Photoluminescence spectra from the Si impurity center in diamond films grown by chemical‐vapor deposition techniques were studied at temperatures between 9 and 300 K. Laser excitation at 514.5 nm, and resonant with the zero‐phonon absorption line at 737 nm (1.6823 eV), were used. Luminescence lines become narrowed at resonance excitation, and the vibrational structure of the Si center was observed with a major phonon replica at 767 nm (1.6165 eV). The observed vibrational energy of 515 cm−1supports a diatomic quasi‐molecular Si2center structure. Temperature dependencies of the linewidth of the zero‐phonon line and its phonon replica were measured and the origins of the broadening are discussed. ©1995 American Institute of Physics.