PHOTOLYSIS OF ETHYNE IN SOLID NEON AND SYNTHESIS OF LONG-CHAIN CARBON CLUSTERS WITH VACUUM-ULTRAVIOLET LIGHT

For an investigation of the VUV photochemistry of a compound, knowledge of the absorption spectrum and electronic states of its molecules in this region serves as a guide for the selection of light to induce dissociation from its excited states. For C₂H₂ in the region 105–220 nm, the absorption features have been identified as transitions to valence states at wavelengths greater than 155 nm and to Rydberg states less than 155 nm. The profiles and features of the absorption of molecules might differ between the gaseous and condensed phases. Although the absorption spectrum of gaseous C₂H₂ is well documented (Suto & Lee 1984; Wu et al. 2001), no absorption spectrum in the condensed phase has been reported.

Figure 1 displays the absorption of a sample of C₂H₂/Ar (1/100) in the region 107–220 nm at 10 K. The absorption features of C₂H₂ exhibit spectral broadening and shifts relative to the gaseous phase; the two known valence transitions show redshifts whereas the Rydberg transitions all display blueshifts. For example, weak maxima of absorption into electronic state B¹B_u show vibrational features at wavelengths 185.09, 182.58, 180.27, 177.66, 175.66, 173.75, 171.05, 169.04, 166.94, 165.13, 163.23, 161.33, 159.53, and 157.63 nm for v = 0–13, respectively; these values average 0.31 nm greater than those in the gaseous phase. In contrast, the discrete series of Rydberg transitions for gaseous C₂H₂ at wavelengths less than 155 nm congest into a strong continuous band with features at 137.82, 134.42, 129.62, 126.83, 124.63, 121.73, and 119.04 nm; these maxima might correspond to Rydberg series 3R₀, 3R₁, 3R'₀, 3R'₁, 3R'₂, 4R'₀, and 4R'₁, respectively. The positions of these Rydberg transitions shift significantly to the blue for the matrix sample.

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The photolysis of ethyne with VUV radiation has been little investigated; because of the difficulty of operating in the VUV region with conventional lamps or lasers, only two excitation wavelengths—193 and 121 nm—were applied for the VUV photolysis of gaseous ethyne. An undulator beam from a synchrotron has the advantage of the wavelength becoming tunable by varying the gap of the magnets; by these means, the VUV undulator readily delivers intense radiation to initiate photolysis. In this work, we investigated the photolysis of ethyne with photons corresponding to wavelengths 171, 130, and 114 nm that excite ethyne into states B (v = 6), Rydberg band 3R₀', and unresolved higher Rydberg states, respectively.

A partial IR absorption spectrum of the sample C₂H₂/Ne (1/1000) after deposition at 3 K is shown in Figure 2. Four intense IR lines of ethyne at 731.8, 1329.9, 3283.6, and 3296.8 cm⁻¹ are associated with vibrational modes ν₅, ν₄+ν₅, ν₃, and ν₂+ν₄+ν₅, respectively. Weak lines observed at 739.5, 745.0, and 3269.7 cm⁻¹ are assigned to a dimer of C₂H₂ (Hirabayashi & Hirahara 2002). Upon VUV photolysis, the intensity of all of these absorptions of ethyne decreased uniformly with increasing duration of photolysis; the extent of depletion was determined by the ratios of absorbances at a particular duration with those of the original sample.

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Figure 3 displays temporal profiles of depletion ratios of ethyne irradiated at various wavelengths. The fact that these curves exhibit diverse profiles indicates that the photolysis mechanism varies with wavelength. The curve for temporal depletion at 171 nm is almost linear up to 50 minutes, but there is no linear relation for 130 nm and 114 nm even for a short period. Upon VUV irradiation for 50 minutes, the intensities of lines due to ethyne decreased by ∼40%, 76%, and 55% at 171, 130, and 114 nm, respectively. For irradiation less than 3 minutes, the ratio of depletion is about 1:40:15 for wavelengths 171, 130, and 114 nm; this ratio is near the ratio of the absorption, 1:52:14, at these wavelengths. The extent of the nascent VUV photolysis for ethyne hence depends on its absorption at particular wavelength: the greater the absorption is, the greater the rate of depletion will be.

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Irradiation of C₂H₂ in solid Ne with VUV light produced various photoproducts. Figure 4(a) displays a partial difference IR absorption spectrum recorded after irradiation of the matrix sample at 130 nm for 50 minutes; in this spectrum, obtained by subtracting the spectrum recorded before VUV irradiation from that recorded after irradiation, lines pointing upward indicate production whereas those pointing downward indicate destruction. On irradiation at 130 nm, ethyne was destroyed efficiently; as shown in Figure 3, C₂H₂ became depleted to an extent of 76% within 50 minutes.

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After photolysis of C₂H₂ in solid Ne at 130 nm, abundant features appear in Figure 4(a). To identify these photoproducts, we undertook experiments with isotopically substituted ethyne—C₂D₂ and ¹³C₂H₂—dispersed in solid Ne under the same conditions of photolysis with 130 nm; the difference IR absorption spectra for these samples are shown in Figures 4(b) and (c), respectively. Based on previous work and experiments on isotopic species, we identified the various photoproducts. For example, the ν₁ and ν₃ modes of the C₂H radical in solid Ne were previously reported at 3293.3 and 1835.7 cm⁻¹, respectively (Wu & Cheng 2008); in the test of photolysis of C₂H₂ in solid Ne upon 130 nm, we observed two lines at 3293.3 and 1835.8 cm⁻¹, which we thus assign to the C₂H species. Upon photolysis of C₂D₂ under the same conditions, the corresponding ν₁ and ν₃ modes of C₂D were recorded at 2536.5 (ν₁) and 1737.6 (ν₃) cm⁻¹, respectively; the results from isotopic experiments thus confirmed these assignments.

We similarly identified photoproducts containing hydrogen species such as C₂H₃, C₄H, C₄H₂, C₈H⁻, and C₈H₂. The observed line at 895.0 cm⁻¹ upon photolysis of C₂H₂/Ne is assigned to ν₇ of C₂H₃ (895.4 cm⁻¹; Wu et al. 2008) and is shifted to 704.0 cm⁻¹ in D-isotopic experiments, corresponding well with previous values of vinyl generated from photolysis of ethylene in solid Ne. The infrared spectra of C₄H and C₄H₂ in solid Ne have been reported by Forney et al. (1995b). They used an H₂ discharged flow lamp to generate VUV light to initiate photolysis of C₂H₂/Ne matrix samples; in their work, they also performed D- and ¹³C-isotopic substitution experiments to confirm their assignment. Our observed line positions of C₄H at 2064.1 cm⁻¹ (ν₃) and C₄H₂ at 3333.1 (ν₄), 2017.5 (ν₅), and 628.6 cm⁻¹ (ν₈) as well as the isotopic variants are similar to their values. For C₈H₂, the pertinent line at 621.5 cm⁻¹ is identical to the reported value of ν₁₄ of C₈H₂ measured in the gas phase (Shindo et al. 2001). This line shifts to 525.0 cm⁻¹ in D-isotopic experiments yielding the D-isotopic ratio of 0.8447, which is close to the value of 0.8411 of C₄H₂. Thus, we tentatively assigned the line at 621.5 cm⁻¹ to ν₁₄ of C₈H₂. The lines at 2106.1 and 2021.7 cm⁻¹ showed the same photolysis behavior and shifted in D-isotopic experiments, suggesting that these two lines may belong to the same species. Grutter et al. (1999) used a mass-selected technique to deposit C₈H⁻/Ne matrix samples at 6 K and measured the infrared spectrum of C₈H⁻; they thus identified the lines at 2106.5 and 2021.2 cm⁻¹ attributed to C₈H⁻. We recorded lines at 2106.1 and 2021.7 cm⁻¹ that are similar to previous values; we thus assigned these two lines to C₈H⁻. In the D-isotopic experiment, the pertinent line at 2106.1 cm⁻¹ shifted to 2101.8 cm⁻¹ yielding the D-isotopic ratio of 0.9980, suggesting that this species contains a long carbon chain framework, supporting our assignment. Vibrational wavenumbers of observed lines and their assignments for samples of C₂H₂ and C₂D₂ are summarized in Tables 1 and 2, respectively.

Besides photoproducts containing H atom(s), carbon clusters are expected to be present in Figure 4. The lines of these carbon clusters show no isotopic shift in the spectra after photolysis of C₂H₂ and C₂D₂, as shown in Figures 4(a) and (b), but shift on photolysis of ¹³C₂H₂, as displayed in Figure 4(c). For example, the ν₃ modes of C₃ (2043.5 cm⁻¹; Weltner et al. 1964), C₄ (1547.0 cm⁻¹; Forney et al. 1995a), and C₅ (2166.4 cm⁻¹; Vala et al. 1989) were recorded at 2043.5, 1546.8, and 2165.3 cm⁻¹, respectively, after photolysis of C₂H₂ and C₂D₂, whereas related lines belonging to ¹³C₃, ¹³C₄, and¹³C₅ after photolysis of ¹³C₂H₂ are found at 1964.9, 1487.2, and 2081.6 cm⁻¹, respectively, as listed in Table 3. The ¹³C-isotopic ratios of these CC stretching modes for C₃, C₄, and C₅ are 0.9615, 0.9615, and 0.9613, respectively; these values are almost the same and support these assignments. For the species C₆, the associated lines of ν₄ and ν₅ were noted at 1958.6 and 1199.0 cm⁻¹, respectively, based on the previous work (1958.7 and 1199.4 cm⁻¹; Smith et al. 1994). The connected lines of ¹³C₆ were found at 1882.4 and 1152.8 cm⁻¹, respectively, in which, the derived ¹³C-isotopic ratios are 0.9611 and 0.9615 for ν₄ and ν₅ of C₆, respectively, further supporting our assignment. Similarly, the ν₄ and ν₅ lines of C₇ (2134.4 and 1897.5 cm⁻¹; Smith et al. 1994; Forney et al. 1996) were recorded at 2134.8 and 1897.2 cm⁻¹, respectively; while, the ν₄ of ¹³C₇ was observed at 2052.2 cm⁻¹, with a ¹³C-isotopic ratio of 0.9613. For the species C₈, the ν₅ and ν₆ modes (2067.8 and 1707.8 cm⁻¹; Freivogel et al. 1995) were recorded at 2067.9 and 1707.5 cm⁻¹, respectively, in the tests of C₂H₂ and C₂D₂, whereas related lines due to ¹³C₈ after photolysis of ¹³C₂H₂ are found at 1986.4 and 1641.8 cm⁻¹, respectively.

The vibrational spectra of larger carbon clusters with a ¹³C-isotopic substitution have been seldom reported in literature; therefore, such ¹³C-isotopic data are lacking. In this work, we found that the ¹³C-isotopic ratios of CC stretching modes for linear C₃ to C₈ are 0.9611 ± 0.0004. Based on isotopic ratios and peak intensities, we might thus identify the lines of larger carbon clusters near the estimated positions. By these means and with careful examination, we accordingly recognized more carbon cluster species C₉, C₁₀, C₁₀⁻, C₁₁, and C₁₂ generated after VUV photolysis of ethyne in solid Ne. Table 3 summarized the line positions and assignments for the carbon hydrides and carbon clusters with fully ¹³C-isotopic substitutions.

A difference spectrum recorded after irradiation of the matrix sample with 171 nm for 50 minutes is presented in Figure 5(a). After irradiation at 171 nm for 50 minutes, intensities of lines due to C₂H₂ decreased by ∼40%. Upon photolysis of ethyne at 171 nm, the most prominent photoproduct is the C₂H radical; its most intense line recorded at 1835.8 cm⁻¹ is the vibrational ν₃ mode. Other weak but abundant absorptions of C₂H in vibronic structures were found in the spectral region 2000–5000 cm⁻¹; these vibronic absorptions of C₂H are discussed in detail elsewhere (Wu & Cheng 2008). Continued subjection of this primary photoproduct C₂H to irradiation is expected to produce C₂ molecules; because of inactivity of the homodiatomic molecules in IR, the spectra showed no line indicative of absorption due to C₂ near 1800 cm⁻¹ (Marenin & Johnson 1970), whereas new lines associated with H containing photoproducts C₂H₃, C₄H, C₄H₂, and C₈H₂ and those coupled to carbon clusters C₄, C₆, and C₈ appeared. The wavenumbers, observed qualitative intensities, and assignments of these lines for various photoproducts are summarized in Table 1. To compare the effect of varied VUV excitation, we present the spectrum upon photolysis at 130 nm in Figure 5(b). The proportions of carbon hydrides C₂H, C₄H₂, and C₈H₂ upon irradiation at 130 nm seemed generally to be less than at 171 nm, but lines due to large carbon clusters including C₃–C₁₂ suggest the contrary. Notably, lines due to species of carbon in clusters with atoms of odd number and of higher even numbers C₁₀ and C₁₂ appeared in the spectrum upon photolysis at 130 nm.

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Figure 4(c) displays a difference IR absorption spectrum recorded after irradiation of a sample at 114 nm for 50 minutes; pertinent data are summarized in Table 1. The efficiency of photolysis at 114 nm is intermediate between those experiments with irradiation at 171 and 130 nm, as reflected in the distribution of photoproducts. Similar to the experiment at 130 nm, abundant lines appear in the spectral region 2000–2200 cm⁻¹, corresponding to CC stretching modes of carbon clusters, but the intensities of observed lines of carbon clusters are less than those observed upon irradiation at 130 nm for the same duration.

The photochemical behavior of species isolated in matrices differs from that in the gaseous phase. At a small density, gaseous fragments have little chance to collide with each other upon photodissociation, whereas photofragments produced in a matrix and with insufficient energy to escape are typically constrained within the immediate environment. In this work, we detected strong IR absorption lines of C₂H radicals upon irradiation at 171, 130, and 114 nm. The results demonstrate that radical C₂H is the primary photoproduct of ethyne at these wavelengths. The dissociation thresholds to form C₂H in the ground state X²Σ⁺ and the excited state A²Π are 46056 cm⁻¹ and 49750 cm⁻¹, which correspond to about 217 nm and 201 nm, respectively (Mordaunt & Ashfold 1994; Lai et al. 1996; Wang et al. 1997). The photoproduct C₂H from ethyne upon irradiation at 171, 130, and 114 nm might thus possess sufficient energy to migrate from the site of production to react with another C₂H to form butadiyne C₄H₂. Figure 6 displays the temporal profiles of the formation of C₂H and C₄H₂ at varied wavelengths; these profiles are all similar at the same wavelength. This effect implies that the secondary photoproduct C₄H₂ was generated from C₂H. Further photodissociation of C₄H₂ might produce the radical C₄H, which might then react with itself to generate C₈H₂. As another possible reactive route, butadiyne C₄H₂ might combine with itself to form C₈H₂ and H₂. The following reactions explain the major carbon hydrides observed at wavelengths 171 nm:

$$\begin{equation*} \begin{array}{@{}l} {\rm C}_2 {\rm H}_2 + h\nu \to {\rm H} + {\rm C}_2 {\rm H} \\ {\rm C}_2 {\rm H} + {\rm C}_2 {\rm H} \to {\rm C}_4 {\rm H}_2 \\ {\rm C}_4 {\rm H}_2 + h\nu \to {\rm H} + {\rm C}_4 {\rm H} \\ {\rm C}_4 {\rm H} + {\rm C}_4 {\rm H} \to {\rm C}_8 {\rm H}_2 \\ {\rm C}_4 {\rm H}_2 + {\rm C}_4 {\rm H}_2 \to ({\rm C}_4 {\rm H}_2)_2 \\ ({\rm C}_4 {\rm H}_2)_2 + h\nu \to {\rm H}_2 + {\rm C}_8 {\rm H}_2. \\ \end{array} \end{equation*}$$

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Other minor carbon-containing photoproducts C₄, C₆, and C₈ were generated at wavelengths 171 nm. These photoproducts containing four or six carbon atoms formed on irradiation of C₂H₂ in Ne at 171 nm might result from photodissociation of a dimer or a trimer of ethyne within the same matrix cage, but the formation of the C₈ species is less likely because of the scarcity of tetramer in a matrix sample with C₂H₂/Ne = 1/1000. These even number carbon clusters might be created through complicated photochemical processes from the major species produced as follows:

$$\begin{equation*} \begin{array}{@{}l} {\rm C}_2 {\rm H} + h\nu \to {\rm H} + {\rm C}_2 \\ \ms {\rm C}_4 {\rm H} + h\nu \to {\rm H} + {\rm C}_4 \\ \ms {\rm C}_2 + {\rm C}_4 \to {\rm C}_6 \\ \ms {\rm C}_2 + {\rm C}_6 \to {\rm C}_8 \\ \ms {\rm C}_4 + {\rm C}_4 {\rm } \to {\rm C}_8. \\ \end{array} \end{equation*}$$

Figure 7(a) displays the temporal profiles of the formation of C₄, C₆, and C₈ at 171 nm; these profiles increase as irradiation time increases. Although the signals of these minor photoproducts are small, we could still have the impression that the slopes of temporal formation of C₄, C₆, and C₈ are almost similar. In addition, the fact that no odd numbers of carbon clusters are observed might suggest that the growth of carbon clusters is mostly through the addition of C₂ step by step upon irradiation at 171 nm.

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Upon photodissociation at 130 and 114 nm, besides the species generated at 171 nm, odd carbon clusters C₃, C₅, C₇, C₉ and C₁₁, and other clusters up to C₁₂ and C₈H⁻ were generated. Temporal profiles of the formation of various even carbon clusters from C₂H₂/Ne = 1/1000 irradiated at 130 and 114 nm are depicted in Figures 7(b) and (c). Upon irradiation with 130 nm, only C₆ and C₈ show similar formation slopes. In contrast, the trends of the formation of C₄, C₆, and C₈ upon irradiation with 114 nm are similar. The depletion ratios of C₂H₂ at these three wavelengths might be the cause of the difference. The large photodissociation efficiency and saturated depletion ratio of C₂H₂ upon irradiation with 130 nm reflects the large formation of C₂H or C₂ in the early periods of irradiation and quick formations for large carbon clusters, as well as the appearance of the turning point of the temporal profile of C₄.

The odd carbon clusters were detected upon photodissociation at 130 and 114 nm; the temporal profiles of the formation of C₃, C₅, and C₇ are shown in Figure 8. Upon irradiation with 130 nm, C₃ was formed rapidly and fruitfully compared with formations of C₅ and C₇. In contrast, in the 114 nm experiments the trends of formations of C₃, C₅, and C₇ are similar and C₃ was formed less. There is no consistency in these temporal profiles of formation of odd carbon clusters. Further analysis of the temporal profiles of the formation of carbon clusters higher than C₈ showed random behaviors. These results suggest that the mechanism of the formation of carbon clusters from ethyne in solid Ne upon VUV photolysis is complicated. The odd carbon clusters might be generated from the addition of carbon atoms with even carbon clusters or photodissociation to odd carbon atoms from large even carbon clusters. Photofragments acquiring excess energy after absorption of VUV photons possess enhanced mobility inside the solid environment that enables them to migrate to other atoms or molecules and to react there. Complicated photochemical reactive paths are thus more likely to consist of sequential construction of long-chain carbon clusters. Also, through an ionic photodissociation, C₈H₂ might become the ion C₈H⁻. Besides the reactions discussed above, the following steps might also occur in this system:

$$\begin{equation*} \begin{array}{l} {\rm C}_2 {\rm H} + h\nu \to {\rm CH} + {\rm C} \\ {\rm C}_8 {\rm H}_2 + h\nu \to {\rm H}^{\rm + } + {\rm C}_8 {\rm H}^ - \\ {\rm C}_8 {\rm H}^ - + h\nu \to {\rm H}^ - + {\rm C}_8 \\ {\rm C} + {\rm C}_2 \to {\rm C}_3 \\ {\rm C}_2 + {\rm C}_3 \to {\rm C}_5 \\ {\rm C}_3 + {\rm C}_4 \to {\rm C}_7 \\ {\rm C}_4 + {\rm C}_5 \to {\rm C}_9 \\ {\rm C}_4 + {\rm C}_6 \to {\rm C}_{10} \\ {\rm C}_5 + {\rm C}_6 \to {\rm C}_{11} \\ {\rm C}_5 + {\rm C}_7 \to {\rm C}_{12}. \\ \end{array} \end{equation*}$$

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The interactions between carbon atoms and molecules are far stronger than those between molecules and Ne. Molecules and atoms of carbon thus combine efficiently to form clusters. The formation of carbon clusters and hydrides with a linear structure indicates a synthesis involving an addition to a carbon chain of one or two carbon atoms in each step.

Titan, the largest moon of Saturn, has a dense atmosphere (160 kPa) composed of N₂ ∼ 98.4%, CH₄ ∼ 1.6%, and other trace gases (Hirtzig et al. 2009). Apart from N₂ and CH₄, identified species in Titan include C₂H₂, C₂H₄, C₂H₆, C₄H₂, C₆H₆, C₆N₂, C₂N₂, HCN, and HC₃N (Hirtzig et al. 2009). These species are likely synthesized by photochemically induced reaction of highly abundant N₂ and CH₄. When the amounts of these trace molecules become large enough, they form aerosol hazes in Titan's atmosphere (Liang et al. 2007), but the composition and mechanism of the formation of these organic layers remain poorly understood. These aerosol hazes likely arise from photochemically initiated reactions involving C₂H₂ present in a significant fraction in Titan's atmosphere. The photochemistry of C₂H₂ thus plays an important role in Titan's atmosphere because it absorbs photons efficiently at wavelengths less than 200 nm.

The temperature in Titan's stratosphere is about 120 K, but less on Titan's surface. At such temperatures, ethyne becomes condensed to a solid; information about the VUV photochemistry of solid ethyne from laboratory is thus needed. In this work, we photodissociated ethyne in solid Ne at three VUV wavelengths through its excited states that generated the radical C₂H as the major species; consecutive photochemical reactions ensue to form various products including large carbon clusters and hydrides. Among these products, we observed carbon hydrides with carbon atoms in even numbers in C₂H_n (n = 1–3), C₄H_n (n = 1–2), and C₈H_n (n = 1–2). We postulate that C₂H_n is the source of the formation of C₄H_n, which is in turn the precursor of C₈H_n. The absence of C₆H_n in the VUV photochemical system of ethyne suggests that a combination of C₂H_n and C₄H_n to form C₆H_n seems less likely in this system.