Fundamentals of Light and Matter Interaction

Module Structure

• The module covers fundamental concepts of light, propagation in waveguides, and light's interaction with matter.
• Topics include lasers, photobiology basics, biophotonics applications (bioimaging, tissue engineering), and more.

Topics to be Covered

• Atoms and Molecules
  • Molecular interactions including absorption, spontaneous and stimulated emission.
  • Fate of excited molecules, specifically fluorescence.
  • Light scattering phenomena like Rayleigh, Mie, and Raman spectroscopy.

At the Start

• The atom was considered an indivisible particle.
• Atoms combine to form different elements.
• Early models of the atom, such as the "plum pudding" model, were proposed.

Failures of Classical Mechanics

• Classical mechanics failed to explain phenomena like blackbody radiation, photoelectric effect, and discrete emission bands.
• Quantum mechanics was developed to address these failures.

Quantization of Energy

• Max Planck proposed that energy is quantized in whole multiples.
• Einstein's explanation of the photoelectric effect further supported the concept of quantized energy.
• Planck's constant (6.626068 × 10⁻³⁴ m² kg / s) is a crucial constant in quantum mechanics.

Rutherford Model

• A "planetary" model, with electrons orbiting a nucleus containing protons and neutrons.
• The model depicts electrons in orbit around the nucleus.

Bohr Model (1913)

• The Bohr model describes electrons orbiting the nucleus at specific distances.
• Electrons occupy different energy levels.
• Radiation is emitted or absorbed when electrons transition between energy levels.
• Different shells hold different numbers of electrons (1st shell = 2, 2nd shell = 8, and 3rd shell = 18).

Electron Transitions

• Electrons can absorb or emit photons during transitions between energy levels.
• Energy of transition is equal to the energy of the absorbed or emitted photon.
• Photon emission, energy of the photon, discrete energy, and spectroscopy are related aspects of electron transitions.

Quantum Mechanics Model

• De Broglie proposed that particles can have wave-like properties.
• Schrödinger's wave equations describe the behavior of electrons in an atom.
• Electrons occupy orbitals that have specific shapes and energies.
• At higher energies, electron orbitals have complex shapes.

Wave-Particle Duality of Matter

• Matter exhibits both particle and wave characteristics.
• Kinetic energy and momentum are particle-like properties.
• Wavelength and quantized energy are wave-like properties.

Parts of the Atom

• Atoms consist of electrons, protons, and neutrons.
• Electrons are negatively charged, protons are positively charged, and neutrons have no charge.
• The mass of an electron is much smaller than the mass of a proton or neutron.
• The number of electrons equals the number of protons in a neutral atom.
• Isotopes have different numbers of neutrons.
• An "ion" is an atom with an unequal number of electrons and protons (charged).

Filling Orbitals with Electrons

• Electrons fill atomic orbitals in a specific order, following the Aufbau principle.
• Subshells (s, p, d, f) and shells (n=1, 2, 3...) are distinct energy levels.
• Electrons occupy different orbitals in a specific order to achieve the lowest energy configuration.

Orbital Shapes

• Different atomic orbitals have different shapes (1s, 2s, 2p, 3s, 3p, 3d).

Allowed Transitions in the Hydrogen Atom

• Hydrogen atom transitions demonstrate discrete energy levels and specific spectral emission bands.
• The allowed transitions between energy levels generate specific wavelengths of light which can be mapped in spectral emission bands.

Molecular Energies

• Molecules have various forms of energy (electronic, vibrational, rotational and translational).
• Quantized effects are observed in all energies levels.
• Rotation and vibration contribute to molecular properties.

Molecular Potential Energy Curve

• A curve detailing energy changes with internuclear separation (distance between nuclei).
• Equilibrium bond length (minimum on the curve) represents the most stable distance.
• The minimum point on the curve indicates the molecule's most stable bond state.

Molecular Vibrational States

• Molecular vibrations are quantized, leading to specific vibrational energy levels.
• Bond dissociation energy (maximum energy to break a bond) is a critical parameter.
• Finite number of quantized vibrational energy states determines the vibrational modes.

Typical Normal Vibrational Modes

• Molecules have specific vibrational modes (e.g., symmetric, asymmetric, bending).
• Linear molecules (3N-5) and non-linear molecules (3N-6) have different numbers of vibrational modes depending on their geometry.

Summary

• Energy is absorbed and emitted in discrete quanta (packets).
• Quantum mechanics describes electron orbitals and their shapes.
• Electrons can transition between energy levels, emitting or absorbing photons.
• Molecular bonds vibrate with quantized energies, affecting their behavior.