H. F. Wellenstein

Bethe surface, elastic and inelastic differential cross sections, Compton profile, and binding effects for H2 obtained by electron scattering with 25 keV incident electrons

Electron impact spectra for H2 have been obtained at scattering angles of 1°, 1.5°, 2°, 3°, 4°, 5°, 7°, and 10° using 25 keV incident electrons. The measured intensities were converted to generalized oscillator strengths and placed on an absolute scale at each scattering angle by the use of the Bethe sum rule for the generalized oscillator strength distribution. The cross section differential with respect to both solid angle and energy loss of the scattered electron was corrected for relativistic and exchange effects and integrated over energy loss to obtain the inelastic differential cross sections. In addition the elastic cross section differential with respect to solid angle was measured. The results are all in excellent agreement with theoretical calculations. The total elastic cross section was determined using additional data from another source. The Compton profile was determined from the 7° scattering data and was found to agree well with the previous x‐ray results. Consistent generalized optical ...

COLLISIONAL RELAXATION PROCESSES FOR THE n = 3 STATES OF HELIUM. II. ASSOCIATIVE IONIZATION.

Rate constants for the associative ionization reaction He * + He → He 2 + + e have been measured, where He* represents the n=3 states of helium. By line absorption from the metastable 21S and 23S states, the n=3 states in the positive column of a dc discharge in helium were selectively and periodically modulated. Using phase‐sensitive detection, the changes in excited‐state populations were determined spectroscopically while the corresponding in‐phase variations in the concentration of He 2 + were measured with a quadrupole mass filter. The cross sections for associative ionization in units of 10−16 cm2 were determined to be: σ (3 3 P) = 1.6 ± 0.1, σ (3 1 P) = 3.1 ± 1, σ (3 3D) = 4.5 ± 0.5, and σ (3 1D) = 20 ± 4. σ (3 3S) and σ(31S) were apparently zero.

Collisional Relaxation Processes for the n=3 States of Helium. I. Excitation Transfer by Normal Atoms and by Electrons

The positive column of a glow discharge through helium served as a reactor where each of the n=3 states of helium was selectively modulated by absorption of radiation. By monitoring the in‐phase component of radiation from adjacent n=3 states over a range of discharge currents and pressures, rate constants were determined for transfer of excitation between n=3 states by collision with normal atoms and with electrons.

Bethe surface and Compton profile of NH3 obtained by 35 keV electron impact

Complete inelastic electron impact spectra have been obtained for NH3 for a momentum transfer range from 0.4 to 12.5 a.u. using 35 keV electrons. These spectra were converted to relative generalized oscillator strengths (GOS), were placed onto an absolute scale by the Bethe sum rule, and higher order sum rules were used to test the accuracy of the GOS. The Compton profile (CP) of NH3 was determined from the GOS by means of the impulse approximation (IA) and it was found that the CP’s were asymmetric and shifted in respect to the free electron theoretical CP. This Compton defect was analyzed in detail and was shown to be due to the failure of the IA. The area under the CP is discussed in terms of the x‐ray incoherent scattering factor and experimental results for the valence, inner shell, and total CP’s are compared to several theoretical profiles.

Simple recording Möllenstedt electron‐velocity analyzer

An electrostatic velocity analyzer of the Mollenstedt type is described in detail, featuring in particular a high efficiency (through‐put) and a resolution adjustable from about 0.2 to 30 eV (or more) full width at half‐maximum (FWHM). The construction is kept simple by using several commercially available parts. A sweep generator together with a multichannel scaler is utilized to record spectra over an energy range from 0 to 16 000 eV. An experimental calibration technique is described and two energy‐loss spectra obtained by electron impact are given to demonstrate the performance of the analyzer.

Determination of optical sums from the small angle electron impact spectra of argon

Abstract A method for obtaining optical sums directly from high energy (25 keV) electron impact spectra in the forward direction is suggested. The electron energy loss spectrum of argon at an energy resolution of ±600 meV was obtained for energy losses from zero to 50 eV and 200 to 300 eV at an energy resolution of 5 eV averaged over an angular range of ±5 × 10 −4 radians. The data were corrected for finite detector size, gas and electron beam spatial distributions to obtain an estimate of zero angle scattering spectrum. A hydrogenic contribution was assumed for the 3p, 3s continuum for energy loss greater than 5- eV and for the 2p continuum for energy loss greater than 280 eV. Various moments of the spectrum were computed and related to optical sums by use of the measured experimental moments of the energy distribution of the incident beam and the scale factor needed to place the data on an absolute scale. The scale factor was obtained by use of the Thomas-Reiche-Kuhn sum rule. The results are in excellent agreement with previous estimates of the various sum rules and the present approach shows promise for providing useful data for a large number of atomic and molecular targets.

A high−voltage telefocus electron gun with remote focusing

A new design for a telefocus electron gun is presented. Features include a nearly constant minimum FWHM of 180 μ with beam currents of 1−400 μA at focal distances from 1/2 to 1 1/2 m. The gun is especially designed to operate under adverse vacuum conditions with background pressures as high as 10−4 Torr with accelerating potentials of 10 to 60 keV. In addition tn can be remotely focused and positioned from outside the vacuum chamber in which the gun is used.

Effect of finite scattering volume and detector geometry on measured electron‐impact spectra in crossed‐beam experiments

Calculations are reported for the effects of finite scattering volume and detector geometry on electron‐impact intensity measurements in crossed‐beam experiments. The target and projectile beams are assumed to be Gaussian in cross‐section shape. Three cases for detector geometry are considered using rectangular apertures. The first case consists of a single rectangular aperture. The second case consists of an additional single slit with slit width perpendicular to the direction of target beam flow located between the scattering volume and the rectangular aperture. The third case consists of two apertures. Correction values for various experimental situations are given and theoretical results for the important case of zero scattering angle are compared with experiment.

High‐energy electron impact spectrometer for absolute triple differential cross sections

A new high‐energy spectrometer for the coincident detection of both outgoing electrons in an electron impact ionization, or (e, 2e), experiment is described. The absolute triple differential cross section can be measured with this instrument to a precision of 10%. Selected results on rare gases are presented, demonstrating the ability of the spectrometer to either probe the target structure in impulsive or binary (e,2e) experiments, or to test the scattering mechanism in nonimpulsive conditions, both for valence or inner shells.

Absolute triple differential cross sections for the 3p ionisation of argon by electron impact

The triple differential cross sections for electron impact ionisation of argon on its 3p orbital are investigated for a primary electron energy near 8 keV. In a coplanar coincidence experiment, both ionisation electrons are analysed with respect to their energy and scattering angles. As in previous low-energy measurements, two lobes are found in the angular distribution of the slow ejected electron. The data are made absolute to an accuracy of better than 10%. This is done at high energy and large momentum transfer, K, by normalising to the impulse approximation Compton profile and assuming a cylindrical symmetry of the data about K. At high momentum transfer where the collision may be considered as a binary electron-electron process, the Ar 3p electron momentum distribution is determined and found to be in very good agreement with the Fourier transform of the Hartree-Fock-Clementi wavefunction.