Volume 64, Issue 5, 01 March 1976
Index of content:

Selective excitation and decay of the fluorescence emission arising from the ^{5} D _{4} state of Tb^{3+} in POCl_{3}:SnCl_{4}
View Description Hide DescriptionLight excitation of Tb^{3+} in POCl_{3}:SnCl_{4} at 488±1 nm giving rise to direct population of the ^{5} D _{4} state, is accompanied by fluorescence emission whose decay is a simple exponential. On the other hand, indirect population of the ^{5} D _{4} state, via excitation with shorter wavelengths and relaxation of higher excited states of Tb^{3+}, gives rise to fluorescence emission characterized by a very complex decay law. The nature of the complex decay and the conditions at which maximum fluorescence intensities are attained are discussed. The fluorescence lifetime of the ^{5} D _{4} state, τ_{4}, is independent of [Tb^{3+}] but decreases slightly at higher temperatures. This is attributed to a temperature dependent quenching process of the ^{5} D _{4} state of Tb^{3+} by the solvent.

A theoretical investigation of the solid‐state synthesis of (SN)_{ x }
View Description Hide DescriptionThe solid‐statepolymerization of S_{2}N_{2} to (SN)_{ x } is investigated as it relates to the molecular, crystallographic, and defectstructures of this superconductingpolymer. The least motion principle predicts that polymerization should occur by a nonunique reaction to form c i s–t r a n spolymer chains normal to the 2_{1} axis of the precursor phase. The origin of the disordered structure observed in partially polymerized S_{2}N_{2} is explained by the occurrence of two symmetry equivalent reaction modes, which produce chains related by a center of symmetry. Both monomer and polymer phases have 2/m point symmetry, but the twofold axes are orthogonal. Consequently, the statistical symmetry of (SN)_{ x } is m m m, which is manifest (as a function of thermal history) either by twinning, molecular scale disorder, the statistical nature of the relationship between S_{2}N_{2} and (SN)_{ x }crystal orientations, or a combination of these possibilities. In agreement with prediction, this symmetry is produced by equal population of the twin crystals previously observed in (SN)_{ x }. Coordinates associated with disordered chains in partially polymerized S_{2}N_{2} are successfully predicted by this reactiontheory. Fractionally occupied sites are observed in the polymer corresponding to (SN)_{ x } chains with different orientations, but the same chain axis direction. The geometry of one of these chains is substantially different from that of the major chain component.

Molecular oxygen enhanced fluorescence of organic molecules in polymer matrices: A singlet oxygen feedback mechanism
View Description Hide DescriptionThe surprising observation that molecular oxygen, the universal quencher of luminescence of organic molecules, can reverse its role and act as an enhancer of fluorescence in polymer matrices was originally recorded by Geacintov e t a l. Bolton, Kenner, and Khan investigated the phenomenon of oxygen e n h a n c e d fluoresence of organic molecules in polymer matrices and attributed the enhancement to a singlet oxygen feedback mechanism in which there is a repopulation of the singlet manifold of the organic molecule via energy transfer from singlet molecular oxygen. In this paper we examine the preliminary results and consider the relative merits of the singlet oxygen feedback mechanism and a trivial mechanism involving ground state depletion of the organic fluorescer in the evacuated samples. Optical cross beamexperiments, designed to compare the extent of ground state depopulation of the organic molecule with the amount of enhancement observed, indicate that the depopulation mechanism can account for only a small part of the fluorescence enhancement. Furthermore, we report the discovery of oxygen s t i m u l a t e dfluorescence in similar systems under conditions in which the singlet manifold of the organic molecule can be populated only by energy transfer from singlet oxygen. Oxygen stimulated fluorescence is a direct and unequivocal confirmation of the basic idea of the singlet oxygen feedback mechanism and raises a fundamental question about the mechanism of oxygen quenching of the excited singlet states of organic molecules.

Effect of vibrations on the internal rotation barrier in ethane
View Description Hide DescriptionThe effect on the ethane internal rotation barrier of (1) zero‐point vibrational averaging and (2) distortions in equilibrium geometry during internal rotation is determined. We find that the (previously ignored) averaging term is 374±90 cal/mole which is substantially larger, and opposite in sign, to the distortion correction of −161±20 cal/mole. The isotope shift, other experimental tests, and some important implications of our results are discussed.

On the long‐range configuration interaction between ionic and covalent states
View Description Hide DescriptionThe problem of long‐range configuration interaction between ionic and covalent states is considered. The existing methods for calculation of this interaction are discussed. An asymptotically exact expression for the long‐range configuration interaction at the crossing point of diabatic ionic and covalent potentials is derived. Using this expression the adiabatic energy splitting at the crossing point for H*+H, M+H, and M+O systems (M = alkali atom) is calculated. The results are compared with those obtained by LCAO and variational calculations.

Light scattering by impurities at a liquid interface
View Description Hide DescriptionWe discuss the scattering of light by anisotropic impurities located at a fluid interface. Our aim is to determine which type of information about the arrangement and motion of the impurities can be obtained from precise light scattering experiments. Since the paper is intended to be exploratory rather than exhaustive, we concentrate our attention on the simplest nontrivial case; a number of possible complications are discussed in a more qualitative way.

Detailed quantum transition state theory
View Description Hide DescriptionThe detailed quantum transition state theory (DQTST) is presented. It allows quantum effects to be obtained from greatly simplified calculations by applying boundary conditions corresponding to flux in the product direction at the transition state and solving for nonreactive scattering S‐matrix elements. These are used to obtain the reaction probability from specific initial states. It is compared to the transition state theory(TST) and the complete quantum calculations for the planar H+H_{2}reaction. The convergence of DQTST to the exact results is shown to be very good.

Molecular beam kinetics: Optical model for reactive scattering of alkali atoms and alkyl iodides
View Description Hide DescriptionAngular distributions of reactively scattered alkali iodides have been measured for reactions of thermal beams of Rb and Cs atoms with beams of alkyl iodides RI, where R=CH_{3}, C_{2}H_{5}, n‐ and i‐C_{3}H_{7}, n‐C_{4}H_{9}, and n‐C_{5}H_{11}. When shifts expected from kinematic differences are taken into account, the qualitative features are found to confirm the results previously obtained for the analogous K atom reactions. The reaction cross sections increase with the size of the alkali atom, from ∼35 Å^{2} for K+CH_{3}I to ∼45 Å^{2} for Rb and ∼75 Å^{2} for Cs. The fall off in intensity of elastically scattered alkali atoms at wide angles also increases for K→Rb→Cs. The anisotropic angular distribution of products, the magnitude of the reaction cross section, and the fall off of the wide‐angle elastic scattering for the K, Rb, and Cs systems are correlated in terms of an extended optical model which simply assumes hard sphere interactions for both the entrance and exit trajectories. The reaction probability derived from the data by use of the optical model is large only for collisions which reach the repulsive wall region but even there it is well below unity for thermal energy collisions.

Elastic scattering measurements of the Ar_{2}* (^{3}Σ^{+} _{ u }) well depth
View Description Hide DescriptionAngular distributionmeasurements of the scattering of Ar(^{3} P _{2}) from Ar(^{1} S _{0}) at center of mass collision energies from 5 to 10 eV yield a prominent rainbow maximum at τ=Eϑ=81±3 eV‐deg. Combined with recent high energy ground state scattering data and information derived from the Ar_{2}* emission continua, the result implies that D _{ e }=0.78±0.04 eV and R _{ e }=2.33±0.02 Å in an assumed Morse form for the lowest 1_{ u }, 0^{−} _{ u }(^{3}Σ^{+} _{ u })Ar_{2}* excimer potential.

Spectra and structure of organophosphorus compounds. XV. Microwave spectrum of ethylphosphine
View Description Hide DescriptionThe microwave spectra of CH_{3}CH_{2}PH_{2}, CH_{3}CH_{2}PHD, and CH_{3}CH_{2}PD_{2} have been recorded in the range 18.0 to 40.0 GHz. A‐type transitions were observed and assigned for both g a u c h e and t r a n s conformers of ethylphosphine, and b‐type transitions observed and assigned for the g a u c h e conformer. The dipole moment components for the g a u c h e conformer were determined from the Stark effect to be: ‖μ_{ a }‖= 0.86±0.01, ‖μ_{ b }‖=0.73±0.07, ‖μ_{ c }‖=0.45±0.05, and ‖μ_{ t }‖=1.22±0.07 D. For the t r a n s conformers, there was only one component of the dipole moment and it had a value of ‖μ_{ a }‖= ‖μ_{ t }‖=1.226±0.005 D. The relative intensity of nine pairs of lines for the g a u c h e and t r a n s were measured at room temperature, and from the ratio of intensities it was found that ethylphosphine exists as a 45% g a u c h e and 55% t r a n s mixture with an energy difference of 200±100 cm^{−1} and the t r a n s the more stable conformer. With reasonable assumptions for the ethyl moiety, the following structural parameters were calculated: &CCP=110.1±0.2°, r _{PC}=1.880±0.0002 Å and PH_{2} dihedral angle = 61±2° and for the g a u c h e conformer &CCP= 115.2±0.2°, r _{PC}=1.876±0.002 Å and PH_{2} dihedral angle = 180±2°.

Scattering of thermal helium beams from high Miller index (stepped) platinum crystal surfaces
View Description Hide DescriptionHelium beams generated by a 300 K source were scattered from single crystalplatinum surfaces. The angular distribution of the scattered beam was monitored as a function of azimuthal angle φ and incident angle ϑ_{ i }. The effect of the presence of atomic steps on the angular distribution of scattered helium atoms was studied on a crystal surface composed of terraces on the average five atoms in width separated by steps of monatomic height. Rainbow scattering is observed when the steps are oriented perpendicular to the incident beam direction indicating strong modulation of the angular distribution by the periodic step potential. Only specular scattering is observed when the incident beam impinges on the crystal along the step edges which indicates minimal effects of the presence of steps on the scattering distribution. These results as well as the absence of diffraction are in good agreement with previous theoretical calculations.

A simple approach to the equilibrium statistical mechanics of two‐dimensional fluids
View Description Hide DescriptionA simple statistical mechanical theory previously developed for three‐dimensional hard spheres is applied to a system of two‐dimensional hard disks to obtain analytical equations for activity and pressure. The first order result, based on only the third virial coefficient, fits the molecular dynamics data for fluid disks significantly better than the seven‐term virial series, but (unlike the case in three dimensions) not so well as the scaled particle theory. The second order result, involving the fourth virial coefficient, is the equal of the Padé approximant (3,4) with seven correct virial coefficients built in, and is significantly better than the scaled particle theory. The simple theory is surprisingly accurate even for the hard disk crystal. A simple theory of the two‐dimensional Lennard‐Jones fluid is obtained by incorporation of attractive wells with a hard core of temperature‐dependent diameter. Comparison of the theory with the 17 high‐density, supercritical pressures obtained by Fehder using molecular dynamics shows deviations averaging only 3 1/2%. Agreement with the high‐density, low‐temperature data obtained by Tsien and Valleau using Monte Carlo techniques is not so good. Calculations for the Lennard‐Jones 6–12 potential are compared with calculations for the 6–12–3 potential, proposed as more suited than the 6–12 for adsorbed gases.

A simple approach to the equilibrium statistical mechanics of Lennard‐Jones fluids
View Description Hide DescriptionA simple statistical mechanical theory previously developed for hard spheres and for two‐dimensional fluids is applied to three‐dimensional particles interacting through a Lennard‐Jones 6–12 potential. The results are compared with the molecular dynamics experiments of Verlet, with surprisingly good agreement. Employing the potential parameters obtained for argon from second virial coefficient data results in pressure isotherms which fit the experimental data of Michels with greater accuracy than the choice of potential warrants.

Statistical mechanics of polar systems. II
View Description Hide DescriptionThe results of Part I of this work [J. Chem. Phys. 61, 562 (1974)] are extended. A new formally exact expression for the dielectric constant ε of a polar nonpolarizable fluid is derived. It involves a ’’core parameter’’ Θ that depends upon the way one decomposes the pair potential into a reference and perturbing term, reducing to our earlier expression given in Part I for Θ=0 and to the expression proposed by Nienhuis and Deutch for Θ=1. It is shown how the Clausius–Mosotti, Onsager, and Wertheim expressions for ε can all three be regarded as mean‐field approximations, each associated with a different value of Θ. Each expression becomes exact for a somewhat different model of a continuum fluid. A mean‐field theory is given for phase transitions that involve dipolar ordering; the theory becomes exact (as do the other mean‐field results of the paper) for a model in which the Kac inverse‐range parameter γ goes to zero.

The infrared and Raman spectra of matrix isolated binary and mixed mercury halides
View Description Hide DescriptionRaman and ir spectra in the stretching region of all HgX_{2} and HgXY molecules (X,Y=Cl, Br, I) trapped in solid Kr at 20 °K are reported. Bands are assigned to modes of monomers and dimers. Bond stretching and interaction force constants are calculated and compared. The possibility of a slightly bent structure for several monomers is indicated. The dimer structure is shown to be centrosymmetric in all cases and the identity of the bridged halogen is suggested for the ’’mixed’’ dimers.

Potential energy curves from the electron gas model. II. The ion–rare gas interactions
View Description Hide DescriptionThe electron gasmodel already proposed in the literature for computing potential energy curves between neutral, spherical systems is here extended to nonneutral interactions by employing a physically intuitive and numerically simple form of the induction forces within the overlap region. Calculations are performed for Li^{+}, Na^{+}, and K^{+} interacting with heavy rare gases, from neon through xenon, and the results are compared with the available molecular beam data. The relevant potential parameters are also presented for all the examined cases.

High‐temperature fast‐flow reactor kinetic studies. The AlO/O_{2} reaction near 1400 K
View Description Hide DescriptionA high‐temperature fast‐flow reactor (HTFFR) has been adapted to the study of the kinetics of diatomic radicals via laser induced fluorescence. In this work, the homogeneous gas‐phase reaction of AlO with O_{2} has been investigated near 1400 K. The reaction proceeds via AlO(v)+O_{2}→AlO_{2} +O; k (v) = (3.1±1.7) ×10^{−13} ml molecule^{−1}⋅sec^{−1}, with no discernible difference between AlO in the v=0 and 1 vibrational levels.

The spectrum of Nd^{3+}:LaCl_{3}
View Description Hide DescriptionNew measurements of the optical spectrum of trivalent neodymium in a hexagonal lanthanum chloride host have been made at 4 and 77 K at wavelengths down to 2500 Å (40 000 cm^{−1}). Crystal quantum number assignments for new energy levels were derived from these results and for most of the earlier levels not already classified. A least‐squares fit with a mean error of 8.1 cm^{−1} was carried out for 101 levels, using 20 adjustable parameters in a Hamiltonian containing operators for two‐ and three‐particle electrostaticinteractions; spin–orbit, spin–other‐orbit and spin–spin magnetic interactions; two‐particle pseudomagnetic configuration‐mixing interactions; and single‐particle crystal‐field interactions.

Infrared spectra of the M^{+}HBr_{2} ^{−} and the M^{+}HClBr^{−} ion pairs and their deuterium analogs isolated in argon matrices at 15 K
View Description Hide DescriptionThe matrix reactions of alkali bromide salts and HBr and DBr have been investigated in argon matrices. In each case, an intense band was observed from the reaction product, which was assigned to the antisymmetric stretch, ν_{3}, of the HBr_{2} ^{−} and DBr_{2} ^{−} anions in the M^{+}HBr_{2} ^{−} and M^{+}DBr_{2} ^{−} ion pairs. Comparisons to literature spectra, in conjunction with quartic deuterium shifts, indicate that the anion has a linear, centrosymmetric geometry in the isolated ion pair. A less intense band was also observed which has been assigned to the combination band ν_{1}+ν_{3} for the HBr_{2} ^{−} anion. Reaction of alkali bromide salts with HCl and alkali chloride salts with HBr have been shown to yield identical absorptions for the M^{+}HClBr^{−} ion pairs. The spectra suggest that both type I and type II HClBr^{−} anions exist, and the resulting form may be determined by cation position.

The magnetic structure of the rare earth tetraborides ErB_{4} and DyB_{4}
View Description Hide DescriptionNeutron diffraction studies were performed on the compounds ErB_{4} and DyB_{4} at 4.2 and 300 K. Both compounds are antiferromagnetically ordered at 4.2 K. A determination of the magnetic structure type is given for ErB_{4} and DyB_{4}. The magnetic intensities were studied as a function of temperature, leading to Néel temperatures T _{ N }=16.3 K (ErB_{4}) and T _{ N }=21 K (DyB_{4}). The relative stability of several simple magnetic structures, appropriate to RB_{4}, are discussed in terms of the RKKY model, together with previously reported results of magnetization and NMR studies on these compounds.