Energy Surface and Generalized Oscillator Strength of the A″1 Rydberg State of H2O

The energies and wavefunctions of the lowest singlet and triplet Rybderg states in H₂O were calculated in the expansion basis self‐consistent‐field procedure for a single configuration. A portion of the energy surface of the ${}^1\mathrm{A\prime }$ Rydberg state for an HOH angle of 105° was determined that shows the asymmetric dissociation of this state into ground‐state $\mathrm{H}{(}^2\mathrm{S)}$ and OH(²Π) fragments. The energy along the reaction coordinate is almost separable into a repulsion depending only on the center‐of‐mass separation and an attractive potential that depends only on the OH internuclear separation. For the $C_{\text{2υ}}$ conformation a large basis set was used to approach the Hartree–Fock limit. Using these functions a minimum was calculated in the generalized oscillator strength for the ${}^1{A}_1{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{B}_1$ transition as a function of the momentum transfer function or the electron scattering angle. This behavior was then confirmed experimentally for an energy loss of 7.4 eV with an incident electron energy of 500 eV. Such a minimum is a general characteristic of Rydberg transitions and is not an essentially molecular phenomena. The good agreement between experiment and theory indicates a reasonable fit to the Rydberg wavefunction by the approximate Hartree–Fock calculation.