Approximate Wave Functions for the U Center by the Point-Ion-Lattice Method

Wave functions for the ground state and the first excited state of the U center are calculated in the point-ion-lattice approximation, and the term values are obtained. The transition energy and the oscillator strengths are computed, and the former are compared with experimental data. They are approximately 20% smaller than the observed transition energies. A qualitative argument is presented, suggesting that this discrepancy can be removed by inclusion of the exchange energy.