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

Abstract

We present the first measurements of ion rotational distributions for photoionization over an extended range [0≤EK≤200 eV for N2 (2σ−1u) and 3≤EK≤125 eV for CO (4σ−1)]. The N2 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 N2 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 2σu→kσg continuum in the case of N2 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 N2 results are also compared with electron bombardment ionization data. This comparison demonstrates that previous interpretations of electron bombardment data are prone to errors.