The initial yield of a hydrated electron (eaq(-)) in a solution under laser pulse irradiation was investigated by pump-probe transient absorption spectroscopy. The initial quantum yield of eaq(-) varies with the concentration of uridine monophosphate (UMP). The variation of the concentration of eaq(-) is often used to study the prehydrated electron (e(pre)(-)) and eaq(-) attachment to UMP. The results of 320 and 260 nm excitations were compared. It was found that with the increase of UMP concentration, the initial yield of eaq(-) increases at 320 nm excitation, but decreases at 260 nm excitation. The further analysis indicates that some of the e(pre)(-) attachments to UMP before solvation at 260 nm excitation result in the decrease of the eaq(-) yield. In addition, the absorption of UMP to 260 nm also causes the decrease of the eSaq(- ) yield. After the excitation at 320 nm, the phosphate group of UMP can release electrons more easily than that of water molecules by two-photon absorption, and therefore the eaq(-) yield increases. With the increase of UMP concentration, the decay rate of eaq(-) increases because eaq(-) is captured by UMP. The change of excitation photon does not affect the reaction rate of eaq(-) attachment to UMP. The longer lifetime of eaq(-) obtained at 260 nm excitation than 320 nm excitation is induced by the larger eaq(-) escape probability at 260 nm excitation. Our results show that the femtosecond pulse pump-probe transient absorption spectroscopy method should be cautiously used because of its complexity in studying the eaq(- )attachment to nucleotides in an aqueous solution.