Thermal reaction and luminescence of long-lived N ²D in N₂ ice

Photochemistry of an N₂ ice and thermal reaction of the irradiated sample were studied with vacuum-ultraviolet (VUV) light from a synchrotron. Concurrent detection of infrared absorption and visible emission spectra provide evidence for the generation of energetic products N (²D) and N (²P) atoms, N₂ (A) molecule and linear-N₃ (l-N₃) radical after excitation of icy N₂ at 121.6 nm. Irradiation at 190 nm is shown to be an effective way to eliminate the l-N₃ radical. After the photolysis and photoelimination of the l-N₃, we initiate synthesis of l-N₃ via the thermal ramping of the sample in temperature range 3.5 to 20 K. In addition, the emission from the N (²D) atom was observed during the thermal ramping process. These behaviors indicate that a long-lived N (²D_long) atom is generated in the VUV-photolyzed N₂ ice. A comparison of the variations of the visible emission of N (²D) and the infrared absorption of l-N₃ with time indicates that the long-lived N (²D_long) dominated the thermal synthesis of l-N₃ The results have enhanced suggestion and understanding of the conversion for nitrogen species in cold astrophysical environments with VUV irradiation.