NEAR-THRESHOLD INFRARED PHOTODETACHMENT OF AL- - A DETERMINATION OF THE ELECTRON-AFFINITY OF ALUMINUM 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 i on-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 W igner threshold law and to the zero-core-contribution theory of photod etachment. The electron affinity of aluminum was determined to be 0.44 094(+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 w ith experiment within a few percent for photon energies within 3% of t hreshold. A proposed leading correction to the Wigner law is discussed .