Near-threshold infrared photodetachment of Al-: A determination of the electron affinity of aluminium and the range of validity of the Wigner law

The relative photodetachment cross section of Al- has been measured in the wavelength range 2420-2820 nm (0.440-0.512 eV), using a coaxial ion-laser beams apparatus, in which a 2.98-keV Al- beam is merged with a beam from an F-center laser. The cross-section data near the P-3(0,1,2)-->P-2(1/2,3/2) photodetachment threshold have been fitted to the Wigner threshold law and to the zero-core-contribution theory of photodetachment. The electron affinity of aluminum was determined to be 0.44094(+0.00066/-0.00048) eV, after correcting the experimental threshold for unresolved fine structure in the ground states of Al- and Al. The new measurement is in agreement with the best previous measurement (0.441+/-0.010 eV) and is 20 times more precise. The Wigner law agrees with experiment within a few percent for photon energies within 3% of threshold. A proposed leading correction to the Wigner law is discussed.