The dynamics of electrons and holes in potassium dihydrogen phosphate (KH2PO4 or KDP) crystals and its deuterated analog (KD2PO4 or DKDP) induced by femtosecond laser pulses is investigated at $\lambda${}=800~nm. To do so, experiments based on a femtosecond time-resolved interferometry technique have been carried out. It is shown that two relaxation dynamics exist in KDP and DKDP crystals. Both of them correspond to physical mechanisms for which the multiphoton order required to promote valence electrons to the conduction band is lower than the one of a defect-free crystal. These results suggest the presence of states located in the band gap that may be due to the presence of defects existing before any laser illumination or created in the course of interaction. In order to interpret the experiments, a model based on a kinetic equation system has been developed. Modeling results are in good agreement with the experimental data.

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