On the basis of variable-temperature ultrahigh vacuum scanning tunneling microscopy data, we propose a two-dimensional phase diagram of monolayer decanethiol on Au(111). Four triple-point temperatures were determined:  T 1 at ≈27 °C, T 2 at ≈33 °C, T 3 at ≈35 °C, and T 4 at ≈56 °C. T 1 defines the lowest temperature melting point, and T 4 defines the temperature above which striped phases are metastable. These data provide a fundamental framework to understand and control mesoscale monolayer structure; moreover, they provide fundamental insight into the two-dimensional phase behavior of molecules with many degrees-of-freedom.

The effects that a reconstructed (herringbone) and stepped Au(111) surface have on the structure of submonolayers of decanethiolate were studied by variable-temperature scanning tunneling microscopy (VT-STM) between 25 and 60 °C. At 25 °C, formation of lattice gas (R) species at low coverages alters the herringbone structure by shortening the periodicity of the elbows from 25 to 15 nm. In addition, small β-phase islands nucleate and grow anisotropically in regions of fcc stacking. These domains grow by incorporating nearby lattice gas species, consuming herringbone ridges and altering the remnant ridges that surround them. In a given domain, the β-phase rows take one of the three 〈121〉 directions of the Au(111) surface. No β-phase is found in regions of hcp stacking or in the bridge atom regions separating the fcc and hcp stacked regions. Increasing the coverage increases the β-phase island size at the expense of herringbone ridges that bound the domains. For a coverage that saturates the β-phase (∼0.25 of the closest packing achievable), raising the temperature to 30 or 40 ο C increases the average size of the β-phase islands by condensation of neighboring islands with no evidence, at the selected coverage, for the presence of any other thiolate phase. At 60 °C, well above the thiolate melting point to form the -phase, small β-phase domains remain. These are stabilized by boundaries of two types remnant herringbone and step edges and for a given domain, fluctuations of the distribution between the -and β-phases were observed on a time scale of minutes.

The coverage-dependent phase behavior of the thiolate formed from decanethiol, CH3(CH2)9SH, on Au(111) was studied at 0 °C using variable-temperature scanning tunneling microscopy and compared to analogous results for temperatures between 25 and 65 °C. At 0 °C, the lowest density striped phase, β, converts to higher density striped phases, δ and χ, at exposures that are significantly less than those required at room temperature. The upright saturation phase, φ, is also obtained with a lower relative exposure. We discuss these results using an extrapolation of the schematic two-dimensional pressure (π) versus temperature phase diagram developed in previous work. The observed low-temperature phase behavior is rationalized on the basis of thermodynamic considerations. By use of a schematic plot of phase chemical potential versus lateral pressure, the range of exposures over which various phases are thermodynamically stable is assessed as a function of temperature between 0 and 65 °C.

The atomic-scale surface structural evolution of Si(100) exposed to gas-phase thermal hydrogen atoms, H(g), has been investigated by scanning tunneling microscopy and temperature-programed desorption mass spectrometry. For the substrate temperature (Ts) between 420 and 530 K, dihydride species in 3×1:H domains were selectively etched upon extensive exposures to H(g). As a result, etch pits grew laterally along Si surface dimer rows. The presence of these pits correlates with the absorption of H(g) into the bulk of Si(100), confirming our earlier suggestion that atomic-scale surface roughening caused by etching is a prerequisite for H(g) absorption.

A physical chemistry laboratory experiment is described in which proton and deuteron FT-NMR spectra are used to determine the equilibrium constant for the isotope exchange reaction DCl + HBr <==> DBr + HCl . The measurement illustrates an unusual gas phase application of NMR spectroscopy. The results are in good accord with a 297K value of Kp = 0.818, calculated from statistical thermodynamics using vibrational-rotational constants. The latter can be taken from the literature or can be conveniently determined in a high resolution FTIR experiment commonly done in most physical chemistry laboratories.