The Vernier Flash Photolysis Spectrometer is a time-resolved spectrometer designed
for measuring photochemistry in the college classroom. In the teaching
laboratory, kinetic studies usually involve mixing two solutions which starts
the reaction of interest. The reaction is then followed in time by monitoring the
change in a physical property of the reaction system. This process is suitable
for reactions which take place on the scale of minutes
or longer, but many reactions take place much faster than this and needs special
instrumentation to examine. Examples of these reactions include excited-state
dynamics, triplet-state phosphorescence, isomerization, reactive free radicals and
photocatalysis. The Vernier Flash Photolysis Spectrometer is an example of
a specialized instrument used for studying these fast light-induced reactions.
A block diagram of this system appears on the top of the instrument.
In the system, a xenon flash lamp provides the photoexcitation pulse.
Then white light from an LED source probes any spectral changes made in the system by
the excitation light pulse. The white light from the LED source is focused on a
sample. From the sample, this beam goes through a wavelength filter while being
focused on a photodiode which detects the light intensity. If the flash lamp causes
changes in the absorption at the filter's wavelength, the detector
will measure these changes. The voltage from the detector is collected,
digitized and stored versus time in a CSV file which may be analyzed in Logger Pro
software. To use the Vernier Flash Photolysis Spectrometer, you first need
to download the free software from our website. From here, you can see the
photodiode level, the type of experiment, the time window or time duration and the
number of samples to average. The reaction I will be demonstrating today
is the excited state Cis-Trans isomerization of a substituted
carbon-carbon double band in the Congo red dye. This is one of the free experiments
available in the full reference manual on our website. I've prepared the sample as
described in the experimental write-up. The instrument needs to be plugged into
AC power and into the USB of the computer and turned on. At this point,
the photodiode level should be at max about two and a half volts. I'm going to
be using the 600-nanometer filter that ships with the device and once I set it
in the filter slot, the photodiode level should drop to about half.
I can then insert my Congo red sample into the sample cell and the photodiode level
should drop a little bit further. The time window for this experiment should be one
second. For preliminary runs, 10 averages should be sufficient.
The more averages you collect, the less noisy your data will be.
Now I'm ready to collect data by pressing Run.
Here is the kinetic trace for the photoisomerization
of Congo red. By looking at the trace, I can estimate the time constant
to be about 800 milliseconds. I can save this data as a CSV file and import it into
Logger Pro for analysis including exponential fits to get more precise time
constants. Once I've saved this data as the experiment describes,
this Cis-Trans isomerization can be further catalyzed by adding a base to
the solution. My next sample contains two millimolar sodium hydroxide.
If I put that sample in the instrument and measure the isomerization
on the same time scale, you'll see that it happens much faster.
In fact, it happens in less than a hundred
milliseconds. This instrument is great for physical chemistry,
biochemistry, inorganic chemistry and instrumental analysis courses.
It is also ideal for student projects. For more information, see our website or email
chemistry@vernier.com.