  Quantitative knowledge of electron-phonon coupling is important for many
applications as well as for the fundamental understanding of nonequilibrium
relaxation processes. Time-resolved diffraction provides direct access to this
knowledge through its sensitivity to laser-induced lattice dynamics. Here, we
present an approach for analyzing time-resolved polycrystalline diffraction
data. A two-step routine is used to minimize the number of time-dependent fit
parameters. The lattice dynamics are extracted by finding the best fit to the
full transient diffraction pattern rather than by analyzing transient changes
of individual Debye-Scherrer rings. We apply this approach to platinum, an
important component of novel photocatalytic and spintronic applications, for
which a large variation of literature values exists for the electron-phonon
coupling parameter $G_\mathrm{ep}$. Based on the extracted evolution of the
atomic mean squared displacement (MSD) and using a two-temperature model (TTM),
we obtain
$G_\mathrm{ep}=(3.9\pm0.2)\cdot10^{17}\frac{\mathrm{W}}{\mathrm{m}^3\hspace{1pt}\mathrm{K}}$
(statistical error). We find that at least up to an absorbed energy density of
$124\hspace{2pt}\frac{\mathrm{J}}{\mathrm{cm}^3}$, $G_\mathrm{ep}$ is not
fluence-dependent. Our results for the lattice dynamics of platinum provide
insights into electron-phonon coupling and phonon thermalization and constitute
a basis for quantitative descriptions of platinum-based heterostructures in
nonequilibrium conditions.
