Formation of dimethyl sulfide (CH₃SCH₃) and ethanethiol (CH₃CH₂SH) in interstellar analog ices of methane (CH₄) and hydrogen sulfide (H₂S)

Abstract

Hitherto unidentified abiotic formation pathways leading to the organosulfur molecules ethanethiol (C₂H₅SH), methanethiol (CH₃SH) and, dimethyl sulfide (CH₃SCH₃) were investigated through a series of laboratory simulation experiments. Interstellar analog ices of methane (CH₄) and hydrogen sulfide (H₂S) were exposed to proxies of galactic cosmic rays (GCRs) in the form of energetic electrons released in the GCR track in interstellar ices simulating typical cold molecular cloud lifetimes of a few 10⁶ to 10⁷ years. During the temperature-programmed desorption phase, the molecules subliming fractionally from the ice mixtures were photoionized with vacuum ultraviolet (VUV) photons at energies both above and below the adiabatic ionization energies of the product molecules of interest. Exploiting photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS) and isotopically labelled ice experiments, the reaction products were selectively photoionized to discriminate between isomers. Ethane (C₂H₆) and methanethiol (CH₃SH), as first-generation irradiation products, along with second-generation dimethyl sulfide (CH₃SCH₃), were identified via infrared spectroscopy and PI-ReToF-MS. The formation of ethanethiol (C₂H₅SH) was further confirmed by matching the photoionization efficiency (PIE) curve to the experimental PI-ReToF-MS data. Our findings instigate a deeper understanding of interstellar sulfur chemistry linking interstellar and cometary ices to the gas-phase detection of sulfur bearing organics in star-forming regions.

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Introduction

In the search of carbonaceous life as we know it on extraterrestrial planets, terrestrial molecules of sole biotic origins are exploited as biosignatures.^1,2 Dimethyl sulfide (CH₃SCH₃), dimethyl disulfide (CH₃SSCH₃), and methanethiol (CH₃SH) have been discussed as such biomarkers and organosulfur compounds of early Earth-like biospheres^2–4 based on their functional and/or chemical characteristics derived from biological entities in extraterrestrial rocky or aquatic planetary environments.^5,6 A recent investigation of the exoplanet K2-18 b, located 120 light years away from Earth, reported a possible detection of dimethyl sulfide (CH₃SCH₃) utilizing the James Webb space telescope (JWST) near-infrared spectrograph (NIRSpec) and near-infrared imager and slitless spectrograph (NIRISS).⁷ However, the observations of these molecules in the interstellar medium (ISM) and in comets have challenged the designation of those sulfur-bearing molecules as biomarkers considering plausible, but hitherto unknown abiotic formation routes in deep space. Furthermore, the interstellar detection of thiols (RSH), with R being an alkyl chain,^8–10 like methanethiol (CH₃SH) and ethanethiol (C₂H₅SH) suggest the extraterrestrial formation of these molecules and their subsequent delivery to early Earth and other Earth-like biospheres.

As one of the simplest sulfur-bearing complex organic molecules (COMs), methanethiol (CH₃SH), also known as methyl mercaptan, was first observed toward the Sagittarius B2 molecular cloud¹¹ and in multiple other star-forming regions such as the hot cores Orion KL and G327.3-0.6.^12,13 Singly deuterated methyl mercaptan (CH₂DSH) was detected toward the protostar IRAS 16293-2422 B.¹⁴ Ethyl mercaptan, also known as ethanethiol (C₂H₅SH), was tentatively assigned toward Orion KL¹² and later confirmed toward the galactic center quiescent cloud G+0.693–0.027.¹⁰ The structural isomer of ethanethiol, dimethyl sulfide (CH₃SCH₃), was recently observed toward G+0.693–0.027 as well.¹⁵ In addition to the detections of these molecules in the interstellar medium, ionized methanethiol (CH₃SH; mass-to-charge ratio (m/z) = 48), and ethanethiol and/or dimethyl sulfide (C₂H₆S; m/z = 62) were identified in the coma of comet 67P/Churyumov–Gerasimenko with the double focusing mass spectrometer (DFMS) of Rosetta orbiter spectrometer for ion and neutral analysis (ROSINA) instrument onboard the Rosetta mission, with hydrogen sulfide (H₂S) being the dominant sulfur species in the coma.^16,17 The carrier of the signal at m/z = 62 remained elusive until Hänni et al. revisited the electron ionization fragmentation patterns of both ethanethiol and dimethyl sulfide, thus identifying the carrier of m/z = 62 as dimethyl sulfide.¹⁸ Furthermore, organosulfur molecules of C_nH_mS_p (n = 0–4, m = 0–6, and p = 1, 2) have been identified in the cometary coma 67P/CG as well.¹⁹

With the observational confirmation of dimethyl sulfide, along with forty neutral organosulfur molecules in extraterrestrial environments (Fig. 1), attention has been devoted to the elucidation of the formation pathways of these sulfuretted molecules. Previous laboratory experiments have incorporated sulfur-containing ices such as hydrogen sulfide (H₂S) or sulfur dioxide (SO₂),^19–30 and sulfur atom bombardments^31,32 to probe the astrophysically relevant sulfur chemistry. On these interstellar analog ices, sulfanes (H₂S_n), sulfur allotropes,^20,22,24,33 and organosulfur molecules as complex as thioacids²⁶ and alkylsulfonic acids have been reported.³⁰ Gas phase photochemical experiments with hydrogen sulfide (H₂S) containing gas mixtures of carbon dioxide (CO₂) and methane (CH₄) probing the atmosphere of planet K2-18b observed the abiotic formation of dimethyl sulfide as an alternative abiotic formation route on planetary bodies with pronounced volcanic activity.³⁴

Fig. 1 Up-to-date inventory of neutral sulfur containing molecules identified in the interstellar environments. The atom colors are of the following correspondence. Hydrogen: white, carbon: grey, nitrogen: blue, oxygen: red, silicon: teal and sulfur: yellow.

Here, laboratory simulation experiments were conducted at the W. M. Keck Research Laboratory in Astrochemistry exploiting the effects of proxies of galactic cosmic rays (GCRs) on interstellar ices composed of hydrogen sulfide (H₂S) and the simplest alkane, methane (CH₄). Ices were processed at low temperatures of 5 K with energetic electrons to mimic the interaction of secondary electrons that are generated in the tracks of galactic cosmic rays that penetrate the icy interstellar dust grains over a typical cold molecular cloud lifetime of a few 10⁶ to 10⁷ years.^35–37 Fourier transform infrared spectroscopy (FTIR) was utilized to characterize the deposited and processed ices. Tunable vacuum ultraviolet (VUV) photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS)^35,38 enabled isomer-specific identification of molecules in the gas phase as they sublimed from the icy frost during temperature programed desorption (TPD) phase of the ice mixtures. This phase simulates the post-gravitational collapse stage and the inherent gradual warm up of the neighboring environments of dust grains. These studies present the first laboratory evidence for the abiotic formation of dimethyl sulfide (CH₃SCH₃) and ethanethiol (C₂H₅SH) in interstellar analog ices exploiting modern, isomer-selective photoionization techniques. At m/z = 62, the dimethyl sulfide isomer (CH₃SCH₃) was confirmed via PI-ReToF-MS, while ethanethiol CH₃CH₂SH) was identified through the analysis of the photoionization efficiency (PIE) curve. Furthermore, formation pathways were investigated utilizing partially deuterated ices of methane–hydrogen sulfide (CD₄–H₂S) and deuterium sulfide–methane (CH₄–D₂S). Our findings are crucial for linking the detection of methyl- and ethyl thiol along with dimethyl sulfide in the interstellar medium,^10,11,15 in comet 67P/CG,^16,18 and potentially on exoplanet K2-18 b,⁷ to a viable abiotic formation route in the solid state.

Experimental section

The experiments were conducted in a hydrocarbon-free stainless steel ultrahigh-vacuum chamber at pressures of a few 10⁻¹¹ Torr generated by two magnetically suspended turbomolecular pumps in series (Osaka, TG420MCAB and TG1300MUCWB) and backed by an oil free scroll pump (Edwards, GVSP30). In the chamber, a highly-polished silver wafer is placed on a freely-rotatable cold-finger, which can be cooled down to 5.2 ± 0.2 K using a two-stage, closed-cycle helium compressor (Sumimoto Heavy Industries, RDK-415E) as described previously.^38,39 A premixed hydrogen sulfide (H₂S, Sigma Aldrich, ≥99.5%) and methane (CH₄, Airgas, >99.9%) gas mixture of H₂S–CH₄ at a 3 : 7 gas-phase ratio was introduced into the main chamber via a glass capillary array and condensed on the silver wafer at a pressure of 4 × 10⁻⁸ Torr. The ice deposition on the silver wafer was monitored in situ using a He–Ne laser (MellesGriot, 25-LHP-230, 632.8 nm). Employing refractive index (n) of 1.41 for hydrogen sulfide and 1.34 for methane, the thickness of the ices was calculated to be 1100 ± 200 nm with an average refractive index of 1.37 ± 0.05 for the mixed ice.^40–42 To simulate the secondary electrons formed in the track of GCRs penetrating the ices, each deposited ice was processed with 5 keV energetic electrons isothermally at 5 K. Two irradiation doses were used for the two main goals of the experiment. First, a high irradiation dose of 60 minutes at 1000 nA, was used to identify the irradiation products formed in these ices, and secondly, a low dose of 10 minutes at 10 nA, was used to identify the isomeric mass shifts in retrosynthetic pathways via intermediate identifications (Table 1). Energetic electrons were generated via an electron gun (Specs PU-EQ 22). The average penetration depth of the electrons was found to be 400 ± 50 nm using CASINO simulations.⁴³ Ices were deposited thicker than the simulated penetration depth to eliminate reactions at the interface of the ice and the silver substrate. The ratios of the deposited ices of methane and hydrogen sulfide were determined utilizing the IR bands and band absorption coefficients of fundamentals v₁/v₃ (H₂S, 1.12 × 10⁻¹⁷ cm molecule⁻¹), v₄ (CH₄, 1.3 × 10⁻¹⁸ cm molecule⁻¹), combination bands v₁ + v₄ (CH₄, 2.9 × 10⁻¹⁹ cm molecule⁻¹), and v₃ + v₄ (CH₄, 4.2 × 10⁻¹⁹ cm molecule⁻¹) (Table 1).^40,42 A Fourier transform infrared spectrometer (FTIR, Nicolet 6700) with a mercury–cadmium–telluride (Thermo, MCT-B) detector was used in the range of 6000–500 cm⁻¹ and a spectral resolution of 4 cm⁻¹ to monitor the chemical changes of the deposited ices in situ after deposition and irradiation. After the irradiation, the ices were warmed up from 5 to 330 K exploiting temperature–programmed desorption (TPD) at rate of 1 K min⁻¹ with the help of a temperature programmable controller (Lake Shore 336) simulating the transformation of cold molecular clouds to star-forming regions.

Table 1 Parameters and dosage calculations of methane–hydrogen sulfide ices

Irradiated area (cm^2)	1.6 ± 0.1
Initial kinetic energy (keV)	5.0
Average energy of transmitted electrons (keV)	0.0
Irradiation current (nA)	10 ± 1			100 ± 11
Total number of electrons	(3.74 ± 0.06) × 10^13			(2.25 ± 0.04) × 10^15
Fraction of transmitted electrons	0.0			0.0
	Low dose, at 10.49 eV			High dose, at 9.34 eV, 8.75 eV, and 8.17 eV
Ice composition	CH4–H2S	CD4–H2S	CH4–D2S	CH4–H2S
Ratio	0.2 ± 0.1 : 1	0.2 ± 0.1 : 1	0.4 ± 0.2 : 1	0.2 ± 0.1 : 1
Density of mixed ice (g cm^−1)	0.561	0.621	0.543	0.561
Average thickness (nm)	1570 ± 200			1100 ± 200
Average penetration depth (nm)	400 ± 50			400 ± 50
Dose per molecule of methane (eV molecule^−1)	0.12 ± 0.04	0.12 ± 0.04	0.16 ± 0.04	7.3 ± 0.3
Dose per molecule of hydrogen sulfide (eV molecule^−1)	0.27 ± 0.04	0.26 ± 0.04	0.28 ± 0.04	15.6 ± 0.3

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During TPD, molecules subliming to the gas phase were analyzed via tunable, single photon ionization exploiting pulsed (30 Hz) vacuum ultraviolet (VUV) light. VUV photons at energies 10.49, 9.34, 8.75, and 8.17 eV were generated via resonant or non-resonant four-wave mixing (FWM) of two pulsed laser beams overlapped in space and time with xenon (99.999%) or krypton (99.999%) as non-linear medium (Fig. 2). A reflectron time-of-flight (ReToF) mass spectrometer (Jordan TOF Products, Inc.) analyzed the photoionized species isomer-specifically. The third harmonic of a pulsed neodymium yttrium-aluminum garnet (Nd:YAG, Spectra-Physics, Quanta Ray PRO 250-30) laser was utilized to generate the 10.49 eV photons via frequency tripling in xenon. The third harmonic of a second pulsed Nd:YAG laser (Spectra-Physics, Quanta Ray PRO 270-30) was used to pump a dye laser (Sirah Lasertechnik, Cobra-Stretch) containing Stilbene-420 dye to obtain 425.112 nm, which undergoes second harmonic generation to produce 212.556 nm (ω₁). Coherent VUV light of 9.34 eV energy was generated by spatially overlapping and time synchronizing colinear beams of ω₁ (212.556 nm) with the third harmonic of the other pulsed Nd:YAG laser (ω₂ = 355 nm) in an evacuated chamber with pulsed jets of krypton gas (30 Hz). Likewise, in generating other wavelengths listed in Table 2, four-wave mixing was utilized with the corresponding dye solutions indicated. The VUV light of relevant energy was spatially separated from interfering wavelengths utilizing a biconvex lithium fluoride lens (ISP Optics) positioned off axis to the beam. The VUV beam of desired energy was passed through a 1 mm aperture, 2.0 ± 0.5 mm above the substrate surface, to photoionize the subliming molecules during TPD. The resulting ion signals were collected by a reflectron time-of-flight mass spectrometer (ReToF-MS; Jordan TOF Products, Inc.) and detected by microchannel plate (MCP) detector. A preamplifier (Ortec 9305) was incorporated to amplify the signal. The signal arriving in 4 ns bin width times were analyzed with a multichannel scaler (FAST ComTec, MCS6A).⁴⁴ The integration of mass spectra of 3600 sweeps at 30 Hz corresponds to an integration time of 2 minutes.

Fig. 2 Ranges of adiabatic ionization energies of methanethiol (CH₃SH, 1), dimethyl sulfide (CH₃SCH₃, 2) and its isomer ethanethiol (C₂H₅SH, 3) incorporating thermal and stark effect are denoted in grey, while the black lines indicate their measured ionization energies. Colored dashed lines indicate the different photoionization energies at which the experiments were conducted to collect ReToF-MS data.

Table 2 Photoionization experiments utilize four-wave mixing schemes to generate vacuum ultraviolet (VUV) photons. At least one dye laser pumped by a Nd:YAG laser was used with appropriate harmonic (355 or 532 nm) in accordance with the dye

Medium	ω VUV	Nd:YAG wavelength (nm)	ω 1 (nm)	ω 1 dye	Nd:YAG wavelength (nm)	ω 2 (nm)	ω 2 dye	Energy (eV)
a Nd:YAG harmonic.
Xenon	3ω1	355^a	—	—	—	—	—	10.49
Krypton	2ω1 − ω2	355	212.556	Stilbene 420	532	532^a	—	9.34
Krypton	2ω1 − ω2	355	212.556	Stilbene 420	—	425.112	—	8.75
Krypton	2ω1 − ω2	355	212.556	Stilbene 420	355	355^a	—	8.17

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Photon energies were chosen to distinguish between the adiabatic ionization energies (IEs) of methanethiol (1, CH₃SH), dimethyl sulfide (2, CH₃SCH₃), and ethanethiol (3, CH₃CH₂SH). The IEs of these molecules are well known from the literature. The IEs of methanethiol, ethanethiol, and dimethyl sulfide are reported as 9.439 ± 0.005 eV,⁴⁵ 9.31 ± 0.03 eV,⁴⁵ and 8.69 ± 0.02 eV,⁴⁵ respectively. The observable IEs indicated in Fig. 2 include the errors and corrected for the Stark effect of the ReToF acceleration field by −0.03 eV (Table 3). Only VUV photons with energies above the ionization threshold of a given molecule can produce the corresponding molecular ion; the resulting radical cation may undergo dissociation in the gas phase at above their threshold for dissociative photoionization.⁴⁶ In principle, molecules undergoing dissociative photoionization may contribute ion signal at corresponding fragment masses as they sublime; this can be evidenced or disproven by, e.g., comparing the TPD profiles of the parents with the fragments. At 10.49 eV, all the three molecules if present can be ionized, whereas 9.34 eV photons can ionize both isomers dimethyl sulfide (2) and ethanethiol (3) at m/z = 62, but not methanethiol (1) at m/z = 48. VUV photons of 8.75 eV can aid in differentiating between the two isomers at m/z = 62 and can only ionize dimethyl sulfide (2). The experiment carried out at 8.17 eV photon energy is below the ionization energies of all three molecules of interest. The photon energies of 8.75 eV and 9.34 eV employed in the experiments are close to the ionization energies of dimethyl sulfide (2, IE = 8.69 ± 0.02 eV) and ethanethiol (3, IE = 9.31 ± 0.03 eV), respectively, thereby excluding the fragmentation of isomers 2 and 3 upon photoionization. An additional experiment with no irradiation (blank) photoionizing with 10.49 eV photons was conducted to confirm that no thermal reactions caused the formation of products. Isotopically labeled ice mixtures CH₄–D₂S (D₂S, Sigma Aldrich, 97 atom % D, Fig. S1), and CD₄–H₂S (CD₄, Sigma-Aldrich, 99.9 atom % D, Fig. S2) were compared to product masses observed on CH₄–H₂S ice processed with a low irradiation dose at 10.49 eV, to confirm the assignments and the formation pathways.

Table 3 Error analysis of adiabatic ionization energies (IEs) of methanethiol (1), dimethyl sulfide (2), and ethanethiol (3). The observable IE ranges are obtained via correcting for the thermal and Stark effects by −0.03 eV

	Molecule/isomer	m/z	Structure	IE (evaluated per eV)	IE range with error (eV)	Corrected IE with Stark effect (eV)
1	CH3SH	48		9.439 ± 0.005^45	9.434–9.444	9.40–9.41
2	CH3SCH3	62		8.69 ± 0.02^45	8.67–8.71	8.64–8.68
3	C2H5SH	62		9.31 ± 0.03^45	9.28–9.34	9.25–9.31

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Results and discussion

Infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy was used to characterize the deposited ices. All absorption features observed in pristine ices were associated with fundamentals or overtone bands of hydrogen sulfide and methane (Fig. 3A). A combination of symmetric and antisymmetric stretching modes of S–H bonds (v₁ and v₃) gave a strong characteristic peak for hydrogen sulfide in the range of 2581–2560 cm⁻¹. The bending mode of H₂S (v₂) emerges as a peak of weak intensity at 1169 cm⁻¹.^47,48 Methane displayed the fundamentals v₃ (3010–3003 cm⁻¹), v₄ (1300–1295 cm⁻¹), and a small peak for infrared inactive v₂ (1528 cm⁻¹) vibration.^42,49 After the irradiation, new infrared (IR) absorption features were observed and deconvoluted in cyan in Fig. 3B. A difference IR spectrum was obtained between after and before irradiation is included in Fig. 3C. Table 4 summarizes the IR absorption features of the deposited pristine ices compared to the features after irradiation.

Fig. 3 Infrared spectra of methane (CH₄) and hydrogen sulfide (H₂S) ice. Original spectrum (grey) is deconvoluted showing the peaks assigned to hydrogen sulfide (pink) and methane (green). The red dashed line is the sum of deconvoluted Gaussian peaks. (A) Pristine ice at 5 K. (B) New features emerged after irradiation are indicated in cyan (Table 4). Insets include the magnified regions 4600–4150 cm⁻¹ and 1550–1400 cm⁻¹. (C) A difference IR spectrum was obtained between after and before irradiation. A blue shift is observed in the reactant peaks. The new IR features observed after irradiation are indicated in red.

Table 4 Infrared absorption assignments of methane (CH₄) and hydrogen sulfide (H₂S) ices before irradiation and the products formed after electron irradiation

Assignment		Position (cm^−1)	Ref.
Before irradiation CH4–H2S ice (10 K)
v 2 + v3	CH4	4525	49
v 3 + v4	CH4	4309, 4299, 4295	49
v 1 + v4	CH4	4201, 4197	49
v 3 + vL	CH4	3074, 3028	49
v 3	CH4	3010, 3008, 3003	49
v 1	CH4	2902	49
v 2 + v4	CH4	2837, 2818, 2812	49
v 1/v3 + vR	H2S	2644	47
v 1/v3 + vT	H2S	2607	47
v 1/v3	H2S	2581, 2560	47
v 2	CH4	1528	49
v 4 + vL	CH4	1334	42
v 4	CH4	1300, 1295	49
v 2	H2S	1174	47
Before irradiation CH4–D2S ice (10 K)
v 2 + v3	CH4	4525	49
v 3 + v4	CH4	4312, 4300, 4295	49
v 1 + v4	CH4	4199	49
v 3 + vL	CH4	3083, 3025	49
v 3	CH4	3010, 3007, 3003	49
v 1	CH4	2901	49
v 2 + v4	CH4	2820, 2812	49
v 1/v3 + v2	D2S	2733, 2709	47
v 1/v3 + vT	H2S	2602	47
v 1/v3	H2S	2590, 2560	47
v 1/v3 + vR	D2S	1945	47
v 1/v3 + vT	D2S	1893	47
v 1/v3	D2S	1873, 1855	47
v 2	CH4	1528	49
v 4 + vL	CH4	1361, 1325	42
v 4	CH4	1300, 1296	49
v 2	D2S	849	47
Before irradiation CD4–H2S ice (10 K)
2v3	CD4	4474	72
…	…	4308	72
v 3 + 2v4 + vL	CD4	4221	72
v 3 + 2v4	CD4	4191	72
v 3 + v4	CD4	3239, 3234	72
v 1 + v4	CD4	3086	72
v 1	CHD3	2978	72
v 1/v3 + vR	H2S	2653	47
v 1/v3 + vT	H2S	2608	47
v 1/v3	H2S	2579, 2560	47
…	…	2339	72
v 3 + vL	CD4	2305	72
v 3	CD4	2262, 2254, 2251, 2248, 2236	72
v 2 + v4	CD4	2090, 2072	72
2v4	CD4	1980, 1974	72
v 1/v3	D2S	1879, 1859	47
v 2	H2S	1175	47
v 4 + vL	CD4	1026	72
v 4	CD4	991, 986	72
New features after irradiation CH4–H2S ice (10 K)
v 1	C2H6	2974	49
v 10	C2H6	2960	49
v 8 + v11	C2H6	2939	49
v 5	C2H6	2881	49
v(S–H)	H2Sn (n > 2)	2495	24
v 3	CS2	1513	51 and 73
v 11	C2H6	1464	49
v 12	C2H4	1437	49
δ(CH3–S–)	CH3–S–/–CH2S–	1429, 1414	52

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Ethane (C₂H₆) was identified via its fundamentals v₁ (2974 cm⁻¹), v₁₀ (2960 cm⁻¹), v₅ (2881 cm⁻¹), v₁₁ (1462 cm⁻¹)⁴⁹ and v₈ + v₁₁ overtone at 2939 cm⁻¹. Absorption feature adjacent to v₁₁ (C₂H₆, 1462 cm⁻¹), at 1437 cm⁻¹ is assigned to scissoring CH₂ vibrations (v₁₂) of C₂H₄.⁴⁹ Broadening of S–H stretching was observed in 2495 cm⁻¹ due to the formation of disulfane (v₅, H₂S₂) and other higher order sulfanes (H₂S_n; n > 2).^22,24,50 Carbon disulfide (CS₂) was identified by its strongest absorption feature, v₃, at 1513 cm⁻¹.⁵¹ In the range 1460–1400 cm⁻¹, methyl residues connected to sulfur atoms (–SCH₃) show symmetric CH₃ deformation vibration features and in the range 1435–1410 cm⁻¹, CH₂ deformation vibrations by –CH₂–S– moieties.⁵² Thus, the features observed at 1429 and 1414 cm⁻¹ are assigned to these deformation vibrations. Since FTIR limits the identifications to functional groups in case of complex icy mixtures, an alternative isomer specific technique is required to investigate the individual products formed.

Photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS)

Tunable vacuum-ultraviolet (VUV) photoionization coupled with reflectron time-of-flight mass spectrometry (PI-ReToF-MS) was utilized to identify the molecules sublimed into the gas phase during TPD. PI-ReToF-MS provides a unique approach in an isomer-selective detection with soft photoionization of molecules of interest via selection of photon energies that would not fragment the molecular parent ions, unlike in electron impact (EI) mass detections with fixed electron energies 70–100 eV.^53,54 A species can only be detected by choosing a photon energy higher than its IE; no parent ion signal is expected with photons of energy below its IE.^39,54 The PI-ReToF-MS data collected during TPD of irradiated and non-irradiated ices of methane and hydrogen sulfide are compiled in Fig. 4. The first sublimation event in both irradiated and non-irradiated (blank experiment) ices photoionized at 10.49 eV is the desorption of hydrogen sulfide with a peak sublimation temperature of 90 K. The H₂S desorption peak is absent in photoionization energies employed below the IE of H₂S, 10.453 ± 0.008 eV.⁵⁵ Methane desorption event at 40 K is not recorded via mass spectrometry as the ionization energy of methane is higher than the photon energies utilized to ionize the subliming molecules (IE = 12.61 ± 0.01 eV).⁵⁶ Only the H₂S sublimation event is observed in the unirradiated CH₄–H₂S ice, confirming all other ion signals observed are a result of energetic electron irradiation on the ices. It should be noted that H₂S trapped amongst the product molecules in the matrix can co–sublime into the gas phase at temperatures higher than 90 K with the gradual sublimation of corresponding molecules (Fig. 4, CH₄–H₂S ice, 10.49 eV, lower irradiation dose). This sublimation behavior may differ in isotopic labeled ices, as previously observed in hydrogen sulfide–carbon monoxide ices.²⁶ Multiple sublimation events reaching to high molecular masses as high as m/z = 322 are observed with the higher irradiation dose compared to the lower irradiation dose at 10.49 eV. The carriers corresponding to these sublimation events observed were investigated utilizing isotopically labeled experiments. In the irradiated CH₄–H₂S ices, possible species for ion signal of m/z = 62 are C₅H₂, and/or C₂H₆S; for m/z = 48, only C₄ and/or CH₄S have to be considered. Isotopically labeled ices CH₄–D₂S and CD₄–H₂S were used to confirm the molecular formula associated with these mass channels by comparing their TPD profiles and identifying the expected mass shifts resulting from substitution of hydrogen with deuterium as outlined below.

Fig. 4 PI-ReToF-MS data collected during TPD of processed methane–hydrogen sulfide ices. Data were collected for a non-irradiated CH₄–H₂S ice (blank), high irradiation dose processed CH₄–H₂S ices at 10.49 eV, 9.34 eV, 8.75 eV, 8.17 eV, and a low irradiation dose processed CH₄–H₂S, CH₄–D₂S, and CD₄–H₂S ices.

Methanethiol (CH₃SH)

Methanethiol (1, CH₃SH, 48 amu) has an adiabatic ionization energy (IE) of 9.439 ± 0.005 eV.⁴⁵ Therefore, VUV photons of 10.49 eV were utilized to ionize – if present – methanethiol among the subliming species in the gas phase for CH₄–H₂S ices; photons at 9.34 eV are below the IE of methanethiol and therefore any signal at 10.49 eV should disappear at 9.34 eV for m/z = 48. Fig. 5A and Fig. S3 compare the TPD profile recorded at m/z = 48 in the CH₄–H₂S system at 10.49 eV to the signal at m/z = 50, where CH₃³⁴SH is expected. The signal for CH₃³⁴SH was predicted based on the signal at m/z = 48 and the natural isotopic distribution of sulfur ³²S to ³⁴S (4.45%). Overlapping the calculated and the observed signals at m/z = 50 confirms that signal at m/z = 48 belongs to a molecule with a single sulfur atom. In the high dose study, a sublimation peak with an onset at 95 K with an integrated total of 242 730 ± 430 counts, was observed at 10.49 eV, but no ion signal was detected at 9.34 eV (Fig. 5B). Therefore, ion signal of m/z = 48 can be linked to methanethiol (1). It should be noted that the broader sublimation event observed at m/z = 48 in the high dose experiment indicates that methanethiol (1) was formed in higher abundance and likely trapped in the ice mixtures and also co-sublime with other irradiation products. To confirm this assignment and its formation mechanisms, low dose irradiation experiments were carried out for partially deuterated ices. Fig. 5C–E compare the sublimation peaks corresponding to (partially) deuterated products of CH₄S in ice mixtures CH₄–H₂S (Fig. 5C), CH₄–D₂S (Fig. 5D), and CD₄–H₂S (Fig. 5E). In the partially deuterated ices, mass shifts up to 4 amu are expected for each deuterium atom replacing hydrogen atoms of CH₄S, where CD₃SD is expected at m/z = 52. In the CH₄–D₂S ice, partially deuterated and/or ³⁴S-labeled methanethiol species observed at m/z = 49 (CH₃SD) and m/z = 50 (CH₂D₂S and CH₃³⁴SH) are plotted in Fig. 5D, while m/z = 50 (CH₂D₂S and CH₃³⁴SH), m/z = 51 (CD₃SH and CH₃³⁴SD), and m/z = 52 (CD₃SD and CH₂D₂³⁴S) in the irradiated CD₄–H₂S ice are indicated in Fig. 5E. TPD profiles of the isotopically shifted methanethiol species match the sublimation profile of m/z = 48 in the CH₄–H₂S ice (Fig. 5C), indicating that the sublimation peak with an onset at 95 K can be assigned to methanethiol (1). It should be noted that the weak sublimation event observed at 90 K is resulted from the saturation of H₂S signal at the detector.

Fig. 5 ReToF-MS data collected for methanethiol identification. (A) Signal at m/z = 48 in the CH₄–H₂S ice photoionized at 10.49 eV during TPD was identified as a single sulfur species by comparing with m/z = 50. (B) TPD profiles of m/z = 48 at 10.49 eV and 9.34 eV in the high irradiation dose experiments. (C)–(E) Comparison of the partially deuterated methanethiol products from the low dose irradiated ices.

Dimethyl sulfide (CH₃SCH₃)

Above the IEs of C₂H₆S isomers—2 (dimethyl sulfide, IE = 8.69 ± 0.02 eV) and 3 (ethanethiol, IE = 9.31 ± 0.03 eV)—photon energies of 10.49 eV and 9.34 eV can ionize both isomers if formed (Fig. 2). At 10.49 eV, a sublimation peak at m/z = 62 with an onset of 107 K with a total of 2096 ± 200 counts, was observed in the low-dose irradiated CH₄–H₂S ice (Fig. 6A, peak II). This signal can be attributed to formula(e) C₅H₂ and/or C₂H₆S. The weak sublimation event observed at 90 K is caused by the saturation of the detector upon the sublimation of H₂S (Fig. 6A and B: peak I). At 10.49 eV, signal at m/z = 64 cannot be exploited to check if the signal at m/z = 62 carries a single sulfur atom with natural isotopic distribution of ³⁴S due to the subliming S₂⁺ and other fragments. Thus, the mass shifts in the deuterated ice mixtures were investigated to confirm the identity of the signal at m/z = 62. In the low-dose irradiated deuterated ice mixtures of CH₄–D₂S and CD₄–H₂S, ion signals from m/z = 62 to m/z = 68 were investigated (Fig. 6A–C). TPD profiles of m/z = 63 and 64 were observed in the CH₄–D₂S ice with an onset sublimation temperature of 109 K, corresponding to C₂H₅SD (m/z = 63), and C₂H₄SD₂/C₂H₆³⁴S (m/z = 64) (Fig. 6B). Sublimation event III in Fig. 6B is due to S₂⁺ fragment ion, as S₂ (IE = 9.356 ± 0.002 eV)⁵⁷ can be photoionized at 10.49 eV. In the irradiated CD₄–H₂S ice, sublimation profiles with an onset of about 110 K were observed at m/z = 66, 67, and 68; these can be linked to partially deuterated products C₂D₄H₂S, C₂D₅HS, and C₂D₆S and ³⁴S isotopically labeled molecules C₂H₄D₂³⁴S, C₂H₃D₃³⁴S, and C₂H₂D₄³⁴S (Fig. 6C). Hence the presence of C₂H₆S isomers at m/z = 62 signal is confirmed.

Fig. 6 ReToF-MS data for C₂H₆S isomers photoionized at 10.49 eV during TPD. Panels (A)–(C) depict the ion counts observed for each indicated mass channel in deuterated and non-deuterated ices. Onset sublimation temperature for C₂H₆S isomers were identified as 111 K. (D) TPD profiles of m/z = 62 at 10.49 eV, 9.34 eV, 8.75 eV and 8.17 eV observed two sublimation peaks with the higher irradiation dose. First sublimation peak with an onset at 111 K originated form subliming dimethyl sulfide (CH₃SCH₃), whereas the second sublimation peak observed at m/z = 62 was identified as fragmentation from higher mass channels m/z = 94 and m/z = 96 at both photon energies (B) 9.34 eV and (C) 8.75 eV. Further analysis on the signal at m/z = 94 and m/z = 96 confirms that C₂H₆S₂ fragments contribute to the second sublimation peak in m/z = 62 at (G) 9.34 eV and (H) 8.75 eV.

Having identified the molecular formula for the ions signal at m/z = 62 as C₂H₆S, we are now focusing on elucidating the nature of the isomer formed. TPD profiles of m/z = 62 (C₂H₆S) recorded with photon energies of 10.49 eV, 9.34 eV, 8.75 eV, and 8.17 eV are shown in Fig. 6D at the high irradiation dose. At 10.49 eV and 9.34 eV, at which both isomers 2 and 3 can be ionized, two distinct sublimation peaks were observed with peak sublimation temperatures at 135 K and 171 K. The first peak has an onset at 111 K, similarly to the C₂H₆S peak identified at m/z = 62 in the low dose experiment. The TPD profile recorded at 8.75 eV, which is above the IE of dimethyl sulfide isomer (2, IE = 8.64–8.68 eV), but below the IE of ethanethiol (3, IE = 9.25–9.31 eV), revealed that two sublimation peaks remain. Lowering the photon energy to 8.17 eV, which cannot ionize dimethyl sulfide (2, IE = 8.64–8.68 eV), no sublimation events were detected, indicating that both sublimation peaks are likely associated to 2. Signal at m/z = 64 was investigated at multiple photon energies to explore potential signal from naturally occurring sulfur isotopes incorporated in C₂H₆³⁴S molecules (Fig. 7). At 10.49 eV, only the onset of m/z = 64 signal matches with the expected signal for C₂H₆³⁴S ions. But at photon energies 9.34 eV and 8.75 eV, i.e. below the IE of S₂ (9.356 ± 0.002 eV),⁵⁷ observed signal at m/z = 64 matches the predicted signal for C₂H₆³⁴S⁺ ions, thus confirming the presence of a single sulfur species in both sublimation peaks (Fig. 7). Notably, the second sublimation peak (171 K) overlaps with TPD profiles of m/z = 94 and m/z = 96 suggesting that this signal may originate from fragmentation of ions associated with m/z = 94 and 96 (Fig. 6E and F). Signal at m/z = 96 was found to be 8.9% of m/z = 94, confirming a two-sulfur system at both 9.34 eV (Fig. 6G) and 8.75 eV (Fig. 6H). Therefore, only the first sublimation event with an onset temperature of 111 K can be clearly assigned to dimethyl sulfide (2).

Fig. 7 Signal observed at m/z = 62 and m/z = 64 were used to confirm the assignment C₂H₆S. The expected signal for C₂H₆³⁴S (calculated using the signal at m/z = 62), was compared with the signal observed at m/z = 64. (A) At 10.49 eV, the mass channel m/z = 64 comprises of contributions from other ions, whereas at (B) 9.34 eV and (C) 8.75 eV, the observed signal at m/z = 64 is identified as C₂H₆³⁴S.

Ethanethiol (CH₃CH₂SH)

The PI-ReToF-MS data confirmed the presence of dimethyl sulfide (2) among the sublimed gas-phase products at 111 K, whereas ethanethiol (3) could not be unambiguously identified based on the ionization energies alone. However, it is possible that ethanethiol (3) forms in the irradiated ices and co-sublimes at the same temperature as dimethyl sulfide (2). But the contribution of 3 – if any – cannot be quantified via PI-ReToF-MS data alone. Therefore, as an additional analytical technique, the photoionization efficiency (PIE) curve, which depicts the relationship between the ion counts at m/z = 62 and the photon energy, was utilized to infer contribution of 3. PIE curves for both 2 and 3 were obtained from their photoelectron (PE) spectra (Fig. 8).^58,59 This plot can account for 50 ± 25% contribution of dimethyl sulfide (2) and ethanethiol (3), thereby suggesting the formation of isomer 3 in these interstellar analog ices.

Fig. 8 Integrated signal counts at m/z = 62 subliming in the range from 109–151 K for C₂H₆S isomers for photoionization experiments at VUV energies; 8.17 eV, 8.75 eV, 9.34 eV and 10.49 eV (black) are compared to the individual photoionization efficiency (PIE) curves of dimethyl sulfide (2, green, obtained from Frost et al. photoelectron spectrum) and ethanethiol (3, blue, obtained from Ogata et al. photoelectron spectrum) and the total fit (red) of the PIE curves obtained by taking the sum of individual curves.

Potential mechanistical pathways

Having confirmed the formation of methanethiol (1), dimethyl sulfide (2) and its isomer ethanethiol (3) in irradiated CH₄–H₂S ices, we now focus on their potential formation pathways. Reaction pathways leading to the formation of 1, 2, and 3 in the astrophysically relevant icy conditions are investigated in Fig. 9via reactions (1)–(13). Proxies for the effect of GCRs on the icy grains over a typical lifespan of cold molecular clouds in form of energetic electrons are exploited in these experiments.^37,38 These simulate secondary electron cascades generated by the GCR penetrating the ices. The energetic electrons deposit the energy that may also drive classically forbidden endoergic reactions in the ice. During the irradiation, electrons deposit doses up to 7.3 eV molecule⁻¹ for methane and 15.6 eV molecule⁻¹ for hydrogen sulfide that can easily break the C–H bonding in methane (CH₄; bond energy = 4.5 eV) and S–H bond of hydrogen sulfide (H₂S; bond energy = 3.9 eV) generating suprathermal hydrogen atoms.^49,54 These suprathermal hydrogen atoms are mobile since they bear excess kinetic energy to overcome the diffusion barrier.

Fig. 9 Retrosynthetic formation pathways of dimethyl sulfide (2, CH₃SCH₃) and ethanethiol (3, C₂H₅SH) in the methane and hydrogen sulfide (CH₄–H₂S) ice.

Methane (CH₄) can decay upon interaction with energetic electrons through endoergic homolytic C–H bond cleavage forming suprathermal hydrogen atoms (H) and methyl (CH₃, X²A″) radicals (reaction (1)); this process is endoergic by 432 kJ mol⁻¹ (4.48 eV).^49,60 Methylene (CH₂, a¹A₁) radicals can be formed viareaction (2), endoergic by 458 kJ mol⁻¹ (4.75 eV).^49,61,62

CH₄ → CH₃ + H (1)

CH₄→ CH₂ + H₂ (2)

Endoergic (376 kJ mol⁻¹ or 3.90 eV) homolytic bond cleavages of hydrogen sulfide (H₂S) can form sulfanyl (HS, X²Π) and hydrogen (H) radicals. Likewise, this leads to sulfur in its first electronically excited state (S,¹D) and molecular hydrogen; this process is endoergic by 405 kJ mol⁻¹ (4.20 eV) (reactions (3) and (4)).^61,63

H₂S → HS + H (3)

H₂S → S + H₂ (4)

These formed radicals and atoms can undergo barrierless recombination or insertion to form first generation of irradiation products. Ethane (C₂H₆) can be formed by the combination of two methyl (CH₃) radicals^49,64,65 or via a methylene (CH₂) radical inserting into a C–H bond of methane^49,64–66 (Fig. 9: reactions (5) and (6)). A sulfanyl (HS) radical combining with a methyl (CH₃) radical can easily form methanethiol (CH₃SH) barrierlessly (Fig. 9: reaction (7)). Barrierless insertions of methylene (CH₂) into the S–H bonds of hydrogen sulfide (H₂S) and sulfur atoms (S(¹D)) insertion to C–H bonding in methane (CH₄) can also form methanethiol (Fig. 9: reactions (8) and (9)). Formation of ethane (C₂H₆) was identified on these CH₄–H₂S ices via multiple infrared absorption bands. The IE of ethane is 11.52 ± 0.04 eV,⁴⁵ higher than the maximum photon energy of 10.49 eV employed in this study; therefore, ethane cannot be ionized at 10.49 eV, whereas the formation methanethiol (1) was confirmed via PI-ReToF-MS.

Branching ratios can be investigated to distinguish which mechanisms are preferred in the formation of methanethiol (1). In the irradiated CH₄–D₂S ice, partially deuterated methanethiol (1) molecules are detected at m/z = 48 (CH₃SH), m/z = 49 (CH₃SD), and m/z = 50 (CH₂D₂S and CH₃³⁴SH). PI-ReToF-MS data collected with 10.49 eV photons for CH₃SH, CH₃SD and CH₂D₂S molecules in irradiated CH₄–D₂S ice formed via sulfur atom insertion, methyl (CH₃) plus sulfanyl (SH) radical combination, and methylene (CH₂) insertion into the S–D bond of deuterium sulfide (D₂S) are compared in the top panel of Fig. 10. Considering the 4.21% natural isotopic abundance of ³⁴S, signal expected for CH₃³⁴SD (m/z = 51) based on CH₃SD (m/z = 49), and CH₂D₂³⁴S (m/z = 52) based on CH₂D₂S (m/z = 50) were calculated. The projections for CH₃³⁴SD (m/z = 51) and CH₂D₂³⁴S (m/z = 52) were compared to signal observed at the respective mass channels m/z = 51 and m/z = 52 (Fig. 10: top). Observed branching ratios between the three reaction mechanisms, i.e. radical recombination, methylene or sulfur atom insertions, were calculated by integrating the signals observed for each molecule in the CH₄–D₂S ice. Radical combination of methyl and sulfanyl radicals contributed to 68 ± 2% of methanethiol (1) formation, while 26 ± 2% formed via CH₂ radical insertion and a 6 ± 1% via S(¹D) insertion to ethane molecules. Likewise, methanethiol (1) formation in the CD₄–H₂S ice is observed at m/z = 50 (CH₂D₂S), m/z = 51 (CD₃SH), and m/z = 52 (CD₃SD and CH₂D₂³⁴S) (Fig. 5E). Signal observed at m/z = 53 and m/z = 54 were compared with the expected signals of CD₃³⁴SH (m/z = 53) and CD₃³⁴SD (m/z = 54) to confirm the assignments (Fig. 10: bottom). Note that the sublimation peak at 90 K is due to co-sublimation with H₂S. In the CD₄–H₂S ice, radical combination amounted to 70 ± 2% of methanethiol (1) formation, 20 ± 2% via CD₂ insertion to H₂S and only a 10 ± 2% via sulfur atom insertion to deuterated ethane. Branching ratios for 1 amongst the two deuterated ices are compared in Table 5.

Fig. 10 TPD profiles for methanethiol (CH₃SH) in top: methane (CH₄) and deuterated hydrogen sulfate (D₂S) and bottom: deuterated methane (CD₄) and hydrogen sulfide (H₂S) irradiated ices. The percentages of each isotopologue observed are listed on each panel along with the mass-to-charge.

Table 5 Observed ratios of deuterated methanethiol isotopologues in each experiment and the proposed branching ratios for each of the three mechanisms in the irradiated methane and hydrogen sulfide ices

Observed m/z of methanethiol in CH4 : H2S ice	Observed m/z of methanethiol in CH4 : D2S ice	Proposed formula in CH4 : D2S ice	Observed m/z of methanethiol in CD4 : H2S ice	Proposed formula in CD4 : H2S ice
m/z 48	m/z = 48 : 6 ± 1%	CH3SH	m/z = 52 : 10 ± 2%	CD3SD
	m/z = 49 : 68 ± 2%	CH3SD	m/z = 51 : 70 ± 2%	CD3SH
	m/z = 50 : 26 ± 2%	CH2DSD	m/z = 50 : 20 ± 2%	CD2HSH

---

Dimethyl sulfide (2) and ethanethiol (3) are second-generation irradiation products formed in these ices. Formation of isomer 2 can proceed either via S(¹D) insertion into the C–C bond of ethane or via methylene inserting into the S–H bond of methanethiol (Fig. 9: reactions (10) and (11)). Ethanethiol (3) is statistically more likely to be formed in these ices via S(¹D) insertion into any of the six C–H bonds of ethane and methylene radical insertions into the C–S or C–H bonds of methanethiol (Fig. 9: reactions (12) and (13)). In both partially deuterated ices of CH₄–D₂S (Fig. 6B) and CD₄–H₂S (Fig. 6C), methylene insertion to methanethiol route showed the highest signal intensities: C₂H₄SD₂ at m/z = 64 (Fig. 6B) and C₂D₄H₂S at m/z = 66 (Fig. 6C).

In addition to 1 and 2, sulfanes (H₂S_n: n = 2–11), and octasulfur (S₈)—which were identified forming on electron irradiated pure hydrogen sulfide ices²²—were also observed in these methane–hydrogen sulfide ices, amongst the plethora of molecules subliming to the gas phase (Fig. S4–S7). Natural isotopic ratio between ³²S and ³⁴S was employed in demonstrating the signal corresponding to sulfanes (H₂S_n⁺) ions in the mass channels shared with sulfur fragments (³²S_n³⁴S⁺). Furthermore, ion signals corresponding to mass channels of CH₄S_n: n = 1–9, and C₂H₆S_n: n = 1–9 that can be either alkylsulfanes or alkylsulfur-ethers were also observed to be formed on these ices exposed to the high irradiation dose (Fig. S8).

Conclusions

Our results propose the abiotic formation of methanethiol (1), dimethyl sulfide (2) and ethanethiol (3) under interstellar conditions via a series of laboratory experiments using methane (CH₄) and hydrogen sulfide (H₂S) ices that simulate astrophysically relevant temperatures and pressures. The reaction pathways leading to 2 and 3 were investigated using isotopically labeled ices CH₄–D₂S and CD₄–H₂S with a low irradiation dose. Ethane (C₂H₆) and methanethiol (1, CH₃SH), which are the first-generation irradiation products of crucial importance to the provided reaction mechanisms were also identified via infrared spectroscopy and photoionization reflectron time-of-flight mass spectrometry. Radical combination of methyl (CH₃) and sulfanyl (HS) radicals proved the major formation pathway towards methanethiol formation accounting for 68 ± 2% in CH₄–D₂S ice and 70 ± 2% in CD₄–H₂S ice.

Similar to the sulfanes and alkyl sulfanes forming in hydrogen sulfide (H₂S) and methane (CH₄) ices, the homologous series of phosphanes (P₂H₄ to P₈H₁₀) and methylphosphanes (CH₃PH₂ to CH₃P₈H₉) were reported forming in phosphine (PH₃) and methane (CH₄) ices.^41,67 But oxygen, which is isovalent to sulfur, was only found to form chains up to three atoms such as hydroxyperoxymethane (H₃COOOH), in extraterrestrial conditions.⁶⁸ The complex organosulfur molecules of C_nH_mS_p (n = 0 to 4, m = 0 to 6, and p = 1, 2) in cometary coma of 67P/CG¹⁹ may have been formed following mechanistic processes similar to such discussed here. Since these organosulfur molecules are feasible to form abiotically in interstellar or cometary ices, methanethiol or dimethyl sulfide cannot be simply interpreted as biomarkers for extraterrestrial life. Moreover, organosulfur molecules can be another possible sulfur sink for interstellar sulfur in ices that can be accounted towards the missing sulfur budget^69–71 in the interstellar medium.

Conflicts of interest

The authors declare no conflicting interests.

Data availability

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5cp04456a.

Essential data are provided in the main text. Additional data are available from the corresponding author upon reasonable request.

Acknowledgements

The authors would like to thank the US National Science Foundation (NSF) Division for Astronomy (NSF-AST 2403867) for support, W. M. Keck Foundation (R. I. K.) for financing the experimental setup, and the University of Hawai’i for providing Teaching Assistantships (A. H., M. M.).

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