Carbon-doped silicon irradiated with electrons at room temperature and subsequently implanted with protons has been studied by infrared absorption spectroscopy. Isochronal annealing in the temperature range from 400 degrees C to 800 degrees C has revealed an absorption line at 2967 cm(-1). When protons are substituted by deuterons, the line shifts down in frequency to 2211 cm(-1) and co-implantation of protons and deuterons gives rise to an additional line at 2218 cm(-1) Isotope shifts and similar annealing characteristics of the three lines show that the 2967 cm(-1) line represents a local vibrational mode of a defect, which contains two equivalent hydrogen atoms. In samples co-doped with C-12 and C-13, an additional mode is observed at 2963 cm(-1), which is about halfway between the modes involving only C-12 or C-13. From the relative intensities of the isotope-shifted modes in silicon co-doped with C-12 and C-13, it is concluded that the center contains two equivalent carbon atoms. The center is tentatively identified as two equivalent hydrogen atoms bound to two neighboring carbon atoms at substitutional sites. (C) 1999 Elsevier Science B.V. All rights reserved.