock heating at the leading edge of the pulse.
The origin of off-resonance Raman intensity
Authors: Robinson G. Wilse , Auerbach Roy A.
Published: February 2003

This paper re-examines methods originally proposed in a paper by Berg and Robinson for the calculation of absolute Raman cross sections, on and far off resonance. In contrast to previous theories of the Raman effect, the concept of polarizability derivatives is hidden: The Franck–Condon effect is the dominant factor in determining Raman cross sections. When sums over the vibrational fine structure are carried out and only a single electronic transition is used, large differences in calculated scattering cross sections occur when using Raman formulas related by various transformations. This is in agreement with conclusions in the earlier paper. However, resonance Raman intensity is not very sensitive to the various methods discussed in the paper. When the sums are extended over more than a single electronic state or when the nuclear dependence of electronic transition moments is taken into account, interference effects found in the Berg–Robinson paper can be subdued, shifted, or eliminated. The need for precise off-resonance Raman excitation spectra of simple gas phase diatomic molecules is stressed.
ESR study of the oxidation–reduction products in an irradiated α-glycerol phosphate single crystal
Authors: Rakvin B. , Herak J. N.
Published: February 2003

The radicals in an irradiated single crystal of α-glycerol phosphate have been studied by ESR spectroscopy. Two distinctive doublets have been observed. One, characterized by g1=1.9974, g2=2.0216, g3=2.1074 and A1=−2.61, A2=−4.30, A3=0.42 mT, is attributed to the trapped hydroxyl radical. The other, with the g-tensor elements g1=2.0375, g2=2.0063, and g3=2.0063 and the hyperfine couplings A1=2.84, A2=3.00, and A3=2.78 mT, is assigned to the PO2−4 radical. The distinction between the PO2−4 radical and the alkoxy radical has been made on the basis of the mutual orientations of the g and A tensors.
Experimental and theoretical shakeup studies. IV. Main group Me2M (M=Zn,Cd,Hg) compounds
Authors: Tse J. S. , Bancroft G. M. , Creber D. K.
Published: February 2003

We report the first observation of shakeup in the core level ESCA of main group metal compounds. By recording the C 1s and metal (Zn 2p, Cd 3d, Hg 4f) level of Me2M (M=Zn, Cd, and Hg) to high statistics, satellite structures on the higher binding energy side of the respective primary photolines are observed. The peaks are assigned to shakeup transitions by theoretical Xα–SW calculations. Good agreement for both energies and intensities is obtained for the metal core level spectra. The calculations yield C 1s intensities which are much smaller than those observed experimentally.
Luminescence quenching and exciton dynamics in quasi-one-dimensional mixed crystals: Ba[Pt1−xNix(CN)4]⋅4H2O
Authors: Holzapfel W. , Yersin H. , Gliemann G.
Published: February 2003

Doping of [Ni(CN)4]2− complexes into Ba[Pt(CN)4]⋅4H2O single crystals quenches selectively the host emission. We have studied the concentration dependence at different temperatures up to one mole percent [Ni(CN)4]2−. It is shown that (at 80 K and x=10−2) the E⊥c-polarized emission (E=electric field vector, c=stack axis) is quenched by a factor of about 103 without any intensity change in the E∥c-polarized emission. The quenching mechanism is described by a phonon-assisted hopping process towards a quenching region with subsequent isotropic radiationless energy transfer to the quencher.
On the role of shallow traps in electron transport in liquid Ar, Kr, and Xe
Authors: Freeman Gordon R.
Published: February 2003

The drift velocity of electrons in gaseous or liquid argon, krypton, or xenon is roughly independent of applied electric field strength in the vicinity of 1×10−18 V cm2/molecule. The drift velocity ’’saturation region’’ is not completely flat and varies somewhat with density. The value of the saturation drift velocity is independent of density in the gas phase nearly up to critical, but in the subcritical liquid phase it increases roughly in proportion to the density. The behavior is not consistent with the participation of thermal electrons and shallow traps. It is consistent with electron heating by the field, and the energy dependence of the elastic scattering cross section of the fluid. (AIP)
Correlated prolate–spheroidal wave function for two-electron diatomic systems
Authors: Aubert-Frécon M. , Le Sech C.
Published: February 2003

For two-electron diatomic systems, the determination of a prolate–spheroidal wave function containing explicitly the interelectronic distance as a coordinate is investigated from the exact resolution of an approximate Schrödinger equation separated in such a way that the true potential is rigorously taken into account, the perturbing term being a kinetic one. Calculations are carried out for the total energy of H2, He2+2, and HeH+ in their ground state. Results are seen to be in good agreement with the best ones. Extension to effective two-electron diatomic problems such as alkali dimers is considered.
Foundations of Walsh’s rules for molecular shape
Authors: March N. H.
Published: February 2003

Combining density functional theory with the form of the virial theorem established by Nelander for polyatomic molecules, the basic foundation of Walsh’s rules for molecular shape is clarified.
An accurate integral equation for the pair and triplet distribution functions of a simple liquid
Authors: Haymet A. D. J. , Rice Stuart A. , Madden William G.
Published: February 2003

We present an accurate integral equation for the pair and triplet distribution functions of a simple classical liquid. The only approximation in the theory, the representation of the triplet distribution function, is improved systematically beyond the superposition approximation by a graphical resummation of the density expansion of the correction terms. In a sample calculation for liquid sodium near its triple point, the triplet distribution function calculated from our integral equation is compared with the results of a molecular dynamics simulation, and excellent agreement is obtained.
The use of a discriminative window in deconvolution method applied to dielectric data
Authors: Salefran J. L. , Dutuit Y.
Published: February 2003

A deconvolution procedure for the analysis of dielectric data is presented which involves a suitable integration window, known as a Kaiser–Bessel window. It is shown that its use by making the emergence of smaller contributions possible allows one to resolve unsuspected multiple dispersions for distribution functions. Several examples, both theoretical and experimental, are given and illustrate the viability of the procedure. Its limitations are examined and its advantages and disadvantages as compared to others of the same and different kinds are considered.
Conformational effects in alkoxy radicals
Authors: Box Harold C. , Freund Harold G. , Budzinski Edwin E.
Published: February 2003

An alkoxy radical was identified in single crystals of cytidine-3′- monophosphate x irradiated at 4.2 °K. Proton hyperfine couplings associated with the alkoxy radical absorption were determined from ENDOR measurements. The magnitudes of β as well as γ proton couplings are dependent upon the conformation of the radical. The conformation of alkoxy radicals is analyzed in terms of two dihedral angles and related to observed coupling tensors.
A spectroscopic study of the G 1Πg state of 7Li2 by pulsed optical–optical double resonance
Authors: Bernheim R. A. , Gold L. P. , Kelly P. B. , Tipton T. , Veirs D. K.
Published: February 2003

The results of a pulsed optical–optical double resonance spectroscopic study of the G 1Πg state of 7Li2 are presented. Observations were made on the v*=0 through v*=20 levels, representing about 60% of the dissociation energy. A set of Dunham molecular constants was derived. The RKR potential was generated and Franck–Condon factors were determined for the range of observed vibrational levels. A dissociation energy was estimated from a Birge–Sponer extrapolation of the Gv values which suggests a correlation with the (2 2P)+(2 2P) atomic lithium states at the dissociation limit.
Emission spectrum of rare gas dimers in the vacuum UV regions. III. Ne2
Authors: Tanaka Y. , Walker W. C.
Published: February 2003

Emission spectra of the neon dimer have been studied in the VUV region. Liquid nitrogen cooled neon was excited with a Tesla coil or transformer discharge. Three band systems I, II, and III were observed in the 735–1000 Å range and classified as produced by electronic transitions to the common ground state X0+g(1S0) from the three low lying excited states 1u(3P2), 0+u(3P1), and 0+u(1P1), respectively. Evidence for the existence of potential humps in the lowest two upper states were observed and the hump heights were calculated to be 142±10 and 670±50 cm−1, respectively. Observations of additional spectra, a newly observed emission continuum and two diffuse band groups, are presented.
Cobalt carbene ion: Reactions of Co+ with C2H4, cyclo-C3H6, and cyclo-C2H4O
Authors: Armentrout P. B. , Beauchamp J. L.
Published: February 2003

An ion beam apparatus is employed to study the formation of the cobalt carbene ion, CoCH+2. This ion is produced in the endothermic reaction of cobalt ions with ethene and cyclopropane and in an exothermic reaction with ethylene oxide. A model is proposed to account for the dependence of experimental cross sections on relative kinetic energy for the endothermic reactions. Using this model to interpret the experimental results, a bond dissociation energy D °(Co+–CH2)=3.7±0.3 eV is derived.
Dissociation of CH3Cl by 1 MeV H+, He+, and O+
Authors: Monce M. N. , Edwards A. K. , Wood R. M. , Steuer M. F. , Shah A. V. , Tabor P.
Published: February 2003

The dissociation of methyl chloride produced by bombardment of 1 MeV H+, He+, and O+ projectiles was investigated. H+, H+2, H+3, C+, CH+2, CH+3, and Cl+ fragments were observed and their respective kinetic energy spectra recorded. Results suggest that one particular channel involved in the dissociation is CH3Cl2+→CH+3+Cl+ with a dissociation energy of 6.6 eV. Results also indicate a possible mechanism in CH+n production involving the dissociation of a CHnClm+ fragment.
State-to-state studies of the collisional quenching of electronically excited Cd(1P1) atoms
Authors: Breckenridge W. H. , Malmin O. Kim
Published: February 2003

A comprehensive study of the quenching of electronically excited Cd (5s5p 1P1) by a variety of simple molecules has been conducted using pulsed laser techniques. Except for He, Ar, and C2F6, deactivation occurs at essentially every encounter with all quenchers studied. The following process occurs with high efficiency with most of the quenching molecules studied, in striking violation of the Wigner spin rule and in spite of other available chemical or energy-transfer exit channels in many cases:         Branching ratios for total Cd(5s5p3PJ) production were determined using an indirect method calibrated by earlier absolute measurements. Initial distributions of individual Cd(3P2,1,0) quantum states in process (1), determined by a pump-and-probe laser-induced fluorescence technique, show wide variations for different types of molecular quenchers. A qualitative model of bond-specific interactions of Cd(1P1) with quenching molecules has been developed which successfully rationalizes all the experimental results. For several of the molecules, and electronically statistical (5:3:1) distribution of Cd(3P2,1,0) is observed and attributed to a preponderance of bond sites at which there is a net attractive interaction of Cd(1P1), caused either by intersection of ionic surfaces (N2,CO) or by chemical bonding (alkyl C–H bonds), so that crossings occur with equal probability to all repulsive states correlating with Cd(3P2,1,0). Cases for which the Cd(3P2,1,0) distribution is skewed away from statistical towards Cd(3P2) [Ar, CH4, C2H6, C(CH3)4, C3H8] result from a less attractive potential with Cd(1P1) (due to stronger C–H bonds and/or weaker dispersion forces) and preferential crossings with the most repulsive ’’S’’-like states, which correlate with Cd(3P2,1,0). Cases for which the Cd(3P2,1,0) distribution is skewed away from statistical towards Cd(3P1) and Cd(3P0) (NO, C3F6, C2H2, C2H4, butadiene, propylene) are postulated to result from formation of long-lived complexes and the resultant formation of products which approach truly statistical distributions of vibrational, rotational, translational, and electronic energy. For the higher alkenes (isobutylene, 2-butene), Cd(1P1) interaction with the alkly C–H bonds completely dominates over the double bond interaction and electronically statistical Cd(3P2,1,0) distributions are observed.
Heat capacity measurements of xenon on graphite
Authors: de Rouffignac Eric , Wade William H.
Published: February 2003

Experimental results are reported on the measurement of the heat capacity of xenon absorbed on a graphitized carbon black (Graphon) in the range 60–120 K. At low temperatures (∼60 K) the heat capacity tends to the one calculated by lattice dynamics, but a pronounced maximum centered at 109 K was observed at higher temperatures, with a width at mid-height of approximately 10 K. Comparison of these results with vapor pressure isotherms of Thomy and Duval shows this maximum to be probably due to a melting transition in the adsorbed xenon layer.
The chemical shift, dipolar, and quadrupolar tensors of deuterium in potassium bicarbonate
Authors: Achlama Abraham M.
Published: February 2003

The full quadrupolar, chemical shift, and dipolar tensors of deuterium in potassium bicarbonate have been determined. The principal directions of the quadrupolar tensor reflect the hydrogen bond directions to within 1°, while the chemical shift deviates by 6°. The dipolar tensor provides a means for determining the sign of the quadrupolar constant and the vector joining the two deuteriums within the dimer. The results are compared with the corresponding ones from zero field, multipulse, and neutron diffraction studies.
The spin state of Fe+3 in (morpholyldithiocarbamate)3Fe⋅CH2Cl2 determined by Mössbauer spectroscopy
Authors: Malliaris Angelos , Papaefthimiou V.
Published: February 2003

The spin nature of the title solvated complex has been studied by Mössbauer spectroscopy at low temperature. It is concluded that below 4.2 K the ferric ion is in the ’’high’’ spin state with S = 5/2, contrary to the previously made S = 3/2 assignment on the basis of magnetic susceptibility measurements.
Analytic atomic shielding parameters
Authors: Bessis N. , Bessis G.
Published: February 2003

It is shown that, using very simple mathematics, one can derive expressions of the screening constants in terms of the quantum numbers of a given electronic configuration. (AIP)
Analysis of the coexistence curve of Na+NH3
Authors: Das B. K. , Greer S. C.
Published: February 2003

We analyze data from the literature on the coexistence curbe of Na+HN3 in terms of the extended scaling expression derived by Wegner. The data over the temperature range 4×10−4<t%1.7×10−2, where t = (Tc−T)/Tc, T is the temperature, and Tc is the critical temperature, can be fitted to a two-term expansion using Ising exponents. The diameter of the xoexistence curve is linear within experimental error. (AIP)
Comment on ‘‘IR lines broadened by chemical exchange’’
Authors: Weston Ralph E.
Published: February 2003

In their recent publication1 entitled ’’IR lines broadened by chemical exchange,’’ the authors claim to be the first to observe line broadening in IR spectra due to chemical exchange. Their claim is disputed, it is argued that this phenomenon is widely reported in literature. (AIP)
Reply to a comment by Weston on ‘‘IR lines broadened by chemical exchange’’
Authors: Cohen Benyamin , Weiss Shmuel
Published: February 2003

The authors agree with Weston that IR and Raman line broadening due to chemical exchanges has been mentioned in the literature. But they contend that their measurements are the first experimental evidence for exchange broadening in the infrared. (AIP)
Energy dependence of the rate constant of the reaction N++NO at collision energies 0.04 to 2.5 eV
Authors: Fahey D. W. , Dotan I. , Fehsenfeld F. C. , Albritton D. L. , Viehland L. A.
Published: February 2003

The rate constant for the reaction N++NO was measured for center-of-mass collision energies from thermal to 2.5 eV. The measurements were made in a flow–drift tube apparatus in which the N+ ions were made by secondary reactions in the flow tube or were injected into the flow tube from a separate source. The measured rates were found to be in good agreement with previous room-temperature measurements and with crossed beam data at 2.5 eV. An unexplained disagreement exists between the present results and the crossed beam data below 2.5 eV.
A study of the ion–molecule half reactions O+2(ã4Πu, v)⋅⋅⋅(O2)m→O+2m+1+O, m = 1, 2, or 3, using the molecular beam photoionization method
Authors: Linn S. H. , Ono Y. , Ng C. Y.
Published: February 2003

The photoionization efficiency (PIE) curve for (O2)+2 has been obtained in the region 650–1 080 Å using the molecular beam photoionization method. The ionization energy (IE) for (O2)2 is determined to be 11.66±0.03 eV. From the measured IE for (O2)+2, the known IE for O2, and the estimated dissociation energy (0.01 eV) of (O2)2, the binding energy for (O2)+2 is deduced to be 0.42±0.03 eV. Comparisons of the PIE spectra for O+2 and (O2)+m, where m = 2, 3, and 4 indicate that the excited dimer complexes O*2(n,v)⋅(O2)m (m = 1, 2, and 3) formed in this wavelength region are almost completely dissociative, and the cluster ions are predominately formed by the direct photoionization processes (O2)m = 2, 3, or 4+hn→(O2)+m = 2, 3, or 4+e. The PIE curves for O+3, O+5, and O+7 are measured in the region 650–780 Å. The appearance energy 16.66±0.03 eV (744±1.5 Å) for O+3 is found to be consistent with a zero activation energy for the ion-molecule reaction O+2(X̃2Rg)+O2→O+3+O. The appearance energy for O+5 is determined to be 16.41±0.06 eV (755.5±3 Å). This value has allowed the determination of a binding energy of 0.26 eV for O+3⋅O2. The nearly structureless PIE spectra observed for O+3, O+5, and O+7 also suggests that these ions originate mainly from (O2)+2, (O2)+3, and (O2)+4 which are formed by direct ionization processes. Using the relative Franck–Condon factors for the O2 ã4Ru←X̃3S−g transitions, the relative reaction probabilities for the ion–molecule half reactions O+2(ã2Ru, v)⋅(O2)m→O+2m+1+O(m = 1, 2, and 3)—(1)—as a function of the vibrational quantum number v have been determined. The relative intensities of O+3, O+5, O+7, and (O2)+m = 2–4 observed in this experiment support the conclusion that the reaction probabilities of (1) with m = 2 and 3 are substantially larger than those with m = 1.
A diatomics-in-molecules case study on the system Be+HF→BeF+H. I. Bonding models and the use of valence bond information
Authors: Kuntz P. J. , Roach A. C.
Published: February 2003

DIM calculations on the ground state potential energy surface for Be+HF→BeF+H are approached on the basis of a variety of bonding models, involving from three up to 18 structures. A considerable input of diatomic fragment information is required. To this end two sets of ab initio valence bond calculations have been carried out on the fragments, the first restricted to just those structures contributing to the largest DIM model and the second containing many additional fragment structures. VB calculations at the first level were extended to compute the full triatomic potential surface and this is found to agree qualitatively fairly closely with that from a DIM model based on the same structures and fragment curves. A preliminary survey highlights two practical difficulties in using data from large VB calculations to support smaller DIM models (a) in numerical interpolation of coupling constants and (b) in identification of the appropriate fragment curves to use. It is also shown that inclusion of additional structures in a DIM calculation may in certain examples lead inevitably to a raising of the calculated interaction energy.
INDO and MINDO/3 atom-in-molecule polarizabilities
Authors: Metzger Robert Melville
Published: February 2003

The INDO and MINDO/3 finite-perturbation algorithms for computing the molecular static electric dipole polarizability tensor aM were applied to obtain a set of atom-in-molecule polarizability tensors ai useful for crystal lattice energy calculations. The decomposition of aM into the ai must yield symmetric tensors, and requires the invention of a scheme to assign bond moments for closed-ring atoms. The resulting ai depend on the molecular geometry and seem to describe local bonding effects rather well. They underrate the polarizability perpendicular to covalent bonds (because of a lack of polarization functions in the INDO and MINDO/3 basis set) but the MINDO/3 ai values are rather close to the atom polarizability tensors derived from experimental data by Applequist.
Theoretical characterization of the potential energy curve for hydrogen atom addition to molecular oxygen
Authors: Dunning Thom. H. , Walch Stephen P. , Goodgame Marvin M.
Published: February 2003

Large scale polarization configuration interaction (POL-CI) calculations are reported on the HO2 molecule. For the structure of the stable complex the calculations predict (with experimental values in parentheses) ROH = 0.99 Å(0.971 Å), ROO = 1.37 Å (1.335 Å), and qHOO = 103°(104.1°). The calculated harmonic frequencies (3655, 1416, and 1181 cm−1) are also in good agreement with those derived from experiment. On the other hand, the H–O2 bond energy obtained in the calculations (42 kcal/mole) is too small by ∼10 kcal/mole (an inherent defect in the POL-CI method). Although both the GVB and GVB-CI calculations predict barriers of ∼3 kcal/mole to the addition of a hydrogen atom to molecular oxygen, the POL-CI calculations reduce the barrier to less than 0.4 kcal/mole. Taking the computational limitations into account, there may well be no barrier at all to the addition reaction.
Two dimensional order in a solid solution of molecules of opposite chirality
Authors: Lajzerowicz Janine , Chion Bernadette , Lajzerowicz Joseph
Published: February 2003

d and l molecules of 3-carboxy-2,2,5,5-tetramethyl pyrrolidinyl-1-oxyl are completely miscible in the solid state and thus form solid solutions for all d/l ratios. At room temperature the racemic (50/50 ratio) solid solution has two dimensional order consisting of almost random stacking of locally ordered planes and accordingly is a two dimensional racemate. We propose a particular planar molecular arrangement based on comparison of our diffuse x-ray scattering results with model calculations employing transfer matrix techniques. We also calculate and use the empirical intermolecular (van der Waals) potential energy. The correlation area within a plane is approximately 60×95 Å2 and thus contains about 140 molecules. Nearest neighbor planes are uncorrelated for symmetry reasons but a weak correlation of next nearest neighbor planes exists. Our knowledge of the molecular pair interactions (calculated from van der Waals energies) allows description of the system as a two dimensional Ising model. By making some simplifying assumptions we are able to use exact results from statistical mechanics and obtain very favorable agreement between experimental results and directly calculated energies.
Initial and final molecular states as ‘‘virtual states’’ in two-photon processes
Authors: Mortensen O. Sonnich , Svendsen E. No/rby
Published: February 2003

The expression for the cross section of a two-photon process is derived. It is shown, in contrast to the usual statements in the literature, that both initial and final molecular states should be included in the summation over virtual states. For the case of vibrational Raman scattering these extra terms contribute very little to the scattering tensor.
Chemical shielding tensor and 13C–14N dipolar splitting in single crystals of L-alanine
Authors: Naito A. , Ganapathy S. , Akasaka K. , McDowell C. A.
Published: February 2003

Chemical shielding tensors have been determined for three chemically distinct carbons in a single crystal of L-alanine using proton-enhanced 13C NMR. The results indicate that the most shielded direction is perpendicular to the sp2 plane for the carboxyl carbon and the C3 axis for the methyl carbon. 13C–14N dipolar splittings have been observed for the Ca carbon, causing further complication in spectral analysis. Since the 14N quadrupole coupling constant is of comparable order with the 14N Zeeman interaction, 14N quadrupole interaction is effective in the 13C–14N dipolar splittings. The 14N quadrupole effect is observed and demonstrated by calculation and the sign of the quadrupole coupling constant e2Qq has been determined to be positive. The chemical shielding tensor for the Ca carbon is determined by considering the 13C–14N dipolar interaction and the 14N quadrupole interaction. It is shown that the 14N quadrupole interaction causes the asymmetric doublet pattern in the 13C NMR signal for the Ca carbon in magic angle spinning (MAS) experiments.
Asymmetry of the quadrupole satellites and determination of the sign of the quadrupole coupling constant of 23Na in NaNO3
Authors: Hughes D. G.
Published: February 2003

The satellite resonances in the quadrupole-split NMR spectrum of 23Na in a single crystal of NaNO3 have been studied at various crystal orientations. They are quite asymmetric near an orientation where the dominant magnetic dipole interaction is between adjacent 23Na nuclei situated in linear chains lying parallel to the external magnetic field. The orientation dependence of the third moment of the satellites is in quite good agreement with the theoretical dipolar third moment, and the transition associated with each satellite has been unambiguously determined. It is found that the quadrupole coupling constant of 23Na in NaNO3 is negative, rather than positive as previously believed. The temperature and pressure dependence of the quadrupole coupling constant measured by others is more easily understood when account is taken of the negative sign. The discrepancy between the measured and calculated third moment values was found to be mainly due to a small admixture of x′ in the spectrometer output. No significant asymmetry of the inhomogeneous quadrupole broadening was found.
Electron paramagnetic resonance and electron nuclear double resonance of 237-neptunium hexafluoride in uranium hexafluoride single crystals
Authors: Butler James E. , Hutchison Clyde A.
Published: February 2003

The EPR and ENDOR spectra of 237NpF6 molecules dilutely substituted for host molecules in single crystals of UF6 at temperatures between 1.2 and 2.1 °K have been obtained at microwave frequencies, ∼9.4 and ∼9.7 GHz. Approximate values are given for the parameters in a spin Hamiltonian formalism that describes the measurements. The results are discussed.
Two-photon fluorescence excitation spectra of trialkylamines
Authors: Kasatani K. , Kawasaki M. , Sato H. , Murasawa Y. , Obi K. , Tanaka I.
Published: February 2003

The two-photon fluorescence excitation spectra of trimethylamine, triethylamine, tri-n-propylamine, and tri-n-butylamine in the gas phase were measured in the excitation wavelength range of 450–530 nm. The effect of the polarization (linear or circular) indicates that the transitions are vibrationally induced ones coupled with ’’umbrella-inversion’’ motion in the longer wavelength region and electronically allowed ones in the shorter wavelength region.
The Jahn–Teller effect in C6 F+6
Authors: Sears Trevor J. , Miller Terry A. , Bondybey V. E.
Published: February 2003

Recent results from the laser spectroscopy of C6 F+6 are collected to derive the positions of ∼15 Jahn–Teller active vibronic levels of the four e2g vibrational modes of the ground X 2E1g state. Intensity information is also collected for a number of transitions, both in absorption and emission, involving these modes. The data are analyzed by a model including linear and quadratic Jahn–Teller coupling, and coupling among all four of these active e2g modes. The positions of these levels are predicted by the eigenvalues, and the transition intensities by the eigenvectors, of matrices of order ∼7000×7000. Quantitative agreement is obtained for both frequencies and intensities, thereby determining unperturbed oscillator frequencies, distortion constants, and stabilization energies for all the possible Jahn–Teller active modes of C6 F+6. Combination of these results with a normal coordinate analysis gives the distorted geometry of the ion at the minimum of its potential function.
A spectroscopic study of the E 1Σ+g and F 1Σ+g states of 7Li2 by pulsed optical–optical double resonance
Authors: Bernheim R. A. , Gold L. P. , Kelly P. B. , Tomczyk C. , Veirs D. K.
Published: February 2003

The results of pulsed optical–optical double resonance spectroscopic studies of the E 1S+g and F 1S+g states of 7Li2 are presented. Observations were carried out on the v = 0 through v = 30 levels of the F 1S+g state, representing about 60% of the dissociation energy. A set of Dunham molecular constants was derived and an RKR potential was generated. Franck–Condon factors were determined for the range of observed vibrational levels. The spectroscopic results on the E 1S+g state are preliminary and are included because of a probable homogeneous anticrossing between it and the F 1S+g state. It is suggested that the E 1S+g state correlates with the (2 2S)+(3 2S) atomic lithium states while the F 1S+g correlates with the (2 2P)+(2 2P) states.
Double ionization of van der Waals dimers: NeXe and ArXe
Authors: Helm H. , Stephan K. , Märk T. D. , Huestis D. L.
Published: February 2003

Simple consideration of atomic first and second ionization potentials show that for certain atomic combinations stable diatomic ions carrying two positive charges should exist below the lowest dissociation limit. One such ion NeXe++ has been observed experimentally by double ionization of the van der Waals dimer NeXe. Similar experiments also resulted in production of a long-lived ArXe++ ion. The stability of these doubly charged molecular ions is rationalized using semiquantitative potential curves derived from the isoelectronic neutral molecules XeO and ICl. The appearance potentials found experimentally for both doubly charged molecular ions are consistent with the predictions from the potential energy curves. Some observations of other investigators on the kinetics of the low lying states of Xe++ in Ar and Ne are reinterpreted.
Softness expansion of gaseous transport properites. II. Moderately dense gases
Authors: Rainwater J. C.
Published: February 2003

The first density corrections to gaseous transport properties for the inverse power potential f = f0(s/r)n are evaluated according to the theory of Curtiss, Snider, and co-workers. A linear softness expansion predicts that the first density corrections are positive for large n but negative for small n, and that the changes of sign occur at n = 32.75 and n = 8.42 for the viscosity and thermal conductivity, respectively. Direct numerical integration yields n = 29.60 and n = 7.07, respectively. These results explain the opposite signs of density corrections to the two transport properties seen in high temperature experimental data, where, effectively, n = 12. Predictions of the modified Enskog theory are compared and contrasted with the above results. It is shown that some of the Snider–Curtiss (collisional transfer) collision integrals may be simplified, one of them being the usual dilute gas omega integral with Smith’s collision lifetime included in the integrand.
Dynamics of freely suspended lyotropic films. I. An inelastic light scattering study of thermal surface fluctuations
Authors: Young Charles Y. , Clark Noel A.
Published: February 2003

We have studied the spectrum and intensity of light scattered by thermal surface displacement fluctuations on freely suspended lyotropic films. Films consisted of a liquid core and surface soap layers and were drawn from solution containing water, glycerol, NaCl, and the ionic surfactant hexadecyltrimethyl ammonium bromide (HTAB). Two modes were observed: a propagating undulation mode in which the film surfaces move together and a damped peristaltic mode having oppositely moving surface soap layers. Dispersion relations for these modes, obtained from the dependence of the scattered light intensity correlation function on film thickness h and wave vector k, confirm the macroscopic hydrodynamic description of film motion. In particular, the overdamped peristaltic mode is shown to involve Poiseuille flow of the fluid core with the flow velocity zero within 2 Å of the surfactant–solution interface, indicating no significant slip or rigid interfacial water layer. No evidence of dispersion in the effective viscosity of the fluid core h(k,w) over the range 0<k<106 cm−1, 0<w<103 sec−1 was found. The undulation mode was found to be underdamped, with frequency in the range of 400 kHz to 1 MHz, the largest observed to date for surface waves in fluids. The dynamic surface tension term s(k,w) for k∼106 cm−1 and w∼6×10 sec−1 was found to be the same as the static value within experimental error. Analysis of the total scattered intensity and of the peristaltic mode dynamics allows the determination of R(h), that part of the film pressure due to electrostatic repulsive and van der Waals attractive forces. The measured R(h) are well represented by the sum of a repulsive screened electrostatic interaction and an attractive van der Waals term. The screened electrostatic interaction is consistent with the concentration of NaCl used and the attractive part of R could be fitted equally well by the simple nonretarded van der Waals form for a uniform dielectric slab, or the Ninham–Parsegian form for a three layer hydrocarbon–water–hydrocarbon slab.
Thermal evolution and decomposition of ethylene on Pt(111)
Authors: Baró A. M. , Ibach H.
Published: February 2003

We have used temperature programed thermal desorption and electron energy loss spectroscopy to characterize the thermal evolution and decomposition of C2H4 on Pt(111). The relative amount of hydrogen desorbing during the conversion to the stable phase with respect to the total hydrogen desorbed depends on the initial C2H4 exposure and crystal temperature. In most cases this amount is larger than 25%. Analysis of the vibration spectra taken at selected temperatures of the desorption process provides additional arguments favoring the interpretation of the stable species as CHCH3. The analysis of the vibration spectrum out of the specular direction permits us to suggest that a CCH2 species is coexisting with CHCH3. A model for ethylene evolution is proposed with CCH2 as intermediate. The decomposition of the stable species at higher temperatures is studied by taking the corresponding vibrational spectra and a sequence of the cracking process is suggested.
The structure of CCH+
Authors: Krishnan R. , Frisch M. J. , Whiteside R. A. , Pople J. A. , Schleyer P. v. R.
Published: February 2003

Molecular orbital theory is used to examine relative energies of low-lying electronic states of the ethynyl cation CCH+. A triplet 3ru is found to be the ground state at the highest level of theory with another triplet 3𝒥− only 2.4 kcal/mole higher in energy. The closed-shell 1𝒥+ state lies 59 kcal/mole above the ground state.
A quantitative test of unimolecular rate theory in the multiphoton dissociation of CF2CFCl
Authors: Stephenson John C. , Bialkowski Stephen E. , King David S. , Thiele Everett , Stone James , Goodman Myron F.
Published: February 2003

We have measured the distribution of total energy in reaction products for the CO2 laser-induced MPD: CF2CFCl→CF2+CFC1. From a separate study of MPD rates as a function of laser intensity and inert buffer gas pressure, reliable estimates of the radiative pumping rates are known for this reaction. These results, when analyzed together, allow us to extract a unimolecular A factor from the MPD data. The determined value A = 3×1016 sec −1 agrees well with estimates based on independent thermal data.
A comparison of detailed lattice model gas–solid theory with molecular-beam data: Scattered velocity distributions for an Ar/W system.
Authors: Lorenzen J. , Raff L. M.
Published: February 2003

Scattered velocity distributions and energy transfer results obtained from detailed lattice model calculations on the Ar/W(110) system are reported and compared to recently reported molecular-beam data using a polycrystalline W target. The computed velocity distributions are found to be in good accord with the beam data provided the results from the lattice model are averaged over scattering angle. The variation of the average energy of scattered Ar with surface temperature and incident beam energy is also correctly predicted by the lattice model. The Ar–W binding energy obtained from recent hard-cube calculations on this sytem is in accord with that previously reported using the lattice model. If comparisons are made between the beam data and computed distributions that have not been averaged over scattering angle, the results do not agree. This is interpreted to be due to the polycrystalline nature of the surface used in the beam experiments, but it may be a result of a defect in the lattice model description of the interaction.
Coupled states study of nonreactive F(2P)+H2(1Σ+g) collisions
Authors: Fitz D. E. , Kouri D. J.
Published: February 2003

The coupled states approximation is extended to treat combined fine structure and rotational state transitions in 2P atom–1Sg+ diatom scattering. CS calculations of opacities and degeneracy averaged integral cross sections for F(2P)+H2(1Sg+) nonreactive collisions agree well with the CC results of Lester and Rebentrost. The present, nonunitarized form of the CS approximation leads to a selection rule which forbids j′1 j′2 j12m12→j1 j2  j12-m12 transitions when j12 = half-odd integer values. This selection rule is not present in an exact treatment.
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o calculations as a source of intermolecular potential functions. Ethanol–water with a minimal basis set
Authors: Alagona Giuliano , Tani Alessandro
Published: February 2003

The potential energy surface of the (trans)ethanol–water dimer has been explored with ab initio quantum mechanical calculations employing a minimal basis set to obtain a potential function suitable to computer simulations of the relevant aqueous solution. In all, 410 interaction energy values have been calculated. A part of the complex geometries has been produced approaching the water molecule to the ethanol along 17 different axes, and a part with an iterative random procedure. On this data base a fit with the experimental geometry of the water molecule and one with the ST2 model have been performed, with standard deviations of 0.69 and 0.73 kcal/mole, respectively. A limited test concerning the role of the three body term in open chain trimers has also been carried out. Finally, these SCF calculations have been used to check the possibility of the electrostatic approximation and of the pair potential library given by Clementi, to provide reliable intermolecular potential energy values in these kinds of systems.
The fcc (111) and (100) crystal–melt interfaces: A comparison by molecular dynamics simulation
Authors: Broughton J. Q. , Bonissent A. , Abraham F. F.
Published: February 2003

The Lennard-Jones fcc (111) and (100) crystal–melt interfaces have been simulated by molecular dynamics with a view to understanding their thermodynamic and structural similarities and/or differences. The atomic and potential energy density profiles look very similar for the two faces, suggesting that any structural and thermodynamic differences are very subtle. Diffusion coefficient data show that the transition from crystal to melt for the (100) face occurs over essentially the same distance as for the (111) face. A careful calculation of excess surface potential energies indicates the (111) interface to be less stable than the (100), but that the energy quantities involved are very small (∼5% of the crystal–vapor values). The excess configurational entropy is negligible in both cases. However, no estimate of the communal entropy has been made, and it could be very important. The Gibbs equimolar dividing surface for the (111) interface coincides with that indicated by the layerwise radial distribution functions and atomic trajectory plots. This is not true of the (100) face; here the dividing surface occurs before the last crystalline layer.