Photoelectrochemical process

Photoelectrochemical processes are reactions and processes in photoelectrochemistry. They transfer light into other forms of energy. These processes play a role in photochemistry , in optically pumped lasers , in sensitized solar cells , in luminescence and in photochromism .

Electronic stimulation

When energy is absorbed, an electron jumps from the ground state into an excited state of higher energy.

With electronic excitation, an electron is transferred from an energetically lower energy state such as the ground state to a higher "excited state". This happens either through light, whereby the electron absorbs a photon and takes up the energy of the photon, or through electronic excitation, whereby the electron takes up the energy of another electron. Another example are semiconductors , in which vibrations are excited by thermal excitation ( phonons ), which transport electrons to a higher energy level, for example the conduction band. When an excited electron falls back into a lower energy state, this is called relaxation . This happens either through radiation or through the fact that the energy is given to another particle.

Photochemical excitation

Military laser experiment

Photochemical excitation is the mechanism of electronic excitation by absorption of photons when the energy of the photon is too low to cause ionization. The absorption of a photon follows the laws of quantum theory . A subsequent reaction is, for example, photoisomerization. Photochemical excitation is used in photochemistry , in dye-sensitized solar cells , in luminescence , in optically pumped lasers and in photochromism experiments.

Photoisomerization

Photoisomerization of azobenzene

In chemistry, photoisomerization is the structural change of a molecule that can be converted into another isomer by exposure to light , with most isomerizations being reversible. Photoisomerizable molecules are used in pigments for rewritable CDs , DVDs and optical 3D storage media. In addition, photoisomerizable molecules are of interest as molecular switches , molecular motors , and molecular electronics.

The most important classes of photoisomerization are the cis - trans - or ( Z ) - ( E ) isomerization and the ring-closing / ring-opening mechanism. Examples are the photoisomerizations of stilbene and azobenzene . These types of molecules have double bonds and the rotation around this double bond is based on isomerization between two energy states. Examples of the cyclic molecules are fulgides and diarylethenes : With this type of molecule, bonds are cleaved and bonds are re-formed when irradiated with light of a specific wavelength. Another type of isomerization is the di-π-methane rearrangement .

Photoionization

See also : Ultraviolet Photoelectron Spectroscopy

Photoionization is a physical process in which a photon knocks one or more electrons out of an atom, ion or molecule. This is essentially the same process as the photo effect on metals; in the case of gases or individual atoms, the term photoionization is more common.

The ejected electrons, also called photoelectrons , contain information about the bound state before ionization. A photoelectron has a kinetic energy that corresponds to the energy of the absorbed photon minus the binding energy. Photons with an energy lower than the binding energy can be absorbed under certain circumstances, but cannot ionize the atom or molecule.

To ionize hydrogen, for example, photons need an energy of more than 13.6 eV , which corresponds to a photon wavelength of 91.2  nm or less  . The energy of the emitted photoelectron can be described by the following equation:

${\displaystyle {\frac {mv^{2}}{2}}=h\nu -13{,}6\,\mathrm {eV} }$

where is Planck's constant and the frequency of the photon. ${\displaystyle h}$ ${\displaystyle \nu }$

For individual photons with an energy below the ionization energy , the photoionization is zero. With the development of pulsed lasers, it has become possible to generate intense coherent light, which enables multiphoton ionization (absorption of multiple photons). At higher intensities (around 10 ¹⁵ –10 ¹⁶  W / cm ² ) of the infrared or visible light range, there are also phenomena such as barrier-free ionization.

Photo semiconductors

→ Main article : Photo semiconductors

After the generation of charge carriers by ultrafast flashes of light, electrical dipoles form on semiconductor surfaces . This phenomenon is known as the December effect or the Photo-December effect. The dipoles are formed due to the different velocities or diffusion constants for electrons and holes in combination with a disturbance of the surface symmetry, which leads to an effective charge separation perpendicular to the surface.

Grotthuss-Draper law

The Grotthuss-Draper law (often also written Grotthus-Draper law) states that only electromagnetic radiation that is absorbed by a system can be photochemically effective. This law forms the basis for the phenomena fluorescence and phosphorescence . It was first proposed by Theodor Grotthuss in 1817 and independently by John William Draper in 1842 .

This is one of the basic laws of photochemistry .

Einstein-Stark law

The Einstein-Stark law is named after the two physicists Johannes Stark and Albert Einstein , who formulated this law independently of one another between 1908 and 1913. It is also known as the law of photochemical equilibrium. Simply put, it says that every photon that is absorbed by a body triggers a chemical or physical process.

A photon is a light quantum or a single unit for radiation. Therefore, each individual light quantum is a multiple of Planck's quantum of action, multiplied by the light frequency. Are symbols of this unit , or . ${\displaystyle \gamma }$ ${\displaystyle h\nu }$ ${\displaystyle \hbar \omega }$

The equation for this photochemical law is as follows: every mole of a substance that reacts corresponds to one mole of light quanta that are absorbed. Expressed in a formula:

${\displaystyle \Delta E_{\mathrm {mol} }=N_{\mathrm {A} }h\nu }$

where is Avogadro's constant . ${\displaystyle N_{\mathrm {A} }}$

This law for photochemical equilibrium applies to light-induced reactions and relates to primary processes such as absorption and fluorescence .

In most photochemical reactions, the primary process is followed by a secondary photochemical process, i.e. secondary reactions that do not require light. This means that such reactions do not correspond to the one-photon-one-molecule relationship.

The law is limited to normal photochemical processes in light sources of medium intensity; so-called bi-photon processes take place in light sources with high intensity such as laser flash photolysis and in laser experiments, which means absorption of two photons.

absorption

→ Main article : absorption (physics)

In physics, the absorption of electromagnetic radiation is the way in which the energy of a photon is absorbed by matter, typically the electrons of an atom. Then the electromagnetic energy can be converted into other forms of energy, such as heat. Normally, the absorption of waves does not depend on their intensity, although under certain conditions the medium changes its transmittance depending on the intensity.

Photosensitization

→ Main article : Photosensitizer (chemistry)

Photosensitization is a process in which the energy from the light absorbed is transferred to other molecules. After absorption, the energy is transferred to a reactant. This option is used when the light required for the reaction is not available in the desired wavelength, but the sensitizer can, however, emit the light of the required wavelength after absorbing the incident light. Sensitizers are mostly aromatic compounds with many conjugated double bonds, they absorb favorably in the range of the irradiated light (often in the UV range) and emit light in the longer wave range back to other molecules.

Sensitizers

Photosensitizers differ in the absorption wavelength, the brightness and the emission duration, so different sensitizers can be used for different reactions: here are some examples:

• Fluorescein
• Rubren
• 9,10-diphenylanthracene
• Tetracene
• Levulinic acid

Chemiluminescence

→ Main article : chemiluminescence

When molecules emit light after a reaction has occurred, this is known as chemiluminescence. The photochemical excitation takes place here through the chemical reaction. A good example:

When alkaline sodium hypochlorite solution is mixed with hydrogen peroxide , the following happens:

${\displaystyle {\ce {ClO_{(aq)}^{-}{+}H2O2_{(aq)}->H_{(aq)}^{+}{+}Cl_{(aq)}^{-}{+}OH_{(aq)}^{-}{+}O2_{(g)}^{\ast }}}}$

${\displaystyle {\ce {O2^{\ast }}}}$is the excited singlet oxygen . This energy state is unstable, so it will revert to the ground state with the emission of a photon.

There are always several ways of deactivating excited energy states:

1. radiationless deactivation
2. Energy transfer to another molecule, as in photosensitization
3. Emission of light

The intensity, duration and color of the emitted light depends on several factors, including the structure of the molecules and the kinetics.

Fluorescence spectroscopy

Fluorescence spectroscopy is a spectroscopic analysis method in which the fluorescence of a sample is analyzed. A beam of light, usually ultraviolet light, is used to make the electrons of a molecule vibrate, thus producing a lower energy emission (in the visible range). A complementary technique is absorption spectroscopy . The corresponding measuring devices are called fluorimeters .

Absorption spectroscopy

Absorption spectroscopy is one of the spectroscopic methods that measure the absorption of radiation as a function of the wavelength or the frequency of light, depending on the interactions with a sample. The sample absorbs energy or photons from a radiation source. The intensity of the absorption is recorded as a function of the frequency, this recorded curve is called the spectrum. Absorption spectroscopy can be performed at all wavelengths of the electromagnetic spectrum .

See also

• Photoionization detector

Individual evidence

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2. ^ RP Madden, K. Codling: Two electron states in helium . In: Astrophysical Journal . 141, 1965, p. 364. bibcode : 1965ApJ ... 141..364M . doi : 10.1086 / 148132 .
3. ↑ A. Mammana: A Chiroptical Photoswitchable DNA Complex . In: Journal of Physical Chemistry B . 115, No. 40, 2011, pp. 11581-11587. doi : 10.1021 / jp205893y .
4. ^ J. Vachon: An ultrafast surface-bound photo-active molecular motor . In: Photochemical and Photobiological Sciences . 13, No. 2, 2014, pp. 241–246. doi : 10.1039 / C3PP50208B .
5. ↑ ^{a b c d} Radiation . In: Encyclopædia Britannica Online . Retrieved November 9, 2009.
6. ↑ BW Carroll, DA Ostlie: An Introduction to Modern Astrophysics . Addison-Wesley , 2007, ISBN 0-321-44284-9 , p. 121.
7. ↑ NB Delone, VP Krainov: Tunneling and barrier-suppression ionization of atoms and ions in a laser radiation field . In: Physics-Uspekhi . 41, No. 5, 1998, pp. 469-485. bibcode : 1998PhyU ... 41..469D . doi : 10.1070 / PU1998v041n05ABEH000393 .
8. ↑ : Cross-shell multielectron ionization of xenon by an ultrastrong laser field . In: Proceedings of the Quantum Electronics and Laser Science Conference . Optical Society of America , pp. 1974-1976. ISBN 1-55752-796-2 doi : 10.1109 / QELS.2005.1549346
9. ↑ T. Dekorsy: THz electromagnetic emission by coherent infrared-active phonons . In: Physical Review B . 53, No. 7, 1996, p. 4005. bibcode : 1996PhRvB..53.4005D . doi : 10.1103 / PhysRevB.53.4005 .
10. ↑ Einstein-Stark Law , Spectrum Lexicon of Physics
11. ↑ ^{a b c d e} Photochemical equivalence law . In: Encyclopædia Britannica Online . Retrieved November 7, 2009.
12. ^ Photosensitization . In: Encyclopædia Britannica Online . Retrieved November 10, 2009.
13. ↑ ^{a b} Hollas: Modern Spectroscopy , 4th. Edition, John Wiley & Sons, 2004, ISBN 0-470-84416-7 .
14. ^ ^{A b} D. C. Harris, MD Bertolucci: Symmetry and Spectroscopy: An introduction to vibrational and electronic spectroscopy , Reprint. Edition, Dover Publications, 1978, ISBN 0-486-66144-X .