N J Bowring

Differential cross sections for electron impact excitation of the n = 2 states of helium at intermediate energies (80, 100 and 120 eV) measured across the complete angular scattering range (0–180°)

Differential cross sections (DCS) for inelastic electron scattering to the n = 2 states in helium have been measured at incident energies of 80, 100 and 120 eV. These DCS have been determined across the complete angular scattering range (0–180°) using a magnetic angle changer (MAC) with a soft-iron core. The convergent close-coupling (CCC), R-matrix with pseudostates (RMPS), and B-spline R-matrix (BSR) methods have been used to calculate these DCS. Agreement between the experimental data and the predictions from these highly sophisticated theoretical methods is generally good. The remaining discrepancies mainly occur at small and large angles for the triplet states 23S and 23P, whereas excellent agreement is found between 30° and 150°. The small-angle differences are likely due to contamination of the observed experimental signal from the neighbouring 21S and 21P states. The present results demonstrate the effective use of a soft-iron core MAC for DCS measurements at intermediate energies, extending the operational energy range of such devices by a factor of approximately 25.

Charge-tube method for space charge in beams

The charge-tube method is an accurate and efficient way of assigning the space-charge of a beam in computational simulations of charged particle systems. The method makes use of the trajectory steps that result from the process of trajectory integration. The space-charge associated with each step of each trajectory is assigned to a narrow cylindrical tube that surrounds the step. The total space-charge of a beam is then the sum of the charges in all the resulting the tubes. In systems of 2-dimensional axial symmetry the charge tubes become conical sheets of charge, and for some purposes these need to be given a finite thickness. The charge-tube method is particularly useful for simulating the space-charge of beams that are very narrow compared with their length. The implementation of the method is described and results obtained with it are compared with those obtained by the traditional charge-cell method.

Comparison of argon and helium (e, 2e) differential cross sections at 64.6 eV using symmetric detection energies and angles

Symmetric (e,2e) differential cross section measurements for argon at an incident energy of 64.6 eV are presented from the coplanar to the perpendicular plane geometry. The outgoing electrons were detected at symmetric scattering angles and with the same energy. These measurements are compared with analogous results at the same incident energy using helium as the target. The argon (e,2e) cross section shows a rich and complex structure as the scattering geometry changes, while the very deep interference minimum found in helium is missing.

Near-threshold doubly-symmetric (e, 2e) measurements in helium

Electron-impact ionization of helium has been studied in the energy range from 3 - 10 eV above the ionization threshold by measuring (e, 2e) angular correlations over a wide range of scattering angles from the coplanar to the perpendicular plane geometry. In these measurements the two outgoing electrons are observed at symmetric scattering angles and with equal energies.

Two-electron interference in the helium (e, 2e) differential cross section at 64.6 eV

The unexpected sharp dip which is known to exist in the helium (e, 2e) differential cross section at an incident electron energy of 64.6 eV, and for equal energies and angles of the two outgoing electrons, is known to be sensitive to three of the four available experimental parameters. We have now investigated the sensitivity of the dip to the fourth parameter, the asymmetry of the angles of the outgoing electrons, and have found that the dip exists over a wide range of asymmetric angles, but with an almost constant angle between the two electrons. A two-electron interference model is proposed to explain these results.

The threshold photoelectron spectrum of mercury

The threshold photoelectron spectrum of mercury has been recorded over the energy range (10–40 eV) which covers the region from the lowest state of the singly charged ion, 5d106s(2S1/2), to the double charged ionic state, 5d9(2D3/2)6s(1D2). Synchrotron radiation has been used in conjunction with the penetrating-field threshold-electron technique to obtain the spectrum with high resolution. The spectrum shows many more features than observed in previous photoemission measurements with many of these assigned to satellite states converging to the double ionization limit.

Parametrization of low-energy symmetric (e, 2e) differential cross section measurements

Symmetric (e, 2e) experimental studies on helium over a very wide range of scattering geometries at energies from 1 to 50 eV above the ionization threshold are parametrized in terms of a set of universal irreducible tensorial angular functions. Measurements from the Manchester and Paris experimental groups are used in this parametrization, which provides a complete picture of the symmetric ionization process. The use of the parametrization technique for deconvolution of the experimental angular resolution from measured results is discussed.

(e,2e) Coincidence Measurements at Low Energy

Electron-atom ionisation event processes can be classified in three regions as defined by the excess incident electron energy above the ionisation threshold. The definition and range of these regions is necessarily somewhat arbitrary, however in this paper these regions are considered to be (figure 1): The threshold region, ranging from 0eV to around 4eV above threshold. The intermediate energy region, ranging from around 4eV to 100eV above threshold. The high energy region, where the incident electron energy exceeds 100eV above threshold.

Novel coherence effects in microwave-microwave double resonance pulse Fourier transform spectroscopy

A two pulse sequence in a microwave-microwave double resonance experiment has exhibited a new set of coherences between rotational energy levels in 35ClCN. The derivation of a set of five-level Bloch equations is presented and used to interpret the experimentally observed dependence on pump power and source detuning. Applications to spectral assignment are outlined.