Assessing thermal spike model of swift heavy ion-matter interaction via Pd$_{1-x}$Ni$_x$/Si interface mixing

Thermal spike model (TSM), a widely accepted mechanism of swift heavy ion (SHI) - matter interaction, provides explanation to 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$_{1-x}$Ni$_x$ of a completely solid soluble binary, and then observing Pd$_{1-x}$Ni$_x$/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 $\Delta \sigma^2$ 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 TSM, and then an equivalent quantity L$^2$, proportional to $\Delta \sigma^2$, has been calculated. Both $\Delta \sigma^2$($x$) and L$^2$($x$) show non-linearities, albeit in slightly dissimilar ways, leading to a conjecture that the present work at least does not invalidate the TSM.