对于自由电子，因此没有什么势能。

It is a free electron so there is no potential energy to be accounted for.

这是附加在自由电子的普通吸收之上的。

This is in addition to general absorption characteristic of the free electrons.

这种处理方法也适用于自由电子激光器的谐波辐射。

This method can also be used with the harmonic wave radiation in FEL.