Page 26 - Topic 3
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CONCEPT CHECK – STUCTURE OF THE The frequencies of the absorption
ATOM lines are a subset of those in the line
emission spectrum of the same
DO YOU KNOW? element.
When an atom absorbs a photon, it is
Concept While Before elevated to an ‘excited state’, which
studying the has a higher energy. Excited states
test are generally short-lived and the
A continuous spectrum contains a atom returns spontaneously to its
continuous range of frequencies. ground state, often by emitting a
series of lower-energy photons
Solid, liquid, or dense gaseous
objects radiate a continuous ‘Fluorescence’ is the process where
spectrum, which may extend into or an atom absorbs a photon to reach
beyond the visible region. an excited state, but then reverts to
the ground state emitting two or
The process is known as more photons with lower energy and
incandescence. The frequency longer wavelength.
distribution, and hence the dominant
colour, depends on the temperature Lasers use the process of stimulated
of the object. emission to produce electromagnetic
radiation. In many lasers stimulated
Atoms can be raised to excited emission occurs from atoms that are
states by heating or by in a higher-energy state.
bombardment with light or particles When a photon with energy
such as electrons. corresponding to a transition from a
The heated vapour of a pure element higher-energy state to a lower-
emits light of discrete frequencies, energy state is incident on an atom in
resulting in a line emission spectrum the higher state, it can stimulate a
when the light is viewed with a transition to the lower state. This
spectrometer. results in two identical photons; the
original photon and a second photon
The presence of discrete frequencies that results from the transition.
in the spectra of atoms is evidence
for the existence of different states in Compare the process of stimulated
atoms. The states have their own emission with that of ordinary (or
specific energies. spontaneous) emission.
The different energies can be The photon emitted in stimulated
represented on an energy-level emission is identical (in energy,
diagram. direction, and phase) to the incident
photon.
When an electron makes a transition
from a higher-energy state to a A population inversion is produced in
lower-energy state in an atom, the a set of atoms whenever there are
energy of the atom decreases and more atoms in a higher-energy state
can be released as a photon. than in a lower-energy state. For
practical systems, the higher-energy
The energy of the emitted photon is state must be metastable if a
given by the difference in the population inversion is to be
electron energy levels of the atom. produced.
An atom is in its ground state when
its electrons have their lowest The energy carried by a laser beam is
energy. concentrated in a small area and can
travel efficiently over large distances,
If an electron is in any of the higher- giving laser radiation a far greater
energy states, the atom is said to be potential to cause injury than light
in an excited state. from other sources.
The line emission spectrum of atomic
hydrogen consists of several series of
lines. CAN YOU?
The ionisation energy of hydrogen is
the minimum energy required to Application While Before
Studying
the test
remove the electron from hydrogen
in its ground state • Describe the changes in the
spectrum of a filament globe
When light with a continuous as the temperature of the
spectrum is incident on a gas of an filament increases.
element, discrete frequencies of light • Describe the general
are absorbed, resulting in a line characteristics of the line
absorption spectrum. emission spectra of elements
