Absolute photoionization cross section for C+

The absolute photoionization cross section for the astrophysically important C+ ion has been measured for the first time thus allowing a test of the predicted cross section from the Opacity Project. The measurements are performed with a new ion - photon merged beam setup at the ASTRID storage ring utilizing an undulator beam line. In addition to the predicted 2s2p(P-3) np D-2 and S-2 autoionizing resonances, the P-2 states which are not included in the theoretical predictions also contribute significantly to the ionization yield below the 2s2p(3P) limit. The cross section is determined with a precision of 10% and lies 5-25% below the theoretical prediction at 24-37 eV.