Cavity Absorption Cells

Waveguide cells exhibit a broad bandwidth and consequently find favour in the spectroscopic study of molecules. They are, however, too large for any sort of process analysis or for use on mobile platforms and suffer badly from memory effects due to their high surface area to volume ratio. Whilst not dismissing their importance to the subject in general we have focused on the more compact cavity spectrometers in this work. 

A common alternative configuration is the optically equivalent combination of a spherical with a plane mirror, a semi-confocal cavity. This is more convenient in that the length of the cavity is reduced by a factor of two, but it does mean doubling the number of reflections and therefore the transmission loss for a given transit path of the radiation through the sample. 

In the example given above such a reflection coefficient would yield a transit distance of 500 m before the beam became attenuated by a factor 1/e. That distance would be traversed in 1.67 s, corresponding to a cavity-0 10 at 100 GHz. Such a high-0 would mean that the cavity bandwidth over which it could respond to any input would be in that case 100kHz. Some degree of frequency stabilisation of the MMW source would consequently be called for, to ensure it remained within this bandwidth for the duration of any measurement (Section 3.3). 

There are in addition, further modes in which the lateral MMW field also varies. These are usually less prominent because they are not so readily excited by the mechanism that couples power into the cavity. They are described by the label TE m and n are further small integers. The general expression describing their resonant frequencies is  

For a confocal cavity the arccos term takes on the value nil and so the lateral 

---

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. 

The idea of using the same medium as absorber and active material has been proposed and realized by several authors 340-343) Leg and Skolnick 40) used a neon gas discharge at low current and low pressure as saturable absorber inside the cavity of a He-Ne laser oscillating at X = 6328 A. The Lamb-dip halfwidth obtained was 30 Mc/sec compared to 1500 Mc/sec for the doppler line. The disadvantage of this arrangement is that the frequency of the neon transitions depends upon pressure and current 341) in the absorption cell, and this limits the stability and reproducibility of the Lamb dip center frequency. 

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. 

Figure 4. Principle of CRD absorption measurements, (a) light decaying in an empty cavity, (b) cell is filled with an absorber.

A FIGURE 6-4 Principal types of epithelium. The apical and basolateral surfaces of epithelial cells exhibit distinctive characteristics, (a) Simple columnar epithelia consist of elongated cells, including mucus-secreting cells (in the lining of the stomach and cervical tract) and absorptive cells (in the lining of the small intestine), (b) Simple squamous epithelia, composed of thin cells, line the blood vessels (endothelial cells/endothelium) and many body cavities, (c) Transitional epithelia, composed of several layers of cells with different shapes, line certain cavities subject to expansion and contraction (e.g., the urinary bladder). 

An ESR spectrometer thus measures the energy required to reverse the spin of an electron in an external magnetic field. Experimentally, this is performed by placing the substance containing unpaired electrons in an absorption cell in the form of a resonance cavity. The sample is... 

For a single-pass FM spectrometer working at 150 GHz the best operating condition is to minimise background, by controlling power reflection within and at each end of a long-path absorption cell, and to apply a modulation depth of 240 kHz p-p at a sample pressure 8-13 Pa. By contrast in a cavity spectrometer, the optimum modulation depth is governed by the cavity width, rather than the sample linewidth (Section 2.4). 

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. 

Assume that the reflectivities of the two resonator mirrors are = 1 and R2 = I — T2 (mirror absorption is neglected). At the laser output power Pout the power inside the cavity is Pint = Fout with q = I/T2. For aL 1, the power AP(co) absorbed at the frequency co in the absorption cell (length L) is... 

External passive resonators may become advantageous when the absorption cell cannot be placed directly inside the active laser resonator. However, there also exist some drawbacks the cavity length has to be changed synchronously with the tunable-laser wavelength in order to keep the external cavity always in resonance. Furthermore, one has to take care to prevent optical feedback from the passive to... 

The experimental setup is shown in Fig. 1.18. The laser pulses are coupled into the resonator by carefully designed mode-matching optics, which ensure that only the TEMoo modes of the cavity are excited. Diffraction losses are minimized by spherical mirrors, which also form the end windows of the absorption cell. If the absorbing species are in a molecular beam inside the cavity, the mirrors form the windows of the vacuum chamber. For a sufficiently short input pulse (Tp < 7r), the output consists of a sequence of pulses with a time separation Tr and with exponentially decreasing intensities, which are detected with a boxcar integrator. For longer pulses (Tp > 7r), these pulses overlap in time and one observes a quasi-continuous exponential decay of the transmitted intensity. Instead of input pulses, the resonator can also be illuminated with cw radiation, which is suddenly switched off at f = 0. 

Fig. 2. Graphic illustration of the saturation resonance observed in CO2 fluorescence at 4.3 pm. The figure shows an internal absorption cell within the laser cavity. External cells may also be used.

If the absorbed power can be measured directly, for example, through the resulting pressure increase in the absorption cell (Sect. 6.3.3) or through the laser-induced fluorescence (Sect. 6.3.1), the signal will be q times larger than for the case of single-pass absorption outside the cavity. 

Intracavity absorption cells are particularly advantageous if the absorption is monitored via the laser-induced fluorescence. Since the radiation field inside the active resonator or inside the mode-matched passive cavity is concentrated... 

Fig. 13.4a-c. Line profiles of Lamb peaks of a HeNe laser at A = 3.39 xm with intra-cavity CH4 absorption cell (a) pure CH4 at 1.4mbar (b) addition of 30mbar He and (c) 79mbar He [13.20]... 

If the saturation condition is met, the velocity-tuned three-photon processes bum holes in the velocity profile of molecules at one third the velocities Uq at which normal single-photon processes can create holes. Freund et aL studied these effects in CH3F. The setup was similar to an earlier experiment carried out by the same group (see above. Ref. 215), and made use of a CO2 laser with the absorption cell installed inside the cavity. Lamb dips were detected as sharp variations of the output power of the laser. 

---