Selective population of spin-orbit levels in the autoionization of a polyatomic molecule: Branching ratios and asymmetry parameters for the Tanaka-Ogawa Rydberg series in CO₂

The spin-orbit selectivity of angle-resolved photoelectron spectra was used to provide new information on the electronic structure, symmetry, and decay dynamics of members of the autoionizing Tanaka-Ogawa Rydberg series in CO ₂. This represents the first time that spin-orbit selectivity has been used to obtain such information for a polyatomic molecule. The spin-orbit photoelectron branching ratios were used to show that the angular momentum quantum number λ of the excited Rydberg electron does not change upon autoionization. Furthermore, a consideration of the present results together with previous calculations of the relative intensities of the discrete and continuum ionization channels shows that the most probable electron configuration for the Tanaka-Ogawa Rydberg series is ...(π_u) ³(π_g)⁴ndδ_g and that autoionization proceeds primarily via a dδ_g→εfδ _u process for the totally symmetric vibronic components of the ion. The asymmetry parameter β was determined for individual spin-orbit components of the various vibronic bands of the X̃ ²Π _g state and is discussed in terms of recent theoretical calculations. The Rydberg series appears to be well described by Ω_cω coupling, even for relatively low principal quantum numbers. The general utility of this technique for autoionizing Rydberg states and its extension to multiphoton ionization of Rydberg states that lie below the first ionization threshold are discussed.