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2 Emission Spectrum 3 Absorption Spectrum 4 Temperature 5 Raman spectroscopy 6 Analysing the solar spectrum |
Atoms consist of a nucleus surrounded by
electrons. When an inelastic collision with
energetic molecules or electrons, or when the atom
absorbs a photon of light the atom can become excited.
This happens if the energy it receives is enough to raise it to
a higher energy state. Atoms can hold energy in the following
forms (in order of increasing energy needed):
Cause
The energy level the atom goes in to is proportional to the
frequency of the electromagnetic radiation it receives. Excited
atoms are unstable and quickly drop down to ground state again
giving off the energy they have received as electromagnetic
radiation.
Atomic spectrum can be classified in to two groups: absorption and emission spectra:
The potential energy stored in the atom in any form is
quantized, as there are discrete levels where electrons can jump
to. As the photons frequency is proportional to the energy
stored in the atom:
(Where e = emergy, h = Planck's constant and f = frequency)
Emission Spectrum
e = hf
the frequency can only be of certain values. An atomic emission
spectrum can be obtained by plotting the
wavelengths emitted by an atom, obtained by
diffracting the electromagnetic radiation given
off. Diffraction splits up the light as EM radiation travels
faster or slower through glass depending on its wavelength,
resulting in different degrees bent for each wavelength.
Separate lines on the EM spectra are obtained where quantised
wavelengths of electromagnetic radiation are emitted. As each
atom has different electron and energy level configurations,
each elements atomic spectrum are different.
The change in energy levels of an atom when it absorbs a photon is explained in spontaneous emission.
When a continuous spectrum of electromagnetic radiation is
passed through sodium gas, certain frequencies are absorbed
which enable the atoms to move up to higher energy levels. When
the atom returns to a ground state it emits an EM wave of
the same frequency as the initial photon, but equally in all
directions, drastically reducing the intensity of the radiation
in the direction of the incident photon (or any one direction). When the spectrum is
analysed these frequencies show up as black lines in an
otherwise continuous spectrum, and as they correspond exactly
with the emission spectrum lines they can be
used to identify atoms. This is explained in detail in atomic absorption spectroscopy.
A continuous spectrum is one in which every wavelength of the
electromagnetic spectrum is observed. [Explanation of continuous spectrum required].
The temperature of the environment where the atoms are present can affect the radiation given out. Hotter objects give out radiation approaching shorter
wavelengths. This is because the hotter objects are, the more
inelastic collisions there are between atoms making atoms
excited into higher energy states. The resulting radiation
reflects this and using:
By using a high-intensity light source such as a laser, it is possible to use the nonlinear optical process of Raman scattering to excite vibrational modes of molecules. The scattered photons are reduced in energy by amounts corresponding to the energy of the vibrational modes, and by observing wavelength of the scattered photons, the vibrational spectrum of the molecules can be deduced. This method is called Raman spectroscopy. It is particularly useful for finding the spectra of organic molecules in the so-called fingerprint region (500-2000 cm-1).
The black lines observed in the solar spectrum are where
elements in the chromosphere of the sun have absorbed
electromagnetic radiation which have the correct frequency to
excite them to higher energy levels. We can compare these to
known spectra and deduce which elements are present in the sun.
The fact that these elements have absorbed the radiation
indicates that they are colder than the photosphere.
However absorption spectra can not give us information about the
abundance of the various elements. This is because Hydrogen
and Helium (the main constituents of the sun) need much more
energy to excite them enough to absorb radiation than other
elements (such as Calcium) present. So even though H and He
are more abundant, a much smaller percentage of them get excited
enough to produce a high intensity. To get a better
understanding of abundance of these elements it is necessary to
study the emission spectrum of elements in the chromosphere. It
is only possible to assess this when the photosphoric radiation is totally obscured during an eclipse. At this
time the emission spectrum of the chromosphere is highly
dominated by hydrogen, which is the main constituent of the sun.
Further readiong
Absorption Spectrum
Temperature
E/h = f
we can see that the greater the energy the higher the frequency.
To analyse the temperature of the sun, the more the peak of the
electromagnetic spectrum approaches higher frequencies of
visible light, then the hotter the object. The sun is
estimated to be around 6000K.Raman spectroscopy
Analysing the solar spectrum
See also: atomic absorption spectroscopy, spontaneous emission