Dye-laser photodetachment studies of Au−, Pt−, PtN−, and Ag−

Photodetachment of Au−, Pt−, PtN−, and Ag− ions is studied in a crossed-beam experiment, in which a 2 keV negative ion beam is intersected by a pulsed tunable dye laser (Δ λ ≈ 1–2 Å). The threshold behavior of the photodetachment cross section σ for Au− and Pt− is found to agree with Wigner's Law σ ∝ k2L+1 (k: momentum of outgoing electron with orbital angular momentum L) over about 50 meV above threshold for this case, in which L=1. The electron affinity of Au is (2.3086 ± 0.0007) eV, that of Pt (2.128 ± 0.002) eV, whereas the one for Ag is 1.78 eV and very probably ≲ 1.5 eV. The PtN− cross section exhibits sharp peaks due to single-photon processes; these features are ascribed either to autodetachment or predissociation of excited long-lived (τ ≳ 5 × 10−13 sec) PtN−* molecules; the present experiment measures the total cross section for the production of neutral atoms (or molecules) and therefore cannot distinguish the two possibilities. Further, saturation experiments are described in which the laser flux is so high as to saturate partially the photodetachment. This method is applied to the case of Pt−, for which the absolute cross section of an excited component Pt−* in the beam is measured and thereby its fractional population is determined.