Microwave spectroscopy Stark-modulated

A second use of microwave spectroscopy is the measurement of dipole moments. These are obtained by measuring the frequency shifts of lines in the applied electric field of a Stark-modulated spectrometer. This method of dipole-moment determination is superior to the older method of measuring dielectric constants. For example, impurities in the sample will not affect the dipole moment as measured by microwave spectroscopy. The dipole moment of a substance present to the extent of a few percent can be accurately measured if its microwave spectral lines can be assigned. The components of d can be determined, thus giving its orientation in the molecule, in addition to its magnitude. 

Although the only direct modulation process to be discussed in detail in this monograph is frequency modulation it is by no means the only detection method possible. The most commonly employed method in anal3ftical microwave spectroscopy has been Stark modulation, but as has been discussed above, it is not well suited to cavity MMW spectrometry. 

We have already discussed the use of electric field modulation as a means of providing the selective detection of those molecular absorption lines having the strongest Stark effects, using tunable lasers as sources ( 2.4). As in microwave spectroscopy, where field modulation is routine, magnetic fields may also be used for this purpose, as was demonstrated by Urban and Herrmann (1978). The spectrum of NO was recorded with extremely high sensitivity using Zeeman modulation with a spin-flip Raman tunable infrared laser. 

To achieve large electric fields, the separation of the Stark electrodes is made as small as possible (typically about 1 mm). This generally excludes an intracavity arrangement because the diffraction by this narrow aperture would introduce intolerably large losses. The Stark cell is therefore placed outside the resonator, and for enhanced sensitivity the electric field is modulated while the dc field is tuned. This modulation technique is also common in microwave spectroscopy. The accuracy of 10 " for the Stark field measurements allows a precise determination of the absolute value for the electric dipole moment. 

In the earlier decades of microwave spectroscopy, and in all commercial spectrometers on the market at that time, the method of choice was that of Stark modulation. A uniform electric field was applied to the entire sample and switched off and on, typically at a 33 kHz rate. The resulting phase coherently detected spectral frequency scan (Figure 1) shows a spectral line in phase with the switching voltage and a number of electric-field shifted (Stark) components in antiphase with it. 

The technique of Fourier-transform microwave spectroscopy (FTMS) has been applied to the study of a number of weakly bonded complexes, the observation of weak isotopic species, and the resolution of hyperfine structure. It is characterized by higher resolution and sensitivity than conventional Stark-modulated spectroscopy. The superior resolution is demonstrated in Figs. 24 and 25. 

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