3P005
Ultraviolet Absorption Spectra of N3 in Solid Nitrogen(NSRRC, Taiwan1, Caltech, USA2)
Yu-Jong Wu,1 Shiang-Jiun Chiang,1 Tzu-Ping Huang1, and Wei-Chen-Chen2[Introduction] Molecular nitrogen (N2) is known to be abundant in theatmospheres of Earth, Titan, Triton, and Pluto in the solar system and also has beenfound to dominate as surface ices on several trans‐Neptunian objects (TNOs), such as
Triton, Pluto, and Eris. The study of excitation of N2 is particularly important inunderstanding the related nitrogen chemistry of these planetary atmospheres andicy surfaces. In our previous work, we have investigated the effect of secondary
electrons generated by irradiation of pure N2 ice with 500‐eV electrons and therecorded IR and UV spectra clearly showed the formations of N3 and N3+ [1]. In thepresent work, gaseous N2 was bombarded with 250 or 1000‐eV electrons and thenfollowed by condensation onto the cold target to form a N2 ice. The IR absorption
spectra of the icy sample were recorded to identify the radiolysis products and theUV absorption spectra of the same matrix sample were subsequently recorded forcomparison with the recent astronomic observations of Titan and Pluto in the UV
spectral region.
[Experiment] A nickel‐plated copper flat cooled to 10 K served as a cold substrate
for matrix samples. The substrate was cooled with a ARS DE‐204 closed‐cycle helium
refrigerator system and a turbomolecular pump provided the cryo‐chamber vacuum,
which was backed with a scroll pump, typically yielding a pressure of less than 1 ×
10‐8 Torr. The IR absorption spectra were recorded with a Fourier‐transform infrared(FTIR) spectrometer (Bruker, Vertex 80) equipped with a KBr beamsplitter and aHg‐Cd‐Te detector (cooled to 77 K), covering the spectral range of 500–4000 cm−1.
For measurement of far‐UV spectra of matrix samples covering the spectral region
110‐350 nm, the UV light was dispersed with a 6‐m monochromator on the high‐flux
beam line at National Synchrotron Radiation Research Center (NSRRC). A spectral
resolution of 0.1 nm was used with an accuracy of ±0.1 nm in the measurements ofspectral positions. An electron gun (Kimball Physics, Model EFG‐7) generated
electron beams with energy of 250 and 1000 eV and beam current of 200 µA forelectron bombardment of gaseous N2 during matrix deposition.
[Result and Discussion] The IR spectra recorded after electron bombardment ofgaseous N2 during deposition show the absorption features of N3 at 1657.6 and1652.4 cm‐1 assigned to the asymmetric stretching (ν3) mode of N3 and weak peaks
at 2003.3 (2005.7) cm‐1 corresponding to the asymmetric stretching (ν3) mode of N3–.
In addition, green luminescence was observed during matrix deposition indicating
the N(2D) atoms were produced via radiolysis and subsequently relaxed to N(4S). We
found no absorption appearing around 1170 cm‐1 relating to the formation of N3+.
The UV absorption spectrum of the same sample shows the single sharp band at272.7 nm is readily assigned to the transition A 2Σu+← X 2Пg of N3 [2]. The weak
progression appearing around 190 nm with an average interval 884 cm‐1 is attributed
to the transition of D 2Πg ← X 2Σg+ of N2+ [3]. The complex progressions distributing inthe 225–192 nm region correlated well to the transition A 2Σu+← X 2Пg of N3 at 272.7
nm. Based on the isotopic (15N‐)
experiment, the observed progressions
might be classified to three groups asshown in Figure 1. We employedtime‐dependent density functional
theory (TD‐DFT) to calculate the verticalexcitation energies of low‐lying
electronic states of N3 with thePW91PW91/aug‐cc‐pV5Z. According tothe TD‐DFT calculations of verticalexcitations of the three of the first fiveexcited doublet states, 2 2Πu, 1 2Σg+, and 1 2Σg–, of N3 lying in 5.79, 5.83, and 6.43 eVabove the ground state X 2Πg, respectively; are in a good agreement with ourobservations of three progressions in the spectral region 223–192 nm. Also
considering vibronic symmetry of D∞h molecules, only the transitions involving thebending modes of πu symmetry are allowed. The observed intervals might
correspond to the bending modes of the upper states with one or two quanta
excitations, since the ν2 (bending) mode of N3 in the ground state is determined
472.0 cm‐1 experimentally and the ν2 mode in the state 1 2Σg+ was predicted to be1044 cm‐1 by MRCI‐SD. The calculations with MRCI‐SD(Q) method are performed forfurther information of the upper states of N3 [4].
[Reference][1] Wu, Y.-J.; Chen, H.-F.; Chuang, S.-J.; Huang, T.-P. Astrophys. J. 2013, 768, 83.
[2] Tian, R.; Facelli, J. C.; Michl, J. J. Phys. Chem. 1988, 92, 4073.
[3] Tanaka, Y.; Namioka, T.; Jursa, A. S. Can. J. Phys. 1961, 39, 1138.
[4] Wu, Y.-J.; Chen, H.-F.; Chuang, S.-J.; Huang, T.-P.; Chen, W.-C. 2013 submitted