Experimental absolute total cross sections for low-energy electron collisions with tetrahedral compounds of germanium:

Abstract

Absolute total cross section (TCS) of the electron collision with GeF 4 molecules has been measured in a linear transmission experiment for impact energies from 0.5 to 250 eV. The TCS function is dominated by a steep rise towards low energies below 3 eV. At energies above the minimum at 3 eV the TCS shows another enhancement spanned up to about 70 eV on which some resonant-like features are visible: the maximum centred near 6.5 eV and two weak peaks at 16 and 25 eV, respectively. A comparison with the recent measurements taken in the same laboratory for GeH 4 and GeCl 4 and for other targets of the same symmetry is made to search for similarities and/or differences between electron scattering TCSs for tetrahedral molecules. The data demonstrate that the general shape of the low-energy TCS for tetrahedral molecules depends on external rather than on central atoms.

Elastic cross sections for electron scattering from GeF4: Predominance of atomic-F in the high-energy collision dynamics

“…This suggests to us that the dipole polarisability of the target molecule plays an important role in the direct scattering process, albeit diminished at the lower energies, throughout the kinematical range of the present measurements. Indeed even at 3 eV, which is at the nadir of the Ramsauer-Townsend minimum, 6 we see evidence for its contribution to the collisional dynamics. The angular distribution of the 3 eV DCS is very different to those at the other energies, being almost isotropic for scattering angles greater than about 60 • .…”

“…We note that this same behaviour is semi-quantitatively reproduced by our IAM-SCAR calculations. The total cross section experiments of Szmytkowski et al 6 (see also Fig. 2) predict a shape resonance with a peak centred at about 6.5-7 eV impact energy.…”

“…In addition, in Fig. 2, we also plot the sum of all the integral inelastic (electronic state excitation and ionisation mainly but not accounting for rotations and vibrational excitation) IAM-SCAR cross sections, the IAM-SCAR total cross section, and the measured TCS from Szmytkowski et al 6 Given our previous discussion at the DCS level, it is not surprising to find that our experimental and theoretical elastic ICS are in very good agreement with one another, to within the experimental uncertainties, for energies between 10 and 200 eV. At lower energies, however, the current calculation overestimates somewhat the magnitude of the measured elastic ICS.…”

“…This is no moot point here, as in particular at 3 eV there is a Ramsauer-Townsend minimum in the TCS. 6 Thus, if the situation in CF 4 (Ref. 17) can be taken as a guide, the vibrational excitation cross sections at this energy may make a substantial contribution to the TCS.…”

“…That calculation was conducted at intermediate and high impact energies, using the independent atom model (IAM) approach with a model static-polarisation-exchange potential. 1 Results for excitation of other scattering channels are fragmentary but include dissociative attachment, 2 vacuum ultraviolet fluorescence, 3 an electron energy loss spectroscopy (EELS) investigation into discrete inelastic channels by Kuroki et al, 4 an electron impact ionisation study from Mason and Tuckett, 5 and finally total cross sections (TCSs), from 0.5-250 eV impact energies, using a linear transmission approach, due to Szmytkowski et al 6 Hence, one rationale for the present work is to improve, at least in part, this deficiency in the literature in terms of our knowledge and understanding for electron-GeF 4 scattering. a) Electronic mail: Michael.Brunger@flinders.edu.au.…”