Absorption and emission spectra reveal how molecules interact with light. These spectra show peaks, intensities, and shapes that give clues about molecular structure and behavior. Understanding these patterns helps scientists analyze chemicals and study reactions.

The Beer-Lambert law connects absorbance to concentration, enabling quantitative analysis. Spectral features like band position, shape, and shifts provide insights into molecular properties and environmental effects. These tools are crucial for various scientific and practical applications.

Absorption and Emission Spectra

Key features of spectra

• Absorption spectra capture light absorbed by molecules across wavelength range showing peak positions intensity variations and band shapes (narrow or broad)
• Emission spectra reveal light emitted by excited molecules featuring Stokes shift longer wavelength emission and anti-Stokes shift shorter wavelength emission
• Fluorescence rapid emission from singlet excited states and phosphorescence slower emission from triplet states characterize emission processes
• Absorption and emission spectra often exhibit mirror image relationship due to similar vibrational energy levels in ground and excited states (Franck-Condon principle)

Beer-Lambert law for concentration

• $A = \varepsilon bc$ relates absorbance (A) to molar absorption coefficient (ε) path length (b) and concentration (c)
• Linear relationship between absorbance and concentration enables creation of calibration curves for unknown sample analysis
• Limitations arise at high concentrations (> 0.01 M) due to molecular interactions and light scattering effects

Insights from spectral bands

• Band position correlates with energy transitions affected by molecular structure (conjugation) and environment (solvent polarity)
• Band shape reveals vibrational and rotational fine structure broadened by homogeneous (temperature) and inhomogeneous (sample heterogeneity) effects
• Spectral shifts indicate changes in electronic transitions bathochromic (red shift to longer wavelengths) and hypsochromic (blue shift to shorter wavelengths)
• Intensity changes hyperchromic (increase) and hypochromic (decrease) reflect alterations in transition probability or sample concentration

Applications in chemical analysis

• Qualitative analysis identifies functional groups (carbonyl ~1700 cm⁻¹) and elucidates molecular structure
• Quantitative analysis determines concentration (pharmaceutical active ingredients) and assesses sample purity
• Kinetics studies monitor reaction progress and determine rate constants for chemical reactions (enzyme kinetics)
• Thermodynamic measurements derive equilibrium constants and binding affinities (protein-ligand interactions)
• Environmental applications detect pollutants (heavy metals in water) and assess water quality (organic contaminants)
• Biomedical applications analyze drugs (HPLC-UV detection) and provide diagnostic tools (fluorescence microscopy)