Change of Initial Yield of a Hydrated Electron with Uridine Monophosphate Concentration Is Related to the Excitation Photon Energy in Transient Absorption Spectroscopy

Abstract

The initial yield of a hydrated electron (e_aq^-) in a solution under laser pulse irradiation was investigated by pump-probe transient absorption spectroscopy. The initial quantum yield of e_aq^- varies with the concentration of uridine monophosphate (UMP). The variation of the concentration of e_aq^- is often used to study the prehydrated electron (e_pre^-) and e_aq^- 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 e_aq^- 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 e_aq^- yield. In addition, the absorption of UMP to 260 nm also causes the decrease of the e_aq^- 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 e_aq^- yield increases. With the increase of UMP concentration, the decay rate of e_aq^- increases because e_aq^- is captured by UMP. The change of excitation photon does not affect the reaction rate of e_aq^- attachment to UMP. The longer lifetime of e_aq^- obtained at 260 nm excitation than 320 nm excitation is induced by the larger e_aq^- 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 e_pre^- attachment to nucleotides in an aqueous solution.