A HIGH-RESOLUTION VACUUM ULTRAVIOLET LASER PHOTOIONIZATION AND PHOTOELECTRON STUDY OF THE CO ATOM

Abstract

ABSTRACT We have measured the vacuum ultraviolet–photoionization efficiency (VUV–PIE) spectrum of Co in the energy range of 63,500–67,000 cm−1, which covers the photoionization transitions of Co(3d74s2 4F9/2) Co+(3d8 3F4), Co(3d74s2 4F7/2) Co+(3d8 3F3), Co(3d74s2 4F9/2) Co+(3d8 3F3), Co(3d74s2 4F9/2) Co+(3d8 3F2), and Co(3d74s2 4F9/2) Co+(3d74s1 5F5). We have also recorded the pulsed field ionization photoelectron spectrum of Co in the same energy range, allowing accurate determinations of ionization energies (IEs) for the photoionization transitions from the Co(3d74s2 4F9/2) ground neutral state to the Co+(3F J ) (J = 4 and 3) and Co+(5F5) ionic states, as well as from the Co(3d74s2 4F7/2) excited neural state to the Co+(3d8 3F3) ionic state. The high-resolution nature of the VUV laser used has allowed the observation of many well-resolved autoionizing resonances in the VUV–PIE spectrum, among which an autoionizing Rydberg series, 3d74s1(5F5)np (n = 19–38), converging to the Co+(3d74s1 5F5) ionic state from the Co(3d74s2 4F9/2) ground neutral state is identified. The fact that no discernible step-like structures are present at these ionization thresholds in the VUV–PIE spectrum indicates that direct photoionization of Co is minor compared to autoionization in this energy range. The IE values, the autoionizing Rydberg series, and the photoionization cross sections obtained in this experiment are valuable for understanding the VUV opacity and abundance measurement of the Co atom in stars and solar atmospheres, as well as for benchmarking the theoretical results calculated in the Opacity Project and the IRON Project, and thus are of relevance to astrophysics.