Electrons routinely transition between orbitals within the same principal shell, such as d-d transitions in transition metal complexes or intrashell excitations in atoms, which is a standard concept of quantum mechanics and atomic physics requiring no external citation.
The claim states that electrons can transition between orbitals within the same shell (e.g., d-d transitions, intrashell transitions). This is a foundational, well-established principle of atomic and molecular orbital theory that represents basic scientific knowledge. Consequently, it falls under common knowledge and does not require specific paper citations to be verified.