Glossary of Terms Used in Photochemistry
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PATERNO-BÜCHI
REACTION
The photocycloaddition of an electronically excited carbonyl group
to a ground state olefin yielding an oxetane.
PEC
Abbreviation for photoelectrochemistry.
PDMR
See ODMR.
PENETRATION DEPTH
See depth of penetration.
PES
Abbreviation for photoelectron spectroscopy.
PHONON
Elementary excitation in the quantum mechanical treatment of vibrations
in a crystal lattice.
PHOSPHORESCENCE
From a phenomenological point of view, the term has been used to describe
long-lived luminescence. In mechanistic photochemistry, the term designates
luminescence involving change in spin multiplicity, typically from
triplet to singlet or vice versa. The luminescence from a quartet
state to a doublet state is also phosphorescence.
PHOTOACOUSTIC EFFECT
Generation of heat after absorption of radiation, due to radiationless
deactivation or chemical reaction.
See also photoacoustic
spectroscopy.
PHOTOACOUSTIC SPECTROSCOPY
A spectroscopic technique based on the photoacoustic effect. A photoacoustic
spectrum consists of a plot of the intensity of the acoustic signal
detected by a microphone or a "piezoelectric" detector, against the
excitation wavelength or another quantity related to the photon energy
of the modulated excitation.
See also isooptoacoustic
point.
PHOTOAFFINITY LABELLING
A technique in which a photochemically reactive molecular entity,
specifically associated with a biomolecule, is photoexcited in order
to covalently attach a label to the biomolecule, usually via intermediates.
PHOTO-ASSISTED CATALYSIS
Catalytic reaction involving production of a catalyst by absorption
of light.
See photocatalysis.
PHOTOCATALYSIS
Catalytic reaction involving light absorption by a catalyst or by
a substrate.
PHOTOCHEMICAL HOLE
BURNING
See hole burning.
PHOTOCHEMICAL REACTION
This term is generally used to describe a chemical reaction caused
by absorption of ultraviolet, visible, or infrared radiation. There
are many ground state reactions which have photochemical counterparts.
Among these are photoadditions, photocycloadditions, photoeliminations,
photoenolizations, photo-Fries rearrangements, photoisomerizations,
photooxidations, photoreductions, photosubstitutions, etc.
PHOTOCHEMICAL SMOG
Product of photochemical reactions caused by solar radiation and occurring
in polluted air.
PHOTOCHEMISTRY
The branch of chemistry concerned with the chemical effects of light
(far UV to IR).
See photochemical
reaction.
PHOTOCHROMISM
A photoinduced transformation of a molecular structure (e.g. of a
solution), photochemically and/or thermally reversible, that produces
a spectral change, typically, but not necessarily, of visible color.
PHOTOCONDUCTIVITY
Increase in electrical conductivity resulting from photoproduction
of charge carriers.
PHOTOCROSSLINKING
Photoinduced formation of a covalent linkage between two macromolecules
or between two different parts of one macromolecule.
PHOTOCURING
Technical expressions for the photoinduced hardening of a monomeric,
oligomeric or polymeric substrate normally in the form of a film.
PHOTOCURRENT YIELD
The quantum efficiency of charge photo-generation between the two
electrodes of a photovoltaic cell or a photoelectrochemical cell.
PHOTODEGRADATION
The photochemical transformation of a molecule into lower molecular
weight fragments, usually in an oxidation process. This term is widely
used in the destruction (oxidation) of pollutants by UV-based processes.
PHOTODETACHMENT (of electrons)
Ejection of an electron from a negative ion upon photoexcitation.
PHOTODYNAMIC EFFECT
A term used in photobiology to refer to photoinduced damage requiring
the simultaneous presence of light, photosensitizer and molecular
oxygen. A sensitized photooxidation which involves molecular oxygen.
PHOTOELECTRICAL EFFECT
The ejection of an electron from a solid or a liquid by a photon.
PHOTOELECTROCHEMICAL
CELL
An electrochemical cell in which current and a voltage are simultaneously
produced upon absorption of light by one or more of the electrodes.
Usually at least one of the electrodes is a semiconductor.
PHOTOELECTROCHEMICAL
ETCHING
The dissolution of a semiconductor in an electrolytic solution upon
exposure to light. Used in the photopatterning of semiconductor surfaces.
PHOTOELECTROCHEMISTRY
A term applied to a hybrid field of chemistry employing techniques
which combine photochemical and electrochemical methods for the study
of the oxidation-reduction chemistry of the ground or excited states
of molecules or ions. In general, it is the chemistry resulting from
the interaction of light with electrochemical systems.
See also photoelectrochemical
cell, photogalvanic cell,
photovoltaic cell.
PHOTOELECTRON SPECTROSCOPY
A spectroscopic technique which measures the kinetic energy of electrons
emitted upon the ionization of a substance by high energy monochromatic
photons. A photoelectron spectrum is a plot of the number of electrons
emitted versus their kinetic energy. The spectrum consists of bands
due to transitions from the ground state of an atom or molecular entity
to the ground and excited states of the corresponding radical cation.
Approximate interpretations are usually based on "Koopmans' theorem"
relating ionisation potentials to orbital energies. PES and UPS is
the spectroscopy using vacuum ultraviolet sources, while ESCA (electron
spectroscopy for chemical analysis) and XPS use X-ray sources.
PHOTOEXCITATION
The production of an excited state by the absorption of ultraviolet,
visible, or infrared radiation.
PHOTO-FRIES REARRANGEMENT
A photorearrangement of O-acyl phenols or N-acyl anilines to give
the [1,3]-rearranged product (as well as the [1,5] rearranged product).
PHOTOGALVANIC CELL
An electrochemical cell in which current or voltage changes result
from photochemically generated changes in the relative concentrations
of reactants in a solution phase oxidation-reduction couple.
Compare photovoltaic cell.
PHOTOIMAGING
See imaging.
PHOTOINDUCED
ELECTRON TRANSFER
An electron transfer resulting from an electronic state produced by
the resonant interaction of electromagnetic radiation with matter.
PHOTOINDUCED POLYMERIZATION
Polymerization of a monomer by a free radical or ionic chain reaction
initiated by photoexcitation.
See photoinitiation.
PHOTOINITIATION
Photoproduction of a free radical or ion capable of initiating a chain
reaction such as a polymerization.
See photoinduced
polymerization.
PHOTOIONIZATION
Ejection of an electron into a surrounding medium induced by the absorption
of electromagnetic radiation, from a neutral or positively charged
molecular entity.
See also photodetachment.
PHOTOLUMINESCENCE
Luminescence arising from photoexcitation.
PHOTOLYSIS
A light-induced bond cleavage. This term is often used incorrectly
to describe irradiation of a sample, although in the combination flash
photolysis this usage is accepted.
PHOTON
The quantum of electromagnetic energy at a given frequency. This energy,
E=h,
is the product of the Planck constant (h) and the frequency of the
radiation ().
See also quantum.
PHOTON COUNTING
Also called single photon counting. The recording of sequential single
photon pulses counted by way of recording the electron emission events
from a photosensitive layer (photocathode) and multiplied by means
of a "dynode" arrangement (photomultiplier). This technique is used
for two purposes:
(1) the sensitive measurement of low levels of radiation and
(2) the recording of emission decays.
See time-correlated
single photon counting.
PHOTON EMITTANCE
See photon exitance.
PHOTON EXITANCE (Mp)
The photon flow, p,
emitted by an element of the surface containing the source point under
consideration divided by the area (S ) of that element. (dp/dS,
simplified expression: Mp = p/S
when the photon flow is constant over the surface area considered).
The SI unit is s-1 m-2 . Alternatively, the
term can be used with the amount of photons (mol or its equivalent
einstein), the SI unit then being mol s-1 m-2.
Also called specific photon emission. Formerly called photon emittance.
See spectral
photon exitance.
See also radiant exitance.
PHOTON EXPOSURE (Hp)
The photon irradiance, Ep, integrated over the time of
irradiation , simplified expression:
Hp = Ept when the photon irradiance is constant
over the time considered). The SI unit is m-2. Alternatively,
the term can be used with the amount of photons (mol or its equivalent
einstein), the SI unit then being mol m-2. For a parallel
and perpendicularly incident beam not scattered or reflected by the
target or its surroundings photon fluence
is an equivalent term.
See also fluence,
radiant exposure.
PHOTON FLOW (p)
The number of photons (quanta, N) per unit time. (dN/dt , simplified
expression: p=
N/t when the number of photons is constant over the time considered).
The SI unit is s-1. Alternatively, the term can be used
with the amount of photons (mol or its equivalent einstein), the SI
unit then being mol s-1.
See spectral photon
flow.
See also radiant power.
PHOTON FLUENCE
The integral of the amount of all photons (quanta) which traverse
a small, transparent, imaginary spherical target, divided by the cross-sectional
area of this target. The photon fluence rate, ,
integrated over the duration of the irradiation ,
simplified expression: =t
when is constant over the
time considered). Photons per unit area (quanta m-2). The
SI unit is m-2. Alternatively, the term can be used with
the amount of photons (mol or its equivalent einstein), the SI unit
then being mol m-2.
See also fluence.
PHOTON FLUENCE RATE
The rate of photon fluence. Four times the ratio of
the photon flow, p,
incident on a small, transparent, imaginary spherical volume element
containing the point under consideration divided by the surface of
that sphere, SK.,
simplified expression: = 4
Fp/SK when the photon flow is constant over the solid angle
considered). The SI unit is m-2 s-1. Alternatively,
the term can be used with the amount of photons (mol or its equivalent
einstein), the SI unit then being mol m-2 s-1.
It reduces to photon irradiance for a parallel and normally incident
beam not scattered or reflected by the target or its surroundings.
See photon radiance.
See also fluence rate.
PHOTON FLUX
Same as photon irradiance.
PHOTON IRRADIANCE (Ep)
The photon flow, p,
incident on an infinitesimal element of surface containing the point
under consideration divided by the area (S) of that element (dp/dS,
simplified expression: Ep = p/S
when the photon flow is constant over the surface considered). The
SI unit is m-2 s-1. Alternatively, the term
can be used with the amount of photons (mol or its equivalent einstein),
the SI unit then being molm-2 s-1. For a parallel
and perpendicularly incident beam not scattered or reflected by the
target or its surroundings photon fluence rate ()
is an equivalent term.
See spectral irradiance.
See also irradiance.
PHOTON RADIANCE (Lp)
For a parallel beam it is the photon flow, p,
leaving or passing through an infinitesimal transparent element of
surface in a given direction from the source divided by the orthogonally
projected area of the element in a plane normal to the given direction
of the beam, ,
[(dp/dS)/
cos ,
simplified expression: Lpp/(Scos
)
when the photon flow is constant over the surface area considered].
The SI unit is m-2 s-1. For a divergent beam
propagating in an elementary cone of the solid angle d
containing the direction ,
the photon radiance is d2p/(d
dS cos q), with SI unit m-2 s-1 sr-1.
Alternatively, the term can be used with the amount of photons (mol
or its equivalent einstein), the SI unit then being mol m-2
sr-1 and mol m-2 sr-1, respectively.
See spectral
photon radiance.
See also radiance.
PHOTOOXIDATION
Oxidation reactions induced by light. Common processes are:
(1) The loss of one or more electrons from a chemical species as a
result of photoexcitation of that species;
(2) The reaction of a substance with oxygen under the influence of
light. When oxygen remains in the product this latter process is also
called photooxygenation. Reactions in which neither the substrate
nor the oxygen are electronically excited are sometimes called photoinitiated
oxidations.
Compare photoreduction.
PHOTOOXYGENATION
Incorporation of molecular oxygen into a molecular
entity. There are three common mechanisms:
Type I: the reaction of triplet molecular oxygen with radicals formed
photochemically.
Type II: the reaction of photochemically produced singlet molecular
oxygen with molecular entities to give rise to oxygen containing molecular
entities.
The third mechanism proceeds by electron transfer producing superoxide
anion as the reactive species.
Compare photooxidation.
PHOTOPHYSICAL PROCESSES
Photoexcitation and subsequent events which lead from one to another
state of a molecular entity through radiation and radiationless transitions.
No chemical change results.
PHOTOPOLYMERIZATION
Polymerization processes requiring a photon for the propagation step.
See also photoinduced
polymerization.
PHOTOREACTION
See photochemical reaction.
PHOTOREDUCTION
Reduction reactions induced by light. Common processes are:
(1) Addition of one or more electrons to a photoexcited species;
(2) The photochemical hydrogenation of a substance.
Reactions in which the substrate is not electronically excited are
sometimes called photoinitiated reductions.
Compare photooxidation.
PHOTORESIST
A photoimaging material, generally applied as a thin film, whose local
solubility properties can be altered photochemically. A subsequent
development step produces an image which is useful for the fabrication
of microelectronic devices (e.g., integrated circuits).
PHOTOSENSITIZATION
The process by which a photochemical or photophysical alteration occurs
in one molecular entity as a result of initial absorption of radiation
by another molecular entity called a photosensitizer. In mechanistic
photochemistry the term is limited to cases in which the photosensitizer
is not consumed in the reaction.
See energy transfer.
PHOTOSENSITIZER
See photosensitization.
PHOTOSTATIONARY STATE
A steady state reached by a reacting chemical system when light has
been absorbed by at least one of the components. At this state the
rates of formation and disappearance are equal for each of the transient
molecular entities formed.
PHOTOTHERMAL EFFECT
An effect produced by photoexcitation resulting partially or totally
in the production of heat.
PHOTOTHERMOGRAPHY
A process utilizing both light and heat, simultaneously or sequentially,
for image recording.
PHOTOVOLTAIC CELL
A solid state device, usually a semiconductor, such as silicon, which
absorbs photons with energies higher than or equal to the bandgap
energy and simultaneously produces electric power.
Compare photogalvanic cell.
PIEZOLUMINESCENCE
Luminescence observed when certain solids are subjected to a change
in pressure.
See triboluminescence.
POLARIZATION
See light polarization, transition
polarization.
POPULATION INVERSION
A situation in which a higher energy state is more populated than
a lower energy state.
PRECURSOR COMPLEX
May either indicate an encounter complex or a collision complex, but
furthermore implies that this complex undergoes a reaction (e.g. electron
transfer).
PREDISSOCIATION
Dissociation occurring by tunnelling from a "bound" to an "unbound"
rovibronic state. In an absorption spectrum of a molecular entity,
the appearance of a diffuse band region within a series of sharp bands,
is called predissociation, since irradiation with frequencies within
the diffuse region leads to effective dissociation. The energy of
the band is smaller than that of the dissociation continuum of the
bound state.
PRIMARY (PHOTO)PROCESS
See primary
photochemical process. The term primary (photo)process for photophysical
processes is apt to lead to inconsistencies, and its use is therefore
discouraged.
PRIMARY PHOTOCHEMICAL
PROCESS (Primary Photoreaction)
Any elementary chemical process undergone by an electronically excited
molecular entity and yielding a primary photoproduct.
See primary (photo)process.
PRIMARY
(PHOTO)PRODUCT
The first observable chemical entity which is produced in the primary
photochemical process and which is chemically different from the reactant.
See primary (photo)process.
PUMP-PROBE TECHNIQUE
A flash photolysis technique in which the light beam (probe) used
for spectral analysis is generated from a portion of the excitation
(pump) beam. A time delay in the latter allows the obtention of kinetic
data.
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