Q O

[CH3SCH3] (1013molecules cm 3)

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, f992). (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).

[CH3SCH3] (1013molecules cm 3)

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, f992). (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).

ated by the electron bombardment of H2, H, and OH using HzO and O from C02 radiolysis.

The reaction cell has a White cell optical system (see Chapter ff.A.lc) with a pulsed xenon lamp light source. Once the radicals are formed, they are detected by their absorptions in the UV using the Xe lamp and a monochromator-photomultiplier or photodiode array detector. Thus the absorption spectra of the free radicals generated in the system can be measured and the absorption at a particular wavelength used to follow their reaction kinetics.

5. Cavity Ring Down Method

The cavity ring down method was first described by O'Keefe and Deacon (f988) and has been reviewed by Paul and Saykally (1997) and Scherer et al. (1997). This technique is based on the sequential loss of light intensity as a light pulse repeatedly traverses the length of a cell during multiple reflections between two mirrors. Loss of intensity occurs both during reflection at the mirrors and, if an absorbing gas is present, by its absorption as well. As will be seen shortly, the change in the time profile of the light transmitted through mirror B when an absorber is present can be used to follow the concentration of the absorber and hence to carry out kinetic studies.

Figure 5.9 is a schematic diagram of a typical cavity ring down apparatus. A laser pulse enters a reaction cell that has two highly reflecting mirrors. The distance between the mirrors, L, must be large compared to the pulse width to avoid multibeam interference in the cell. After traveling through the length of the cell the first time, the laser pulse strikes mirror B. If the reflectivity of the mirrors is R (defined as the fraction of light reflected), then (1 - R) is the fraction of light lost by reflection at this surface. If the incident light intensity is /', then the intensity of light lost by the reflection is (1 — R)I'. Assuming that both mirrors A and B have the same reflectivity, then in one round trip in the cell, from mirror B to mirror A and back, the lost intensity is dl ~ 2(1 — R)I'. The time to make this one round trip is tx (where tr = 2L /c, and c is the speed of light). Under these conditions, dl —2(1 — R)I' -(l-R)r dt~ tr ~ tjl or

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