Influence of coadsorbed potassium on the electron-stimulated desorption of ${\mathrm{F}}^{+}$, ${\mathrm{F}}^{\mathrm{-}}$, and ${\mathrm{F}}^{\mathrm{*}}$ from ${\mathrm{PF}}_3$ on Ru(0001)

The electron-stimulated-desorption ion angular distributions (ESDIAD) of positive ions, negative ions, and metastable species from ${\mathrm{PF}}_3$ 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 (${\mathrm{F}}^{+}$, ${\mathrm{F}}^{\mathrm{-}}$) from ${\mathrm{PF}}_3$, which demonstrates that PF bonds are inclined away from the surface normal. In the presence of K, ESDIAD patterns of ${\mathrm{F}}^{+}$, ${\mathrm{F}}^{\mathrm{-}}$, and metastable ${\mathrm{F}}^{\mathrm{*}}$ 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, ${\mathrm{PF}}_3$ 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 ${\mathrm{F}}^{\mathrm{-}}$ and ${\mathrm{F}}^{\mathrm{*}}$ are interpreted in terms of a charge-transfer model.