The investigation on quantum radar requires accurate computation of the state vectors of the single-photon processes of the two-level system in free space.However,the traditional Weisskopf-Wigner (W-W) theory fails to deal with those processes other than spontaneous emission.To solve this problem,we provide a new method based on the renormalization theory.We evaluate the renormalized time-ordered Green functions associated with the single-photon processes,and relate them to the corresponding state vectors.It is found that the ultraviolet divergences generated by the Lamb shift and higher-order interactions can be systematically subtracted in the state vectors.The discussions on spontaneous emission and single-photon absorption are then presented to illustrate the proposed method.For spontaneous emission,we obtain the same results of the W-W theory.For single-photon absorption where W-W theory fails,we find that the two-level electric dipole first gets excited rapidly and then decays exponentially,and that the efficiency of the single-photon absorption declines as the bandwidth of the incident photon becomes narrow.The proposed method can improve the investigation on quantum radar.