Amorphous carbon nitride (CNx) thin films are recognized to have potential for applications like hard coating and electron field emission device. In this paper we have optically studied the dynamics of the plasma plume during preparation of CNx thin films by KrF excimer laser (λ=248 nm) ablation of pure graphite target in a pure NH₃ gas. It was found that carbon atoms emitted from the target react with ambient gas. CN and N₂ molecules are formed in the gas phase. We have prepared the CNx thin films at various NH₃ gas pressure and laser fluence on Si (100) and quartz substrates. The N/C ratios of the CNx films depended on the ambient NH₃ gas pressure and laser fluence. We obtained the maximum N/C ratio of 0.62 at NH₃ pressure of 100 mTorr. The typical absorption of CN bonds such as sp² C-N, sp³ C-N, G band and D band were detected from the infrared absorption measurement by FTIR for the deposited CNx thin films.