Microwave absorption cells

Figure 10.12. Free space microwave absorption cell used by Yamada, Fujitake and Hirota [9] for the study of NaO at a temperature of 350 °C.

Radicals (see also Section IV.E.2) are very short-lived, reactive species. They are often produced as products of an RF electric discharge. Alternatively, the products of an electric discharge are allowed to react with another substance to produce the desired radical. In these production methods, a continuous flow of radicals is supplied to the microwave absorption cell. The use of glass absorption cells with Teflon windows is particularly useful for such studies. The large volume-to-surface ratio possible... 

In the microwave region tunable monochromatic radiation is produced by klystrons, each one being tunable over a relatively small frequency range, or a backward wave oscillator, tunable over a much larger range. Both are electronic devices. Absorption experiments are usually carried out in the gas phase, and mica windows, which transmit in this region, are placed on either end of the absorption cell, which may be several metres in length. Stark... 

Both microwave and millimetre wave radiation can be channelled in any direction by a waveguide made from metal tubing of rectangular cross-section, the dimensions depending on the frequency range. The absorption cell is also made from waveguide tubing. 

A particular problem arises when aqueous solutions are studied because of the high dielectic absorption of water in the microwave region. In order to prevent extensive microwave absorption, and hence loss of sensitivity, a region of the cavity is selected where the electric component is low but the magnetic component is high. For a standard H0i2 cavity, thin cells holding ca. 0.1 ml are used. These can be made demountable for tissue studies or can be part of a flow system. 

Thus, the desire was to design a narrow, robust, easily cleaned, flat cell in which the pathlength is short enough such that microwave absorption is minimal. The cell was also required to be spacious enough such that the three electrodes for electrogeneration could be incorporated. 

In a typical experiment, monochromatic microwave radiation with a bandwidth less than 10 Hz 48) at a frequency of 10 GHz is slowly swept over the interesting frequency range of the rotational spectrum. Highly accurate frequency markers are superimposed automatically on the recorded spectrum. After having passed an attenuator, a crystal mixer which is part of the frequency stabilization system, and a second attenuator, the microwave radiation enters the absorption cell through a mylar window. The temperature of the absorption cell is con-... 

Since the electric field vector of the incident microwave radiation, Emw, is linearly polarized perpendicular to the broad face of the waveguide absorption cell (TEio-mode of propagation), the M-selection rules may be selected by the orientation of the waveguide cell with respect to the magnetic field. This may be... 

Alternatively one may modulate the sample concentration by periodically removing it from and replacing it within the absorption cell as done, albeit rather slowly, in pulsed supersonic jet absorption spectroscopy. The sample absorption coefficient can be modulated by double resonance using a second microwave source to pump a transition that has one energy level in common with the... 

In this chapter, we describe the technique of Fourier transform microwave spectroscopy. We distinguish here two rather different types of sample absorption cells which require somewhat different theoretical descriptions. First, we describe the theory for the relatively broad-band waveguide absorption cell in which the radiation is described as a traveling wave. Second, we describe the narrow-band Fabry-Perot cavity absorption cell in which the radiation is described as a standing wave. 

In this section, we replace the broadband waveguide absorption cell with a narrow-band Fabry-Perot cavity. The traveling wave is then replaced with a standing wave. We consider a static gas polarization and subsequent coherent emission in the Fabry-Perot cavity.7.8 However, the use of a Fabry-Perot cavity and the pulsed Fourier transform microwave method is also well-suited for the measurement of the resonant transitions of transient or otherwise short-lived species. 

The combination of favorable properties of PANI and TiO opens the possibility for various applications of PANI/TiO nanocomposite materials, such as piezoresistivity devices [41], electrochromic devices [99,118], photoelectrochemical devices [43,76], photovoltaic devices/solar cells [44,50,60,61,93,119], optoelectronic devices/UV detectors [115], catalysts [80], photocatalysts [52,63,74,75,78,84,87,97,104,107,121,122,125], photoelectrocatalysts [122,123], sensors [56,61,65,69,85,86,95,120,124], photoelectrochemical [110] and microbial fuel cells [71], supercapacitors [90,92,100,109,111], anode materials for lithium-ion batteries [101,102], materials for corrosion protection [82,113], microwave absorption materials [77,87,89], and electrorheological fluids [105,106]. In comparison with PANI, the covalently bonded PANI/TiO hybrids showed significant enhancement in optical contrast and coloration efficiency [99]. It was observed that the TiO nanodomains covalently bonded to PANI can act as electron acceptors, reducing the oxidation potential and band gap of PANI, thus improving the long-term electrochromic stability [99]. Colloidal... 

So far, we have considered only experiments with continuous-wave lasers under steady state conditions. With time-resolved experiments, on the other hand, energy transfer rates and transition probabilities can be obtained. Such measurements were carried out by mechanically chopping the laser beam directed into an external absorption cell together with the microwave radiation. Later, Levy et at reported time-resolved infrared-microwave experiments with an N2O laser Q-switched with a rotating mirror to produce pulses less than 1 /tsec in duration. They observed a transient nutation of the inversion levels of the molecule following the infrared laser pulse. Based on the Bloch equations, the observed phenomena could be explained quantitatively. From the decay envelope of the oscillations a value for the transverse relaxation time T2 was determined. Similar effects were produced by rapidly switching a Stark field which brings the molecules into resonance with the cw microwave radiation. 

---