Microwave field, power dissipated

One last factor to consider is the saturation behavior of the carbon radical - its response to an applied microwave field. At low microwave powers the energy which a carbon radical absorbs from the microwave field is dissipated to its environment in a relatively short period of time in a process called spin-lattice relaxation. At sufficiently high microwave powers this relaxation process is no longer able to transfer the microwave energy absorbed by the spins to the environment rapidly enough for the spins to remain in thermodynamic equilibrium. This... 

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