Doświadczenie Francka-Hertza

Franck-Herts experiment (1913) was researching for current flow by the triode filled with mercury vapours. In spite of its simplicyty it has essential meaning for quantum mechanics. The point are light quanta emitted of absorber by atoms and energy quanta wich are received or transferred by electrons.

1. It was known since Kirchoff, Bunsen and Angström times (last one as first researched spectra of molecular hydrogen in 1853), that light emission spectra is in form of well defined lines,
2. Similary light absorption by the gases is in shape of portions, or specified lines (from this we know chemical compositon of Sun's surface),
3. In photoelectric phenomena (in solid state of in the eye) electrons are absorbing light energy in the form of potrions, for this explanation Eistein received his Nobel prize,
4. According to Planck light emmision from the perfect black body also is by energy portions,
5. It was missing check if electron in gas (ie. electrical discharge) are loosing their energy also in form of portions. Franck-Hertz experiment gave positive answer: yes! These portions (in energy units electronovolts, for example about 4.8 eV for the mercury) corresponds to wavelenghts emitted by excited gas (ultraviolet in case of the mercury vapours).

Schematics and result of Franck-Hertz experiment Electrons are emitted from cathode. At grid S is given adjusted voltage (from zero to teenth V) accelerating electrons. Between grid and anode A, small, constant stopping voltage, which don'a allow to too slow electrons to reach anode. Without collisions, with increase of grid voltage, electron current should grow (of in saturation conditions it should be constant). In triode filled with the mercury vapour this increase is not monotonic, and curve falls down when electrons have sufficient energy to excite mercury atoms to higher electronic state. It means: mercury atoms are not only emitting energy as a (light) quanta, but also are absorbing energy (from free electrons) as a quanta.  This pictrure is from: H. Haken i H.C. Wolf, Atomy i kwanty, PWN, 2002

Irony is that in the same year when construction of quantum mechanics was closed, appeared first scratch: Bohr's postulates, almost quantum but nor understable without wave mechanics of deBroglie.