Role of Electron Spin Coupling in Molecular Photoionization: ThebΣg−4←XΣg−3Photoelectronic Transition ofO2

The electron spin splittings of the rotational levels of the $b$ and $X$ states have been resolved in the pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of the ${\mathrm{O}}_2^{+}b{}_{}{}^4{}_{}{}^{}\Sigma _g^{-}{}_{}{}^{}\leftarrow {\mathrm{O}}_{2}X{}_{}{}^3{}_{}{}^{}\Sigma _g^{-}{}_{}{}^{}$ transition. Photoionization leads to a very selective population of specific spin components of ionic rotational levels. Of the nine $b{}_{}{}^4{}_{}{}^{}\Sigma _g^{-}{}_{}{}^{}(v^{+}{=0,N}^{+}{=1,F}_j,j=1,2,3\mathrm{})\leftarrow X{}_{}{}^3{}_{}{}^{}\Sigma _g^{-}{}_{}{}^{}(v^{\prime \prime }{=0,N}^{\prime \prime }{=1,F}_i,i=1,2,3\mathrm{})$ components that constitute the $N^{+}=1\mathrm{}\leftarrow N^{\prime \prime }=1\mathrm{}$ rotational transition, only those with $i=j$ are observed with significant intensity. The results demonstrate the importance of a detailed consideration of the electron spin for a satisfactory description of molecular photoionization.