Relative dissociation fractions of SF6under impact of 15-keV to 30-keV H−and C−negative ions

Abstract

The relative dissociation fractions for the production of fragment ions and ion pairs of SF${}_{6}$ are studied for H${}^{\ensuremath{-}}$ and C${}^{\ensuremath{-}}$ impact in the energy range from 15 to 30 keV. Recoil ions (${\mathrm{SF}}_{4}^{+}$, SF${}_{3}^{+}$, SF${}_{2}^{+}$, SF${}^{+}$, S${}^{+}$, F${}^{+}$, SF${}_{4}^{2+}$, SF${}_{2}^{2+}$) and ion pairs $({\mathrm{SF}}_{3}^{+}+{\mathrm{F}}^{+},{\mathrm{SF}}_{2}^{+}+{\mathrm{F}}^{+},{\mathrm{SF}}^{+}+{\mathrm{F}}^{+},{\mathrm{S}}^{+}+{\mathrm{F}}^{+}$, F${}^{+}+{\mathrm{F}}^{+})$ are detected and identified in coincidence with scattered projectiles in two charge states ($q=0$ and $q=+1$) by using a time-of-flight spectrometer. The relative dissociation fractions are energy dependent for both single-electron-loss (SL) channel and double-electron-loss (DL) channel processes for certain negative ions. It is also found that the relative dissociation fractions for DL are larger than those for SL. In addition, the degree of fragmentation will become greater with a larger mass number of the projectiles at the same impact energy for the same electron-loss channel. A comparison of the time-of-flight spectra is made between that under negative-ion impact and that under electron impact, and it is found that the probability of production of SF${}_{n}^{+}$ ions with $n$ odd is higher than that of similar ions with $n$ even, and the probability of production of SF${}_{n}^{2+}$ ions with $n$ even is higher than that of similar ions with$n$ odd under H${}^{\ensuremath{-}}$, C${}^{\ensuremath{-}}$, positive-ion, and electron impact. We analyze this interesting phenomenon from the bond-dissociation energies of SF${}_{n}^{+}$ and SF${}_{n}^{2+}$. We also analyze the coincident time-of-flight spectra of two fragment ions resulting from double ionization of SF${}_{6}$ by H${}^{\ensuremath{-}}$ and C${}^{\ensuremath{-}}$ impact and describe the major dissociation pathways of SF${}_{6}^{2+}$ for H${}^{\ensuremath{-}}$ and C${}^{\ensuremath{-}}$ impact in the energy range from 15 to 30 keV.