Optical Ramsay Fringes

The problem of time-of-flight broadening was recognized many years ago in electric or magnetic resonance spectroscopy in molecular beams [13.1], In these Rabi-type experiments the natural linewidth of the radio-frequency or microwave transitions is extremely small [because the spontaneous transition 

A considerable reduction of time-of-flight broadening could be achieved by realization of Ramsay s ingenious idea of separated fields [13.2]. The molecules in the beam pass two phase-coherent fie lds which are spatially separated by a distance x = L Ax = d large compared with the extension AX = d of each field (Fig.13.1). The interaction of the molecules with the first field creates a dipole moment of each molecular oscillator with a phase depending on the interaction time t = d/v and the detuning of the 

The interaction between dipole and second field depends on their relative 

When we assume that all N molecules passing the field per second have the same velocity v, we obtain the signal 

Fig 13 2 Signal power, ab-sorbed in the second field, as a function of detuning = 0) - o)q (Ramsay fringes for a narrow velocity distribution) 

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The interesting question of whether there exists a fundamental resolution limit, is discussed in Chap.13. Some recently developed techniques such as optical Ramsay fringes and trapping and cooling of atoms, illustrate how laser spectroscopy approaches this ultimate limit of spectral resolution. 

Some techniques of laser spectroscopy, such as the method of separated fields (optical Ramsay fringes, see Sect.13.1) or coherent transient spectroscopy (see Sect.11.4) allow one to distinguish between phase changing, velocity changing, or orientation changing collisions. 

The capability of this combination of optical Ramsay fringes with saturation spectroscopy has been impressively demonstrated by BERQUIST et al. [13.8],... 

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