Sputtering yields, definition

It is more convenient, especially in the surface analysis field, to define preferential sputtering using the surface versus the bulk composition. At steady state an ion beam simply moves the atoms of a solid from one place to another. The composition of the sputtered flux must be equal to the bulk composition at steady state. From the definition of the sputter yield, the surface concentration can be related to the bulk... 

The sputter yield defines the absolute number of atoms, ions, or molecules removed per impinging primary ion. As discussed in Section 3.2.2, this absolute value is differentiated from the analytical value to avoid confusion. The absolute definition is used throughout this text unless otherwise specified. 

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