In the photon energy range interrogated directly by the FWM experiment, 17.5 eV to 17.9 eV above the ground state, the calculations of the photoionization cross sections found two series of Rydberg autoionizing resonances leading to the 𝐵 ̃ Σ 𝑢 + 2 ionization threshold. The positions and widths extracted from the Fano profile analysis for these resonances are given in Table . The resonances in the diffuse Σ 𝑢 ] series come in pairs with the diffuse state having quantum number n being very close to the sharp state with quantum number n-1 as is also seen in the synchrotron one-photon photoabsorption experiment . In order to extract the Fano parameters for these resonances, we fit each pair of resonances independently of the other pairs. We then assumed that the fit of the Fano profile for a given pair had a common non-resonant background term, which was quadratic in the photon energy, and we further assumed that the contributions of the photoionization cross sections of the two resonances of a given pair were additive. The position and width of the