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The molecular and dissociative adsorption of NH3 on a clean Si( 111 )-(7 X 7) surface has been studied using high-resolution electron-energy-Ioss spectroscopy (HREELS), Auger electron spectroscopy, and temperature-programmed desorption (TPD). All NHx (3)x> 1) species have been observed vibrationally under varying experimental conditions. Adsorbed molecular ammonia is observed by both HREELS and TPD below 200 K. The absence of a strong {js ( NH3 ) mode in our vibrational spectra suggests that NH3 is bound as a tilted species to the Si(111)-(7X7) surface. Ammonia exposures above 1.7X 10 14 NH3/cm2 at 110 K populate a weakly bound NH3 (a) state which desorbs upon heating with a peak desorption temperature of 115 K. The NH2 (a) species is observed to form at 80 K at all coverages and exhibits an enhanced thermal stability at higher ammonia exposures. The NH(a) species is detected above 200 K by an energy-loss feature at 1100 cm -I, which we assign to the {j(NH) mode. NH(a) is stable to 750 K on high coverage layers. Finally, we observe an increase in the Si-H stretching frequency as the concentration ofNH (a) and N(a) species is increased by heating.7826

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