A potential energy curve for the (A) over tilde＇ (2)Pi(g,1/2) state of I-2(-) is constructed based on femtosecond photoelectron spectroscopy of the I-2(-) (A) over tilde＇ 2 Pi(g,1/2) <-- (X) over tilde (2)Sigma(u)(+) transition at 780 nm. The experiment is sensitive to the slope of the repulsive potential wall, the well depth, equilibrium bond length, and the long- range attractive portion of the upper state potential. The (A) over tilde＇ (2)Pi(g,1/2) potential is fit to a piecewise potential which is flexible in each of these regions. Simulations of the spectrum using a previously determined Morse potential for the (X) over tilde (2)Sigma(u)(+) state of I-2(-) [J. Chem. Phys. 107, 7613 (1997)] yields a well depth (D-e) of 0.017 +/- 0.010 eV for the (A) over tilde＇ (2)Pi(g,1/2) state with an equilibrium bond length (R-e) of 6.2 +/- 0.6 Angstrom. These values differ significantly from previous semiempirical results.