Electromagnetic spectroscopy

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
2. rotational
3. vibrational
4. energy associated with electrons

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:

Emission Spectrum

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:

  e = hf

 (Where e = emergy, h = Planck's 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. 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].

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).

Analysing the solar spectrum

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

• Advanced Level Physics Nelkon and Parker Page 855+
• Heinemann Advanced Chemistry Fullick Page 211+
• chemweb.calpoly.edu (http://www.chemweb.calpoly.edu/)

See also: atomic absorption spectroscopy, spontaneous emission