The present status of infrared synchrotron radiation (IRSR) at UVSOR-II and the application to an infrared reflection-absorption spectroscopy (IRAS) in the IR and terahertz (THz) regions using IRSR are reported. At UVSOR-II, the IRSR beam line has been upgraded to make a powerful IR-THz light source. An IRAS experiment covering the wave number range below 1,000 cm⁻¹ was started to investigate adsorbed molecules on surfaces. The Alq₃/Ag system was investigated using the apparatus. The Al-N stretching mode at around 420 cm⁻¹ was clearly observed and the peak frequency changes to 440 cm⁻¹ after the potassium doping. This is the evidence for the charge transfer to the Alq₃ molecule from the potassium ion.