A theoretical model for electron‐impact ionization cross sections, which has been developed primarily for atoms and atomic ions, is applied to neutral molecules. The new model combines the binary‐encounter theory and the Bethe theory for electron‐impact ionization, and uses minimal theoretical data for the ground state of the target molecule, which are readily available from public‐domain molecular structure codes such as GAMESS. The theory is called the binary‐encounter Bethe (BEB) model, and does not, in principle, involve any adjustable parameters. Applications to 19 molecules, including H2, NO, CH2, C6H6, and SF6, are presented, demonstrating that the BEB model provides total ionization cross sections by electron impact from threshold to several keV with an average accuracy of 15% or better at the cross section peak, except for SiF3. The BEB model can be applied to stable molecules as well as to transient radicals.

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New model for electron-impact ionization cross sections of molecules

Establishing the presence and state of organic matter, including its possible biosignatures, in martian materials has been an elusive quest, despite limited reports of the existence of organic matter on Mars. We report the in situ detection of organic matter preserved in lacustrine mudstones at the base of the ~3.5-billion-year-old Murray formation at Pahrump Hills, Gale crater, by the Sample Analysis at Mars instrument suite onboard the Curiosity rover. Diverse pyrolysis products, including thiophenic, aromatic, and aliphatic compounds released at high temperatures (500° to 820°C), were directly detected by evolved gas analysis. Thiophenes were also observed by gas chromatography–mass spectrometry. Their presence suggests that sulfurization aided organic matter preservation. At least 50 nanomoles of organic carbon persists, probably as macromolecules containing 5% carbon as organic sulfur molecules.

/pdf/organic-matter-preserved-in-3-billion-year-old-mudstones-at-4m8xgkn183.pdf

Organic matter preserved in 3-billion-year-old mudstones at Gale crater, Mars.

Cross sections and rate coefficients are provided for inelastic collision processes of electrons and protons with CxHy and CxHy+ (x=2,3; 1⩽y⩽2x+2) hydrocarbon species in a wide range of collision energies and plasma (gas) temperatures The considered processes include electron-impact ionization and dissociation of CxHy, dissociative excitation, ionization and recombination of CxHy+ with electrons, and both charge transfer and atom exchange in proton channels are considered separately The presented cross sections are based upon a critical assessment of available experimental data and upon an extensive use of a number of semi-empirical, physically well grounded cross section scaling relationships Information is also provided for the energetics of each individual reaction channel The cross sections and rate coefficients are presented in compact analytic forms

Collision processes of C2,3Hy and C2,3Hy+ hydrocarbons with electrons and protons

The reactivity of slow electrons with molecules at different degrees of aggregation: gas phase, clusters and condensed phase

The fabrication of organized molecular-scale structures is key to realizing high-performance molecular- and conjugated-polymer devices. Here, we demonstrate a unique single-molecular processing technique using electrochemistry, termed 'electrochemical epitaxial polymerization'. This technique is based on step-by-step electropolymerization of the monomer by applying voltage pulses to a monomer–electrolyte solution that also contains iodine. Using this technique, we observe the formation of high-density arrays of single conjugated-polymer wires as long as 75 nm on the surface of a Au(111) electrode. Our findings unveil the mechanism of electropolymerization, showing that the conjugated polymer wires grow from nuclei adsorbed on the iodine-covered Au(111) surface. The results may also open the door to mass-production of molecular-scale devices based on conjugated polymers.

Electrochemical epitaxial polymerization of single-molecular wires.

Electron impact ionization of propane (C3H8) was studied using the ion beam deflection technique and a double focussing mass spectrometer in conjunction with a recently developed correction procedure that accounts for discrimination due to the initial kinetic energy of fragment ions. The relative corrected partial ionization cross sections for the production of C3H8+, C3H7+, C3H6+, C3H5+, C3H4+, C3H3+, C3H2+, C3H+, C3+; C2H5+, C2H4+, C2H3+, C2H2+, C2H+, C2+; CH3+, CH2+, CH+, C+; C3H52+, C3H42+, C3H32+ and C3H22+ were determined from threshold up to 950 eV. Absolute partial ionization cross sections were obtained by charge weighted summing of all the observed partial ionization cross sections and by normalizing to a recent accurate determination of the total ionization cross section at 100 eV by Djuric et al. 1991. The absolute total ionization cross section curve obtained is in excellent agreement in shape and magnitude with the low energy data (<200 eV) of Djuric et al. 1991 and the high energy data points of Schram et al. 1966.

Absolute partial and total electron impact ionization cross sections for C3H8 from threshold up to 950 eV

Stoichiometric and non-stoichiometric negatively charged CO2 cluster ions have been produced in a crossed neutral cluster/electron beam ion source. The abundance and stability of these ions have been studied with a double focussing sector field mass spectrometer. The observed abundance anomalies (“magic numbers”) in the mass spectra of (CO2)
 −
 and (CO2)
 n
 O− ions correlate with corresponding small and large metastable fractions of these ions (for loss of one CO2 unit). Variation of the measured metastable fractions as a function ofn are related to corresponding changes in the monomer binding energies. In addition, we have observed for the first time (CO2)
 n
 O
 2
 −
 ions (i.e. at electron energies above 8 eV with an energy resonance at about 14 eV) and we discuss possible production mechanisms for these ions. Relative electron attachment cross sections have been determined in the energy regime O<E≦20 eV for (CO2)
 −
 , (CO2)
 n
 O− and (CO2)
 n
 O
 2
 −
 withn=1 to 20. The shape of the cross section function for (CO2)
 n
 O− is strongly dependent on the cluster sizen.

Production and properties of CO 2 cluster anions

Interaction of free electrons with C60: ionization and attachment reactions

Stoichiometric and non-stoichiometric, positive and negative oxygen cluster ions (n up to 70) have been produced in a crossed neutral beam/electron beam ion source. The abundance and stability of the ions formed have been analyzed with a double focussing sector field mass spectrometer in a series of experiments. Positive and negative ion mass spectra observed exhibit distinct abundance anomalies, however, at different cluster sizes. Abundance maxima and minima correlate with correspondingly small and large metastable fractions of (O2)
 +
 and (O2)
 −
 ions, respectively. (O2)
 +
 ions may also lose up top=(n−1) monomers by collision induced dissociation with monotonously decreasing probability with increasingp. Metastable fractions determined for (O2)
 −
 ions produced with appr. zero eV electrons are in general larger than those for ions produced with appr. 7 eV electrons. (O2)
 −
 ions are also observed to decay via autodetachment, with lifetimes increasing with increasing cluster size. Finally, here we were able to prove that an apparent loss of the monomer fragment O (and higher homologues) observed in the metastable time regime is due to ordinary metastable monomer evaporation in the acceleration region. Moreover, we will also present here some new data and interpretation concerning the electron attachment cross section function for O2 clusters.

T. Rauth

Papers

Absolute partial and total electron impact ionization cross sections for C3H8 from threshold up to 950 eV

Production and properties of CO 2 cluster anions

Interaction of free electrons with C60: ionization and attachment reactions

Production and stability of oxygen cluster cations and anions, revisited

Interaction of electrons with C60+ and C60−: post-ionization, dissociation and kinetic energy release