  Upon progressive refinement of energy resolution, the conventional RIXS
instrumentation reaches the limit where the bandwidth of incident photons
becomes insufficient to deliver an acceptable photon-count rate. We show that
the RIXS spectra as a function of energy loss are essentially invariant to
their integration over incident energies within the core-hole lifetime. This
fact permits the RIXS instrumentation based on the hv2-concept to utilize
incident synchrotron radiation over the whole core-hole lifetime window without
any compromise on the energy-loss resolution, thereby breaking the photon-count
limit.
