The atomic structure of the InP͑001͒ reconstructions has been identified by scanning tunneling microscopy and infrared spectroscopy of adsorbed hydrogen. The four phases, in order of decreasing phosphorous coverage, are c(2ϫ2)/p(2ϫ2), P ϭ2.0 ML; (2ϫ1), P ϭ1.0 ML; (2ϫ4), P ϭ0.25 ML, and ␦(2ϫ4), P ϭ0.125 ML. The (2ϫ2) consists of phosphorous dimers adsorbed onto a layer of P atoms. Removal of the P adatoms at 300°C, produces the (2ϫ1), which is terminated with a complete layer of buckled phosphorous dimers. Further annealing at 400 to 500°C, yields the indium-rich (2ϫ4) and ␦(2ϫ4) reconstructions. The (2ϫ4) reconstruction comprises a single phosphorous dimer in the top layer and four indium dimers in the second layer. The ␦(2ϫ4) differs from the (2ϫ4) reconstruction in that one of the P atoms is replaced with an In atom to make an In-P heterodimer. This difference is evident by comparison of the intensities of P-H, In-H, and In-H-In stretching vibrations for the two surfaces.

The relationship between the reflectance difference spectra and the atomic structure of arsenic-rich reconstructions of GaAs͑001͒ has been investigated. Scanning tunneling micrographs reveal that a roughening process occurs as the surface structure changes with decreasing arsenic coverage from 1.75 to 0.75 monolayers ͑ML͒. At 1.65 ML As, small pits, one bilayer in depth and having the same c(4ϫ4) reconstruction as the top layer, form in the terraces. At the same time, gallium atoms are liberated to the surface, disrupting the c(4 ϫ4) ordering. At about 1.4 ML As, (2ϫ4) domains nucleate and grow on top of the c(4ϫ4). Further desorption of arsenic causes the underlying layer to gradually decompose into a metastable (2ϫn) phase (n ϭ2, 3, or 4͒, and finally into the (2ϫ4). In the reflectance difference spectra, negative peaks at 2.25 and 2.8 eV correlate with the c(4ϫ4)-type arsenic dimers. However, the intensity of the latter feature strongly depends on the presence of adsorbates, such as alkyl groups and gallium adatoms. By contrast, the intensity of the positive peak at 2.9 eV is directly proportional to the density of (2ϫ4)-type dimers.

Hydrogen adsorption on the InP (001)-(2ϫ1) reconstruction has been characterized by vibrational spectroscopy and ab initio calculations with density functional theory. The (2ϫ1) surface is covered with a complete layer of phosphorus dimers. The clean and hydrogen-terminated dimers have been modeled by In 5 P 4 H x clusters with the proper number of covalent and dative bonds to accurately represent the surface of interest. The optimized molecular cluster of the unreacted dimer reveals that it has one filled and one partially filled dangling bond. Hydrogen atoms attack the P-P dimers forming terminal and coupled phosphorushydrogen bonds, and the predicted vibrational frequencies of these species are in excellent agreement with the infrared spectra. All the observed vibrational modes can be assigned to species containing one, two, and three hydrogen bonds ͑i.e., PH, HP-PH, and PHϩPH 2 ͒ per surface dimer.

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