Ab initio study of the potential energy surface of the B 1A1 excited state of H2O

A large part of the potential energy surface of the B̃ 1A1 (1A′) second excited state of the H2O molecule for dissociation into OH+H has been calculated using the SCF orbitals of the corresponding triplet state as the basis for a limited CI calculation. The form of the calculated surface is dominated by a conical intersection with the ground state surface in the linear configuration. Cross sections of the surface also show either a small potential maximum or a point of inflexion. These features appear to be related to the rapid transformation of the highest occupied orbital from a Rydberg orbital to an antibonding orbital. A cross section of the surface leading to O+H2 has also been calculated. This shows a very high (5 eV) potential barrier owing to an avoided crossing. The dissociation of H2O in the B̃ 1A1 state to give the OH (2Σ+) fragment with a highly abnormal rotational distribution is discussed in light of the calculated surface.