The identification of two entrance channels in the reaction of metastable helium He(23S,21S) with SO2

The energy spectrum of electrons emitted in the reaction of He(2³S) and He(2¹S) with SO₂ indicates that in addition to the covalent entrance channel a strongly attractive ionic channel plays a role. We find upper limits for the well depth of the potential curve that corresponds to the latter channel of D_e ? 3.5 eV and D_e ? 4.3 eV for He(2³S) and He(2¹S) as the reaction partner, respectively. The partitioning between the two entrance channels can be understood by assuming the coupling matrix between both channels to depend strongly on the orientation of the SO₂ molecule with respect to the incident metastable He*.     

The temperature dependence of penning ionization electron energy spectra: He(23S)—ar,N2, NO,O2, N2O,CO2

Using two intense thermal energy He(2³S) sources of different temperatures (≈400 and ≈ 1000 K, resp.) and a transmission-calibrated electron energy analyzer with about 30meV resolution, the dependence of He(2³S) Penning ionization electron spectra on collision energy for the target species Ar, N₂, NO, O₂, N₂O and CO₂ have been studied. The energy shifts of the Penning electron energy distributions and the branching ratios for the population of different electronic states in the molecular ions are determined quantitatively and compared for the two different collision temperatures. These results and the shapes of the observed Penning electron energy distributions are discussed in the light of current models for the Penning procen; the observed temperature dependences are correlated with the nature of the ionized orbitals in cases of only one entrance channel (closed shell targets) and, in addition, to the existence of qualitatively different entrance channels (open shell targets).     

Ionization of hydrogen and deuterium atoms in thermal energy collisions with state-selected Ne(3s 3 P 2,3 P 0) metastable atoms

High resolution energy spectra of electrons and ions resulting from thermal energy collisions of hydrogen and deuterium atoms with state-selected metastable Ne(Ne(3s ³ P ₂,³ P ₀) atoms are reported. The electron spectra for Ne(³ P ₂)+H(D) are very broad: The high energy part due to formation of NeH⁺ (NeD⁺) bound states (associative ionization), amounts to about 30% of the ionizing events, whereas the dominant part of the spectrum including a prominent low-energy peak is due to Penning ionization out of a strongly-attractive entrance potential curve. Comparison of the spectra with quantum mechanical fit calculations yields fairly accurate information on this potential, in particular its well depthD _e [Ne(³ P ₂)?H,D]= 2.0(1) eV. The spectra for Ne(³ P ₀)+H, D are comparatively narrow with much lower cross sections than the one for the Ne(³ P ₂) state. The corresponding entrance channel is a weakly bound van der Waals molecule with a well depth below 0.1 eV. A perturbation calculation of the Ne(3s)+H(1s) potential energy curves at large distances explains the observed difference between the Ne(³ P ₂)+H(D) and Ne(³ P ₀)+H(D) systems. Symmetry arguments are given that the major contribution to the Ne(³ P ₂)+H(D) spectra is due to the² Σ potential.     

Partial cross sections for Rb− photodetachment in the region of the Rb(5p 2 P 1/2 3/2) thresholds and their analysis by multichannel quantum defect theory

A crossed ion-laser beam apparatus has been used to measure accurate relative total and partial cross sections for photodetachment from Rb^? ions with high photon energy resolution (0.1–0.6 cm^?1) in the region of the Rb(5p ² P _1/2,3/2) thresholds (photon energy range 16,350–16,820 cm^?1). Satisfactory fits to these data by multichannel quantum defect theory (MQDT) have been obtained, resulting in a reliable set of seven MQDT parameters. The electron angular distribution parameter for the Rb(5_s) channel was found to beβ(5s)=2, independent of photon energy. MQDT predicts a sharp, window-type variation of?(5s) around the minimum of the 5s-cross section below the Rb(5p ² P _1/2) threshold (where an accurate experimental measurement ofβ(5s) was not possible) and a similar behaviour ofβ(6s) in the case of Cs^?photodetachment below the Cs(6p ² P _1/2) threshold.     

SiO2 surface defect centers studied by AES

A theoretical model is proposed on how a Si dangling bond associated with an oxygen vacancy on a SiO₂ surface (E_s′ center) should be observed by Auger electron spectroscopy (AES). The Auger electron distribution N_A(E) for the L₂₃VV transition band is calculated for a stoichiometric SiO₂ surface, and for a SiO_x surface containing Si-(e^?O₃) coordinations. The latter is characterized by an additional L₂₃VD transition band, where D is the energy level of the unpaired electron e^?. The theoretical N_A(E) spectra are compared with experimental N(E) spectra for a pristine, and for an electron radiation damaged quartz surface. Agreement with the theoretical model is obtained if D is assumed to lie ≈2 eV below the conduction band edge. This result shows that AES is uniquely useful in revealing the absolute energy level of localized, occupied surface defect states. As the L₂₃VD transition band (main peak at 86 eV) cannot unambiguously be distinguished from a SiSi₄ coordination L₂₃VV spectrum (main peak at 88 eV), supporting evidence is presented as to why we exclude a SiSi₄ coordination for our particular experimental example. Application of the Si-(e^?O₃) model to the interpretation of SiO₂Si interface Auger spectra is also discussed.     

Low-field mobilities of rare-earth metals

The KLL Auger spectrum of Ni generated in the electron capture decay of radioactive ⁶⁴Cu in a solid state matrix was measured for the first time using a combined electrostatic electron spectrometer adjusted to a 7 eV instrumental resolution. Energies and relative intensities of the all nine basic spectrum components were determined and compared with data obtained from X-ray induced spectra of metallic Ni and with theoretical results as well. Absolute energy of 6562.5 ± 1.3 eV (related to the Fermi level) measured for the dominant KL₂L₃(¹D₂) than a value obtained from the X-ray induced spectra which is probably caused by the effects of chemical bonding and physico-chemical environment. Moreover, it is higher by 20.4 eV (16??) than a prediction of the semi-empirical calculations by Larkins which indicates an influence of the ??atomic structure effect?? on absolute energies of the Auger transitions following the electron capture decay and, possibly, some imperfections in the calculations. Good agreement of the measured and predicted KL₁L₂(³P₀/¹P₁) transition intensity ratios indicates perceptible influence of the relativistic effects on the KLL Auger spectrum even at Z = 28.     

High-field EPR-detected shifts of magnetic tensor components of spin label side chains reveal protein conformational changes: The proton entrance channel of bacteriorhodopsin

Continuous-wave high-field electron paramagnetic resonance (95 GHz, 3.4 T) is performed on a spin label side chain located at residue position 171 in the proton entrance channel of bacterior-hodopsin The conformational differences of three bacteriorhodopsin mutants, the single mutant F171C, the double mutant D96G/F171C, and the triple mutant D96G/F171C/F219L, are reflected in different g_xx and A_zz tensor component shifts of the nitroxide side chain. The most polar microenvironment is found in the single mutant, whereas the open proton entrance channel reported for the triple mutant allows a reorientation of the nitroxide group towards a microenvironment of lower polarity and/or reduced hydrogen bonding. The experimental data of the double mutant are explained by a light-independent equilibrium of two nitroxide orientations with different polarities of the local microenvironment. Upon illumination the spectrum of the single mutant revealsg _xx andA _zz tensor component shifts which resemble those determined for the triple mutant in the dark. This result provides strong evidence for a light-induced opening of the proton entrance channel of the single mutant similar to that found in the unilluminated triple mutant, in agreement with electron diffraction data.     

Preequilibrium GDR excitation and entrance channel angular momentum effects

The energy spectra of the γ-rays emitted in the ³⁵Cl + ⁹²Mo reaction at incident energy E = 260 MeV were measured in coincidence with the ejectiles produced in dissipative reaction events. The cumulative energy spectrum of the γ-rays coming from the decay of the ejectiles was calculated within the statistical model and its comparison to the experimental spectrum evidences an excess in the data for E _γ = 8 to 12 MeV. Such an excess, fitted with a Lorentz curve, is attributed to the preequilibrium GDR γ-decay of the intermediate dinuclear system. The centroid energy of the Lorentz curve corresponds to a dipole oscillation along the symmetry axis of the system and its width is found to be comparable to that of the ground state GDR low energy component of the deformed dinucleus. The small quantal dispersion Δ? = (10.3 ± 0.1)? of the entrance channel angular momentum, determined by analysing the dissipative fragment angular distribution in the framework of the Strutinsky model, is suggested to limit the broadening of the preequilibrium GDR width.     

Energy dependence of the 24Mg(16O, 12C)28Si reaction

Excitation functions for the reaction ²⁴Mg(¹⁶O, ¹²C)²⁸Si(g.s., 2⁺₁) were measured at 5°(lab) in the energy range 32 < E_c.m. < 49 MeV. Although the resonant structure, previously observed at lower energies, becomes progressively weaker, three new correlated maxima have been observed near E_c.m. = 37.5, 40.2 and 43.5 MeV. Angular distribution measurements at these energies yield spin assignments, from P²_j(cos θ) comparisons, of 27, 29 and 31, respectively. Attempts to find a consistent optical-model fit to the elastic scattering in the entrance channel and an exact finite-range DWBA fit to the four-nucleon transfer reaction in this energy range were unsuccessful. Such a failure is to be expected if strong couplings between the elastic channel and inelastic channels of either the initial or final system are important. The features of the resonance phenomena in the transfer reaction are discussed within a band crossing model framework.     

Dynamic polarization potential of 12C+12C system at molecular-resonance energies

The nuclear reaction dynamics leading to the formation of recently discovered resonance in the mutual-0₂ ⁺ channel of the ¹²C+¹²C inelastic scattering around E _c.m.≃ 32 MeV is studied in terms of the dynamic polarization potential (DPP) induced by the channel coupling among various excited states in ¹²C. The microscopic 3α cluster-model wave functions are used to generate the ¹²C−¹²C diagonal and coupling potentials in the double-folding model. It is found that DPP for the 0₂ ⁺+ 0₂ ⁺ channel is an unusually strong attractive potential which even exceeds the zeroth-order folding-model potential of this channel around the nuclear surface region and that the strong coupling between the 0₂ ⁺ and 2₂ ⁺ states is predominantly responsible for the unusual DPP in this channel. The effective potential, the sum of the original folding-model potential and the attractive DPP, is found to generates resonance states in the same energy region as that of the resonance states generated by the original folding-model potential but the former states are found to be higher-nodal states having four additional radial nodes. Similar but more moderate property of DPP is also found in the entrance (elastic) channel. These results suggest that the reaction dynamics of generating the resonance in the ¹²C(0₂ ⁺) +¹²C(0₂ ⁺) channel may rather differ from that of the simple crossing of the zeroth-order molecular band generated by the potentials in the entrance and exit channels suggested by the standard band-crossing model. Received: 17 December 1998 / Revised version: 1 March 1999     

甲醛的态选择性多光子电离研究

我们在XeCl准分子激光多光子电离甲醛分子的研究中,获得了甲醛经过2+1和2+2光子共振激发过程而产生的母体离子和高能离解通道的离子CO⁺。在电子轰击和单光子电离中占优势的低能阈离解通道离子HCO⁺,由于态选择性激发的结果而未出现。 关键词：     

Redistribution of electrons between ellipsoids in a magnetic field in the quantum limit range in n-Bi-Sb alloys

The magnetoresistivities ρ₂₂(H) and ρ₃₂(H) and the Hall coefficient R _32.1 for single-crystal samples of the n-Bi_0.93Sb_0.07 semiconducting alloy have been measured at low temperatures in magnetic fields up to H=14 T at H∥C ₂. The samples with three electron concentrations n ₁=1.25 × 10¹⁶ cm^s-3, n ₂=3.5×10¹⁶ cm^s-3, and n ₃=1.6×10¹⁷ cm^s-3 have been studied. The strong anisotropy of the electron spectrum of the alloys has made it possible to observe quantum oscillations of the magnetoresistivity ρ₂₂ (H) at H∥C ₂ for electrons of the secondary ellipsoids with the transition to the quantum limit in high magnetic fields. However, in the same magnetic fields, the quantization condition for electrons of the main ellipsoid is not satisfied. An increase in the energy of electrons of the secondary ellipsoids in the magnetic fields of the quantum limit leads to their migration to the main ellipsoid. After the complete migration, the Fermi energy for the alloy samples with the electron concentrations n ₁, n ₂, and n ₃ increases from 7.0 to 11.3 meV, from 11.0 to 17.1 meV, and from 20.2 to 30.6 meV, respectively. After the migration, the magnetoresistivity for electrons of the main ellipsoid increases with an increase in the magnetic field and the specific features in the behavior of the kinetic coefficients are observed in the vicinity of the magnetic field H=10 T. Therefore, the electronic topological transition from the three-valley electron spectrum to the single-valley electron spectrum occurs in the Bi_0.93Sb_0.07 single crystals for H∥C ₂ at low temperatures in the range of magnetic fields of the quantum limit.     

Many-body effects in l2mm auger electron spectra of Ge(CH3)4 excited by Mg Kα X-rays

The Ge:L₂MM Auger electron spectra excited by Mg Kα X-rays from Ge(CH₃)₄ free molecules have been compared with the corresponding spectra excited by Al Kα X-rays. The Al Kα excited spectra have characteristic features of the diagram Auger transitions, because the excitation energy is far above the L₂ ionization threshold. The energy of Mg Kα photons is 1.21 eV below the Ge:L₂ ionization threshold and thus the Mg Kα excited L₂MM Auger electron spectra indicate many-body effects, post collision interaction (PCI) effects and spectator Auger satellite structures. The L₂M_4,5M_4,5 type spectrum displays both these features but the L₂M_2,3M_4,5 type spectrum has only a spectator Auger satellite structure, because the (3p⁻¹3d⁻¹nl) final state interferes with the (3s) hole state.     

Features of direct cluster stripping in primary fragments from quasielastic20Ne-induced reactions

The primary yield distribution of projectile-like fragments from the²⁰Ne+¹⁹⁷Au reaction has been reconstructed and was found to be similar when the bombarding energy is increased from 150 to 290 MeV. Alternative to theQ _gg-systematics where the relative yields are ruled by exit channel properties, the Friedman model for projectile-fragmentation is successfully applied to the data. It relates the fragment yields essentially to the separation energyQ _F of the clusters removed from the projectile (and transferred to the target) i.e. to entrance channel properties and was successful in describing the relative yields at very high bombarding energy (2 GeV/N). For a good reproduction of the relative yields at the bombarding energy and the heavy target of this experiment it is necessary to make a correction for neutron pick-up which accompanies the cluster stripping process with a certain probability. The predicted momentum widths of the fragment spectra, too, are consistent with the data.     

Multichanel localization and magnetic impurities

The behaviour of an electron in N-coupled conducting chains of the length L is studied in the presence of magnetic impurities. The electron flow can be expanded in an orthonormal set of 2N channels. Each channel has its own transmission coefficient T_n(L). Spontaneous breaking of the symmetry between channels results in 2N different exponential dependences T_n(L) ～ exp (—L/l¹_n) where l¹_n is a localization length in the nth channel. Low frequency kinetic behaviour depends on the channel with the maximum localization length l¹₀.     

Polarization effects in ionizing thermal energy collisions of laser-excited Ne(3p 3 D 3) atoms with Ar atoms

Using transverse and longitudinal excitation of a collimated metastable Ne(3s ³ P _2.0) beam with average velocities of 500, 800, and 1,200 m/s by means of a single mode dye laser on the²⁰Ne(3s ³ P ₂→3p ³ D ₃) transition, we have investigated ionizing collisions of polarized Ne(3s ³ P ₂) and Ne(3p ³ D ₃) atoms with Ar atoms. The product electrons were energy analyzed with high resolution (9–25 meV). The resulting Ne(3p ³ D ₃) electron spectra exhibit a strong dependence on the three types of laser polarization (π _∥,π_⊥, σ_?), chosen to prepare the excited atoms. In contrast, the Ne(3s ³ P ₂) spectra are only weakly dependent on polarization. Detailed model calculations have been carried out for the Ne(3p)+Ar cross sections, using computed excited-state potential curves, semi-empirical ionic potentials, and local autoionization width functions. A semiclassical closecoupling method is applied to describe the evolution of the polarized collision system in the coupled entrance channels. It is found that a single autoionization widthΓ(R) is not sufficient to describe the measured polarization effects properly. The dependence ofΓ on the initial and final state is expressed in terms of few reduced electronic transition matrix elements, which are determined by comparison of measured and calculated total cross sections and Ar⁺(² P _3/2)/Ar⁺(² P _1/2) branching ratios for ionizing collisions of the various Ne(3pJ=1,2,3) multiplet states with Ar. The matrix elements corresponding to Ar(3pσ)→Ne(2pσ) electron transfer during autoionization are found to dominate, but Ar(3pπ)→Ne(2pπ) transfer has also to be included. The resulting calculated electron spectra reproduce the measured polarization effects in a semi-quantitative way.     

Excitation transfer from He(235,21,S) into bound and continuum states of CS2 studied by electron and optical spectroscopy and by coincidence measurements

For the reaction of He(2³ S, 2¹ S) with CS₂ we present a large body of experimental data: electron spectra, fluorescence spectra, electron-ion and electron-photon coincidence spectra. The combination of all these data allows us to characterize the reaction in great detail in a way which is rather direct and free of speculations. As for other molecules with positive electron affinity the charge exchanged channel He⁺ + CS ₂ ^? plays an important role as intermediate state. It is possible to isolate experimentally the electron energy spectra which correspond to formation of the individual electronic CS ₂ ⁺ states out of the intermediate He⁺ + CS ₂ ^? state. This has not been achieved so far for CS₂ or any other molecule by straightforward evaluation of experimental data.     

Die Spektren der Neutronen aus den Reaktionen C12(γ,n)C11 und O16(γ,n)O15

The (γ, n)-reactions on carbon and oxygen were studied using the 30,5 MeV brems-strahlung beam of the Heidelberg betatron. The photoneutrons were detected via the recoil protons in a stilbene scintillator. The pulse height spectrum of scintillations produced in the stilbene crystal by the recoil protons is analyzed to yield the incident neutron energy distribution. Theγ-ray and electron background was strongly reduced through pulse shape discrimination. To test the apparatus the spectrum of the neutrons from a Po-α-Be source was measured. The energy spectrum of the photo-neutrons from O¹⁶ shows clearly two peaks at excitation energies of 22,4 and 24,4 MeV also seen in the photo-proton spectra. The energy distribution of the neutrons from C¹² (γ, n) too contains indication of structure already known from the proton spectra from C¹² (γ, p). The derived cross sections of C¹² (γ, n ₀) and O¹⁶ (γ, n ₀) coincide as well in their shape as in their absolute magnitude with the corresponding (γ, p ₀)-cross sections. In both C¹² (γ, n) and O¹⁶ (γ, n)-reaction the neutrons seem to leave the final nucleus mainly in its ground state.     

Spectral Studies of Vanadyl-Doped Cadmium–Strontium Borosilicate Glass System

Glasses with composition CdO–(20-x) SrO–B₂O₃–SiO₂–x V₂O₅ (CdSBSi) (x = 0.5, 1, 1.5, 2, 2.5 mol%) were prepared by melt quenching technique. The amorphous nature of prepared glasses is confirmed by X-ray diffraction. Optical absorption spectra, electron paramagnetic resonance (EPR) and Fourier transform infrared (FTIR) measurements were also carried out for the prepared glass samples. The optical band gap energy (E _opt) and Urbach energy (?E) were calculated from their ultraviolet edges. The theoretical values of optical basicity (Λ _th) of glasses have been evaluated. The optical absorption spectrum exhibits two band characteristic of VO²⁺ ions in tetragonally distorted octahedral site symmetry. The two bands have been assigned to the transitions ²B₂ → ²B₁ and ²B₂ → ²E in the decreasing order of energy. The spin–Hamiltonian parameters (g and A), bonding parameters (β*² and $ \varepsilon_{\pi }^{*2} $ ), Fermi contact interaction parameter (K) have been evaluated from the EPR spectra. The VO²⁺ site symmetry is ascribed to a tetragonally (C_4v) distorted octahedron. FTIR spectra of these glasses were analyzed in order to identify the contribution of each component to the local structure. The physical properties of these glasses were also evaluated.     

Fragmentation of polyatomic ions produced by electron-loss collisions of butane and isobutane molecules with ions of kiloelectronvolt energy

Fragmentation accompanying the loss of electrons by butane and isobutane (C₄H₁₀) molecules in collisions with energy H⁺, He²⁺, and Ar⁶⁺ ions of kiloelectronvolt energies is studied. The electron density functional technique is applied to C _n H_2n+2 alkane molecules and their respective C _n H _2n+2 ⁺ ions to carry out quantum-chemical calculations of the atomic spacing, electron total energy for the initial configuration of the ionizing molecules and ions in the ground state, and atomic bond breaking energy necessary to produce different ion fragments. The fragmentation energy is correlated with the fragmentation probability. It is shown that the relative cross sections of ion fragmentation depend primarily on the related energy consumption. However, the process cross section is also strongly affected by the initial configuration of C₄H₁₀ isomer molecules, as well as by the amount of dangling and arising atomic bonds involved in the formation of each ion fragment.