Fully quantal description of combined internal conversion and intersystem crossing processes in the smallest Criegee...

A quantal description of nuclear motion using coupled fifteen-state potential energy and spin-orbit coupling surfaces for studying the photodissociation of CH₂OO to H₂CO(X¹A₁) + O¹D and H₂CO(X¹A₁) + O³P channels is presented. For the evaluation of surfaces, multireference electronic wave functions are employed. For the fully quantal description of the nuclear motion, we diabatize the PESs of the two and four lowest excited singlet and triplet states, respectively, within the three sets of vibronically coupled states, i.e. (B¹A', C¹A'), (a³A', b³A') and (a³A'', b³A''), employing the diabatization by ansatz method. This yields three different adiabatic-to-diabatic mixing angles, which are used for the diabatization of the spin-orbit coupling surfaces and allow to investigate simultaneously the internal conversion and intersystem crossing processes in CH₂OO using a nuclear quantum dynamical approach for the first time. Our calculation predicts the presence of a weak spectral band with irregular and discrete structures, which originates from the role of spin-orbit couplings. This band of the spectrum is mainly located between the minimum energy of the a³A' state and the onset of the B¹A' ← X¹A' spectral band. Furthermore, sizable SOC between the B¹A' and a³A'' states mixes them via intersystem crossing. Due to the vibronic interaction between the a³A'' and a³A' states, the molecule finally relaxes to the a³A' state and is dissociated to H₂CO(X¹A₁) and O³P products.