Infrared-visible sum-frequency generation was used to obtain vibrational spectra of hydrogen on C(l I I). From a fully relaxed (I && I) surface, a single sharp peak at -2830 cm ' was observed, which can be identified as the CH stretch mode from H on top sites. With the surface freshly transformed from (2X I) to (I && I), however, another peak at a higher frequency was detected. This is attributed to H adsorbed on a metastable (I X I) structure.Hydrogen on diamond C(l I I) is a subject of great current interest since it plays an important role in the growth of diamond films by vapor deposition. ' However, despite the rather extensive effort in the past, understanding of the system is still not complete. The low-energy electron-diff'raction (LEED) work showed that upon hydrogen adsorption, the reconstructed C(l I I)-(2X I) surface is transformed into a (I X I) bulk-terminated structure, but few details are known. The electron-energy-loss spectroscopy (EELS) results suggested that the adsorption forms CH3 groups truncating the dangling bonds of the (I x I ) surface, while the He scattering and diA'raction data and the angular distribution of protons desorbed by electron stimulation ' from H/C(111)-(I x I) favored a simple hydrogen truncation of the bulk (H on top sites).Recently, infrared-visible sum-frequency generation (SFG) has been proven to be an eA'ective tool for surface vibrational spectroscopic studies.It can be applied to any interface accessible by light and is ideally suited for investigation of hydrogen adsorption on diamond. Here, we report the first results on H/C(111) using this technique. We have found, for H on a fully relaxed (I x I) surface, a single CH stretch peak at -2830 cm ' in the vibrational spectrum with an intrinsic full width less than 10 cm '. This, together with the polarization dependence of the spectrum, assures the monohydride adsorption geometry of H sitting on top C(l I I) sites. We have also found, together with LEED and optical second-harmonic generation work, that adsorption of only a few percent of a full monolayer of H was sufficient to induce the (2X I ) (I x I ) phase transformation.The CH stretch mode observed on a freshly formed (I & I ) surface at a low H coverage without annealing, however, showed up as a peak at a higher frequency (2860 cm '), suggesting the freshly formed surface might not be fully relaxed. The full surface relaxation could be effected by either hightemperature annealing (without hydorgen desorption) or higher surface coverage of H.The experimental setup for the SFG measurements has been described elsewhere. Briefly, the sum-frequency output from a sample surface was generated by overlapping a 25-ps visible light pulse at 0.532 pm and a 25-ps tunable infrared pulse on the surface. It is known that the output is proportional to the absolute square of the surface nonlinear susceptibility g, -, which can be written as (2) (2) + (2) gs gNR gR gR g Q)tR Q)g+ffg where gN2R is the nonresonant contribution, co~R is the infrared laser frequency, and Ag, cog,...
