Cavity-enhanced absorption techniques

Cavity-enhanced absorption techniques are also applied in flames in the near infrared [22-25]. The multiple absorption paths provided with these techniques are the reason for their superb sensitivity in the ppb range. Absolute concentrations can be obtained, provided the absorption coefficient for the respective transition is known. CRDS can be used in conjunction with other laser-based combustion diagnostics for... 

An extremely sensitive technique able to detect the nature of radical pairs in a photochemical reaction is called chemically induced dynamic nuclear polarization (CIDNP), which depends on the observation of an enhanced absorption in a nuclear magnetic resonance (NMR) spectrum of the sample, irradiated in situ, in the cavity of a NMR spectrometer. The background to and interpretation of CIDNP are discussed by Gilbert and Baggott (28). 

Instead of absorption, weak emission lines can also be detected with the intracavity techniques [26]. If this light is injected into specific modes of the multimode laser, the intensity of these modes will increase for observation times t mode-coupling with other modes. Cavity-enhanced spectroscopy in optical fibres have been reported in [32, 33]. 

The sensitivity can be further enhanced by frequency modulation of the laser (Sect. 1.2.2) and by intracavity absorption techniques. With the spectraphone inside the laser cavity, the photoacoustic signal due to nonsaturating transitions is increased by a factor as a result of a -fold increase of the laser intensity inside the resonator (Sect. 1.2.3). The optoacoustic cell can be placed inside a multipath optical cell (Fig. 1.28) where an effective absorption pathlength of about 50 m can be readily realized [74]. 

New absorption methods, like intracavity spectroscopy, cavity-ring-down and cavity-enhanced spectroscopy, have demonstrated very high sensitivities in laboratory measurements with DLs. An ultrasensitive technique that combines external cavity enhancement and FM spectroscopy has been developed recently. This method, which has been called NICE-OHMS. or noise-immune cavity-enhanced optical heterodyne molecular spectroscopy, is based on frequency modulation of the laser at the cavity free-spectral-range frequency or its multiple. The MDA of 5x 10 1 X 10 cm ) in the detection of narrow... 

The third method is based on the sensitive dependence of the laser intensity on absorption losses inside the laser resonator. When the absorbing sample is placed inside the laser resonator, the laser intensity I decreases by Al = q a(o))xI where the factor q which gives the enhancement of the intensity change compared with the same absorption outside the cavity, can become very large. This "intracavity absorption" technique will be discussed in Sect.8.2.3. 

The setup for ESR spectroscopy is a cross between NMR and micro-wave techniques (Section 5.8). The source is a frequency-stabilized klystron, whose frequency is measured as in microwave spectroscopy. The microwave radiation is transmitted down a waveguide to a resonant cavity (a hollow metal enclosure), which contains the sample. The cavity is between the poles of an electromagnet, whose field is varied until resonance is achieved. Absorption of microwave power at resonance is observed using the same kind of crystal detector as in microwave spectroscopy. Sensitivity is enhanced, as in microwave spectroscopy, by the use of modulation The magnetic field applied to the sample is modulated at, say, 100 kHz, thus producing a 100-kHz signal at the crystal when an absorption is reached. The spectrum is recorded on chart paper. 

LIF (Ezekiel and Weiss, 1968 Cruse, et al., 1973 Zare and Dagdigian, 1974 Kinsey, 1977) is an example of an indirect technique for the detection of a one-photon resonant upward transition. There are many other indirect detection techniques (optogalvanic, optothermal, photoacoustic, cavity ringdown), but Multi-Photon Ionization (MPI) is a special type of indirect technique uniquely well suited for combining absorption detection with other useful functionalities (see Section 1.2.1.1). In MPI, photo-ion detection replaces photon detection. The one-color, singly-resonant-enhanced (n + m) REMPI f process consists of an n-photon resonant e, v, J <— e",v",J" excitation, followed by a further nonresonant m-photon excitation into the ionization continuum... 

The CRDS technique uses the same principle as intra-cavity spectroscopy, namely increasing the effective absorption path length. The difference is that in CRDS the absorption coefficient is determined from a time measurement, i.e., the decay time of the ringing cavity , while in intra-cavity spectroscopy the gain competition between different resonator modes is used as the enhancement factor. 

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. 

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