tigating the composition of surfaces,
based on ionization of atoms at the
surfaces. Ionizing radiation (such as
X-rays or ultraviolet radiation) is used
to eject electrons from atoms in the
surface. The ejected electrons are
then focused, thus enabling an image
of the surface to be constructed.
photoelectron spectroscopy
(PES) A technique for determining
the *ionization potentials of molecules. In ultraviolet photoelectron
spectroscopy (UPS) the sample is a
gas or vapour irradiated with a narrow beam of ultraviolet radiation
(usually from a helium source at 58.4
nm, 21.21 eV photon energy). The
photoelectrons produced in accordance with *Einstein’s equation are
passed through a slit into a vacuum
region, where they are deÛected by
magnetic or electrostatic Üelds to
give an energy spectrum. The photoelectron spectrum obtained has
peaks corresponding to the ionization potentials of the molecule (and
hence the orbital energies). The technique also gives information on the
vibrational energy levels of the ions
formed. X-ray photoelectron spectroscopy (XPS), also known as ESCA
(electron spectroscopy for chemical
analysis), is a similar analytical technique in which a beam of X-rays is
used. In this case, the electrons
ejected are from the inner shells of
the atoms. Peaks in the electron spectrum for a particular element show
characteristic chemical shifts, which
depend on the presence of other
atoms in the molecule. See also koopmans’ theorem.
photoemission The process in
which electrons are emitted by a substance as a result of irradiation. See
photoelectric effect; photoionization.
photography The process of forming a permanent record of an image
on specially treated Ülm or paper. In
normal black-and-white photography
a camera is used to expose a Ülm or
plate to a focused image of the scene
for a speciÜed time. The Ülm or plate
is coated with an emulsion containing silver salts and the exposure to
light causes the silver salts to break
down into silver atoms; where the
light is bright dark areas of silver are
formed on the Ülm after development (by a mild reducing agent) and
Üxing. The negative so formed is
printed, either by a contact process
or by projection. In either case light
passing through the negative Ülm
falls on a sheet of paper also coated
with emulsion. Where the negative is
dark, less light passes through and
the resulting positive is light in this
area, corresponding with a light area
in the original scene. As photographic emulsions are sensitive to ultraviolet and X-rays, they are widely
used in studies involving these forms
of electromagnetic radiation. See also
colour photography.
photoionization The *ionization
of an atom or molecule as a result of
irradiation by electromagnetic radiation. For a photoionization to occur
the incident photon of the radiation
must have an energy in excess of the
*ionization potential of the species
being irradiated. The ejected photoelectron will have an energy, E, given
by E = hf – I, where h is the Planck
constant, f is the frequency of the incident radiation, and I is the ionization potential of the irradiated
species.
photoluminescence See luminescence.
photolysis A chemical reaction
produced by exposure to light or ultraviolet radiation. Photolytic reactions often involve free radicals, the
Ürst step being homolytic Üssion of a
chemical bond. (See flash photolyphotoelectron spectroscopy
416
p
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based on ionization of atoms at the
surfaces. Ionizing radiation (such as
X-rays or ultraviolet radiation) is used
to eject electrons from atoms in the
surface. The ejected electrons are
then focused, thus enabling an image
of the surface to be constructed.
photoelectron spectroscopy
(PES) A technique for determining
the *ionization potentials of molecules. In ultraviolet photoelectron
spectroscopy (UPS) the sample is a
gas or vapour irradiated with a narrow beam of ultraviolet radiation
(usually from a helium source at 58.4
nm, 21.21 eV photon energy). The
photoelectrons produced in accordance with *Einstein’s equation are
passed through a slit into a vacuum
region, where they are deÛected by
magnetic or electrostatic Üelds to
give an energy spectrum. The photoelectron spectrum obtained has
peaks corresponding to the ionization potentials of the molecule (and
hence the orbital energies). The technique also gives information on the
vibrational energy levels of the ions
formed. X-ray photoelectron spectroscopy (XPS), also known as ESCA
(electron spectroscopy for chemical
analysis), is a similar analytical technique in which a beam of X-rays is
used. In this case, the electrons
ejected are from the inner shells of
the atoms. Peaks in the electron spectrum for a particular element show
characteristic chemical shifts, which
depend on the presence of other
atoms in the molecule. See also koopmans’ theorem.
photoemission The process in
which electrons are emitted by a substance as a result of irradiation. See
photoelectric effect; photoionization.
photography The process of forming a permanent record of an image
on specially treated Ülm or paper. In
normal black-and-white photography
a camera is used to expose a Ülm or
plate to a focused image of the scene
for a speciÜed time. The Ülm or plate
is coated with an emulsion containing silver salts and the exposure to
light causes the silver salts to break
down into silver atoms; where the
light is bright dark areas of silver are
formed on the Ülm after development (by a mild reducing agent) and
Üxing. The negative so formed is
printed, either by a contact process
or by projection. In either case light
passing through the negative Ülm
falls on a sheet of paper also coated
with emulsion. Where the negative is
dark, less light passes through and
the resulting positive is light in this
area, corresponding with a light area
in the original scene. As photographic emulsions are sensitive to ultraviolet and X-rays, they are widely
used in studies involving these forms
of electromagnetic radiation. See also
colour photography.
photoionization The *ionization
of an atom or molecule as a result of
irradiation by electromagnetic radiation. For a photoionization to occur
the incident photon of the radiation
must have an energy in excess of the
*ionization potential of the species
being irradiated. The ejected photoelectron will have an energy, E, given
by E = hf – I, where h is the Planck
constant, f is the frequency of the incident radiation, and I is the ionization potential of the irradiated
species.
photoluminescence See luminescence.
photolysis A chemical reaction
produced by exposure to light or ultraviolet radiation. Photolytic reactions often involve free radicals, the
Ürst step being homolytic Üssion of a
chemical bond. (See flash photolyphotoelectron spectroscopy
416
p
www.AzShimi.ir www.AzShimi.com