Ring Down Method

FIGURE 5.8 (a) Typical decay of resonance fluorescence from atomic chlorine in the presence of CH3SCH3 (8.6 x 1013 molecules cm 3) at 297 K and in 50 Torr N2 as the carrier gas (adapted from Stickel et al., 1992). (b) Typical pseudo-first-order plot of slopes of plots such as those in part (a) against the initial concentration of CH3SCH3 (adapted from Stickel et al., 1992). 

FIGURE 5.9 Schematic diagram of a cavity ring down apparatus (adapted from Paul and Saykally, 1997). 

In the presence of an absorbing gas with absorption cross section a and concentration N, the fractional change in light intensity is given by A = (1 — e aNL) (see Problem 10). This change in intensity occurs in addition to the (1 — R) loss on the mirror and Eq. (Z) becomes 

Thus measurement of the cavity ring down times in the presence and absence of the absorbing gas allows the absorption of the gas to be obtained, and from A = (1 - e aNL) aNL, the concentration of the gas determined. These concentrations as a function of time therefore allow the reaction kinetics of the absorbing species to be determined. 

Typical examples of its application to processes of interest for atmospheric chemistry are the measurement of the kinetics of the reaction of the vinoxy radical with 02 (Zhu and Johnston, 1995) and the kinetics of the C2H5 + C2H5 and C2H502 + C2H502 reactions (Atkinson and Hudgens, 1997). In addition, it has been shown to be useful for probing surface processes as well when combined with total internal reflection techniques (Pipino et al., 1997). 

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Fig. 1.48. Absorption spectra of small gold clusters Au2, Au4, Aug, and Au2o in comparison to the spectra of gold atoms (lower left frame) and nanometer size gold islands (upper right frame). AU spectra were obtained with the cavity ring-down method. The mass-selected clusters were soft-landed onto an a-Si02 substrate. The gold nanoparticles were grown on the same substrate by thermal evaporation...

Figure 2 Setups used for TSM resonators (A) active mode, (B) impedance anaiysis of the osciiiation (passive mode), and (C) ring down method (QCM-D ).

QCM-D technique Kasemo and co-workers have developed an interesting technique that measures the resonance frequency f and the dissipation factor D, which is the inverse of the Q-factor, of the oscillation simultaneously by a ring down method. The quartz plate is excited every second with a frequency generator followed by switching off the source and recording the free decay of the quartz oscillation. The dissipation factor and resonance frequency are obtained from each cycle by a curve fit of an exponentially damped harmonic oscillator function (Figure 2C). 

Cavity Ring-Down Spectroscopy was introduced in 1988 by O Keefe and Deacon as a spectroscopic method for absorption measurements (O Keefe and Deacon, 1988). It is a versatile high sensitivity absorption technique. One of the most essential advantages of CRDS in contrast to usual absorption methods is that the CRDS signal is not affected by intensity fluctuations of the laser since only the decay time of the signal, which does not depend on the laser intensity, is detected. 

A second independent method of direct absorption spectroscopy has been recently applied to clusters cavity ring down (CRD) spectroscopy. This method, where a sample is introduced into the cavity of a high finesse Fabry-Perot interferometer, and is shown schematically in Fig. 3. 

Cavity ring down (CRD) spectroscopy, having proven to be a very sensitive method for detecting molecular species in a wide variety of environments, has also been applied to the mid infrared vibrational spectroscopy of hydrogen-bonded clusters of water " and alcohols.As a direct absorption method, it can be used to quantitatively measure important molecular properties, such as absorption cross sections and coefficients. Knowing these properties, as a function of cluster size and structure, is useful in making the connection to the condensed phase. The sensitive detection of methanol clusters, as shown in Fig. 13, is of considerable importance. These particular measurements nicely complement the action spectra of methanol clusters, detected by depletion of mass-detected signal via vibrational predissociation. 

G. Berden, G. Meijer, W. Ubachs, Spectroscopic AppUcations using ring-down cavities. Exp. Methods Phys. Sd. 40, 49 (2002)... 

The main part of the book presents various applications of lasers in spectroscopy and discusses the different methods that have been developed recently. Chapter 6 starts with Doppler-limited laser absorption spectroscopy with its various high-sensitivity detection techniques such as frequency modulation and intracavity spectroscopy, cavity ring-down techniques, excitation-fluorescence detection, ionization and optogalvanic spectroscopy, optoacoustic and optothermal spectroscopy, or laser-induced fluorescence. A comparison between the different techniques helps to critically judge their merits and limitations. 

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... 

Laboratory trace gas monitoring Discuss and compare the following methods optoacoiistic spectroscopy, cavity ring-down spectroscopy and frequency modulation spectroscopy. 

Methods based on optical absorbance may also be used for air samples. For example, concentrations of gases such as methane and carbon dioxide may be measured by their optical absorbances at certain infrared wavelengths. A recent implementation of this principle for gases is cavity ring-down spectroscopy (CRDS), in which gas concentrations are inferred from fhe rate at which pulses of monochromatic light, tuned to the optical absorbance peak of fhe sample gas, die away in a reflective cavity. 

The general method is to drive in a ring of vertical pipes and pass chilled brine down through an inner pipe so that it flows up the annulus, to cool and eventually freeze the surrounding wet soil. This process is continued until the ice builds up a continuous wall around the proposed excavation. Depths of over 650 m have been excavated in this way. Calcium chloride brine, cooled by surface plant, is usual, but liquid nitrogen has been used on small shafts [50]. 

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