The Astrophysical Journal, 779:40 (6pp), 2013 December 10 doi:10.1088/0004-637X/779/1/40 C 2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A. + FAR ULTRAVIOLET ABSORPTION SPECTRA OF N3 AND N2 GENERATED BY ELECTRONS IMPACTING GASEOUS N2 Yu-Jong Wu1, Hui-Fen Chen2, Shiang-Jiun Chuang1, and Tzu-Ping Huang1 1 National Synchrotron Radiation Research Center, No. 101, Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan; [email protected] 2 Department of Medical and Applied Chemistry, Kaohsiung Medical University, 100, Shih-Chuan 1st Road, Kaohsiung 80708, Taiwan Received 2013 August 9; accepted 2013 September 27; published 2013 November 25 ABSTRACT + Electron bombardment of gaseous N2 produces N2 and N3, which are subsequently trapped in the N2 matrix at 10 K. Both the infrared and ultraviolet absorption spectra of the matrix sample at various stages of electron irradiation were recorded. Apart from a progression observed below 192 nm, with intervals ∼900 cm−1, corresponding to 2 2 + + the transition of D Πg ← X Σg of N2 , three new progressions were recorded in the range 225–192 nm, with −1 intervals ∼1000 cm , that correlated well with variations in intensities of the electronic absorption band of N3 −17 −1 2 + 2 at 272.7 nm; an absorption coefficient of 3.76 × 10 cm molecule for the transition A Σu ← X Πg of N3 was estimated for the first time. These newly observed progressions were characterized and the vertical excitation energy and oscillator strength were calculated using time-dependent, density-functional theory. This was based on 2 2 + 2 − assigning the three progressions to electronic transitions of N3 from the ground state to 2 Πu,1 Σg , and 1 Σg , respectively. Key words: astrochemistry – ISM: molecules – methods: laboratory: molecular – molecular processes – planets and satellites: atmospheres Online-only material: color figure 1. INTRODUCTION electrons at 25 eV and 100 eV, recorded the emission spectra covering the spectral range 330–1100 nm, and observed emis- 3 + 3 + 3 3 + In our solar system, molecular nitrogen (N2) is abundant sion bands that include B Πg → A Σg and C Πu → B Πg 2 + 2 + 2 2 + + in the atmospheres of the Earth, Titan, Triton, and Pluto and of N2, and B Σu → X Σg and A Πu → X Σg of N2 ,as + dominant also in the surface ice of several trans-Neptunian well as line emissions of N (NI) and N (NII). Moore & Hudson objects (Stern 2010; Tegler et al. 2010). This motivates the (2003) irradiated N2 ice with 0.8 MeV protons and also with a study of the excitation of N2, which is crucial for understanding vacuum ultraviolet (VUV) lamp, consequently discovering that the related nitrogen chemistry of these planetary atmospheres N3 radicals only formed when the ice was bombarded with pro- and icy surfaces. For instance, Saturn’s largest moon, Titan, tons. Bombardment of N2 ice with 5 keV electrons (Jamieson 2+ has a dense atmosphere (160 kPa) composed of ∼98.4%N2, &Kaiser2007) and with 60 keV Ar ions (Baratta et al. 2003) ∼1.6%CH4, and other trace gases (Hirtzig et al. 2009). Apart also produced N3.Wuetal.(2012) investigated the photolysis of from N2 and CH4, species identified on Titan include C2H2, N2 ice at various wavelengths (λ<130 nm) using synchrotron C2H4,C2H6,C4H2,C6H6,C6N2,C2N2,HCN,andHC3N radiation and found N3 to be the only product. (Hirtzig et al. 2009). When these trace molecules are present In previous works, the effect of secondary electrons that were in a sufficiently large amount, they form aerosol hazes in generated by irradiating pure N2 ice with 500 eV electrons were Titan’s atmosphere (Liang et al. 2007), but the composition investigated, and the resulting infrared (IR) and UV spectra + and the formation mechanism of these organic layers are poorly clearly indicate the formation of N3 and N3 (Wu et al. 2013). In understood. These reactions are likely initiated by highly elec- this present work, gaseous N2 is first bombarded with 250 eV or tronically excited N2 and CH4 and their fragments and the exci- 1000 eV electrons, followed by condensation onto the cold target tation energy may possibly derive from solar ultraviolet (UV), to form N2 ice. The IR absorption spectra of the icy sample were solar wind, or Galactic cosmic rays. Cassini recorded far-UV then recorded and the radiolysis products were identified. The (FUV) emission from the thermosphere of Titan at an altitude UV absorption spectra of the same icy sample was consequently of 900–1400 km (Ajello et al. 2008a, 2008b) using the Ul- recorded and compared with recent astronomic observations of traviolet Imaging Spectrograph (UVIS), which revealed emis- Titan and Pluto in the UV spectral region (Stern et al. 2012; 1 1 + sions from the transition a Πg → X Σg of N2; the density Shemansky et al. 2005). 9 of N2 at altitudes around 1000 km was in the range 5 × 10 to 1.7 × 1010 cm−3. Likewise, the Ballistic Missile Defense 2. EXPERIMENTS Organization’s Midcourse Space Experiment detected the flu- + orescence of N2 at a very high altitude (Romick et al. 1999) The experimental setup was similar to that described in over Earth’s north polar cap. These observations of the for- previous literature (Bahou et al. 2012, 2013) for the purpose of mation of activated nitrogen-species in the nitrogen-dominated measuring the spectra of icy samples irradiated with energetic atmospheres of planets clearly indicate that excitation and de- electrons. A nickel-plated copper plate was utilized as a cold excitation of N2 play important roles in planetary atmospheric substrate for icy samples. The substrate was cooled to 10 K using chemistry. a closed-cycle helium refrigerator system (ARS DE-204), and There have been extensive laboratory investigations of a turbomolecular pump, backed with a scroll pump, provided gaseous and solid N2 treated with high-energy photons and a cryo-chamber vacuum with a base pressure of less than 1 × − particles. Mangina et al. (2011) bombarded gaseous N2 with 10 8 Torr. 1 The Astrophysical Journal, 779:40 (6pp), 2013 December 10 Wu et al. (a) (b) Figure 1. IR absorption spectra of N2 ice subjected to electron bombardment (a) during deposition and (b) solid N2 subjected to an electron energy of 250 eV Figure 2. UV absorption spectrum of N2 ice subjected to electron irradiation and a current of 200 μAfor1hr. during deposition for 1 hr with an electron energy of 250 eV and a current of 200 μA. The IR absorption spectra were recorded with a Fourier- 3. RESULTS AND DISCUSSION transform infrared spectrometer (Bruker, Vertex 80) equipped with a KBr beamsplitter and a Hg–Cd–Te detector (cooled to 3.1. Infrared Characterization 77 K), covering the spectral range of 500–4000 cm−1.The Figure 1 compares the partial IR spectra recorded after the copper substrate also served as a mirror to reflect the incident IR − electron bombardment of gaseous N during deposition with that beam to the detector. Four hundred scans at resolution 0.5 cm 1 2 for solid N2. According to the deposition process, the absorption were typically recorded for each stage of the experiments. To − − features of N were observed at 1657.6 cm 1 and 1652.4 cm 1, measure the far-UV (FUV) spectra of the icy samples covering 3 which correspond to the asymmetric stretching (ν ) mode of N the spectral region 110–350 nm, UV light was dispersed using a 3 3 (Jamieson & Kaiser 2007). The absorption coefficient for the 6 m monochromator on the high-flux beam line of the 1.5 GeV − − ν mode of N , calculated to be 4.0 × 10 17 cm molecule 1, storage ring at the National Synchrotron Radiation Research 3 3 − was used (Jamieson & Kaiser 2007) to estimate the amount Center. The flux was in the range 1 to 4 × 1012 photons s 1 in the of N generated for the two different methods. The amount spectral region 300–110 nm, when a resolving power 1000 was 3 of N3 generated during matrix deposition was calculated to applied. The wavelength was calibrated with absorption lines of − be 3.0 × 1015 molecules cm 2, which is approximately one N ,CO,O, and NO. Spectral positions were measured with a 2 2 order of magnitude greater than that generated through electron spectral resolution of 0.1 nm and the accuracy of wavelength is bombardment of solid N . The amount of N increased with limited by the scan step, 0.1 nm in this work. 2 3 deposition time. The largest amount of N3 generated in this An ice sample was deposited onto a target maintained at − experiment reached 5.0 × 1015 molecules cm 2 and was 10 K. UV light reflected from the target was detected by a produced by electron irradiation for a 2 hr deposition period. photomultiplier tube (Hamamatsu R6836) using an amplifier In contrast, in the case of electron irradiation of solid N , (Hamamatsu C7246). An electron gun (Kimball Physics, Model 2 the resulting N radicals were destroyed by further electron EFG-7) was utilized to generate electron beams of energy 3 irradiation, causing N3 production to reach a steady state. 250 eV and 1000 eV and a beam current of 200 μA, for − Apart from N3, the formation of N3 was also confirmed the bombardment of gaseous N2 during the matrix deposition −1 14 −1 by observing weak peaks at 2003.3 and 2005.7 cm , which process. The electron flux, which was ∼7 × 10 electrons s , − correspond to the asymmetric stretching (ν ) mode of N .