The T-line luminescence system is created in Si by annealing at 400-60
0 degrees C. Shifts and splitting of the spectral features with C-13 a
nd D isotope substitution identify the presence of two C atoms and one
H atom in the center. Uniaxial stress and magnetic field measurements
show that the T center has monoclinic I symmetry and possesses an acc
eptor (-/0) level at 0.2 eV below the conduction band. Ab initio clust
er calculations lead to a structure in which an interstitial C-H defec
t binds with a substitutional C atom. The calculated vibrational modes
are in good agreement with those observed.