Dr. T. B. Grimley (Liverpool) said: I would ask Rosch whether these nickelethylene complexes are stable. Have they made any calculations of the total energy? There must always be some doubts about the usefulness of a discussion of chemical bonding unless the theory also leads to a stable complex.Dr. C. R. Brundle (Bradford) said: It is suggested in the paper of Rosch and Rhodin that the Xa calculations of Ni2-C2H4 favour a n-complex assignment for adsorbed C2H4 on Ni. One of the supporting pieces of evidence given is that the n-complex calculation produces a shift of 0.72 eV in the n level on chemisorption, the di-o-complex 0.30 eV, whereas the experimental value of Eastman and Demuth is 0.9 eV. The validity of the experimental value is open to question. The He1 photoelectron spectra of C2H4 condensed (at 77 K) and chemisorbed (at 200 K) on a nickel film are shown in the figure. It is clear that experimentally the position of the n level does not alter but the G levels move uniformly by 0.9 eV. Demuth and Eastman consider that this uniform o shift can only be explained as a difference in the relaxation effect in the final ionized states, between condensed and chemisorbed C2H4 the amount being the same for all ionized o levels. They then make the questionable assumption that the n ionized state suffers the same change in relaxation energy (theoretically this would not be expected, especially if it is the orbital involved in the chemisorption bonding to the metal surface) and therefore shift the whole of the chemisorbed -J E, 2 4 6 8 I0 I 2 binding energy /eV FIG. 1.-He I Photoelectron spectrum of the Ni/C2H4 system. Top : schematic spectrum of gaseous ethylene. -, clean Ni f i l m at 77 K ; ---, Ni + multilayers of condensed ethylene (77 K) ; . . . ., Ni + chemisorbed ethylene (200 K). The gas phase spectrum has arbitrarily been lined up with the condensed spectrum at the 2nd ionisation potential. 59
DISCUSSIONDr. C. M. @inn (Birmingham) said: As discussants are no doubt aware by this time, Lloyd, Richardson and I at Birmingham University have recently begun a study of angular dependence in p.e. of surfaces. I am very impressed by Willis's paper since it seems to go a considerable distance along the road to our understanding electron emission processes at surfaces. I would like to ask him one question and make one remark about the interpretation of off-normal angular resolved emission data.My remark concerns the influence of momentum broadening normal to the surface whenever an evanescent final state occurs in the photoelectric process. Under such conditions the surface photoelectric process applies and the degree of normalmomentum broadening reflects the localization in the normal direction into the crystal of the incoming scattering function. Thus it seems to me that a characteristic of surface photoelectric emission should be an " angular broadening " in which spectral features due to the surface effect occur over a range of angles in angularresolved spectra, a range reflecting the normal-momentum broadening. For copper single crystals, our results to date suggest that we may be observing this type of effect in certain instances.For a copper (001) surface and analyser tracking from (001) to (100) directions, one expects band-structure in the rWKWr section of the Brillouin-zone to be observable. We expected for identical photon angles that we should be able to monitor the TW band-structure at -26" and then again at -63" off-normal. However, the spectra for the analyser settings are different and this is also true over a range on either side of these positions. Can this be due to a large refraction effect? My question concerns the problem of refraction.
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