Coherent spectroscopy

• Quantized interactions in which involved molecules jump from a stationary state to another stationary state. It is the most usual spectroscopy.
• Coherent interactions in which the involved molecules are excited to a non-stationary state during the interaction and return to their initial state. A detectable effect requires the coherence, that is the relations between the molecular parameters allowing the interaction and the local electromagnetic fields are the same for all molecules.
• Mixed interactions: coherent and involving transitions of some molecules; example : He-Ne lasers.