Multipass absorption techniques

Besides the issue of user-friendly alignment, the major design requirements for any multipass cell can be summarized as follows  

Nowadays, three basic designs of multipass cells are used in tuneable laser absorption instruments, i.e. plane-plane mirror designs. White cell designs and the Herriott cell designs. 

Alignment easy difficult relatively easy easy 

Path length poor medium long very long 

Path length adjustment very easy easy difficult relatively easy 

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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. 

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). 

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