Application of inelastic epithermal neutron scattering to the vibrational spectroscopy of adsorbed molecules: Butane physisorbed on graphite (0001) surfaces

Abstract

Inelastic epithermal neutron scattering (IENS) has been used to investigate the intramolecular vibrations of butane [CH3(CH2)2CH3] physisorbed on the (0001) surfaces of a graphite powder. The purpose of these studies was to assess IENS as a vibrational spectroscopy for adsorbed species by using a relatively well‐characterized substrate (Carbopack B). The experiments were performed on the IN1 spectrometer located on a beam from the ''hot'' source at the Institut Laue‐Langevin reactor in Grenoble. Film coverages of 1.0 and 3.6 layers were investigated at a temperature of 78 K. The IENS spectra are rich in structure, containing eight vibrational bands in the energy‐transfer range from 280-3470 cm−1 (35-430 meV). The similarity in the spectra at the two coverages investigated indicates that in this energy‐transfer range the butane intramolecular modes are not strongly perturbed by physisorption on graphite. A detailed comparison is made between the monolayer IENS spectrum and those calculated from models of a free and adsorbed molecule. Both models employ a previously derived intramolecular force field in the normal mode calculation. A third calculation assumes the displacement eigenvectors of the free molecule but replaces the mode eigenfrequencies by their measured Raman and IR values. It reproduces the observed IENS spectra very well. The large number of vibrational bands observed and the close agreement with the calculated spectra suggest butane as a desirable adsorbate for similar experiments on catalytic substrates.