Interpreting vibrational spectra and LEED patterns is crucial for understanding surface structures and adsorbates. Group theory and selection rules help decode molecular vibrations, while LEED reveals atomic arrangements through electron diffraction.

Combining these techniques gives a fuller picture of surface chemistry and structure. Vibrational spectroscopy identifies chemical species and bonding, while LEED shows long-range order. Together, they help build detailed models of complex surface systems.

Vibrational Spectra Interpretation

Group Theory and Selection Rules

• Group theory is a mathematical framework that describes the symmetry of molecules and crystals categorizes symmetry elements into point groups based on the molecule's geometry
• Selection rules determine whether a vibrational mode is IR-active, Raman-active, or both, based on the symmetry of the molecule or crystal
  • For a vibrational mode to be IR-active, it must cause a change in the dipole moment of the molecule during the vibration (water molecule)
  • For a vibrational mode to be Raman-active, it must cause a change in the polarizability of the molecule during the vibration (carbon dioxide molecule)
• The number of vibrational modes for a molecule can be determined by its degrees of freedom depend on the number of atoms and the molecule's geometry (linear, nonlinear)

Surface-Specific Considerations

• Surface selection rules differ from those of bulk materials due to the reduced symmetry at the surface can lead to the appearance of new vibrational modes or the disappearance of modes that are active in the bulk
• The orientation of adsorbed molecules on a surface can be determined by comparing the observed vibrational spectra with those predicted by group theory for different adsorption geometries
  • Example: CO adsorbed on a metal surface can have different vibrational frequencies depending on whether it is adsorbed in an atop, bridge, or hollow site
• Changes in the vibrational spectra as a function of adsorbate coverage can provide information about the adsorption process and intermolecular interactions (shift in CO stretching frequency with increasing coverage on a Pt surface)

Surface Reconstruction Analysis

LEED Pattern Interpretation

• Low-Energy Electron Diffraction (LEED) is a technique used to determine the atomic structure of crystalline surfaces by analyzing the diffraction patterns of low-energy electrons
• The periodic arrangement of atoms on a surface creates a two-dimensional reciprocal lattice determines the positions of diffraction spots in the LEED pattern
• The size and shape of the surface unit cell can be determined from the spacing and arrangement of diffraction spots in the LEED pattern (square, rectangular, or hexagonal patterns)

Identifying Surface Reconstructions and Adsorbate Structures

• Surface reconstructions, which occur when the atomic structure of the surface differs from that of the bulk, can be identified by the presence of additional diffraction spots or changes in the spot intensities compared to the unreconstructed surface (Si(111) 7x7 reconstruction)
• The presence of ordered adsorbate structures on a surface can be detected by the appearance of new diffraction spots or changes in the existing spot intensities
• The size and symmetry of the adsorbate unit cell can be determined from the positions of the adsorbate-related diffraction spots relative to the substrate spots (c(2x2) or p(2x2) adsorbate structures)
• LEED-I(V) analysis, which involves measuring the intensity of diffraction spots as a function of electron energy, can provide quantitative information about the atomic positions and bond lengths in the surface structure

Surface Structure and Composition

Combining Vibrational Spectroscopy and LEED

• Vibrational spectroscopy (IRAS, HREELS) provides information about the chemical identity and bonding of surface species, while LEED reveals the atomic structure and periodicity of the surface
• The adsorption site and orientation of molecules on a surface can be determined by comparing the vibrational frequencies and selection rules with those predicted for different adsorption geometries based on the surface structure obtained from LEED
• Changes in the vibrational spectra as a function of adsorbate coverage can be correlated with changes in the LEED pattern to identify different adsorption phases and structural transitions (formation of ordered overlayers or surface alloys)

Developing Comprehensive Surface Models

• The combined information from vibrational spectroscopy and LEED can be used to develop detailed models of the surface structure, including the positions of adsorbed molecules and any surface reconstructions
• In cases where the surface structure is too complex to be fully determined by LEED alone, vibrational spectroscopy can provide complementary information to constrain the possible structural models
  • Example: Determining the adsorption geometry of complex organic molecules on metal surfaces by combining IRAS and LEED data
• Iterative refinement of surface models based on the agreement between experimental data and theoretical predictions from quantum chemical calculations or dynamical LEED simulations

Limitations of Vibrational Spectroscopy vs LEED

Vibrational Spectroscopy Limitations

• Vibrational spectroscopy is sensitive to the chemical identity and local bonding environment of surface species but does not directly provide information about long-range order or surface periodicity
• The surface sensitivity of vibrational spectroscopy depends on the technique used (IRAS is more surface-sensitive than HREELS) and may be limited for very rough or porous surfaces
• The interpretation of vibrational spectra can be complicated by the presence of overlapping peaks can make it difficult to assign specific vibrational modes to individual surface species (co-adsorption of multiple species)

LEED Limitations

• LEED is sensitive to the atomic structure and periodicity of crystalline surfaces but does not directly provide information about the chemical identity or bonding of surface species
• The surface sensitivity of LEED is limited by the penetration depth of low-energy electrons, which is typically a few atomic layers can make it challenging to study surfaces with thick adsorbate layers or buried interfaces
• The interpretation of LEED patterns can be complicated by the presence of multiple scattering events can make it difficult to determine the exact atomic positions in complex surface structures (reconstructions with large unit cells)

Complementary Nature of Techniques

• The combination of vibrational spectroscopy and LEED provides a more complete understanding of surface structure and composition than either technique alone offer complementary information on both the chemical and structural properties of the surface
  • Example: Identifying the adsorption site and bonding configuration of a molecule on a surface using IRAS, and determining the long-range ordering and surface periodicity using LEED
• The limitations of one technique can often be overcome by the strengths of the other, providing a more comprehensive picture of the surface properties