Photodetachment microscopy to an excited spectral term and the electron affinity of phosphorus

A beam of P− ions produced by a cesium sputtering ion source was photodetached in the presence of an electric field, with a single-mode ring dye laser. Neutral P can be produced at one or the other of the fine-structure sub-levels of its 3s23p3 2Do excited term. This is the first atomic photodetachment microscopy experiment with excitation of the parent neutral atom out of the fundamental spectral term. The background electron signal due to ground-state photodetachment notwithstanding, photodetachment microscopy images produced at the excited thresholds could be analysed to provide a measure of these excited-term thresholds with interferometric precision. Starting from the three possible fine-structure sub-levels of P− 3s23p4 3P, the five fine-structure thresholds that may be detected, taking the selection rules into account, have been measured. They are combined with the spectroscopic data available in the literature on neutral P to produce an improved experimental value of the electron affinity A of phosphorus: 602 179(8) m−1 or 0.746 607(10) eV. Taking all covariances of the optimized energy levels into account, one can merge them with the former measure of the three lowest detachment thresholds of P−, which results in a slightly more precise value of A(P): 602 181(8) m−1, or 0.746 609(9) eV. The accuracy of A(P) is now essentially limited by the uncertainty on the D3/2 and D5/2 energy levels of the neutral atom. The fine-structure intervals of the 3s23p3 2Do doublet of the neutral atom and of the 3s23p4 3P triplet of the negative ion have their accuracy improved by more than one order of magnitude. (Some figures in this article are in colour only in the electronic version)