Light-matter interactions are the foundation of photochemistry. These processes involve absorption, emission, and scattering of light by atoms and molecules. Understanding these interactions is crucial for interpreting spectroscopic data and designing photochemical reactions.

Electronic transitions play a key role in light-matter interactions. When molecules absorb light, electrons jump to higher energy states. This process follows quantum mechanical rules and can lead to various outcomes, including fluorescence, phosphorescence, or non-radiative decay.

Light-Matter Interactions: Fundamental Processes

Absorption vs emission vs scattering

• Absorption
  • Matter takes in light energy causing electrons to jump to higher energy states
  • Decreases intensity of transmitted light (UV-visible spectroscopy)
  • Occurs when photon energy matches energy gap between states
• Emission
  • Matter releases light energy as electrons return to lower energy levels
  • Spontaneous emission happens without external stimuli (fluorescence)
  • Stimulated emission requires interaction with incoming photons (lasers)
• Scattering
  • Light changes direction upon interaction with matter without energy transfer
  • Elastic scattering maintains wavelength (Rayleigh scattering in sky)
  • Inelastic scattering alters wavelength (Raman spectroscopy)

Electronic transitions in light interactions

• Electronic transitions
  • Electrons move between energy levels in atoms or molecules
  • Quantum mechanical selection rules govern allowed transitions
• Absorption process
  • Electron jumps from ground to excited state absorbing a photon
  • Follows Franck-Condon principle maximizing wavefunction overlap
• Emission process
  • Excited electron falls to ground state releasing a photon
  • Energy of emitted photon equals transition energy
• Types of transitions
  • $\sigma \to \sigma^$: high energy UV transitions (alkanes)
  • $n \to \sigma^$: moderate energy transitions in saturated compounds (alcohols)
  • $\pi \to \pi^$ and $n \to \pi^$: lower energy often visible transitions (conjugated systems)

Photoluminescence and Efficiency Factors

Principles of fluorescence and phosphorescence

• Fluorescence
  • Rapid emission process occurring within nanoseconds to microseconds
  • Transitions between same spin multiplicity states typically singlet to singlet
  • Emitted photon has lower energy than absorbed photon due to Stokes shift
  • Vibrational relaxation precedes emission resulting in energy loss
• Phosphorescence
  • Slower emission process lasting milliseconds to hours
  • Involves intersystem crossing to a triplet state before emission
  • Triplet to singlet ground state transition forbidden causing slower decay
  • Exhibits larger Stokes shift compared to fluorescence
• Jablonski diagram
  • Visually represents electronic states and transitions in molecules
  • Shows absorption fluorescence phosphorescence and non-radiative processes

Efficiency factors in light-matter interactions

• Absorption cross-section
  • Probability measure of absorption likelihood
  • Larger cross-section indicates higher absorption probability
  • Depends on molecular structure and incident light wavelength
• Quantum yield
  • Ratio of emitted to absorbed photons measuring photoluminescence efficiency
  • Affected by competing non-radiative decay pathways (internal conversion)
• Factors affecting efficiency
  • Molecular structure and rigidity influence relaxation pathways
  • Solvent effects and temperature impact energy dissipation
  • Quenchers or energy transfer acceptors reduce emission
  • Concentration affects self-quenching at high levels
• Beer-Lambert law
  • Relates absorption to concentration and path length
  • $A = \varepsilon bc$ where $A$ is absorbance $\varepsilon$ is molar absorptivity $b$ is path length $c$ is concentration
• Oscillator strength
  • Dimensionless quantity indicating transition probability
  • Higher values correspond to stronger more likely transitions