Theoretical study of electronic autoionization in CO: Vibrationally resolved results between 17 and 18.3 eV

We have theoretically studied the electronic autoionization between 17 and 18.3 eV of the Rydberg series converging to the B ${\mathrm{}}^2$ ${\Sigma }^{+}$ state of ${\mathrm{CO}}^{+}$ ($R_B$ series). We have obtained the partial and vibrationally resolved differential and total photoionization cross sections by combining ab initio electronic quantities with a variant of the two-step multichannel quantum-defect theory (MQDT). The formalism of the Feshbach projection operators is used to separate the electronic quantities needed in the first and second steps of the MQDT. We have introduced the vibrational motion within the Condon approximation. Our results lead to new unambiguous assignments of the $R_B$ series. The vibrationally resolved cross sections, the associated branching ratios, and the background and resonant features are discussed in detail and compared with the available experimental data. There is an overall agreement between theory and experiment except for the angular distribution of photoelec- trons.