Ammonia and a mixture of nitrogen and hydrogen are used for material processing. Since these gases are chemically active, the processing efficiency is enhanced. This article describes a study to understand the physical properties of ammonia and a mixture of nitrogen and hydrogen plasmas inside and outside an arcjet generator. Spectroscopic measurement is made, and several plasma properties are determined from the data. The result shows that the H-atom electronic excitation temperature and the nitrogen rotational excitation temperature decreased from 7000-11000 K in the constrictor to about 4000 K and to 1000-1500 K, respectively, on the nozzle exit with mass flow rates of 0.1-0.2 g/s at input powers of 7-12 kW. However, the NH rotational excitation temperature did not show a significant axial decrease even in the downstream plume. Paper includes a German-language abstract.