Electromagnetic spectroscopy

Electromagnetic spectra are spectrums which arise out of atoms absorbing and emitting quanta of electromagnetic radiation.

Cause:

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):

1. translational

1. rotational

1. vibrational

1. energy associated with electrons

The energy level the atom goes in to is proportional to the

frequency of the electromagnetic radiation it recieves. 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:

Emission Spectrum

The potential energy stored in the atom in any form is

quantized, as there are discreet levels where electrons can jump

to. As the photons frequency is proportional to the energy

stored in the atom:

 e = hf

(Where e = emergy, h = Plancks constant and f = frequency)

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.

Absorption Spectrum

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.

A continuous spectrum is one in which every wavelength of the

electromagnetic spectrum is observed. [Explanation of continuous spectrum required].

Temperature

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:

 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

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).

Chemical composition of the Sun

The black lines observed in the solar spectrum are where

elements in the chronosphere of the sun have absorbed

electromagnetic radiation which have the same 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 chronosphere. 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 chronosphere is highly

dominated by hydrogen, which is the main constituent of the sun.

References

• Advanced Level Physics Nelkon and Parker Page 855+

• Heinemann Advanced Chemistry Fullick Page 211+

• chemweb.calpoly.edu