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I've read about photoelectric cells, and most of the sources say that Potassium and Cesium can be used as electrodes in the same. But why not Rubidium?

The abundant isotope of Rb (72.2%) is not radioactive either, and is actually quite stable.

The ionization energy of Rb lies in between that of Cs and K, so its pretty easy to remove electrons from Rb too, and does not require much energy.

So, is Rb used in photoelectric cells or not? If not, why?

Buckbitter Inning
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1 Answers1

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All of the alkali elements can be used in photocells, and they are suitable because of the comparatively low first ionization potential.

Consider the photoelectric effect, though; one of the first clues to quantum mechanics. An incident photon must have sufficient energy, E = hν, to knock out that electron. A red light photon is ~1.5 electron volts (ev), and violet ~3 ev, so photocells using cesium are insensitive to red light, and potassium photocells would respond only to midrange UV. Rubidium would work for long-wave UV and shorter wavelengths.

So, it depends on the wavelength for which the photocell is intended (as well as the volatility of the metal at the temperature of use, cost, etc.).

DrMoishe Pippik
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  • yes, im aware of the wavelength argument. but i was wondering if theres any specific reason why only Cs and K are used. the others may be used, but thats not usually the case. why? – Buckbitter Inning Mar 03 '21 at 18:02
  • Misapprehension "Rubidium is also used in the manufacture of photocells." https://www.azom.com/article.aspx?ArticleID=6063 – DrMoishe Pippik Mar 03 '21 at 18:10