A Quantitative Model for the Effects on Secondary Ion Emission of Gaseous Absorption at Solid Surfaces Under Noble Gas Ion Bombardment

Abstract

The origin of SIMS “fingerprint” mass spectra [1,2], in which the yields of molecular ions (B_mA_n ^q) of (B)ulk and (A)dsorbate atoms maximize at different degrees of coverage (θ) of adsorbate gas, is explained in a new mathematical model based on the concept of “complex sputtering centres”[3,4]. From statistical reaction kinetics [4] the fractional abundance (N_mn) of the species B_mA_n in all charge states in the “sputtered assemblage” immediately outside the surface depends on the product of the abundances of the constituent atoms raised to the (integer) powers m and n respectively, multiplied by some velocity-averaged constant for that particular molecule, such that we have:

$${{\rm{N}}_{{\rm{mn}}}} = {\left\{ {{N_B}\left( \theta \right)} \right\}^m}.{\left\{ {{N_A}\left( \theta \right)} \right\}^n}.\left( {{\rm{constant for the molecule }}{{\rm{B}}_m}{A_n}} \right).$$

. (1)