|The effects of pump pulse fluctuation on ∆abs noise.
The pump pulse fluctuates with the same pattern as Is and, on a longer time scale, it can look similar to Is/Ir, see figure. This is not fully obvious in figure because the pump beam is chopped and the detector readings for the blocked pulses are also plotted. If the ∆abs signal is large relative to the probe noise, the pump pulse fluctuation is the major noise source. Under certain circumstances, this noise can be corrected for by scaling each ∆abs measurement with the corresponding pump pulse intensity. In the case that pump pulse energy fluctuation is the major type of pump noise, improvements in signal-to-noise up to a factor of 5 have been attained. If the pump pulse varies due to beam movements and intensity profile fluctuation, it can not be corrected for in this way. On the other hand, if the ∆abs signal is small relative to the probe noise, pump pulse intensity corrections do not improve the signal-to-noise ratio.