Chapter 11: Molecular Spectroscopy

The treatment begins with rotational spectroscopy, which exploits the quantized rotational energy levels of molecules determined by their moment of inertia to reveal information about molecular geometry and structure through microwave and rotational Raman techniques. Vibrational spectroscopy extends this analysis by initially applying the harmonic oscillator model to predict vibrational transitions, then incorporating anharmonic corrections to more accurately represent the behavior of real diatomic molecules. For polyatomic systems, the chapter introduces the concept of normal modes as the fundamental vibrational units, with group theory and symmetry considerations determining which modes are active in infrared or Raman spectroscopy based on whether their symmetry properties align with the dipole moment operator or its quadratic forms. Electronic spectroscopy addresses transitions between different electronic states, with the Franck-Condon principle establishing that nuclear positions remain effectively frozen during electronic excitation, resulting in observable fine structure arising from simultaneous vibrational and rotational transitions that frequently coalesce into distinctive band heads. The discussion of excited state dynamics distinguishes between fluorescence, characterized by rapid radiative decay from excited states of the same multiplicity, and phosphorescence, a slower process involving intersystem crossing to different spin multiplicities, with both phenomena elucidated through Jablonski diagrams. The chapter concludes by addressing predissociation mechanisms and the quantum mechanical basis of laser operation, particularly the roles of stimulated emission and population inversion in generating coherent radiation.