M7-S3: Introduction to Spectroscopy

  • Electrons in their ground state can be excited.
In an atom, when electrons occupy their usual shells or electronic orbits, it is referred to as the ground state. However, when energy is input into the system (usually in the form of radiation), electrons will absorb this energy and are able to ‘jump’ to higher energy states.
  • These higher energy states are called ‘excited state’.
  • This phenomenon is analogous to doing work on satellites to lift them to a higher altitude.
  • Excitation of electron only occurs if the energy exactly matches the energy required to jump from one state to another.


  • If the energy provided does not exactly match the energy an electron needs to reach an excited state, it does not absorb it.



  • When electrons return from their excited state, the released energy produces an atomic emission spectrum.

However, once the energy input ceases, excited electrons will return to the ground state (their usual shells). During this process, electrons will release the energy they absorbed in the form of electromagnetic waves. In a later section of this module, we will learn about this energy in more detail. The released EM wave generates an atomic emission spectrum that is specific to the amount of energy released.


Since electrons may have multiple excitation states, the emitted electromagnetic radiation can also take multiple forms (frequencies)



  • Elements have unique emission and absorption spectra.

This means an electron has multiple specific energy levels that can catalyse its excitation. Consequently, this also means both the absorption and emission spectra contain multiple bands that correspond to these different energy levels.


The number and wavelength/frequency of these bands vary greatly with element. This phenomenon is used to identify elements present in astronomical bodies. (covered later)   

  • Excitation of electrons produces absorption line spectrum
  • Electrons’ return to the ground state produces emission line spectrum



  • Emission Spectra of Noble Gases



  • Frequency of electromagnetic radiation is proportional to the energy of the radiation. Higher the frequency, greater the energy. Again, more on this later in the module.
  • The number of spectral lines in the spectrum corresponds to the number of possible excited states electrons can have in a particular atom.


Practice Question 1 (4 marks) 

(a) In your own words, explain what emission and absorption spectra are. (2 marks)

(b) Explain why elements have different emission and absorption spectra with reference to electronic states and energy levels. (2 marks)


Practice Question 2 (2 marks)

In the image above, explain why Mercury has so few emission bands despite having many more electronic levels compared to other elements such as Argon and Hydrogen. 



Previous section: Determination of the Speed of Light

Next section: Applications of Spectroscopy