Arsenic ions were implanted into silicon-on-insulator (SOI) structures at an incident energy of 100 keV to a dose of 2 x 10(15) cm(-2). Conductive top layers were formed in the SOI structures after annealing at 1200 degrees C for 20 s. Infrared reflection spectra in the wave number range of 1500-5000 cm(-1) were measured and interference fringes, related to free-carrier plasma effects, were observed. By detailed theoretical analysis and computer simulation of infrared reflection spectra, the carrier concentration, the carrier mobility, and the carrier activation efficiency were obtained. The physical interpretation of the results and a critical discussion of the sensitivity of the data, fitted to variation in the parameters, are given.