Collisional Deactivation of K(7s2S) and K(5d2D) by No

Radiative lifetimes and total deactivation cross sections of K(7²S) and K(5²D) by collision with NO are studied. The K atomic vapor in either the 7²S or the 5²D state was prepared by two- photon absorption using a dye laser. The decay signal of the time-resolved fluorescence from the 7²S – 4²P_1/2 or 5²D – 4²P_3/2 transition was then monitored. Based on a Stern-Volmer analysis, the radiative lifetimes are 155 ±8 ns and 561 ± 18 ns for the K(7²S) and K(5²D) states, respectively. The total deactivation cross sections are 88 ±1Ų and 70 ±2Ų for the K(7²S)-NO and K(5₂D)-NO collisions, respectively. In the absence of NO collisions the radiative lifetimes obtained in this work show excellent agreement with those previously reported. The quenching cross sections for NO have been measured for the first time, and have values in a reasonable range, when compared with Na-N₂ collisions.     

Collisional depolarization of excited114Cd 5s5p 3 P 1 atoms by collisions with molecular gases

Depolarization of excited¹¹⁴Cd 5s5p ³ P ₁ atoms induced by collisions with various molecular gases (N₂, H₂, D₂, CO, CO₂, CH₄, C₂H₆, C₂H₄) has been investigated using polarized fluorescence spectroscopy. After pulsed optical excitation of the Cd 5³ P ₁ level with appropriately polarized light the temporal behaviour of Zeeman quantum beats has been observed showing the influence of collisional destruction of orientation and alignment. By analyzing the signal curves at different molecular gas pressures the corresponding depolarization cross sections for¹¹⁴Cd atoms in the 5³ P ₁ state have been obtained. With regard to a test of a nuclear spin decoupling model for the collisions the cross sections were compared with previously measured hyperfine structure transfer cross sections of¹¹³Cd 5s5p ³ P ₁ atoms.     

Collisional deactivation of O2(1Σg+)

Relaxation rates for O₂(¹Σ_g⁺) by nonradiative pathways have been determined using the fast-flow technique. O₂(¹Σ_g⁺) is formed from O₂(¹Δ_g) by an energy pooling process. O₂(¹Δ_g) is generated by passing purified oxygen through a microwave discharge. Oxygen atoms are removed by distilling mercury vapor through the discharge zone. It has been observed that the wall loss rate for O₂(¹Σ_g⁺) decreases with increasing pressure of oxygen and thus appears to be diffusion controlled. Quenching rate constants for O₂, N₂, and He have been determined and found to be (1.5 ± 0.1) × 10⁴, (1.0 ± 0.05) × 10⁶ and (1.2 ± 0.1) × 10⁵ l./mol·sec, respectively.     

Collisional deactivation Of Ca(4p3PJ) by barium atoms

Collisional deactivation of Ca(4p³P_J by barium atoms proceeds with a thermal cross section of 0.26±0.02 nm² at 850 K. No evidence for the corresponding deactivation of electronically excited Ca(4p³P_J) by Ca(4s¹S_o) was obtained. The optical lifetime of Ca(4p³P_J) was measured to be 0.33 ± 0.03 ms, in good agreement with previous studies.     

Lifetime measurements of the 3d 94s(1 D) 4p configuration of Cu I

The radiative lifetimes of the levels in the 3d ⁹4s(¹ D)4p configuration of Cu I are measured. The levels are excited from the metastable 3d ⁹4s ^{2 2} D _3/2,5/2 levels. The metastable Cu atoms are generated in a pulsed hollow cathode discharge. The levels investigated are populated with a 35-ps laser pulse at wavelengths around 220 nm. The laser induced fluorescence signal is detected. The lifetime of the 3d ⁹4s(³ D)4p ⁴ D _1/2 level is also determined by direct excitation from the ground state. A comparison with calculated literature values is given.     

Lifetime measurements of the 2p 3d 3 D 1, 2, 3 0 and 2p 3d 1 P 1 0 levels in NII by beam-foil-laser spectroscopy

Accurate lifetimes measured by means of the cascade-free method based on laser excitation of a fast ion beam preexcited in a carbon foil are reported for four 2p 3d levels in NII. The lifetime results are: τ(2p 3d ³ D ₁ ⁰ )=0.209±0.007 ns, τ(2p 3d ³ D ₂ ⁰ )=0.219±0.007 ns, τ(2p 3d ³ D ₃ ⁰ )=0.217±0.005 ns, and τ(2p 3d ¹ P ₁ ⁰ )=0.410±0.017 ns. These results are compared to theoretical and experimental lifetimes reported previously.     

Population transfer between the fine-structure levels 5s5p 3 P 1 and 5s5p 3 P 0 of114Cd atoms by collisions with molecular gases

Fine-structure mixing of excited¹¹⁴Cd 5³ P _J atoms induced by collisions with various molecular gases (N₂, H₂, D₂, CO, CH₄, C₂H₆) has been investigated by a combined method of absorption and fluorescence spectroscopy. After pulsed optical excitation of the Cd(5³ P ₁) level, the time dependence of the population densities has been measured simultaneously both for the 5³ P ₁ state and for the collisionally populated 5³ P ₀ state. By analyzing the signal curves at different molecular gas pressures cross sections for collisional transfer between the¹¹⁴Cd 5³ P ₁ and 5³ P ₀ levels as well as the quenching cross sections for the 5³ P levels have been obtained.     

Cadmium (5 1 P 1 → 5 3 P J) excitation transfer and 5 3 P J intramultiplet relaxation by collisions with molecular gases

Collisional 5 ¹ P ₁ → 5 ³ P _J spin changing fine structure transfer as well as 5 ³ P _J intramultiplet mixing induced by various molecular gases (H₂, D₂, N₂, CO, CO₂, CH₄, C₂H₆, C₃H₈, C₂H₄) have been investigated using a combined method of fluorescence and absorption spectroscopy. After pulsed optical excitation of the Cd(5 ¹ P ₁) level the time dependence of the population densities has been measured both for the Cd(5 ¹ P ₁) level as well as the three collisionally populated Cd(5 ³ P _J) levels. By analyzing the signal curves at different molecular gas pressures not only the ratios of 5 ¹ P ₁ → 5 ³ P _J population transfer cross sections but also the Cd(5 ¹ P _J), Cd(5 ³ P _J) quenching cross sections and the Cd(5 ³ P _J) intramultiplet population transfer cross sections have been obtained.     

Syndioselective propylene polymerization catalyzed by rac-2,2-dimethylpropylidene (1-η5-cyclopentadienyl) (1-η5-fluorenyl) dichlorozirconium

Syndioselective propylene polymerization has been promoted by rac-2,2-dimethylpropylidene (1-η⁵-cyclopentadienyl) (1-η⁵-fluorenyl) dichlorozirconium ( 1 ). The active catalytic species were generated using either triphenylcarbenium tetrakis (pentafluorophenyl) borate ( 2 ) (Zr⁺ method) or methylaluminoxane (MAO method). The former exhibited much higher activity than the latter, especially at low polymerization temperatures (T_p). Syndiotactic poly (propylene) (s-PP) obtained at T_p = ?20°C has T_m approaching 160°C, [rrrr] pentad fraction of 0.92 to 0.95, and 45% crystallinity (X_c). It crystallized in two antichiral unit cells B and C. The C structure is favored by low temperature of polymerization, slow crystallization from melt, and annealing. The s-PP has M?_w/M?_n ranging from 3.6 to 4.4, which can be separated into stereoregular fractions soluble in heptane and hexane and stereoirregular fractions soluble in pentane, ether, and acetone. Therefore, this system cannot be considered to be a single-site catalyst. A parallel study was made on the isopropylidene (1-η⁵-cyclopentadienyl) (1-η⁵-fluorenyl) dichlorozirconium ( 3 )/MAO catalyst. Molecular mechanics calculations were performed for all combinations of the configuration of asymmetric centers. The steric energy favors syndiotactic enchainment for both catalysts 1 and 3 , with 1 forming the more syndioselective catalyst. © 1994 John Wiley & Sons, Inc.     

Kinetic studies of the deactivation of O2(1Σg+) and O(1D)

The kinetics of the deactivation of O₂(¹Σ_g⁺) is studied in real time. O₂(¹Σ_g⁺) is generated in this system by the O(¹D) + O₂ reaction following O₃laser flash photolysis in the presence of excess O₂, and it is monitored by its characteristic emission band at 762 nm. Quenching rate constants were obtained for O₂, O₃, N₂, CO₂, H₂O, CF₄and the rare gases. Since O(¹D) is the precursor for the formation of O₂(¹Σ_g⁺), the addition of an O(¹D) quencher effectively lowers the initial concentration of O₂(¹Σ_g⁺). By measuring the initial intensity of the 762 nm fluorescence signal, the relative quenching efficiencies were determined for O(¹D) quenching by N₂, CO₂, Xe, and Kr with respect to O₂; the results are in good agreement with literature values.     

Alignment of atoms following photoionisation: Cd+(4d −1 2 D 5/2,2 D 3/2) and Zn+(3d −1 2 D 3/2)

The linear polarisation of fluorescence radiation following the photoionisation in the 4d _5/2 and 4d _3/2 subshells of Cd and in the 3d _3/2 subshell of Zn has been measured in the energy range of primary photons from threshold up to about 40 eV. The experimental values of the linear polarisation found for the fluorescence radiation of Cd⁺(4d ^?1) ions are in essential agreement with theoretical results obtained by an RRPA calculation (Johnson et al.). For the case of Zn⁺(3d ^?1) on the other hand, no theoretical data are presently available. Using the experimental values of linear polarisation of fluorescence radiation the values of the alignment tensorA ₂₀ have been evaluated for the ions Cd⁺(4d ^{?1 2} D _5/2,² D _3/2) and Zn⁺(3d ^{?1 2} D _3/2).     

Crystal structure,dissociation and zwitterion formation in 2,6-diaryl-1(3)-oxo-3(1)-hydroxy-5(7)-imino-7(5)-amino-1 H,5H-(3-H,7H)-pyrazolo[1,2-a]pyrazoles

Single crystal X-ray analysis and quaternization of 2,6-diphenyl-1(3)-oxo-3(1)-hydroxy-5(7)-imino-7(5)-1H,-5H[3H,7H)-pyrazolo[1,2-a]pyrazole is described. The dissociation constants are determined and compared with those of 4-phenyl-1,2-dimethyl-3(5)-oxo-5(3)-hydroxypyrazole and 4-phenyl-3,5-diaminopyrazole. The quaternization of the latter compound is also described. The influence of electron donating substituents at the cationic moiety on the electronic spectra of such paraionic systems is discussed. The title products exist in the solid state as zwitterions and probably as covalent species in solution.     

The Crystal Structures of SrAlF5 and Ba0.43(1)Sr0.57(1)AlF5

Two members belonging to the pseudobinary phase diagram of Ba/SrF₂ and AlF₃ were synthesized. Single domain crystals of SrAlF₅ and Ba_0.43(1)Sr_0.57(1)AlF₅ were prepared from the corresponding metal fluorides. The mixed compound Ba_0.43(1)Sr_0.57(1)AlF₅ has been synthesized and characterized in detail for the first time. The structure of SrAlF₅ was reinvestigated; a superstructure was found which is absent in Ba_0.43(1)Sr_0.57(1)AlF₅. The compounds crystallize at room temperature in the centrosymmetric tetragonal space groups I4₁/a and I4/m. Lattice parameters are a = b= 1988.22(14), c = 1432.24(19) and a = b = 1431.32(14), c = 722.83(7) pm. Both structures differ mainly in the AlF₆ arrangement situated in the cavities of the matrix structure. In SrAlF₅, ordered dimer [Al₂F₁₀]^4– units are found, whereas in the Ba²⁺ mixed sample a linear chain of AlF₆ octahedra is observed. For both compounds no partial occupation was observed. Sm²⁺ doped samples are further characterized by luminescence measurements. Four independent sites of alkaline earth atoms were found in the Sm²⁺ luminescence of the host SrAlF₅.     

Phase-shift studies of the imprisonment lifetime of cadmium(5 3P1) and the effect of the addition of noble gases

The imprisonment lifetime of Cd³P₁), measured by a phase-shift method was found to depend on the modulation frequency at 534 K. This frequency dependence is considered to correspond to the non-exponential decay after a pulsed excitation, and was analyzed on the basis of Milne's diffusion theory. The effect of the addition of noble gases was also examined, and it was found that noble gases scarcely deactivate the Cd(³P₁) atoms at 458 K.     

Inner-valence photoionization of O((1)D): experimental evidence for the 2s(2)2p(4)((1)D)-->2s(1)2p(5)((1)P) transition

Photoionization and autoionization of electronically excited atomic oxygen O((1)D) are investigated in the energy range between 12 and 26 eV using tunable laser-produced plasma radiation in combination with time-of-flight mass spectrometry. A broad, asymmetric, and intense feature is observed that is peaking at 20.53+/-0.05 eV. It is assigned to the 2s(2)2p(4)((1)D)-->2s(1)2p(5)((1)P) transition, which subsequently autoionizes by a Coster-Kronig transition, as predicted by the previous theoretical work [K. L. Bell et al., J. Phys. B 22, 3197 (1989)]. Specifically, the energy of the unperturbed transition occurs at 20.35+/-0.07 eV. Its shape is described by a Fano profile revealing a q parameter of 4.25+/-0.8 and a width of gamma=2.2+/-0.15 eV. Absolute photoionization cross section sigma is derived, yielding sigma=22.5+/-2.3 Mb at the maximum of the resonance. In addition, weak contributions to the O((1)D) yield from dissociative ionization originating from molecular singlet oxygen [O(2)((1)Delta(g))] are identified as well. Possible applications of the 2s(2)2p(4)((1)D)-->2s(1)2p(5)((1)P) transition as a state-selective and sensitive probe of excited oxygen in combination with photoionization mass spectrometry are briefly discussed.     

Photoacoustic study of CH4(ν3) deactivation by collisions with rare gases

CH₄(ν₃) deactivation is studied in the gas phase by the photoacoustic method at 300 K. Rapid vibration-to-vibration transfer holds the adjacent levels in a quasi-equilibrium distribution. The vibrational levels can then be grouped in two sets: (ν₂, ν₄) on the one hand and (ν₃, ν₁, 2ν₂, 2ν₄, ν₂ + ν₄) on the other. By successive dilution of CH₄ in He, Ne, Ar, we determined the vibration-to-translation-rotation rate constants characterizing the deactivation of each set. The vibration-to-vibration intermolecular rate constant which connects the two sets is also obtained.     

The reaction of OH with NO2 and the deactivation of O(1D) by CO

NO₂ was photolyzed with 2288 Å radiation at 300° and 423°K in the presence of H₂O, CO, and in some cases excess He. The photolysis produces O(¹D) atoms which react with H₂O to give HO radicals or are deactivated by CO to O(³P) atoms The ratio k₅/k₃ is temperature dependent, being 0.33 at 300°K and 0.60 at 423°K. From these two points, the Arrhenius expression is estimated to be k₅/k₃ = 2.6 exp(?1200/RT) where R is in cal/mole – °K. The OH radical is either removed by NO₂ or reacts with CO The ratio k₂/k^α is 0.019 at 300°K and 0.027 at 423°K, and the ratio k₂/k⁰ is 1.65 × 10^?5M at 300°K and 2.84 × 10^?5M at 423°K, with H₂O as the chaperone gas, where k^α = k₁ in the high-pressure limit and k⁰[M] = k₁ in the low-pressure limit. When combined with the value of k₂ = 4.2 × 10⁸ exp(?1100/RT) M^?1sec^?1, k^α = 6.3 × 10⁹ exp (?340/RT)M^?1sec^?1 and k⁰ = 4.0 × 10¹²M^?2sec^?1, independent of temperature for H₂O as the chaperone gas. He is about 1/8 as efficient as H₂O.