Work function

The work function (or trigger work, work function ) is the work that needs to be at least expended to solve an electron from an uncharged solid. In general, the work function is measured in electron volts. Significantly, the work function, among others, the outer photoelectric effect, in which electrons are " knocked out " by light.

Description and measurement

The work function is different from the electron binding energy, which is comparable to the ionization energy of the atom or molecule. The electron binding energy is different for electrons of different electron shells: If you want an electron from a lower ( lower energy ) shell release, more energy has to be applied. Here, the ionization energy applies only to the minimum power which must be applied to solve a specific electron from its binding. On the contrary, the work function is generally the minimum energy for the discharge of one electron, so the energy of extracting an electron from the Fermi level.

The work function is therefore the chemical potential depends on the type of the solid body ( material ) to be released from the electrons. It is relatively small for alkali metals such as rubidium (2.13 eV), cesium (2.14 eV), potassium ( 2.25 eV ) or sodium (2.28 eV), while for metals such as aluminum (4.20 eV ), zinc ( 4.34 eV) or platinum ( 5.66 eV ) is much higher.

UV - poor daylight or UV -free incandescent light consists of photons of maximum energy of 3 eV and can be dissolved out of cesium electrons, while the zinc -energy ultraviolet is needed. The released electrons have a certain kinetic energy:

The measurement of the work function by means of the photoelectric effect is generally realized by measuring the kinetic energy of the free electrons. This results from the difference of the input energy (usually the energy of the incident photon ) and the work function. So if you have the kinetic energy of the electrons measured (using an electron spectrometer ), and the wavelength used by filters or laser characteristics is known, one can calculate the work function difference:

A simple way is measuring the retarding field method.

Different work functions result in a contact between two metal potential, which can therefore be used for the measurement of the relative work functions. Significantly, the measurement using a Kelvin probe, even Kelvin vibrator is called.

Application

An electron tube used heated metals as electron source. First, tungsten eV was having a work function of 4.5, then used tungsten with a one atom thick layer of thorium with 2.6 eV. A thin barium on tungsten was 1.7 eV and the oxide cathode mentioned combination of tungsten, barium oxide and barium external gave 1.1 eV to 1.0 eV. The cathode temperature was reduced to 700 ° C at the oxide cathode because of the reduced work function of 2400 ° C for tungsten.