Considering Bohr’s atomic model the nucleus is surrounded of electrons that travel in circular orbits around the nucleus. The Bohr model is a very simple model of the hydrogen atom but can give an intuitively idea of what happens when making photo crystallography. Each electron has a specific energy depending on the orbital. When light (energy) is added to the system, the electron may gain an amount of energy that makes it possible for the electron to reach an orbital having a higher energy. We call it that the electron is excited. Some structures undergo a phase transition and become metastable when light of a certain wavelength is applied. The new metastable structure can then be determined using X-ray diffraction.
The molecules in the crystal are excited using light from diodes. The crystal is cooled down to 10 K to prevent the exited states to undergo decays. X-ray diffraction is then used to measure the excited crystal structure.