  We propose two possible quantum microscopic mechanisms for intervalley
transitions just at the Franck-Condon state in GaAs. The first is the
simultaneously combined coulombic elastic transitions of two electrons,from the
originally photo-excited $\Gamma$ valley to the mutually opposite L and -L
valleys in the Brillouin zone. The second is the "elastic" scattering of an
electron from the $\Gamma$ valley to the L one by the frozen phonon, which is
not newly created in the excited state, but already exists, being inherited
from the starting ground state, according to the Franck-Condon principle of the
photo-excitation. Surprisingly, we can theoretically show that such the elastic
electron-phonon scattering gives extremely fast time constant of the order of a
few tens femtoseconds, as well as the above elastic electron-electron
scattering does.
