Femtosecond stimulated emission pumping in combination with femtosecond photoelectron spectroscopy is used to characterize the potential energy function of the I-2(-)((X) over tilde (2)Sigma(u)(+)) ground state up to vibrational energies within 2% of the dissociation limit. The frequency and anharmonicity of this state are measured at a series of vibrational energies up to 0.993 eV by coherently populating a superposition of ground state vibrational levels using femtosecond stimulated emission pumping, and monitoring the resulting wave packet oscillations with femtosecond photoelectron spectroscopy. The dissociative I-2(-)((A) over tilde' (2)Pi(g,1/2)) state is used for intermediate population transfer, allowing efficient population transfer to all ground state levels. Using the measured frequencies and anharmonicities, the (X) over tilde (2)Sigma(u)(+) state has been fit to a modified Morse potential with the beta-parameter expanded in a Taylor series, and the bond length, well depth, and upsilon=0-1 fundamental frequency set equal to our previously determined Morse potential [J. Chem. Phys. 107, 7613 (1997)]. At high vibrational energies, the modified potential deviates significantly from the previously determined potential.