Resonance effects in the 5σ−1 photoionization channel of CO

Vibrational branching ratios and photoelectron angular distributions are reported for the 5σ−1 photoionization channel of CO in the range 16 eV<hν<45 eV. Striking non-Franck–Condon effects are observed in both the branching ratios and angular distributions as a result of various autoionizing states and a σ shape resonance that lie in this spectral range. The goal of the present measurement was to observe definitive evidence for the σ shape resonance via its non-Franck–Condon effects on the vibrational ionization channels. Guided by recent calculations [Smith, Lynch, and McKoy, J. Chem. Phys. 85, 6455 (1986)], we examined the broad structure in the vibrational branching ratios and angular distributions in the range 25 eV <hν<40 eV. There, we found clear evidence for the σ shape resonance in the quantities β(v+=0 and 1) and σ(v+=2)/σ(v+=0). Substantial differences between theory and experiment for the σ(v+=1)/σ(v+=0) branching ratio, however, serve to define the limitations of the current single-channel picture for this process.