Chemistry Reference
In-Depth Information
and at
E
H
V
, respectively, where
V
is negative and is a measure of the
strength of the covalent interaction between the neighbouring orbitals. By
comparison with the LiH molecule in Chapter 2, the two lowest energy
levels in the H
X
molecule are given by
−
E
X
2
E
X
+
E
H
−
E
H
V
2
E
=
±
+
(4.24)
2
2
The variation in the energy levels of the H
X
molecule with the
X
atom
self-energy,
E
X
, is plotted in fig. 4.7(a). On the upper left-hand side of the
(a)
)
2
E
=
E
X
+
E
H
2
(
E
X
-
E
H
2
+
+
V
2
E
H
+⏐
V
⏐
E
x
E
H
E
H
-
⏐
V
⏐
(
)
2
E
=
E
X
+
E
H
2
E
X
-
E
H
2
-
+
V
2
(b)
E
c
E
g
E
v
0
Electronegativity difference
Figure 4.7
(a) Variation in the energy levels of the H
X
molecule as the
X
atom
self-energy,
E
X
, is varied with respect to the H atom value,
E
H
. (b) In a semi-
conductor, the bonding and anti-bonding levels are broadened into bands.
For small electronegativity differences, defect-related resonant states are
found in the bands (dashed lines), which shift into the energy gap (solid
lines) as the electronegativity difference increases.
