Tunable cavity microwave spectrometer

Figure 10.15. Block diagram of the tunable-cavity spectrometer built by Radford [13], and used by bim to study the microwave spectra of OH and OD.

An unusual example of a microwave spectrometer which uses superheterodyne detection and molecular modulation is a tunable-cavity spectrometer designed and built by Radford [13]. Microwave cavities were described in chapter 9, where they form the heart of a microwave magnetic resonance spectrometer. Compared with... 

A microwave pulse from a tunable oscillator is injected into the cavity by an anteima, and creates a coherent superposition of rotational states. In the absence of collisions, this superposition emits a free-mduction decay signal, which is detected with an anteima-coupled microwave mixer similar to those used in molecular astrophysics. The data are collected in the time domain and Fourier transfomied to yield the spectrum whose bandwidth is detemimed by the quality factor of the cavity. Hence, such instruments are called Fourier transfomi microwave (FTMW) spectrometers (or Flygare-Balle spectrometers, after the inventors). FTMW instruments are extraordinarily sensitive, and can be used to examine a wide range of stable molecules as well as highly transient or reactive species such as hydrogen-bonded or refractory clusters [29, 30]. 

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