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 mol-
ecules. In ultraviolet photoelectron
spectroscopy (UPS) the sample is a
gas or vapour irradiated with a nar-
row beam of ultraviolet radiation
(usually from a helium source at 58.4
nm, 21.21 eV photon energy). The
photoelectrons produced in accor-
dance with *Einstein’s equation are
passed through a slit into a vacuum
region, where they are deÛected by
magnetic or electrostaticÜelds to
give an energy spectrum. The photo-
electron spectrum obtained has
peaks corresponding to the ioniza-
tion potentials of the molecule (and
hence the orbital energies). The tech-
nique also gives information on the
vibrational energy levels of the ions
formed. X-ray photoelectron spec-
troscopy (XPS), also known as ESCA
(electron spectroscopy for chemical
analysis), is a similar analytical tech-
nique 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 spec-
trum for a particular element show
characteristic chemical shifts, which
depend on the presence of other
atoms in the molecule. See also koop-
mans’ theorem.photoemissionThe process in
which electrons are emitted by a sub-
stance as a result of irradiation. See
photoelectric effect; photoioniza-
tion.
photographyThe process of form-
ing a permanent record of an imageon specially treatedÜlm or paper. In
normal black-and-white photography
a camera is used to expose aÜlm or
plate to a focused image of the scene
for a speciÜed time. TheÜlm or plate
is coated with an emulsion contain-
ing 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 theÜlm after develop-
ment (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 negativeÜlm
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 photo-
graphic emulsions are sensitive to ul-
traviolet and X-rays, they are widely
used in studies involving these forms
of electromagnetic radiation. See also
colour photography.photoionizationThe *ionization
of an atom or molecule as a result of
irradiation by electromagnetic radia-
tion. 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 photo-
electron will have an energy, E, given
by E = hf– I, where h is the Planck
constant, f is the frequency of the in-
cident radiation, and I is the ioniza-
tion potential of the irradiated
species.photoluminescenceSee lumines-
cence.
photolysisA chemical reaction
produced by exposure to light or ul-
traviolet radiation. Photolytic reac-
tions often involve free radicals, the
Ürst step being homolyticÜssion of a
chemical bond. (See flash photoly-photoelectron spectroscopy 416p