If linear spectroscopy is asking a person a single question and recording their answer, is eavesdropping on a conversation between three people to find out how they really feel about each other. Mukamel just provided the dictionary to translate that conversation.
Because real-life lab samples are messy. They consist of billions of molecules interacting with a solvent at room temperature. Wave functions cannot handle statistical ensembles or environmental purging (dephasing). The density matrix can. Reading the Matrix For a simple two-level system (ground state and excited state ), the density matrix looks like this: If linear spectroscopy is asking a person a
hits the molecule, creating a final coherence that acts as an antenna. The molecule emits the signal field ( Esigcap E sub s i g end-sub ), which we detect. They consist of billions of molecules interacting with