An EPR spectrum produced in aluminum-doped silicon by 1.5-MeV electron irradiation is described. Labeled Si G9, it is identified as arising from an aluminum-vacancy pair, presumably formed when a mobile lattice vacancy is trapped by substitutional aluminum. The resonance is observed only upon illumination and is identified as a long-lived excited triplet ($S=1\mathrm{}$) state of the defect. The observed hyperfine interactions with ${\mathrm{Al}}^{27}$ and neighboring ${\mathrm{Si}}^{29}$ nuclei, as well as the $g$ tensor and axial fine-structure term $D$, are discussed in terms of a simple model of the defect using a linear combination of atomic orbitals. No Jahn-Teller distortion is observed in this excited state, as is consistent with the predictions of the model. Preferential alignment of the aluminum-vacancy axis direction in the lattice is achieved by stressing the crystal at ∼200°C. The magnitude and sense of the alignment is consistent with the prediction of the model that the ground state is a Jahn-Teller distorted state similar to the phosphorus-vacancy pair previously studied. Abnormally strong $\Delta m\ne 0$ nuclear hyperfine transitions are observed in the EPR spectrum, and the theory of this effect, found only in an even-spin system, is developed. Emission is observed for some of the lines in the spectrum similar to that reported by Tanimoto et al. for another photoexcited $S=1\mathrm{}$ system. The origin of this effect is discussed in terms of the model.

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