A vibrationally resolved fluorescence polarization study of theCO+ A 2Π → X 2Σ+ transition in the 16.5–20 eV excitation range

Abstract

Synchrotron radiation has been used to formCO+ inthe A 2Π state, either through direct photoionization or through excitation into a Rydberg statefollowed by autoionization. The alignment of the resulting ion has been determined bymeasuring the degree of linear polarization of the fluorescence emitted in the or 1 transitions. These vibrationally resolved polarization data, together with thecorresponding fluorescence yields, have been recorded in the 16.5–20 eV excitation range. Inthis energy range the fluorescence yields exhibit prominent structure due to autoionizingRydberg states belonging to series converging onto various vibrational levels of either theA 2Π orthe B 2Σ+ ionization thresholds. Resonant excitation and decay processes produce substantial deviationsin the fluorescence polarization and these changes have been used to deduce the symmetryof the excited state. The vibrationally resolved data are particularly useful in this respectbecause the observed variations in the polarization can be related to the branching ratiobetween a specific vibrational level in the Rydberg state and a particular level in theA 2Π state. This information has allowed the assignments of the Rydberg series converging onto theB 2Σ+ ionization limit to be verified. In addition, the measured polarization has been comparedwith predictions derived from a theoretical approach in which only direct photoionization istaken into account.