Absolute electron backward scattering cross sections are reported for the hydrocarbons CH4, C2H6, C3H6 and C3H8, and for the fluorocarbons CF4, C2F6 and C3F8, over the energy range 10 meV to 0.5 eV. The perfluoroalkanes are an important chemical group, extensively used in plasma processing, high voltage gaseous insulation and, more recently, have found notoriety in atmospheric chemistry. This work has studied the low energy electron scattering behaviour of the early members of the series in comparison to their hydrocarbon counterparts. Previous published work on the perfluoroalkanes includes electron attachment, electron swarm studies and crossed beam scattering experiments [1 and references therein].
The cross sections are derived using a transmission experiment which has a synchrotron radiation photoionisation source to generate the primary electrons. The energy resolution of the resulting electron beam is 5 meV (FWHM). The electron beam is constrained by an axial magnetic field. Thus only backward scattering cross-sections (electrons scattered into the backward hemisphere) are determined. Further experimental details can be found in [2].
The variation of the measured cross-sections has been analysed for non-polar and weakly polar species using Modified Effective Range Theory (MERT), yielding scattering lengths and low energy limiting cross-sections. Rotationally inelastic scattering cross-sections have been calculated for C3H6 and C3H8 using the first Born point-dipole approximation. Details and results will be presented at the conference.
[1] SL Lunt, J Randell, J-P Ziesel, G Mrotzek and D Field 1998 J.Phys.B.
In Press
[2] J Randell, J-P Ziesel, SL Lunt, G Mrotzek and D Field 1993 J.Phys.B. 26 3423