Quantitative correlation of XPS linewidth with dislocation density in shock‐loaded ammonium perchlorate

The linewidths of x‐ray photoelectron spectra have been correlated with dislocation densities in a shock‐damaged crystal of ammonium perchlorate (AP). A centimeter‐size AP crystal was non‐uniformly damaged by a rapidly decaying shock (peak pressure of 24.4 kbar at the entry surface), recovered intact and cleaved. The cleaved planes permitted interior analysis of the crystal by x‐ray photoelectron spectroscopy (XPS) over a pattern of 1 mm × 1 mm areas. The linewidth of the Cl 2p 3/2 peak ranged from 1.70 eV for the region of greatest damage to 1.22 eV for the region demonstrating no visible damage, the same linewidth as that for unshocked AP (control). The correlation between dislocation density and XPS linewidth was quantitatively established by chemically etching and counting dislocations on cleaved planes from the shocked crystal. By this technique, a 100 × increase in dislocation density was determined for the region of greatest shock damage relative to undamaged crystal. Molecular orbital energy variation caused by distortion of the lattice in the vicinity of a dislocation is the physical mechanism responsible for the broadening of the photoelectron lines. Ab initio calculations of the Cl 2p energy level from various distorted perchlorate anion geometries have predicted orbital energy shifts sufficient to produce the observed linewidth broadening.