photoelectron

The story

1. Proto-Indo-European*bʰeh₂-
2. Proto-Indo-European*bʰéh₂os
3. Proto-Hellenic*pʰáwos
4. Ancient Greekφᾰ́ος (phắos)
5. Ancient Greekφῶς (phôs)
6. Ancient Greekφωτο- (phōto-)
7. Englishphoto-
8. New Latinēlectricus
9. Englishelectric
10. Proto-Indo-European*h₁ey-
11. Proto-Indo-European*h₁éyti
12. Proto-Hellenic*éimi
13. Ancient Greekἰόν (ión)
14. Englishion
15. Englishelectron
16. English todayphotoelectron

From photo- + electron.