Changes in Light Intensity

At still shorter time scales other techniques can be used to detenuiue excited-state lifetimes, but perhaps not as precisely. Streak cameras can be used to measure faster changes in light intensity. Probably the most iisellil teclmiques are pump-probe methods where one intense laser pulse is used to excite a sample and a weaker pulse, delayed by a known amount of time, is used to probe changes in absorption or other properties caused by the excitation. At short time scales the delay is readily adjusted by varying the path length travelled by the beams, letting the speed of light set the delay. 

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... 

Recall from Eq. (6.71) that the fractional change in light intensity, dl/I, over a distance x within the absorbing medinm is directly proportional to the linear absorption coefficient,... 

One other problem also arises when the speciation of metal ions is of interest. The nature of the samples with respect to metal ion valence and/or degree of complexation can be expected to vary with changes in light intensity and energy, oxygen content, biological activity, pH, pressure, etc. Thus, the sample would best be maintained at the same dynamic equilibrium conditions from which it was taken, but this would be virtually impossible if a water sample were taken and stored for subsequent analysis. 

Figure 3. Changes in light intensity(A) and temperatures of ambient,culture broth and heat exchanger water in the water bath(B) in the cone-shaped helical tubular photobioreactor outdoor culture experiment (Data of 1996.7.19.)...

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