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| [[File:Military laser experiment.jpg|right|275px|thumb|[[Photons]] emitted in a [[Coherence (physics)|coherent]] beam from a [[laser]]]]
| | Wielu biznesmenów coraz nie przekazuje sobie materii z owego, jak duże znaczenie oddziela Internet zaś wyszukiwarka Google w istnieniu klientów. Z owej nieznajomości wykorzystują „spontaniczni” przedsiębiorcy, jacy w tym okresie zjednują klientów przez wyszukiwarkę. Jak owe produkują? Poprzez pozycjonowanie swoich stronic internetowych.<br><br>Pozycjonowanie strony www owe nic innego, jak przedsięwzięcia mające na zamysłu naprawę pozy witryny w plonach wyszukiwania Google na przesiane słowa krytyczne. Dobierając wygodne wysłowienia wpisywane przez nabywców istniejemy w stanie wyraźnie wskazać adresatów polskich służby i namawiać widzialność naszej stronicy właśnie do nich.<br><br>Dzięki poprawnie prowadzonej wojny SEO ([http://pozycjonowaniestronbialystok.bloog.pl/ pozycjonowanie Białystok] natomiast optymalizacja) nie tylko adiustuje się widzialność witryny w efektach wyszukiwania Google. Ostrym celem istnieje gdyż podwyższenie przychodów konsumenta, czyli posiadacza pozycjonowanej stronic. Obiekt ten urzeczywistniony jest skrajnie przez podniesienie liczebności odwiedzin w gablotce, co skutkuje podwyższenie liczby nabywców.<br><br>Przedsiębiorcy w Białymstoku powinny zastanowić się nad tą krzepą marketingu, bo jest ona nietuzinkowo przebojowa, relatywnie tania, natomiast przede wszelkim nieprowokująca dla końcowych nabywców. Internauci są wszak zadowoleni, że w tworach wyszukiwania Google dostają właściwe rezultaty, prawda? |
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| '''Photoelectrochemical processes''' are processes in [[photoelectrochemistry]]; they usually involve transforming light into other forms of energy.<ref name=photochemelec-process>
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| {{Cite book
| |
| | last=Gerischer| first=Heinz
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| | chapter=Semiconductor electrodes and their interaction with light
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| | year=1985
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| | editor-last=Schiavello | editor-first=Mario
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| | title=Photoelectrochemistry, Photocatalysis and Photoreactors Fundamentals and Developments
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| | publisher=[[Springer (publisher)|Springer]]
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| | pages=39
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| | url=http://books.google.com/?id=rLRMeP1KGhsC&pg=PA39#v=onepage&f=false
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| | isbn=978-90-277-1946-1
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| }}</ref> These processes apply to photochemistry, optically pumped lasers, sensitized solar cells, luminescence, and photochromism.
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| ==Electron excitation==
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| [[Image:Energylevels.png|thumb|210px|right| After absorbing energy, an electron may jump from the ground state to a higher energy excited state.]]
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| '''Electron excitation''' is the movement of an [[electron]] to a higher [[energy state]]. This can either be done by photoexcitation (PE), where the original electron absorbs the photon and gains all the photon's energy or by electrical [[Excited state|excitation]] (EE), where the original electron absorbs the energy of another, energetic electron. Within a semiconductor crystal lattice, thermal excitation is a process where lattice vibrations provide enough energy to move electrons to a higher [[energy band]]. When an excited electron falls back to a lower energy state again, it is called electron relaxation. This can be done by radiation of a photon or giving the energy to a third spectator particle as well.<ref name=2-electron>
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| {{Cite journal
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| |last=Madden | first=R. P.
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| |last2=Codling| first2=K.
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| |year=1965
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| |title=Two electron states in Helium
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| |journal=[[Astrophysical Journal]]
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| |volume=141 |pages=364
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| |bibcode=1965ApJ...141..364M
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| |doi=10.1086/148132
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| }}</ref>
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| In physics there is a specific technical definition for [[energy level]] which is often associated with an atom being excited to an [[excited state]]. The excited state, in general, is in relation to the [[ground state]], where the excited state is at a higher [[energy level]] than the ground state.
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| ==Photoexcitation==
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| {{See also|Photoelectric effect}}
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| '''Photoexcitation''' is the mechanism of [[electron excitation]] by [[photon]] absorption, when the energy of the photon is too low to cause photoionization. The absorption of the photon takes place in accordance with Planck's quantum theory.
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| Photoexcitation plays role in photoisomerization. Photoexcitation is exploited in [[dye-sensitized solar cell]]s, [[photochemistry]], [[luminescence]], optically [[laser pumping|pumped]] lasers, and in some [[photochromic]] applications.
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| ==Photoisomerization==
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| In [[chemistry]], '''photoisomerization''' is [[molecule|molecular]] behavior in which structural change between [[isomer]]s is caused by photoexcitation. Both reversible and irreversible photoisomerization reactions exist. However, the word "photoisomerization" usually indicates a reversible process. Photoisomerizable molecules are already put to practical use, for instance, in [[pigment]]s for [[CD-RW|rewritable CDs]], [[DVD-RW|DVDs]], and [[3D optical data storage]] solutions. In addition, recent interest in photoisomerizable molecules has been aimed at molecular devices, such as molecular switches, molecular motors, and molecular electronics.
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| Photoisomerization behavior can be roughly categorized into several classes. Two major classes are ''trans-cis'' (or 'E-'Z'') conversion, and open-closed ring transition. Examples of the former include [[stilbene]] and [[azobenzene]]. This type of compounds has a double [[chemical bond|bond]], and rotation or inversion around the double bond affords isomerization between the two states. Examples of the latter include [[fulgide]] and [[diarylethene]]. This type of compounds undergoes bond cleavage and bond creation upon irradiation with particular wavelengths of light. Still another class is the [[Di-pi-methane rearrangement]].
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| ==Photoionization==
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| {{See also|Ultraviolet photoelectron spectroscopy}}
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| '''Photoionization''' is the physical process in which an incident [[photon]] ejects one or more [[electron]]s from an [[atom]], [[ion]] or [[molecule]]. This is essentially the same process that occurs with the photoelectric effect with metals. In the case of a gas or single atoms, the term photoionization is more common.<ref name="Grotthuss–Draper-law">
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| {{Cite web
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| | title=Radiation
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| | work=[[Encyclopædia Britannica Online]]
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| | url=http://www.britannica.com/EBchecked/topic/488507/radiation
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| | accessdate =2009-11-09
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| }}</ref>
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| The ejected electrons, known as [[photoelectron]]s, carry information about their pre-ionized states. For example, a single electron can have a [[kinetic energy]] equal to the energy of the incident photon minus the [[electron binding energy]] of the state it left. Photons with energies less than the electron binding energy may be absorbed or [[scattering|scattered]] but will not photoionize the atom or ion.<ref name="Grotthuss–Draper-law"/>
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| For example, to ionize [[hydrogen]], photons need an energy greater than 13.6 [[electronvolt]]s, which corresponds to a wavelength of 91.2 [[nanometer|nm]].<ref>
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| {{Cite book
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| |last=Carroll |first=B. W.
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| |last2=Ostlie |first2=D. A.
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| |year=2007
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| |title=An Introduction to Modern Astrophysics
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| |page=121
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| |publisher=[[Addison-Wesley]]
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| |isbn=0-321-44284-9
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| }}</ref> For photons with greater energy than this, the energy of the emitted photoelectron is given by:
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| : <math> { mv^2 \over 2 } = h \nu - 13.6 eV</math>
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| where ''h'' is [[Planck's constant]] and ''ν'' is the [[frequency]] of the photon.
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| This formula defines the [[photoelectric effect]].
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| Not every photon which encounters an atom or ion will photoionize it. The probability of photoionization is related to the [[Photoionisation cross section|photoionization cross-section]], which depends on the energy of the photon and the target being considered. For photon energies below the ionization threshold, the photoionization cross-section is near zero. But with the development of pulsed lasers it has become possible to create extremely intense, coherent light where multi-photon ionization may occur. At even higher intensities (around 10<sup>15</sup> - 10<sup>16</sup> W/cm<sup>2</sup> of infrared or visible light), [[non-perturbative]] phenomena such as ''barrier suppression ionization''<ref>
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| {{cite journal
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| | last1=Delone | first1=N. B.
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| | last2=Krainov | first2=V. P.
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| | year=1998
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| | title=Tunneling and barrier-suppression ionization of atoms and ions in a laser radiation field
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| | journal=[[Physics-Uspekhi]]
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| | volume=41 | issue=5 | pages=469–485
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| | doi=10.1070/PU1998v041n05ABEH000393
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| |bibcode = 1998PhyU...41..469D }}</ref> and ''rescattering ionization''<ref>
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| {{cite conference
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| |last1=Dichiara |first1=A.
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| |coauthors=''et al.''
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| |year=2005
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| |title=Cross-shell multielectron ionization of xenon by an ultrastrong laser field
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| |booktitle=Proceedings of the Quantum Electronics and Laser Science Conference
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| |volume=3 |pages=1974–1976
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| |publisher=[[Optical Society of America]]
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| |doi=10.1109/QELS.2005.1549346
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| |isbn=1-55752-796-2
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| }}</ref> are observed.
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| ===Multi-photon ionization===
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| {{See also|Fluorescence spectroscopy| Fluorescence| Photoionization mode}}
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| Several photons of energy below the ionization threshold may actually combine their energies to ionize an atom. This probability decreases rapidly with the number of photons required, but the development of very intense, pulsed lasers still makes it possible. In the perturbative regime (below about 10<sup>14</sup> W/cm<sup>2</sup> at optical frequencies), the probability of absorbing ''N'' photons depends on the laser-light intensity ''I'' as ''I''<sup>''N'' </sup>.<ref>
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| {{Cite journal
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| |last1=Deng|first1=Z.
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| |last2=Eberly|first2=J. H.
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| |year=1985
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| |title=Multiphoton absorption above ionization threshold by atoms in strong laser fields
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| |journal=[[Journal of the Optical Society of America B]]
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| |volume=2 |issue=3 |pages=491
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| |doi=10.1364/JOSAB.2.000486
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| |bibcode = 1985JOSAB...2..486D }}</ref>
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| [[Above threshold ionization]] (ATI) <ref>
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| {{Cite journal
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| |last1=Agostini |first1=P.
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| |coauthors=''et al.''
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| |year=1979
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| |title=Free-Free Transitions Following Six-Photon Ionization of Xenon Atoms
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| |journal=[[Physical Review Letters]]
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| |volume=42 |issue=17 |pages=1127–1130
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| |doi=10.1103/PhysRevLett.42.1127
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| |bibcode=1979PhRvL..42.1127A
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| }}</ref> is an extension of multi-photon ionization where even more photons are absorbed than actually would be necessary to ionize the atom. The excess energy gives the released electron higher [[kinetic energy]] than the usual case of just-above threshold ionization. More precisely, The system will have multiple peaks in its [[photoelectron spectrum]] which are separated by the photon energies, this indicates that the emitted electron has more kinetic energy than in the normal (lowest possible number of photons) ionization case. The electrons released from the target will have approximately an integer number of photon-energies more kinetic energy. {{Citation needed|date=September 2011}}
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| ==Photo-Dember==
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| {{Main|Photo-Dember}}
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| In semiconductor physics the [[Photo-Dember]] effect (named after its discoverer H. Dember) consists in the formation of a charge [[dipole]] in the vicinity of a [[semiconductor]] surface after ultra-fast photo-generation of charge carriers. The dipole forms owing to the difference of mobilities (or diffusion constants) for holes and electrons which combined with the break of symmetry provided by the surface lead to an effective charge separation in the direction perpendicular to the surface.<ref name=photodember>
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| {{Cite journal
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| |last1=Dekorsy |first1=T.
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| |coauthors=''et al.''
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| |year=1996
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| |title=THz electromagnetic emission by coherent infrared-active phonons
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| |journal=[[Physical Review B]]
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| |volume=53 |issue=7 |pages=4005
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| |doi=10.1103/PhysRevB.53.4005
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| |bibcode = 1996PhRvB..53.4005D }}</ref>
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| ==Grotthuss–Draper law==
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| The '''Grotthuss–Draper law''' (also called the '''Principle of Photochemical Activation''') states that only that light which is absorbed by a system can bring about a photochemical change. Materials such as [[dye]]s and [[phosphor]]s must be able to absorb "light" at optical frequencies. This law provides a basis for [[fluorescence]] and [[phosphorescence]]. The law was first proposed in 1817 by [[Theodor Grotthuss]] and in 1842, independently, by [[John William Draper]].<ref name="Grotthuss–Draper-law"/>
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| This is considered to be one of the two basic laws of [[photochemistry]]. The second law is the [[Photoelectrochemical processes#Stark–Einstein law|Stark–Einstein law]], which says that primary chemical or physical reactions occur with each [[photon]] absorbed.<ref name="Grotthuss–Draper-law"/>
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| ==Stark–Einstein law==
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| The '''Stark–Einstein law''' is named after German-born physicists [[Johannes Stark]] and [[Albert Einstein]], who independently formulated the law between 1908 and 1913. It is also known as the '''photochemical equivalence law''' or '''photoequivalence law'''. In essence it says that every photon that is absorbed will cause a (primary) chemical or physical reaction.<ref name=StarkEinsteinlaw>
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| {{Cite web
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| |title=Photochemical equivalence law
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| |url=http://www.britannica.com/EBchecked/topic/457732/photochemical-equivalence-law
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| |work=[[Encyclopædia Britannica Online]]
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| |accessdate=2009-11-07
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| }}</ref>
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| The photon is a quantum of radiation, or one unit of radiation. Therefore, this is a single unit of EM radiation that is equal to Planck's constant (h) times the frequency of light. This quantity is symbolized by γ, hν, or ħω.
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| The photochemical equivalence law is also restated as follows: for every [[mole (unit)|mole]] of a substance that reacts, an equivalent mole of quanta of light are absorbed. The formula is:<ref name=StarkEinsteinlaw/>
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| :<ma
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| The photochemical equivalence law applies to the part of a light-induced reaction that is referred to as the primary process (i.e. [[absorption (electromagnetic radiation)|absorption]] or [[fluorescence]]).<ref name=StarkEinsteinlaw/>
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| In most photochemical reactions the primary process is usually followed by so-called secondary photochemical processes that are normal interactions between reactants not requiring absorption of light. As a result such reactions do not appear to obey the one quantum–one molecule reactant relationship.<ref name=StarkEinsteinlaw/>
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| The law is further restricted to conventional photochemical processes using light sources with moderate intensities; high-intensity light sources such as those used in [[flash photolysis]] and in laser experiments are known to cause so-called biphotonic processes; i.e., the absorption by a molecule of a substance of two photons of light.<ref name=StarkEinsteinlaw/>
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| ==Absorption==
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| {{Main|Absorption (electromagnetic radiation)}}
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| In [[physics]], '''absorption''' of electromagnetic radiation is the way by which the [[energy]] of a [[photon]] is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy, for example, to heat. The absorption of light during [[wave propagation]] is often called [[attenuation (electromagnetic radiation)|attenuation]]. Usually, the absorption of waves does not depend on their intensity (linear absorption), although in certain conditions (usually, in [[optics]]), the medium changes its transparency dependently on the intensity of waves going through, and the [[Saturable absorption]] (or nonlinear absorption) occurs.
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| ==Photosensitization==
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| Photosensitization is a process of transferring the [[energy]] of absorbed light. After absorption, the energy is transferred to the (chosen) [[reactant]]s. This is part of the work of [[photochemistry]] in general. In particular this process is commonly employed where reactions require light sources of certain [[wavelength]]s that are not readily available.<ref name=Photosensitization/>
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| For example, [[mercury (element)|mercury]] absorbs radiation at 1849 and 2537 [[angstrom]]s, and the source is often high-intensity [[arc lamp|mercury lamps]]. It is a commonly used sensitizer. When mercury vapor is mixed with [[ethylene]], and the compound is [[irradiated]] with a mercury lamp, this results in the photodecomposition of ethylene to acetylene. This occurs on absorption of light to yield excited state mercury atoms, which are able to transfer this energy to the ethylene molecules, and are in turn deactivated to their initial energy state.<ref name=Photosensitization/>
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| [[Cadmium]]; some of the [[noble gas]]es, for example [[xenon]]; [[zinc]]; [[benzophenone]]; and a large number of organic dyes, are also used as sensitizers.<ref name=Photosensitization>
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| {{Cite web
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| |title =Photosensitization
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| |url=http://www.britannica.com/EBchecked/topic/458153/photosensitization
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| |work=[[Encyclopædia Britannica Online]]
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| |accessdate =2009-11-10
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| }}</ref>
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| Photosensitisers are a key component of [[photodynamic therapy]] used to treat cancers.
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| ==Sensitizer==
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| {{Redirect|Sensitizer|the particulate material used to create voids that aid in the initiation or propagation of an explosive's detonation wave|Explosive sensitiser}}
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| A '''sensitizer''' in [[chemoluminescence]] is a chemical compound, capable of [[light emission]] after it has received energy from a molecule, which became excited previously in the chemical reaction. A good example is this:
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| When an alkaline solution of [[sodium hypochlorite]] and a concentrated solution of [[hydrogen peroxide]] are mixed, a reaction occurs:
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| :ClO<sup>-</sup>(aq) + H<sub>2</sub>O<sub>2</sub>(aq) → O<sub>2</sub>*(g) + H<sup>+</sup>(aq) + Cl<sup>-</sup>(aq) + OH<sup>-</sup>(aq)
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| O<sub>2</sub>*is excited oxygen - meaning, one or more electrons in the O<sub>2</sub> molecule have been promoted to higher-energy [[molecular orbital]]s. Hence, oxygen produced by this chemical reaction somehow 'absorbed' the energy released by the reaction and became excited. This energy state is unstable, therefore it will return to the [[ground state]] by lowering its energy. It can do that in more than one way:
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| *it can react further, without any light emission
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| *it can lose energy without emission, for example, giving off heat to the surroundings or transferring energy to another molecule
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| *it can emit light
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| The intensity, duration and color of emitted light depend on [[quantum mechanics|quantum]] and [[chemical kinetics|kinetical]] factors. However, excited molecules are frequently less capable of light emission in terms of brightness and duration when compared to sensitizers. This is because sensitizers can store energy (that is, be excited) for longer periods of time than other excited molecules. The energy is stored through means of [[quantum vibration]], so sensitizers are usually compounds which either include systems of [[Aromaticity|aromatic]] rings or many conjugated double and triple [[covalent bond|bonds]] in their structure. Hence, if an excited molecule transfers its energy to a sensitizer thus exciting it, longer and easier to quantify light emission is often observed.
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| The color (that is, the [[wavelength]]), brightness and duration of emission depend upon the sensitizer used. Usually, for a certain chemical reaction, many different sensitizers can be used.
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| ===List of some common sensitizers===
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| *[[Violanthrone]]
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| *[[Isoviolanthrone]]
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| *[[Fluorescein]]
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| *[[Rubrene]]
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| *[[9,10-Diphenylanthracene]]
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| *[[Tetracene]]
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| *[[13,13'-Dibenzantronile]]
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| *[[Levulinic acid|Levulinic Acid]]
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| ==Fluorescence spectroscopy==
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| {{Main|Fluorescence spectroscopy| Fluorescence}}
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| '''Fluorescence spectroscopy''' aka fluorometry or spectrofluorometry, is a type of [[electromagnetic spectroscopy]] which analyzes [[fluorescence]] from a sample. It involves using a beam of light, usually [[ultraviolet light]], that excites the electrons in [[molecules]] of certain compounds and causes them to emit light of a lower energy, typically, but not necessarily, [[visible light]]. A complementary technique is [[absorption spectroscopy]].<ref name=Modern-spectroscopy/><ref name=sym-spectroscopy/>
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| Devices that measure [[fluorescence]] are called [[fluorometer]]s or fluorimeters.
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| ==Absorption spectroscopy==
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| {{Main| Absorption spectroscopy}}
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| '''Absorption spectroscopy''' refers to [[spectroscopy|spectroscopic]] techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a function of frequency, and this variation is the [[absorption spectrum]]. Absorption spectroscopy is performed across the [[electromagnetic spectrum]].<ref name=Modern-spectroscopy>
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| {{cite book
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| |author=Hollas |first=J. M.
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| |year=2004
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| |title=Modern Spectroscopy
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| |edition=4th
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| |publisher=[[John Wiley & Sons]]
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| |isbn=0-470-84416-7
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| }}</ref><ref name=sym-spectroscopy>
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| {{cite book
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| |last=Harris |first=D. C.
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| |last2=Bertolucci |first2=M. D.
| |
| |year=1978
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| |title=Symmetry and Spectroscopy: An introduction to vibrational and electronic spectroscopy
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| |edition=Reprint
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| |publisher=[[Dover Publications]]
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| |isbn=0-486-66144-X
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| }}</ref>
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| ==See also==
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| {{colbegin|3}}
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| *[[Ionization energy]]
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| *[[Isomerization]]
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| *[[Photoionization mode]]
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| *[[Photochromism]]
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| *[[Photoelectric effect]]
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| *[[Photoionization detector]]
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| {{colend}}
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| ==References==
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| {{Reflist|2}}
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| {{Use dmy dates|date=September 2010}}
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| {{DEFAULTSORT:Photoelectrochemical Processes}}
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| [[Category:Astrochemistry]]
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| [[Category:Chemical reactions]]
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| [[Category:Electron]]
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| [[Category:Luminescence]]
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| [[Category:Materials science]]
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| [[Category:Optics]]
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| [[Category:Photochemistry]]
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| [[Category:Physical chemistry]]
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| [[Category:Reaction mechanisms]]
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| [[Category:Semiconductors]]
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| [[Category:Albert Einstein]]
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