Albert Crowe

Differential cross sections for elastic scattering of electrons from argon and krypton as a continuous function of energy

Using a low-resolution electron-impact spectrometer in excitation function mode, the energy dependence of differential cross sections for elastic scattering of electrons on argon and krypton was studied. Relative cross sections were put on an absolute scale by normalizing to the previous data of Srivastava et al at one conveniently chosen energy and angle in each gas. Differential cross sections for elastic scattering of electrons on krypton are compared with previously published cross sections for positron scattering. No broad resonance-like structures are observed in the case of electron scattering but the energy dependence, especially in krypton, shows a strong variation with energy which is not observed for positrons.

Simultaneous excitation-ionization of helium to the state

Simultaneous excitation-ionization of helium to the ionic state by electron impact is studied experimentally and theoretically. Electron-photon angular correlations measured at an incident electron energy of 200 eV and electron scattering angles between and are in reasonable agreement with theoretical predictions. The double-differential cross section shows structure due to interference effects arising from autoionization states decaying into this channel. The emission cross section from threshold to 300 eV is also reported and compared with previous data.

(e, 2e) experiments on (2s2)1S, (2p2)1D and (2s2p)1P autoionizing levels of helium in the direction of the binary lobe

The (e, 2e) triple differential cross sections are measured for the electron-impact excitation of the (2s2)1S, (2p2)1D and (2s2p)1P autoionizing transitions in helium. The experiments were performed under coplanar asymmetric kinematics, at an incident electron energy of 250 eV, and for a scattered electron detection angle of −13° in the direction of the binary lobe. The results are compared with the two available experimental data sets under similar kinematical conditions, consequently removing questions raised by the theory on these earlier results. We present evidence that the spectrum for the (2p2)1D and (2s2p)1P states might be dependent on the energy resolution of the scattered-electron energy analyser, especially in the binary region, where the strong interference between the two processes results in the observed broad asymmetric profiles. This possible distortion is not solely related to the present work.

A critical look at electron-photon coincidence experiments with heavy noble gases in the regime of large impact parameters

The electron-photon coincidence technique has been employed in experimental investigations aimed at elucidating the finer details of the excitation of the first excited states of the heavy noble gases by electron impact since the mid 1970s. The agreement between measured data and the predictions of first-order perturbative theories such as the distorted wave Born approximation (DWBA) and the first-order many-body theory (FOMBT) ranges from good in some cases to rather poor in other cases. This article will review the current status of electron-photon coincidence experiments with the heavy noble gases, critically assess the level of agreement and disagreement between experiment and theory, discuss apparent trends in the data, make recommendations for the direction of future experiments and suggest ways to ensure that measured coherence parameters are free from instrumental and physical effects to the maximum extent possible.

Experimental and theoretical investigation of (e, 2e) ionization of Ar(3p) in asymmetric kinematics at 200?eV

Experimental and theoretical studies of electron impact ionization of the 3p orbital of argon are reported. The relative triple-differential cross sections were measured in the coplanar asymmetric geometry. The experimental data at an incident electron energy of 200 eV, ejected electron energies of 15 and 20 eV, and scattering angles of −10°, −15°, and −20° were obtained using a conventional crossed-beam (e, 2e) spectrometer. The experimental data are compared with predictions from first- and second-order hybrid distorted-wave plus R-matrix models, as well as a fully nonperturbative B-spline R-matrix approach.

Electron impact excitation of the 3D states of helium: comparison between experiment and theory at 30 eV

Experimental and theoretical values of the Stokes parameters are reported for electron impact excitation of the state of helium at 30 eV. The theoretical results are from a 111-state convergent close-coupling (CCC) method. Excellent agreement is obtained between theory and experiment for the Stokes parameters and for the derived parameters describing the shape and dynamics of the excited state. By considering the excitation as the incoherent sum of two coherent processes, an ambiguity in constructing the excited state from the experimental dipole radiation pattern is removed. This provides an attractive way of providing a complete description of the excitation compared with a triple coincidence experiment involving the scattered electron and photons from the 3D - 2P - 1S transitions. This analysis is also carried out for the state at 30 eV and comparison is made between 111-state CCC calculations and the experimental data of Donnelly et al.

In-plane polarisation correlation study of the 31D state of helium excited by 40 eV electrons

Electron-photon polarisation correlations measured in the scattering plane defined by the incident and scattered electron momenta are reported for 40 eV electron impact excitation of the 31D state of helium and for electron scattering angles in the range 40°≦ϑ≦120°. When combined with the results of our earlier polarisation analysis on radiation emitted perpendicular to the scattering plane, detailed information on the shape and dynamics of the exicted state is obtained. In the small angular range of overlap (40–60°) there is excellent agreement with a pervious experimental study. The behaviour of the linear polarisation of the radiation emitted in the scattering plane is well reproduced by a multichannel eikonal theory for ϑ≦80°. Otherwise theories totally fail to described the excitation process.

Electron impact double-excitation of helium 2ℓ2ℓ' autoionizing states using the (e, 2e) technique

Coplanar (e, 2e) triple differential cross sections (TDCS) measurements are reported for the helium autoionizing doubly excited states, (2s2)1 S, (2p2)1D, and (2s2p)1P, for an incident electron energy of 250 eV and a scattering angle of −13°, corresponding to a momentum transfer of 1.06 a.u. The presence of autoionization results in a clearly visible recoil peak in the TDCS structure with a shape that is strongly dependent on the orbital angular momentum L of the resonance. The resonance contributions together with the strong background of direct ionization processes lead to a complicated asymmetric structure of the resonance profile in the TDCS.

Experimental and theoretical investigation of (e,2e) ionization of Ar(3p) in asymmetric kinematics at intermediate energies

Experimental and theoretical studies of electron-impact ionization of the 3p orbital of argon are reported. Good agreement is obtained with other benchmark experiments and state-of-the-art calculations.

Theoretical and experimental investigation of electron‐helium scattering

We present both calculations and measurements of the 33,1D state charge clouds after 30 eV electron‐impact excitation of the ground state of helium. The measurements are obtained using the polarization‐correlation technique. The theory is the convergent close‐coupling (CCC) theory. Agreement between theory and experiment is very good indicating that the CCC theory is able to treat the exchange and continuum effects very accurately.