Electron-stimulated desorption from partially fluorinated hydrocarbon thin films: Molecules with common versus separate hydrogen and fluorine bonding sites

We have observed ${\mathrm{H}}^{+}$ and ${\mathrm{F}}^{+}$ yields as a function of low-energy (10-100 eV) electron bombardment from thin films of solid nonafluoroisobutane [${\left({\mathrm{CF}}_3\right)}_3$CH], 1,2,3,5-tetrafluorobenzene (${\mathrm{C}}_6$ ${\mathrm{H}}_2$ ${\mathrm{F}}_4$), 1-fluorobutane (${\mathrm{C}}_4$ ${\mathrm{H}}_9$F), and fluorocyclohexane (${\mathrm{C}}_6$ ${\mathrm{H}}_{11}$F). For the materials with separate H and F bonding sites [${\mathrm{C}}_6$ ${\mathrm{H}}_2$ ${\mathrm{F}}_4$ and ${\left({\mathrm{CF}}_3\right)}_3$CH], ${\mathrm{H}}^{+}$ and ${\mathrm{F}}^{+}$ have different appearance potentials, corresponding, respectively, to excitations of the C $2s$ and F $2s$ levels (~25 and ~40 eV). For the materials with common H and F bonding sites, the ${\mathrm{H}}^{+}$ and ${\mathrm{F}}^{+}$ appearance potentials correspond to excitation of the C $2s$ level. The ${\mathrm{F}}^{+}$ threshold is relatively weak, and a steep increase in the ${\mathrm{F}}^{+}$ yield is observed near 40 eV (F $2s$ excitation). These results indicate that the final-state excitations leading to ion desorption in these systems are localized within functional groups but not necessarily within individual C-H or C-F bonds of the functional group.