The interaction of ammonia with clean and CO‐covered Re(0001) surface was investigated, utilizing optical second harmonic generation (SHG), temperature programmed desorption (TPD), and work function change (Δφ) techniques. The surface coverage of ammonia was continuously monitored during both isothermal desorption and equilibrium measurements as a function of crystal temperature and ambient NH3 pressure. A fast decrease of the activation energy for desorption from 21±1 to 9±1 kcal/mol as coverage increases from 0.2 to 0.9 of the saturation coverage was found. This indicates the existence of very strong repulsive interactions among adsorbed ammonia molecules. The pre‐exponent factor was also determined as a function of coverage, for the first time for any ammonia–metal system. Its fast decrease with increasing coverage, reveals an example for a compensation effect in desorption. The coadsorption system of CO and NH3 was also studied. It was found that while ammonia effectively blocks the adsorption of CO on Re(0001) when it is preadsorbed, the opposite order of adsorption, namely preadsorption of CO, hardly affects the adsorption–desorption kinetics of NH3. It was also found that decrease of work function caused by ammonia of 2.4 eV at saturation coverage, shifts upward exactly by the increase of Δφ by CO on the clean surface if the latter is preadsorbed. These minor effects of CO if preadsorbed, are in sharp contrast to the dramatic decrease of the second harmonic (SH) response due to ammonia on Re(0001), if only small amounts of CO are present on the surface. It is suggested that submonolayer coverage of CO are enough to remove any electronic interaction which otherwise leads to a strong SH response. A large SHG quenching cross section of CO is introduced, implying a nonlocal effect of CO on the adsorbed NH3 on Re(0001). If ammonia is preadsorbed, much of the quenching effect due to the coadsorbed CO is removed. This is discussed in terms of NH3 islands formation.

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