THE EMISSION, LIFETIMES, AND FORMATION THRESHOLD OF THE VEGARD–KAPLAN TRANSITION OF SOLID NITROGEN EXPOSED TO FAR-ULTRAVIOLET RADIATION

The flyby of Pluto for the New Horizons attracted public attention in 2015 (Stern et al. 2015); the mission of this spacecraft is to explore the cold boundary of our solar system in an unprecedented manner. When the spacecraft passed Pluto, it confirmed that its surface is covered with nitrogen ice (Grundy et al. 2016). N₂ exists in a condensed phase in cold outer space (Cruikshank et al. 1984); it has been detected also on Charon, Eris, Triton, and in interstellar clouds (Maret et al. 2006; Tegler et al. 2010). In cold space, chemical reactions from solid molecules might be initiated with various energetic species including far-ultraviolet (FUV) photons. To understand the evolution of nitrogen-bearing species to an amino acid in a cold astronomical environment (Dutuit et al. 2013), the photochemistry of solid nitrogen induced with FUV light is an important issue that warrants detailed development.

In 1924, emission from solid nitrogen was investigated by Vegard (1924) and by McLennan & Shrum (1924). This fascinating subject has since been studied with various excitations, including 50 kV X-rays, 700–5000 V cathode rays, 4–10 keV ions and 1–10 keV electrons (Brocklehurst & Pimentel 1962; Tinti & Robinson 1968; Lofthus & Krupenie 1977; Oehler et al. 1977; Schou 1987), but information about the emission of solid nitrogen excited with FUV radiation is lacking. To remedy this lack, we investigated the emission of solid nitrogen upon excitation with FUV light from a synchrotron source.

The apparatus for the study of the photochemisty of solid molecules using FUV light from a synchrotron undulator was described previously (Lin et al. 2014; Lo et al. 2014, 2015b). A closed-cycle refrigerator (Janis RDK-415) cooled the KBr substrate to ∼4 K within a high-vacuum chamber evacuated to <4 μPa. A gaseous sample was deposited on a KBr window, before its irradiation at a selected wavelength tunable from an undulator on beamline 21A2 attached to the synchrotron in Taiwan Light Source (TLS) at National Synchrotron Radiation Research Center (NSRRC). The TLS provided light source as trains of pulses with a width of about 40 ps and a repetition rate of 500 MHz, the interval between the pulses was 2 ns; by this means, the FUV excitation light was close to a semi-continuous wave in this experiment. The mean flux of photons was ∼1 × 10¹⁵ photons s⁻¹ (bandwidth 2%) at the selected wavelength; the beam image was nearly rectangular with cross section of about 3.5 × 5 mm².

On irradiation of a sample with light in the wavelength range of 120–220 nm, the emission spectra in region 200–450 nm were recorded concurrently with or after excitation. The dispersed emission was measured with a charge-coupled detector (CCD, 1024 × 256 pixels, Horiba Symphony II) cooled with liquid nitrogen and used in image mode; the response time of the CCD was set at 50 μs. For the present work, the entrance slit was set at a width of 0.5 mm; the grating (2400 l mm⁻¹) produced a resolution of about 0.2 nm. IR absorption spectra were recorded with an interferometric spectrometer (Bomem DA8) equipped with a KBr beamsplitter and an MCT detector (cooled to 77 K) to span the spectral range of 500–4000 cm⁻¹; typically, spectra with 400 scans and a resolution of 0.5 cm⁻¹ were performed.

Gaseous nitrogen (Matheson, purity 99.999%), CH₄ (Matheson, nominal purity 99.999%) and C₂H₄ (Matheson, nominal purity 99.99%) were used without purification.

Figure 1 displays the emission spectra obtained with steps of 0.03 nm in the wavelength region 200–440 nm after excitation at 138.7 nm. During excitation with FUV light, the solid nitrogen sample emitted a bright luminescence, which diminished on a timescale of minutes following irradiation.

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The emission lines observed belong to the Vegard–Kaplan (VK) system of N₂, $A{}^{3}{{\rm{\Sigma }}}_{u}^{+}\to X{}^{1}{{\rm{\Sigma }}}_{g}^{+}$ (Tinti & Robinson 1968). Vegard first observed this system in the wavelength region 170–670 nm from the emission of solid nitrogen (Vegard 1924); subsequently, Kaplan reported the system of single-headed bands in the gaseous nitrogen (Kaplan 1934). Since then, numerous authors have observed the VK system of the N₂, the review of the spectroscopic data can be found in the literature (Lofthus & Krupenie 1977). Table 1 summarizes the recorded lines and their assignments based on previous works. The VK system of nitrogen involves the transition from triplet to singlet; therefore, the emission yield is low. In this work, two vibronic progressions, (0, ν'') and (1, ν''), of the VK system were recorded for solid nitrogen; the intensity of the former is about 50–75 times that of the latter. Twelve lines of the vibronic progression (0, ν'') for solid nitrogen were observed, in which the first and last lines are (0, 1) at 212.29 nm and (0, 12) at 427.76 nm, with red shifts within 1.49 ∼ 6.2 nm, from the gaseous transitions (Lofthus & Krupenie 1977). The (0, 0) line of the VK system is at 200.985 nm in the gaseous phase; however, we did not record this line in solid nitrogen. In this progression, the highest vibrational state is from the (0,7) transition at 296.73 nm (with a width 1.24 nm), for which the Franck-Condon factor is the most favorable. Whereas, the weakest one is from the (0, 1) line at 212.29 nm; the intensity of the former is about three orders higher than the latter. For the weaker vibronic progression (1, ν''), five lines (1, 7) to (1, 11) were recorded at 284.87, 302.70, 323.33, 346.79, and 372.69 nm, redshifted 2.9 ∼ 4.36 nm from the corresponding transitions in a gaseous sample.

After being irradiated with FUV light, the solid nitrogen sample still emitted for a significant period; in this work, we recorded 21 spectra separated consecutively by 1.167 s (the integrating time of the signal 1 s, the readout time of the CCD 0.05 s, and the saving time of data in the PC 0.117 s). For demonstration, the plot of the decaying intensities for selected lines (0, 3), (0, 5), (0, 7), (0, 9), and (0, 10) versus time is displayed in Figure 2 on a semi-logarithmic scale; the curves of the relations are linear. From the slopes of these curves, we derived the lifetimes of these transitions, as fitted for lines (0, 1) to (0, 12) listed in Table 1; the values are within 2.12–2.65 s.

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To discover the threshold for the generation VK emission of solid N₂ upon FUV excitation, we monitored the emissive wavelength region near 300 nm, comprising the line (0, 7), as an indicator of its emission, and tuned the wavelength of radiation beyond 121.6 nm. Figure 3 displays the emission spectra near 300 nm from samples of solid N₂ during irradiation with FUV light at 121.6, 130.0, 138.7, 145.6, 155.0, 165.0, 175.0, and 180.0 nm. The emission line (0, 7) at 296.73 nm appeared clearly upon irradiation at wavelengths 121.6, 130.0, 138.7, 145.6, 155.0, and 165.0 nm. For excitation at wavelengths of 175.0 and 180.0 nm, we enlarged the signal 1000 times, as shown in Figures 3(g) and (h), respectively; the resulting curves exhibit a small signal of line (0, 7) also at 175.0 nm, but not at 180.0 nm. We tuned the wavelengths also to 190.0, 200.0, and 220.0 nm and detected no signal of line (0, 7). We thus derive the threshold wavelength as 175.0 ± 3.5 nm (7.08 ± 0.14 eV) to generate the VK emission from the excitation of solid nitrogen at 4 K. We tuned the wavelength of the far-UV light from the synchrotron source on adjusting the gap of the undulator, which delivers the selected light with 2% bandwidth and uncertainty.

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An electronically excited molecule might relax radiatively to its ground state. Cecchi-Pestellini and Aiello suggested that the interaction of galactic cosmic rays with interstellar molecules produces secondary fields of ultraviolet radiation, the so-called cosmic-ray-induced photons (Cecchi-Pestellini & Aiello 1992); this radiation might subsequently initiate photochemistry in interstellar clouds or in other astrophysical environments. For example, excited molecular H₂ can emit far-UV light in wavelength region 140–170 nm (Warneck 1962; Chen et al. 2014). Light of wavelength less than 400 nm is likely to be an important driving force to generate chemical compounds in not only our solar system but also other spatial environments. In this work, our results demonstrate that excited N₂ can be formed and survives on a timescale of seconds in solid nitrogen upon irradiation with FUV light; the long-lived excited N₂ can emit UV radiation in wavelength region 200–400 nm. In this process, the energy of the UV emission is less than that of the FUV excitation, the yield of the UV emission is also low; however, the UV light as-emitted might be possible to provide a secondary UV source for photolysis of molecules in cold astrophysical environments.

Our work also indicates that the generation of N₂ in an excited state might play important roles in photochemical reactions in the nitrogen-ice analogs. The long-lived excited N₂ might be reactive in the icy nitrogen analogs; it might also react readily with astrophysical molecules nearby in the cold space. To investigate further the reactions of excited N₂ with astro-molecules is thus of interest to understand the detailed mechanism, but it is difficult to distinguish the chemistry of excited N₂ from that of other reactive nitrogen species such as the N₃ radical and N (²D) atom in icy nitrogen. In this work, we found the threshold wavelength for formation of excited N₂ upon FUV light to be 175.0 nm; according to our previous works, the threshold wavelength of N₃ radical and N (²D) atom is 145.6 nm (Chou et al. 2014; Lo et al. 2015a). If we can derive the wavelength dependence on studying the photoproducts, the threshold wavelength 175.0 nm can then provide a clue to distinguish the involvement of excited N₂ from nitrogen species N₃ radical and N (²D) atom in the icy nitrogen analogs.

This argument could be learnt from the example of investigation on the formation thresholds of CH₃ and N₃ in solid N₂ in our previous work (Lo et al. 2015a). To enhance the demonstration, we combined the parts of Figures 1 and 4 in our previous report (Lo et al. 2015a) and re-plotted them into a new figure, as shown in Figure 4; which displays the partial difference infrared absorption spectra including the characteristic absorption lines (1657.7, 1655.6, and 1652.4 cm⁻¹ for mode ν₃) of N₃ products of the irradiation of solid N₂ samples and the line (611.1 cm⁻¹ for mode ν₂) of CH₃ as products of the irradiation of solid CH₄:N₂ = 1:500 upon various FUV light at 3 K. In this case, we found that the formations of CH₃ and N₃ in solid N₂ have the same threshold wavelength at 145.6 nm. However, the threshold wavelength of radiation to generate CH₃ from CH₄ dispersed in solid neon, as Equation (1),

$$\begin{eqnarray}&&{\mathrm{CH}}_{4}+{hv}\to {\mathrm{CH}}_{3}+{\rm{H}}\end{eqnarray} \tag{ 1 }$$

was determined at 140.0 nm, different from 145.6 nm on photolysis of CH₄ dispersed in solid nitrogen. Accordingly, we could conclude that the following equation might express a route for the generation of CH₃ in icy nitrogen.

$$\begin{eqnarray}&&{\mathrm{CH}}_{4}+{{\rm{N}}}_{3}\to {\mathrm{CH}}_{3}+{\mathrm{HN}}_{3}.\end{eqnarray} \tag{ 2 }$$

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In the case of C₂H₄ in solid nitrogen, Figure 5 displays the partial different infrared absorption spectra exhibiting the characteristic absorption line (901.3 cm⁻¹ for mode ν₈) of C₂H₃ products from solid C₂H₄:N₂ = 1:500 with various FUV light at 3 K. According to Figure 5, the IR spectral line of C₂H₃ radical appears after excitation at tested wavelengths 121.6, 130.0, 138.7, 145.6, 155.0, 165.0, and 175.0 nm, but not 180.0 nm. From Figures 3 and 5, we thus found that the formations of C₂H₃ and VK N₂ have the same threshold wavelength at 175.0 nm. However, the threshold wavelength of radiation to form C₂H₃ from C₂H₄ dispersed in solid neon, as Equation (3),

$$\begin{eqnarray}&&{{\rm{C}}}_{2}{{\rm{H}}}_{4}+{hv}\to {{\rm{C}}}_{2}{{\rm{H}}}_{3}+{\rm{H}}\end{eqnarray} \tag{ 3 }$$

was determined at 150.0 nm; which is different from 175.0 nm on photolysis of C₂H₄ dispersed in solid nitrogen. Thus, we may suggest that the VK nitrogen can react with C₂H₄ to form C₂H₃ radical in icy nitrogen as the following equation

$$\begin{eqnarray}&&{{\rm{C}}}_{2}{{\rm{H}}}_{4}+{{\rm{N}}}_{2}* \to {{\rm{C}}}_{2}{{\rm{H}}}_{3}+{\rm{H}}+{{\rm{N}}}_{2}.\end{eqnarray} \tag{ 4 }$$

Therefore, our work on the investigation of icy VK nitrogen enhances our understanding of its photochemistry in the astro-environments.

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After excitation of solid nitrogen with FUV light from a synchrotron source at 4 K, the emission of the Vegard–Kaplan system of N₂ was measured in a wavelength range of 200–440 nm. We determined the lifetimes of VK (0, 1) to (0, 12) transitions to be in the range of 2.12 ∼ 2.65 s. The threshold wavelength to generate the VK emission was determined to be 175.0 ± 3.5 nm. The specified threshold wavelength serves as a spectral signature to enable the identification of VK nitrogen involved in the photochemistry of icy nitrogen. This investigation of the generation of VK nitrogen in the solid state provides an indication of its role in the photochemictry of icy nitrogen and can assist our understanding of the evolution of nitrogen species in space.

The National Science Council of Taiwan (grant NSC 102-2113-M-213-005-MY3) and the NSRRC provided support for this research.