In situ characterization of volatile and refractory hydrocarbons produced by UV photolysis of interstellar C2H2 ice

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Volume 705 (January 2026)

A&A, 705 (2026) A229

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Table 2

List of detected organic residues.

No. of C	m/z (decreasing intensity)	Corresponding elemental composition	Possible key substituted functional groups proposed in this work
1	15, 14, 13, 12	${C H}_{3}^{+}, {CH}_{2}^{+}$ $\mathbf{CH}_{3}^{+},\ \text{CH}_{2}^{+}$ , CH⁺, C⁺	m/z=15−R−CH₃
3	39, 38, 41, 40, 37, 36	C₃H₃⁺, C₃H₂⁺, C₃H₅⁺, C₃H₄⁺, C₃H₁⁺, C₃⁺	m/z=39=R−CH₂−C≡CH or R−C≡C−CH₃ or R−CH=C=CH₂
4	51, 50, 52, 53, 49, 54, 55	C₄H₃⁺, C₄H₂⁺, C₄H₄⁺, C₄H₅⁺, C₄H₁⁺, C₄H₆⁺, C₄H₇⁺	m/z=51=R−CH=CH−C≡CH or R−C≡C−CH=CH₂, m/z=53=R−CH=CH−CH=CH₂
5	63, 65, 62, 61, 64, 66, 67	C₅H₃⁺, C₅H₅⁺, C₅H₂⁺, C₅H₁⁺, C₅H₄⁺, C₅H₆⁺, C₅H₇⁺	m/z=63=R−CH₂−C≡C−C≡CH or R−C≡C−CH₂−C≡CH or R−C≡C−C≡C−CH₃, m/z= 65=R−C≡C−CH=CH−CH₃ or R−CH=CH−CH₂−C≡CH or R−CH₂−CH=CH−C≡CH, etc.
6	77, 78, 74, 76, 75, 79, 80	C₆ H₅⁺, C₆ H₆⁺, C₆H₂⁺, C₆H₄⁺, C₆H₃⁺, C₆H₇⁺, C₆H₈⁺	m/z=77=R−C₆H₅ (phenyl group) or R−CH=CH−CH=CH−C≡CH or R−C≡C−CH=CH−CH=CH₂ or m/z=78 (Benzene), etc.
7	91, 89, 87, 86, 88	C₇ H₇⁺, C₇ H₅⁺, C₇H₃⁺, C₇H₂⁺, C₇H₄⁺	m/z=91=R−CH₂−C₆H₅ (benzyl group), m/z = 89= R−CH₂−CH ≡ CH−CH=CH−C≡CH, etc.
8	102, 103, 98, 104, 105, 101, 99	C₈ H₆⁺, C₈ H₇⁺, C₈H₂⁺, C₈H₈⁺, C₈H₉⁺, C₈H₅⁺, C₈H₃⁺	m/z=102=H−C≡C−C₆H₅ (phenylacetylene), m/z= 103=R−CH=CH−C₆H₅ (substituted styrenes)
9	115, 116, 117, 113, 114	C₉ H₇⁺, C₉H₈⁺, C₉H₉⁺, C₉H₅⁺, C₉H₆⁺	m/z=115=R−CH₂−C≡C−C₆H₅ or R−C₉H₇ (substituted indenes)
10	128, 127, 129, 126	C₁₀ H₈⁺, C₁₀ H₇⁺, C₁₀H₉⁺, C₁₀H₆⁺	m/z=128=C₁₀H₈ (naphthalene), 127=R−C₁₀H₇ or R−CH=CH−C≡C−C₆H₅
11	139, 141	C₁₁ H₇⁺, C₁₁ H₉⁺	m/z=139=R−CH₂−C≡C−C≡C−C₆H₅, m/z=141= R−CH₂−C₁₀H₇
12	152, 153, 151, 150, 154	C₁₂ H₈⁺, C₁₂H₉⁺, C₁₂H₇⁺, C₁₂H₆⁺, C₁₂H₁₀⁺	m/z=152=HC≡C−C₁₀H₇ or C₁₂H₈ (acenaphthylene?)
13	165, 163	C₁₃H₉⁺, C₁₃H₇⁺	m/z=165=R−CH₂−C≡C−C₁₀H₇ or R−C₁₃H₉ (substituted phenalenes?)
14	178, 176	C₁₄ H₁₀⁺, C₁₄H₈⁺	m/z=178=C₁₄H₁₀ (phenanthrene) or H₂ C=CH−C≡C−C₁₀H₇
15	189, 191, 187	C₁₅H₉⁺, C₁₅H₁₁⁺, C₁₅H₇⁺	m/z=189=R−CH₂−C≡C−C≡C−C₁₀H₇
16	202, 203, 201	C₁₆H₁₀⁺, C₁₆H₁₁⁺	m/z=202=C₁₆H₁₀ (pyrene), HC≡C-C₁₄H₉

Notes. List of detected organic residues after VUV irradiation of pure C₂H₂ ice at 15 K for a total fluence of 3 × 10¹⁷ photons cm⁻² (16 minutes). A list of ion signals observed in the LDPI ReTOF mass spectra of the organic residue presented in Fig. 4.

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