  Thermal spike model (TSM) is presently a widely accepted mechanism of swift
heavy ion (SHI) - matter interaction. It provides explanation to various SHI
induced effects including mixing across interfaces. The model involves
electron-phonon (e-p) coupling to predict the evolution of lattice temperature
with time. SHI mixing is considered to be a result of diffusion in transient
molten state thus achieved. In this work, we assess this conception primarily
via tuning the e-p coupling strength by taking a series Pd$_{1-x}$Ni$_x$ of a
completely solid soluble binary, and then observing 100 MeV Au ion induced
mixing across Pd$_{1-x}$Ni$_x$/Si interfaces. The extent of mixing has been
parametrised by the irradiation induced change $\Delta \sigma^2$ in variances
of Pd and Ni depth profiles derived from X-ray photoelectron spectroscopy. The
$x$-dependence of $\Delta \sigma^2$ follows a curve that is concave upward with
a prominent minimum. Theoretically, e-p coupling strength 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. L$^2$, however, increases monotonically with $x$ without
any minimum, bringing out a convincing disparity between experiment and theory.
Perhaps some mechanisms more than the TSM plus the transient molten state
diffusion are operative, which can not be foreseen at this point of time.
