A high-density N-2 plasma was generated after mode jump by a helical antenna surrounded by magnetic coils. In the wavelength region of 600-800 nm. the emission lints from the N-2 first positive system and that from N atoms (3p --> 3s) were also observed, in addition to the emission lines from the N-2 second positive and N-2(+) first negative systems in the ultraviolet and violet regions. At the mode jump, the emission peak intensity from N atoms especially increased. Nitridation of GaAs was performed in the N-2 plasma before and after mode jump. N 1s peak intensities from the surface layer nitrided in the N-2 plasma after mode jump measured by X-ray photoelectron spectroscopy were 2-5 times larger than those nitrided before mode jump. which indicates the enhancement of the nitridation. From the spectroscopic ellipsometry, the thickness of nitrided layer was 3-11nm.