Collision-induced and photoinduced electronically diabatic transitions in polyatomic systems are described, starting with an eikonal representation of the molecular wave function and developing a self-consistent eikonal approximation for short deBroglie wavelengths. The approach provides state-to-state transition amplitudes for electronic excitation without requiring any preliminary knowledge of the nature of transitions between potential energy surfaces. The formalism has some similarities to recent semiclassical treatments using the initial value representation. It has been applied to the electronically diabatic dissociation CH3I + phi --> CH3 + I induced by absorption of UV light using previously introduced potential energy surfaces and couplings to compare with accurate quantal results. Results for the model are given for branching ratios in the formation of I and I*, and for the final distribution of vibrational states of CH3 for two light wavelengths.