Total dissociation cross section of CF4and other fluoroalkanes for electron impact

Abstract

Various techniques from the field of surface science have allowed the development of a method for measuring the total absolute cross section for electron-impact-induced dissociation of a number of molecules. The technique is particularly reliable for the fluoroalkanes such as C${\mathrm{F}}_4$, C${\mathrm{F}}_3$H, ${\mathrm{C}}_2$ ${\mathrm{F}}_6$, and ${\mathrm{C}}_3$ ${\mathrm{F}}_8$. The total dissociation cross section for C${\mathrm{F}}_4$ has been measured for energies between threshold (∼ 12.5 eV) and 600 eV. The magnitude of the cross section at its maximum is 5.5×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$. Less extensive data are presented for C${\mathrm{F}}_3$H, ${\mathrm{C}}_2$ ${\mathrm{F}}_6$, and ${\mathrm{C}}_3$ ${\mathrm{F}}_8$. Their cross sections at the maxima are 5.8×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$, 8.6×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$, and 1.18×${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$, respectively. Arguments are presented which suggest that the total dissociation cross section for each of these gases is equal to the sum of the cross sections for excitation to all electronic and ionic states, i.e., the total cross section for electronic excitation. The results are discussed from the point of view of the Bethe theory. It is concluded that the Bethe asymptotic behavior is not yet attained in the energy range of these measurements.