Photoion rotational distributions from near-threshold to deep in the continuum

We present the first measurements of ion rotational distributions for photoionization over an extended range [0 ≤ E_K ≤ 200 eV for N_2 (2σ^(−1)_u ) and 3 ≤ E_K ≤ 125 eV for CO (4σ^(−1))]. The N_2 ion rotational distributions are seen to change dramatically over this energy range, indicating that characteristically molecular behavior of the photoelectron persists far from ionization threshold. In addition, the N_2 and CO results show a strikingly different dependence on energy. 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 2σ_u →kσ_g continuum in the case of N_2 and from an f‐wave shape resonance in the 4σ→kσ channel in CO. Agreement between measured and calculated ion rotational distributions is excellent. The N_2 results are also compared with electron bombardment ionization data. This comparison demonstrates that previous interpretations of electron bombardment data are prone to errors.