Resonance effects in the 5~' photoionization channel of CO

Vibrational branching ratios and photoelectron angular distributions are reported for the 5a’ photoionization channel of CO in the range 16 eV < hv < 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 0 shape resonance that lie in this spectral range. The goal of the present measurement was to observe definitive evidence for the u shape resonance via its nonFranck-Condon effects on the vibrational ionization channels. Guided by recent calculations [Smith, Lynch, and McKay, J. Chem. Phys. 85,6455 ( 1986) 1, we examined the broad structure in the vibrational branching ratios and angular distributions in the range 25 eV < hv < 40 eV. There, we found clear evidence for the (T shape resonance in the quantities /3(l)+ = Oand 1) and a(u + = 2)/a( u + = 0). Substantial differences between theory and experiment for the a( u + = 1 )/a( u + = 0) branching ratio, however, serve to define the limitations of the current single-channel picture for this process.