Lecture 1

Overview of Chapter 8: UV-Vis and Raman Spectroscopy

• Preparation for the exam on April 1 covering:

  • Selection rules for UV-Vis spectra

  • IR and Raman spectroscopy

Introduction to Roman Mosaics

• Exploration of pigments used in Roman mosaics.

• Key Pigments:

  • Cinnabar: Mercury sulfide (reddish-brown).

  • Calcite: Calcium carbonate (white, similar to chalk).

  • Carbon Black: Elemental carbon (black).

  • Good Type: Iron oxide, formula: FeOOH (dark brown/black).

  • Hematite: Iron oxide, formula: Fe2O3 (red).

  • Magnetite: Another iron oxide.

  • Egyptian Blue: Detected in the samples analysed.

Raman Spectroscopy Techniques

• Instruments used to analyze mosaics:

  • Confocal Raman Spectroscopy: High precision technique.

  • Raman Microscope: Used for in situ analysis, though with lower resolution (5 reciprocal cm).

Limitations and Issues

• Resolutions:

  • Quality of measurements in Raman spectroscopy can be poor, e.g., 5 reciprocal cm versus previous cases of 1 or 4 reciprocal cm.

• Fluorescence Concerns:

  • Specified wavelength of laser (632 nm) can induce fluorescence.

  • Need to avoid overlap with absorbance spectra of organic pigments to prevent interference.

Luminescence in Chemistry

• Definitions:

  • Fluorescence: Emission of light from an excited state back to ground state without spin flip.

  • Phosphorescence: Emission involving a spin flip and longer-lived excited state (triplet state).

Spin Multiplicity

• Equation:

  • MS = 2S + 1 (where S is the spin quantum number).

  • For singlet state (fluorescence), S = 0 hence MS = 1.

  • For triplet state (phosphorescence), S = 1 hence MS = 3.

Emission Processes

• Fluorescence:

  • Emission occurs by transitioning from the excited state to the ground state.

  • Quick process (nanoseconds).

• Phosphorescence:

  • Slower process due to spin flipping and longer emission lifetimes (milliseconds to seconds).

  • Delayed fluorescence can occur, indicating complex interactions and lifetimes.

Applications of Luminescence

• Biochemical Applications:

  • Useful in studying cellular membranes, metabolic processes, and detecting trace levels of substances (nanomolar or picomolar limits).

Instrumentation in Luminescence

• Laser-based System Overview:

  • Sources: Generally lasers or xenon arc lamps for excitation.

    • Used to use mercury lamps: line source (254nm for organics)

    • Lasers (monochromatic)

    • Xenon arc (continuum source) (200-800nm)

  • Right Angle Geometry: Excitation and emission at 90 degrees to minimize direct laser interference.

  • In-line Geometry: Useful for solid samples.

• Jablonski Diagram: Shows the processes occurring in a molecule upon excitation and illustrates the potential transitions and relaxations.

Energy and Time Scales of Processes

• Timescales of Emission:

  • Absorption: 10^{-15} seconds (femtoseconds).

  • Internal conversion/vibrational relaxation: 10^{-14} to 10^{-10} seconds.

  • Fluorescence: 10^{-9} to 10^{-7} seconds. (intersystem crossing)

  • Phosphorescence: 10^{-3} to 10^{-2} seconds (very slow due to forbidden transitions).

  • All affect whether or not you get emission

Key Processes

• Conditions for Emission: Fluorescence and phosphorescence occur based on the environment and the specific molecular structure which influences excited state lifetimes and emission.

• Stokes Shift: The difference in wavelength between absorption and emission, representing energy loss.

Challenges and Future Directions

• Continued research on luminescence techniques, including the interaction of molecular environments and the potential for new applications in chemistry and biology.

• Investigation into the effects of different solvents on molecular emissions.

Conclusion

• Preparation for the upcoming exam should focus on the integration of knowledge of spectroscopy techniques, molecular behavior under excitation, and practical applications within the field of chemistry.