Low-energy electron scattering from halomethanes. IV. e-CF₂Cl₂

Abstract

For pt.III see ibid., vol.25, no.7, p.1621-32 (1992). Differential cross sections are presented for vibrationally elastic and inelastic e-CF2Cl2 scattering in the energy range 0.5-10 eV. All four low-lying sigma * valence type resonances are observed in the measurements. The hitherto unidentified B1 (C-F sigma *) resonance has been located at 5.0-5.5 eV. A quite selective excitation of CF2 and CCl2 stretching modes is observed corresponding to the respective valence character of the resonance and in agreement with the symmetry selection rules. Resonance-enhanced vibrational excitation is found to be most pronounced in the totally symmetric resonances of symmetry type A1. The results are discussed in comparison with dissociative attachment experiments. Below 1 eV, similar to the other molecules in this series (CF4, CF3Cl), direct scattering leads to strong excitation of stretching modes according to their IR activity.

Cross sections for elastic scattering of electrons by CF3Cl, CF2Cl2, and CFCl3

“…2(a)-2(c). The present results confirm well the DCSs measured by Mann and Linder 18,19 between 1 and 10 eV, to within the error bars, for both CF 3 Cl and CF 2 Cl 2 . Note that the absolute scale in the work of Mann and Linder was defined by normalization to the TCS value of Jones 22 at 4 eV and cross checked further by using the relative-flow technique.…”

“…2(a)-2(c), are the corresponding results from our IAM-SCAR computations, the results from our modified phase shift analysis fits to the present DCSs and results from the calculations using the SMC method. 2,20,21 For the CF 3 Cl and CF 2 Cl 2 molecules, results from the earlier measurements of Mann and Linder 18,19 are also shown. Figure 3 illustrates the substitution effects as you progressively replace a F-atom with a Cl-atom in the series CF 4-x Cl x (x = 0, 1, 2, 3, 4).…”

“…Note that the data for CF 4 and CCl 4 were reported previously by our group 1,6 and are included here simply for comparison with the present results. For 18,19 Also included in these figures are the available experimental and theoretical total cross sections, [22][23][24][25][55][56][57] elastic ICSs using the SMC approach, 20 and experimental [58][59][60][61][62]64 and BEB theoretical 63 …”

“…In general vibrational excitation is emerging in such resonant energy regions, as well as near an elastic cross section minimum, like the Ramsauer-Townsend effect. 26 Mann and Linder 18,19 observed resonances in their measured vibrational data at 1.7 eV, 5.5 eV, and 8.5 eV for CF 3 Cl and at 1.0 eV, 2.5 eV, 4 eV, and 6 eV for CF 2 Cl 2 . However, the present elastic DCSs (see Figs.…”

“…Elastic and vibrational excitation cross sections for CF 2 Cl 2 and CF 3 Cl have been reported only by Mann and Linder,18,19 using the crossed electron-molecular beam geometry in the energy range from 1 to 10 eV and for scattering angles between 10 • and 100 • . Calculations for those molecules have been performed by Natalense et al 20 and Varella et al, 21 with the Schwinger multichannel (SMC) method in the energy range from 5 eV to 100 eV for CF 3 Cl.…”