Inquantum mechanics, anexcited state of a system (such as anatom,molecule ornucleus) is anyquantum state of the system that has a higherenergy than theground state (that is, more energy than the absolute minimum). Excitation refers to an increase inenergy level above a chosen starting point, usually the ground state, but sometimes an already excited state. Thetemperature of a group of particles is indicative of the level of excitation (with the notable exception of systems that exhibitnegative temperature).[citation needed]
The lifetime of a system in an excited state is usually short:spontaneous orinduced emission of a quantum of energy (such as aphoton or aphonon) usually occurs shortly after the system is promoted to the excited state, returning the system to a state with lower energy (a less excited state or the ground state). This return to a lower energy level is known as de-excitation[1] and is the inverse of excitation.
Long-lived excited states are often calledmetastable. Long-livednuclear isomers andsinglet oxygen are two examples of this.[citation needed]
Atoms can be excited by heat, electricity, or light. Thehydrogen atom provides a simple example of this concept.
The ground state of the hydrogen atom has the atom's singleelectron in the lowest possibleorbital (that is, the spherically symmetric "1s"wave function, which, so far, has been demonstrated to have the lowest possiblequantum numbers). By giving the atom additional energy (for example, by absorption of aphoton of an appropriate energy), the electron moves into an excited state (one with one or more quantum numbers greater than the minimum possible). When the electron finds itself between two states—a shift which happens very fast—it's in asuperposition of both states.[2] If the photon has too much energy, the electron will cease to bebound to the atom, and the atom will becomeionized.
After excitation the atom may return to the ground state or a lower excited state, by emitting a photon with a characteristic energy. Emission of photons from atoms in various excited states leads to anelectromagnetic spectrum showing a series of characteristicemission lines (including, in the case of the hydrogen atom, theLyman, Balmer, Paschen and Brackett series).
An atom in a high excited state is termed aRydberg atom. A system of highly excited atoms can form a long-lived condensed excited state,Rydberg matter.
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 equilibriumBoltzmann distribution. This phenomenon has been studied in the case of atwo-dimensional gas in some detail, analyzing the time taken to relax to equilibrium.
Excited states are often calculated usingcoupled cluster,Møller–Plesset perturbation theory,multi-configurational self-consistent field,configuration interaction,[3] andtime-dependent density functional theory.[4][5][6][7][8][9]
The excitation of a system (an atom or molecule) from one excited state to a higher-energy excited state with the absorption of a photon is calledexcited-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.[10] Excited-state absorption measurements are done using pump–probe techniques such asflash photolysis. 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.[11]
A further consequence of excited-state formation may be reaction of the atom or molecule in its excited state, as inphotochemistry.
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