Secondary ions produced by 400 eV He+ ion impact on N-2 and O-2 thin films deposited on a silicon substrate at 8 K were measured as a function of film thickness using a time-of-flight secondary-ion mass spectrometer. While the N-n(+) ions with n up to 4 were observed for N-2 film, formation of larger cluster ions O-n(+) (n up to 10) were observed for O-2 film. The secondary ion intensities for N-2 reached the plateau with film thickness of about 20 monolayers, whereas those for O-2 with film thickness of about 400 monolayers. The observed marked difference in the film-thickness dependence between N-2 and O-2 films is discussed on the viewpoint of relaxation of electronic energy deposited in the solid films by the 400 eV He+ ion impact.