Multipass absorption

Armerding, W., M. Spiekermann, and F. J. Comes, OH Multipass Absorption Absolute and In Situ Method for Local Monitoring of Tropospheric Hydroxyl Radicals, . /. Geophys. Res., 99, 1225-1239 (1994). 

Fig. 1.2 Multipass absorption cell, (a) Beam geometry, (b) laser beam spots on a mirror surface...

All these techniques may be combined with intracavity absorption when the sample molecules are placed inside the laser resonator to enhance the sensitivity. Cavity ring-down spectroscopy yields absorption spectra with a detection sensitivity that is comparable to the most advanced modulation techniques in multipass absorption spectroscopy. 

Example 10.1 With a diode laser spectrometer and a multipass absorption cell (Fig. 10.1) NO2 concentrations down to the 50-ppt level in air were detected on a vibrational-rotational transition at 1900 cm NO concentrations down to 300 ppt, while for SO2 at 1335 cm a sensitivity limit of 1 ppb was reached [1371]. 

In the region located between the Huggins and the Chappuis bands, the cross-sections are very weak (lO -lO cm ), and the data are either missing (390 nm < A. < 420 nm) or obtained under insufficient resolution (Johnston for X > 420 nm and Wahner for X > 390 nm). To measure such a weak absorption we have built a 1 metre multipass absorption cell, coolable down to -60 °C. The body of the cell, made of stainless steel, is well suited to being filled with corrosive gases such as pure ozone. Measurements are in progress they concern the 440-515 nm and 650-720 nm ranges at ambient temperature. 

The pure rotational spectrum in the far infrared (zero field) was achieved with a tunable IR laser [81] this might, combined with multipass absorption devices, be an interesting method to detect NH(X) radicals. 

Figure 24.8 Experimental schematic of the multiplexed diode-laser sensor system used to measure CO, CO2, CH4, and H2O absorption by sampling hot combustion gases 1 ECDL 1.49-1.58 pm 2 optical isolator 3 — fiber coupler 4 — 1x2 fiber splitter 5 — etalon 6 — InGaAs detector 7 — DEB 1.65 pm 8 — 2 x 1 fiber combiner 9 optical fiber 10 fiber pitch 11 — concave mirror 12 — multipass...

Differential absorption spectroscopy techniques were used to determine absolute species concentrations (C2H4, CO, and CO2) by tuning the wavelength of the lasers across transitions near 1646 nm (i/i +1 9, t 5 + < 9 bands of C2H4), the i 13 transition of CO (3z/ band) near 1564 nm, and the R16 transition of CO2 near 1572 nm 2ui + 2v2 + V9 band). The absorption measurements were recorded in the multipass cell. 

TDLAS measures the absorption of monochromatic light by CO in a multipass flow cell (77). The time response is controlled by the flow of air through the cell. In theory, the instrument is fast enough for application to aircraft measurement of CO flux by eddy correlation. A reported precision of 1 ppbv or 1% is superior to other techniques. The instrument is not commercially available. 

Direct spectroscopic measurements of absorptions could provide substantial and much-needed complimentary information on the properties of BLMs. Difficulties of spectroscopic techniques lie in the extreme thinness of the BLM absorbances of relatively few molecules need to be determined. We have overcome this difficulty by Intracavity Laser Absorption Spectroscopic (ICLAS) measurements. Absorbances in ICLAS are determined as intracavity optical losses (2JI). Sensitivity enhancements originate in the multipass, threshold and mode competition effects. Enhancement factor as high as 106 has be en reported for species whose absorbances are narrow compared to spectral profile of the laser ( 10). The enhancement factor for broad-band absorbers, used in our work, is much smaller. Thus, for BLM-incorporated chlorophyll-a, we observed an enhancement factor of 10 and reported sensitivities for absorbances in the order of lO- (24). 

The analyses were performed on a Jarrell-Ash Model No. 32-360 multipass atomic absorption spectrophotometer equipped with a Beckman total consumption three burner set operated on a hydrogen-air mixture, and Westinghouse single element hollow cathode lamps. 

The more obvious and direct approach would be to develop methods for direct absorption studies of clusters in molecular beams. This would eliminate the need for an action to take place, for example, should the cluster fail to dissociate on the time scale of the experiment. It would also allow for the measurement of the absorption coefficient, a useful quantity for comparison with theory. The challenge here was to overcome the inherent low density of clusters in order to obtain an absorption with reasonable signal to noise. As in the case of conventional absorption spectroscopy, multipassing the laser through the molecular beam was the obvious first step to take. However, two additional technical innovations were necessary before direction absorption studies could be accomplished fast differential optical detection and slit nozzles. The combination of these three methods allowed for direct absorption studies in the mid infrared of van der Waals and hydrogen-bonded... 

Apart from the benefits of the high pressure mixing of SO3 with water to produce sulfuric acid, the proposed cold process for the manufacture of sulfuric acid has also been conceived to avoid the complexity of requiring a sulfur furnace and the related heat recovery system, the multipass static converter, counter current heat exchangers, the interpass absorption tower (IPAT), drying tower (DT), final absorption tower (FAT), mist eliminators, acid coolers, and alkali scrubber. The resulting plant is, as a result, of much lower cost in equipment and land use. 

The first step towards higher sensitivities is the increase of the absorption path length L. This can be for instance achieved when the absorption cell is placed inside a multipass cell. One example is shown in Fig. 1.2. Two spherical mirrors with a high reflectivity R are separated by a distance d which nearly equals the mirror radius r forming a nearly spherical resonator (Vol. 1, Sect. 5.2). 

Fig. 8.24 Schematic diagram of the crossed-jet direct-absorption reactive scattering experiment. Fluorine atoms produced in a discharge puised jet expansion are intersected at 90° 4.5 cm downstream with a pidse of supersonically cooled H2. Tunable single-mode IR laser light is multipassed perpendicular to the collision plane and probes HF(v, J) products by direct absorption [1061]...

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