Ab initio calculation of potential energy surfaces for the three lowest triplet states (1 3A′′,1 3A,2 3A′′) of PH(X,A)–He

Quantum chemical ab initio calculations at the complete active space SCF level and with inclusion of correlation effects have been performed for the potential energy surfaces of PH in its X 3Σ− ground state and its first excited triplet state, A 3Π, colliding with He atoms. The PH distance was fixed at its experimental value (of the A 3Π state), the PH–He distance and the HePH angle were varied. All three potential energy surfaces [1 3A′′ for PH(X)–He and 1 3A,2 3A′′ for the two components of PH(A)–He] are purely repulsive, except for very shallow van der Waals minima with well depths of about 15–40 cm−1. The interaction potentials decay approximately exponentially with increasing PH–He distance and show large angular anisotropies. Legendre expansions for the angular dependence of the potential surfaces converge slowly for V(1 3A′′) and the sum potential 1/2[V(2 3A′′)+V(1 3A)], but rapidly for the corresponding difference potential 1/2[V(2 3A′′)−V(1 3A)]. The present PH(A)–He potentials have been used in the companion paper by Neitsch et al. [J. Chem. Phys. 106, 7642 (1997)], for the calculation of thermal state-to-state rate constants for inelastic PH(A)–He collisions.