excited state

Examples of excited state in the following topics:

• Mechanistic Background

  • Overall bonding in an excited state is usually lower than in the ground state.
  • Thus, the X–X bond length is increased in the excited state.
  • The excited state may return to the ground state by emitting a photon (light blue line).
  • It is termed phosphorescence if it occurs slowly by way of other excited states.
  • Alternatively, an excited state may return to the ground state by emitting a photon (radiative decay).

• Gamma Decay

  • When a nucleus emits an α or β particle, the daughter nucleus is usually left in an excited state.
  • Next, the excited nickel-60 drops down to the ground state by emitting two gamma rays in succession (1.17 MeV, then 1.33 MeV), as shown in .
  • Emission of a gamma ray from an excited nuclear state typically requires only $10^{-12}$ seconds: it is nearly instantaneous.
  • Gamma decay from excited states may also follow nuclear reactions such as neutron capture, nuclear fission, or nuclear fusion.
  • In certain cases, the excited nuclear state following the emission of a beta particle may be more stable than average; in these cases it is termed a metastable excited state if its decay is 100 to 1000 times longer than the average $10^{-12}$ seconds.

• Alkene Isomerization

  • A photochemical reaction occurs when internal conversion and relaxation of an excited state leads to a ground state isomer of the initial substrate molecule, or when an excited state undergoes an intermolecular addition to another reactant molecule in the ground state.
  • This bonding is absent in the π → π* excited state (magenta curve in the diagram).
  • Molecules occupying this new excited state then relax to either DHP or cis-stilbene ground states.
  • The stilbene reactions described above have been attributed to singlet excited states.
  • In order to study the behavior of triplet excited states it is often necessary to generate them by energy transfer from a higher triplet excited state of a suitable sensitizer molecule.

• Glow of Space Shuttles

  • The glow observed as a space shuttle re-enters the atmosphere is due to excited NO2 releasing light to return to its ground state.
  • where NO2* represents the excited state of electrons in NO2.
  • It is the relaxation of these electrons from the excited state back to the ground state that produces the glow that is visible around the space shuttle (see the concept about the emission spectra for more information).
  • When atomic oxygen from the high atmosphere combines with nitric oxide on the surface of the space shuttle, the resulting excited nitrogen dioxide returns to the ground state emitting an apparent glow.
  • Recall that excited-state nitrogen dioxide is responsible for the glow observed as space shuttles re-enter Earth's atmosphere.

• Conjugated π-Orbital Functions

  • The resultant singlet excited states undergo a variety of reactions, as shown in the following diagram for 1,3,5-hexatriene and two 2,5-dialkyl derivatives.
  • Taken together with the very short lifetimes of these excited states (≤10 nsec), this suggests that photochemical products should reflect the rotamer composition of the ground state.
  • Put another way, excited state rotamers are not expected to equilibrate prior to reaction, the NEER principle (Non-Equilibration of Excited Rotamers).
  • ACP may be formed from either the tEt or cZt excited states, but VCB and BHE require the latter rotamer.
  • Here, the rapid radiationless deactivation of the excited state by OBF is impeded and a normally non-fluorescent compound becomes fluorescent.

• Lasers

  • An electron in an excited state may decay to an unoccupied lower-energy state according to a particular time constant characterizing that transition.
  • A material with many atoms in an excited state may thus result in radiation that is very monochromatic, but the individual photons would have no common phase relationship and would emanate in random directions.
  • However, an external photon at a frequency associated with the atomic transition can affect the quantum mechanical state of the atom .
  • As the incident photon passes by, the rate of transitions of the excited atom can be significantly enhanced beyond that due to spontaneous emission.
  • The result is an atom in the ground state with two outgoing photons.

• Lasers

  • It is excited by an external source of energy into an excited state (called "population inversion"), ready to be fired when a photon with the right frequency enters the medium.
  • In most lasers, this medium consists of a population of atoms which have been excited by an outside light source or an electrical field which supplies energy for atoms to absorb in order to be transformed into excited states.

• Aurora Borealis and the Aurora Australis

  • Auroras result from emissions of photons in the Earth's upper atmosphere (above 80 km, or 50 mi), from ionized nitrogen atoms regaining an electron, and from oxygen and nitrogen atoms returning from an excited state to ground state.
  • The excited particles' energy is lost by the emitting photon or colliding with another atom or molecule.
  • This energy serves to move the electrons in nitrogen and oxygen from their ground state up to an excited state, where they can then decay back to the ground state by emitting photons of visible light (see the concept on emission spectra for more information).
  • Collisions with other atoms or molecules can absorb the excitation energy and prevent emission.
  • Nitrogen emissions are blue if the atom regains an electron after it has been ionized and red if the atom returns to ground state from an excited state.

• Photochemistry

  • The first law of photochemistry, the Grotthuss-Draper law, states that light must be absorbed by a compound in order for a photochemical reaction to take place.
  • The second law of photochemistry, the Stark-Einstein law, states that for each photon of light absorbed by a chemical system, only one molecule is activated for subsequent reaction.
  • Thus, we may define quantum yield as "the number of moles of a stated reactant disappearing, or the number of moles of a stated product produced, per einstein of monochromatic light absorbed
  • Here the asterisk represents an electronic excited state, the nature of which will be defined later.
  • The biacetyl product, formed in the third reaction, may itself be excited by light or accept excitation energy from an excited acetone molecule, further complicating this process.

• Fluorescence and Phosphorescence

  • Fluorescence and phosphorescence are photoluminescence processes in which material emits photons after excitation.
  • Fluorescence occurs when an orbital electron of a molecule or atom relaxes to its ground state by emitting a photon of light after being excited to a higher quantum state by some type of energy.
  • Excitation of electrons to a higher state is accompanied with the change of a spin state .
  • Once in a different spin state, electrons cannot relax into the ground state quickly because the re-emission involves quantum mechanically forbidden energy state transitions.
  • As these transitions occur very slowly in certain materials, absorbed radiation may be re-emitted at a lower intensity for up to several hours after the original excitation.