Modulation frequencies, range

As is also seen, the dielectric constant does not exhibit substantial dispersion in the modulating frequency ranges of interest. 

Theory. If two or more fluorophores with different emission lifetimes contribute to the same broad, unresolved emission spectrum, their separate emission spectra often can be resolved by the technique of phase-resolved fluorometry. In this method the excitation light is modulated sinusoidally, usually in the radio-frequency range, and the emission is analyzed with a phase sensitive detector. The emission appears as a sinusoidally modulated signal, shifted in phase from the excitation modulation and partially demodulated by an amount dependent on the lifetime of the fluorophore excited state (5, Chapter 4). The detector phase can be adjusted to be exactly out-of-phase with the emission from any one fluorophore, so that the contribution to the total spectrum from that fluorophore is suppressed. For a sample with two fluorophores, suppressing the emission from one fluorophore leaves a spectrum caused only by the other, which then can be directly recorded. With more than two flurophores the problem is more complicated but a number of techniques for deconvoluting the complex emission curve have been developed making use of several modulation frequencies and measurement phase angles (79). 

Here, results are shown from experiments performed in ASTER, reported by Biebericher et al. [512. 519], A SiH4-H2 (50 50 flow ratio, total flow 60 seem) plasma was generated at an RF excitation frequency of 50 MHz. The substrate temperature was 250°C. The RF signal was ampitude modulated (AM) by a square wave. The modulation frequency has been varied in a range of 1-400 kHz. The modulation depth was always 90%. The duty cycle was fixed at 50%. The pressure amounted to 0.2 mbar, and the average power was kept at 10 W. With a duty cycle of 50%, this leads to a power of 20 W during the plasma-on period. 

For spin-f nuclei, dipolar interactions may be modulated by intramolecular (DF, reorientation etc.) and/or intermolecular (TD) processes. In general, the intra- and inter-molecular processes can produce quite different Tj frequency dispersion curves. In practice, NMR field cycling experiments are often needed to extend the frequency domain from those employed in conventional spectrometers to a lower frequency range (i.e., the kHz regime) for unambiguous separation (and identification) of different relaxation mechanisms. The proton spin relaxation by anisotropic TD in various mesophases has been considered by Zumer and Vilfan.131 133,159 In the nematic phase, Zumer and Vilfan found the following expression for T ... 

The two techniques, ENDOR and ESE envelope modulation, supplement each other. ESE envelope modulation seems to be more sensitive in detecting nuclear transitions at very low frequencies but is limited in the frequency range by yeB , where ye denotes the gyromagnetic ratio of the electron and Bj the microwave pulse amplitude. ENDOR, whose sensitivity increases with frequency, suffers on the other hand from the small transition probability at low frequencies. 

In the upper panel of Figure 13.6, the emission is drawn assuming a modulation frequency of 30 MHz and a lifetime of 9 nsec. Using the equations above, the phase angle is 59.5° and the demodulation factor is 0.5. (For further details, the reader is referred to Lakowicz(66)). Additionally, multifrequency phase and modulation instruments that operate over a range of frequencies have been described(67, flS) and simple instruments are possible if only one or several discrete frequencies are required (Figure 13.6, lower panel). 

V at 100 MC/sec compared with 10 kV in Kerr cells) and they transmit over a wide spectral range (2500-12000 A). When designed as travelling wave modulators, they can be used even up to micro-wave modulation frequencies... 

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