ENERGY-DEPENDENCE OF PHOTOION ROTATIONAL DISTRIBUTIONS OF N2 AND CO

We present the first measurements of rotational distributions for photoionization over extended energy ranges [0 less-than-or-equal-to E(k) less-than-or-equal-to 200 eV for N2 (2sigma(mu)-1) and of 3 less-than-or-equal-to E(k) less-than-or-equal-to 125 eV for CO (4sigma-1)]. The N2 and CO results show a strikingly unusual and different energy dependence. Although differences are expected due to the absence of a center of symmetry in CO, detailed calculations reveal that this behavior arises from the presence of Cooper minima in the photoelectron continuum (ksigma(g)) in the case of N2 and from an f-wave shape resonance for 4sigma-1 photoionization in CO.