what is ground state and excited state in chemistry
Excited state The excitation of a system (an atom or molecule) from low-energy excited state to a high-energy excited state with the absorption of a photon is called excited state absorption (ESA). Excited state absorption is possible only when an electron has been already excited from the ground state to a lower excited state. The excited state absorption is usually an undesired effect, but it can be useful in upconversion pumping.[7] The excited state absorption measurements are done using pump-probe techniques. However, it is not easy to measure them compared to ground-state absorption and in some cases complete bleaching of the ground state is required to measure excited state absorption.[8]