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Atomic spectroscopy is a chemical analysis technique and it is used to identify what
elements are in a compound. It uses the idea of a photon being absorbed or emitted
whenever an electron changes from one energy level to another.
The diagram below shows sodium salt being sprinkled onto a flame and yellow light being
emitted.
Emission spectra are produced by thin gases in which the atoms do not experience many
collisions (because of the low density). The emission of yellow light occurs because the
electrons of the sodium salt have been promoted to a higher electronic energy state but
have then fallen back down and emitted the energy as an electromagnetic wave, in the
wavelength corresponding to yellow which is ? 600 nm. The more intense that the yellow
band is the more abundant the sodium salt present.
The diagram (left) shows an atomic excitation 
caused by the absorption of a photon and an atomic
de-excitation caused by emission of a photon. 
In each case the wavelength of the emitted or absorbed
light is exactly such that the photon carries the energy 
difference between the two orbits. This energy may be 
calculated by multiplying the Plank constant by the
wavelength of the light. Thus, an atom can absorb
or emit only certain discrete wavelengths
(or equivalently, frequencies or energies). 
This diagram shows white light being shone through sodium vapour and the resulting
spectra on a board.
An absorption spectrum occurs when light passes through a cold, dilute gas and atoms in
the gas absorb at characteristic frequencies; since the re-emitted light is unlikely to
be emitted in the same direction as the absorbed photon, this gives rise to dark lines
(absence of light) in the spectrum. 
Absorption spectroscopy can only be carried on a substance in solution or gaseous form.
The presence of the dark band shows that the sodium vapour had absorbed the light in the
yellow region. Sodium salt has absorbed energy but it is not re-emitted or just not
re-emitted efficiently and so the wavelength of the light increases, leaving the observed
dark band where yellow was expected. 
The Sun appears yellow because that is the main wavelength the sun emits radiation at.
This is shown in a graphically below. 
For each hot object there is a corresponding colours. Those stars with colours of lower
wavelengths are lower in temperatures. For example something that appears red has a
temperature of ? 3000 K. But something blue has a temperature of ? 10000 K. So finding
the predominant colour of the sun then its temperature could be determined. The Suns
spectrum resembles that of something around 5000 K.
By studying the emission spectra captured on the photographic film for dark bands , the
composition can be found.
Because interstellar clouds have a temperature of between 10-50 K, radiation emitted has
much shorter wavelengths, so different techniques to the ones above have to be used. The
wavelengths are in fact in the order of 0.001 m so the can be picked up by radio
telescopes on earth. Here is a picture of one below.
A neutral hydrogen atom (H I) consists of 1 proton and 1 electron. The proton and
electron spin like tops with their spin axes either parallel or anti-parallel. When
hydrogen atoms switch from the parallel to the anti-parallel configuration they emit
radio waves with a wavelength of 21 centimetres and a corresponding frequency of exactly
1420 MHz. This is called the 21-centimetre line. Thus, radio telescopes tuned to this
frequency can be used to map the great clouds of neutral hydrogen found in interstellar
space. 
Radio telescopes identify which elements are present and how abundant they are and then
the conditions are replicated here on earth. But this requires keeping the elements in
gaseous form at low temperatures (as low as 7 K) without condensing. This requires using
CRESU apparatus .It takes advantage of the flow properties of gaseous expansions from
convergent-divergent Laval nozzles into low-pressure environments, producing a flow of
gas, which is uniform in temperature, density and velocity, and carries on for hundreds
of millimetres and hundreds of microseconds after leaving the nozzle exit. Frequent
collisions occur during the controlled expansion within the nozzle .The expansion is slow
enough to maintain thermal equilibrium, but rapid enough that condensation is avoided. A
uniform, 'collimated' flow results at the exit of the nozzle. This uniform supersonic
flow provides a good environment in which to perform experiments on "collisional"
processes at extremely low temperatures.
The said gas is made up of three components: the source of the radicals (which are to be
broken up by a generating laser), a molecule to react with the radicals and a chemically
inert gas to carry the other two gases.
Removal of the radicals is followed by another laser which exites the radical and
fluorescence can be observed .The rate constant can be determined by increasing the time
delay between the generating laser and the detecting laser because the fluorescence will
fall as the radical is being removed due to reaction.
Abstract
A photon is absorbed when an electron is raised to a higher energy level and emitted when
falling to a lower level. Each element has discreet energy levels, which can be
identified by looking at absorption and emission spectra so the composition of anything
which emits radiation can be found.
Acknowledgements
The sources of additional information used were: 
Salters Advanced Chemistry "Chemical Ideas".
Internet resource pages / www.sp.uconn.edu
www.gly.ac.uk
www.csep10.phys.utk.edu
Encarta 2000