<P> A collection of molecules forming a gas can be considered in an excited state if one or more molecules are elevated to kinetic energy levels such that the resulting velocity distribution departs from the equilibrium Boltzmann distribution . This phenomenon has been studied in the case of a two - dimensional gas in some detail, analyzing the time taken to relax to equilibrium . </P> <P> Excited states are often calculated using coupled cluster, Møller--Plesset perturbation theory, multi-configurational self - consistent field, configuration interaction, and time - dependent density functional theory . </P> <P> 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 . 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 . </P> <P> A further consequence is reaction of the atom in the excited state, as in photochemistry . Excited states give rise to chemical reaction . </P>

What is ground state and excited state in chemistry
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