Photoion Alignment: Chemical Signatures 200 eV above Threshold

Abstract

We present results of experiment and theory for the alignment of CO^+(B^2Σ^+) and N_2^+(B^2Σ^+_u) photoions over an extended energy range (0 ≤ E_k ≤ 210 eV for CO and 0 ≤ E_k ≤ 250 eV for N_2). The polarization of CO^+(B^2Σ^+ → X^2Σ^+) and N_2^+(B^2Σ^+ → X^2Σ^+_g) fluorescence is used to interpret the oscillator strength distributions for normally unresolved degenerate ionization channels. The results show the influence of a CO 4σ → kσ shape resonance clearly and agreement between theory and experiment is excellent. However, agreement between the calculated and measured values is less satisfactory for N_2. 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 N_2 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 (i.e., polarization) is very sensitive to the molecular environment for photoejection. Such behavior provides useful insight into fundamental scattering phenomena in chemical physics.