MATERIALS SPECIFICITY, QUANTUM LENGTH SCALES, AND STOPPING POWERS

Standard arguments, based primarily on behavior at high projectile ene rgies and classical notions of thickness as a continuous parameter, as sert that stopping powers are only modestly affected by target chemist ry and aggregation and -by sample thickness if the thickness is ''suff iciently small'' (and channeling is avoided). Sufficient thinness usua lly is defined in terms of freedom from multiple scattering and from p rojectile charge-state changes. The growing technological importance o f extremely thin material layers (e.g. microelectronics) has motivated re-examination of both the thickness and aggregation-state assertions . We give arguments to show that both are inadequate and reappraise re cent computations in confirmation. A particular focus is the proper de finition of thickness for an ultrathin film of nu atomic planes (nu = 1, 2, 3....).