Surface structure rotated

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

Figure 4.4 Surface structure of a spore of Bacillus polymyxa. From left to right side view same rotated a quarter turn from right to left same rotated a further quarter turn view, of a pole.

Another study of the surface structure of CdSe NCs of 3.7 nm size used 31P MAS-NMR and 31P/77Se rotational-echo double-resonance (REDOR) to identify overlapping broad peaks from two surface species trioctylphosphine oxide (TOPO) at 29.3 ppm and trioctylphosphine selenide (TOPSe) at 22.2 ppm [343]. Both the isotropic chemical shift and CSA of the surface-bound TOPO were substantially different from those of the free ligand. Spin-echo experiments on 31P were stated to indicate an average P-P distance of 8-10 A at the surface, consistent with capping at alternate atomic sites (all Cd but not Se). 

Fig. 6. Schematic representation of the normal modes of an adsorbed diatomic molecule neglecting the surface structure, after Richardson and Bradshaw . In parentheses the experimentally measured values for CO in the ontop position on Pt(lll). (a) A frustrated translation (60 cm (b) A frustrated rotation (not yet detected), (c) The metal-molecule stretch (460cm ) . (d) The intramolecular stretch model (2100cm" ) . ...

Table 5.2. Surface structures on substrates with threefold rotational symmetry...

LEED studies have revealed that the atoms in this platinum surface are in the positions expected from the projection of the X-ray unit cell to the surface (5). The diffraction pattern that is exhibited (Fig. 4) clearly indicates a sixfold rotational symmetry that is expected in such a surface. Calculations of surface structure from LEED beam intensities indicate that atoms are in those positions in the surface layer (with respect to the second layer) as indicated by the X-ray unit cell within 5% of the interlayer distance (6,7). 

In addition to the general behaviour outlined above, rotation spectra can contain signatures from such features as cell walls, large organelles, surface structure and membrane mobile charges. This allows the spectra to be used in a variety of investigations... 

Combined Rotation and Multiple Pulse Spectroscopy (CRAMPS) is a technique in which the dipolar interaction is averaged through a multiple-pulse sequence [54, 55]. The simultaneous spinning around the magic angle, as in MAS NMR, averages the chemical shift anisotropy. Under appropriate conditions, CRAMP spectra can be of greater resolution than MAS NMR spectra. While CRAMPS is not exclusively a surface-sensitive technique, the majority of catalytic applications have focused on the study of adsorbed species, and the information on surface structure that can be extracted from their spectra. 

Coexisting domains of expanded and condensed surface structures are observed as shown in Fig. 3.61. The expanded surface structure should be formally described by an Ag(lll)-(-s X VS) R 30° structure, but can be attributed to 2D Me-S surface alloy formation starting at monatomic steps. The condensed surface structure corresponds to the compressed and rotated hep structure of the 2D Meads overlayer. A similar model for 2D Me-S surface alloy formation in the system Au(lll)/Pb, H, CIO4 was suggested by Green [3.195]. 

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