Molecular reorientation on liquid surfaces: a MIES study of pentadecane segregation in formamide

Metastable-induced electron spectra (MIES) of binary liquid mixtures can often be interpreted as linear combination of the pure compound spectra, S-1 and S-2, for the whole range of molar fraction. In the language of linear algebra, i.e., if the spectra are viewed as elements of a vector space, the spectra of the mixture belong to the two-dimensional subspace spanned by the two reference vectors S-1 and S-2. Since this simple assumption failed in the case of the pentadecane/formamide system, we determined the dimensionality of a series of spectra by means of singular value decomposition (SVD) to d = 3. This indicates that the series of spectra can be explained by three reference spectra identified as the spectrum of pure formamide, that of flat-lying pentadecane and that of pentadecane molecules exposing their methyl end groups to vacuum. With this assignment, the reorientation of alkanes during the course of the segregation process can be traced quantitatively. (C) 1997 Elsevier Science B.V.