The anomalous shape of the cross section for the formation of SF_3~+ fragment ions produced by electron impact on SF_6 revisited

摘要

The partial ionization cross section for the formation of SF_3~+ fragment ions following electron impact on SF_6 is known to have a pronounced structure in the cross section curve slightly above 40 eV.We used the mass-analyzed ion kinetic energy (MIKE) scan technique to demonstrate the presence of a channel contributing to the SF_3~+ partial ionization cross section that we attribute to the Coulomb explosion of doubly charged metastable SF_2~(2+) ions into two singly charged ions SF_3~+ and F~+,with a threshold energy of about 45.5 eV.Thus the observed unusual shape of the SF_3~+ partial ionization cross section is the result of two contributions,(i) the direct formation of SF_3~+ fragment ions via dissociative ionization of SF_6 with a threshold energy of 22 eV and (ii) the Coulomb explosion of metastable SF_4~(2+) ions with a threshold energy of about 45.5 eV.A detailed analysis of the MIKE spectrum reveals an average kinetic energy release of about 5 eV in the Coulomb explosion of the SF_4~(2+) ions with evidence of a second channel corresponding to an average kinetic energy release of about 1.1 eV.