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 (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 (epre-) 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 epre- 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 eaq- 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 epre- attachment to nucleotides in an aqueous solution.