We report an experimental and theoretical investigation of electron-impact single ionization of the highest occupied molecular orbital 1t₂ and the next highest occupied molecular orbital 2a₁ states of CH₄ at an incident electron energy of 250 eV. Triple differential cross sections measured in two different laboratories were compared with results calculated within the molecular 3-body distorted wave and generalized Sturmian function theoretical models. For ionization of the 1t₂ state, the binary peak was observed to have a single maximum near the momentum transfer direction that evolved into a double peak for increasing projectile scattering angles, as has been seen for ionization of atomic p-states. A detailed investigation of this evolution was performed. As expected because of its s-type character, for ionization of the 2a₁ state, only a single binary peak was observed. Overall, good agreement was found between experiment and theory.

The double-bound analog of the double-continuum three-body Coulomb (C3) wave function, recently introduced by the authors, allows us to propose two different basis sets of correlated functions which can be used for describing double-bound states. The components of the two basis diagonalize the Coulomb interactions but differ in the definition of the distortion factor which depends only on the interelectronic coordinate. In the first set, this factor includes a nonlinear parameter, while the second set contains parameter-free functions. Through an angular correlated configuration interaction method it is possible to generate highly correlated wave functions for $S$ bound states of heliumlike systems. By construction, these states form an orthogonal set, satisfy all two-body cusp conditions, and retain the C3 double-continuum character. Even when a limited number of configurations is included, rather good energy values for the ground and excited states are obtained. A systematic improvement by inclusion of more configurations can be easily achieved, in particular with the computationally efficient, parameter-free basis.