THE NATURE OF HYDROGEN IN X-RAY PHOTOELECTRON-SPECTROSCOPY - GENERAL PATTERNS FROM HYDROXIDES TO HYDROGEN-BONDING

Important progressive alterations in chemical bonding are often realiz ed through correlations with shifts in thp x-ray photoelectron spectro scopy (XPC) binding energies nf key elements Fnr example there are use ful general XPS shifting schemes for such systems as oxides, nitrides, halides, and even various functional groups in organics. Very general patterns, based upon location in the periodic table, exist for many o f these materials even when the structure is not strongly considered. Unfortunately, apparently because of the lack of direct XPS detection of hydrogen, there seems to be no general statements in the literature for describing hydrogen-containing compounds, despite the fact that s ynergistic shifts obviously exist in the XP-S spectra of elements atta ched to hydrogen (e.g., for M-O-H vs M-O-M units, where M is a typical metal). While not attempting a complete review paper, in the present work we use XPS shifting patterns to evolve a series of interrelated c ovalency/ionicity arguments to help explain the progressive, periodic changes in XPS peak locations for such common cases as M-O-H- and M-N- H-containing systems. These arguments are followed by consideration of the less dramatic XPS shifting patterns exhibited by metal and metall oid hydrides, including organic bonding. The formalism concludes with a discussion of hydrogen bonding detected by XPS. After a select revie w of the infrequent use made by others to attribute XPS peak shifts to hydrogen bonding, we consider in some detail two cases recently publi shed by members of our group. One case involves the formation of -N-H- N- bonds in proton sponge organic systems, while the other uses XPS to examine the formation of surface oriented -O-H---O- bonds in the adso rption of peptides on oxidized metals. In the present article, the XPS patterns for these two seemingly divergent cases are explained by clo sely related arguments that may have far reaching generalities. (C) 19 96 American Vacuum Society.