Nitriding of a tool steel was carried out with an electron-beam-excited plasma (EBEP). EBEP is sustained with energetic electron beams over the pressure range of 10(-3)-10(1) Pa by electron-impact ionization. Samples whose temperatures were controlled by electric radiant heater were exposed to EBER A nitrided layer of 100 mu m and a surface hardness of 1000 HV(0.1) were achieved for tool steel SKD61 (JIS) at 800 K and a treatment time of 3 h. In order to measure the density of nitrogen atoms in EBEP, a vacuum ultraviolet absorption spectroscopy system was used. It was found that the density of nitrogen atoms increased from loll to 10(12) cm(-3) linearly with an increase of electron beam current from 2 to 20 A. (c) 2006 American Vacuum Society.