In 1914, James Franck and Gustav
Hertz performed an experiment which clearly demonstrated
the presence of excited states (as predicted by Bohr) in
mercury atoms. Electrons were accelerated towards a positively
charged grid mounted in a glass tube filled with mercury
vapour. Behind the grid 'A' was a collection plate 'B' held
at a more positive voltage than the grid.
While the drop at (1) corresponds to the electron exciting one mercury atom, the drop at (2) corresponds to the electron having sufficient energy to excite two mercury atoms. It is in fact possible to observe up to 10 sequential bumps with equal spacings.
This data provided direct evidence for Bohr's idea that
electrons only occupy discrete, quantised energy levels
around the nucleus. They won the Nobel Prize in Physics
in 1925 for this work. |
The Nobel Prize in Physics 1922 - Niels
Henrik David Bohr »
The Nobel Prize in Physics 1925 - James
Franck »
The Nobel Prize in Physics 1925 - Gustav
Ludwig Hertz »
