Quantum Physics Tutorial 3 - Ionisation and Excitation of Atoms



Emission Spectra
Absorption Spectra



When we heat a gas or pass an electric current through it we can make it glow.  We have ionised the gas. Ionisation is due to collisions with the outer shell electrons from other electrons. One or more of the outer electrons are removed.  This can happen due to collisions with an electron or a high energy photon.  An electron is removed if the collision energy is greater than the ionisation energy.  Once the electron is removed, any left over energy is kinetic.



In this case, the electron has knocked off an outer shell electron of this lithium atom, leaving a lithium 1+ ion (carrying a charge of +1.6 10-19 C).  If we put enough energy in, we could remove all the electrons. 

Question 1

Describe how an atom becomes ionised.                                                         



A plasma is a gas where electrons have been removed.  The gas is charged, and a current flows.  Plasmas are often very hot.

The picture shows a discharge tube:

The ionised gas glows dimly.  The brightness depends on the current.  The more electrons knocked off, the bigger the current.  A positive ion is clearly very attractive to other electrons.  The attraction will occur rapidly through the electromagnetic force.


Question 2

Explain what happens when an electron interacts with an ionised atom.         



Emission Spectra

If we look at the glowing gas through a spectrometer, we see the spectrum of the gas which is distinctive for that gas.  Unlike the spectrum of the Sun, in which we see all the colours of the rainbow, we only see certain colours, while others are absent.  We call this kind of spectrum a line emission spectrum.  The colours are discrete wavelengths.



When a gas is ionised, one or more outer electrons are ripped off.  The molecule has become positive.  It will recombine with an electron and lose energy, giving that energy back in the form of a photon.  Other atoms may not have been ionised, but are still in a very excited state.  The atoms have interacted with the photon and the electrons have moved to a higher energy level.


 About a microsecond later, the electrons lose their energy as a photon and return to the stable state, called the ground state.  The important thing to remember is that electrons can only exist at permitted energy levels.  Its like a person standing on a ladder; he can stand at one rung up, two rungs, etc., but NOT at a height of 1.5 rungs.

 As we consider energy levels in atoms, we will look at hydrogen which fits this model well.  (Hydrogen has one electron.)  More complex atoms with several electrons do not.


Question 3

In a spectrum of an ionised gas, only certain coloured lines can be seen.  Why does this happen? 



Absorption Spectra

If we drop sodium chloride crystals into a Bunsen flame we see an orange flame.  If we do this while shining a sodium light onto the flame, we see a shadow.

This is because the photons are scattered in all directions, reducing the intensity of the sodium light.


In an absorption spectrum, we shine the whole spectrum of visible light through the glowing gas.  We see black lines.  This is because the photons emitted in the gas are scattered in all direction.  The light is much dimmer, giving the impression of a shadow.



Gases in stars are analysed in this way.  Each gas has a distinctive and unique pattern of spectral lines.  Each line represents an energy level, which can be thought of as a rung on the energy ladder.  We will look at this next.



Atoms can interact with photons of lower energy than is required to remove electrons from them.  The photons we looked at in the photoelectric effect could remove the electrons from very reactive metals like caesium.  Photons can interact with other atoms to give them extra energy, which makes them excited.  But the electron is NOT removed.



Excited atoms have very particular energy levels to which the electron rises.  The incoming electron has to have exactly the right amount of energy to raise the electron to a higher energy level.  If it does, the electron is raised to the new energy level.  Almost immediately the electron falls back to its ground state, emitting a photon.


If the incoming electron does not have the right energy, there is no excitation.  No photon is emitted.


Photons can also excite electrons to higher energy levels.  Again they have to have exactly the right energy.  If they do, they are absorbed, and a photon of the same wavelength is emitted.  If not, they are not absorbed at all.


If we look at a spectrum of hydrogen, we find lines at several discrete wavelengths.


Each line represents the energy of a photon as the electron makes a transition from a higher energy level to a lower level.  This we can show in a diagram below:



The electron does a job of work in releasing a photon; it has lost potential energy.  Therefore we start at the highest level which we give a value of zero.  Therefore the electron falls from the zero point to the 3.41 eV level.  Notice that it can do this in three possible leaps or transitions.  One is straight from 0 to -3.41 eV.  The other is from 0 to -0.22 eV and -0.22 eV to -3.41 eV.  These transitions result in photons of 0.22 eV, 3.19 eV and 3.41 eV.  We would see these as coloured lines.


Question 4

What is the difference between an ionised and excited atom?