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

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 (2Dlong) 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 (2Dlong) 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.

N2 is a fundamental molecule widely distributed in space but is perceived to be a volatile substance above 77 K; however, it is able to exist as an amorphous solid in cold environments below 60 K. Beyond Uranus, N2 ice is observed on many objects such as Triton (1–3), which is the largest satellite of Neptune, the dwarf planet Pluto and its satellite Charon, etc. (3–6). These icy objects continuously undergo photolysis and radiolysis to produce active atoms or radicals which may react to form N-bearing complex molecules. In addition, those active nitrogen species might be subject to temperature variations according to daily or seasonal patterns, and endogenic activity. Coupled with these variations, active species in N2 ice could interact with contaminants to synthesize an N-bearing complex observed in space.

Although N2 is recognized to be a primary source for the synthesis of much more complicated N-containing complexes in astroenvironments, its large dissociation energy, 9.75 eV (7, 8), in the gaseous phase provides a great potential barrier to be overcome. The first excited state, A, of N2 has a long lifetime, on the order seconds (9), and seems to serve as a steppingstone of energy for further reactions, or an energy reservoir to transfer its energy to its surroundings (10–13). Although the lowest resonance absorption of N2 is the Lyman–Birge–Hopfield system (9), for a N2 ice at 145.6 nm (8.52 eV) (14), the measured threshold of emission for A → X, the Vegard–Kaplan system (VK) (9), extends to 175 nm (7.08 eV) (15). With this steppingstone in state A, the dissociation barrier of icy N2 becomes greatly decreased; the important evidence is that emission from the photofragment N (2D → 4S) (16, 17) has a threshold energy of 8.52 eV (18). The energetics and transitions of the photolysis of icy N2 are summarized in Fig. 1.

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 (2Dlong) atoms.

Other than the nitrogen atom, the azide radical, linear-N3 (l-N3), is the other nitrogen complex able to be synthesized from photolysis of the N2 ice in the laboratory; this radical and excited N atom seem active so as to be readily able to induce other reactions with their surroundings (19). In icy N2, the N and l-N3 are hence the only products that can be identified from photolysis, but their identifications differ. The presence of l-N3 can be directly verified through absorption spectra in the mid-infrared (IR) (19–23) and UV (24, 25) spectral ranges, whereas, the N can be identified through a visible emission spectrum, like the luminescence from the transition N (2D → 4S) observed in photolysis (18) and radiolysis (16) experiments.

In this work, we applied vacuum-ultraviolet (VUV) light at 121.6 nm from a synchrotron (see for experiments) to synthesize l-N3 and atomic N in a N2 ice and then used light at 190 nm to irradiate the ice to eliminate l-N3 so that the ice contained only N-atom doping (26). Our recorded absorption and emission spectra served to monitor the temporal variations of l-N3, N2, and N. During the variation of temperature of the N-atom–doped ice after in darkness 1 h, α-lines of transition N (2D → 4S) were observed again, indicating that N (2D) still existed in the N2 ice. After that heating, the absorption of l-N3 was observed again in the IR spectrum. This long-lived N in state 2D seems to have played a major role in the thermal-activated emission and thermal-activated synthesis of l-N3; it seems that whereby raising the temperature can induce further reactions in the irradiated icy environment.

Icy N2 upon 121.6-nm Illumination

Fig. 2 shows the experimental procedure; the horizontal axis indicates a timescale representing the sequence of the experiment. The graphic trend, shown in Fig. 2, indicates variations of the photocurrent of the VUV light and symbols beside the trend indicate the time to record spectra–“IR-#”, “iEm-#,” and “Em-#”; Fig. 2 shows the temperature of the N2 ice. Fig. 2 shows temporal profiles of the peak height of emission (“iEm” and “Em”) at 522.6 nm. After recording an IR absorption spectrum labeled “IR-1” for the deposited icy N2 as a basic reference, we irradiated the N2 ice with VUV light at 121.6 nm, Lyman-α line of atomic H.

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.

Upon 121.6-nm illumination, we simultaneously recorded series emissions, labeled “iEm-1.#” from the icy N2; for examples, the spectra labeled “iEm-1.1” and “iEm-1.45” in Fig. 3 were recorded as the 1st and 45th of the set of “iEm-1” emission spectra, respectively. These emissions show abundant and complicated features identified as transitions of N2 (A → X, VK series) (9, 15), N (2D → 4S, α series) (16), N (2P → 2D, δ series) (17) and impurity O [1S → 1D (27), β series (16)]; the details of identifications were presented in our previous work (26).

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.

Excitation at 121.6 nm (10.20 eV) could pump ground-state N2 to state B 3Πg; which rapidly crossed to state A 3Σu+ with radiationless transition, then relaxed to its ground state X 1Σg+ through the luminescent process with emissions of VK lines as shown in Fig. 1. According to previous work (14), the formation of threshold wavelength for N (2D) and l-N3 from the icy N2 is 145.6 nm, 8.52 eV, as Eq. .Generation of N (2D) and l-N3 could occur and the emission from the excited N (2D) hence appeared upon 121.6 nm as displayed in Fig. 1 . In addition, considering that the energy of N (2P) is about 1.2 eV greater than that of N (2D), the formation of N (2P) from photolysis of the icy N2 at 121.6 nm might also be possible as summarized in Eq. .In addition, the N (2D) might be pumped to the N (2P) by light. The above discussion explains observations of emissions from N (2D), N (2P), and N2 (A 3Σu+) in Fig. 3. Among these emissions, the most intense emission of atomic N was the α-line from 2D at 522.6 nm as depicted in Fig. 3; details of spectral profiles are shown in Fig. 4. Their identifications were discussed in our previous work (26). Subsequently, the atomic N (2D) might play an important role in the photochemical system of icy N2; we thus primarily monitored its emission and display the intensity profiles in the Fig. 2.

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.

After irradiation with VUV light at 121.6 nm, we recorded an IR absorption spectrum labeled “IR-2” in Fig. 5; “IR-1” is the spectrum of precursor N2 before irradiation. Due to inactive absorption of N2, the curve IR-1 shows only background. Two sites of characteristic absorption of l-N3 were observed at 1,652.6 and 1,657.8 cm−1 in IR-2; this result reflects the formation of l-N3 from the reaction Eqs. or upon 121.6 nm. Two characteristic lines may represent either the same l-N3 molecule with different vibrational quantum numbers or the different l-N3 molecules associated with the different structural arrangements stored in icy N2. There is no other evidence for the ν3 mode of l-N3 possessing different vibrational quantum numbers in this work. More likely, considering the host molecule N2 is not spherical that may result in different structural sites for store the l-N3 molecules in icy N2.

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.

Icy N2 Containing As-Generated l-N3 Irradiated at 190 nm

To engage the photochemical system simply, we intended to eliminate the as-generated l-N3 at this stage. The vibronic absorption progression of l-N3 in the icy N2 was measured by Chin et al. in wavelength region 190 to 225 nm (28); therefore, we photolyzed the target icy sample containing the as-generated l-N3 upon light at 190 nm for 1 h. Also, because the threshold wavelengths of emission of the N α-line and N2 (A → X) are 145.6 and 175 nm (16), respectively, we used VUV light at 190 nm, which initiates no reaction of N2 ice, to irradiate the icy N2 containing as-generated l-N3 radicals. After we irradiated at 190 nm, we then recorded the IR spectrum of the icy sample shown as spectrum “IR-3” in Fig. 5 which discloses only background curve without signal of l-N3. This result indicates that as-generated l-N3 radicals were totally dissociated upon 190 nm (26). During upon 190 nm, we could only detect the emission of the N α line as displayed in Fig. 2. These observations combined allow us to conclude that the photodissociation of l-N3 generates N2 (X 1Σg+) and atomic N (2D) as Eq. (26).At that point in the experiment the as-generated l-N3 radicals were eliminated; the target ice contained only N and N2, which might be not all in their ground states. Considering that the decay lifetimes of N (2D) and N2 (A) are 25 and 2.5 s due to parity-forbidden transitions (15, 17), we thus waited 1 h with the icy sample (after irradiated at 190 nm) in darkness at 3.5 K to ensure that those 2 species had relaxed to their ground states; we hence supposed that there was nearly no excited-state species in the target N2 ice at this stage due to no emission observed at all.

Thermal Ramping of the Target Ice and Observation of the Long-Lived N (2D)

The target ice was then subjected to a series of temperature variations and concurrently recorded emission spectra labeled “Em-1” to “Em-3,” of which temporal and spectral profiles of the α-line from N (2D) are shown in Figs. 2 and 4, respectively. Sequentially, we raised the temperature of the icy sample from 3.5 to 15 K over a period of 8.5 min and maintained it at 15 K for 45 min (Em-1), raised from 15 to 20 K over a period of 5 min and maintained it at 20 K over a period of 15 min (Em-2), cooled to 3.5 K again for 20 min, and raised from 3.5 to 20 K over a period of 16.5 min and maintained it at 20 K for 20 min (Em-3).

From spectra Em-1 to Em-3, the observation of N α-line emission indicated that some portion of N was still in state 2D and not all in ground state as previously supposed. After elimination of l-N3, we waited until no emission of the α-line was measurable as “iEm-2” in Fig. 2; this result indicates that no N was in state 2D. But, the temporal profiles of Em-1 to “Em-3” in Fig. 2 recorded the α line, the results indicate that the target ice still contained N in an excited state 2D which might be different from N (2D) in emissions of “iEm-1” and iEm-2. To distinguish the N (2D) with a lifetime of 25 s, we thus label this long-lived state as N (2Dlong).

After the thermal ramping and recording visible spectra Em-1 to Em-3, we then recorded the IR spectrum “IR-4” in Fig. 5, in which the absorption of l-N3 appeared again at 1,657.8 cm−1, but notably corresponding to only 1 site, whereas, the other site of feature absorption at 1,652.6 cm−1 is absent here. Considering the nascent threshold wavelengths of l-N3 and N to be 145.6 nm, we reasonably suppose that l-N3 was synthesized from N2 combining with atomic N that might be in an excited state. First considering N2, no trace of N2 (A) was found as the A-X emission was not observed; it is hence reasonable to exclude the effect of N2 (A) in further formation of l-N3 and thermal warming. For atomic N, emissions of series of N α-lines were observed. N might hence be present in configuration 2s22p3 with 4S, 2D, or other terms. According to a calculation of Galvão (29), a synthesis of l-N3 has a greater probability from N (2D) than from N (4S). We thus suggest that the formation of l-N3 at 1,657.8 cm−1 in the warming process follows Eq. .

Comparison between the VUV-Synthesized l-N3 with the Thermal-Synthesized l-N3

At this stage in our experiment, we had found 2 methods to synthesize l-N3; one was VUV irradiation and the other was warming the ice containing atomic N. Only one method, VUV irradiation, can decompose l-N3 into N and N2, which could be in their ground states, N (4S) and N2 (X), or in excited states, N (2D, 2P) and N2 (A). In the elimination of l-N3, if its fragments of decomposition are in luminescently excited states, we have the possibility to observe these emissions. Upon 190-nm illumination, we observed strong emission of N (2D → 4S) as in iEm-2 and “iEm-5” shown in Fig. 2, but not in “iEm-3;” these phenomena indicate the states produced were different. The IR absorption spectra might provide some information of states in the icy N2 at 3.5 K. For the VUV-synthesized l-N3 with 121.6 nm light, the l-N3 was formed in 2 sites corresponding to absorption lines at 1652.6 and 1657.8 cm−1 as “IR-2” and “IR-7”, but only one site, corresponding to 1657.8 cm−1, was observed for the thermal synthesis of l-N3 as “IR-4”. Therefore, we suppose that there are 2 channels for the reaction Eq. as follows:The decomposed fragment N (2D) from the reaction Eq. for l-N3 at the site corresponding to 1652.6 cm−1 luminescently relaxes to 4S with lifetime 25 s, but not that of the N (2Dlong) associated with 1657.8 cm−1 from reaction Eq. . According to these schemes we were able to observe strong emissions from the N (2D) at the beginning of “iEm-2” and “iEm-5” but not “iEm-3” in the VUV elimination of as-synthesized l-N3.

Due to thermal effect, 2 excited N atoms in 2D may involve the translation between them; the conversion of N (2Dlong) to N (2D) may proceed during thermal ramping. Upon 121.6-nm irradiation, the reaction Eq. produces not only 2 sites of l-N3 in 2Πg1,657.8 and 2Πg1,652.6 but also 2 excited N atoms in 2D and 2Dlong at low temperature. The excited N (2D) atom luminescently relaxes to ground-state N (4S) with lifetime about 25 s, whereas N (2Dlong) keeps stable for a long time in N2 ice. Induced by thermal, the long-lived N (2Dlong) can convert to N (2D), which subsequently relaxes to N (4S) with emission as described in Eq. :In this process, the conversion of N (2Dlong) to N (2D) can thus be indirectly monitored by the emission of N α-line, as recorded in Fig. 2. When the temperature of icy sample containing the N (2Dlong) raised from 3.5 to 15 K, the emission of N α-line appeared above 3.5 K, its intensity kept increasing until reaching the final temperature 15 K; then, the intensity of N α-line decayed to none at 15 K in 45 min, shown as Em-1 in Fig. 2. In the next step, the temperature of target icy sample raised from 15 to 20 K, the N α-line emitted again above 15 K and increased to a maximum until 20 K; after that, the intensity decreased to almost none at 20 K in 20 min as shown in Em-2 profile. From profiles of “Em-1” and “Em-2,” the curves demonstrate that the conversion rate from N (2Dlong) to N (2D) increased during temperature ramping from 3.5 to 20 K. This phenomenon hints that the stability of N (2Dlong) may relate to temperature; it becomes more stable at lower temperature. Interestingly, the conversion of N (2Dlong) to N (2D) is an irreversible process. When the target icy sample cooled from 20 to 3.5 K, and then raised from 3.5 to 20 K again, almost no emission was observed as depicted in Em-3. Combined, these results suggest that N (2Dlong) atoms persist in stable sites of the icy N2. The sites may depend on temperature; the more sites possibly exist at lower temperature. When the temperature of icy N2 goes up, either the sites of store for N (2Dlong) atoms are disrupted by thermal energy irreversibly or the N (2Dlong) atoms have enough thermal energy to physically translocate from these sites irreversibly is possible.

Considering that the relaxations of N (2D) and N2 (A) to ground states involve parity-forbidden transitions (17), the processes of deexcitations may be strongly perturbed from the inter- and intramolecular environments; thus, the unusually long lifetime of N (2Dlong) may be related to a physical character of the N2 ice lattice and could also be influenced by the temperature. It is also possible that the process of conversion from N (2Dlong) to N (2D) depends on temperature; the less active process may occur at lower temperature but that is faster at higher temperature.

To further consider that thermally synthesized l-N3 is dominated by N (2Dlong), one portion of which relaxes to 4S and another portion of which reacts with N2 to synthesize l-N3. This assumption is provable from a comparison of the ratio between the total amount of emission 2D → 4S and the yield of l-N3. For the emission part, we integrated the temporal profiles and obtained (Em-1 to Em-3)/(Em-4) = 6.3; for yields of l-N3, we integrated the IR absorptions of l-N3 and made a comparison of (“IR-4” – “IR-3”)/(“IR-6” – “IR-5”) = (7.8–0)/(3.2–2) = 6.5; these 2 ratios of emission (N α line) and IR absorption (l-N3) are nearly the same. This correlation indicates that the thermal synthesis of l-N3 can be represented with this equation:The reactions of Eq. involved are displayed in the diagrams of Fig. 1.

Implication to Astroscience

The mid-UV reflectance spectra of Pluto and Charon were recorded by the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS); Stern et al. (30) reported the Pluto’s spectrum contains a strong mid-UV absorption feature. Recently, Chin et al. (28) suggested that the carrier of absorption feature of Pluto’s surface observed by HST/COS might be l-N3. According to this work, the photochemical process of icy N2 upon VUV light may possibly produce l-N3 and N (2D) in the outer Solar System. For evolution of nitrogen species in cold astroenvironments, the reactive l-N3 radical and N (2D) atom may initiate many chemical reactions to form complicated N compounds at low temperature. For example, the l-N3 radical can react with hydrocarbons like CH4 and ultimately form HCN and HNC in the icy N2 (18). Therefore, these reactive nitrogen species l-N3 and N (2D) may play important roles in the chemical reactions of the nitrogen-ice analogs. Understanding the formation and elimination of N (2D) and l-N3 in icy N2 are thus important and urgent. In this work, our results demonstrate that photolysis of icy N2 with VUV light can produce reactive nitrogen species N (2D) and l-N3. Other than photogeneration of the excited N (2D) with definite lifetime about 25 s, formation of the long life of excited N (2Dlong) has been first confirmed in icy N2; remarkedly, the N (2Dlong) can react with N2 to produce l-N3 by thermal-activated reaction. On the contrary, photodissociation of l-N3 can generate N (2D) in icy N2. Hence, the transformation between these 2 nitrogen species can occur in photochemical process and thermal reaction. The universe is full of VUV source including the Lyman-α 121.6 nm; in addition, the icy N2 may exist in cold interstellar medium. Our results have thus opened a window to understand the nitrogen transformation in cold astrophysical environments.

Summary

In summary, photolysis of a N2 ice with VUV light at 121.6 nm produced N and l-N3; VUV light at 190 nm eliminated l-N3 leaving the N2 ice with only N-doping. During VUV synthesis of l-N3, emissions in the VK series of N2, α, and δ lines of N and the β-line of O from impurity O2 were observed. In the elimination of l-N3, only the α-line was observed; also, the α-line was recorded in the thermal synthesis l-N3. Although 1 h of darkness prevented N2 and N from existing in their excited states A and 2D, respectively, observing the α-line during warming indicated that there existed long-lived N (2Dlong), having a lifetime much greater than 25 s, which dominated the thermal synthesis of l-N3. This long-lived N (2Dlong) was stored in the N2 ice and could be triggered to release emission or to induce a reaction on raising the temperature.

Data Availability.

All data discussed in the paper are presented in the figures and will be made available to readers.