N-2 and HF vibrations on LiF(001): the effect of surface coverage

Adsorption energies and stretching vibrational frequency shifts for N-2 and HF molecules adsorbed on a LiF(001) surface have been obtained from

periodic Hartree-Fock calculations. Surface coverages between 6 and

100% investigated. A one-layer and a three-layer description of the

LiF slab are found to yield virtually the same adsorption energies,

adsorbate-substrate distances and vibrational frequencies at all

coverages. The relative importance of the molecule-molecule and

molecule-surface interactions at the optimized surface-adsorbate

distance are presented. For an isolated N-2 molecule physisorbed on

Li-s(+), we obtain a shift upwards of +9 cm(-1), only one wavenumber

away from the published experimental value. For low surface coverages,

the frequency shift of N-2 on Li-s(+) stems entirely from the

surface-adsorbate interaction, whereas for HF physisorbed on Li-s(+) (F

towards surface), the small upshift found is entirely due to the

molecule-molecule interaction within the adlayer. HF molecules

hydrogen-bonded on top of F-s(-) are strongly downshifted in frequency.

(C) 1998 Elsevier Science B.V. All rights reserved.