Single pass absorption

Intracavity enhancement, relative to conventional single pass absorption spectroscopy, is due to mode competition and to threshold effects. A simple calculation of the latter for a single mode laser, starting with... 

Compared with the single-pass absorption of a sample with the absorption coefficient a and the absorption pathlength L2 outside the laser resonator where AP/P = -aL2 = - Ay, the intracavity absorption represents a sensitivity enhancement by the factor... 

The enhanced sensitivity of intracavity absorption may be utilized either to detect minute concentrations of absorbing components or to measure very weak forbidden transitions in atoms or molecules at sufficiently low pressures to study the unperturbed absorption line profiles. With intracavity absorption cells of less than 1 m, absorbing transitions have been measured that would demand a path length of several kilometers with conventional single-pass absorption at a comparable pressure [15, 19]. 

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. 

Two types of water absorption processes are commonly used for the upgrading of biogas single pass absorption and regenerative absorption [48], A high consumption of water is needed in the case of absence of regeneration steps. 

In dimethyl sulfoxide, a single neutral species was detected for both 43 and 44. However, in acetonitrile, an acid-base process between the neutral and an acidic form was detected and characterized through pK values in the ground state and p.A( values in the excited state (44 pA" = 2.0, pAl = 2.3 43 pAT = 2.9, pAl = 4.0). Absorption maxima in the range of 326-304 nm were observed. Fluorescence maxima in the range of 413—469 nm, with relatively high quantum efficiencies (0.3-0.7), were obtained <1995JPH35>. 

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). 

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