where 𝜐 NIR1 and 𝜐 NIR2 are the frequencies of the NIR photons involved in the wave-mixing pathway, 𝜃 NIR1 and 𝜃 NIR2 are the crossing angles at which the NIR beams intersect the XUV in the target cell, and 𝜐 XUV is the frequency of the initial XUV excitation. Multiple wave-mixing pathways composed of photons of different frequencies can emit at the same energy, augmenting the angular divergence of the wave-mixing signals . However, the intensity of emission is enhanced when each photon involved in the wave-mixing process is resonant with a transition in the medium , resulting in a preference for pathways mediated by one-photon, dipole forbidden (dark) states. Two NIR photons couple one photon, dipole allowed (bright) states via dark states, leading to the emission of XUV photons at the energies of the Rydberg states observed in the photoabsorption spectrum. Calculations described in Section IIB suggest that the viable intermediary dark states are located below the emitting state in energy, as depicted in Fig. .