Experimental parameters and instrumentation

the proportionality constant c(A,r) is time-independent and drops out, as first noted by Bilderback et al (1984) and used by Hajdu et al (1987b). F(h,0) may be quantitated in the Laue experiment itself if c(A,r) has been determined, or it may have been obtained in a prior monochromatic experiment. A time-dependent difference Fourier will utilise [ F(h,r) — F(h,0) ] as coefficients to yield Ap(t), the change in electron density as a function of time. 

A basic weakness of the difference ratio method is that each coefficient involves a division by FL(h,A,0), which will very often be weak or even zero, leading to a large error in the coefficient. In the earlier methods, where the A-curve is explicitly determined, only the strong intensities in the pattern are used hence a reflection is scaled by a factor determined from these more precise measurements alone. 

There are also practical problems with the difference ratio method if c(A,r) is not strictly constant. It is essential to maintain the crystal in a fixed orientation during the sequence of photographs. In practice, this can be very difficult to do. Hence, for all these reasons the method based on explicitly deriving a A-curve from symmetry related reflections (Campbell et al 1986) is nearly always used rather than the difference ratio method. 

From the above considerations we can see that the relevant experimental parameters are  

To achieve the short wavelength may require use of a wiggler in the case of a relatively low energy machine. The use of a multipole device also leads to further reductions in exposure time. However, even a multipole wiggler has some undulator character, i.e. has a long fundamental wavelength of emission and so the smoothness of the emitted spectrum needs to be checked in a given case. 

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