Flop-out

Figure 8.25. Schematic diagram of amolecular beam electric resonance spectrometer, employing dipole A and B fields [48]. The trajectories of levels characterised by J, M quantum numbers are shown. Transitions occurring in the C field region which obey the selection rule AM= 1 result in a reduction in the number of molecules reaching the detector this is therefore an example of flop-out detection.

The spectra of LiBr were again for molecules in the J = 1 rotational level, and quadrupole lenses were operated to focus molecules in the My = 0 state. AM/ = 1 resonances were therefore detected using the flop-out mode. We shall consider the details of the weak field investigation first, subsequently describing the strong magnetic field studies. 

Figure 8. (a) Flop out C (>fyi- Hin resonance on a sweep of decreasing frequency, (b) flop in Cs 16/5/2-16g7/2 resonance on a sweep of increasing frequency. The slight offset is due to the time constant used in signal averaging. ... 

These atoms will not be refocussed at the detector in this so-called flop-out arrangement of the polarizer and analyser fields. Consequently when the detector current is recorded as a function of oJq for a fixed value of B, a series of sharply defined minima will be observed. The width of these magnetic resonance lines is often as narrow as 300 Hz and permits very precise measurements of the spins and magnetic moments of both atoms and nuclei. We shall now discuss some aspects of the atomic beam apparatus in more detail before considering the application of this technique to hyperfine structure measurements in one-electron atoms. 

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