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pericyclic reaction A chemical reaction in photon flow, ) ) ) p p p The number of photons
which concerted reorganization of bonding takes (quanta, N) per unit time. (dN/dt, simplified
place throughout a cyclic array of continu- expression: ) = N/t when the number of pho-
p
ously bonded atoms. It may be viewed as a tons is constant over the time considered.) The
−1
reaction proceeding through a fully conjugated SI unit is s . Alternatively, the term can be used
cyclic transition state. The number of atoms in with the amount of photons (mol or its equivalent
−1
the cyclic array is usually six, but other num- Einstein), the SI unit then being mol s .
bers are also possible. The term embraces a 0 0 0
photon fluence, H H H p p p The integral of the
variety of processes, including cycloadditions,
amount of all photons (quanta) which traverse
cheletropic reactions, electrocyclic reactions,
a small, transparent, imaginary spherical target,
and sigmatropic rearrangements, etc. (provided
divided by the cross-sectional area of this tar-
they are concerted). 0
get. The photon fluence rate, E , integrated over
p

0
the duration of the irradiation ( E dt, simpli-
p
0
0
0
photoelectrical effect The ejection of an fied expression: H = E t when E is constant
p
p
p
electron from a solid or a liquid by a photon. over the time considered). Photons per unit area
−2
−2
(quanta m ). The SI unit is m . Alternatively,
the term can be used with the amount of photons
photoelectron spectroscopy (PES) A spec- (mol or its equivalent Einstein), the SI unit then
−2
troscopic technique that measures the kinetic being mol m .
energy of electrons emitted upon the ionization 0 0 0
of a substance by high energy monochromic photon fluence rate, E E E p p p The rate of photon
photons. A photoelectron spectrum is a plot fluence. Four times the ratio of the photon flow,
) , incident on a small, transparent, imaginary
of the number of electrons emitted vs. their p
spherical volume element containing the point
kinetic energy. The spectrum consists of bands
under consideration divided by the surface of that
due to transitions from the ground state of an
sphere, S .( L dw, simplified expression:
K
p
atom or molecular entity to the ground and 0 4π
E = 4) /S when the photon flow is constant
p
K
excited states of the corresponding radical cation. p
over the solid angle considered). The SI unit
Approximate interpretations are usually based on −2 −1
is m s . Alternatively, the term can be used
“Koopmans theorem” and yield orbital energies.
with the amount of photons (mol or its equivalent
PES and UPS (UV photoelectron spectroscopy) −2 −1
Einstein), the SI unit then being mol m s .It
refer to the spectroscopy using vacuum ultra-
reduces to photon irradiance for a parallel and
violet sources, while ESCA (electron spec-
normally incident beam not scattered or reflected
troscopy for chemical analysis) and XPS use by the target or its surroundings.
X-ray sources.
photon flux Synonymous with photon irra-
diance.
photoemissive detector A detector in
which a photon interacts with a solid surface, photon irradiance, E E E p p p The photon flow,
which is called the photocathode, or a gas, ) , incident on an infinitesimal element of sur-
p
releasing a photoelectron. This process is called face containing the point under consideration
the external photoelectric effect. The photoelec- divided by the area (S) of that element (d) /ds,
p
trons are collected by an electrode at positive simplified expression: E p = ) /S when the
p
electric potential, i.e., the anode. photon flow is constant over the surface con-
−2 −1
sidered). The SI unit is m s . Alternatively,
the term can be used with the amount of photons
photon Particle of zero charge, zero rest (mol or its equivalent Einstein), the SI unit then
mass, spin quantum number 1, energy hν, and being mol s . For a parallel and perpendicu-
−2 −1
momentum hν/c (h is the Planck constant, ν the larly incident beam not scattered or reflected by
frequency of radiation, and c the speed of light); the target or its surroundings photon fluence rate
0
carrier of electromagnetic force. (E ) is an equivalent term.
p
© 2003 by CRC Press LLC