Picosecond Pulse Radiolysis Study on the Radiation-Induced Reactions in Neat Tributyl Phosphate

The ultrafast radiolytic behavior of tributyl phosphate, TBP, has been investigated using 7 ps electron pulses with 7 MeV kinetic energy, from which two key species have been observed and characterized: the TBP solvated electron (e_TBP^-) and the TBP triplet excited state TBP* (³a) or its fragmentation products. The e_TBP^- exhibits a broad absorption band in the visible and near-infrared (NIR) spectrum, with a maximum beyond our 1500 nm detection limit. Nitromethane was used to scavenge e_TBP^- to confirm its absorption spectrum and to determine its associated rate coefficient, 1.0 × 10¹⁰ M^-1 s^-1. The electron's molar extinction coefficients were found by an isosbestic method using biphenyl as a solvated electron scavenger. The time-dependent radiolytic yield of e_TBP^- was also determined directly from 7 ps to 7 ns and compared with those in water, tetrahydrofuran, and diethyl carbonate. In less than 10 ns, the decay is not due to the reaction with other solvent molecules and is instead predominantly due to the reactions with cations issued from the proton transfer by the TBP radical cation (TBP^•+). In addition to e_TBP^-, another absorption band, stable up to 7 ns, was identified in the visible range. This has been attributed mainly to the TBP triplet excited state, TBP*(³a), by a combination of molecular modeling methodologies. Interestingly, we did not observe any absorption band in the visible nor in the NIR range arising from TBP^•+. Calculations suggest that TBP^•+ undergoes rapid proton transfer to yield a UV-absorbing species, TBP(-H⁺). Experimental results and supporting molecular simulations provide detailed identification of the earliest species yielded from the radiolysis of neat TBP.