INFRARED-SPECTROSCOPY OF MODEL ELECTROCHEMICAL INTERFACES IN ULTRAHIGH-VACUUM - INTERFACIAL CATION SOLVATION BY AMMONIA ON PT(111)

Infrared reflection-absorption spectroscopic (IRAS), along with work-f unction (rp), measurements are reported for ammonia dosed onto potassi um-predosed as well as clean Pt(111) in ultrahigh vacuum (UHV) with th e objective of assessing the nature of interfacial cation sorvation, w hich is of fundamental relevance to electrochemical systems. Ammonia w as selected as a strongly coordinating as well as chemisorbing solvent , for comparison with other dipolar hydrogen-bonding media examined pr eviously in this manner. In the absence of potassium, ammonia chemisor ption at 90 K yields several vibrational features in the N-H stretchin g (v(NH)) and especially the symmetric H-N-H bending (delta(HNH)(s)) r egions: their identification with distinct chemisorbed, second-layer a nd multilayer ammonia states is aided by comparison of temperature-dep endent spectra with earlier temperature-programmed desorption (TPD) da ta. These chemisorbate states were absent for ammonia dosed onto a sat urated CO adlayer. In the presence of predosed potassium, distinctly d ifferent v(NH) and delta(HNH)(s) spectral features were obtained for i nitial ammonia dosages (corresponding to small NH3/K stoichiometries, approximate to 1. Comparison with vibrational spectra for bulk-phase K +-NH3 species support the occurrence of K+-induced reorientation of am monia, involving also NH3-surface hydrogen bonding. Further ammonia ex posure yielded frequency downshifts in the delta(HNH) mode, ostensibly similar to that observed for second-shell solvation of gas-phase Naand indicative of primary-secondary shell H bonding. However, completi on of the K+ primary solvation shell requires markedly higher ammonia exposures, reflecting further the competition between K+- and surface- ammonia interactions. In contrast to other solvents, Phi was observed to be essentially independent of the K coverage (approximate to 3.0 eV ) in the presence of ammonia multilayers. Comparison with solution-pha se electrochemical data suggests strongly that solvated electrons alon g with K+ are formed at the Pt(111)-UHV interface under these conditio ns.