Cumulative distribution (in percentage) of known MWDs with mean field strengths greater than B (thick blue line). The field strength ranges analyzed in photoionization studies are indicated. The insert shows a cross section evaluated using a fully quantum-mechanic method (Zhao & Stancil 2007) and the results obtained with the RWA approach (see text). The dark line represents a Gaussian convolution of the Zhao & Stancil evaluation with a full width at half maximum of 3.5 nm (≈10⁻³ Ry).