Measurements of sub-nanosecond lifetimes by means of nanosecond laser pulses

It is shown that the lifetimes of excited states of molecules which give a reasonably strong absorption (molar decadic extinction coefficient ∈ > 10⁴M^?1cm^?1) at the frequency of the ruby laser can be determined from the saturation of the optical transitions. This is illustrated for DDI (1,1'-diethyl-2,2'-dicarbocyanine iodide) in glycerol, which appears to have a lowest excited state with a lifetime of 110 psec. This value agrees fairly well with that calculated from the formula for the rate of internal conversion derived from Siebrand's work. Under favourable conditions lifetimes as short as 10 psec can be measured.