Elastische und unelastische Streuung von Elektronen an Gasen

The theoretical differential cross section for the elastic scattering and for the excitation of optical transitions in helium by electron impact has been refined in Born approximation by use of the two parameter Eckart eigenfunction for the ground state and for the excited states. The angular distributions of 25 kev electrons scattered elastically and inelastically by helium were measured in the angular range 2·3·10^?4≦?≦4·10^?2. The intensity distribution of the elastically scattered electrons is in accordance with the theoretical curve for?>7·10^?3 and is disturbed at smaller angles by the primary beam. Normalization of the experimental values to the theoretical elastic differential cross section leads to agreement between the experimental differential cross section for the excitation of the 2¹ P and 3¹ P state and the scattering formulae given in this paper. There are small systematic deviations (<20%) for the 2¹ P differential cross section in the angular range 3·10^?3<?<1·10^?2 only. The oscillator strength of these two transitions has been determined from the scattering measurements:f ₂₁=0·312±0·04 andf ₃₁=0·0898±0·006.     

Spinpolarisation und Winkelverteilung der Elektronen nach Anregung verschiedener Zustände des Hg-Atoms

At energies above ～ 50 eV, electrons scattered inelastically from mercury atoms show a similarity to elastically scattered electrons in both spin polarization and angular distribution. This similarity vanishes at primary energies below ～ 30 eV for the 6p′³ P ₁-channel (11.0 eV energy loss) and below～ 25 eV for the 6¹ P ₁-channel (6.7 eV energy loss).     

Angular-distribution neutron-emission spectra of niobium following bombardment by fast neutrons

Neutron-emission spectra at ten angles between 20 and 160 degrees and incident neutron energies of 5.9, 7.1 and 8.4 MeV were measured relative to the neutron field emitted after spontaneous fission of ²⁵²Cf. The angular distribution of inelastically scattered neutrons appears to be essentially isotropic where it can be separated from the dominant elastic scattering peak. Above an excitation energy of 4 MeV the spectra are well described by a Maxwellian temperature distribution.     

Elastische und unelastische Streuung von Deuteronen an Sr,Zr, Nb,Pd und Ag bei 11,8 MeV

Continuing previous experiments on the elastic and inelastic scattering of 11·8MeV deuterons on medium weight nuclei, Sr, Zr, Nb, Pd, and Ag targets have been studied. In addition to elastically scattered deuterons, angular distributions of inelastically scattered deuterons have been obtained for one group in Sr, and three groups in Zr and Nb, respectively, between 20° and 165°. The correlation between the diffraction maxima is discussed in terms ofBlair's phase rule. As in our previous results, this rule works poorly in the comparison between elastic and inelastic angular distributions. However the strongly excited inelastic groups which are interpreted as collective vibrational states do follow this rule. In the case of Pd and Ag, energy spectra at several scattering angles have been measured. The integrated differential cross sections of the inelastically scattered deuterons atΘ=90° of all investigated elements are compared with those of previous measurements. They show the influence of closed shells but no systematic decrease or increase with increasing atomic weight.     

Generation of VUV radiation in Lyman and Werner bands of pulsed space discharge in hydrogen mixtures with helium

A self-consistent numerical model for the self-sustained high-current pulsed discharge is constructed based on the solution of the equations of the population of H₂ electronic and vibrational states. The model accounts for electronic, ion-molecular, and vibrational kinetics, electron attachment to and detachment from the H₂ molecule, Lyman and Werner band emission, and their radiation trapping. The equations of electron-vibrational kinetics are solved simultaneously with the Boltzmann equation for the electron energy distribution function and the external electrical circuit equations.     

Ein quantitativer Vergleich der Energieverlustspektren von Elektronen in Aluminium und Kalium mit der dielektrischen Theorie

The energy loss spectra of 34 kV electrons inelastically scattered by thin foils of Aluminium and Potassium were measured and compared with loss intensities, calculated by the dielectric theory from the dielectric constantsε ₁ andε ₂ depending on energy. One obtains a good quantitative agreement of the measured spectra with the calculated energy loss intensities.     

Excitation of lowest electronic states of the uracil molecule by slow electrons

The excitation of lowest electronic states of the uracil molecule in the gas phase has been studied by electron energy loss spectroscopy. Along with excitation of lowest singlet states, excitation of two lowest triplet states at 3.75 and 4.76 eV (±0.05 eV) and vibrational excitation of the molecule in two resonant ranges (1?C2 and 3?C4 eV) have been observed for the first time. The peak of the excitation band related to the lowest singlet state (5.50 eV) is found to be blueshifted by 0.4 eV in comparison with the optical absorption spectroscopy data. The threshold excitation spectra have been measured for the first time, with detection of electrons inelastically scattered by an angle of 180°. These spectra exhibit clear separation of the 5.50-eV-wide band into two bands, which are due to the excitation of the triplet 1³ A?? and singlet 1¹ A?? states.     

Investigation of fast neutron interaction with235U

The angular distribution of neutrons emitted by elastic, inelastic and fission processes on²³⁵U were measured at the incident neutron energies of 1.5, 1.9, 2.3, 4.0, 4.5, 5.0 and 5.5 MeV using nanosecond time-of-flight technique. The differential elastic scattering cross sections and their angular distributions at all the seven energies are presented. The total elastic scattering cross sections, angle and energy integrated cross sections for the inelastically scattered neutrons in energy bands of 200 keV, fission cross sections and the angular distributions of fission neutrons were extracted at 1.5, 1.9 and 2.3 MeV incident neutron energies. The energy distributions of the prompt fission neutrons and of the inelastically scattered neutrons are given at the incoming neutron energies of 1.5, 1.9 and 2.3 MeV; and the average fission neutron energies and the inelastic neutron evaporation temperatures were also evaluated at these energies.     

Spin-flip probability for 7.6 MeV neutrons scattered inelastically (Q = −2.23 MeV) by a sulfur target

The spin-flip probability for 7.6 MeV (ΔE = 0.250 MeV) neutrons scattered inelastically (Q = ?2.23 MeV) from naturally occuring sulfur has been measured at scattering angles between 40° and 160° using a γ-correlated neutron time-of-flight method. The scattering by the natural sulfur target was assumed to be characteristic of ³²S. The results of the measurement were compared with an incoherent sum of statistical compound-nucleus (CN) and direct-interaction (DI) contributions. The DI contributions were obtained using either a DWBA calculation or the coupled-channel (CC) formalism. Neither combination of CN plus DI contributions reproduced the experimental spin-flip probability angular distribution which was peaked near 110° with a maximum value of 0.33 ± 0.08.     

Eine Gegenfeldanordnung zur Messung von Energie- und Winkelverteilungen gestreuter Elektronen

In a previous paper direct electron-intensity measurements in Debye-Scherrer diagrams of aluminium have been described. A retarding field apparatus was used for investigations into the influence of crystal size and electron energy (15 to 50 keV) on the elastic diffraction intensities. The validity range of the kinematical theory could be determined. By means of a retarding field the inelastically scattered electrons were separated from the elastically scattered electrons (energy loss below 2 eV). In this paper the retarding field apparatus by which scattering intensities can be measured with good accuracy (≈ 2%) is described in detail. It is compared with other arrangements for the measurements of angular and energy distributions of scattered electrons.     

Differentielle Streuquerschnitte bei der Wechselwirkung neutraler Moleküle Messungen der Kleinwinkelstreuung

A molecular beam apparatus of high angular resolution is described, and measurements of the angular intensity distribution of alkali beams scattered at small angles (15 seconds of arc to 1 degree) by crossed beams of various gases and vapours are presented. Formulas, connecting the measured intensity with the differential cross section in the center of mass system, are derived, considering the velocity distribution in the beams and the geometry of the experimental set-up. The measurements show, that the differential cross section can be calculated down to very low angles (order of magnitude 10 minutes of arc) by means of classical mechanics, taking only into account the long-range attractive van der Walls interaction. At smaller angles, deviations, predicted by quantum mechanics, can be observed. If the potential of the van der Waals forces is written in the formV(r)=?C/r ^s, a method is outlined to determine bothC ands from the measured angular intensity distribution.     

On the influence of lattice vibrations on the tunnel-current in normal metals

Energy spectra of 50 kev-electrons scattered inelastically in thin Al foils of various thicknesses have been measured as a function of scattering angle. The measured angular distribution of electrons which have excited one or more volume plasmons is compared with theoretical calculations and good agreement is obtained. From the integrated intensities, the mean free path of 50 kev-electrons for plasmon excitation is determined with an accuracy of about 20%. The resulting value Λ=820 Å is in good agreement with the theoretical value calculated fromFerrell's free electron model (Λ=760Å).     

Resonance scattering of electrons from orientated O2 physisorbed on graphite

We have studied inelastic electron scattering from a saturated monolayer of O₂ physisorbed on graphite at 25K. The cross-section for excitation of the v = 0–1 vibrational transition in O₂ peaks at 8.5–9eV, and is attributed to the ⁴Σ_u negative ion resonance. The angular distribution of electrons inelastically ejected out of the resonance orbital has been measured, and is found to peak at 15° from the normal to the crystal for several different incident electron beam angles, in accord with the theory of resonant electron scattering by an orientated molecule. We discuss the implications of this measurement for a determination of the orientation of the O₂ molecule on the surface, taking account of possible multiple scattering effects.     

Die Winkelabhängigkeit des Energieverlustes von 1 MeV-Elektronen durch Bremsstrahlung

The angular dependence of bremsstrahlung energy loss of 1 MeV electrons has been measured at 30°, 60° and 90°. The electrons were scattered on aluminium, silver and gold foils of thickness from 0·1 to 2·3 mg/cm². The shape of the energy loss distribution between 20 and 100 keV has been found in accordance with theory which gives

$$\frac{{d^2 \sigma }}{{dQ d\Omega }} = q_{Mott} \cdot \frac{{c(E_{0,} \vartheta _0 )}}{Q} \cdot F$$     

Verhältnis des differentiellen (e,N)- zum totalen (γ,N)-Wirkungsquerschnitt

The ratio of the differential cross section for the (e, N)-process to the total (γ,N) -cross section is derived with the use of the relativistic Coulomb Eigenfunctions for the continuous spectrum. For electric and magnetic dipole transitions the Born approximation, the Coulomb correction, the effect of screening and that of finite nuclear size are calculated. In this angular distribution there should be no interference of electron waves scattered by different multipoles, where the inelastically scattered electrons are detected. Numerical calculations have been done for nuclei withZ=6, 29, and 82 and scattering anglesθ=1Ω, 132Ω, 160Ω and 180Ω of the electron. The result of this theory is compared with the experiments of W.C.Barber et al.     

The scattering of hydrogen from a cesiated tungsten surface

The reflection of protons from a partially cesiated tungsten surface is studied in the energy domain between 100 and 2000 eV and in the angular domain between 75° and 85° with respect to the surface normal. The study is performed by measuring the angular and energy distribution of the scattered negative ions. The reflection can take place along two paths. One path is reflection from the cesium surface layer, the other one is reflection from the tungsten substrate. A dependence of the final charge state on the path is observed. It is inferred that this phenomenon is due to incomplete neutralization of the protons scattered from the cesium layer. The energy loss of the reflected ions cannot be accounted for by using only the binary collision model. Also the electronic stopping of the atoms by the metal electrons is shown to be an important energy loss mechanism. Total conversion measurements of H⁺ to H^- combined with the measurements of the negatively charged fraction of the scattered particles, as reported in the proceeding paper, yield the particle reflection coefficient as a function of the angle of incidence. These reflection coefficients show that for angles of incidence less than 75° already more than 50% of the particles do not reflect from the surface. Total conversion efficiency measurements with H^- ions as primary ions show that the influence of the initial charge state on the total conversion is very small.     

Low energy electron spectroscopy of CO in the 10.600–14.400 eV energy loss range

Excitation spectra of CO have been obtained at low electron impact energy in the 10.600–13.400 eV energy loss range for scattering angles from 10 to 120°, with a 35 meV experimental resolution. The angular behaviour of the observed peaks is used to discriminate singlet-singlet and singlet-triplet transitions. Previously calculated Rydberg states are observed, in particular the triplet analogue of the F¹Σ⁺ state. A new high energy valence triplet state is identified; the first observed vibrational level is at 11.595 eV and the vibrational spacing is 90 meV. Upper levels are strongly affected by predissociation.     

Inelastische Positronenstreuung am Atomkern

The ratio of the total and differential cross section for the inelastic positron-nucleus scattering (ē, N)-process to the total (γ, N) -cross section is derived in Born approximation for electric and magnetic dipole transitions. The result agrees with that obtained for the (e, N)-processes. Using the relativistic Coulomb Eigenfunctions for the continuous spectrum of the positrons, the Coulomb correction, the effect of screening and that of finite nuclear size agree with the (e, N)-process, when the annihilation of positrons with atomic electrons is neglected, and for positron energiesE _1,2 ⁺ >10 MeV. The effect of finite nuclear size is only calculated in Born approximation. ForE _1,2 ⁺ ≦2 MeV only the Coulomb correction differs from that obtained for the (e, N)-process. In the angular distribution for the (ē, N)-process there should be no interference of positron waves scattered by different multipoles, where the inelastic scattered positrons are detected. Numerical calculations have been carried out for nuclei withZ=6.29 and 82 and scattering angles ?=1°, 132°, 160° and 180° of the positron. This theory can be compared with the experiments in progress by W.C.Barber et al. using positrons for the inelastic scattering process at nuclei. The two-and three-virtual quanta-exchange effect in the (ē, N)-cross section is below 1.3% for positron energies between 10≦E ₁ ⁺ ≦300 MeV, and decreases rapidly for higher energies. This theory is also valied for inelastic scattering processes with positiveμ-mesons at nuclei; one has only to change the mass in the following equations.     

Mehrfachstreuung von Elektronen im Energiebereich von 0,52 bis 2,0 MeV

The plural nuclear scattering of electrons with kinetic energies of 0.52 MeV up to 2.0 MeV has been measured for angles between 0° and 20°. Pb, Ag and Al foils of various thicknesses (mean number of collisions between 1 and 21) had been used as targets. The experimentally derived angular distribution of the scattered electrons has been compared with a numerical computation byKeil, Zeitler andZinn based on theMolière theory of plural and multiple scattering and his approximation of the single scattering law. His corrected values of the shielding angle and the mean number of collisions describe the distribution of the scattered electrons better than the original ones derived from a Born approximation calculation. The intensity of the remaining unscattered electrons must be calculated with the Born approximation value for the mean number of collisions in order to obtain agreement with the experiment.     

Calculation of radiative transition probabilities and radiative recombination rate coefficients for H2, OH, H 2+ and OH+ molecules

A method is presented to calculate the radiative transition probabilities and the radiative recombination rate coefficients between electronic molecular states. Total transition probabilities are determined from vibrational transition probabilities without considering the detailed rotational structure of the molecular electronic states. Radiative recombination rate coefficients are obtained from the computation of vibrational photo-ionisation cross sections. Concerning spontaneous emission, Lyman (B → X) and Werner (C → X) band systems of H₂ and Meinel (A → X), (B → A) and (B → X) band systems of OH are investigated. For radiative recombination, transitions between H₂⁺ (X) and H₂(X), and between OH⁺(X, a, A, b, and c) and OH(X) are considered. Transition probabilities and recombination rate coefficients are calculated as a function of temperature in the range 1500–15 000 K.