Speaker
Description
The study of gas-phase ion-neutral reactions is a subject of significant research interest for astrochemistry, due to their higher reaction rates at low temperatures and pressures than the equivalent neutral-neutral processes [1]. Furthermore, following recent increases in observational capabilities, a growing appreciation has developed regarding the importance of isomers in astrochemistry [2,3], with the low-temperature conditions in many environments making barriers to interconversion inaccessible. This means that the relative densities of different isomers can deviate significantly from what would be predicted by a thermochemical equilibrium, instead reflecting the relative rates of formation and destruction. As a result, to accurately reproduce the chemistry of these astrochemical environments, modelers require isomer-specific experimental data.
While some isomeric species can be synthesized selectively, allowing isomeric ions to be generated through direct ionization of the isomer-specific neutral samples, many relevant ionic isomers lack stable neutral equivalents, and so alternative methods must be developed. One particularly promising approach to this has been the use of dissociative ionization processes [4,5], where larger neutral molecules are ionized into excited states that can then dissociate into the ionic isomer of interest along with a corresponding neutral fragment or fragments, with the target ion typically being selected based on their mass-to-charge (m/z) ratio. However, this doesn’t allow for any discrimination against isobaric ions, including other isomers that can result from structural rearrangement prior to fragmentation. Spectroscopic measurements are therefore needed for structural identification of the ions formed by such processes.
Sulphur is among the ten most abundant elements in astrochemical environments, and S-containing species have been detected in environments ranging from photodissociation regions and protoplanetary disks to dense cores and exoplanetary atmospheres [6-8]. An environment where a diverse range of S-containing species have been detected is the carbon-rich cold, dark cloud TMC-1, with the recent observation of HCCS+ marking the first observation of a protonated radical species in a cold dark cloud [9]. While the H2CCS•+ ion has not yet been observed, HCCS neutral has been [10], with the protonation of HCCS to give H2CCS•+ potentially proceeding similarly to the protonation of CCS, which is believed to be the major pathway for HCCS+ formation in TMC-1 [9]. Importantly, extremely limited experimental data for both HCCS+ and H2CCS•+ is currently available, largely due to a lack of known generation methods for either species [11].
In these studies, we have identified methods for the selective generation of both HCCS+ and H2CCS•+, using 2,5-dibromothiophene and thiophene, respectively. These formation methods have been probed through both computational potential energy surface (PES) calculations and experimental vibrational spectroscopy. Experimental measurements utilised the FELion instrument [12], in conjunction with the FELIX beamline [13], to perform infrared pre-dissociation (IR-PD) spectroscopy of the ions formed by dissociative ionization. Comparison of these experimental spectra with theoretical predictions and the calculated fragmentation PESs allows us to conclusively identify the fragment ions as the target species.
These results not only expand our understanding of fragmentation dynamics but enable future reactivity measurements of these ions. These measurements also provide vibrational spectra that can be used as the basis for future observations, in particular by the vibrational spectrometer of the James Webb Space Telescope.
References
[1] M. Larsson, W. D. Geppert, and G. Nyman, “Ion Chemistry in Space”, Reports on Progress in Physics, 75, 066901 (2012)
[2] M. A. Cordiner, N. A. Teanby et al. “ALMA Spectral Imaging of Titan Contemporaneous with Cassini’s Grand Finale”, The Astronomical Journal, 158, 76 (2019)
[3] R. C. Woods, C. S. Gudeman et al. “The [HCO+]/[HOC+] abundance ratio in molecular clouds”, The Astrophysical Journal, 270, 583-588 (1983)
[4] V. Richardson, C. Alcaraz et al. “The reactivity of methanimine radical cation (H2CNH•+) and its isomer aminomethylene (HCNH2•+) with methane”, Chem. Phys. Lett., 775, 138611 (2021)
[5] V. Richardson, L. Alcock et al. “Experimental Characterization of the Isomer-Selective Generation of the Astrochemically Relevant Hydroxymethylene Radical Cation (HCOH•+/DCOH•+)”, J. Phys. Chem. Lett., 15, 10888-10895 (2024)
[6] J . R. Goicoechea, J. Pety et al. “Low sulfur depletion in the Horsehead PDR”, A&A, 456, 565-580 (2006)
[7] C. Vastel, D. Quénard et al. “Sulphur chemistry in the L1544 pre-stellar core”, Monthly Notices of the Royal Astronomical Society, 478, 5514-5532 (2018)
[8] R. Le Gal, K. I. Öberg et al. “Sulfur Chemistry in Protoplanetary Disks: CS and H2CS”, The Astrophysical Journal, 876, 72 (2019)
[9] C. Cabezas, M. Agúndez et al. “Discovery of the elusive thioketenylium, HCCS+, in TMC-1”, A&A, 657, L4 (2022)
[10] J. Cernicharo, C. Cabezas et al. “TMC-1, the starless core sulfur factory: Discovery of NCS, HCCS, H2CCS, H2CCCS, and C4S and detection of C5S”, A&A, 648, L3 (2021)
[11] T. J. Millar, C. Walsh et al. “The UMIST Database for Astrochemistry 2022”, A&A, 682, A109 (2024)
[12] P. Jusko, S. Brünken et al. “The FELion cryogenic ion trap beam line at the FELIX free-electron laser laboratory: Infrared signatures of primary alcohol cations” Faraday Discuss. 217, 172-202 (2019)
[13] D. Oepts, A. F. G. Van der Meer, and P. W. Van Amersfoort, “The free-electron-laser user facility FELIX”, Infrared Phys. Technol. 36, 297-308 (1995)
[14] J. A. Diprose, K. Steenbakkers et al. “Selective formation and spectroscopic characterization of the H2CCS•+ radical cation via dissociative ionization of thiophene” J. Chem. Phys. 162, 164304 (2025)
