Key roles of conical intersections in the photolysis of phosphine and diphosphine

The photochemical reactions of phosphine (PH3) and diphosphine (P2H4) play important roles in interstellar chemical evolution. Herein, the multistate complete active space second-order perturbation theory has been employed to investigate the photolysis mechanisms of PH3 and P2H4 in gas phase. Our results show that upon light irradiation, the PH3 molecule splits to form the PH2 and H radicals, with PH2 dimerizing to P2H4. The excited-state P2H4 either isomerizes to PH3 and PH or undergoes dehydrogenation to yield P2H3 and H radicals involving an unusual three-state crossing point (S1/T2/T1)x. Importantly, we found that the isomerization is governed by an extended (S1/S0)x conical intersection seam, first observed in phosphorus-containing compounds. These mechanisms explain the early experimental observation about the changing trend of P2H4 yield in the photolysis of PH3 [Ferris and Benson, Nature 285, 156-157 (1980)], advance phosphorus hydride photochemistry, and provide a theoretical framework for interstellar phosphorus molecule evolution.