The absolute absorption strength and vibrational coupling of CH stretching on diamond C(111)

Abstract

This research investigates the infrared absorption intensity and isotope-dependent frequency shifts of CH stretching on diamond C(111) single-crystal surfaces by Fourier transform infrared spectroscopy (IRS). By employing single-pass direct absorption and in situ surface oxidation methods, a single sharp feature at νm=2832.2±0.9 cm−1 with a FWHM of Γ≈6 cm−1 is observed at 800 K. Systematic measuring of how band intensity depends on hydrogen etching time indicates that a well hydrogen-terminated C(111)-1×1 can be prepared only after prolonged exposure of the surface to H, generated by hot W filaments, at 1100 K. A study of the band intensity at saturation, and assuming an electronic polarizability of αe=0.65 Å3 for the CH bond as that in CH4, yields an integrated cross section σ̄z=5.5×10−18 cm for the CH stretching motion along the internuclear axis. Additional measurements of band position as a function of mixed isotope concentrations afford a stretching frequency of νi=2816.2±0.9 cm−1 for a single CH isolated in a monolayer of CD oscillators at 800 K. The frequency shift of νm−νi=16.0 cm−1 is too large for dipole coupling theories to explain. The implications of the present findings with regard to applying IRS for quantitatively characterizing CVD diamondlike carbon films are discussed.