INFLUENCE OF COADSORBED POTASSIUM ON THE ELECTRON-STIMULATED DESORPTION OF F+, F-, AND F-ASTERISK FROM PF3 ON RU(0001)

The electron-stimulated-desorption ion angular distributions (ESDIAD) of positive ions, negative ions, and metastable species from PF3 and K coadsorbed on a Ru(0001) surface are measured. In the absence of K, only positive- and negative-ion ESDIAD is observed, showing highly anisotropic, off-normal fluorine-ion emission (F+, F-) from PF3, which demonstrates that PF bonds are inclined away from the surface normal. In the presence of K, ESDIAD patterns of F+, F-, and metastable F* display only normal emission, suggesting a reorientation or chemical reaction which results in fluorine bonding perpendicular to the surface. Thermal desorption spectrometry, Auger electron spectroscopy, and soft-x-ray photoemission (SXPS) are used to determine the surface chemistry as an aid in intepreting the ESDIAD results. The SXPS data show that in the presence of potassium, PF3 dissociates, possibly forming KF-like species, which suggests that an alkali-metal-induced chemical reaction is reponsible for the changes observed in ESDIAD. The yields of the various desorbed fluorine species vary strongly with potassium coverage; the results for F- and F* are interpreted in terms of a charge-transfer model.