Reactive scattering of alkali dimers: K2 + HgCl2, HgBr2, HgI2

Angular distribution measurements of KX reactive scattering of a potassium dimer K₂ beam by mercuric halide molecules HgX₂ are reported. All the reactions exhibit strong forward peaking in the centre of mass differential cross sections and large total reaction cross sections Q_r ～ 150 Ų. However, there is substantial backward peaking ( forward peak) for HgBr₂, HgI₂. Despite the direct stripping dynamics, both alkali atoms of the K₂ dimer become bound alkali halide molecules in most (perhaps all) reactive collisions. A major fraction of the reaction exoergicity is disposed into vibrational excitation of the product KX molecules. A mechanism involving a first electron jump in the entrance valley and a second electron jump in the exit valley of the potential surface is suggested to explain the rapid transfer of both K₂ valence electrons.     

Reactive scattering of alkali dimers K2 + I2, CH2I2, CHI3, CBr4

Angular distribution measurements of KX reactive scattering of a potassium dimer K₂ beam by I₂ and by a series of halomethane molecules are reported. The K₂ + I₂ reactive scattering is similar to that previously observed for K₂ + Br₂. The predominant reaction path yields K + KI + I with the K and KI product recoiling in the forward direction. However, the forward peak of the KI differential cross section is lower than that for K from K₂ + I₂ and is broader than that observed for KBr from K₂ + Br₂. This is attributed to slow dissociation of the I ₂ ^- ion formed in the electron jump mechanism previously proposed for K₂ + Br₂. In the halomethane reactions, both alkali atoms of the K₂ dimer become bound alkali halide molecules in all reactive collisions, despite the direct dynamics of the corresponding supersonic K atom reactions. Thus, these reactions provide compelling evidence for a second electron jump mechanism, previously proposed for the reactions of K₂ dimers with polyhalide molecules. The differential cross sections for the K₂ dimer plus halomethane reactions indicate an osculating collision complex with a lifetime at least comparable to its rotational period, perhaps much longer. This reaction complex is identified with the doubly ionic state formed by the second electron jump transition.     

Reactive scattering of alkali dimers: Product K atoms from K2+Br2, IBr,BrCN, SnCl4

Approximate angular distribution measurements of reactively scattered K atoms from a potassium dimer K₂ beam by cross beam molecules Br₂, IBr, BrCN, SnCl₄ are reported. Forward peaking in the centre of mass differential cross section for K₂+Br₂, IBr, BrCN unambiguously confirms that the reaction path at large impact parameters yields K + KX + Y products. Differential cross sections at wide angles for K compared with KX products, suggest that the reaction path yielding KX + KY products becomes important at small impact parameters. Comparison of the reactive K atom scattering with the KX scattering lends further support to a schematic mechanism of the reaction dynamics suggested previously. The K₂ + SnCl₄ reaction does not yield reactively scattered K atoms.     

Measurements of K-shell ionization cross sections of35Br,37Rb,39Y using low energy protons

K-shell ionization cross section measurements are reported for₃₅Br,₃₇Rb and₃₉Y targets caused by protons over 300–400 keV energy range in 20 keV increment. The K-shell ionization cross sections (σ _k ^l ) at different energies were deduced from the K_α and K_β X-ray production cross sections which were obtained from X-ray yields of the K_α and K_β transitions. The experimental values are compared with the calculated values of ECPSSR theory and empirical reference cross sections. The resultant K-shell ionization cross sections are found to be in reasonable agreement with the ECPSSR theory. The K_α/K_β intensity ratios are also presented and compared with other experimental values and also with the theoretical one-hole values given by Scofield.     

The oscillator strength of the πg shape resonance in the absorption K-spectrum of a nitrogen molecule

Magnitudes of the absorption cross sections of a nitrogen molecule are measured by the transmission method near the ionization K-threshold. Based on these data, the partial absorption NK cross sections are obtained and the oscillator strength of the π_g shape resonance is determined. It is found that the oscillator strength of the π_g-resonance increases as the optical density of nitrogen decreases. This phenomenon is interpreted as a result of the instrumental distortion effect (the thickness effect). By extrapolating to zero pressure, the oscillator strength of the π_g shape resonance is determined to be 0.29.     

K-shell X-ray production for fluorine ions on target elements Ti to La

The production of target atomK-shell X-rays has been studied for 2 to 28 MeV fluorine ions incident on thin solid targets of 14 elements with atomic numbers Z₂=22 to 57. Total X-ray production cross sections, energy shifts ofK _α andK _β lines andK _α/K _β intensity ratios were measured with a Si(Li) detector. The results of cross section measurements are compared with theoretical predictions of inner shell ionization. In most cases, satisfactory agreement between measured cross sections and theoretical Coulomb ionization cross sections, corrected for the perturbation of the target atom by the projectile charge and for relativistic effects, was obtained.     

Reactive scattering of a supersonic alkali atom beam: K + Br2, BrCN,SnCl4, PCl3, CCl4, CH3I

Angular distribution measurements of reactive scattering of a supersonic potassium atom beam by a series of molecules are reported with initial kinetic energy (～ 5 kcal mole^-1) above the thermal energy range. The narrow Laval nozzle velocity distribution gives improved resolution over thermal energy measurements. Total reaction cross sections are found to decrease with energy. Differential reaction cross sections for Br₂, BrCN and CCl₄ show increased forward scattering compared with thermal energies but for CH₃I there is no change to within experimental error. The BrCN scattering is compatible with spectator-stripping dynamics, though this limit has not been reached in the Br₂ scattering. The SnCl₄ differential reaction cross section appears not to be compatible with a single peak in the centre of mass recoil velocity distribution. It is suggested that high and low velocity contributions to the intensity may arise from a long-lived collision complex dissociating by two reaction paths.     

Electron impact ionisation cross sections derived from total inelastic cross section for CF3X and CF2X2 (X = H,Cl, Br and I) molecules

We report here the total ionisation cross sections for CF₃X and CF₂X₂ (X = H, Cl, Br and I) molecules by electron impact from ionisation threshold to 5 keV. The total inelastic cross section is obtained employing a quantum mechanical approach called spherical complex optical potential formalism. Then, using a semi-empirical complex scattering potential-ionisation contribution method, the ionisation cross section is derived from the inelastic cross section. The results obtained are compared with previous measurements and theoretical values, wherever available and a satisfactorily agreement is observed. The ionisation cross section values for CF₂I₂ molecule are reported for the first time.     

Differential scattering and rotational excitation in low-energy positron-hydrogen-molecule collisions

Differential cross sections are calculated for positron-hydrogen-molecule scattering below the positronium formation threshold usingK-matrix elements obtained by the Kohn variational method. The internuclear separation of the hydrogen molecule is fixed at its equilibrium value, 1.4a₀. The trial function used in the Kohn calculation is very flexible. It contains separable and Hylleraas-type basis functions and also basis functions to take into account longrange polarization. The cross sections for rotational excitation are also calculated, using the adiabatic-rotation approximation. Preliminary results are presented in this paper; full details will be published elsewhere.     

Flavour structure of low-energy hadron-pair photoproduction

We consider the process γγ→H₁H̄₂, where H₁ and H₂ are either mesons or baryons. The experimental findings for such quantities as the pp̄ and K_SK_S differential cross sections, in the energy range currently probed, are found often to be in disparity with the scaling behaviour expected from hard constituent scattering. We discuss the long-distance pole–resonance contribution in understanding the origin of these phenomena, as well as the amplitude relations governing the short-distance contribution, which we model as a scaling contribution. When considering the latter, we argue that the difference found for the K_SK_S and the K⁺K^- integrated cross sections can be attributed to the s-channel isovector component. This corresponds to the ρω→a subprocess in VMD (vector-meson-dominance) language. The ratio of the two cross sections is enhanced by the suppression of the φ component, and it is hence constrained. We give similar constraints to a number of other hadron-pair production channels. After writing down the scaling and pole–resonance contributions respectively, the direct summation of the two contributions is found to reproduce some salient features of the pp̄ and K⁺K^- data. PACS 11.30.Hv; 12.40.-y; 12.40.Nn; 12.40.Vv; 13.66.Bc     

Oxygen inducedK-shell ionization of selected elements from sulphur to bromine

K-shell X-ray production cross sections for oxygen ions on thin solid targets of 13 selected elements with atomic numbers between 16 and 35 were measured by a Si(Li) detector at incident ion energies from 7 to 24 MeV. Ionization cross sections are compared with calculations assuming Coulomb-ionization. Best agreement is found with theoretical cross sections that include corrections for binding energy and Coulomb deflection effects. Energy shifts ofK _α andK _β X-rays andK _α /K _β intensity ratios were also measured and are used to deduce information about outer shell ionization.     

Inclusive V production cross sections from 920 GeV fixed target proton-nucleus collisions

Inclusive differential cross sections and for the production of K ⁰ _S , , and particles are measured at HERA in proton-induced reactions on C, Al, Ti, and W targets. The incident beam energy is 920 GeV, corresponding to GeV in the proton-nucleon system. The ratios of differential cross sections (K and are measured to be and , respectively, for x_F . No significant dependence upon the target material is observed. Within errors, the slopes of the transverse momentum distributions also show no significant dependence upon the target material. The dependence of the extrapolated total cross sections on the atomic mass A of the target material is discussed, and the deduced cross sections per nucleon are compared with results obtained at other energies. Received: 16 December 2002 / Revised version: 6 March 2003 / Published online: 11 June 2003 RID="a" ID="a" Supported by the Bundesministerium für Bildung und Forschung, FRG, under contract numbers 05-7BU35I, 05-7DO55P, 05 HB1HRA, 05 HB1KHA, 05 HB1PEA, 05 HB1PSA, 05 HB1VHA, 05 HB9HRA, 05 7HD15I, 05 7HH25I, 05 7MP25I, 05 7SI75I     

一种考虑几何屏蔽效应的计算“电子-分子”散射总截面的可加性规则修正方法

在考虑分子内原子间的几何屏蔽效应随电子入射能量变化的基础上, 提出了一种能够在中、高能区准确计算“电子-分子”散射总截面的可加性规则修正方法. 利用这一修正后的可加性规则并使用“电子-C, H, O, N原子”散射总截面的实验数据, 在50—5000 eV内计算了电子被NO, N₂O, NO₂和C₂H₆分子散射的总截面, 且将计算结果与实验结果及其他理论结果进行了比较. 结果表明, 利用这一方法修正过的可加性规则进行计 关键词： 电子散射 可加性规则 总截面 几何屏蔽效应     

Effects of potassium on the reaction pathways of CH2 fragment over Mo2C/Mo(1 0 0)

The adsorption and surface reactions of CH₂I₂ on the K-dosed Mo₂C/Mo(1 0 0) have been studied by high resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy and thermal desorption spectroscopy. Potassium is an effective promoter for the rupture of C-I bond in the adsorbed CH₂I₂. A partial dissociation of this compound occurred even at 100 K and was completed at 190 K at monolayer K coverage. The dissociation was further promoted by the illumination of coadsorbed layer at 100 K. As revealed by HREELS and XPS measurements the primary products of the dissociation are CH₂ and I. Methylene was converted to π-bonded ethylene characterized by T_p = 160 K, and di-σ-ethylene with T_p = 350 K. Other products of the surface reaction are hydrogen and methane. The coupling reaction of CH₂ species was clearly facilitated by potassium. The effect of potassium was explained by the extended electron donation to adsorbed alkyl iodide in one hand, and by the direct interaction between potassium and I on the other hand.     

Measurements ofK-shell ionization with132Xe and208Pb projectiles in the energy range of 1.4–5.9 MeV/u

Projectile and targetK-shell ionization cross sections induced by 3.6-, 4.7-, and 5.9 MeV/u¹³²Xe ions and 1.4-, 3.6-, 4.7-, and 5.9 MeV/u²⁰⁸Pb ions from the UNILAC in thin solid targets between C and U are measured. The cross sections are discussed in terms of the molecular model of innershell vacancy production in heavy ion-atom collisions. The sharing of 2p _1/2σ vacancies between theK shells of the two collision partners in these very heavy ion-atom collisions is found to deviate from the Meyerhof-Demkov formula forR≦10^?2. The measured ionization cross sections are compared with theoretical calculations for 1sσ and 2p _1/2σ excitation cross sections. AZ _UA=Z₁+Z ₂ dependence is found independent ofZ ₁/Z ₂. Outer-shell vacancy configurations measured in these close encounters are reported.     

K-K electron capture from adenine and CO2 molecule by fast carbon ions using KLL-Auger electron technique

The experimental measurement of total electron transfer cross section in Bragg peak energy region is important to understand energy loss in the biomolecular system. In this study, we have measured state selective, K-K electron capture and K-ionization cross sections for adenine (C ₅H ₅N ₅) in collisions with fast (2.5–5 MeV/u) C ions. These are compared with the data for smaller gas molecule, CO ₂. These are derived from a study of the KLL-Auger electron emission yields as a function projectile charge state. The K-ionization cross-section (σ_KI) data are compared with the ECUSAR (united and separated atom [USA] approximation with energy loss [E], Coulomb deflection [C], and relativistic [R] corrections) model calculation. The measured σ_KI data and the calculations are in good agreement. The K-K transfer cross-section (σ_K−K) data are compared with the CPSSR (perturbed stationary state [PSS] with Coulomb deflection [C] and relativistic corrections [R]) calculation that underestimates the measured data for such symmetric collision system. The energy dependence of σ_K−K for adenine is found to be flat in contrast to a sharp variation predicted by the model. The K-transfer cross section is found to be substantial fraction of the K-ionization.     

Theoretical study of the <Emphasis Type="Italic">K</Emphasis>-photoabsorption cross section of carbon in the acetylene molecule

The magnitudes and the shape of the K-absorption spectrum of carbon in the C₂H₂ molecule are found by the one-center method. Within this method, one-electron wave functions of occupied and unoccupied states are calculated in the approximation of nonorthogonal orbitals. The processes of single and double excitation/ionization with additional excitation of an electron from the external 1π_u shell to the 1π_g state are taken into account. The mechanism of formation of a fine structure in the photoionization spectrum is refined. The calculated cross sections are compared with experimental and theoretical results obtained by other researchers. The results of calculation of the oscillator strengths of single photoexcitation are predictive.     

Vacuum UV photoelectron intensity of gaseous compounds: I. HeI spectra of simple compounds

A new method for determining absolute photoionization cross sections for gaseous compounds is proposed. In this method a mixture of a sample and a standard gas (N₂) is used in photoelectron intensity measurements so that the relative intensity of the component is obtained with respect to N₂ The relative photoelectron band area is converted to the absolute photoionization cross section on the basis of the absolute cross-section data of N₂ recently reported by Samson et al. This method has been applied to various aliphatic compounds to study the effect of alkyl substitution on photoionization cross sections of O- and N-nonbonding electrons for 584-Å radiation.     

Inner-shell ionization ofZ=25–32 elements by 5-48 MeV32S ions

TheK-shell ionization cross sections of Mn, Ni, Cu, Zn and Ge under³²S bombardment have been measured in the energy range from 5 to 48 MeV. The cross sections are compared with available theories based on a direct Coulomb ionization mechanism and with the predictions of theK-vacancy sharing process. This last process can reasonably account for the measured cross sections at high bombarding energies. The energy shifts of theK _α- andK _β-lines and theK _α/K _β-intensity ratios have also been measured. This information is used to deduce the defect configuration of the atoms. The mechanisms responsible for the multiple vacancy production are discussed.     

Application of atom-photon coincidence techniques to the determination of the differential cross section for excitation ofK(42 P) in low-energy K-N2, CO collisions

The differential cross section for the processK(4² S)+N₂, CO→K(4² P)+ N₂, CO is presented for reduced scattering angles τ<2·10⁴ eV deg. The inelastic process is identified by determining the time-correlation between the inelastically scattered potassium atom and the emittedK(4² P→4² S)photon. The performance of different coincidence techniques is compared. From the differential cross sections values for the position of the curve crossing responsible for the excitation process are derived (R _c=2.77, 2.48 Å andV(R _c )=0.55, 1.00 eV for K-N₂, CO respectively). The values indicate that the lowest ionic intermediate state of the form K⁺-N ₂ ^? , CO^? is responsible for the excitation process.