Longitudinal spin relaxation time of electrons photodetached from rubidium anion (Rb-) in tetrahydrofuran (THF) solution was found to depend on the excitation wavelength (lambda = 532, 640, and 720 nm). Spin relaxation rate vs photon energy exhibits a parabolic dependence, which corresponds to the conduction band profile of THF. This unique behavior is attributed to the spin-polarized photoelectrons interacting with rubidium atoms (Rb.) generated in the same photoprocess. Analysis of the results suggests that the photoelectrons exhibit high mobility, an essential feature for spin injection through interfaces.