We have demonstrated the existence of longitudinal- and transverse-like optical modes of Si–N bond in vibrational absorption spectrum of hydrogenated amorphous silicon nitride thin films. One of the longitudinal-like optical resonances coincides with the transverse-like mode of Si–O bond, and the other closely neighbors the bending mode of N–H bond. We have also shown that the conventionally assigned asymmetric stretching mode of Si–N bond is merely a transverse-like mode of the bond. The microstructures of both longitudinal- and transverse-like modes can well be apprehended by a p-polarized beam at an oblique incidence light, especially at Brewster angle incidence. The spectrum distortion induced by interference fringes can be eliminated at this condition. © 1998 American Institute of Physics.