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We report the obsewation of orthorhombic Au-Li and Pi-Li pairs in Si using EPR and ENDOR techniques and also MCDA spectroscopy. The EPR spectra alone could be mistaken as being due to orthorhombic isolated point defects and ENDOR is required to detect the Li partner of the pair. Comparison of the MCDA spectra with conventional infrared absorption shows that the optical spectra generally associated with the isolated substitutional Au point defect are excited from a paramagnetic ground state. We show that the interpretation of the Si-NL50 centre as the isolated substitutional Au; point defect is unlikely in view of our results.

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