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

A quantal description of nuclear motion using coupled fifteen-state potential energy and spin-orbit coupling surfaces for studying the photodissociation of CHOO to HCO(XA) + OD and HCO(XA) + OP 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, (BA', CA'), (aA', bA') and (aA'', bA''), 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 CHOO 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 aA' state and the onset of the BA' ← XA' spectral band. Furthermore, sizable SOC between the BA' and aA'' states mixes them intersystem crossing. Due to the vibronic interaction between the aA'' and aA' states, the molecule finally relaxes to the aA' state and is dissociated to HCO(XA) and OP products.