Scaled apparent charge density

In the low-charge-density Debye-Hiickel limit, the scaled apparent charge density si (and S2) reduces to the scaled charge density di of Eq. [201]. 

We can estimate the time scale in which the whole surface layer is affected by the primary ions. The lifetime of a surface may be simply estimated from the primary ion flux (Ip) and damage cross-section (er) generated by each impact. Ip is commonly measured in A cm-2 (1 A = 6.2 x 1018 charged particles per second). Assume that each primary ion generates a = 10-13 cm2. Then, 1013 primary ions cm-2 will affect the whole surface area of 1 cm2. It means that the lifetime of a surface with the flux density Ip= 1 pA cm-2 (= 6.2 x 1012 ions cm-2) is less than 1 second. Apparently, 1 p A cm-2 of flux density for primary ions is too high for static SIMS. Since it is commonly accepted for the static SIMS condition to limit the total amount of primary ions up to 1013 ions cm-2, for a 10-min duration of static SIMS examination a primary flux density of about 2.7 nA cm-2 is required to preserve the chemical structure of the surface top layer where the secondary ions are emitted. This flux is extremely low compared with that of dynamic SIMS, which requires a flux density of greater than 1 pA cm-2 to ensure a reasonable erosion rate of surface for depth profiling. 

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