  We have measured the magnetic excitation spectrum of the model square-lattice
spin-1/2 antiferromagnet $\rm Sr_2CuO_2Cl_2$ over a broad range of energy and
momentum using high-resolution inelastic neutron scattering (INS). The magnon
dispersion along the zone boundary was accurately measured to be a 43 meV
between (1/2,0) and (3/4,1/4) indicating the importance of coupling beyond
nearest neighbors in the spin Hamiltonian. We observe a strong momentum
dependent damping of the zone-boundary magnons at (1/2,0) revealing a high
energy continuum of magnetic excitations. A direct comparison between our INS
measurements and resonant inelastic X-ray scattering (RIXS) measurements shows
that the RIXS spectrum contains significant contributions from higher energy
excitations not previously considered. Our observations demonstrate that this
high-energy continuum of magnetic fluctuations is a ubiquitous feature of
insulating monolayer cuprates, apparent in both inelastic neutron and light
scattering measurements.
