Quantum Mechanical Potential Energy Surfaces and State Couplings for Photodissociation and Collision-Induced Dissociation Reactions: New Methods and Applications

Abstract

Potential energy surfaces (PESs) play essential roles in the study of chemical dynamics. The adiabatic ground-state PES of N4 was constructed with permutationally invariant polynomials and is suitable for treating high-energy vibrational-rotational energy transfer and collision-induced dissociation in N2-N2 collisions. Our adiabatic PES reproduces the ab initio data well but adiabatic surface fitting is not designed to reproduce the cuspidal ridges at state crossings. This motivates working with the diabatic representation for the photodissociation where state crossing seams are key global features. Diabatic representations are also very convenient for fitting state couplings. A new diabatization scheme based on complete-active-space self-consistent-field diabatic molecular orbitals and the fourfold way was proposed to obtain smooth diabatic potentials and couplings at the multi-configurational quasi-degenerate perturbation theory level of electronic structure theory. The new scheme has been used to study the photodissociation of phenol in which three electronic states are involved. A new method for fitting global potential energy surfaces of multi-dimensional reactive systems was developed and is called the anchor points reactive potential (APRP) scheme. The full-dimensional 3 x 3 matrix of diabatic potential energy surfaces and couplings for the nonadiabatic photodissociation of phenol was constructed with the APRP method. Multidimensional tunneling calculations through the barrier on the shoulder of the conical intersection of the S1 and S2 states of phenol suggest the adiabatic nature of the early dynamics of phenol photodissociation and the importance of tunneling in the photodissociation.