Dynamics of NF3 in a condensed film on Au(1Â 1Â 1) as studied by electron-stimulated desorption

We report a low-temperature dynamics study of condensed layers of NF3 on Au(1 1 1) by time-of-flight electron-stimulated desorption ion angular distribution (TOF-ESDIAD), temperature-programmed desorption (TPD) and low-temperature scanning tunneling microscopy (LT-STM). Upon adsorption at 30 K, molecular NF3 adsorption occurs first at the step edges and at minor terrace defect sites with the formation of 2D islands. Within the islands, NF3 is adsorbed in an upright conformation via the nitrogen lone pair electrons projecting fluorine atoms away from the surface as judged by the presence of only a sharp F+ central beam in the ESDIAD pattern. At higher coverages, 3D islands start to populate the surface. Electron bombardment of a thick NF3 (∼6 ML) layer adsorbed on the Au(1 1 1) surface leads to emission of F+, N+, NF+, NF2+ and NF3+ ions as observed in the TOF-ESD distribution. Upon heating to ∼37 K, a sudden decrease of the NF2+ and NF3+ ion yield, which is not related to thermal desorption, is observed which reflects the surface migration of NF3 molecules, leading to local thinning of the film. The thinning process occurs at the temperature of onset of molecular rotations and self-diffusion in the bulk NF3 crystal. In this process, some NF3 molecules move closer to the surface which results in higher efficiency for ion neutralization by the underlying metal surface. In the TPD spectra, the monolayer desorption is observed to begin at ∼65 K, exhibiting zero-order kinetics with an activation energy of 21 kJ/mol.