Energy dependence of photoion rotational distributions of N_2 and CO

We present the first measurements of rotational distributions for photoionization over extended energy ranges [0 ≤ E_k ≤ 200 eV for N_2 (2σ^(-1)_u) and of 3 ≤ E_k ≤ 125 eV for CO (4σ^(−1))]. The N_2 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 (kσ_g) in the case of N_2 and from an f-wave shape resonance for 4σ^(−1) photoionization in CO