1. Electronic surface states

The investigation of surface electronic states has a long history. Their existence was first postulated theoretically in the 1930s by I. Tamm , then by W. Shockley , who identified specific electronic states called "surface states", with an amplitude maximum near the edge and an asymptotic exponential decay in vacuum and inside the solid. The angularly resolved photoemission technique enabled us to confirm their existence and study their properties in ever greater detail, as devices and paradigms for interpreting results improved.