Electron-impact single ionisation of W ^q+ions: experiment and theory for 11 ≤ q ≤ 18

Abstract

Absolute cross sections for electron-impact single ionisation (EISI) of multiply charged tungsten ions (W q+ ) with charge states in the range 11 ≤ q ≤ 18 in the electron-ion collision energy ranges from below the respective ionisation thresholds up to 1000 eV were measured employing the electron-ion crossed-beams method. In order to extend the results to higher energies, cross section calculations were performed using the subconfiguration-averaged distorted-wave (SCADW) method for electron-ion collision energies up to 150 keV. From the combined experimental and scaled theoretical cross sections rate coefficients were derived which are compared with the ones contained in the ADAS database and which are based on the configuration-averaged distorted wave (CADW) calculations of Loch et al. [Phys. Rev. A 72, 052716 (2005)]. Significant discrepancies were found at the temperatures where the ions investigated here are expected to form in collisionally ionised plasmas. These discrepancies are attributed to the limitations of the CADW approach and also the more detailed SCADW treatment which do not allow for a sufficiently accurate description of the EISI cross sections particularly at the ionisation thresholds.

Hybrid subconfiguration-average and level-to-level distorted-wave treatment of electron-impact single ionisation of W$^{15+}$ and W$^{16+}$

“…This less than 1% deviation is almost within the quoted uncertainties of the NIST values 361.9±1.5 and 387.9 ± 1.2 eV. Comparison of the present SCADW cross sections (lines) for EISI of (a) W 15+ and (b) W 16+ with the corresponding experimental results of Schury et al [7] (large open symbols: absolute cross-section data points with the error bars comprising statistical and systematic uncertainties; small filled symbols: scan measurements with the error bars comprising statistical uncertainties only). Contributions by different direct ionisation (DI) and excitation-autoionisation (EA) processes are represented by the labelled shaded curves.…”

“…Figure 1 presents comparisons of our SCADW cross sections for EISI of W 15+ and W 16+ ions in their ground-configurations with the corresponding experimental data of Schury et al [7]. In the SCADW calculations we assumed a statistical population of the subconfigurations.…”

“…Obviously, the individual levels contribute differently to the averaged EA cross section (thick black dashed line in figure 3). For the averaging we assumed a statistical population of all levels, since their lifetimes are much longer than the microsecond flight time of the ions in the experiment of Schury et al [7]. This is because the transitions from the excited levels to the ground level are dipole forbidden.…”

“…Consequently, EISI of tungsten ions has been repeatedly studied by experiment [3][4][5][6]. The most recently published experimental data are those of Schury et al [7] for EISI of W q+ ions with initial charge states 11 ≤ q ≤ 18.…”

“…On the theory side, configuration-averaged distorted wave (CADW) calculations have been widely used for EISI calculations because of the efficiency of this method in generating large numbers of cross sections and plasma rate coefficients [7][8][9][10]. More elaborate level-resolved calculations have been carried out only for a limited number of ion species such as W 5+ [11], W 17+ [12], W 25+ [13], W 26+ [14], and W 27+ [15].…”