lated emission of radiation) A light
ampliÜer (also called an optical
maser) usually used to produce
monochromatic coherent radiation
in the infrared, visible, and ultravio-
let regions of the electromagnetic
spectrum. Lasers that operate in the
X-ray region of the spectrum are also
being developed.
Nonlaser light sources emit radia-
tion in all directions as a result of the
spontaneous emission of photons by
thermally excited solids (Ülament
lamps) or electronically excited
atoms, ions, or molecules (Ûuores-
cent lamps, etc.). The emission ac-
companies the spontaneous return of
the excited species to the ground
state and occurs randomly, i.e. the ra-
diation is not coherent. In a laser, the
atoms, ions, or molecules areÜrst
‘pumped’ to an excited state and
then stimulated to emit photons by
collision of a photon of the same en-
ergy. This is called stimulated emis-
sion. In order to use it, it isÜrst
necessary to create a condition in the
amplifying medium, called popula-
tion inversion, in which the majority
of the relevant entities are excited.
Random emission from one entity
can then trigger coherent emission
from the others that it passes. In this
way ampliÜcation is achieved.
The laser ampliÜer is converted to
an oscillator by enclosing the ampli-
fying medium within a resonator. Ra-
diation then introduced along the
axis of the resonator is reÛected back
and forth along its path by a mirror
at one end and by a partially trans-
mitting mirror at the other end. Be-
tween the mirrors the waves are
ampliÜed by stimulated emission.
The radiation emerges through the
semitransparent mirror at one end
as a powerful coherent monochro-
matic parallel beam of light. The
emitted beam is uniquely parallel be-
cause waves that do not bounce back
and forth between the mirrors
quickly escape through the sides of
the oscillating medium without
ampliÜcation.
Some lasers are solid, others are
liquid or gas devices. Population in-
version can be achieved by optical
pumpingwithÛashlights or with
other lasers. It can also be achieved
by such methods as chemical reac-
tions and discharges in gases.
Lasers have found many uses since
their invention in 1960. In chemistry,
their main use has been in the study
of photochemical reactions, in the
spectroscopic investigation of mol-
ecules, and in *femtochemistry. See
also dye laser; four-level laser;
pockels cell.laser spectroscopy Spectroscopy
that makes use of lasers. The beams
of coherent monochromatic radia-
tion produced by lasers have several
signiÜcant advantages compared
with other spectroscopic techniques,
particularly in those that employ the
*Raman effect.
Lassaigne’s test A method of test-
ing for the presence of a halogen, ni-
trogen, or sulphur in an organic
compound. A sample is heated in a
test tube with a pellet of sodium. The
hot tube is dropped into pure water
and the fragments ground up in a
mortar. The presence of a halogen
(now in the form of a sodium halide)
is detected by precipitation with sil-
ver nitrate solution. Nitrogen is
revealed by the formation of a pre-
cipitate of Prussian blue on heating
part of the solution with iron(II) sul-
phate solution containing hydrochlo-
ric acid and a trace of iron(III) ions.
Lead ethanoate or sodium nitroprus-
side gives a precipitate with any sul-
phur present.
latent heatSymbol L. The quantity
of heat absorbed or released when a
substance changes its physical phase317 latent heat
l