Minimum detectable absorption , laser

With conventional spectroscopy for a pathlength L = 10 cm and = 10Aco we obtain = 10 cm With single-mode lasers one may reach Au> < Sm and a long absorption path with L = 10 m, which yields a minimum detectable absorption coefficient of ofmm — I0 cm i.e., an improvement of a factor of 1000 ... 

Laser PAS has been used to monitor air samples. Minimum detectable concentrations of gaseous pollutants are often in the parts per billion (ppb) or sub-ppb range depending on the molecular absorption cross section and on possible absorption interferences. Most studies have been devoted to investigations on collected air samples of different origin. A CO laser-based PAS has been used for analysis of vehicular exhausts. A number of compounds such as nitric oxide, nitrogen dioxide, H2O vapor, and other volatile organic compounds such as alkenes, aromatic hydrocarbons, and aldehydes have been reported. A mobile PAS system has been developed for analysis in the field. 

If near-infrared diode lasers have low-noise characteristics similar to those of mid-infrared diode lasers, and thus minimum absorbances of 10 5 or less are possible, then an approximate detection limit can be calculated for an absorption experiment. For a 200-m optical path, the calculated detection limit is 5 x 1010 molecules/cm3, which is well above levels of H02 expected to be found in the atmosphere. An absorption experiment in this spectral region apparently would require extremely long optical path lengths, and, indeed, a calculation with a 5-km path yields a calculated detection limit of 2 x 109 molecules/cm3, still rather high for tropospheric measurements. Other issues associated with the use of diode lasers in absorption spectroscopy are discussed in the next section. 

We have seen in Sect. 1.4.1 that resonant two-photon ionization can reach a detection efficiency of 100 %, which means that every absorbed photon can be detected. The question is now how few molecules can be still detected According to Eq. (1.4a) the minimum number density of detectable absorbers is Ni > AP/(PoctikL), where L is the absorption length and AP the absorbed power. If single absorbed photons can be detected, AP = h-v and we obtain for the incident laser power Pq = n hv... 

---