The angularly resolved O 1s ion-yield spectrum of O₂ revisited

The high-resolution photoabsorption spectrum of O₂ below the O $1sσ−1$ ionization threshold has been reanalyzed by using a sophisticated fit approach. For the vibrational substates of the O $1sσu−1(4∑u−)3sσg(3∑u−)$ Rydberg state Fano lineshapes are observed indicating an interaction with the O $1sσg−1(4∑g−)3σu∗(3∑u−)$ core-to-valence excited state. In the angularly resolved ion-yield spectrum recorded at 90° relative to the polarization direction of the synchrotron radiation clear evidence for the O $1sσg−1(2∑g−)3σu∗(3∑u−)$ and the O $1sσu−1(2∑u−)3sσg(3∑u−)$ state was found. This observation clearly suggests thatΛ, the projection of total orbital angular momentum on the molecular axis, is not well separated in the angularly resolved ion-yield spectrum due to a partial breakdown of the axial-recoil approximation for these transitions.