ENDOR - Relaxation

energy level relaxation With ENDOR, the system with one electron and one nucleus of spin ω has four spin-lattice relaxation times controling the populations of the energy levels. This contrasts with normal EMR spectrum where only the one electron spin-lattice relaxation time is important, and unless this is very short measurements between 77K and 300K are possible.

In order to measure an ENDOR spectrum, it is necessary to (at least partially) saturate the EMR signal, and also for the nuclear relaxation to compete with the electron relaxation. This is usually best achieved by working at temperatures below 30K. This has the effect of lengthening the electron relaxation time so that the signal is easier to saturate.

The optimum power for saturation of the EMR signal, so that a steady state ENDOR spectrum can be observed can be obtained via the Bloch equationsand is given by ttgB3.gif - 1351 bytes (Atherton, Principles of Electron Spin Resonance, Ellis Horwood, 1993, p402), involving the spin lattice and spin-spin relaxation times and the microwave power for the electron system.