228 Spectroscopy
A study of how light interacts with matter that is used to probe, identify, and measure the structural properties of atoms and molecules.
The Bohr frequency condition. A molecule exchanges a photon with light only when the photon energy matches the gap between two allowed energy states. A photon is a discrete packet of light energy. Quantized energy states are the discrete energies the system may have. This principle is used to calculate the frequencies and wavelengths that appear as spectral lines.
The electric-dipole transition rule. Light couples to a molecule only when the transition briefly redistributes charge and creates an oscillating electric dipole. The transition dipole moment measures that change in charge distribution. A forbidden transition has a zero transition dipole moment and stays dark to photon exchange. This principle is used to predict which transitions are visible and which are silent.
The Born-Oppenheimer approximation. Nuclei move much more slowly than electrons, so electronic motion can be treated separately from vibration and rotation. This separation is the Born-Oppenheimer approximation. This principle is used to split a molecule’s total energy into electronic, vibrational, and rotational parts so spectra can be analyzed piece by piece.
The Franck-Condon principle. When a molecule jumps to a new electronic state, the jump is so fast that the nuclei keep their positions and speeds during the jump. An electronic state is the arrangement of the electrons at a given energy. This principle is used to predict the intensities of vibrational fine structure in electronic spectra.
The rotational gross selection rule. A gas-phase molecule changes its rotation by absorbing microwave radiation only if it has a lasting uneven charge distribution. That permanent electric dipole moment is a built-in charge asymmetry. This principle is used to decide which molecules show microwave spectra and to extract bond lengths and angles.
The vibrational gross selection rule. A molecule changes its vibration by absorbing infrared radiation only if that motion continually changes its charge asymmetry. A normal mode is a coordinated vibration in which all atoms move at one shared frequency. This principle is used to decide which stretches and bends appear in the infrared and to identify chemical groups.
The Raman gross selection rule. A molecule can scatter light with a shifted frequency and change its nuclear motion only if that motion periodically changes how easily its electron cloud is deformed. Polarizability measures that ease of deformation. Raman scattering is inelastic photon collision that shifts the frequency. This principle is used to observe motions in symmetric molecules silent in ordinary infrared and microwave absorption.