0198506961.pdf

6.4 Measurement of hyperfine structure 117

atomic-beam experiment are as follows.

• Atoms emerge from an oven to form an atomic beam in an evacuated
  chamber. The atoms have a mean free path much greater than the
  length of the apparatus (i.e. there are no collisions).


• The deflection of atoms in the A and B regions depends onMJ.If
  these two regions have magnetic field gradients in the same direction,
  as indicated in Fig. 6.15, then atoms only reach the detector if their
  MJquantum number changes in the C region, i.eMJ=+^12 ↔MJ=
  −^12. This is known as the flop-in arrangement.^3737 The flop-out arrangement is shown in
  Fig. 6.14.


• As the atoms travel from A into the C region their state changes
  adiabatically to that in a low magnetic field as shown in Fig. 6.16
  (and similarly as the field changes between the C and B regions). The
  transitions in the low-field region that can be observed are those that
  connect states with different values ofMJin the high-field regions,
  e.g. the transitions:

low frequency (∆F=0):

F=1,MF=0 ↔ F=1,MF=−1with∆E=gFμBB;

higher frequency (∆F=±1):

F=0,MF=0 ↔ F=1,MF=0 with ∆E=A,

F=0,MF=0 ↔ F=1,MF=1 with ∆E=A+gFμBB.

In this example, theMF =1↔MF= 0 change between the Zee-

man sub-levels of the upperF= 1 level cannot be detected, but this

(b)

D

Oven

(a)

Detected

flux of

atoms

r.f.

Flop-in

Fig. 6.15(a) The trajectories of atoms in an atomic-beam apparatus similar to that shown in Fig. 6.14, but magnetic field
gradients in the A and B regions have the same direction. Atoms only reach the detector if theirMJquantum number is
changed in the C region by the interaction with radio-frequency radiation. This is known as the flop-in arrangement and gives
a signal as in (b).