Hahn echo experiments on a single triplet electron spin

We present results on Hahn echo experiments with a single molecular triplet electron spin. The surprising possibility of observing an echo phenomenon with an individual electron spin arises as a result of a spectral diffusion process of the magnetic resonance transition frequency in the course of the optical excitation cycle and illustrates the equivalence of time and ensemble average. The spin dephasing times as measured by the decay times of the Hahn echo for different molecules varies between 2 and 7 μs. By using standard theories of nuclear spin diffusion these differences may be attributed to variations in the local environment close to the chromophore which can be estimated quantitatively.