Assessing a thermal spike model of swift heavy ion-matter interactions via Pd_1-xNi_x/Si interface mixing

The thermal spike model (TSM), a widely accepted mechanism of swift heavy ion (SHI)-matter interactions, provides explanation for various SHI induced effects, including mixing across interfaces. We assess the validity of the model via tuning the electron-phonon coupling strength (G) by taking a series Pd_1-xNi_x of a completely solid soluble binary, and then observing Pd_1-xNi_x/Si interface mixing induced by a combination of 100 MeV Au ion irradiation and 4 keV Ar ion sputtering. If the TSM truly describes the SHI-matter interaction mechanism, any non-linearity in x-variation of G must also result in a similar non-linearity in the x-dependence of mixing. Experimentally, the extent of mixing has been parametrized by the irradiation induced change Δσ² in variances of Pd and Ni depth profiles derived from XPS. Computationally, G determined using density functional theory has been used to solve the equations appropriate to the TSM, and then an equivalent quantity L², proportional to Δσ², has been calculated. Both Δσ²(x) and L²(x) show non-linearities, albeit in slightly dissimilar ways, leading to a conjecture that the present work at least does not invalidate the TSM.