Spectral how-to: Continuous photolysis

DRAFT

Continuous photolysis uses Exposure acquisition mode to alternate timed exposure of sample to light with spectral measurements. Read description of Exposure mode. Before setting up Exposure measurement prepare the following:

1. Exposure requires operation of a light source and Exposure shutter. Turn on and prepare light source. Do not forget to adjust light source to desired wavelength after you set up spectral monitoring. Note: startup of Photomax arc lamp sometimes causes a lockup of shutter controller. We do not know why this happens, but it will require restart of controller and possibly the software. To avoid having to do setup twice it is best to start with turning on arc lamp if it is to be used.  
2. If photolysis is to be done cryogenically, follow recommendations on setting up cryogenic temperature control. Reaching and stabilizing target temperature make take some time, set it up in advance.
3. Since spectral measurements in Exposure mode are identical to multiple Single acquisitions, you will need to follow recommendations for Startup and Simple measurements first. 

After preliminary steps have been completed, proceed with setting up exposure measurement.

1. Open Shutter control and set up Timing parameters:
1. Select shutter channel. Photomax source is wired on channel #1, HPD2 source is on channel #2;
2. Setup initial exposure. Value depends on sample being used, but 100-500 ms may be a good starting point. You will need to adjust this value after you know how fast your photoreaction is. Optimal setting will allow to record 5-10 spectra before changes begin to take place. For fast photoreaction exposure as short as 20 ms can be used, for slow reactions it may be several seconds.
3. Setup cycle factor. Values of 1.02-1.05 are acceptable in most cases. Smaller values will record more details but will make overall measurement much longer and size of a dataset larger. Larger value will give a faster “snapshot” of the reaction with lesser resolution. For the initial overview run use short exposure (50 ms) and larger cycle factor value (1.10), but count on repeating this measurement with adjusted parameters.
4. Set the number of blanks. One blank is enough for stable and most cryogenic samples because reference is measured separately. For samples that change in the background relatively fast (drift)  5-20 blanks will work better by providing instant reference that can be later subtracted.
5. Shutter pre-roll can be left at default 500 ms in most cases. Post-roll value depends on the sample and conditions. At room temperature post-roll can usually be short – 500-1000 ms just to allow vibrations to fade. Longer post-roll values are needed in cryogenic condition to allow for fluorescence decay. In preliminary experiments observe and time fading of blue fluorescence immediately after closing of the shutter. Double this time and use it as post-roll delay time.
6. Switch to shutter setup page. Check shutter operation using Open and Close buttons. If slow shutter does not operate in response to commands, check position of switch on a gray shutter control box labeled Oriel – it should be in Ext position. This switch normally should not be changed. If neither shutter respond to commands controller may be locked up. Locate a gray, unlabelled box that usually resides on top of Symphony controller and find connected USB cable. Disconnect USB cable and reconnect after 5-10 sec. Check shutter operation and if it still does not work restart xDSoft.
   After operation is confirmed, make sure shutter is left in closed state.
7. Set occurrence mode to every cycle.
8. Set synchronization mode to Sequential.
9. Return to Timing page, Shutter setup is complete.
2. Set up wavelength of photolysis source. This usually can be done by placing a blank sample and observing Scope signal with photolysis shutter open and probe shutter closed. Do not use your actual sample for wavelength tuning – otherwise you will photolyze it. Small amount of scattered photolysis light will be picked up by optics and appear as a band in the spectrum. Make necessary adjustment to photolysis dispersion optics until photolytic band is at the wavelength of interest. Keep in mind that intensity of photolytic light is not uniform across the spectrum. Intensity of observed band (in scope counts) will be proportional to intensity of photolytic light. Sometimes you may get much higher intensity by shifting position slightly to center on a plasma line of the source.
3. Follow recommendation for setting up a simple measurement. Photolytic measurements are usually done in a difference absorption mode. Therefore, during alignment and optimization you need to use the sample that you will be photolyzing.
   Photolysis typically implies that chemical reaction occurs only while sample is exposed to light. If this assumption is not true for your sample, you need to follow recommendations for recombination kinetics, simple kinetics or other acquisition cycles.
   Since no chemical reactions occur with shutter closed when accumulation takes place, you can setup as long integration as necessary to obtain required S/N ratio. In this case there is little reason to make accumulation of white and dark spectra any longer than sample acquisition. Once Detector/Integrate duration has been set based on intensity in Scope mode, try different number of averages by changing either Detector/AcqCount or Integrate/Average values while observing level of noise in the Absorption mode. Very weak samples may require long accumulations. It is OK to use larger number of acquisition for White/Dark data than sample measurements, but not vice versa.
4. Set up total run time. Enter time in for field of acquisition control. If desired time is not known, enter a negative time, such as -1 – this will start unlimited cycle. Keep in mind that total time can be adjusted during measurement when it becomes clear that initial setting was too shot or too long. It is better to stay on the longer side as you always can stop acquisition or discard unnecessary data.  
5. In the options tab enable display as acquire option. There is no need to suspend display in photolytic studies.
6. Create a new blank document in Destination.
7. Start exposure acquisition cycle from Exposure tab of Acquisition control.
8. Observe the progress of acquisition. Acquisition window status bar will display current information about acquisition. You can also switch to the destination document and analyze current cumulative results. Keep in mind that document is locked for changes while acquisition is on-going. If you want to do evaluation analysis that requires modification, copy current data into a new document and process that copy. Of course, copy will not receive new data.