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# Excited state

In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). [1]

## Atom

An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element.

## Atomic nucleus

The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment.

## Atomic orbital

In quantum mechanics, an atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom.

## Boltzmann distribution

In statistical mechanics and mathematics, a Boltzmann distribution (also called Gibbs distribution Translated by J.B. Sykes and M.J. Kearsley. See section 28) is a probability distribution, probability measure, or frequency distribution of particles in a system over various possible states.

## Bound state

In quantum physics, a bound state is a special quantum state of a particle subject to a potential such that the particle has a tendency to remain localised in one or more regions of space.

## Configuration interaction

Configuration interaction (CI) is a post-Hartree&ndash;Fock linear variational method for solving the nonrelativistic Schrödinger equation within the Born&ndash;Oppenheimer approximation for a quantum chemical multi-electron system.

## Coupled cluster

Coupled cluster (CC) is a numerical technique used for describing many-body systems.

## Electromagnetic spectrum

The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.

## Electron

The electron is a subatomic particle, symbol or, whose electric charge is negative one elementary charge.

## Energy

In physics, energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object.

## Energy level

A quantum mechanical system or particle that is bound—that is, confined spatially—can only take on certain discrete values of energy.

## Ground state

The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system.

## Hydrogen atom

A hydrogen atom is an atom of the chemical element hydrogen.

## Hydrogen spectral series

The emission spectrum of atomic hydrogen is divided into a number of spectral series, with wavelengths given by the Rydberg formula.

## Møller–Plesset perturbation theory

Møller–Plesset perturbation theory (MP) is one of several quantum chemistry post-Hartree–Fock ab initio methods in the field of computational chemistry.

## Mechanistic organic photochemistry

Mechanistic organic photochemistry is that aspect of organic photochemistry which seeks to explain the mechanisms of organic photochemical reactions.

## Metastability

In physics, metastability is a stable state of a dynamical system other than the system's state of least energy.

## Molecule

A molecule is an electrically neutral group of two or more atoms held together by chemical bonds.

## Multi-configurational self-consistent field

Multi-configurational self-consistent field (MCSCF) is a method in quantum chemistry used to generate qualitatively correct reference states of molecules in cases where Hartree–Fock and density functional theory are not adequate (e.g., for molecular ground states which are quasi-degenerate with low-lying excited states or in bond breaking situations).

## Negative temperature

In physics, certain systems can achieve negative temperature; that is, their thermodynamic temperature can be expressed as a negative quantity on the Kelvin or Rankine scales.

## Nuclear isomer

A nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons (protons or neutrons).

## Phonon

In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, like solids and some liquids.

## Photoionization

Photoionization is the physical process in which an ion is formed from the interaction of a photon with an atom or molecule.

## Photon

The photon is a type of elementary particle, the quantum of the electromagnetic field including electromagnetic radiation such as light, and the force carrier for the electromagnetic force (even when static via virtual particles).

## Quantum mechanics

Quantum mechanics (QM; also known as quantum physics, quantum theory, the wave mechanical model, or matrix mechanics), including quantum field theory, is a fundamental theory in physics which describes nature at the smallest scales of energy levels of atoms and subatomic particles.

## Quantum number

Quantum numbers describe values of conserved quantities in the dynamics of a quantum system.

## Quantum state

In quantum physics, quantum state refers to the state of an isolated quantum system.

## Repulsive state

In quantum mechanics, a repulsive state is an electronic state of a molecule for which there is no minimum in the potential energy.

## Rydberg atom

A Rydberg atom is an excited atom with one or more electrons that have a very high principal quantum number.

## Rydberg formula

The Rydberg formula is used in atomic physics to describe the wavelengths of spectral lines of many chemical elements.

## Rydberg matter

Rydberg matter is an exotic phase of matter formed by Rydberg atoms; it was predicted around 1980 by É. A. Manykin, M. I. Ozhovan and P. P. Poluéktov.

## Singlet oxygen

Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemical with the formula O.

## Spectral line

A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies.

## Spontaneous emission

Spontaneous emission is the process in which a quantum mechanical system (such as an atom, molecule or subatomic particle) transitions from an excited energy state to a lower energy state (e.g., its ground state) and emits a quantum in the form of a photon.

## Stationary state

A stationary state is a quantum state with all observables independent of time.

## Stimulated emission

Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level.

## Temperature

Temperature is a physical quantity expressing hot and cold.

## Time-dependent density functional theory

Time-dependent density functional theory (TDDFT) is a quantum mechanical theory used in physics and chemistry to investigate the properties and dynamics of many-body systems in the presence of time-dependent potentials, such as electric or magnetic fields.

## Two-dimensional gas

A two-dimensional gas is a collection of objects constrained to move in a planar or other two-dimensional space in a gaseous state.

## Wave function

A wave function in quantum physics is a mathematical description of the quantum state of an isolated quantum system.

## References

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