Theoretical and experimental investigation of $(e,2e)$ ionization of argon $3p$ in asymmetric kinematics at intermediate energy

The field of electron-impact ionization of atoms, or $(e,2e)$, has provided significant detailed information about the physics of collisions. For ionization of hydrogen and helium, essentially exact numerical methods have been developed which can correctly predict what will happen. For larger atoms, we do not have theories of comparable accuracy. Considerable attention has been given to ionization of inert gases and, of the inert gases, argon seems to be the most difficult target for theory. There have been several studies comparing experiment and perturbative theoretical approaches over the last few decades, and generally qualitative but not quantitative agreement is found for intermediate energy incident electrons. Recently a nonperturbative method, the $B$-spline $R$-matrix (BSR) method, was introduced which appears to be very promising for ionization of heavier atoms. We have recently performed an experimental and theoretical investigation for ionization of argon, and we found that, although the BSR gave reasonably good agreement with experiment, there were also some cases of significant disagreement. The previous study was performed for 200-eV incident electrons and ejected electron energies of 15 and 20 eV. The purpose of the present work is to extend this study to a much larger range of ejected electron energies (15–50 eV) to see if theory gets better with increasing energy as would be expected for a perturbative calculation. The experimental results are compared with both the BSR and two different perturbative calculations.

Figure 1

Schematic diagram of the coincidence electronics used to accumulate a coincidence timing spectrum at each kinematics.

Figure 2

Experimental and theoretical TDCS for 200 eV electron-impact ionization of argon. The projectile scattering angle is 10° and the ejected electron energies are noted in each subsection of the figure. The theoretical calculations are 3DW: solid line; dash-dot: DWB2-RM; and dashed: BSR. The experimental data are triangles: Ren et al. [46]; open circles: Ulu et al. [48]; and solid circles: present results. All theories and experiment were normalized to 1.0 at the maximum of the binary peak (see text).

Figure 3

Experimental and theoretical TDCS for 200 eV electron-impact ionization of argon. The projectile scattering angle is 15° and the ejected electron energies are noted in each subsection of the figure. The theoretical calculations are 3DW: solid line; dash-dot: DWB2-RM; and dashed: BSR. The experimental data are triangles: Ren et al. [46]; open circles: Ulu et al. [48]; stars: Stevenson et al. [45]; and solid circles: present results.

Figure 4

Same as Fig. 3 except that absolute values of the theories are shown in atomic units.