A Combined Experimental and Computational Investigation on the Synthesis of Acetaldehyde [CH₃CHO(X1A′)] in Interstellar Ices

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Abstract

The synthetic routes to form acetaldehyde [CH3CHO(X1A')] in extraterrestrial ices were investigated experimentally in a contamination-free ultrahigh vacuum scattering machine. Binary ice mixtures of carbon monoxide [CO(X1Σ+)] and methane [CH4(X1A1)] were condensed at 10 K onto a silver monocrystal and irradiated with 5 keV electrons to mimic the electronic energy transfer processes initiated by MeV cosmic-ray particle-induced δ-electrons in the "ultratrack" of MeV ion trajectories; the carbon monoxide-methane ices served as model compounds to simulate neighboring CO–CH4 molecules in astrophysical ices, as present in cold molecular clouds and in cometary matter. Upon completion of the high-energy processing, the ice samples sublimed during the heating phase to 293 K, thus releasing the remaining reactants as well as the newly formed molecules into the gas phase. The experiment was monitored on line and in situ via a Fourier transform infrared (FTIR) spectrometer in absorption-reflection-absorption mode (solid state) and a quadrupole mass spectrometer (gas phase). Our investigations were combined with electronic structure calculations. At 10 K, the primary reaction step involved the cleavage of the carbon-hydrogen bond of the methane molecule via an electronic energy transfer process from the impinging electron to the methane molecule to form a methyl radical [CH3(X2A)] plus a hydrogen atom [H(2S1/2)]. The H atom contains the excess energy in the form of translational motion; suprathermal hydrogen atoms can add to the carbon-oxygen triple bond of the carbon monoxide molecule, overcoming the entrance barrier, to yield the formyl radical [HCO(X2A')]. Depending on the reactant geometry inside the matrix cage, the formyl radical recombined barrierlessly with the neighboring methyl radical inside the ices at 10 K. Upon warming of the ice sample, the acetaldehyde molecules sublime into the gas phase. This process mimics the sublimation of molecules from the grain mantles into the gas phase upon the transition of the molecular cloud to the hot molecular core phase. This mechanism to form acetaldehyde inside interstellar ices (cold molecular clouds; 10 K) upon high-energy processing, followed by a radical-radical recombination and sublimation in the hot core phase (molecular cores; few 100 K), presents a compelling route to account for high fractional abundances of acetaldehyde of a few times 10-9 toward star-forming regions, as compared to abundances of only some 10-10 in the cold cloud TMC-1, where solely gas-phase reactions are supposed to synthesize acetaldehyde.

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