Photoexcitation
Photoexcitation is a phenomenon in physics where an excited state of a quantum system 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.
A photon's energy is directly proportional to the frequency of its associated electromagnetic wave. 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.
Photoexcitation plays a role in different subjects of physics and chemistry:
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. For example, the BIPS molecule 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.