Index of content:
Volume 69, Issue 9, 01 November 1978

Radiative lifetimes and two‐body deactivation rate constants for Ar(3p ^{5}, 4p) and Ar(3p ^{5},4p′) states
View Description Hide DescriptionThe radiative lifetimes and two‐body deactivation rate constants of the Ar(3p ^{5}4p and 4p′) levels have been measured by a time‐resolved laser‐induced fluorescence technique in a flowing afterglow apparatus. The flowing afterglow produced the Ar(3p ^{5}4s,^{3} P _{2}) metastable atoms, which subsequently were pumped by the pulsed dye laser. The measured radiative lifetimes are in excellent agreement with the calculated transition probabilities based upon intermediate‐coupling theory. Radiative branching ratios of these states also were measured in order to assign the transition probabilities and the absorption oscillator strengths of the transitions between the 4s and 4p levels. The two‐body deactivation rate constants range from 1×10^{−10} to 2×10^{−11} cm^{3} atom^{−1} sec^{−1}. Specific product states from the two‐body collisions between Ar(4p and 4p′) atoms and Ar were identified from the laser‐induced emission spectra and rate constants for individual product states were assigned for each level. Our results show that two‐body collisions do not necessarily result in cascade down the p and p′ manifold, and for the Ar(4p and 4p′) levels more than 50% of the atoms are directly converted to the Ar(4s and 4s′) manifold.

On the efficiency of rate processes. Power and efficiency of heat engines
View Description Hide DescriptionWe analyze the power and efficiency of heat engines which operate subject to irreversible heat flow. First, we consider a specific model, with a cycle for an ideal gas similar to that of a reversible Carnot engine (’’isothermal cycle’’), and find the maximum power, and efficiency at the point of maximum power (η_{ m }), for given heat bath temperatures and compression ratio. We prove that the cycle chosen produces more power than any other conceivable cycle in the limit of large compression ratio; the derivation is made for an ideal or van der Waals gas as a working fluid, but this is not restrictive in this limit. We use these results to obtain a general formulation, of upper bounds on power and η_{ m }, valid for isothermal cycles to study the dependence of these quatities on the form of the law of irreversible heat conduction. We find that η_{ m } depends only on the heat bath temperatures and the form of the irreversible rate process, but is independent of the material properties of the system. The dependence of η_{ m } on the form of the rate process suggests the concept of ’’efficiency of rate processes.’’

Molecular distortion in the pyrazine triplet state. Analysis in terms of the MIDP experiments under polarized light
View Description Hide DescriptionThe distortion of the pyrazine molecule in its triplet excited state, as revealed by the appearance of the group theoretically forbidden bands in the sublevel phosphorescence, is analyzed in terms of the polarization of the originally forbidden bands in the sublevel phosphorescence. The experimental results together with some theoretical estimates lead to the conclusion that the pyrazine triplet state is distorted to C _{2h }[C _{2}(y)] due to the pseudo Jahn–Teller type vibronic coupling involving b _{2g }(ν_{4}) vibrational mode.

Electron‐impact excitation of low‐lying electronic states in CS_{2}, OCS, and SO_{2}
View Description Hide DescriptionVariable angle, electron‐impact energy‐loss spectra of CS_{2}, OCS, and SO_{2} have been obtained at incident electron energies of 25, 40, and 70 eV for CS_{2}, and 30 and 55 eV for OCS and SO_{2}. Singlet→triplet excitations are observed with peaks at transition energies of 3.36 eV in CS_{2}, 4.94 eV in OCS, and 3.40 eV in SO_{2}. A feature which peaks at 3.65 eV in CS_{2} is observed to have singlet→triplet character. The CS_{2} and OCS spectra do not confirm the existence of several spin‐forbidden transitions reported in solid phase ultraviolet absorption studies of these molecules. In SO_{2}, no evidence is found of transitions to the 1 ^{3} A _{2} and 1 ^{3} B _{2} states, believed to lie near the well‐known ? ^{3} B _{1} state.

Scattering of thermal He beams by crossed atomic and molecular beams. I. Sensitivity of the elastic differential cross section to the interatomic potential
View Description Hide DescriptionThe ability of diffraction oscillations in atomic beamscattering experiments to uniquely determine interatomic potentials for highly quantal systems is examined. Assumed but realistic potentials are used to generate, by scattering calculations and incorporation of random errors, differential cross sections which are then treated as if they were ’’experimental’’ data. From these, attempts are made to recover the initial potential by varying the parameters of assumed mathematical forms different from the original one, until a best fit to the ’’experimental’’ results is obtained. It is found that the region of the interaction potential around the van der Waals minimum is accurately determined by the ’’measured’’ differential cross sections over a range of interatomic separations significantly wider than would be expected classically. It is also found, for collision energies at which the weakly repulsive wall is appreciably sampled, that the SPF–Dunham and double Morse–van der Waals types of potentials lead to accurate determinations of the interatomic potential, whereas many other mathematical forms do not. Analytical parameterizations most appropriate for obtaining accurate interatomic potentials from thermal DCS experiments, for a given highly quantal system, may depend on the collision energy used.

Electron energy loss spectroscopy and the optical properties of polymethylmethacrylate from 1 to 300 eV
View Description Hide DescriptionEnergy lossspectra of 80 keV electrons transmitted through thin films polymethylmethacrylate (PMMA) were measured with a resolution of 0.1 eV for energy losses from 1 to 300 eV. From the loss spectra, the dielectric response function of PMMA was obtained from 1 to 100 eV and compared with recent synchrotron radiation results. The spectrum of valence excitations from 5 to 13 eV is shown to be characteristic of the pendant group and is compared to experimental gas phase spectra and molecular orbital (CNDO/S) calculations of model molecules. The spectrum of core electron excitations above 285 eV provides a measure of the distribution of empty molecular orbitals and, when the carbon 1s binding energies are taken into account, a qualitatively accurate description of the observed spectrum is made from the CNDO ground state calculation. The energy lossspectra of 20, 40, and 100 eV electrons reflected from the surface of PMMA exhibit a triplet excitation at 4.2 eV and indicate the extreme sensitivity of this material to radiation damage. Finally, the spectrum of electron damage is shown to be similar to that reported for uv photolysis but with a strong previously unreported peak at 6.3 eV assigned here to excitation of radiation induced isolated unsaturated bonds.

Low‐energy electron diffraction study of the surface of thin crystals and monolayers of normal paraffins and cyclohexane on the Ag(111) crystal surface
View Description Hide DescriptionThe surfaces of the normal paraffins (C _{4}–C _{8}) and cyclohexane in the form of thin single crystal films (10^{2} Å) condensed upon an Ag(111) single crystalsurface have been studied using low‐energy electron diffraction(LEED). The physical adsorption of monolayers of these molecules was also studied on the Ag(111) surface. The data were compared with similar data obtained from experiments on the Pt(111) surface (Ref. 1) to yield the following results : n‐Octane condenses on either substrate with its unreconstructed (101̄) plane parallel to the substrate. Cyclohexane similarly condenses exposing its (001). n‐Heptane multilayers on either substrate give the same LEEDpattern, but the surface unit mesh is different from any of the expected planes of the n‐heptane crystal structure.n‐Hexane and n‐pentane multilayers form pseudomorphic crystal structures, that are dependent upon the structure of the substrate. Physically adsorbed monolayers of n‐octane, n‐heptane, and n‐hexane adsorb in ordered structures with their molecular axes parallel to each other and the Ag<11̄0≳. Cyclohexane adsorbs in a (9×9) structure in a hexagonal array with its molecular plane parallel to the Ag(111) surface. The generally lower temperatures necessary to order monolayers on Ag(111) indicate that unsaturated hydrocarbons are less strongly adsorbed on this surface than on the Pt(111).

Energy dependence of the differential photoelectron cross sections of molecular nitrogen
View Description Hide DescriptionThe angular distribution of photoelectron intensity for molecular nitrogen was studied using He i and Ne i resonance line discharge light sources. Studies of photoelectron angular distributions covering a range of photon energies, and thus a range of photoelectron energies, are possible using the weaker high order lines in each discharge as well as the principal lines. Peaks in three photoelectron bands of N_{2} were studied at the photon energies 16.85, 19.78, 21.22, 23.09, and 23.74 eV, where possible. We find that the v′=0 peak of the X ^{2}Σ^{+} _{ g } band has abnormally high intensity and, at the higher photon energies, an abnormally low angular distribution asymmetry parameter, β. Several mechanisms for this anomaly are discussed, including autoionization, the variation of electric dipole transition moments with internuclear distance, and possible shape resonance phenomena. None of these explanations is completely in agreement with all theoretical and experimental evidence.

Cation dynamic studies in n‐propylammonium chloride, CH_{3}(CH_{2})_{2}NH_{3}Cl, by NMR
View Description Hide DescriptionPulsed and continuous wave protonmagnetic resonance techniques have been used to study the low and room temperature modifications of CH_{3}(CH_{2})_{2}NH_{3}Cl (n‐propylammonium chloride or PACl) and CH_{3}(CH_{2})_{2}ND_{3}Cl (d _{3}‐PACl) over the temperature region 77 °K to ∼ 346 °K. The proton static spin–lattice relaxation time (T _{1}) and second moment data in the ordered low temperature phase are consistent with a model where only CH_{3} and CH_{3}+NH_{3} ^{+} group rotation about their three‐fold axes take place in d _{3}‐PACl and PACl, respectively. At each temperature the methyl group rotation is faster than that of NH_{3} ^{+} and the respective activation energies are 2.4 (±0.15) and 3.7 (±0.2) kcal/mole for CH_{3} and NH_{3} ^{+} rotation. For the room temperature phase of both salts relatively fast chain rotation about their long chain axis with independent CH_{3} +NH_{3} ^{+} group rotation was found from second moment analysis. The activation energy obtained for this motion from the T _{1} data is 2.5 (±0.2) kcal/mole. The reported phase transition exhibits an appropriate discontinuity in the T _{1} curve at 186±2 °K and seems to be associated with the dynamic disorder. The reorientations and phase transitions in the corresponding modifications of the n‐alkylammonium chloride series is discussed in the light of our results and other published data.

The radiative and nonradiative decay of the triplet state of F _{2} centers in KCl and KBr
View Description Hide DescriptionThe temperature variation of the decay time of the triplet state of the F _{2} center in KCl and KBr has been measured from 4 to 240 K using the linear dichroism technique. These results are interpreted in terms of several mechanisms. At low temperature, a radiative process is competing with a nonradiative one, while at high temperature, a thermally activated one (level crossing process) competes with the nonradiative one. The radiative mechanism can be calculated assuming a spin orbit mixing of the ^{3}Σ_{ u } ground triplet state with a ^{1}Π_{ u } singlet state and the result is in good agreement with experiment. The nonradiative decay can be roughly accounted for by the mixing of non‐pure‐spin Born–Oppenheimer singlet and triplet states (spin–orbit mixed) by the nonadiabatic terms of the potential; this theory gives a satisfactory description of the temperature behavior of the triplet decay time and an order of magnitude quantitative prediction of the rate.

Semiconducting behavior in simple and complex charge‐transfer salts of tetracyanoquinodimethane with substituted pyridinium donors
View Description Hide DescriptionA series of eight anion–radical salts has been prepared, each with tetracyano‐p‐quinodimethane (TCNQ) as the acceptor and a substituted pyridinium ion as the donor. Four species of donor substitution were made as follows: placement of a CN group at either the 3 or 4 position on the pyridinium ring, and use of either N‐methyl or N‐benzyl substituents. The simple (1:1) and complex (1:2) salts were crystallized for each species. Compaction conductivity measured between 100 and 300 K showed that the simple salts are weakly conducting semiconductors, of activation energy ≳0.3 eV. Single crystal conductivity data for two of these showed essentially isotropic conduction, compatible with alternating stack structures. The four 1:2 salts are all semiconductors, with considerably enhanced conductivity and with activation energies ?0.15 eV. Single crystal data for one of these showed the large conductivity anisotropy characteristic of structures with segregated TCNQ stacks.

An accurate analytic method for the calculation of the Franck–Condon factors: Its application to the study of rotational dependence
View Description Hide DescriptionWe incorporated the perturbed Morse oscillator function and Manneback’s formulas into the matrix method to calculate the Franck–Condon factors. The method is exact in the context of the variational principle and is presented as a viable alternative to the numerical integration method where computational facilities are limited. Its accuracy is demonstrated by computing the eigenvalues of CO and Franck–Condon factors of the (B–X) band system of Na_{2}. It is then applied to the study of rotational dependence of the Franck–Condon factors in the (A–X) band system of MgH.

Perturbation calculation of molecular correlation energy using Gaussian‐type geminals. Second‐order pair energies of LiH and BH
View Description Hide DescriptionThe second‐order pair energies of LiH and BH are calculated using Rayleigh–Schroedinger perturbation theory. The first‐order perturbed pair functions are expanded in terms of explicitly correlated Gaussian functions. The nonlinear parameters entering the Gaussian geminals are optimized with reference to crude SCF functions according to the method proposed previously. The final values of the second‐order pair energies are then calculated using accurate SCF orbitals. At this final stage only the linear parameters are reoptimized. The calculated second‐order correlation energies of LiH and BH are compared with recent diagrammatic many‐body perturbation theory results. The basis sets composed of two optimized Gaussian geminals for each spin‐adapted pair function are shown to give quite reliable second‐order correlation energies. The results obtained with four geminals for each pair function are superior to the most accurate many‐body perturbation theory data. It is stressed that the nonlinear parameters of Gaussian geminals can be given a simple physical interpretation which facilitates their reasonable guess. The first‐order pair functions represented in terms of Gaussian geminals have a very attractive compact and simple form. If properly optimized they can also provide highly accurate values of the second‐order molecular correlation energies.

Optical properties of organic dye monolayers by surface plasmon spectroscopy
View Description Hide DescriptionThe spectroscopicproperties of dye monolayers on silverfilms were determined by using the technique of attenuated total reflection(ATR). The dyes N,N′‐di(methyloctadecyl) squarylium and N,N′‐dioctadecyloxacarbocyanine were deposited on silverfilms by the Langmuir–Blodgett monolayer technique. From a least squares fit of the experimental ATR curves to the exact reflection formula, the dielectric function of the dye monolayers was determined in the visible wavelength region. It was found that squarylium forms ordered layers with a regular array of molecules and a film thickness equal to the length of the molecule (2.66 nm). These films were characterized by a strong absorption band at λ_{ s }=530 nm (dimer band) and were highly anisotropic, the transition dipole moment lying in the plane of the monolayerfilm. Undiluted cyanine dye films on silver, on the other hand, showed a thickness less than that expected from the length of the molecule and a high degree of disorder. Increasing regularity in arrangement and growing thickness were observed when the cyanine dye was diluted with cadmium arachidate. For both the pure and diluted cyanine dye monolayers, two absorption bands have been measured in the visible region (a monomer band at λ_{ s }=500 nm and a dimer band at λ_{ s }=464 nm) and it was confirmed that their relative intensities depended on the mixing ratio of cyanine dye to Cd–arachidate.

Lattice model studies of the effect of chain flexibility on the nematic‐isotropic transition. IV. Semiflexible chain solutes in solvent molecules composed of rigid cores and semiflexible tails
View Description Hide DescriptionA steric, mean field, simple cubic lattice model, with and without attractive forces, for mixtures of semiflexible chain solutes in solvent molecules composed of rigid cores and semiflexible tails is presented. The calculated values and absolute trends of the following properties at the nematic–isotropic transitions of the mixtures are compared with relevant experimental data: coexistence curve slopes, solvent and solute order parameters, and solute excess properties as a function of the length and flexibility of the solute and of the solvent. With results intermediate between those for the completely flexible chain solute model and the rigid rod solute model, this semiflexible chain solute model is adequate to describe the general qualitative features and trends observed experimentally in mixtures of n‐alkanes in liquid crystalsolvents at the nematic–isotropic transition. This model predicts four general curvature types for the coexistence curves as a function of solute length and flexibility. It also predicts maxima in coexistence curve slopes as a function of solute length for sufficiently long and sufficiently stiff chains. While steric repulsions alone are sufficient in this model to generate the general features and trends, attractive forces permit some adjustments in trends and produce significant improvements in energetics.

Infrared absorption intensities of methyl acetylene
View Description Hide DescriptionThe infrared absorption intensities of the fundamental bands of CH_{3}CCH, CD_{3}CCH and CH_{3}CCD were measured in the gas phase by the pressure broadening technique. Out of 32 possible sign combinations of (∂p/∂Q _{ i }) for each of a _{1} and e species, only one combination was selected for both species, utilizing the isotopic data, Coriolis information, and the CNDO calculation. Thus, the polar tensor and the effective charges were obtained, as well as the dipole moment derivatives with respect to primitive coordinates. Comparison of these parameter values with those of other molecules suggests that the methyl hydrogens in methyl acetylene resemble those in methyl halides rather than those in saturated hydrocarbons, while the acetylenic hydrogen seems to retain the property in acetylene molecule itself. On the other hand, the CNDO method was not found very successful in this molecule as in propane, cyclopropane, or benzene. In particular, its capability to predict intensity parameter values concerned with the acetylenic group of this molecule was found fairly poor.

Self‐consistent field molecular orbital wavefunctions for the ground and excited states of the PMDA molecule
View Description Hide DescriptionWe report the results of a b i n i t i o molecular orbital self‐consistent field calculations on the ground state and low lying excited states of the PMDA molecule. Emphasis is placed on the consequences of localization of the molecular orbitals and on the relative positions of the n→π* and π→π* excited states. It is shown that reduction of the symmetry constraint from D _{2h } to C _{2v } or C _{ s } has a large effect on the n→π* excitation but only a small one on π→π*. A study of the effect of using basis sets of different size is also presented.

Hydrodynamic boundary conditions at the surface of an ablating material
View Description Hide DescriptionThe boundary conditions for a vapor at the surface of an ablating or vaporizing material are obtained. The vapor pressure, temperature, and mass flow rate are obtained when the initial state of the solid or liquid material and power absorbed at the phase transition layer are specified. These boundary conditions are derived from the hydrodynamic jump conditions and one equation from the thermodynamics of irreversible processes for a single component system. The results show that, for small mass flow rates, the vapor temperature rise is linearly proportional to the mass flow rate and the pressure drop varies as the square of the mass flow rate. These dependencies are the reversal of Vulliet’s results. Two application examples are considered: the ablation of iron, and the ablation of condensed hydrogen. It is shown that these boundary conditions for the ablation of condensed hydrogen are useful in studies of the refueling of present‐day tokamak plasmas by injection of hydrogen pellets.

Proton and deuteron magnetic resonance study of single crystals of diamminesilver dinitroargentate, [Ag(NH_{3})_{2}]Ag(NO_{2})_{2}
View Description Hide DescriptionProton and deuteron NMR spectra of single crystals of diamminesilver dinitroargentate, [Ag(NH_{3})_{2}]Ag(NO_{2})_{2} and [Ag(ND_{3})_{2}]Ag(NO_{2})_{2}, were studied at room temperature. Rapid reorientational motions of the ammine groups around their C _{3} axes were found. The orientations of the C _{3} rotation axes within the crystal coincide (deviation=0.9°) with the directions Ag–N in the diamminesilver ions as found by an x‐ray crystal structure determination of the compound. A small deviation from linearity of the bond angle N–Ag–N within the ion [H_{3}NAgNH_{3}]^{+} found röntgenographically is supported by the NMR experiments. The deuterium quadrupole coupling constant e ^{2} q Q h ^{−1} at room temperature is 71.6±0.5 kHz. A comparison of this value with the data reported for solid ND_{3} supports the assumption of only small changes in the geometrical and electronical structure of the ammonia molecule by the coordination to the silver ion.

Observations on O(^{1} D→^{3} P) and O_{2}(b ^{1}Σ_{ g } ^{+} →X ^{3}Σ_{ g } ^{−}) following O_{2} photodissociation
View Description Hide DescriptionTime‐resolved studies have been made on the energy transfer process O(^{1} D)+O_{2}→O(^{3} P)+O_{2}(b ^{1}Σ_{ g } ^{+})_{ v }. O(^{1} D) was generated by O_{2}photodissociation at ?1600 Å, using an H_{2} laser, and observed by its emission at 6300 Å. Only the v=0 and 1 levels are produced in O_{2}(b ^{1}Σ_{ g } ^{+}) by energy transfer, in the ratio k _{1}/k _{0}=0.7. The quantum efficiency of converting O(^{1} D) electronic energy into O_{2}(b ^{1}Σ_{ g } ^{+}) is 0.77±0.2, in agreement with the most recent determination. O_{2}(b ^{1}Σ_{ g } ^{+})_{ v=1} is quenched very rapidly by O_{2}, with a rate coefficient of 2.2×10^{−11} cm^{3} molec^{−1} s^{−1}. This is almost six orders of magnitude faster than O_{2} quenching of O_{2}(b ^{1}Σ_{ g } ^{+})_{ v=0} and accounts for the weakness of emission from the v=1 level in the terrestrial airglow.