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

Abstract

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.

Keywords: atomic N (2D); azide radical; icy N2; photochemistry; vacuum-UV radiation.

Fig. 1.

(A) Scheme of energetics and transitions for excitation of icy N₂ at 121.6 nm, 10.20 eV. (B) Scheme of energetics and generation l-N₃ for excitation of icy N₂ at 121.6 nm. (C) Scheme of N (²P) and N (²D) atoms relax to their lower states N (²D) and N (⁴S), respectively, through emissions. (D) During thermal ramping (marked with symbols ▲), scheme of conversion and reaction of N (²D_long) atoms.

Fig. 2.

Experimental procedure related to the photon current of irradiation, temporal profiles of the N α-line and temperature of the N₂ ice. The graphic trend in A shows photon currents (logarithmic scale) of irradiated light; the scatters show the time line of recording various spectra, in which the triangle(▼), circle (●) and star(★) indicate when recording spectra IR, iEm, and Em, respectively. B shows temporal profiles of the height of the atomic N α-line monitored at 522.6 nm; both temporal profiles of iEm-1 and iEm-4 are flat because of saturation of the charge-coupled device (CCD) detector. C shows the variation of temperature of the N₂ ice.

Fig. 3.

Emission spectra during irradiation at 121.6 nm. A and B are spectra of the 1st (recorded at 157 min) and 45th (recorded at 194 min) measured spectra in series spectra of iEm-1; both are labeled as iEm-1.1” and “iEm-1.45,” respectively. C indicates the 7th spectrum (recorded at 607 min) in series spectra of iEm-4; it is labeled as “iEm-4.7”. The notations “N₂ VK” and “N₂ VK, 2nd” represent the first- and second-order lines of N₂ VK transition (26). The notations N (α) and N (α′) indicate lines of (0,0) and (0,1) from transitions N (²D → ⁴S), whereas N (δ) and N (δ″) indicate lines of (0,0) and (n,n-1) from transitions N (²P →²D) (26). The notation O (β) represents lines due to O (¹S → ¹D) from impurity O₂.

Fig. 4.

Profiles of emissions near N α-line region for spectra iEm-# and Em-#. The iEm-# were the 1st of serial emission spectra, in which iEM-1 to iEM-5 were recorded at 155, 222, 497, 600, and 637 min, respectively. Em-# were averaged spectra from serial emission spectra, in which the 1st spectra of Em-1 to Em-4 were recorded at 354, 412, 456, and 550 min, respectively.

Fig. 5.

IR absorption spectra. The labels are those used in Fig. 1; the direct times of IR-1 to IR-8 were recorded at 142, 208, 300, 495, 540, 590, 630, and 710 min, respectively. The IR-1 is the spectrum of precursor N₂ before irradiation, the IR-2 is the spectrum after irradiated 121.6 nm, the IR-3 is the spectrum after irradiated 190 nm, the IR-4 is the spectrum after thermal ramping, the IR-5 is the spectrum after irradiated 190 nm again, the IR-6 is the spectrum after thermal ramping again, the IR-7 is the spectrum after irradiated 121.6 nm again, and the IR-8 is the spectrum after irradiated 190 nm finally. The area ratios are among IR-2: IR-4: IR-5: IR-6: IR-7: IR-8 = 33: 7.8: 2: 3: 22: 1.