External photoelectric effect

Photoelectric effect
refers to the electrical effect produced when an object absorbs light energy and converts it into the energy of certain electrons in the object. The working principle of photoelectric sensors is based on the photoelectric effect. The photoelectric effect is divided into two categories: external photoelectric effect and internal photoelectric effect.

Photoelectric effect
The phenomenon that electrons in an object escape from the surface of the object and are emitted outward under the action of light is called photoelectric effect. Electrons emitted outward are called photoelectrons. Photoelectric devices based on the photoelectric effect include phototubes, photomultiplier tubes, etc.
Photons are particles with energy. The energy of each photon is: E=hv

According to Einstein's hypothesis, an electron can only receive the energy of one photon, so in order for an electron to escape from the surface of an object, the energy of the photon must be greater than the surface work function of the object, and the excess energy is expressed as the kinetic energy of the escaping electron. The photoelectric effect mostly occurs in metals and metal oxides. The time required from the beginning of light irradiation to the release of electrons by the metal does not exceed 10 ^-9 s.
According to the law of conservation of energy

Where m is the mass of the electron and v ₀ is the electron escape velocity.
This equation is called the Einstein photoelectric effect equation.

Whether photoelectrons can be generated depends on whether the energy of photoelectrons is greater than the surface electron work function A0 of the object. Different substances have different work functions, that is, each object has a corresponding light frequency threshold, called the red limit frequency or wavelength limit. When the frequency of light is lower than the red limit frequency, the photon energy is not enough to cause the electrons in the object to escape. Therefore, for incident light with a frequency lower than the red limit frequency, no matter how strong the light intensity is, photoelectrons will not be emitted. On the contrary, when the frequency of incident light is higher than the red limit frequency, photoelectrons will be emitted even if the light is weak.
When the spectral components of the incident light remain unchanged, the photocurrent generated is proportional to the light intensity. That is, the greater the light intensity, the more incident photons there are, and the more electrons escape.
The surface of the photoelectron escaping the object has an initial kinetic energy of mv02 /2, so even if the external photoelectric effect device (such as a phototube) does not apply an anode voltage, photoelectrons will be generated. In order to make the photocurrent zero, a negative cutoff voltage must be applied, and the cutoff voltage is proportional to the frequency of the incident light.