LAUSR

Significance N2 is present as ice in cold outer space. In this work, photochemical experiments with concurrent detection of infrared absorption and visible emission spectra provide evidence for the generation of energetic products N 2D atom and l-N3 radical after photolysis of icy N2 with vacuum-ultraviolet radiation. We found that the photolysis of icy N2 may also produce the long-lived N 2Dlong atom which can be triggered to release emission or to induce the synthesis of l-N3 upon raising of the temperature. This work enhances our understanding of the transformation of nitrogen species in cold astroenvironments; the results also open a window into photochemical processes in solid state. 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 (2Dlong) 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 (2Dlong) 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.