On the Radiolysis of Ethylene Ices by Energetic Electrons and Implications to the Extraterrestrial Hydrocarbon Chemistry

Abstract

The chemical processing of ethylene ices (C 2 H 4 ) by energetic electrons was investigated at 11 K to simulate the energy transfer processes and synthesis of new molecules induced by secondary electrons generated in the track of galactic cosmic ray particles. A combination of Fourier transform infrared spectrometry (solid state) and quadrupole mass spectrometry (gas phase) resulted in the identification of six hydrocarbon molecules: methane (CH 4 ), the C2 species acetylene (C 2 H 2 ), ethane (C 2 H 6 ), the ethyl radical (C 2 H 5 ), and-for the very first time in ethylene irradiation experiments-the C4 hydrocarbons 1-butene (C 4 H 8 ) and n-butane (C 4 H 10 ). By tracing the temporal evolution of the newly formed molecules spectroscopically online and in situ, we were also able to fit the kinetic profiles with a system of coupled differential equations, eventually providing mechanistic information, reaction pathways, and rate constants on the radiolysis of ethylene ices and the inherent formation of smaller (C1) and more complex (C2, C4) hydrocarbons involving carbon-hydrogen bond ruptures, atomic hydrogen addition processes, and radical-radical recombination pathways. We also discuss the implications of these results on the hydrocarbon chemistry on Titan's surface and on ice-coated, methane-bearing interstellar grains as present in cold molecular clouds such as TMC-1.

Implications for Extraterrestrial Hydrocarbon Chemistry: Analysis of Ethylene (C2H4) and D4-Ethylene (C2D4) Ices Exposed to Ionizing Radiation via Combined Infrared Spectroscopy and Reflectron Time-of-flight Mass Spectrometry

“…, and n-butane (C 4 H 10 ) ( Table 1; Figure 3). These infrared identifications agree with the only two other studies that incorporated in situ infrared analysis of the sample (Kaiser & Roessler 1998;Zhou et al 2014).…”

“…Finally, it is important to point out that none of these later sublimation events or any signal detected in this group (C n H 2n+2 ) can be due to fragmentation of larger molecules. These results are much more diverse than previous experiments, as Wagner (1962) was only able to detect products with even carbon units, and multiple other experiments (Gorden & Ausloos 1971;Strazzulla et al 2002;Compagnini et al 2009;Ennis et al 2011;Zhou et al 2014) did not detect any alkanes with an odd carbon unit. However, Kaiser & Roessler (1998) were able to assign alkane products containing both even and odd carbon units from C3 to C14.…”

“…Products detected online and in situ via FTIR spectroscopy and QMS were acetylene and vibrational modes attributed to larger aliphatic hydrocarbon species. Recently Zhou et al (2014) used energetic keV electrons to process solid ethylene to explore hydrocarbon chemistry pathways on Titan and on methanebearing interstellar grains. Using FTIR and QMS, products were determined to be methane, acetylene, ethane, the ethyl radical, 1-butene, and n-butane.…”

Formation and High-order Carboxylic Acids (RCOOH) in Interstellar Analogous Ices of Carbon Dioxide (CO 2 ) and Methane(CH 4 )

“…The premixed gases were then deposited to the silver wafer at 10 K via a precision leak valve and glass capillary array for 15 minutes at a pressure of 2×10 −7 Torr in the main chamber. The thickness of the ice was determined in situ via laser interferometry (Zhou et al 2014) with one helium-neon (He-Ne) laser operating at 632.8 nm. The light from the laser was reflected at an angle of 2°relative to the ice surface normal.…”

On the formation of niacin (vitamin B3) and pyridine carboxylic acids in interstellar model ices

“…The thickness of the ice was determined to be 540 ± 80 nm. Here, the ice thickness was determined in situ through via laser interferometry [29,30] with two helium-neon (HeNe) lasers operating at 632.8 nm. Computation of the thickness also required a value for the index of refraction for the ice mixture, to which the value for carbon dioxide (݊ మ = 1.245 [31])…”