A MASS-SPECTROMETRIC METHOD FOR PROBING SURFACE-STRUCTURE

Coincidence counting mass spectrometry (CCMS) has been used to study t he negative and positive cluster ion formation from two inorganic comp ounds, NaBF4 and alpha ZrP. Coincidence spectra are obtained from conv entional mass spectra by collecting only those desorption events that include an ion of interest. Direct comparison of the cluster ion compo sitions to the stoichiometry and structure of the original solid were made. Negative ions reflect the original solid composition and structu re while the positive cluster ions do not. Also, new evidence supports an intact emission for negative cluster ions and a recombination mech anism for positive cluster ions. Changing the crystallinity of the sol id affects the negative spectrum, with larger cluster ions observed in the more crystalline samples. No difference is observed with the posi tive clusters, consistent with a gas phase formation process. In the n egative ion mode CCMS can be an effective fool for studying the compos ition and the nanoscale order of a solid surface.