The neutral vacancy V0 gives an optical spectra labelled GRl and the n
egative vacancy V- a spectra labelled ND1. Between these are lines lab
elled GR2-8 associated with both V0 and V-. GR2-8 relate to bound exci
ton states coupled via strong electron-electron correlation into multi
plet molecular states of V0. Optical intensities associated with GR2-8
relate more to V- than V0.