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 less than or equal to E(K) l ess than or equal to 200 eV for N-2 (2 sigma(u)(-1)) and 3 less than o r equal to E(K) less than or equal to 125 eV for CO (4 sigma(-1))]. Th e N-2 ion rotational distributions are seen to change dramatically ove r this energy range, indicating that characteristically molecular beha vior of the photoelectron persists far from ionization threshold. In a ddition, 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 behav ior arises from the presence of Cooper minima in the 2 sigma(u)-->k si gma(g) continuum in the case of N-2 and from an f-wave shape resonance in the 4 sigma-->k sigma channel in CO. Agreement between measured an d calculated ion rotational distributions is excellent. The N-2 result s are also compared with electron bombardment ionization data. This co mparison demonstrates that previous interpretations of electron bombar dment data are prone to errors. (C) 1995 American Institute of Physics .