The apparatus for flash photolysis studies is shown schematically in the figure.
The sample is subjected to a very powerful flash having energy, of the order of 10⁵ and a duration of 10μ seconds.
The flash is so intense that in some cases almost all the molecules of the sample are excited and as a result, most of the molecules are dissociated into free radicals.
Powers of the order of 50 megawatt may be obtained for a few microseconds.
In recent years, high power pulsed laser light sources have been used in flash photolysis.
The concentration of the intermediates produced by flash photolysis is followed as a function of time absorption photometry.
A shown in figure, a condenser of high micro farad capacity is charged to 10,000 volt by a high voltage supply.
By means of a trigger signal, a spark is produced in the spark gap which permits rapid passage of current through the discharge lamp.
The condenser discharge in a few micro seconds.
The lasers can produce a flash of the duration of a few nanoseconds.
Soon after the capacitor discharges, a flash lamp which can produce analysing light, is automatically triggered off.
The rate of the disappearance of excited molecules or free radicals is followed by the rate of increase in the monochromatic transmitted light as measured with the oscilloscope and photomultiplayer.
The spectrometer is so set that the light passing through the illuminated cell is of the wavelength that is observed by the excited molecule or free radical.
This technique is called kinetic spectroscopy.
Flash, photolysis has been used for determining the absorption spectrum of free radicals such as .NH₂ .CH₃ and .ClO whose concentration may be as small as
10⁻⁶ M.
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