Vibronic coupling and other many-body effects in the 4σ−1g photoionization channel of CO2

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Abstract
Vibrational branching ratios and photoelectron angular distributions were measured for 4σ−1g photoionization of CO2 in the energy range 20–28 eV. Of particular interest are three vibrational components of the resulting CO+2 C̃ 2Σ+g state—the allowed (000) and (100) bands and the forbidden (101) band. The wavelength dependence of the beta parameter for the forbidden band deviated significantly from that of the two allowed bands, showing instead a strong resemblance to that of the B̃ 2Σ+u state. This behavior suggests that vibronic coupling to the B̃ 2Σ+u state is responsible for the appearance of the forbidden (101) band in the C̃ 2Σ+g state photoelectron spectrum. We also observe evidence for other many‐body effects—shape‐resonance‐induced continuum–continuum coupling and doubly excited autoionizing resonances—in the present data.