The optical and morphological properties of hydrogenated carbon nitride thin films obtained by ArF excimer laser photodeposition (lambda = 193 nm) from different ammonia/acetylene mixtures were investigated. The optical properties of the films depend on the nitrogen content. The band gap deduced from measurements of the optical absorption coefficient in the UV-VIS spectrum is found to be superior to 2.3 eV. Film thicknesses and refraction indexes were investigated by m-lines spectroscopy The refraction index varies from 1.66 to 1.72. The microstructure of the deposited films as revealed by scanning and transmission electron microscopy is discussed as a function of the deposition parameters. 2000 Elsevier Science B.V. All rights reserved.