electromagnetic coupling to one of
the bound electrons of that atom
rather than being released as a pho-
ton. The coupling is usually with an
electron in the K-, L-, or M-shell of
the atom, and this conversion elec-
tron is ejected from the atom with a
kinetic energy equal to the difference
between the nuclear transition en-
ergy and the binding energy of the
electron. The resulting ion is itself in
an excited state and usually subse-
quently emits an Auger electron or
an X-ray photon.
internal energy Symbol U. The
total of the kinetic energies of the
atoms and molecules of which a sys-
tem consists and the potential ener-
gies associated with their mutual
interactions. It does not include the
kinetic and potential energies of the
system as a whole nor their nuclear
energies or other intra-atomic ener-
gies. The value of the absolute inter-
nal energy of a system in any
particular state cannot be measured;
the signiÜcant quantity is the change
in internal energy, ∆U. For a closed
system (i.e. one that is not being re-
plenished from outside its bound-
aries) the change in internal energy
is equal to the heat absorbed by the
system (Q) from its surroundings, less
the work done (W) by the system on
its surroundings, i.e. ∆U = Q – W. See
also energy; thermodynamics.
internal resistance The resistance
within a source of electric current,
such as a cell or generator. It can be
calculated as the difference between
the e.m.f. (E) and the potential differ-
ence (V) between the terminals di-
vided by the current being supplied
(I), i.e. r = (E – V)/I, where r is the in-
ternal resistance.
interstellar moleculesSee astro-
chemistry.
interstellar moleculesSee astro-
chemistry.interstitialSee crystal defect.interstitial compoundA com-
pound in which ions or atoms of a
nonmetal occupy interstitial posi-
tions in a metal lattice. Such com-
pounds often have metallic
properties. Examples are found in
the *carbides, *borides, and *sili-
cides.intersystem crossing A process in
which a singlet excited electronic
state makes a transition to a triplet
excited state at the point where the
potential energy curves for the ex-
cited singlet and triplet states cross.
This transition is forbidden in the ab-
sence of *spin–orbit coupling but oc-
curs in the presence of spin–orbit
coupling. A triplet formed in this
way is frequently in an excited vibra-
tional state. This excited triplet state
can reach its lowest vibrational state
by collisions with other molecules.
The transition from this state to the
singlet state is forbidden in the ab-
sence of spin–orbit coupling but al-
lowed when there is spin–orbit
coupling. This gives rise to the slow
emision of electromagnetic radiation
known as *phosphorescence.intrinsic factorSee vitamin b com-
plex.InvarA tradename for an alloy of
iron (63.8%), nickel (36%), and carbon
(0.2%) that has a very low expansivity
over a a restricted temperature
range. It is used in watches and other
instruments to reduce their sensitiv-
ity to changes in temperature.inverse Compton effect The gain
in energy of low-energy photons
when they are scattered by free elec-
trons of much higher energy. As a
consequence, the electrons lose en-
ergy. See also compton effect.287 inverse Compton effect
i