The nitrogen plasma generated by microwave or electrical discharge in gaseous N-2 was quenched on a cold window (6.2-35 K) and resulting matrix was examined by IR and UV-Vis absorption spectroscopies. In samples prepared with N-14(2) we observe a weak infrared transition at 936.7 cm(-1). It shifts to 900.0 cm(-1) when N-15(2) is used. Both peaks do not correlate with any other features in the spectra and are best explained as originating from tetrahedral tetrazetes (N-4). Their positions are compatible with quantum chemical estimates for N-14(4), at 936.0 cm(-1) and N-15(4), at 904.4 cm(-1).