MODULE 22 (701) Absorption Spectrophotometry of Excited States S1S1 S0S0 T3T3 T2T2 T1T1 S 1  S n T 1  T n

 is the molar decadic extinction coefficient (units of M -1 cm -1 ) at the wavelength, and ℓ is the optical path length of the sample. When the species concentration changes with time, then Hence the time profile of the change in A  is directly related to the time profile of the absorber concentration. MODULE 22 (701) The underpinning of all absorption measurements, time-resolved or not, is the Beer-Lambert law

MODULE 22 (701) A (t) is obtained by monitoring the time dependence of the change in the transmitted light intensity, I(t), at a constant value of I 0. When I 0 cannot be kept constant, its value must be determined for every I(t) value. I(t) is a light fluence quantity which is converted to electrical charge by a photosensitive detector (e.g. PMT). The measurement we make is a voltage time profile, V(t), which is referenced to V 0, derived from I 0. Thus From the time dependence of V, thence of A and eventually of c, the rate parameters being sought can be determined.

MODULE 22 (701) There are two ways of obtaining absorptiometric information. 1: Continuous photoelectric method: A photo-detector (e.g. PMT) monitors the intensity of the transmitted light through the sample at particular wavelength ( ) in a continuous manner, before, during, and after the initiating light pulse is absorbed. The detector output is fed to the input amplifier of a waveform- recording device such as a digital oscilloscope (ADC). In this way recording of the V(t) profile is done in real time. Repeating the process over a series of wavelengths allows the investigator to build up the dynamic surface (A(,t)) of the photo- induced transient.

MODULE 22 (701) Schematic of nanosecond laser flash photolysis instrument

Wavelength / nm Time / ns MODULE 22 (701) An A(,t) surface

MODULE 22 (701)

A major limitation of photoelectric recording is the nanosecond barrier. From the intrinsic time response of the electronic devices that are used to process the output current of the detector. All such devices have impedance and even the best-designed circuitry has stray capacitance of typically 20 pF. Combining this with the 50  industry-standard of high bandwidth electronic amplifiers, yields a RC time constant of 1 ns. Hence instruments that are built from conventional electronic units will have waveform rise times in the ns region and therefore absorption changes having lifetimes in this time regime will be severely deformed. We need to find ways around this problem.