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The Lund de-airing technique is safer, simpler, and more effective compared with the carbon dioxide insufflation technique. The technique is also more cost-effective because the de-airing time is shorter and no extra expenses are incurred.

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Experimental vibration spectra for ordered overlayers of O and S on Ni(lOO) are analyzed in terms of surface-lattice dynamics in relation to rigid-lattice calculations. It is shown how spectral differences for the consecutive ^(2x 2) and c(2x 2) structures are primarily due to differences in the coupling to the substrate phonon modes.PACS numbers: 68.30. + Z, 68.20.+t Vibration spectra of adsorbates on solid surfaces provide a most important source of information about the state of adsorption and possible adsorption geometries. Measured spectra are in general analyzed qualitatively^ with use of selection rules, symmetry arguments, and empirical comparison of spectral features with data for suitable gauge substances. In some cases a direct comparison of measured and calculated vibrational energies has resulted in a determination of the adsorption geometry, e.g., for hydrogen adsorption on nickel surfaces, where vibrational energies derived from calculated potential curves^'^ compare very favorably with those observed in electron-energy-loss measurements (EELS). For oxygen adsorption on Ni(lOO) a similar analysis has proven rather intricate. The unexpectedly large shift of the experimentally observed vibrational energies for oxygen in the consecutive /)(2x2) and c(2x2) structures'*^ has been suggested to be the consequence of a drastic change in the electronic structure of the adsorbate resulting in quite different oxygen positions, dj^^ above the nickel surface.^'^ This picture, however, is not supported by experimental structure investigations which rather yield d^'^ 0.8-0.9 A for both structures.^ "^ In this Letter we present EELS spectra for ordered overlayers of O and S on Ni(lOO) and discuss them in relation to rigidlattice calculations. We show how spectral differences between the /?(2x2) and c(2x2) O spectra are primarily due to the stronger coupling to substrate modes in the/>(2x2) case.The Ni(lOO) specimen was cleaned by argon-ion bombardment and annealing according to standard procedures. The ordered O and S structures were formed and monitored by low-energy electron diffraction as described previously.' The EELS spectra were recorded at an ambient pressure of 2x10"^^ Torr with use of a high-resolution electron spectrometer that has been described elsewhere.^° The scattering plane containing the incident and the scattered electron beams is defined by the specimen surface normal and the [lOO] direction in the surface plane. The experimental spectra shown in Figs. 1 and 2 have been measured in the specular direction for an angle of incidence around 50° and an energy of the incident electrons around 2 eV. Angular measurements have been carried out routinely and show that the spectral features of interest are excited by dipole scattering, i.e., the loss intensity peaks sharply close to the specular direction.^°The experimental EELS spectra have been analyzed by a calculational scheme for lattice dynamics of dipole-active surface vibrational modes of a semi-infinite substrate.^^ The measured EELS spectrum is ...