Revisiting the electron stimulated desorption of anions from thin films of CF2Cl2

A recently constructed TOF mass spectrometer was used to measure the electron-stimulated desorption of anions from thin films of vapor-deposited CF2Cl2 in the 0–20 eV energy range. In addition to the previously observed signals of F− and Cl−, the anions CF−, F2−, CF2−, ClF−, and Cl2− are detected. Measurements as a function of both incident current and CF2Cl2 coverage on a Kr substrate indicate that all desorbed ions are the result of unimolecular dissociation, initiated by the impact of a single electron. Both dissociative electron attachment (DEA) and dipolar dissociation (DD) processes are found to contribute to the desorption yield, but to an extent that is anion dependent. The DEA and DD derived yields of the anions demonstrate distinctly different dependencies on molecule–metal distance, as has been observed in other molecular solids. As previously reported, prolonged bombardment of the CF2Cl2 film leads to the production of Cl2 molecules as evidenced by an enhanced desorption signal of Cl− at incident electron energies near 5 eV. We have used this effect used to measure the relative cross-section for production of Cl2 as a function of energy and observe a threshold energy for this process of 7 eV.