EELS of CH 4, CD 4 and CD 2H 2 physisorbed on NaCl (100)

Abstract

Abstract We report on the potential use of EELS to obtain information about molecules weakly adsorbed on an insulator. Previous work has shown that it is possible to observe EELS spectra of adsorbates on semiconductors and metallic oxides (refs. 1, 2) but none have been reported for adsorbates on a wide band-gap insulator. Methane was adsorbed on NaCl (100) with background pressures in the range 10−8 -10−7 mbar and at a temperature of 40K. EELS spectra were recorded in the specular direction for the clean crystal face, and for different coverages of methane. It was found that surface charging effects were dependent on coverage. Increasing the beam energy to 30 to 50 eV was sufficient to overcome charging effects for the clean crystal. However for methane NaCl, charging effects caused rapid signal losses regardless of the incident beam energy. Spectra could be recorded for only a few minutes before complete loss of signal occurred. The clean NaCl (100) is characterized by the appearance of clearly resolved loss peaks corresponding to a surface optical phonon frequency of 237 cm−1 (29.4 meV, Fig. 1), which is very close to the value predicted by dielectric theory (ref. 3). When methane is admitted several changes to the 40K EELS spectrum are observed. Broad relatively weak loss peaks corresponding to the vibrational frequencies of the isotope of methane adsorbed become apparent. The linewidth (fwhh) of these bands is typically of the order of 400–600 cm−1. Given that the width of the elastic peak varies from 125 cm−1 for clean NaCl through to 320 cm−1 for methane multilayers, the width of the methane loss peaks is probably due to the overlap of individual bands which are separated by less than 200 cm−1. The CH4 EELS spectrum (Fig. 2), for example, shows two broad bands at 1540 cm−1 (v2 and v4) and 2970 cm−1 (v1 and v3) (ref. 4). If the degeneracies of v4(f2) and v2(e) are split by using CD2H2, the width of the corresponding loss peak increases. We conclude that all four methane modes contribute to the loss spectrum. Some of the broadening in the region of the elastic peak and phonon losses which is observed even at low methane coverages may be due to the external modes of NaCl-methane, which would be expected to occur in the region of about 100 to 200 cm−1. Further work is required to see if improved resolution may be attained by working at lower temperatures and pressures to reduce scattering by gas phase methane. If this is possible, then the questions of exact band positions and external mode frequencies may be answered.