  This paper describes the implementation and performance of CsI(Tl) pulse
shape discrimination for the Belle II electromagnetic calorimeter, representing
the first application of CsI(Tl) pulse shape discrimination for particle
identification at an electron-positron collider. The pulse shape
characterization algorithms applied by the Belle II calorimeter are described.
Control samples of $\gamma$, $\mu^+$, $\pi^\pm$, $K^\pm$ and $p/\bar{p}$ are
used to demonstrate the significant insight into the secondary particle
composition of calorimeter clusters that is provided by CsI(Tl) pulse shape
discrimination. Comparisons with simulation are presented and provide further
validation for newly developed CsI(Tl) scintillation response simulation
techniques, which when incorporated with GEANT4 simulations allow the particle
dependent scintillation response of CsI(Tl) to be modelled. Comparisons between
data and simulation also demonstrate that pulse shape discrimination can be a
new tool to identify sources of improvement in the simulation of hadronic
interactions in materials. The $K_L^0$ efficiency and photon-as-hadron
fake-rate of a multivariate classifier that is trained to use pulse shape
discrimination is presented and comparisons are made to a shower-shape based
approach. CsI(Tl) pulse shape discrimination is shown to reduce the
photon-as-hadron fake-rate by over a factor of 3 at photon energies of 0.2 GeV
and over a factor 10 at photon energies of 1 GeV.
