Page 265 - Essentials of physical chemistry

Early Experiments in Quantum Physics 227

Scattering of 75 V electrons from
a block of nickel (many small crystals)

Scattering of 75 V electrons from
several large nickel crystals
FIGURE 10.12 The Davisson–Germer discovery after annealing Ni crystals. (Reprinted with permission
from Davisson, C. and Germer, L.H., Phys. Rev, 30, 705, 1927. Copyright 1927 by the American Physical
Society.)

d

(3.5238 Å)
FIGURE 10.13 The Ni (fcc) unit cell showing the {1, 1, 1} planes.

The most pronounced scattering peak occurred for 54 eV electrons and was observed at an angle
of about 508–558 from the incoming beam, which was perpendicular to the metal surface. This is a
different orientation from the usual Bragg angle with the angle of incidence equal to the angle of
reflection but when the phase of the waves is right for reinforcement such diffraction can occur
using the equation of nl ¼ 1d sin(u) instead of nl ¼ 2d sin(u).
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
mV 2 2(54 eV)(1:60217653 10 19 J=eV) 6
¼ 4:358354871 10 m=s:
E ¼ ¼ 54 eV so V ¼ 31
2 9:1093826 10 kg
h 6:6260693 10 34 J s 10
¼ 1:668954288 10 m
¼
mV (9:1093826 10 kg)(4:358354871 10 m=s)
l ¼ 31 6