Modulation Stark

Fig. 3. Stark modulation spectrum of HDCO around 2850.62 cm", obtained with a Zeeman-tuned Xe laser line at 3.50 fim. The Stark field is perpendicular to the optical field and increases from the bottom towards the top of the figure resulting in an increasing splitting of the Stark levels therefore more and more components are separated. (From Uehara, K.T., Shimizu, T., Shimoda, K., ref. 85))...

By modulating the electric field and using phase-sensitive detection methods, Uehara et al. 8 ) were able to increase the sensitivity considerably and they could even detect Stark splittings of less than the doppler width of the components. Fig. 3 shows the Stark spectrum of HDCO for different electric field strengths. Because of the Stark modulation technique the absorption lines appear differentiated the zero points represent the center of each line. 

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. 

Typically, a Stark-modulated microwave spectrometer will readily resolve lines separated by at least 0.3 MHz. For CF3C = CH, what is the lowest value of J for which the microwave absorption spectrum will show more than one line Calculate the 7+1 theoretical frequencies for this value of 7. Use the data of (5.86). 

Figure 10.6. The effects of Stark modulation and phase-sensitive detection.

Figure 10.7. Methods for introducing Stark modulation into a rectangular waveguide cell ...

There was at one time some uncertainty as to the planarity of the selenophene molecule,9-12 and the electric moment was also in doubt (0.41 or 0.77 D).13 14 Therefore, Pozdeev and others15 undertook a microwave spectral investigation of selenophene, 2,5-dideuterioseleno-phene and tetradeuterioselenophene, at — 40°C on spectrographs with Stark modulation (frequency 62.5 or 90 kHz). The sensitivity of the two spectrographs was about 10-9 cm-1. 

Finally, we must mention the excellent work of Carrington (100), who with his coworkers developed the Stark modulation cavity for electron resonance study in the gas phase. 

The vibration-rotation spectra of the v, and v bands of CO C1 j have been measured (using a tunable semiconductor-diode laser), and assigned with the aid of Stark modulation spectra [2224]. The precise values of these bands were determined to be 1828.2012 and 851.0105 cm", respectively, and the equilibrium rotational constants for CO Clj were calculated as = 7950.35, Bg = 3490.22, and Cg = 2425.44 MHz cf. 

The diamagnetic susceptibility anisotropies and molecular second moments of the electronic charge distributions for COCIF have been calculated by Maksic and Mikac [1303a], and these authors suggested that their calculated values showed that the experimentally derived second moments (calculated from the Zeeman effect in the Stark-modulated microwave... 

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. 

The simplest spectra to understand and analyse are those obtained from molecules which exhibit good case (a) coupling, and for which the d-doublets are degenerate to within the spectroscopic resolution. Typical examples are CIO [20] and BrO [21], the spectra of which are shown in figure 9.9. In both cases the Stark modulation cavity described in figure 9.3 was used CIO was obtained flowing a mixture of chlorine and... 

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