When atoms come closer together, a tunnel transition of outer electrons from one atom to another is possible, which leads to broadening of the levels (formation of bands in a solid). Assuming that the electron is in a rectangular potential well of width a = 2rB ~ 1A at a depth equal to the ionization energy U0 = 10 eV, and the barrier width d is equal to the average distance between atoms, estimate the energy broadening in a crystal with d = 1 A.

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