In the present lecture, you have been introduced to the general principles of spectroscopy and electronic spectroscopy of atoms and molecules. Molecular energies are quantized and molecular energy levels may be conveniently classified as electronic, vibrational, rotational and other energy levels. While there are “cross” interactions between these levels (electronic and vibrational, vibrational and rotational, and so on), the classification is a great aid in studying different aspects of molecular behaviour. A transition between levels is caused by the absorption of electromagnetic radiation. Transitions between energy levels are governed by selection rules. Selection rules are in turn, governed by conservation of energy and angular momentum in addition to the nature of the initial state, the final state and the process occurring within a molecule that connects the two states.

A block diagram of an absorption spectrometer is described. Electronic spectra of hydrogen like atoms are well described by the Bohr's model. Interaction between electron spin and electron angular momenta brings about new levels and doublets and multiplets appear in electronic spectra. For diatomic molecules, an additional quantum number, corresponding to the angular momentum along the internuclear axis is defined and used in selection rules. Term symbols for characterizing the states involved in molecular spectra are briefly outlined. In complex molecules, associating electronic transitions to specific groups such as the ethylenic double bond or the carbonyl group allows one to use electronic spectra for quantitative and qualitative analysis. Photoelectron spectroscopy described towards the end of the lecture allows one to measure the binding energies of different electrons in a molecule and use the atomic binding energy data to characterize molecules as well as surfaces.