Liquid photocathodes were studied intensely for a number of years around 1988. Initially it was observed that the gaseous compounds used as photocathodes were absorbed on surfaces, making them slightly photosensitive. This opened up the dream of a well-defined photosensitive layer. In a photosensitive detector with a gaseous photoelectric converter it is difficult to know where each individual photoelectron is actually emitted. The conversion volume has to be made thick enough to allow efficient conversion of the incoming photons. This smears the position resolution, and reduces the time resolution. A thin layer photocathode would eliminate this smearing in space and time. Furthermore, the gas system might be simplified, or even removed with such a liquid cathode. We summarize the results of these studies, which led to the important development of solid photocathodes in gaseous detectors.
Top1. Introduction: Early Observations
In the previous chapter we described highly efficient UV sensitive gaseous detectors filled with photosensitive gases. These detectors ionize vapours with low ionization potential, so they are sensitive to wavelengths:λ < λion, (1) where λion is the ionization threshold of the vapour.
Experiments have shown that these detectors may also have some very small photosensitivity (usually several orders of magnitude less) also for wavelengths longer than λion, if the light hits the cathode of the detector. This is mainly due to the classical photoelectric effect on the metallic surface.
The photoelectric effect in a metallic surface happens in there steps:
- 1.
Absorption of the light in the thin surface layer of the cathode where the photon transfers its energy to one of the electrons in the conduction band,
- 2.
Ballistic transport of the excited/hot electron to the surface,
- 3.
The electron escapes from the metal surface into the gas.
The maximum possible kinetic energy of the electron after it has escaped from the metallic surface is described by the famous Einstein formula mentioned earlier:Ek = hν − φ, (2) where h is the Plank constant and ν is the frequency of the incident light.
Metals have high reflectivity in the visible and the near ultraviolet region of the spectrum, so the majority of the photons are reflected on the surfaces and to not cause any photoelectric effect. The good reflectivity is one of the explanations why metals have low quantum efficiency to light of these wavelengths. The quantum efficiency is usually expressed in terms of electrons per incident photon.
Metals have a high abundance of free electrons why the exited hot electron will experience many collisions where it gradually loses its excess kinetic energy. It will be “thermolized” within a short distance. As a result only electrons created very close to the surface will have some chance to escape if, of course, its kinetic energy is still above the metal’s work function. The combination of good reflectivity and energy losses within the metal before the electron has a chance to escape from the surface leads to very low quantum efficiency for metals to visible and UV light.
The quantum efficiency also depends on which type of gas the metallic surface is in contact with (Mc. Daniel, 1964; Funfer, 1965).
There are two reasons for this:
- 1.
Some atoms and molecules change the surface barrier/work function of the metal.
- 2.
Some gases can form a dielectric layer on the metallic surface. Electrons escaping from the metal into the gas must penetrate through this layer, where the electrons can be absorbed before escaping into the gas.
Photosensitive gaseous detectors practically never use a metallic cathode as a photo-converter due to the unacceptably low quantum efficiency.
A breakthrough idea came from David Anderson (Anderson, 1982). He suggested using a photosensitive metallic cathode covered with a thick layer of absorbed TMAE. He called it a “liquid photocathode”. He used the fact that the threshold energy for photoionization in liquids is generally lower than in gases owing to the polarization energy P+ of the positive ion and the lowering of the electron conduction band energy, V0.The relation between the gas phase ionization energy Ei and the liquid threshold Eliq, as was mentioned earlier, is given by the relation (Schmidt, 1997):
E
lid = E
i + P
+ + V
0 + E
val,
(3)Sometimes Eval can be neglected, the polarization energy is always negative, whereas V0 may be of either sign. In TMAE, V0=-0.26 eV which extends the photosensitivity of its liquid phase to 350 nm, whereas gaseous TMAE is only photosensitive up to wavelengths of 230 nm (Anderson, 1982).
One way to form a liquid photocathode is to cool a metallic plate and expose it to TMAE vapour.