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). Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being raised to an excited state. The temperature of a group of particles is indicative of the level of excitation (with the notable exception of systems that exhibit negative temperature).

The lifetime of a system in an excited state is usually short: spontaneous or induced emission of a quantum of energy (such as a photon or a phonon) 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 often loosely described as decay and is the inverse of excitation.

Long-lived excited states are often called metastable. Long-lived nuclear isomers and singlet oxygen are two examples of this.

Energy levels
After absorbing energy, an electron may jump from the ground state to a higher energy excited state.
CuO2-plane in high Tc superconductor
Excitations of copper 3d orbitals on the CuO2-plane of a high Tc superconductor; The ground state (blue) is x2-y2 orbitals; the excited orbitals are in green; the arrows illustrate inelastic x-ray spectroscopy

Atomic excitation

A simple example of this concept comes by considering the hydrogen atom.

The ground state of the hydrogen atom corresponds to having the atom's single electron in the lowest possible orbit (that is, the spherically symmetric "1s" wave-function, which, so far, has demonstrated to have the lowest possible quantum numbers). By giving the atom additional energy (for example, by the absorption of a photon of an appropriate energy), the electron is able to move into an excited state (one with one or more quantum numbers greater than the minimum possible). If the photon has too much energy, the electron will cease to be bound to the atom, and the atom will become ionized.

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 an electromagnetic spectrum showing a series of characteristic emission lines (including, in the case of the hydrogen atom, the Lyman, Balmer, Paschen and Brackett series.)

An atom in a high excited state is termed a Rydberg atom. A system of highly excited atoms can form a long-lived condensed excited state e.g. a condensed phase made completely of excited atoms: Rydberg matter. Hydrogen can also be excited by heat or electricity.

Perturbed gas excitation

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.

Calculation of excited states

Excited states are often calculated using coupled cluster, Møller–Plesset perturbation theory, multi-configurational self-consistent field, configuration interaction,[1] and time-dependent density functional theory.[2][3][4][5][6]

Excited state absorption

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]

Reaction

A further consequence is reaction of the atom in the excited state, as in photochemistry. Excited states give rise to chemical reaction.

See also

References

  1. ^ Hehre, Warren J. (2003). A Guide to Molecular Mechanics and Quantum Chemical Calculations (PDF). Irvine, California: Wavefunction, Inc. ISBN 1-890661-06-6.
  2. ^ Glaesemann, Kurt R.; Govind, Niranjan; Krishnamoorthy, Sriram; Kowalski, Karol (2010). "EOMCC, MRPT, and TDDFT Studies of Charge Transfer Processes in Mixed-Valence Compounds: Application to the Spiro Molecule". The Journal of Physical Chemistry A. 114 (33): 8764–8771. doi:10.1021/jp101761d. PMID 20540550.
  3. ^ Dreuw, Andreas; Head-Gordon, Martin (2005). "Single-Reference ab Initio Methods for the Calculation of Excited States of Large Molecules". Chemical Reviews. 105 (11): 4009–37. doi:10.1021/cr0505627. PMID 16277369.
  4. ^ Knowles, Peter J.; Werner, Hans-Joachim (1992). "Internally contracted multiconfiguration-reference configuration interaction calculations for excited states". Theoretica Chimica Acta. 84: 95. doi:10.1007/BF01117405.
  5. ^ Foresman, James B.; Head-Gordon, Martin; Pople, John A.; Frisch, Michael J. (1992). "Toward a systematic molecular orbital theory for excited states". The Journal of Physical Chemistry. 96: 135. doi:10.1021/j100180a030.
  6. ^ Glaesemann, Kurt R.; Gordon, Mark S.; Nakano, Haruyuki (1999). "A study of FeCO+ with correlated wavefunctions". Physical Chemistry Chemical Physics. 1 (6): 967–975. Bibcode:1999PCCP....1..967G. doi:10.1039/a808518h.
  7. ^ {url = https://www.rp-photonics.com/excited_state_absorption.html}
  8. ^ Dolan, Giora; Goldschmidt, Chmouel R (1976). "A new method for absolute absorption cross-section measurements: rhodamine-6G excited singlet-singlet absorption spectrum". Chemical Physics Letters. 39 (2): 320–322. Bibcode:1976CPL....39..320D. doi:10.1016/0009-2614(76)80085-1.

External links

3-Hydroxyflavone

3-Hydroxyflavone is a chemical compound. It is the backbone of all flavonols, a type of flavonoid. It is a synthetic compound, which is not found naturally in plants. It serves as a model molecule as it possesses an excited-state intramolecular proton transfer (ESIPT) effect to serve as a fluorescent probe to study membranes for example or intermembrane proteins. The green tautomer emission (λmax ≈ 524 nm) and blue-violet normal emission (λmax ≈ 400 nm) originate from two different ground state populations of 3HF molecules. The phenomenon also exists in natural flavonols. Although 3-hydroxyflavone is almost insoluble in water, its aqueous solubility (hence bio-availability) can be increased by encapsulation in cyclodextrin cavitiies

Baird's rule

In organic chemistry, Baird's rule estimates whether a lowest triplet state of planar cyclic structure will have aromatic properties. The quantum mechanical basis for its formulation was first worked out by physical chemist N. Colin Baird at the University of Western Ontario in 1972.The lowest triplet state of a ring structure is aromatic according to Baird's rule when it has 4n π-electrons, where n is any positive integer. A 4n π electron count makes a ring system antiaromatic in the ground state by Hückel's rule, but that rule is for the ground state, which is usually the lowest singlet state. Baird's rule instead looks at the lowest triplet excited state, where the electron-count patterns for aromaticity and antiaromaticity are reversed.

Chemiluminescence

Chemiluminescence (also chemoluminescence) is the emission of light (luminescence), as the result of a chemical reaction. There may also be limited emission of heat. Given reactants A and B, with an excited intermediate ,

[A] + [B] → [] → [Products] + light

For example, if [A] is luminol and [B] is hydrogen peroxide in the presence of a suitable catalyst we have:

where:

Chromophore

A chromophore is the part of a molecule responsible for its color.

The color that is seen by our eyes is the one not absorbed within a certain wavelength spectrum of visible light. The chromophore is a region in the molecule where the energy difference between two separate molecular orbitals falls within the range of the visible spectrum. Visible light that hits the chromophore can thus be absorbed by exciting an electron from its ground state into an excited state.

In biological molecules that serve to capture or detect light energy, the chromophore is the moiety that causes a conformational change of the molecule when hit by light.

Edging (sexual practice)

Edging, peaking, or surfing, is an orgasm control sexual technique which may be practiced either alone or with a partner and involves the maintenance of a high level of sexual arousal for an extended period of time without reaching climax.

When practiced by males, orgasm control allows the practitioner to enjoy direct sexual stimulation without waiting through the refractory period common after orgasm. When the decision is made to allow orgasm, the physical sensations may be much greater and more pleasurable than if the orgasm were experienced conventionally. Orgasm control is referenced as 'slow masturbation' in Alex Comfort's The New Joy of Sex (1993) and 'extended massive orgasm' in Vera and Steve Bodansky's 2000 book of the same name, and is similar to the Venus Butterfly technique used in the volume The One Hour Orgasm (1988) by Leah and Bob Schwartz.

Since orgasm control prolongs the experience of powerful sexual sensations occurring during the final build-up to orgasm, the physical demands of being kept or keeping oneself in this highly excited state for an extended time can induce a pleasurable, almost euphoric state. Orgasm control can involve either sex partner being in control of the other partner's orgasm, or a person delaying their own orgasm during sexual activity with a partner or by masturbation. To experience orgasm control, any method of sexual stimulation can be used.

Excimer

An excimer (originally short for excited dimer) is a short-lived dimeric or heterodimeric molecule formed from two species, at least one of which has completely filled valence shell by electrons (for example, noble gases). In this case, formation of molecules is possible only if such atom is in an electronic excited state. Heteronuclear molecules and molecules that have more than two species are also called exciplex molecules (originally short for excited complex). Excimers are often diatomic and are composed of two atoms or molecules that would not bond if both were in the ground state. The lifetime of an excimer is very short, on the order of nanoseconds. Binding of a larger number of excited atoms form Rydberg matter clusters, the lifetime of which can exceed many seconds.

Forbidden mechanism

In spectroscopy, a forbidden mechanism (forbidden transition or forbidden line) is a spectral line associated with absorption or emission of light by atomic nuclei, atoms, or molecules which undergo a transition that is not allowed by a particular selection rule but is allowed if the approximation associated with that rule is not made. For example, in a situation where, according to usual approximations (such as the electric-dipole approximation for the interaction with light), the process cannot happen, but at a higher level of approximation (e.g. magnetic dipole, or electric quadrupole) the process is allowed but at a much lower rate.

An example is phosphorescent glow in the dark materials, which absorb light and form an excited state whose decay involves a spin flip, and is therefore forbidden by electric dipole transitions. The result is emission of light slowly over minutes or hours.

Although the transitions are nominally forbidden, there is a small probability of their spontaneous occurrence, should an atomic nucleus, atom or molecule be raised to an excited state. More precisely, there is a certain probability that such an excited entity will make a forbidden transition to a lower energy state per unit time; by definition, this probability is much lower than that for any transition permitted or allowed by the selection rules. Therefore, if a state can de-excite via a permitted transition (or otherwise, e.g. via collisions) it will almost certainly do so before any transition occurs via a forbidden route. Nevertheless, most forbidden transitions are only relatively unlikely: states that can only decay in this way (so-called meta-stable states) usually have lifetimes on the order milliseconds to seconds, compared to less than a microsecond for decay via permitted transitions. In some radioactive decay systems, multiple levels of forbiddenness can stretch life times by many orders of magnitude for each additional unit by which the system changes beyond what is most allowed under the selection rules. Such excited states can last years, or even for many billions of years (too long to have been measured).

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. An excited state is any state with energy greater than the ground state. In the quantum field theory, the ground state is usually called the vacuum state or the vacuum.

If more than one ground state exists, they are said to be degenerate. Many systems have degenerate ground states. Degeneracy occurs whenever there exists a unitary operator that acts non-trivially on a ground state and commutes with the Hamiltonian of the system.

According to the third law of thermodynamics, a system at absolute zero temperature exists in its ground state; thus, its entropy is determined by the degeneracy of the ground state. Many systems, such as a perfect crystal lattice, have a unique ground state and therefore have zero entropy at absolute zero. It is also possible for the highest excited state to have absolute zero temperature for systems that exhibit negative temperature.

Indirect DNA damage

Indirect DNA damage occurs when a UV-photon is absorbed in the human skin by a chromophore that does not have the ability to convert the energy into harmless heat very quickly. Molecules that do not have this ability have a long-lived excited state. This long lifetime leads to a high probability for reactions with other molecules—so-called bimolecular reactions. Melanin and DNA have extremely short excited state lifetimes in the range of a few femtoseconds (10−15s). The excited state lifetime of these substances is 1,000 to 1,000,000 times longer than the lifetime of melanin, and therefore they may cause damage to living cells that come in contact with them.

The molecule that originally absorbs the UV-photon is called a "chromophore". Bimolecular reactions can occur either between the excited chromophore and DNA or between the excited chromophore and another species, to produce free radicals and reactive oxygen species. These reactive chemical species can reach DNA by diffusion and the bimolecular reaction damages the DNA (oxidative stress). It is important to note that indirect DNA damage does not result in any warning signal or pain in the human body.

The bimolecular reactions that cause the indirect DNA damage are illustrated in the figure:

1O2 is reactive harmful singlet oxygen:

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). "Metastable" describes nuclei whose excited states have half-lives 100 to 1000 times longer than the half-lives of the excited nuclear states that decay with a "prompt" half life (ordinarily on the order of 10−12 seconds). The term "metastable" is usually restricted to isomers with half-lives of 10−9 seconds or longer. Some references recommend 5 × 10−9 seconds to distinguish the metastable half life from the normal "prompt" gamma emission half life. Occasionally the half-lives are far longer than this and can last minutes, hours, or years. For example the 180m73Ta nuclear isomer survives so long that it has never been observed to decay (at least 1015 years).

Sometimes, the gamma decay from a metastable state is referred to as isomeric transition, but this process typically resembles shorter-lived gamma decays in all external aspects with the exception of the long-lived nature of the meta-stable parent nuclear isomer . The longer lives of nuclear isomers' metastable states are often due to the larger degree of nuclear spin change which must be involved in their gamma emission to reach the ground state. This high spin change causes these decays to be forbidden transitions and delayed. Delays in emission are caused by low or high available decay energy.

The first nuclear isomer and decay-daughter system (uranium X2/uranium Z, now known as 234m91Pa/23491Pa) was discovered by Otto Hahn in 1921.

Phosphorescence

Phosphorescence is a type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur very slowly in certain materials, absorbed radiation is re-emitted at a lower intensity for up to several hours after the original excitation.

Everyday examples of phosphorescent materials are the glow-in-the-dark toys, stickers, paint, and clock dials that glow after being charged with a bright light such as in any normal reading or room light. Typically, the glow slowly fades out, sometimes within a few minutes or up to a few hours in a dark room.The study of phosphorescent materials led to the discovery of radioactivity in 1896.

Photochemistry

Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400 nm), visible light (400–750 nm) or infrared radiation (750–2500 nm).In nature, photochemistry is of immense importance as it is the basis of photosynthesis, vision, and the formation of vitamin D with sunlight. Photochemical reactions proceed differently than temperature-driven reactions. Photochemical paths access high energy intermediates that cannot be generated thermally, thereby overcoming large activation barriers in a short period of time, and allowing reactions otherwise inaccessible by thermal processes. Photochemistry is also destructive, as illustrated by the photodegradation of plastics.

Photodisintegration

Photodisintegration (also called phototransmutation) is a nuclear process in which an atomic nucleus absorbs a high-energy gamma ray, enters an excited state, and immediately decays by emitting a subatomic particle. The incoming gamma ray effectively knocks one or more neutrons, protons, or an alpha particle out of the nucleus. The reactions are called (γ,n), (γ,p), and (γ,α).

Photodisintegration is endothermic (energy absorbing) for atomic nuclei lighter than iron and sometimes exothermic (energy releasing) for atomic nuclei heavier than iron. Photodisintegration is responsible for the nucleosynthesis of at least some heavy, proton-rich elements via the p-process in supernovae.

Population inversion

In science, specifically statistical mechanics, a population inversion occurs while a system (such as a group of atoms or molecules) exists in a state in which more members of the system are in higher, excited states than in lower, unexcited energy states. It is called an "inversion" because in many familiar and commonly encountered physical systems, this is not possible. The concept is of fundamental importance in laser science because the production of a population inversion is a necessary step in the workings of a standard laser.

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. The liberated energy transfers to the electromagnetic field, creating a new photon with a phase, frequency, polarization, and direction of travel that are all identical to the photons of the incident wave. This is in contrast to spontaneous emission, which occurs at random intervals without regard to the ambient electromagnetic field.

The process is identical in form to atomic absorption in which the energy of an absorbed photon causes an identical but opposite atomic transition: from the lower level to a higher energy level. In normal media at thermal equilibrium, absorption exceeds stimulated emission because there are more electrons in the lower energy states than in the higher energy states. However, when a population inversion is present, the rate of stimulated emission exceeds that of absorption, and a net optical amplification can be achieved. Such a gain medium, along with an optical resonator, is at the heart of a laser or maser.

Lacking a feedback mechanism, laser amplifiers and superluminescent sources also function on the basis of stimulated emission.

Stokes shift

Stokes shift is the difference (in wavelength or frequency units) between positions of the band maxima of the absorption and emission spectra (fluorescence and Raman being two examples) of the same electronic transition. It is named after Irish physicist George Gabriel Stokes.When a system (be it a molecule or atom) absorbs a photon, it gains energy and enters an excited state. One way for the system to relax is to emit a photon, thus losing its energy (another method would be the loss of energy as heat). When the emitted photon has less energy than the absorbed photon, this energy difference is the Stokes shift.

The Stokes shift is the result of two actions: vibrational relaxation or dissipation and solvent reorganisation. A fluorophore is a dipole, surrounded by water molecules. When a fluorophore enters an excited state, its dipole moment changes, but water molecules can't adjust so quickly. Only after vibrational relaxation do their dipole moments realign.

Triple-alpha process

The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon.

Ultraviolet–visible spectroscopy

Ultraviolet–visible spectroscopy or ultraviolet–visible spectrophotometry (UV–Vis or UV/Vis) refers to absorption spectroscopy or reflectance spectroscopy in part of the ultraviolet and the full, adjacent visible spectral regions. This means it uses light in the visible and adjacent ranges. The absorption or reflectance in the visible range directly affects the perceived color of the chemicals involved. In this region of the electromagnetic spectrum, atoms and molecules undergo electronic transitions. Absorption spectroscopy is complementary to fluorescence spectroscopy, in that fluorescence deals with transitions from the excited state to the ground state, while absorption measures transitions from the ground state to the excited state.

YAMBO code

Yambo is a computer software package for studying many-body theory aspects of solids and molecule systems. It calculates the excited state properties of physical systems from first principles, e.g., from quantum mechanics law without the use of empirical data. Parts of it are open-source software released under a GNU General Public License (GPL).

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