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The isolated vacancy in silicon has been studied with magnetic resonance spectroscopy. The EPR spectrum labeled Si-G2, identified as arising from the negative charge state of the vacancy, has been investigated by electron-nuclear double resonance. Hyperfine interactions between the unpaired defect electron and Si nuclei were determined for 51 shells of surrounding atoms. These shells contain 152 lattice sites. They can be divided into four different symmetry classes. From a linear combination of atomic orbitals (LCAO) analysis of the hyperfine interactions together with the division in classes, we found that the defect wave function is primarily localized in one mirror plane of the vacancy. In this plane it could especially be assigned to lattice sites on a particular (011) lattice chain. This one-dimensional character of the defect confirms the preference for charge transfer along (011) chains which was found in theoretical calculations. This picture leads to the identification of hyperfine interactions with atoms in the chain and tentatively even in a side chain. The very small localization on the other mirror plane of the vacancy is in agreement with a one-electron defect-molecule description which predicts it to be a nodal plane of the wave function. The remaining small localization allows an estimate of the importance of many-electron effects. Because of this small localization, small discrepancies of the LCAO description become prominent in this plane. In a number of cases dipole-dipole interaction with spin density on nearby lattice sites can explain the observed hyperfine interactions. Also exchange polarization effects have to be considered there.The positive charge state of the vacancy has been studied with EPR only. Incomplete hyperfine data for three shells of lattice sites are reported.

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