K. Roessler

Theoretical and Laboratory Studies on the Interaction of Cosmic-Ray Particles with Interstellar Ices. III. Suprathermal Chemistry-Induced Formation of Hydrocarbon Molecules in Solid Methane (CH4), Ethylene (C2H4), and Acetylene (C2H2)

Methane, ethylene, and acetylene ices are irradiated in a ultra high vacuum vessel at 10 K with 9.0 MeV α-particles and 7.3 MeV protons to elucidate mechanisms to form hydrocarbon molecules upon interaction of Galactic cosmic-ray particles with extraterrestrial, organic ices. Theoretical calculations focus on computer simulations of ion-induced collision cascades in irradiated targets. Our experimental and computational investigations reveal that each MeV particle transfers its kinetic energy predominantly through inelastic encounters to the target leading to electronic excitation and ionization of the target molecules. Here electronically excited CH4 species can fragment to mobile H atoms and nonmobile CH3 radicals. The potential energy stored in Coulomb interaction of the CH+4 ions release energetic H and C atoms not in thermal equilibrium with the 10 K target (suprathermal species). Moderated to 1-10 eV kinetic energy, these carbon atoms and those triggered by the elastic energy transfer of the MeV projectile to the target are found to abstract up to two H atoms to yield suprathermal CH and CH2 species. C and CH, as well as CH2, can insert into a CH bond of a CH4 molecule to form methylcarbene (HCCH3), the ethyl radical (C2H5), and ethane (C2H6). HCCH3 either loses H2/2H to form acetylene, C2H2, rearranges to ethylene, C2H4, or adds two H atoms to form ethane, C2H6. C2H5 can abstract or lose an H atom, giving ethane and ethylene, respectively. C2H2 and C2H4 are found to react with suprathermal H atoms to form C2H3 and C2H5, respectively. Overlapping cascades and an increasing MeV ion exposure transforms C2Hx (x = 2, ..., 6) to even more complex alkanes up to C14H30. These elementary reactions of suprathermal species to insert, abstract, and add in/to bonds supply a powerful pathway to form new molecules in icy grain mantles condensed on interstellar grains or in hydrocarbon rich bodies in our solar system even at temperatures as low as 10 K.

THEORETICAL AND LABORATORY STUDIES ON THE INTERACTION OF COSMIC-RAY PARTICLES WITH INTERSTELLAR ICES. II. FORMATION OF ATOMIC AND MOLECULAR HYDROGEN IN FROZEN ORGANIC MOLECULES

Methane ices are irradiated at 4 × 10-10 mbar at temperatures between 10 and 50 K with 9.0 MeV α-particles and 7.3 MeV protons to elucidate the formation of atomic as well as molecular hydrogen via interaction of Galactic cosmic-ray particles with extraterrestrial organic ices. Theoretical calculations focus on computer simulations of ion-induced collision cascades in irradiated targets. Our data reveal that more than 99% of the energy is transferred via inelastic interactions to the electronic system of the target to form electronically excited CH4 molecules decomposing to a CH3--H radical pair. Two H atoms recombine in a diffusion limited step to H2. Further, secondary dissociation of CH3 to H and CH2 contributes to H production. To a minor amount, implanted ions generate C and H knock-on atoms via elastic encounters which abstract hydrogen atoms or insert into chemical bonds (carbon atoms only). Fourier transform infrared spectroscopy (FTIR) and quadrupole mass spectrometry (QMS) analyses indicate that if these energy-loss processes accumulate up to 6 ± 3% H atoms in the CH4 target, more than 90% of the ice is released in an explosive ejection into the gas phase. This mechanism represents a powerful pathway to supply newly formed molecules from interstellar grains back to the gas phase of the interstellar medium even at temperatures as low as 10 K.

Theoretical and Laboratory Studies on the Interaction of Cosmic-Ray Particles with Interstellar Ices. I. Synthesis of Polycyclic Aromatic Hydrocarbons by a Cosmic-Ray-Induced Multicenter Mechanism

Methane, ethylene, and acetylene ices were irradiated in a ultra-high vacuum vessel between 10 K and 50 K with 7.3 MeV protons as well as 9.0 MeV He2+ nuclei to simulate the interaction of galactic cosmic-ray particles with extraterrestrial, organic ices and to elucidate a mechanistic model to synthesize experimentally detected polycyclic aromatic hydrocarbons (PAHs). Theoretical calculations center on computer simulations of ion-induced collision cascades in irradiated methane targets. MeV ions induce hydrogen and carbon knock-on particles in elastic encounters with the target atoms. Each primary knock-on triggers one collision cascade with up to 70 suprathermal carbon atoms concentrated in one to two subcascades in 0.6-5 × 103 A3. At the end point of each single trajectory, every suprathermal carbon atom can form an individual reaction center of hydrogen abstraction and insertion in or addition to chemical bonds of a reactant molecule. In the relaxation phase of this energized volume, overlapping reaction zones likely form observed PAHs napthalene, phenanthrene/azulene, and coronene. This multicenter mechanism establishes a versatile route to synthesize complex molecules in extraterrestrial ices even at temperatures as low as 10 K within cosmic-ray-initiated single collision cascades.

Non-equilibrium chemistry in space

Abstract Chemical processes by accelerated species do not proceed in thermal equilibrium with the target molecules. The hot or suprathermal reactions are characterized by the fact that a kinetic energy ranging from about 1 to few 10 eV is imparted to the collision complex. Reaction rates are by orders of magnitude higher and products are more diversified than in classic thermally controlled reactions. Space is rich in energetic atoms, ions, molecules, fragments, clusters, and grains from cosmic rays, solar or stellar radiation, shock waves and many secondary acceleration processes with energies ranging from a few eV (photodissociation) to some TeV (cosmic rays). Non-equilibrium chemistry is, thus, typical for space. Suprathermal reactions in the solid state can effectively compete with radiolysis or photolysis and ion molecule interactions, particularly in view of the formation of complex organic matter by multicenter reactions in the collision cascades.

On line and in situ quantification of gas mixtures by matrix interval algebra assisted quadrupole mass spectrometry

A novel, efficient technique to identify and quantify complex gas mixtures is described. This approach can be applied on line and in situ and is extendible to samples with reactive and thermally labile species. Complex hydrocarbon mixtures are prepared in test experiments by irradiating frozen methane targets with 9 MeV α particles in an ultrahigh vacuum chamber and releasing them during successive heating of the solid samples from 10 to 293 K after each ion bombardment. A quadrupole mass spectrometer monitors time‐dependent ion currents of selected m/z values, which are proportional to partial pressures in the case of a nonoverlapping fragmentation pattern. Predominantly, parent molecules and fragments of different molecular species add to a specific m/z value, i.e., C2H+4, N+2, and CO+ contribute to m/z=28. Programmed m/z ratios are chosen to result in an inhomogeneous system of linear equations including the measured ion current (right‐hand vector), partial pressures (unknown quantity), and the calibra...

Cosmic ray simulator: A versatile apparatus for quantitative studies on the interaction of cosmic rays with frozen solids by on line and in situ quadrupole mass spectrometry and Fourier transform infrared spectroscopy

The cosmic ray simulator consists of a 50 l cylindrical stainless steel chamber. A rotable cold finger milled of a silver (111) monocrystal optimizes heat conductivity and is connected to a programmable, closed cycle helium refrigerator allowing temperature control of an attached silver wafer between 10 and 340 K (±0.5 K). Oil‐free ultrahigh vacuum (UHV) conditions of ≊10−10 mbar are provided by a membrane, drag, and cryopump, hence guaranteeing a vacuum system free of any contamination. Ice layers of defined crystal structures and reproducible thickness of (5±1) μm are achieved by depositing gases, e.g., CH4, CD4, CD4/O2, and CH4/O2, with a computer‐assisted thermovalve on the cooled wafer. These frosts are irradiated at 10 and 50 K with 7.3 MeV protons and 9 MeV α particles of the compact cyclotron CV28 in Forschungszentrum Julich up to doses of 150 eV per molecule, i.e., simulating the distribution maximum of galactic cosmic ray particles interacting with primordial matter in space during 0.7×109 yr. D...

Emission of organic products from the surface of frozen methane under MeV ion irradiation

Abstract 10 μm layers of CH 4 freshly condensed onto a cold finger at 10–15 K were irradiated with 10–20 MeV protons and 3 He 2+ ions. The gases emitted during irradiation and successive warming to ambient temperature were monitored by quadrupole mass spectrometry (QMS). C 2 H 2 and C 2 H 4 were the primary volatile products at low temperatures. They were converted with increasing irradiation time and dose into C 2 H 6 , C 3 H 6 and heavier hydrocarbons up to C 8 . During the warmup phase even more complex hydrocarbons up to C 12 were emitted including substituted benzenes (xylols), naphthalene derivates and anthracene and/or phenanthrene. The preferential formation of unsaturated compounds in the first reaction steps underlines the role of hot carbon atoms in the radiation induced complexation of solid organic matter, starting with their insertion into CH bonds. The interaction of cosmic rays with organic solids in space includes these suprathermal reactions as one of the most prominent processes.

Energy density effects in the formation of organic residues in frozen methane by MeV ions

Abstract Carbonaceous residues were formed by irradiation of thin layers of frozen methane at 10 to 15 K by 17 MeV protons and 3He2+ ions and successive warming to ambient temperature. Analysis was performed by optical microscopy, scanning electron microscopy (SEM), Rutherford backscattering spectroscopy (RBS), elastic recoil detection analysis (ERDA), infrared spectroscopy (IR) in transmission, hydrogen nuclear magnetic resonance (1H-NMR), high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS). Long chain aliphatic and olefinic hydrocarbons constituted the main products. The formation of aromatic and polycyclic compounds, PAHs and amorphous carbon increased with the energy deposited in one collision cascade. The linear energy transfer with respect to CH4, LT(CH4), was varied in the systems p//Ar/CH4 (12:1), p//CH4 and 3He2+//CH4 from 160 to 10810 eVμm−1, respectively. A threshold LT for the formation of PAHs and related structures seems to range between 2 and 10 keVμm−1. The experiments give evidence for a fast multicenter combination of intermediate radicals formed by the secondary suprathermal carbon atoms from knock on processes in one collision cascade. The experiments were aimed to simulate the effects of cosmic rays on primordial frozen matter. The results underline the role of heavier ion irradiation (He, etc.) in the prebiotic buildup of complex organic matter in space.

Interaction of MeV ions and VUV photons with polymers and high molecular hydrocarbons

Abstract Organic solids such as polymethylene, polyethylene, polyoxymethylene, aliphatic and cyclic paraffins, naphthalene, anthracene and kerogen were irradiated at 77 K with 10–20 MeV cyclotron ions (p, 3 He) and up to 10 eV photons. The products were analysed by quadrupole mass spectrometry (QMS). Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy and gas chromatography (GC). Besides radiolytic fragmentation, also synthesis of new and complex compounds was observed as a consequence of hot carbon chemistry. Vacuum ultraviolet (VUV) irradiation proved to be less effective in the buildup of new structures than MeV ions, due to its lower linear energy transfer.