While (1) gas-phase reactions, (2) surface reactions on bare carbonaceous or siliceous dust grains, (3) thermal reactions, and (4) atom addition reactions contribute to interstellar chemistry, the energetic processing of ice mantles inside dark, dense molecular clouds via photochemistry and radiation chemistry is thought to be the dominant mechanism for the synthesis of prebiotic molecules. We explore differences in flux of low energy (&lt; 20 eV) secondary electrons—agents of radiation chemistry—and low-energy photons—instigators of photochemistry. Photons produced via excitation of gaseous hydrogen within dense molecular clouds have a flux of ~ 103 photons cm−2 s−1, whereas our preliminary Fermi-type calculations indicate fluxes as high as ~ 102 electrons cm−2 s−1 for low-energy secondary electrons produced within interstellar ices due to incident cosmic rays. Given that electron flux does not diminish with depth, we suggest that the fluxes of photons and electrons within ices are approximately equal. Because (1) electron-induced singlet-to-triplet transitions are allowed, (2) electrons can be captured into resonant negative ion states that may then dissociate, (3) electron impact excitation is not a resonant process, reaction cross-sections can be several orders of magnitude larger for electrons than for photons, particularly at incident energies corresponding to resonances associated with dissociative electron attachment. Therefore, these order-of-magnitude calculations suggest that the effects of low-energy secondary electrons are at least as significant as those of photons in the interstellar synthesis of prebiotic molecules. Moreover, given the orders of magnitude higher penetration depth of galactic cosmic rays compared to UV photons on the Martian surface, electrons likely dominate photons in the subsurface Martian radiolysis, which has recently been hypothesized as a possible source of metabolic energy.

Investigating the relative importance of low-energy (< 20 eV) electrons in astrochemistry

ACS Spring Meeting 2021

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