We present the first measurements of ion rotational distributions for photoionization over an extended range ͓0рE K р200 eV for N 2 ͑2 u Ϫ1 ͒ and 3рE K р125 eV for CO ͑4 Ϫ1 ͔͒. 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 2 u →k g continuum in the case of N 2 and from an f -wave shape resonance in the 4 →k 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.

%e present the first measurements of rotational distributions for photoionization over extended energy ranges [0~Et,~200 eV for N2 (2a"') and of 3~E t,~l25 eV for CO (4o ')]. 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 (kas) in the case of N2 and from an f wave shape resonance for 4' ' photoionization in CO.

We present results of experiment and theory for the alignment of CO+(B2Z+) and N2+(B2&+) photoions over an extended energy range (0 I E k 1. 210 eV for CO and 0 1. Ek I 250 eV for N2). The polarization of CO+(B2Z+-X2Z+) and N2+(B2&+-X2Z,+) fluorescence is used to interpret the oscillator strength distributions for normally unresolved degenerate ionization channels. The results show the influence of a CO 40 -ka shape resonance clearly and agreement between theory and experiment is excellent. However, agreement between the calculated and measured values is less satisfactory for N2. This behavior is somewhat surprising, as previous rotationally resolved fluorescence experiments have shown excellent agreement between theory and experiment. This comparison helps to illustrate the complementarity of alignment studies relative to alternative probes of ionization. For both N2 and CO, the data indicate that the photoions retain significant alignment even at high energies. The results demonstrate that even well above threshold the spectral dependence of the alignment $e., polarization) is very sensitive to the molecular environment for photoejection. Such behavior provides useful insight into fundamental scattering phenomena in chemical physics.

We present results of measurements and calculations of the alignment for CO+(B 2Σ+) photoions over an extended energy range (0≤Ek≤210 eV). The polarization of CO+(B 2Σ+→X 2Σ+) fluorescence indicates that the photoions retain significant alignment even at high energies. Agreement between the measured and calculated polarization of the fluorescence is excellent.