The Henning diffuse and Henning sharp series are assigned Rydberg electron characters of 𝑛𝑠𝜎 𝑔 and 𝑛𝑑𝜎 𝑔 , respectively. Vibrational progressions up to the 𝜈 1 = 1 symmetric stretch are observed in the photoionization spectra of the ionic 𝐵 ̃ Σ 𝑢 + 2 state, but the 𝜈 1 = 0 transition dominates. Photoionization quantum efficiency measurements provide insight into the channels relevant in the decay dynamics of these series . For the Henning diffuse series, the photoionization quantum efficiency is close to unity, indicating that autoionization is the dominant decay pathway for this series. On the other hand, in the same work, the photoionization quantum efficiency of the Henning sharp series is notably less than unity, possibly reflecting competition between neutral predissociation and autoionization. While these photoionization measurements and the spectral linewidths of the states suggest that members of the Henning diffuse series decay more quickly than those of the Henning sharp series, more work is necessary to establish the lifetimes of these states.