Photochemical reactions of gaseous ethylene in femtosecond laser filaments are studied. It is shown that hydrogenated amorphous carbon (a-C:H) films and nanoparticles are produced by the strong interactions with the intense laser fields (ca. 10(13)Wcm(-2)). The laser field intensity is found to be an important parameter to determine the sp(2)/sp(3) carbon ratio of the films and the size and number of nanoparticles.