Rotationally resolved photoelectron spectroscopy of the 2Σ− Rydberg states of OH: The role of Cooper minima

Abstract

We have measured rotationally resolved photoelectron spectra of the OH radical using (2+1) resonance enhanced multiphoton ionization spectroscopy via the D ²Σ⁻(3pσ) and 3 ²Σ⁻(4sσ) Rydberg states. For the D ²Σ⁻(3pσ) state, we observe primarily ΔN=even distributions of ionic rotational states, in contrast to the ΔN=odd distribution expected for ionization of a 3pσ Rydberg electron. The observations are described quantitatively by ab initio calculations which predict a Cooper minimum in the 3pσ→kπ(l=2) channel, whose occurrence determines the ΔN=even ion rotational distribution. In contrast, the 3 ²Σ⁻(4sσ) photoelectron spectra reveal a broad distribution in rotational levels, arising from greater l mixing in the higher Rydberg orbital and much weaker Cooper minima in the continuum.