Molecular packing in Langmuir Blodgett films by core level loss spectroscopy

Abstract

Molecular packing in lead arachidate [(C₁₉H₃₉COO)₂Pb] Langmuir Blodgett monolayer films transferred to glass substrates has been investigated using X-ray photoemission spectroscopy. It was observed that the degree of salt formation was a sensitive function of the subphase pH reaching a maximum at a pH of nearly 5.8. The loss features in the C 1s core level spectra of the films were clearly defined for films consisting of purely metal salt of arachidic acid while they became progressively diffused as the subphase pH was reduced. The blurring of the C 1s loss features has been explained as arising due to disorder introduced into the alkyl chains of lead arachidate molecules by the evaporation of acid molecules under ultrahigh vacuum. This leads to changes in the quasi one-dimensional electronic band structure in such molecular crystal films. A comparison is made with results obtained on cadmium arachidate and arachidic acid monolayer Langmuir Blodgett films. It is thus shown that molecular packing in Langmuir Blodgett films may be investigated using core level loss spectroscopy.