Pure amorphous silicon thin films have been deposited by ion beam sputtering onto borosilicate glass substrates at ambient temperature. Optical constants were derived from spectral transmittance measurements in the range of 350-2500 nm. All the samples, regardless of thickness, show a Raman peak centred between 460 and 480 cm-1. The root-mean-square variation in the bond angles, Δθ, obtained from Raman spectra, indicates the presence of relatively good short range order in amorphous silicon thin films. The refractive index at 1.4 eV decreases from 4.35 to 3.52 with an increase in thickness from 30 to 130 nm. The band gap varies with thickness and at a thickness of 130 nm the band gap is 1.3 eV, which increases to 1.8 eV at 30 nm. The narrow band gap values of ion beam sputtered a-Si thin films indicate that they can be explored for photovoltaic applications. © 2008 IOP Publishing Ltd.