A CASSCF/MR-CI Study toward the Understanding of Wavelength-Dependent and Geometrically Memorized Photodissociation of Formic Acid

The S0, T1, and S1 potential energy surfaces for the HCOOH dissociation and isomerization processes have been mapped with different ab initio methods. The wavelength-dependent mechanism for the HCOOH dissociation was elucidated through the computed potential energy surfaces and the surface crossing points. The HCOOH molecules in S1 by excitation at 248 nm mainly decay to the ground state via the S0 and S1 vibronic interaction, followed by molecular eliminations in the ground state. The S1 direct dissociation to HCO(2A‘) + OH(2Π) is the dominant pathway upon photoexcitation at 240−210 nm. Meanwhile, there is a slight probability that the system relaxes to the ground state via the S0 and S1 vibronic interaction at these wavelengths. After irradiation of HCOOH at 193 nm, the S1 direct dissociation into HCO(2A‘) + OH(2Π) is energetically the most favorable pathway. In view of high IC efficiency at the S0/S1 conical crossing, the S1 → S0 internal conversion via the S0/S1 point can occur with considerable efficiency. In addition, the S1 isomerization probably plays a dominant role in the partially conformational memory of the HCOOH photodissociation, which has been discussed in detail.