Theory of Rydberg Series in Polyatomic Molecules: H2O

The characteristic features of the Rydberg transitions in a polyatomic molecule are examined with emphasis on their extravalence-shell nature. It is made plausible that in molecular orbital perturbation theory the corresponding Hartree—Fock atomic orbital of principal quantum number n is a zero-order approx-imation to the Rydberg orbital. Calculations without integral approximations are carried out for the low-lying states of H2O. The ASP—SCF—LCAO—MO treatment of the H2O ground state provides the core. The mean singlet—triplet absorption spectrum of H2O is fairly well reproduced by the calculation. The calculated oscillator strength of B1(3s)←A1, 1×10—2, and the calculated symmetry-induced splitting of the 3p Hartree—Fock atomic orbital in C2v symmetry into B1 and A1 components, 2200 cm—1, are also in satisfactory agreement with experiment, 3×10—2 and 1500 cm—1, respectively.