Fourier transform Fabry-Perot cavity

The most important and unique part of a Fourier transform microwave spectrometer is the Fabry Perot cavity. A fairly complete description of the principles has been given by Balle and Flygare [14] and we here summarise the main aspects, with the aid of figure 10.19. We use the cavity built by Balle and Flygare as a typical example. It is formed by two parallel, spherical concave mirrors made from solid aluminium. The mirrors are 36 cm in diameter, have a radius of curvature (R) of 84 cm, and are situated... 

Fig. 5. Pulsed-nozzle FT microwave measurements. A molecule-radiation interaction occurs when the gas pulse is between mirrors forming a Fabry-Perot cavity. If the transient molecule has a rotational transition of frequency vm falling within the narrow band of frequencies carried into the cavity by a short pulse (ca. 1 (is) of monochromatic radiation of frequency v, rotational excitation leads to a macroscopic electric polarization of the gas. This electric polarization decays only slowly (half-life T2 = 100 (is) compared with the relatively intense exciting pulse (half-life in the cavity t 0.1 (is). If detection is delayed until ca. 2 (is after the polarization, the exciting pulse has diminished in intensity by a factor of ca. 106 but the spontaneous coherent emission from the polarized gas is just beginning. This weak emission can then be detected in the absence of background radiation with high sensitivity. For technical reasons, the molecular emission at vm is mixed with some of the exciting radiation v and detected as a signal proportional to the amplitude of the oscillating electric vector at the beat frequency v - r , as a function of time, as in NMR spectroscopy Fourier transformation leads to the frequency spectrum [reproduced with permission from (31), p. 5631.

In this chapter, we describe the technique of Fourier transform microwave spectroscopy. We distinguish here two rather different types of sample absorption cells which require somewhat different theoretical descriptions. First, we describe the theory for the relatively broad-band waveguide absorption cell in which the radiation is described as a traveling wave. Second, we describe the narrow-band Fabry-Perot cavity absorption cell in which the radiation is described as a standing wave. 

In this section, we replace the broadband waveguide absorption cell with a narrow-band Fabry-Perot cavity. The traveling wave is then replaced with a standing wave. We consider a static gas polarization and subsequent coherent emission in the Fabry-Perot cavity.7.8 However, the use of a Fabry-Perot cavity and the pulsed Fourier transform microwave method is also well-suited for the measurement of the resonant transitions of transient or otherwise short-lived species. 

This new development combines the principles of a standing wave pulsed Fourier transform spectrograph in a Fabry-Perot cavity with a pulsed molecular source, where a high-pressure gas expands into a... 

We will now solve Equation 23 for P-j, P, and aN, using functional forms of (r, t) appropriate for the pulsed Fourier transform experiment carried out in a Fabry-Perot cavity.7... 

Balle TJ, Flygare WH (1981) Fabry-Perot cavity pulsed Fourier-transform microwave spectrometer with a pulsed nozzle particle source. Rev Sci Instnun 52 33-45... 

The CW microwave spectrometer just described is a typical frequency-domdim instrument. In the late 1970s it was demonstrated that pulsed /m -domain microwave spectroscopy could be practically performed in analogy to the techniques already well known in other fields such as NMR spectroscopy. Figure 2 depicts a block diagram of a modern version of a pulsed Fourier-transform microwave spectrometer. The particular instrument shown utilizes a Fabry-Perot cavity and a pulsed-gas nozzle, and is especially useful for detecting microwave... 

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