Ponderomotive threshold shift and phase control in multiphoton detachment of negative ions

Recently an excitation scheme has been proposed for observing interference effects caused by simultaneous action of a two-colour low-frequency field w ith commensurate frequencies during the photodetachment of a negative ion. In this letter, we find out the physical conditions that give rise to the n arrow peaks in the electron energy spectra when the photoelectrons absorb f rom the low frequency field an amount of energy almost equal to the pondero motive energy. We use this information to determine the ponderomotive thres hold shift of photodetachment in a highly nonlinear regime.