Cavity Ring-Down Spectroscopy CRDS

1 Doppler-Limited Absorption and Fluorescence Spectroscopy with Lasers 

The time delay between successive transmitted pulses equals the cavity round-trip time Tr = 2L/c. The nth pulse therefore is detected at the time t = 2 L/c. If the time constant of the detector is large compared to Pr, the detector averages over subsequent pulses and the detected signals give the exponential function 

Without an absorbing sample inside the resonator a = 0), the decay time of the resonator will be lengthened to 

The absorption coefficient a can be obtained directly from the difference 

What level of accuracy can be achieved for the determination of a Assume that the decay times t, can be measured within an uncertainty 5t,-. From (1.31) we then obtain for the uncertainty Sa of a 

During the last ten years a new, very sensitive detection technique for measuring small absorptions, cavity ring-down spectroscopy (CRDS), has been developed and gradually improved. It is based on measurements of the decay times of optical resonators filled with the absorbing species [6.31]. We can understand its general principle as follows  

Assume a short laser pulse with input power Pq is sent through an optical resonator with two highly reflecting mirrors (reflectivities R = R2 = R, and transmission T = 1 — R — A 1, where A includes all losses of the cavity from absorption, scattering, and diffraction, except those losses introduced by the absorbing sample). The pulse will be reflected back and forth between the mirrors (Fig. 6.12), while for each round-trip a small fraction will be transmitted through the end mirror and reach the detector. The transmitted power of the first output pulse is 

The experimental setup is shown in Fig. 6.13. The laser pulses are coupled into the resonator by carefully designed mode-matching optics, which ensure that only the TEMqo modes of the cavity are excited. Diffraction losses are minimized by spherical mirrors, which also form the end windows of the absorption cell. If the absorbing species are in a molecular beam inside the cavity, the mirrors form the windows of the vacuum chamber. For a sufficiently short input pulse (Tp 7r), the output consists of a sequence of pulses with a time separation of Jr and with exponentially decreasing intensities, which are detected with a boxcar integrator. For longer pulses (Tp 7r), these pulses overlap in time and one observes a quasi-continuous exponential decay of the transmitted intensity. Instead of input pulses, the resonator can also be illuminated with cw radiation, which is suddenly switched off at f = 0. 

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The fact that the transmitted intensity decreases exponentially with time forms the basis of cavity ring-down spectroscopy (CRDS). 

In contrast, absorption spectroscopy allows for more direct and more accurate, absolute concentration and temperature measurements. An increasingly popular approach for the detection of (minor) species in flames is cavity ring-down spectroscopy, CRDS. It combines the advantage of common absorption techniques, i.e. the direct determination of number density, with an effective absorption path of up to a few kilometres. Therefore, the detection of species of very low concentrations is possible. For a description of the principles of CRDS see Section 7.2. A typical experimental setup for the quantitative measurement of species concentration and temperature in flames is shown in Figure 29.7. 

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. 

Fluorescence (LIF), Cavity Ring Down Spectroscopy (CRDS)/ Photoacoustic Spectroscopy (PAS) and Quartz Crystal Microbalance (QCM), which do not have all the advantages possessed by electrochemical methodologies. In this chapter we consider those applying dynamic electrochemistry rather than potentiomet-ric or those similarly related using resistivity. 

Materials CRDS cavity ring down spectroscopy... 

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. 

CRDS Cavity ring-down spectroscopy OPO Optical parametric oscillator... 

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. 

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