We have measured the optical-absorption properties of plasma-enhanced chemically vapor deposited films of hydrogenated amorphous silicon nitride (a-SiNx:H) over the energy range from 0.54 to 2 eV using photothermal deflection spectroscopy (PDS). Typical PDS spectra show a broad absorption tail with a large number of narrow absorption peaks. Using the phase sensitive feature of this spectroscopy we are able to separate nitride-related absorption from that arising from the fused quartz substrates. In addition to the broad, featureless band tail, whose magnitude is quite sensitive to optical excitation with light above ∼3.9 eV, we observe a number of narrow absorption peaks which are most likely overtones of localized vibrational modes. Specifically, we see peaks at 0.82 and 1.20 eV which we identify as the first and second harmonics of N-H stretching vibrations, and several other peaks whose origins remain undetermined. Prolonged UV excitation produces no detectable change in these vibrational modes, although large changes are seen in the underlying absorption tail. This reinforces previous suggestions that photoinduced changes in these films do not arise from rearrangement of the bonded hydrogen. © 1992 The American Physical Society.