Index of content:
Volume 67, Issue 9, 01 November 1977

H(2s) and H(2p) branching ratio in the photodissociation of H_{2} near threshold
View Description Hide DescriptionThe excitation spectrum for producing Lyman‐α fluorescence by photodissociating H_{2} has been obtained with 0.12 Å bandwidth in the dissociation threshold region. At 839 Å, the peak in the fluorescence cross section, one observes Lyman‐α emission from several continuums and from the J=1 and J=2 predissociated rotational levels of the D ^{1}Π^{+} _{ u }(v′=3) state. At this wavelength the branching ratio σ (2s)/[σ (2s)+σ (2p)] was obtained using a quenching field method. It was determined that 57% of the excited atoms are produced in the metastable 2 ^{2} S _{1/2} state. From a study of the fluorescence signal versus pressure a cross section of 0.9×10^{−14} cm^{2} was derived from the chemi‐ionization reaction H_{2}+H(2s) →H^{+} _{3}+e ^{−}.

A comparison of the Becker–Döring and Lothe–Pound theories with exact cluster free energies from Monte Carlo simulations
View Description Hide DescriptionThermodynamic functions for small clusters of atoms interacting with 6:12 potentials are calculated using the classical Becker–Döring theory and the revised Lothe–Pound theory. Comparisons are made with the results of Monte Carlo studies on such clusters.

The effect of reagent translation on product internal energy distributions: Laser fluorescence study of Al+O_{2}
View Description Hide DescriptionThe effect of incident translational energy on the product internal state distribution has been investigated for the reaction Al+O_{2}→AlO+O. The incident energy is varied through the use of time‐of‐flight velocity selection of an Al atomic beam, and dye laser fluoresence excitation spectra are reported for AlO produced in a ’’beam‐gas’’ arrangement under single‐collision conditions. Relative cross sections and effective rotational temperatures are derived for the observed vibrational levels. On the average about 20% of the available energy appears as product vibration and 30% as product rotation. It is found that the AlO internal excitation increases with increasing incident kinetic energy but that ∼80% of the additional kinetic energy is disposed of as product translation and rotation. Comparison of the present results with previous theoretical work suggests that the Al+O_{2}reaction proceeds on an attractive surface with mixed energy release channeled primarily into product translation and rotation. The observation of a translational energy threshold for the production of AlO in the v=2 level has been employed to derive a lower bound to the AlO dissociation energyD _{0}° (AlO) ?121.0±0.6 kcal/mole. With the help of previous work it is concluded that this lower bound is an equality and a value of 121–122 kcal/mole is recommended for the dissociation energy of aluminum monoxide.

Phase transitions of (1:1) alkaline salts of TCNQ as studied by vibronic intensity enhancement in the infrared spectra
View Description Hide DescriptionThe results of a study of the temperature dependence of the infrared absorption spectra of M^{+}TCNQ^{−} (M^{+}=Na^{+}, K^{+}, Rb^{+}, Cs^{+}, and NH_{4} ^{+}) salts are reported and exemplified in detail for the case of triclinic RbTCNQ(II). On lowering the temperature, the infared absorptions which have already been recognized as due to vibronic activation of the totally symmetric modes of the TCNQ anion display a striking intensity enhancement. The plot of the vibronic band intensities against temperature displays an abrupt variation at nearly the same transition temperatures reported in the literature and determined by magnetic susceptibility and dc conductivity measurements. This result suggests that the vibronic intensity enhancement follows the distortion of the regular TCNQ anion stacks in the high temperature phases to alternating stacks in the low temperature ones.

Mass dependence of the ionic dynamics in molten salts
View Description Hide DescriptionThe dynamical properties of condensed media are studied on the basis of molecular dynamics in a two component system of charged particles. The short‐time behavior of the particle motion is described in terms of velocity autocorrelation functions. In order to get a better understanding of the mechanism of the transport process the effect of isotope substitution is examined. It is shown that the collision theory does not provide a suitable model; the results are interpreted within the framework of perturbation theory. Expressions are derived which give a correct representation of the influence of a mass change on the self‐diffusion coefficients; this approach is used to calculate the isotope effects in naturally occurring systems. Comparisons with experimental results are interpreted by introducing strongly correlated motions.

Unified treatment of intermolecular and thermodynamic potentials of polar fluids. I. Preaveraged intermolecular potential
View Description Hide DescriptionA unified treatment is presented of intermolecular and thermodynamics potentials of polar fluids. The treatment is based on a time‐evolution approach which traces the propagation of electrical—and molecular density—fluctuations from the onset of the perturbation to the time equilibrium is fully established. Of prime importance in this development are the relative time scales for establishing electronic, rotational, and translational correlations. The theory is specialized to a system of axially symmetric polar molecules in which the translational correlation time greatly exceeds the rotational correlation time. It is shown that the latter condition leads naturally to an intermolecular potential which is preaveraged over the rotational states of the molecules. An approximate closed‐form expression is derived for the fluidfree energy in terms of the translational susceptibility of the unperturbed fluid and the intermolecular potential. The latter is formulated in terms of the rotational and dispersion susceptibilities of the molecules and the static (Coulomb) potential. The intermolecular potential can be decomposed into generalized dispersion, orientation, and induction potentials, from which the standard (dipolar) expressions are obtained as special cases. It is shown that in the classical limit, the fluidfree energy is consistent with the standard statistical formula for the free energy except that the potential which appears in the configuration integral is angle independent and temperature dependent.

Unified treatment of intermolecular and thermodynamic potentials of polar fluids. II. Angle‐dependent intermolecular potential
View Description Hide DescriptionA unified treatment is presented of intermolecular and thermodynamic potentials of polar fluids. The theory is specialized to the case in which the rotational and translational correlation times are comparable. It is shown that the latter condition leads naturally to an intermolecular potential which can be decomposed into repulsion, dispersion, electrostatic, and polarization terms, all of which are angle dependent. Expressions are developed for the fluid free energy in a perturbation series and the first few terms are considered in detail. Special attention is focused on the classical limiting form of the free energy for a system of axially‐symmetric molecules. It is shown that the free energy expression is consistent with a Boltzmann distribution in which the intermolecular potential is, within the dipolar approximation, represented by a set of infinite order pair potentials, the leading terms of which reduce to the Stockmayer potential plus polarization terms. Comparison is made between our series expansion of the fluid free energy and other perturbation schemes.

Raman study of the ferroelectric phase transition in PbHPO_{4} and PbHAsO_{4}
View Description Hide DescriptionThe Raman spectra of PbHPO_{4} and PbHAsO in the p a r a‐ and ferroelectric phases and also PbDAsO_{4} (room temperature only) have been measured. Mode assignment of most predicted modes has been made. Weak underdamped soft modes have been observed below 100 cm^{−1} in the ferroelectric phase of both hydrogen compounds which confirms the role of the protontunnelling in the double minimum potential. PbDAsO_{4} seems not to be isomorphous to PbHAsO_{4} at room temperature.

On the equivalence between the space fixed and the body fixed formulations of the j _{ z }‐conserving approximation
View Description Hide DescriptionIt is shown that space fixed and the body fixed formulations of the j _{ z }‐conserving approximation are equivalent if the partial wave parameter ? is chosen equal to the final orbitral angular momentuml′. We also examine the replacement of ? by l, the initial orbital angular momentum in the space fixed formulation. Only these two choices of ? give simple expressions for the scattering amplitude.

Monopole effects on electronic excitation interactions between large molecules. I. Application to energy transfer in chlorophylls
View Description Hide DescriptionThe transition monopole theory of electronic excitation interaction is described and applied to the theory of resonance transfer for chlorophyll molecules. It is then compared with analogous results based on transition dipole theory. The correction to the Förster theory of resonance transfer is evaluated in various geometries for the case of short range transfer. The enhancement or reduction factor for the transfer rate in various chlorophylls is of the order of two to one tenth for the range that we are interested in, i.e., 12–30 Å, and anisotropy of the transfer rate is greatly increased. A possible enhancement of the transfer rates in the case of forbidden transitions in porphin is discussed.

Second‐order nuclear quadrupole coupling in the Br^{−} ion
View Description Hide DescriptionThis paper describes a theoretical treatment of the second‐order Stark effect on the nuclear quadrupoleinteraction of a closed‐shell atomic system. A method of calculation is presented which applies a dipolar perturbation in the uncoupled Hartree approximation. It is suggested in the context of ionic solids that the unperturbed anion be represented by Hartree–Fock wavefunctions appropriate to the free neutral atom rather than by the more extended free‐ion functions. The first‐order perturbation to the wavefunction is obtained by numerical solution of a differential equation (the method of Kotani and Sternheimer), and the second‐order perturbation is approximated by projecting it onto the unperturbed wavefunction. The Br^{−} ion is considered in detail and it is shown that, in the strong electrostatic fields to be found in ionic solids, the second‐order nuclear quadrupole coupling can be considerable. The calculation yields a dipolarizability of 6.19 Å^{3}, a dipole shielding factor of 4.45, a second‐order Stark quadrupolarizability of 0.544 Å^{5} V^{−1}, and a second‐order Stark effect on the field gradient of γ_{24}=−3.23 V^{−1}. The second‐order response is smaller in this case than in the free Cl^{−} ion of earlier, variational calculations.

HX, N_{2} double doping experiments in monatomic matrices: Near infrared spectra and symmetry properties of the intermolecular potential
View Description Hide DescriptionThe infrared spectra of monomeric and dimeric hydrogen halides trapped in mixed nitrogen‐rare gas matrices are recorded for various nitrogen–rare gas ratios. The analysis of the displacement of the monomer and dimer band frequencies strongly suggests the formation of HX–nN_{2} aggregates inside the monatomic crystal when varying this ratio from pure rare gas to pure nitrogen. A convenient analytical form is proposed in order to describe the interaction potential energy between the foreign molecules and the matrix. Numerical procedures based on realistic but tractable models for the representation of the dopedcrystal structure are then used in order to calculate the conformation of some aggregates inside the crystal and to prove the strong tendency for foreign molecules to agglomerate each other. We can thus explain the behavior of the HX monomer and dimer bands frequencies that decrease when doping with N_{2} molecules down to a minimum for a N_{2}–rare gas ratio 60%–70% and then increase up to the HX–pure nitrogen frequencies, in terms of a symmetrization process in the conformation of the aggregates around the HX [or (HX)_{2}] molecule.

Temperature dependence of the total reaction rates for Cl+HI and Cl+HBr
View Description Hide DescriptionThermal reaction rate constants have been determined for the reactions Cl+HI and Cl+HBr in the temperature range 220–400 °K. The rates vary slowly with temperature. For Cl+HI the effective reaction cross section reaches a maximum of 31 Å^{2} near 300 °K. A tentative reaction model is proposed in which the attacking halogen atom is attracted to the halogen end of the hydrogen halide and then rotation of the hydrogen, with little or no activation energy, completes the reaction.

Molecular Rydberg states. XI. Quantum defect analogies between molecules and rare gases
View Description Hide DescriptionThe s and dRydberg series of hydrogen iodide and methyl iodide are reported. These series are assigned using analogies to the corresponding series in xenon. It is shown that a comparison between molecular spectra and pertinent atomic data provides a facile assignment technique for molecular Rydbergspectra. Using the phase amplitude method, the Rydberg data are analyzed in an attempt to provide information on the residual atomic and molecular potentials and to categorize them qualitatively as attractive/repulsive, long range/short range. It is shown that, for l?2, the centrifugal barrier inhibits ingress of the Rydberg electron into the core and that the molecule is effectively spherically symmetric.

Nitrogen‐14 nuclear quadrupole resonance study of substituted nitrobenzenes
View Description Hide DescriptionNitrogen‐14 nuclear quadrupole resonance (NQR) spectra of 26 C–nitro compounds are reported and analyzed in the framework of the Townes and Dailey theory. Six different possibilities for the principal axis system of the electric field gradient (efg) tensor at the nitrogen of the NO_{2} group are considered. The results lead to the conclusion that the z direction of the principal axes is in the plane of the molecule and perpendicular to the C–N bond, whereas the x direction is along the C–N bond. The variations of the π electron density are in very good agreement with the ideas of classical chemistry. Satisfactory correlations of NQR data with the Hammett σ and σ_{ R } constants have been found. σ_{ p } and σ_{ R } for the cholesteryl group are determined which suggest that the cholesteryl group, COOC_{27}H_{45}, is an electron attracting group. The nitrogen‐14 quadrupole coupling constant in the solid phase of nitromethane (1695 kHz) is found to be substantially larger than that found in the gaseous phase (1187 kHz) by microwave measurements. This result cannot be explained by invoking the presence of hydrogen bonds in the solid phase of nitromethane.

The laboratory microwave spectrum of the cyanide radical in its X ^{2}Σ^{+} ground state
View Description Hide DescriptionThe microwaveabsorptionspectrum of the CN molecule in the v=0 and v=1 vibrational states of the electronic ground state has been obtained in glow discharges in nitrogen–cyanogen mixtures at room temperature. Zeeman modulation is used for detection of the signal, and the microwave source is phase locked and digitally programmed by a computer. For each vibrational state the frequencies of the seven strongest hyperfine components of the N=0→1 rotational transition have been extracted from a careful regression analysis of the complex line shapes observed in the digitized spectral data. For the v=0 state the resulting rest frequencies and molecular parameters are in good agreement with, but more precise and accurate than, the values obtained from earlier radioastronomical studies. For the excited vibrational state the present work provides the first determination of the hyperfine parameters. The precision of the results for both states is sufficient to give a reliable measure of the variation of the spin–rotation constant (γ) and the hyperfine constants (b, c, and e Q q) with vibrational quantum number.

An electron‐gas study of the bonding, structure, and octahedral ligand‐field splitting in transition‐metal halides
View Description Hide DescriptionWe present a discussion of the structure and the ligand‐field splitting parameter 10D q for some d ^{1}titanium (III) halides. Our approach is based on an ionic model allowing for the inclusion both of electrostatic and of non‐Coulombic repulsive contributions. The point‐Coulombic forces are treated exactly using the Madelung constant for each specific crystal. Only the metal–ligand and ligand–ligand short‐range repulsions are taken to be of importance, and are calculated using the electron‐gas model. Our results show that for the chloride the model is poor and hence that covalency is essential for understanding the heavier halides. For the fluoride systems, however, we obtain values for the bond distances, and the lattice energy for the trifluoride, which are accurate within five per cent of experiment. As previous approaches have shown, we find the negative sign for 10D q when only the purely electrostatic interaction is included. We show in addition that overlap repulsion provides the major part of the balance which corrects the ordering of the e _{ g } and t _{2g } states. We conclude that for fluorides at least the ionic model is an excellent starting‐point for an understanding of their properties.

Density functional theory and molecular bonding. I. First‐row diatomic molecules
View Description Hide DescriptionThe density functionalequations of Hohenberg, Kohn, and Sham (HKS) are solved self‐consistently for simple molecules using a method described originally by Andersen and Woolley. Spectroscopic constants, calculated for B_{2}, N_{2}, O_{2}, F_{2}, CO, and BF and for the four lowest lying states of C_{2}, are in good agreement with experiment. Results are consistently better than Hartree–Fock and comparable with those of configuration interaction calculations.

Thermodynamics of internal variables in the context of the absolute reaction rate theory
View Description Hide DescriptionIn this paper we present a nonlinear internal constitutive equation which we have obtained by unifying the theory of ’’The Thermodynamics of Internal Variables’’ and a modified version, presented here, of ’’The Theory of Absolute Reaction Rates’’. The chief characteristic of this equation is the explicit form of the nonlinear relation between the internal forces and the rates of change of the internal variables. In addition, we have given the internal variables a precise physical interpretation. An internal variable in effect represents the average displacement of a group of atoms whose motion is impeded by a potential energy barrier of a specific height. As a consequence, the microforce encountered in the Eyring theory is identified as the free energy gradient causing the displacement of this particular group; the precise relationship between the microforce and the externally applied force field is also established. It is deduced that the single‐well model, characteristic of theories with one internal variable, represents the mechanical response of a p e r f e c t crystal to externally applied force fields. The theory developed, herein, is anticipated to have wide applicability in the mathematical representation of the rate dependent behavior of metals and other materials.

Temperature dependence of the rate constant for the reaction between carbon monosulfide and atomic oxygen
View Description Hide DescriptionThe rate of the reaction CS+O→CO+S was investigated from 150 to 300 K using a flow‐tube technique. The rate was determined both by monitoring the CO formation rate in the presence of excess CS and by measuring the CS reaction rate in the presence of excess O. The reaction S+O_{2}→SO+O could interfere with the first method when CS and O react in the presence of O_{2}. Experimental evidence indicates, however, that the effect of the O_{2}reaction is not important at room temperature because of a fast chain‐terminating reaction for S. The data from both methods fit an Arrhenius form with an activation energy of 6.3±1.2 kJ/mole and a pre‐exponential factor of 2.6±0.4×10^{−10} cm^{3}/molecule s, where the errors quoted are for the 95% confidence level.