Electronic Energy Levels. Electrons in an atom are arranged into a number of different shells and sub-shells. An electron can move from one of these sub-shell levels to another by absorbing or emitting radiation of an appropriate energy. The system is then said to have undergone an electronic transition. Electronic transitions may involve electrons that are in inner shells (higher energy) or outer shells (lower energy) of atoms.
� Vibrational Energy Levels. Molecules (but not atoms) can vibrate in a number of different modes, e.g., the atoms can compress or stretch along the axis of a bond, or they can bend symmetrically or asymmetrically. Each of these vibrations occurs at a characteristic frequency (energy) which depends on the mass of the atoms and the strength of the bonds involved.
� Rotational Energy Levels. Molecules often contain chemical groups that are capable of rotating around certain bonds at fixed frequencies (and therefore energies). Each group has a specific number of frequencies at which it rotates and therefore has a specific number of quantized rotational energy levels. The rotation frequency is determined by the mass of the atoms involved and their distance from the axis of rotation.
� Nuclear Energy Levels. The nuclei of certain atoms have a property known as spin. A (charged) spinning nucleus generates a small magnetic field and can be thought of as being a small magnet. Normally, this magnet can be orientated in any direction, but in the presence of an external magnetic field it can only align itself either with or against the field, i.e., it is quantized. Transitions between the different energy levels within the nuclei can be made to occur by applying radiation of a specific energy to the sample.
� Translational Energy Levels. Atoms and molecules are in continual translational motion because of the thermal energy of the system. Translational energy levels are quantized, however, the differences between the energy levels are so small that the molecules act as though the energy is distributed continuously.