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On the CCD scope the spectrum of the white light does not have the same shape along the delay track. Our VIS detector itself is responsive up to 1000 nm. The spectral range of detection is determined If it the white light spectrum is not the same throughout the delay track range it means that your delay stage needs to be realigned. Please refer to the probe alignment section of the manual. If the delay stage is misaligned the 800 nm beam focus walks off on the sapphire crystal. This results in the displacement of the origin of the white light. Therefore the white light that gets into the fiber is not the same for the different positions of the delay stage. We did some experiments with the samples that we know very well and we noticed that the kinetics are distorted. If the kinetics appear to be distorted (non-exponential, wrong lifetimes) it usually means that the delay stage needs to be realigned. Please refer to the probe alignment section of the manual. If the delay stage is misaligned the 800 nm beam focus walks off on the sapphire crystal. When this happens the focal spot of the white light in the sample gets displaced as well. Therefore the overlap between pump and probe changes as you scan the delay track. This will result in distorted, usually non-exponential, kinetic profiles. Another possible reason for weird kinetic profiles is sample degradation. Why in some experiments are we obtaining transient spectra that appear to be inverted? This problem is most likely related to the alignment of the pump beam in the chopper. If the beam is positioned closer to either side of the chopper aperture rather than being central, the synchronization becomes half a period out of phase, p, which means the signal is negated. If the synchronization is p/2 (quarter period) out of phase, the signal becomes close to 0 (very low or no signal). The practical advice is, when you change the pump wavelength or anything else in the pump beam alignment, make sure it goes through the center of the chopper aperture. If you completely open the iris before the chopper, you should be able to see two thin lines around the center of the hole, the size of one sector of the blade; the reason you see them in a constant position is that the chopper is synchronized with the laser. Make sure the beam is centered between the two lines, and reduce the iris aperture so that beam diameter is not larger than the distance between the two lines. We are seeing a negative signal below 500 nm (which exists before time zero and remains constant for the whole experiment). The signal you are seeing can come from two sources. Some data files may have a non-zero spectral baseline, which appears throughout the whole data set even before t0. Such background is usually a result of long lived species present in the sample (species with lifetime longer or on the order of the temporal interval between the laser pulses). Additionally such background may be a result of thermal lensing caused by the excitation beam. Simply subtracting the pre-zero signal from the data set is acceptable in such case. Please refer to the Subtract Background section of the Surface Xplorer Pro manual. In some cases your detector picks up scattered light from the sample. This often happens when the excitation wavelength falls into the probing range of your experiment. To correct for such scattered pump or scattered emission (not stimulated emission) it is not correct to simply subtract the pre-zero background from the data, particularly if the amount of scattered light is significant. The corrected transient absorption is expressed as follows:  A=-log(10 -S-10 -S(t<0)+1) Where S is the experimentally observed A, and S (t<0) is the average experimentally observed A before time zero. Please refer to the Subtract Intensity Background section of the Surface Xplorer manual. Would is be possible to have both the CaF2 and sapphire crystals to be able to span a probe range of 350 to 900nm? How difficult would it be to swap the two crystals? You cannot use CaF2 and sapphire simultaneously. Additionally the CaF2 plate needs to be constantly translated during the experiment in order to prevent thermally induced instabilities. However switching between the two substrates is very easy. Normally Silicon detectors are sensitive up to 1100 nm, why is your system cut at 800nm? Our VIS detector itself is responsive up to 1000 nm. The spectral range of detection is determined mainly by the grating we use. We normally configure the VIS spectrometer to cover 350-950 nm, the range of the white light continuum. It is also possible to move this range by rotating the grating. In an actual transient absorption experiment you will need to use a short-pass filter to cut off the residual 800 nm light in the continuum. This is what limits the detection range to 800 nm. It is possible to put a different grating into the spectrograph in order to extend the spectral range. For available grating options refer to our web site http://www.ultrafastsystems.com/high_speed_spectrometer.htm .
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