Frequency modulation spectroscopy monitoring

Laboratory trace gas monitoring Discuss and compare the following methods optoacoiistic spectroscopy, cavity ring-down spectroscopy and frequency modulation spectroscopy. 

The pyrolysis of CR NH (<1 mbar) was perfomied at 1.3 atm in Ar, spectroscopically monitoring the concentration of NH2 radicals behind the reflected shock wave as a fiinction of time. The interesting aspect of this experiment was the combination of a shock-tube experiment with the particularly sensitive detection of the NH2 radicals by frequency-modulated, laser-absorption spectroscopy [ ]. Compared with conventional narrow-bandwidth laser-absorption detection the signal-to-noise ratio could be increased by a factor of 20, with correspondingly more accurate values for the rate constant k T). 

Electron spin echo spectroscopy (ESE) monitors the spontaneous generation of microwave energy as a function of the timing of a specific excitation scheme, i.e. two or more short resonant microwave pulses. This is illustrated in Fig. 7. In a typical two-pulse excitation, the initial n/2 pulse places the spin system in a coherent state. Subsequently, the spin packets, each characterized by their own Larmor precession frequency m, start to dephase. A second rx-pulse at time r effectively reverses the time evolution of the spin packet magnetizations, i.e. the spin packets start to rephase, and an emission of microwave energy (the primary echo) occurs at time 2r. The echo ampHtude, as a fvmction of r, constitutes the ESE spectrum and relaxation processes lead to an irreversible loss of phase correlation. The characteristic time for the ampHtude decay is called the phase memory time T. This decay is often accompanied by a modulation of the echo amplitude, which is due to weak electron-nuclear hyperfine interactions. The analysis of the modulation frequencies and ampHtudes forms the basis of the electron spin echo envelope modulation spectroscopy (ESEEM). 

The method of phosphorescence microwave double resonance (PMDR) spectroscopy is based, like the two other methods discussed above, on c.w. excitation of the Pd(2-thpy)2 compound at low temperature. Additionally, micro-wave irradiation is applied, whereby the frequency is chosen to be in resonance with the energy separation between the two substates I and III of 2886 MHz. With this set-up, one monitors the phosphorescence intensity changes in the course of scanning the emission spectrum. Technically, the phosphorescence spectrum is recorded by keeping the amplitude-modulated microwave frequency at the constant value of 2886 MHz and by detecting the emission spectrum by use of a phase-sensitive lock-in and signal averaging procedure (e.g. see [61, 75,90]). 

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