FIGURE 5.9 Schematic diagram of a cavity ring down apparatus (adapted from Paul and Saykally, 1997).
where r0 is a constant known as the cavity ring down time in the absence of an absorbing gas. Figure 5.9 also illustrates the resulting measured signal intensity as a function of time, from which the cavity ring down time is obtained.
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
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