Sean A. Harris

Absolute electron impact ionization cross sections for , where X = H, F, Cl, Br, and I

Measurements of electron impact ionization cross sections have been made for methane and the series methyl fluoride to methyl iodide. The results for methane and methyl fluoride to methyl bromide have been compared with ionization efficiency curves calculated using Deutsch - Mark (DM) and binary-encounter-Bethe (BEB) methods, and also with the results of an ab initio model which gives the maximum cross section as a function of molecular orientation. In addition, the ab initio and DM methods have been used to calculate the steric ratios for the electron impact ionization of methyl chloride which have been compared with experimental measurements made previously.

Frontside versus backside reactivity in electron transfer to oriented tert butyl bromide and methyl bromide

Electron transfer collisions between beams of neutral K atoms and neutral alkyl bromide (R–Br) molecules (R=CH3,t-C4H9) are observed by detecting positive and negative ions in coincidence for energies ≳4 eV, the minimum energy for overcoming the Coulomb attraction between ions. The molecules are state selected by a hexapole electric field and oriented prior to the electron transfer. The steric asymmetry for both molecules above ≈6 eV shows that “frontside,” or Br end attack, is favored to form Br−, with t-C4H9Br being more asymmetric than CH3Br. The asymmetry maximizes near 5 eV and as the energy decreases, apparently changes sign to favor “backside,” or alkyl-end attack. Free electrons (and K+) are detected from t-C4H9Br and show a similar change in preferred orientation: at low energies alkyl end attack is favored, and at high energies Br end is favored. These observations suggest that the electron is transferred into different orbitals with different spatial distributions as the energy is varied. Steri...

Electron transfer to oriented molecules: Surprising steric effect in t-butyl bromide

Collisions between neutral K atoms and oriented t-butyl bromide molecules produce the ions K+ and Br− at energies high enough to separate charged particles (≳4 eV). Ions are detected by coincidence tof mass spectrometry for orientation of the t-butyl bromide such that the K atom attacks either the Br end or the t-butyl end of the molecule. At high energies the steric asymmetry factor is larger than that for CH3Br. But at energies near threshold, the steric asymmetry factor reverses sign and attack at the t-butyl end becomes more reactive than attack at the Br end. The electron is apparently transferred into different orbitals at different ends.

Electron transfer collisions with spatially oriented CH3CN

Positive and negative ion pairs are detected in collisions of fast [5–30 eV center of mass (CM)] K atoms with CH3CN molecules oriented in space. Different products are favored by attack at different ends of the molecule. The principal negative ion is CN−, which is produced preferentially upon attack at the negative, or CN-end of the molecule. The cyanomethyl ion, CH2CN−, is observed as the energy is raised, and is preferentially produced upon attack at the positive (methyl) end of the molecule. Formation of these ions appears to proceed by electron transfer into the LUMO at either end of the molecule, followed by a curve crossing into the most exoergic channel. Neither parent ions nor electrons are detected.