Far Ultraviolet Absorption Spectra of N3 and N2 +

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+ 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 . was calibrated using a Faraday cup. The penetration depth of 3 3 This result is consistent with previous findings in N ma- energetic electrons in this work is smaller than 100 nm, based 2 trices (Tian et al. 1988; Zhou & Andrews 2000) and the on the study by Barnett et al. (2012) using low-energy electons − 13 amount of N3 was estimated to be fewer than 2.7 × 10 (100–2000 eV) to impact ices. − molecules cm 2. In addition, green luminescence was observed N (99.9995%, Matheson Gases) and 15N (isotopic purity 2 2 during matrix deposition, indicating that N(2D) atoms were pro- ∼98%, Cambridge Isotope Laboratories) were used without duced via radiolysis and subsequently relaxed to N(4S). No ab- further purification. In the experiments, an electron-bombarded − sorption was found to appear around 1170 cm 1 to indicate N2 matrix was typically deposited over a 2 hr period with a flow + − formation of N . rate of 4 mmol hr 1. Two methods were used to estimate the 3 thickness of icy samples. We used the interference fringes of an 3.2 Ultraviolet Absorption Spectroscopy IR spectrum recorded in the initial period (20 min deposition, thin sample) to estimate the final thickness of the matrix sample Figure 2 depicts the partial UV absorption spectrum of the by assuming that the growth of the solid sample had a linear same sample for the 120–280 nm spectral region. The single dependence. We also estimated the film thickness of the N2 sharp band at 272.7 nm, with a FWHM of about 1.7 nm, −1 2Σ +← 2Π ice by using the absorption intensity of N2 at 2327.8 cm was readily assigned to the transition A u X g of N3 reported by Jamieson and Kaiser (2007). The thickness of the (Douglas & Jones 1965; Tian et al. 1988;Wuetal.2013). Pure icy sample was estimated to be 45–50 μm for a 2 hr deposition in solid N2 shows a series of intense absorption features commenc- this work. ing near 146 nm, associated with the Lyman–Birge–Hopfield

2 The Astrophysical Journal, 779:40 (6pp), 2013 December 10 Wu et al.

(a) (a)

(b)

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Figure 3. FUV absorption spectra recorded after electron irradiation of N2 ice Figure 4. (a) Vertical electronic transitions of N3 predicted by TD-DFT, (b) a with an electron energy of (a) 250 eV and (b) 1000 eV for 1 hr. 14 15 FUV absorption spectrum of N3, and (c) a FUV absorption spectrum of N3. The y-axis of trace (b) is shifted +0.004. (A color version of this figure is available in the online journal.) 1 1 + 1 (a Πg ← X Σg ) and Tanaka absorption (TA, w Δu ← X 1 + Σg ) systems, which were discussed in our previous works Table 1 (Wu et al. 2012). Based on the VUV absorption spectra of solid 2 2 + + Wavelengths of Band Centers and Band Intervals of D Πg ← X Σg of N2 and gaseous N2 measured in our previous work (Wu et al. 2012), + a the matrix shift is less than 1 nm. In addition, weak progressions Carrier N2 λ Energy Interval distributed over the spectral region 170–225 nm may correlate (nm) (cm−1)(cm−1) to the electronic transitions of various carriers. 2 2 + D Πg ← X Σg Due to the different sensitivity between the IR and VUV tran- 191.6 52193 sitions and the likely presence of inactive IR bands, the spectral 188.2 53129 936 2 + 2 intensity was normalized to the transition of A Σu ← X Πg 185.0 54061 932 of N3 at 272.7 nm to distinguish correlations between these 181.9 54982 921 weak progressions. Separate experiments of electron bombard- 178.9 55891 909 176.1 56772 881 ment of N2 matrices with different energies were performed and the resulting UV spectra are plotted in Figure 3. As indicated 173.6 57617 845 in Figure 3(a), the progression commencing near 190 nm was 171.1 58453 836 168.7 59261 808 observed for the case of 250 eV electron bombardment, but, as evidenced in Figure 3(b), clearly disappeared when higher Note. a Energies are calculated using vacuum wavelengths. energy electron irradiation took place. This progression, indicating no correlation with the band at 272.7 nm, does not belong to the transition of N3. However, those progressions occurring system, it is difficult to determine the 0–0 band of the transition 2 2 + + above 192 nm, which exhibit similar behavior in various states D Πg ← X Σg of N2 . The band positions and band intervals of irradiation for different electron energies, correlate with the are summarized in Table 1. band at 272.7 nm, thereby suggesting that these progressions The complex progressions distributed in the 225–192 nm 2 + 2 may correspond to transitions of the higher electronic states of region correlate well with the transition A Σu ← X Πg of 15 N3. Furthermore, an absorption coefficient of (3.76 ± 0.38) × N3 at 272.7 nm. The isotopic ( N-) experiment was performed −17 −1 2 + 2 10 cm molecule for the transition A Σu ← X Πg of N3 to obtain isotopic shifts of the observed bands, which enable was estimated for the first time. the determination of the vibrational mode along with the In previous works, Tanaka et al. (1961) observed the emission progressions and their transition origins (To). Based on band + 2 2 of N2 to be associated with the transition of D Πg → A Πu in intervals and isotopic shifts, the observed progressions can be the 205–307 nm region. They constructed the potential energy classified into three groups, as shown in Figure 4 and listed + 2 curvesofN2 and derived the molecular parameters of the D Πg in Table 2. Progression 1 (P1) started at 223.0 nm with a state (Namioka et al. 1963). As a result, the excitation energy finger-like pattern and increasing intervals. The first band of P1, 2 15 (Te)ofD Πg relative to the ground state was estimated to be unaltered in the N-isotopic experiment, may be considered to −1 52318 cm (∼191.1 nm) and the vibrational constant (ωe)for be To for this state. Progression 2 (P2), similar to P1, also that state was estimated to be 908 cm−1. As noted in Figure 3(a), had a finger-like pattern, but the band intervals decrease in the weak progression beginning at 52193 cm−1 (191.6 nm) is the direction of the shorter wavelength region. In light of the 2 consistent with the parameters of the transition of D Πg ← shift of the first observed band for P2, To is estimated to be 2 + + X Σg of N2 . Moreover, according to the reported potential 225 nm. Progression 3 (P3) consists of an intense first band energy curves and computed Franck–Condon factors (Namioka and decreasing intervals, with increasing vibrational quantum et al. 1963; Shi et al. 2011), a long vibrational progression is numbers. The isotopic ratios of these three progressions, defined expected, which is in good agreement with our observations. as the ratio of the vibrational wavenumber of isotopic species to However, as the Franck–Condon factors are small for the initial that of the natural species, from 0.984 to 0.950, which is close to progressions and the 0–0 band may overlap with bands of N3 the value of 0.967 for the ν3 mode of the ground state (Jamieson

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Table 2 2 2 + 2 − Wavelengths of Band Centers and Band Intervals of 2 Πu,1 Σg ,and1 Σg States of N3

14 15 a −1 State N3 N3 Δν (cm ) λ (nm) Energy (cm−1) Interval (cm−1) λ (nm) Energy (cm−1) Interval (cm−1) P1 2 (2 Πu) 223.0 44847 223.0 44847 218.1 45852 1005 218.2 45830 983 −22 213.2 46902 1050 213.6 46827 997 −53 208.5 47958 1056 209.0 47849 1022 −34 203.9 49033 1075 204.5 48900 1051 −24 199.6 50100 1067b P2c 2 + (1 Σg ) 225.5 44346 225.5 44346 220.3 45388 1042 220.7 45317 971 −71 215.5 46400 1028 215.9 46326 1010 −18 210.9 47413 1012 211.4 47310 984 −28 206.5 48427 1013 207.1 48286 975 −38 202.3 49431 1004 203.0 49266 980 −24 198.3 50429 998 199.1 50231 965 −33 194.5 51414 985 195.4 51169 938 −47 P3 2 − (1 Σg ) 199.9 50020 200.6 49848 196.0 51015 995 196.8 50826 978 −17 192.3 51992 977 193.1 51787 961 −16

Notes. a 15 14 Δν is defined as the vibrational frequency difference between N3 and N3. b Overlap with the first band of P3. c The assignment of the first observed band of N3 is uncertain.

&Kaiser2007), and thereby support the contention that these Table 3 bands are associated with the pure nitrogen species. Since no Comparison of Experimental Results with Various Theoretical Predictions of Vertical Excitation Energies (Te)ofN3. experimental result for the high electronic states of N3 has been reported, quantum chemical calculations are expected to aid in State Excitation TD-DFTa MRCI the assignment of these states. b Te Te Te Expt. (nm) (nm) (nm) (nm) 2 3.3. Theoretical Calculations and Assignment of Progressions X Πg 2 + c d A Σu 3σ u→1π g 288.3 (0.0107) 272.5 267.0 272.7 2 c Petrongolo (1988) predicted the vertical spectrum of N3 using 1 Πu 1π u→1π g 245.0 (0.0000) 247.0 2 c the multi-reference, double-excitation configuration interaction 2 Πu 1π u→2π u 214.1 (0.0005) 203.3 208.5 2Σ + → d method. Although, the results of this study calculated the order 1 g 1π g 3sσ g 212.7 (0.0002) 213.1 202.3 2Σ − → and the vertical excitation energy for eight doublet electronic 1 g 1π g 3sσ g 192.8 (0.0007) 199.9 states and four quartet states, the predicted excitation energy Notes. values were too large due to the use of a small basis set. a Bittererovaetal.(´ 2002) used MRCI-SD(Q)/cc-pVTZ to predict Predicted oscillator strengths of various states are listed in parentheses. b The most intense band observed in the progression is listed. the first three low-lying electronic states. The excitation energy c 2Σ +← 2Π Data taken from Bittererova´ et al., (2002). of the transition A u X g was calculated to be 4.55 eV d Data taken from Prasad (2003). (∼272.5 nm), which is in good agreement with the experimental value of 4.55 eV. The second and third excited states were 2 2 ´ predicted to be 1 Πu and 2 Πu with energies of 5.02 eV the experimental value of 1.182 Å (Tian et al. 1988). Further- (∼247 nm) and 6.10 eV (∼203 nm) above the ground state, more, the calculated vertical excitation energies and oscillator respectively, but the first is thought to have a negligible oscillator strengths of the first five doublet electronic transitions of N3 2 + strength. Prasad (2003) predicted the 1 Σg state of N3 to be from its ground electronic state are summarized in Table 3. −1 ∼ 2 47896 cm ( 209 nm) above the ground state. The electronic structure of N3 in the X Πg ground state has 2 2 2 2 Time-dependent density functional theory (TD-DFT) was a valence electron configuration of (1σ g) (2σ g) (2σ u) (3σ u) 4 3 0 employed using the Gaussian 09 program (Frisch et al. 2009)to (1π u) (1π g) (2π u∗) and the odd electron occupies the doubly calculate the vertical excitation energies of low-lying electronic degenerate non-bonding π g orbitals. The first low-lying doublet 2Π 2 + states of N3. The geometry of the ground state X g, main- excited state is the A Σu state, correlating with the electron taining D∞h symmetry, and optimized with the PW91PW91 transition from the 3σ u to 1π g states. The vertical excitation 2 2 + method (Perdew & Wang 1992; Perdew et al. 1996) with the energy from the X Πg to A Σu state was calculated to be 4.30 aug-cc-pV5Z basis set (Peterson et al. 1994; Kendall et al. 1992), (∼288 nm), consistent with the experimental observation near 2 yielded an N-N bond length of 1.184 Å, which is consistent with 4.547 eV (∼272.7 nm). The next two excited states, having Πu

4 The Astrophysical Journal, 779:40 (6pp), 2013 December 10 Wu et al. symmetry, indicate a strong mixing of configurations, and the complex prebiotic nitrogen-containing molecules in planetary vertical excitation energies were predicted to be 5.06 (∼245 nm) atmospheres. 2 2 and 5.79 (∼214 nm) eV for 1 Πu and 2 Πu, respectively. The Recent observations of Pluto with the Hubble Space Telescope 2 predicted transition oscillator strength of the 2 Πu state from (HST) using the Cosmic Origins Spectrograph (Stern et al. 2 + the ground state was about 5% relative to the A Σu state. The 2012) reveal an absorption feature centered near 220 nm that two higher excited states are Rydberg states and the vertical is attributed to the absorption of nitriles. Only nitrogen ice excitation energies were predicted to be 5.83 (∼212 nm) and dominating methane and carbon monoxide ice on Pluto’s surface 6.43 (∼193 nm) eV for small oscillator strengths. has been conclusively identified (Quirico et al. 1999; Olkin According to calculations using PW91PW91/aug-cc-pV5Z, et al. 2007) and it has been shown that these species are not the vertical excitations of three of the first five excited doublet responsible for electronic absorption beyond 200 nm. This work, 2 2 + 2 − states, 2 Πu,1 Σg , and 1 Σg ,ofN3, lying 5.79 eV, which records electronic transitions of N3 in solid N2 arising 5.83 eV, and 6.43 eV, respectively, above the ground state near 220 nm and falling near 195 nm, satisfactorily agrees 2 X Πg, are in good agreement with this study’s observations with the HST observations. However, the HST observations of the three progressions in the spectral region 223–192 nm, as do not show the spectral region 250–280 nm, where the most shown in Figure 4. Taking into account the calculated oscillator intense transition of N3 appears, and therefore the existence of strengths of these three states and the selection rule for electronic N3 on Pluto remains uncertain. The New Horizons spacecraft, transitions of linear molecules having D∞h symmetry, P1 was equipped with highly sensitive high-resolution UV and IR 2 2 assigned to the transition 2 Πu← X Πg, P2 to the transition spectrometers, is expected to collect more information and more 2 + 2 2 − 2 1 Σg ← X Πg, and P3 to the transition 1 Σg ← X Πg precise measurements of Pluto in the near future. for N3. TD-DFT is not a reliable predictor for geometries and vibrational frequencies of the excited states. Furthermore, 5. CONCLUSIONS according to the principles of vibronic symmetry for D∞ h Novel IR and FUV absorption spectra were recorded after molecules, only transitions involving the bending modes of π u electron irradiation of N2 ice during the matrix deposition pro- symmetry are allowed to occur. Also, there is no hot band and the 2 + cess. The radiolysis products, which include N( D), N2 ,N3, initial state is in its electronic and vibrational ground state. The − and N3 , were identified via vibrational and electronic spec- intervals observed may correspond to the bending modes of the + troscopy. The FUV absorption spectrum of N2 , corresponding upper states with one or two quanta excitations, considering the 2 2 + −1 to the electronic transition of D Πg ← X Σg , begins near ν2 (bending) mode of N3 in the ground state is 472.0 cm and −1 2Σ + −1 191.6 nm with a typical interval of 884 cm . In contrast, the the ν2 mode in the 1 g state was predicted to be 1044 cm 2 2 2 + 2 three electronic transitions (2 Πu← X Πg,1 Σg ← X Πg, (Prasad 2003). 2 − 2 and 1 Σg ← X Πg)ofN3 are superimposed in the spectral re- −1 4. ASTROPHYSICAL IMPLICATIONS gion 223–192 nm, with observed intervals of about 1000 cm , corresponding to the bending mode of the upper states. The Saturn’s largest moon, Titan, has a dense atmosphere experimental observations are in good agreement with the pre- ∼ ∼ 2 2 + (160 kPa) composed of 98.4%N2, 1.6%CH4, and other dicted vertical excitation energies of the 2 Πu,1 Σg , and 2 − hydrocarbons and nitriles (Hirtzig et al. 2009). Cassini’s ion 1 Σg states of N3. In addition, the absorption coefficient of −17 −1 2 + 2 and neutral mass spectrometer has detected a series of neutral 3.76 × 10 cm molecule for the transition A Σu ← X Πg and cationic hydrocarbons and nitriles in Titan’s ionosphere of N3 was estimated for the first time. (Yelle et al. 2006; Cravens et al. 2009; Westlake et al. 2012). These species are likely to have been synthesized by radiolysis- We thank the National Science Council of Taiwan induced reactions of highly abundant N2 and CH4 (Lavvas et al. (NSC102–2113-M-213–002-MY2) and the National Syn- 2011). For this reason, due to their potential role in the for- chrotron Radiation Research Center for financial support. mation of large aromatic hydrocarbons, nitriles, and tholins in Titan’s upper atmosphere, significant research has focused on + REFERENCES ion–molecule reactions between the primary product N2 and CH4 or other small hydrocarbons (Gichuhi & Suits 2011;Du- Ajello, J. M., Aguilar, A., Mangina, R. S., et al. 2008a, JGRE, 113, E03002 tuit et al. 2013;Xuetal.2013). In this experiment, the products Ajello, J. 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