| |
In a liquid sample, molecular motion causes the electric field gradient to average to zero and NQR is observed only in solid samples. 14N and 35Cl are the common nuclei which exhibit NQR absorption in the frequency range of 20 – 40 MHz. For axially symmetric systems, the quantized energy levels due to the quadrupole-electric field gradient interaction are
Em = e2 q Q [3 m2 – I ( I + 1) / 2 I ( 2 I -1 ) (15.5)
In the above equation, I is the nuclear spin quantum number, q = ∂2V/ ∂z2, the electric field gradient in the direction of the axis of symmetry and m, the projection of I is given by
m = I, I - 1, …… - I + 1, - I (15.6)
Note that the energy levels depends on m2 and + m and – m values correspond to the same energy. The selection rule for an NQR transition is,
Δ m = ± 1 (15.7)
The frequency for a transition from m - 1 to m level is given by
 ) (15.8)
The quantity e2qQ/h is called the nuclear quadrupole coupling constant. The energy levels for spins 1, 1½ and 2½ are shown in Fig 15.4 |
| |
Figure 15.4. The energy levels for I = 1(a), I = 3/2(b) and I(5/2) (c) and the allowed NQR transitions. |
