Low-energy electron scattering from halomethanes. III. e-CF₃Cl

Abstract

Differential cross section measurements for the elastic scattering of electrons from CH 3 Cl at energies from 0.5 to 9.5 eV are reported for scattering angles of 30° and 100°. The angular scattering dependence is determined at selected energies over this range. At energies below 1.0 eV, the cross sections are in excellent agreement with calculations using the Born dipole approximation. At large angles and higher energies, the scattering is dominated by a 2 A 1 temporary negative ion state near 3.5 eV. Energy loss data at 3.5, 5.0, and 8.5 eV are reported and the relative contributions of various vibrational modes determined. Differential cross sections for vibrational excitation of the 3 a 1 C-Cl and 4 (e) CH stretching modes have been measured. The latter reveals a broad shape resonance of 2 E symmetry peaking near 5.5 eV. Angular distributions for excitation of these same modes are also reported. Using fittings to the vibrational excitation functions, the resonance parameters have been extracted and used in a mixed semiempirical ab initio calculation to compute the relative strengths of the vibrational energy loss peaks. These compare favorably with the results of the experiment. Elastic cross sections integrated over angle are reported for low energies. They are substantially larger than the results from recent measurements of the total cross section.

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

“…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 Figs. 4(a) and 4(b) we compare the present elastic ICS to corresponding results from Mann and Linder, 18,19 where the energy ranges of the two sets of data overlap. Generally, good agreement is found between the results from both groups, to within the error limits of their measurements.…”

“…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.…”