Optical absorption spectra in the low-energy region 0.4-1.2 eV is reported for muc-Si:H using a photothermal deflection spectroscopy technique. Absorption coefficient spectra in the low-energy region contain important information related to defects and hydrogen. It is demonstrated that there is a good correlation between electron spin densities and integrated absorption coefficient spectra from 0.7 to 1.2 eV. The amount of the hydrogen molecules in microvoids is much larger in muc-Si:H than that in a-Si:H. Light illumination effects in PDS spectra has also been studied from a view point of photo degradation of the muc-Si:H. (C) 2001 Elsevier Science B.V. All rights reserved.