The main goals of the 2023 visit were to study the electron impact emission cross sections, spectral features, and dissociation thresholds of CS2 gas. The products of CS2 – atomic sulfur and its ions, CS, excited CS2, and CS2+ – make CS2 a rich target of inquiry. Further, the products CS and atomic S are routinely observed in near-nucleus observations of comets (see e.g. discussion in Noonan et al. 2023). Measurements of sulfur abundances in comets show discrepancies between remote and in-situ observations, and improved electron-impact data for CS2 may help resolve this discrepancy. The present experiments are part of a long-term campaign to understand diagnostic electron-impact driven emission and ionization of diatomic/polyatomic molecules in cometary atmospheres. We expect these data will provide valuable insights in one of our ongoing projects to investigate sulfur abundances through analyses of 100+ archived comet observations. In the first week of our visit to the EIF lab, we measured the electron-impact spectrum of CS2 gas at various electron energies between 0 – 100 eV, with energies chosen based on known thresholds for CS, CS2+, and atomic fragment production. During this time, we also began developing an emission model for CS in order to simplify the future analyses of these data. In the second week of the visit, higher-resolution spectra and several cross sections were measured in order to begin comparisons to existing literature. We also identified, for the first time, the emissions of atomic fragments (S I, S II) in the near-infrared red-ward of 600 nm.
The main objective of the project was to study electron impact processes of small organic compounds, using acetone as the first target.
Record and analyse emission spectra of acetone induced by electron impact at several different energies in the range 10 – 100 eV.
Determine emission cross sections corresponding to selected most intensive transitions in range of impact energies (from the thresholds of selected process to 100 eV).
Identify neutral products of electron impact fragmentation of acetone.
Determine reaction kinetics parameters such as threshold energies for selected electron impact excitation reactions of acetone.
During the first half of the visit, we measured electron-impact spectra of acetone at multiple electron energies and generated partial spectral electron energy map which provides the spectral information at various electron energies and thus the efficiency curves (relative emission cross-section curves). During the second half of the visit, the measured data was partially analysed. The emission band in the range of 415 – 445 nm corresponds to the radiation of CH (A2Δ–X2Π) (ν,ν) fragment. Less intensive radiation of CH (B2Σ−–X2Π) (0,0) fragment was identified within 386 – 402 nm. Several emission lines of hydrogen’s Balmer series Hγ – Hη were detected throughout the spectrum as well. Individual rotational transitions from P, Q, R branches of both CH fragments were identified according to LIFBASE 2.1.1 spectroscopy tool, which is software to chart the spectroscopy of diatomic molecules. The rotational temperature of the LIFBASE spectrum was set to ~ 5000 K.
Report Summary: The goal of the 2022 visit was to study and measure the electron-impact induced emission from dissociation and/or ionisation of CO and CO2 between 0 – 100 eV electron energy. These experiments are part of a longer-term plan to characterize the electron-impact-induced emission features of oxygen-containing molecules found in cometary environments. These data are expected to be used in future modelling and analyses of data acquired in situ during the Rosetta mission to comet 67P/Churyumov-Gerasimenko. We aim to understand the conditions in the inner coma and how electron-impact-induced emission features can probe the physical and chemical processes occurring in the near-nucleus coma environment.
During the first half of the visit, we measured electron-impact spectra of CO2 gas at multiple electron energies. Electron impact of CO2 can give rise to emission from CO, CO+, CO2+, and excited states of C and O atoms. Since the probabilities of the different reaction channels depend strongly on the collision energy, these spectral features offer a way to diagnose the conditions of plasmas containing CO2. The collected spectra and threshold measurements are in reasonable agreement with the limited data in the literature. During the second half of the visit, we measured electron-impact spectra of CO gas at numerous electron energies. Many of the spectral features for neutral CO, CO+, and atomic C and O were characterised, as a function of electron energy, for the first time. Given the time-consuming nature of the measurements, data analysis and additional measurements will continue remotely.
Report Summary: Auroral emissions from electron impact processes provide the opportunity to remotely characterize the physical properties of plasma and neutral gases surrounding small bodies. Surprisingly, Rosetta found that outside 2 AU, atomic and molecular emission features in the inner coma were predominantly caused by dissociative electron impact excitation. These emission features provide a wealth of information on local plasma conditions and through excited fragment species, it can allow for the measurement of chemical abundances of species that may otherwise not be easily detected remotely (CO2, O2).
We conducted electron impact experiments at the electron induced fluorescence laboratory at Comenius University (Bratislava, Slovak Republic) to characterize electron-impact induced emission of fragment species in the neutral gas surrounding comets and other small bodies in our solar system. For this project, we studied collisions between electrons up to 100 eV and CO2 and CO molecules. We measured velocity-dependent emission cross sections, determine activation thresholds of relevant reactions, and construct a spectral atlas that will aid observers and astrophysical modelers.
Europlanet 2024 RI has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 871149.
Europlanet AISBL (Association Internationale Sans But Lucratif - 0800.634.634) is hosted by the Department of Planetary Atmospheres of the Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Avenue Circulaire 3, B-1180 Brussels, Belgium.