Determination of rotational alignment ofN2+BΣu+2and CO+BΣ+2following photoionization ofN2and CO

When a low-pressure (${10}^{-2\mathrm{}}$-Torr) sample of ${\mathrm{N}}_2$ or CO is exposed to a beam of linearly polarized synchrotron radiation (18-34 eV), electronically excited molecular ions are formed whose subsequent unresolved emission is found to be partially polarized. The degree of polarization of the ${\mathrm{N}}_2^{+}$ $B{}_{}{}^2{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}-X{}_{}{}^2{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ fluorescence (∼391 nm) and the C${\mathrm{O}}^{+}$ $B{}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}-X{}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ fluorescence (∼220 nm) is studied as a function of the energy of the ionizing radiation from threshold to higher energies. This information determines the alignment of the photoion, which in the case of ${\mathrm{N}}_2$ must be corrected for depolarization caused by spin-rotation and hyperfine interaction. From the polarization of the fluorescence we deduce $\gamma$, the ratio of parity-unfavored to the sum of parity-favored contributions for photoelectron ejection. Comparison of the dynamical channel ratio $\gamma$ with theoretical calculations for $\sigma$- and $\pi$-photoelectron ejection shows poor agreement for ${\mathrm{N}}_2$ and fair agreement for CO.