Absolute partial and total electron impact cross sections of fluorine and chlorine based atmosphere-damaging molecules*

Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons  (HCFCs) molecules pose a serious environmental threat [1].  Once released into the earth  atmosphere, they accumulate in the stratosphere, where they contribute to the depletion of the ozone layer.  Reactions that may occur in the stratosphere were detailed in the literature [2]  . Here we present experimental non-dissociative and dissociative cross-sections for electron impact of CFCs and HCFCs atmosphere-damaging molecules. The experiments, based on a Reaction Microscope (ReMi), were carried out at several electron energies up to 1 keV. A noble gas and CFC (HCFC) mixing setup was implemented to convert the relative cross sections measured by the ReMi setup into absolute values. Using these techniques, ion collection and calibration uncertainties  were minimized. The presented results on CF₄ and  HCFCs were compared with theoretical and experimental studies available in the literature [3,4]. Previous electron impact experiments mostly present relative cross sections and add simulated corrections to obtain the absolute cross sections due to the difficulty in the determination of absolute data. We elucidate the differences of the new measurement method from the existing ones in the literature and explain why the new method is needed. Further we show how reducing correction terms affects the results obtained.

References

1 L. M. Western et al. , Nat. Geosci. 16, 309–313 (2023).

2 Scientific Assessment of Ozone Depletion, Chapter 2: Hydrofluorocarbons, (2022).

3 L. G. Christophorou, J. K. Olthoff and M. V. V. S. Rao J. Phys. Chem. Ref. Data 25, 1341–1388 (1996).

4. L. G. Christophorou and J. K. Olthoff, J. Phys. Chem. Ref. Data 28, 967–982 (1999).