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.