Principal Investigator Sylvia Ceyer
By raising the energy of the incident molecule we were able to synthesize adsorbed CH3 radicals for the first time under low pressure, ultrahigh vacuum conditions. More recent, higher resolution and higher sensitivity spectra of CH3, CH2D and CD3, as described in a manuscript J. Chem. Phys. 102, 7734 (1995) have confirmed a Fermi resonance between the overtone of the asymmetric deformation and the “soft” C-H symmetric stretch. These spectra have also allowed a symmetry analysis to be carried out that is sufficient to establish that the CH3 species is adsorbed with C3v symmetry in a threefold hollow site with the hydrogens either eclipsed over or staggered between the surrounding Ni atoms.
Because of this unique ability to produce a CH3 species, we have been able to probe its stability. Above 150 K, CH3 begins to dissociate to adsorbed CH. An unambiguous identification of the spectrum as that of CH is only possible because of the high resolution (32 cm-1 FWHM) and high sensitivity (5×106 counts/sec for the elastically scattered electron beam) of our spectrometer. This assignment is also supported by the spectra measured after the thermal decomposition of the mixed isotope CH2D. This work rectifies a previous assignment of a spectrum in the literature to a CH species.
Because of the presence of coadsorbed hydrogen resulting from the decomposition of the hydrocarbon, it was necessary to determine the frequencies of the Ni-H vibrations in separate studies in which only hydrogen was adsorbed. We found that we could observe and identify not only the symmetric and antisymmetric Ni-H stretch modes and their overtones but also resolve these modes for each type (hcp or fcc) of threefold site on which hydrogen is bound. Since the structures of the two threefold sites differ only by the presence of a Ni atom in the second layer, the difference in the local potential at the two sites is subtle. Accordingly, this is the first observation, again made possible by the high resolution and sensitivity of the spectrometer, of any experimentally determined distinction in the physical properties of the hcp and fcc threefold sites on any metal surface. These results provide a critical test of theory. This work is described in a manuscript in preparation.