Lecture 27 : Experimental methods in chemical kinetics
27.4
Flash Photolysis:
Introduced by Norrish and Porter in 1950, this method was one of the early departures from the classical methods. A Schematic diagram illustrating the principles involved is given in Fig 27.3.
Figure 27.3 A flash photolysis apparatus.
The reaction is initiated by an intense flash of visible or ultraviolet light generated by a photoflash lamp. The duration of the flash, about 5 to 15 microseconds (s) is shorter than the reaction time scale. The intermediates generated can be probed in real time using spectroscopic methods. The absorption spectrum of the intermediate has to be determined at fixed intervals after the initial flash. This is best accomplished by repeating the experiment after different time delays.
The reaction cell (RC) is a quartz cylinder of length up to 1 m. Both the ends of the cylinder are plane windows. The photoflash lamp tube is parallel to the reaction cell. Suitably placed reflectors increase the intensity of light falling on the sample. The photoflash lamp has a capacitor connected to the two electrodes of the lamp. For short flashes (ns) the energy dissipated from the lamp is small (J, microjoules) for millisecond flashes, the energy dissipated is several hundred joules.
The absorption spectrum of intermediates/products is obtained using a second lamp (the spectroflash) at one end of the reaction cell. The spectroflash energy and duration have to be adjusted so that it does not interfere with the ongoing photochemistry in the reaction cell, but has sufficient intensity to be detected in the detector, kept at the other end.
Today, pulsed lasers have replaced the conventional photoflash lamps, as they give light with higher power and of a much greater time resolution. This is in addition to the coherent and monochromatic light beams of the laser. The trigger enables in starting the discharge of the capacitor as well as timing the spectroflash output signals of the spectrum. Laser flash photolysis has been used to study a number of reactions including radical radical recombinations (e.g., oCH3+
oCH3 C2 H6 ), unimolecular decay, atom - atom, atom - molecule and radical - molecule reactions. It has also helped in detecting excited states of atoms, molecules, ions and radicals.
s) is shorter than the reaction time scale. The intermediates generated can be probed in real time using spectroscopic methods. The absorption spectrum of the intermediate has to be determined at fixed intervals after the initial flash. This is best accomplished by repeating the experiment after different time delays. 
C2 H6 ), unimolecular decay, atom - atom, atom - molecule and radical - molecule reactions. It has also helped in detecting excited states of atoms, molecules, ions and radicals. 
