Photoexcitation

An illustration of electron excitation, showing excitation by photon (left) and by particle collision (right). This is the simplest case of photoexcitation, sinca a single photon excites a single quantum particle.

Photoexcitation is a phenomenon in physics where an excited state of a quantum system (an atom or a molecule) is created by photon absorption. The excited state originates from the interaction between a photon and the quantum system when the energy of the photon is too low to cause photoionization. A very simple example of this process is electron excitation. ^[1]

A photon's energy is directly proportional to the frequency of its associated electromagnetic wave. ^{[2] [1]} Thus, light with lower frequencies is associated to photons with a lower energy. In contrast, light with higher frequencies is associated to photons with a higher energy. The absorption of the photon takes place in accordance with the theory of quantum mechanics. ^[1]

Photoexcitation plays a role in different subjects of physics and chemistry:

• Photochemistry
• Luminescence
• Photoisomerization
• Photoelectrochemistry

Moreover, photoexcitation is exploited by many different devices, such as:

• Solar cells, electronic devices that convert the energy of light directly into electricity by means of the photovoltaic effect.
• Optically pumped lasers, devices that emit light through a process of optical amplification based on the stimulated emission of electromagnetic radiation.
• Photochromic applications. Photochromism causes a transformation of two forms of a molecule by absorbing a photon. ^[3] For example, the BIPS molecule(2H-l-benzopyran-2,2-indolines) can convert from trans to cis and back by absorbing a photon. The different forms are associated with different absorption bands. In a cis-form of BIPS, the transient absorption band has a value of 21050 cm⁻¹, in contrast to the band from the trans-form, that has a value of 16950 cm⁻¹. The results were optically visible, where the BIPS in gels turned from a colorless appearance to a brown or pink color after repeatedly being exposed to a high energy UV pump beam. High energy photons cause a transformation in the BIPS molecule making the molecule change its structure.

On the nuclear scale photoexcitation includes the production of nucleon and delta baryon resonances in nuclei.

See also

• Electron excitation
• Photon
• Photochemistry
• Photoelectrochemistry
• Photovoltaic effect
• Two temperature model

References

1. Griffiths, David Jeffrey (1995). Introduction to quantum mechanics (1 ed.). Englewood Cliffs (N.J.): Prentice Hall. pp. 306–307. ISBN   978-0-13-124405-4.
2. Pelc, J. S.; Ma, L.; Phillips, C. R.; Zhang, Q.; Langrock, C.; Slattery, O.; Tang, X.; Fejer, M. M. (2011-10-17). "Long-wavelength-pumped upconversion single-photon detector at 1550 nm: performance and noise analysis". Optics Express. 19 (22): 21445–56. Bibcode:2011OExpr..1921445P. doi: 10.1364/oe.19.021445 . ISSN   1094-4087. PMID   22108994. S2CID   33169614.
3. PRESTON, D.; POUXVIEL, J.-C.; NOVINSON, T.; KASKA, W. C.; DUNN, B.; ZINK, J. I. (1990-09-11). "ChemInform Abstract: Photochromism of Spiropyrans in Aluminosilicate Gels". ChemInform. 21 (37). doi:10.1002/chin.199037109. ISSN   0931-7597.