Fine structure excitation transfer between the lithiumD-lines by collisions with cesium atoms

Applying resonant Doppler-free 2-photon laser spectroscopy with thermionic diode detection, the cross sections for the excitation energy transfer of the collisional process⁷Li*(2P _1/2+Cs(6S _1/2)→⁷Li*(2P _3/2)+Cs(6S _1/2) have been measured. The experimental cross sections, σ^Li-Cs (1/2→3/2)=890 Ų and σ^Li-Cs (3/2→1/2)=430 Ų, are compared with theoretical data obtained by a sudden impact approximation approach taking into account the long-range interaction potentials only. The calculated cross sections show an excitation mixing process at large internuclear distances where Li-Cs dipole-dipole and dipole-quadrupole interaction forces are predominant.     

Laser-induced-fluorescence study of A Na/Na2 free jet. II. State-dependent effective inelastic cross sections for rotational and vibrational relaxation

Laser-induced-fluorescence measurements along the jet axis are used to study the relaxation of Na₂ molecules in a Na/Na₂ expansion. From the fluorescence intensity, the evolution of the occupancies of several vibration—rotation levels of the Na₂ ground state is obtained. The whole relevant region along the jet axis is covered: from the equilibrium region close to the nozzle down to the downstream part of the jet, where the internal-state distribution is non-Boltzmann and “frozen”. The relaxation of Na₂ molecules is analysed using first-order relaxation equations. Vibrational and rotational relaxation are treated separately by means of similar equations but with different inelastic cross sections determining the relaxation rates. The evolution of the vibrational levels can be described by means of a single effective cross section of = 100 Ų; the rotational cross sections are strongly J-state dependent and range from = 250 to 15 Ų, for J = 13-55.     

Measurements of inelastic cross sections for the (j,m) = (2,0)→(3,0) rotational transition of CsF in collisions with atoms and molecules

Individual state-to-state rotational transitions have been resolved in small angle scattering of polarized CsF molecules on Ne, Ar, C₂, H₆, N₂, CO, CO₂, CHF₃ at center of mass energies of about 0.1 eV. The absolute inelastic cross sections range from 5Ų up to 600Ų.     

Photoabsorption spectrum of radicals produced by a pulsed discharge of trace H2O in Ar

The absorption spectrum of radicals produced by a pulsed discharge of few mTorr H₂O in 3 Torr Ar has been measured in the 1200–1370 Å region. The radicals are mainly OH in low vibrational levels of the ground state. The observed spectrum is consistent with previous report that OH has a high absorption cross section in the 1223 Å region and a small one at 1216 Å. The spectrum is tentatively interpreted with the OH potential curves preliminarily calculated by van Dishoeck. Langhoff, Wetmore, and Dalgarno.     

The molecular structure of tris-2,2,6,6-tetramethyl-heptane-3,5-dione indium: gas-phase electron diffraction and quantum chemical calculations

The molecular structure of tris-2,2,6,6-tetramethyl-heptane-3,5-dione indium, or In(thd)₃, has been determined by gas-phase electron diffraction monitored by mass spectrometry (GED/MS) and quantum chemical (DFT) calculations. Both the DFT calculations and the GED data collected at 387(8) K indicate that the molecules have D ₃ symmetry with a distorted anti-prismatic InO₆ coordination geometry. According to GED refinements, the twist angle θ, i.e. the angle of rotation of the upper and lower O₃ triangles in opposite directions relative to their positions in a regular prism is θ = ±24.9(1.2)° and the bond distances (r _h1) in the chelate ring are In–O = 2.127(4) Å, C–O = 1.268(3) Å and C–C = 1.411(3) Å, respectively. The DFT calculations yielded structure parameters in close agreement with those found experimentally.     

Measurements of fast neutron capture cross sections on <Superscript>63</Superscript>Cu and <Superscript>186</Superscript>W

Neutron capture cross sections on ⁶³Cu and ¹⁸⁶W were measured by fast neutron activation method at neutron energies from 1 to 2 MeV. Monoenergetic fast neutrons were produced by ³H(p,n)³He reaction. Neutron energy spread by target thickness, which was assumed to be the main factor of neutron energy spread, was estimated to be 1.5% at neutron energy of 2.077 MeV. Neutron capture cross sections on ⁶³Cu and ¹⁸⁶W were calculated by reference comparison method on those of ¹⁹⁷Au(n,γ). Not only statistical errors of gamma-counts from samples but also systematic errors in the counting efficiency for HP Ge detector and the uncertainty of areal density of samples were considered in calculating neutron capture cross section. Estimated neutron capture cross sections on ⁶³Cu and ¹⁸⁶W were also compared with ENDF-6 data.     

Crystal Structure of the Ordered Double Perovskite,Sr2NiTeO6

The crystal structure of the ordered double perovskite Sr₂MnTeO₆ has been refined at ambient temperature from high resolution neutron and X‐ray powder diffraction data in the monoclinic space group I 1 2/m 1 with a = 5.6166(1) Å, b = 5.5807(1) Å, c = 7.8797(1) Å and β = 90.048(2)°. The structure is the result of out‐of‐phase (–) rotations of virtually undistorted NiO₆ and TeO₆ octahedra in the (0 – –) sense about two of the axes of the ideal cubic perovskite. Electron diffraction measurements have been used to confirm the proposed space group and structure.     

Free NH3 quantum rotations in Hofmann clathrates: structure factors and line widths studied by inelastic neutron scattering

Quantum rotations of NH₃ groups in Hofmann clathrates Ni–Ni–C₆H₆ and Ni–Ni–C₁₂H₁₀ have been studied using inelastic neutron scattering. Calculations of the dynamical structure factor for a free uniaxial quantum rotor reproduce the neutron scattering data with respect to their Q- and T-dependence as well as the relative intensities for the 0 → 1, 0 → 2 and 1 → 2 transitions. Though the effective NH₃ rotation constant is different from the gas phase value, the effective radius of rotation (i.e., the average distance of protons from the rotation axis) is equal or very close to the geometrical value r = 0.94 Å for a NH₃ group. Comparing the experimental data with the calculated dynamical structure factor for the 0 → 3 transition it could be shown, that the corresponding transition line, in contrast to transitions between j = 0,1,2 levels measured so far, has a finite width at T = 0 K.     

The crystallographic and magnetic structure of Ni2O3H

A nickel oxide hydroxide, nominally Ni₂O₃H, has been synthesized using hydrothermal techniques. The crystallographic and magnetic structures of the material have been determined at 4.5 K from powder neutron diffraction data. The material, which is nonstoichiometric with respect to Ni and H, contains Ni²⁺ and Ni⁴⁺ rather than Ni³⁺ and is orthorhombic (space group Pnmn; a = 5.084(1) Å, b = 2.9103(6) Å, c = 13.954(3) Å). Magnetic moments are aligned along the c axis with antiferromagnetic order. Powder neutron diffraction data collected above the Néel temperature, at 298 and 473 K, revealed no significant changes to the structure or localized electron model.     

Low‐dimensional compounds containing cyano groups. I. catena‐Poly[[[dicyanoargentato(I)‐κN]bis(4‐methylpyridine‐κN)copper(II)]‐μ‐dicyanoargentato(I)‐κ2N:N′]

The structure of the title compound, [Cu(C₆H₇N)₂{Ag(CN)₂}₂]_n, is made up of neutral zigzag chains of [–NC–Ag–CN–Cu(4‐Mepy)₂{Ag(CN)₂}–NC–Ag–CN–] (4‐Mepy is 4‐methyl­pyridine). Neighbouring chains are linked by weak argentophilic interactions, with Ag?Ag distances of 3.2322 (12) Å. The Cu atom, which lies on a twofold rotation axis, is pentacoordinated by one monodentate Ag(CN)₂^? anion [Cu—N 1.985 (3) Å], the atoms of which lie on the same rotation axis, and by bridging di­cyano­argentate anions [2 × Cu—N 2.0827 (19) Å], with Ag atoms on inversion centres. The coordination polyhedron is completed by two 4‐Mepy mol­ecules [2 × Cu—N 2.038 (2) Å], which occupy the axial positions of a distorted trigonal bipyramid.     

Collision cross sections for excitation energy transfer in Na* (3P 1/2)+K(4S 1/2)⇔Na*(3P 3/2)+K(4S 1/2) processes

The cross sections for the excitation energy transfer between the 3² P _J states of sodium atoms by collisions with ground-state potassium atoms have been measured by resonant Doppler-free two-photon spectroscopy, where the population densities of directly pumped and collisionally excited Na(3P _J )(J=1/2, 3/2) levels were probed by counter-propagating Na(3P _J ) → Na(4D _{3/2, 5/2}) excitation and detected with the thermionic diode. Cross sections of σ(3P _1/2 → 3P _3/2)=190 Ų±20% and σ(3P _3/2 → 3P _1/2)=100 Ų±20% were found. The theoretical calculations taking into account the long-range interaction termsR ^?6,R ^?8 andR ^?10 yield a value σ(3P _1/2 → 3P _3/2)=165 Ų. On the basis of these long-range interaction potentials the differential cross section has been calculated and compared with recently published experimental data. Very good agreement between the theoretical and experimental data was found.     

Cross sections for quenching of Cd(3PJ) as determined by resonance radiation flash photolysis

Absolute quenching cross sections of several gases for the equilibrated set of excited states Cd(³P₀, ³P₁) have been determined at 280°C. The excited atoms were generated by 3261 Å resonance radiation flash photolysis of cadmium vapor in excess bath gas, and monitored by kinetic absorption spectroscopy. Cadmium monohydride and cadmium monofluoride have been detected in absorption as primary products in certain CD(³P₀, ³P₁) quenching reactions.     

Neutron diffraction determination of the crystal structure of Ce7O12

A neutron diffraction study has been made on polycrystalline and single crystal samples of CeO_1.714. The results confirm that the compound is isostructural with ternary oxides of the type UY₆O₁₂. The space group is $\text{R}\text{3}$ with hexagonal unit cell dimensions a = 10.37 Å and c = 9.67 Å (rhombohedral cell a = 8.60 Å and α = 99.4°). The hexagonal unit cell contains three formula units of Ce₇O₁₂. Totals of 79 and 24 independent reflections from the single crystal were measured at neutron wavelengths of 1.185 and 2.37 Å, respectively. Simultaneous refinement of the two sets of data yielded a weighted R factor of 0.144. The structure is a rhombohedral defect type of fluorite arrangement in which pairs of oxygen vacancies are ordered along the [111] axis.     

Powder neutron diffraction of SrNbO2N at room temperature and 1.5 K

The structure of strontium niobium dioxygen nitride, SrNbO₂N, has been solved by powder neutron diffraction at room temperature and 1.5 K. SrNbO₂N crystallizes in the tetragonal space group I4/mcm, with a = 5.7056 (4) and c = 8.1002 (9) Å at room temperature, and a = 5.6938 (4) and c = 8.0974 (8) Å at 1.5 K. The crystal structure is derived from the cubic perovskite archetype by a slight rotation of the Nb(O,N)₆ octahedra with respect to the tetragonal axis. A partially ordered distribution of oxygen and nitrogen on the anionic sites was found.     

High resolution study of photoabsorption photodissociation and fluorescence in H2

Extensive excitation fluorescence spectra for both the molecule and the atomic fragment are presented with the absorption spectrum in the 765–850 Å spectral range with a resolution varying from 0.07 to 0.05Å. A review is made of the previously published results on photodissociation cross sections, photodissociation rates, predissociation probabilities. New results on the predissociation of then p σ Σ⁺ _u levels are presented. The competitions between the various decay processes: radiation, predissociation and autoionisation, are discussed.     

Deltafunction model for the helium dimer

The helium dimer ⁴He₂ has recently been detected, confirming earlier ab initio predictions of stability for a single bound state with binding energy of 1.310 mK. The predicted potential minimum is at 2.96 Å, with a radial distribution function peaking at 6.96 Å. We model this system using a Dirac bubble potential, which also admits just one bound j = 0 state. With the bubble located at 6.96 Å, an overlap of 0.9994 with the ab initio wave function is obtained. An average internuclear distance of 52.6 Å is calculated, in good agreement with the ab initio result. The root mean square deviation from the mean, 48.0 Å, indicates an enormous spread of the radial wave function. Also consistent with our model is the absence of bound states for the isotopic variants ³He⁴He and ³He₂. Cross sections for helium-helium scattering are also computed, using both a partial-wave expansion and the Born approximation. General trends in the energy dependence of the total cross section are accounted for, in qualitative agreement with experimental results. © 1995 John Wiley & Sons, Inc.     

Fully converged sudden rotation total integral cross sections for 4He + H2 (ν = 0, j = 0) → 4He + H2 (ν′ = 1, j′)

Total integral cross sections for ⁴He + H₂ (ν = 0, j = 0) → ⁴He + H₂ (ν′ = 1, j′ = 0, 2) have been calculated in the total energy range 1.2 to 5.5 eV, according to a quantal sudden approximation for the H₂ rotational degrees of freedom and a close coupling expansion of the vibrational degree of freedom. Convergence of the above cross sections is investigated by employing four vibration basis sets in the close coupling calculations, i.e., ν = 0,1, ν = 0,1, 2, ν = 0, 1, 2, 3 and ν = 0, 1, 2, 3, 4. Between 4.2 and 5.5 eV calculations were done with three vibration basis sets; ν = 0.–4, ν = 0–5, and ν = 0–6. It is found that at least four vibrational basis functions are needed to converge (to within 5–10%) these cross sections in the above energy range. Comparison of breathing sphere calculations and summed sudden rotation results shows good agreement for the (weakly anisotropic) Mies-Krauss potential. However, as expected the former results underestimate the vibrational 0 → 1 total integral cross sections.     

Structure Determination of a Low Temperature Phase of Calcium and Strontium Amide by means of Neutron Powder Diffraction on Ca(ND2)2 and Sr(ND2)2

Calcium and Strontium amide are ionic compounds crystallising in a tetragonally distorted anatase structure-type at ambient temperatures. The amide ions (NH₂^–/ND₂^–) resemble water molecules in structure and in charge distribution. By means of temperature dependent neutron diffraction investigations weak super-structure reflections were observed at temperatures below 90 K (Ca(ND₂)₂) and 60 K (Sr(ND₂)₂), respectively, indicating the existence of a so far unknown low-temperature (LT) phase. Using high resolution neutron powder diffraction at temperatures below 10 K the structure was determined for both compounds. The LT-phases are isotypic and crystallise monoclinic in the space group P2₁/c with four formula units within the unit cell: Ca(ND₂)₂ at 10 K a = 7.257(2) Å, b = 7.2434(2) Å, c = 6.300(1) Å, β = 124.73(1)° Sr(ND₂)₂ at 5 K a = 7.6950(1) Å, b = 7.68374(9) Å, c = 6.6324(3) Å, β = 124.917(2)°. Their structure is closely related to the tetragonal HT-phase, but an ordering of the amide ions occurs due to freezing of a lattice mode which is dominated by the librational motion of the amide ions in the {1 0 0} planes of the HT-phase.     

Vibrational deactivation of CO2(0001) by ethane

The deactivation of CO₂(00⁰1) by ethane in the temperature range 300–600 K has been studied using a laser induced fluorescence technique. The energy transfer cross section decreased from 0.23 Ų at 300 K to 0.16 Ų at 600 K. The magnitude of the cross section is consistent with the expectation that near resonant V-V energy transfer processes are responsible for the energy transfer between CO₂(00⁰1) and ethane during collisions even though the observed temperature dependence of the energy transfer cross section does not follow that predicted by the existing theories.     

Conformation of comb‐like liquid crystal polymers in isotropic solution probed by small‐angle neutron scattering

The conformational characteristics of a comb‐like side‐chain liquid crystal polysiloxane (SCLCP), dissolved in deuterated chloroform, were evaluated by small‐angle neutron scattering (SANS) measurements over a wide q range. SANS studies were carried out on specimens with constant backbone length (DP = 198) and variable spacer length (n = 3, 5, and 11), and with constant spacer length (n = 5) and variable DP (45, 72, 127, and 198). The form factor P(q) at high q was analyzed using the wormlike chain model with finite cross‐sectional thickness (R_c) and taking into account the molecular weight polydispersity. The analysis generated values of persistence length in the range l_p = 28–32 Å, considerably larger than that of the unsubstituted polysiloxane chain (l_p = 5.8 Å), with contour lengths per monomer comparable to the fully‐extended polysiloxane backbone (l_m = 2.9 Å). This indicates a relatively rigid SCLCP chain due to the influence of the densely attached mesogenic groups. The SCLCP with n = 11 is more flexible (l_p = 28 Å) than those with n = 3 and n = 5 (l_p = 32 Å). The cross‐sectional thickness increases with spacer length, R_c ～ n^0.21±0.02 (3 ≤ n ≤ 11), and the contour length per monomer decreases with increasing spacer length, l_m ～ n^?0.35±0.01. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2412–2424, 2006