Volume 82, Issue 6, 15 March 1985
Index of content:

A study of the extreme ultraviolet spectrum of O_{2} by electron impact
View Description Hide DescriptionWe have measured in the laboratory the electron impact emission cross sections for O_{2} at 200 eV. Included in the study are all emission features in the extreme ultraviolet from 40 to 131 nm at a resolution of 0.5 nm. The features are entirely from the dissociation products (OI, OII, OIII). Additionally we have measured the excitation functions from 0 to 400 eV for characteristic OI multiplets at 98.9 and 102.6 nm and for OII multiplets at 53.9 and 83.3 nm. We find the OI multiplets are formed near the dissociation limit whereas the OII multiplets have a threshold about 10 eV above the dissociation limit. We also determine the total VUV emission cross section of O_{2} from 40 to 200 nm and indicate the effects of autoionization to the measured emission spectrum.

Proton nuclear magnetic resonance and line shape analysis in the alkali metal hydroxides: LiOH, NaOH, KOH, and RbOH
View Description Hide DescriptionThe proton NMR line shapes of the anhydrous alkali metal hydroxides at 27 and 200 MHz are presented. The structure in the protonresonances of NaOH, KOH, and RbOH is explained using a four‐spin planar zig–zag chain model calculation. Second moment calculations show that the observed line shapes are consistent with known crystal structures. The crystal structures of KOH and RbOH, as determined by x‐ray powder diffraction, are confirmed.

Microwave and infrared characterization of several weakly bound NH_{3} complexes^{a)}
View Description Hide DescriptionWe present the results of microwave and infrared spectroscopic studies of several van der Waals complexes of NH_{3}. These results were obtained with a molecular beam electric resonance spectrometer. The microwave spectroscopy of the complexes (NH_{3})_{2} and Ar–NH_{3} show that both systems are nonrigid. The observed dipole moments for (NH_{3})_{2}[0.74(2) D] and (ND_{3})_{2}[0.57(1) D] are not compatible with the presently accepted theoretical structure. Ar–NH_{3}, which has a complicated and currently unassigned microwave spectrum, exhibits Q branch inversion transitions near 19 GHz which indicate that the NH_{3} subunit is likely to be a near‐free rotor. Infrared studies of the complexes NH_{3}–HCCH, NH_{3}–CO_{2}, (NH_{3})_{2}, Ar–NH_{3}, NH_{3}–OCS, NH_{3}–N_{2}O, and NH_{3}–HCN have been carried out with a line tunable CO_{2} laser. Only for NH_{3}–HCN were no infrared resonances discovered. Photodissociative transitions are observed in all of the other systems. Band origins for the photodissociative infrared transitions involving the ν_{2} umbrella motion of NH_{3} were determined for NH_{3}–HCCH [984.4(9) cm^{−} ^{1}], NH_{3}–CO_{2} [987.1(2) cm^{−} ^{1}]. NH_{3}–OCS [981.5(15) cm^{−} ^{1}], and NH_{3}–N_{2}O [980(2) cm^{−} ^{1}]. The observation of an infrared transition for Ar–NH_{3} at 938.69 cm^{−} ^{1}, which is 40 cm^{−} ^{1} lower than the band origins in the other NH_{3} complexes, lends support to the model of Ar–NH_{3} mentioned above. NH_{3}–HCCH, NH_{3}–CO_{2}, (NH_{3})_{2}, and Ar–NH_{3} were studied in microwave‐infrared double resonance experiments in order to eliminate much of the inhomogeneous broadening present in their infrared spectra and to aid in the rotational assignment of the infrared spectra. Linewidths were determined for NH_{3}–HCCH (0.15 GHz) and for NH_{3}–CO_{2} [14(6) GHz]. An important result of this study is that the dissociation energies of all the complexes studied, except for NH_{3}–HCN, are established to be less than 990 cm^{−} ^{1}, i.e., 2.8 kcal/mol.

The A(^{2}Π_{ i })–X(^{2}Σ^{+}) transition of the SiN radical by infrared diode laser spectroscopy
View Description Hide DescriptionThe A(^{2}Π_{ i })–X(^{2}Σ^{+}) 1,0 band of SiN has been observed in the 5 μm region by diode laser spectroscopy in a silane/nitrogen electrical discharge plasma. A detailed analysis of about 170 observed lines has led to a v=1 term value for the A state of 1972.443 73(24) cm^{−} ^{1} with 2.5 standard deviations in parentheses, which, when combined with optical data, was converted to the equilibrium term value of 993.9(2) cm^{−} ^{1}. The analysis has clearly confirmed that the spin‐orbit coupling constant in the A state is negative. The Λ‐type doubling constants p and q in the A state have been determined for the first time, and other parameters including the rotational and spin‐orbit coupling constants have been improved in accuracy by a factor of 100.

Pressure dependence of the 2.3 μm laser‐induced fluorescence (LIF) of room temperature PuF_{6}
View Description Hide DescriptionWhen PuF_{6} is excited in the 800 nm region, fluorescence peaked at 2.296 μm with less intense peaks at 2.207 and 2.126 μm is observed. Unlike the previous works that reported no self‐quenching of the 2.3 μm LIF of PuF_{6}, our data indicate a pressure dependence for the 2.3 μm LIF lifetime with a self‐quenching rate constant of 162±4 Torr^{−} ^{1} s^{−} ^{1} and a collision‐free lifetime of 218±5 μs. Both fluorescence and excitation spectra are in good agreement with the previously reported absorptionspectrum of PuF_{6}.

Rotation of a large molecule in liquids: A resonance‐enhanced light scattering study
View Description Hide DescriptionDepolarized resonance‐enhanced light scatteringspectra of the cationic dye auramine‐O have been obtained in methanol, ethanol, and 2‐propanol solutions at high dilution and with different counterions. The spectra could all be fitted with a single Lorentzian. The rotational correlation times thus obtained conform to the Arrhenius equation and the activation energies are equal to the activation energies of the macroscopic shear viscosity. The Debye–Stokes–Einstein equation applies to the data, but the hydrodynamic volumes of gyration obtained by this procedure are dependent upon the solvent and the nature of the counterion. This effect is discussed in terms of solute–solvent interactions and of ion pairing.

The lower Rydberg states of trans‐hexatriene
View Description Hide DescriptionThe one photon (optical), and two and three photonresonantmultiphoton spectra of trans‐hexatriene have been recorded and analyzed. Transitions to the 3s, 3p _{ x }(A _{ g }), 3p _{ ȳ}(B _{ g }), 3d _{ ȳz̄}(A _{ u }), and 4s(3d) orbitals (states) have been observed. Vibrational modes a _{ g }: C=C stretch, CH_{2} rock, and CH and CCC deformations, a _{ u }: CH and skeletal torsions, b _{ u }: C=C stretch have been observed in many of the studied Rydberg states. The assignments of the electronic states and vibrational modes are discussed.

Monte Carlo simulation of electron thermalization in gases. V. Subexcitation electrons in rare gases
View Description Hide DescriptionThe thermalization of subexcitation electrons in rare gases is studied by the Monte Carlo simulation(MCS). The electron velocity distribution is found to deviate significantly from the local Maxwellian distribution (MD) even for the initial Maxwellian distribution. Comparisons with available experimental results of the electron energy degradation as well as the electron thermalization time are made. A considerable discrepancy between the theoretical results obtained by the MCS and the approximate theory based on the MD assumption and the experimental results is revealed especially for rare gases with the Ramsauer minimum. The measured electron energy degradation is likely to be much less sensitive to the Ramsauer minimum than the theoretical one.

A semiclassical theory in phase space for molecular processes: Formalism based on dynamical characteristic function
View Description Hide DescriptionA new semiclassical theory is formulated by introducing a new distribution function to be propagated in phase space. Primarily this theory is intended to describe molecular collisions such as reactive scattering, but is also applicable to intramolecular processes including bound state problems. This new distribution function is a kind of characteristic function in the terminology of the phase‐space‐quantum theory developed by Weyl, Wigner, Moyal and others, and is called a dynamical characteristic function (DCF). The equation of motion and some properties of the DCF are presented. The essential feature of our theory is that the semiclassical DCF can be propagated using classical trajectories. This distribution function carried information not only on the density, but also on the phase (action integral). In this sense this theory can be regarded as a kind of phase‐space‐path‐integral formalism. The semiclassical approximation is analyzed from the viewpoint of path integrals. The Wigner distribution function is also discussed in the context of our analysis. As a theoretical application, the semiclassical DCF is shown to reproduce the expression of the periodic orbit theory for bound states. Also, as a simple numerical application, a propagation of the DCF on a nonquadratic (Morse) potential is presented.

Collisional quenching of excited iodine atoms (5p ^{5} ^{2} P _{1/2}) by Cl_{2} in a flow system
View Description Hide DescriptionTime‐resolved infrared emission from photolytically generated I*(^{2} P _{1/2}) has been studied in a slow flow apparatus. The total rate of deactivation of I*(^{2} P _{1/2}) by Cl_{2} has been measured to be no more than 8×10^{−15} cm^{3} molecule^{−} ^{1} s^{−} ^{1}, substantially slower than previous reports. Evidence is presented for a very fast (k≂2×10^{−10} cm^{3} molecule^{−} ^{1} s^{−} ^{1}) relaxation of I* by Cl atoms, which can account for both the earlier and the present observations.

Floquet theory analysis of stable and unstable motion
View Description Hide DescriptionFloquet theory is applied to the stability analysis of linear triatomic molecules having local Morse bonding potentials. We identify conditions for the existence of a symmetric mode and analyze its stability as a function of the energy and symmetric to antisymmetric normal mode frequency ratio. In the stable region we apply a special phase normalization to the Floquet eigenvalues that allows us to identify factors in the Floquet index with a red‐shifted generalized antisymmetric mode frequency. Instabilities set in when the ratio of this frequency to the symmetric mode frequency is integer or half‐integer. Analytic forms for Poincaré surfaces of section valid for the linearized theory are derived and compared with actual trajectory intersections for both stable and unstable cases. In the stable (quasiperiodic) cases, surfaces corresponding to different sections are ellipses with varying eccentricity but constant area. Hyperbolas are obtained in unstable cases.

The monoenergetic vibrational predissociation of expansion cooled NCNO: Nascent CN(V,R) distributions at excess energies 0–5000 cm^{−} ^{1}
View Description Hide DescriptionWe report detailed vibration, rotation distributions for nascent CN(X ^{2}∑^{+}), following the one‐photon photodissociation of expansion cooled NCNO via π*←n excitation throughout the region 450–585 nm. At the observed threshold for dissociation (585.3 nm), >90% of the CN product is in v″=0, N ^{″}=0, with the remainder in N ^{″}=1, corresponding to 〈E _{rot}〉 <0.4 cm^{−} ^{1}. CN(X ^{2}∑^{+}, v ^{″}=0) rotational distributions are obtained at many photolysis wavelengths and rotational levels are observed up to, but never above, the limit imposed by energy conservation: [B ^{″} _{ v } N″(N″+1)]<E _{ p }−D _{0}(v″), where D _{0}(v″) is the dissociation energy to produce CN(X ^{2}∑^{+},v″) and E _{ p } is the photonenergy. CN(X ^{2}∑^{+},v″=1) and CN(X ^{2}∑^{+},v″=2) thresholds are observed at photolysis wavelengths which correspond exactly to E _{ p }−D _{0}(v″=1) and E _{ p }−D _{0}(v″=2). These observations can only be reconciled with a vibrational predissociation mechanism and spectroscopic observations suggest that this occurs following internal conversion to the ground state surface. With E _{ p }−D _{0}(v″) less than ∼2000 cm^{−} ^{1}, the phase space theory of unimolecular reactions (PST) predicts the CN rotational distributions with high accuracy. However, when product vibrations are accessible, PST cannot be used, since it does not take proper account of the parent being vibrationally excited but rotationally cold. When explicitly taking this into account, we are able to reconcile the present experimental findings with a statistical model and we believe that the behavior observed for NCNO has a sound physical basis and is quite general.

Effect of temperature on the dissociative electron attachment to CClF_{3} and C_{2}F_{6} ^{a)}
View Description Hide DescriptionThe total electron attachment rate constant k _{ a }(〈ε〉) for CClF_{3} and C_{2}F_{6} has been meausured using an electron swarm technique in the mean electron energy range 0.41 to 4.81 eV and over the range of temperature T from 300 to 750 K. At each value of T the total electron attachment cross section σ_{ a }(ε) was determined from the measured k _{ a }(〈ε〉) using the swarm unfolding technique and was compared with the results of a mass spectrometric study. The σ_{ a }(ε) for C_{2}F_{6} shows a single peak (due to F^{−} and CF_{3} ^{−}) which shifts from 3.9 eV at 300 K to ∼3.3 eV at 750 K. (The onset shifts correspondingly from 2.3 to 1.5 eV.) For CClF_{3} the σ_{ a }(ε) shows two peaks: at ∼1.5 eV (due to Cl^{−}) and at ∼4.7 eV (due to Cl^{−}, F^{−}, CClF_{2} ^{−}, and ClF^{−}). The peak at ∼1.5 eV is especially sensitive to changes in T. The peak value of σ_{ a }(ε) increased by a factor of ∼3, and the energy position of the peak and onset shifted to progressively lower energies when T increased from 300 to 700 K. The analysis of these results led us to conclude that the changes in k _{ a }(〈ε〉) and σ_{ a }(ε) for the dissociative attachment processes of these molecules with increasing T result from the increase with T of the total internal (≂ vibrational) energy of the molecule.

Differential nonreactive scattering of He*(2 ^{1} S, 2 ^{3} S) by D_{2} and H_{2}: Anisotropic optical potentials and comparison with ab initio theory
View Description Hide DescriptionAngular distributions of He*(2 ^{1} S, 2 ^{3} S) scattered by D_{2} and H_{2}measured in crossed supersonic molecular beams at collision energies in the range 1.0–2.4 kcal/mol are analyzed to yield anisotropicoptical potentials that simultaneously reproduce these data along with quenching rates and ionization cross sections. Comparison with ab initio calculations of the potentials by Cohen and Lane and Hickman, Isaacson, and Miller shows very good agreement. The results are combined with one‐electron model potential calculations to probe the nuclear and electronic dynamics involved in these collisions. Quenching or Penning ionization is found to occur mainly through broadside attack of He* on H_{2}, despite the fact that the occupied σ_{ g } orbital on H_{2} has greater spatial extent along the bond axis than perpendicular to it. An implication is that Penning ionization electron spectroscopy (PIES) experiments on larger molecules cannot be interpreted simply on the basis of van der Waals radii and spatial extent of various molecular orbitals; the nature of the excited‐state potential surface may play a dominant role in determining these spectra.

Observation of activation energies in the chemiluminescent reactions of Sr with Cl_{2} and Br_{2} at collision energies below 4 eV
View Description Hide DescriptionIn a molecular beam experiment the chemiluminescence M+X_{2} → MX*+X in the reactive scattering of strontium atoms and halogen molecules Cl_{2} and Br_{2} has been studied below 4 eV collision energy. Relative integral reactive cross sections as a function of collision energy reveal large activation energies. Dynamical aspects of endothermic reactions are discussed. A simple kinematic transformation yields a classical limit for the rotational excitation of the product molecule.

Quantum mechanical study of the D+H_{2}→HD+H reaction
View Description Hide DescriptionA quantum mechanical study is made of the D+H_{2}(v _{ i }=0,1)→ HD(v _{ f }=0,1,2)+H reactions within the infinite order sudden approximation (IOSA) for the total energy interval 0.28≤E _{ t }≤1.28 eV. Results at various stages of the calculation are given ranging from most detailed reactive transition probabilities through opacity functions and γ‐dependent cross sections to total and state‐to‐state integral and differential cross sections, as well as rate constants. The cross sections and rate constants are compared with other available theoretical results and experiments. It is found that the IOSA total cross sections for v _{ i }=0,1 overlap very nicely with the corresponding quasiclassical trajectory cross sections, except for the tunneling region. A less satisfactory fit is obtained with the distorted wave born approximation results. The calculated rate constants are compared with experiment and a rather good fit is obtained, in particular for rate constants from the ground state.

Quantum chemistry by random walk: A faster algorithm
View Description Hide DescriptionTwo extensions of the fixed‐node random walk method of solving the Schrödinger equation are described. A simple iterative procedure is found to reduce time‐step error and the use of two trial wave functions in place of one for importance sampling is found to reduce computation effort. In combination the two modifications reduce computation effort for a fixed accuracy by a factor of about 10.

Ab initio relativistic effective potentials with spin‐orbit operators. I. Li through Ar
View Description Hide DescriptionA refined version of the ‘‘shape consistent’’ effective potential procedure of Christiansen, Lee, and Pitzer was used to compute averaged relativistic effective potentials (AREP) and spin‐orbit operators for the atoms Li through Ar. These are tabulated in analytic form. Small optimized Gaussian basis sets with expansion coefficients for the lowest energy state for each atom are given and the reliability of the potentials relative to all electric calculations is discussed. Finally a procedure for computing molecular moments and Breit corrections is suggested.

Configuration interaction studies of low‐lying valence and Rydberg states of NS
View Description Hide DescriptionUsing a better than double‐zeta‐plus‐polarization basis set, the multireference‐double‐excitation‐configuration‐interaction (MRD‐CI) method has been employed to calculate the potential curves for the X ^{2}Π, a ^{4}Π (unobserved), b ^{4}Σ^{−} (unobserved), B ^{2}Π, ^{2}Φ (unobserved?), A ^{2}Δ, C ^{2}Σ^{+}(Rydberg),H ^{2}Π, I ^{2}Σ^{+}, and J ^{2}Σ^{+}(Rydberg) states of the NS radical. Except for the B′ ^{2}Σ^{+} state, which is not found in our calculations, all the calculated T _{ e } and r _{ e } values for experimentally observed states are found to be in good agreement with experiment, whereas the calculated ω_{ e }’s are only satisfactory. Arguments have been advanced to identify the experimental observed B′ ^{2}Σ^{+} state as the calculated ^{2}Φ state. The radiative lifetime for the v’=0 level of the C ^{2}Σ^{+} state has been calculated to be about 30 ns, indicating the level predissociates since the experimental radiative lifetime is only 6.5 ns.

Investigation of the differences in stability of the OC⋅⋅⋅HF and CO⋅⋅⋅HF complexes
View Description Hide DescriptionThe structure and energetics of the isomeric H‐bonded complexes OC⋅⋅⋅HF and CO⋅⋅⋅HF have been investigated by ab initio molecular orbital theory and by natural bond orbital analysis. Only with the inclusion of electron correlation is a significant preference for the experimentally observed OC⋅⋅⋅HF isomer found. The large effect of correlation upon the relative stability of the two isomers is apparently entirely an electrostatic effect caused by the correlation‐induced sign reversal of the dipole moment of CO. Nevertheless, a molecular multipole expansion is found inadequate to account for the principal features of these H‐bonded complexes and their relative stability. Contrary to a recent study, we find that ‘‘charge transfer’’ effects are highly significant contributions to the binding in these complexes. The differences in stability of OC⋅⋅⋅HF and CO⋅⋅⋅HF are attributed primarily to differences in the interaction of carbon and oxygen lone pairs of CO donating into the unfilled antibond on HF, i.e., to differences in n _{C}→σ_{HF} ^{*} and n _{O}→σ_{HF} ^{*} matrix elements.