Occupation probability as a function of effective principal quantum number n^∗ for conditions representative of three depths in the solar atmosphere. The depths are labelled by their approximate temperatures, and these correspond to particle number densities of N_H and N_e, of 10¹⁴ and 10¹³ cm^-3 at 4000 K, of 10¹⁷ and 3 × 10¹³ cm^-3 at 6000 K, and of 10¹⁷ and 7 × 10¹⁴ cm^-3 at 8000 K, respectively. In each case, N_He = 0.1 × N_H.