  We propose a novel mechanism for exchange-induced exciton-magnon absorptions
via hopping between two antiferromagnetically coupled sites and a simultaneous
magnetic dipole transition to an excited orbital state. The obtained selection
rules correspond to ones for magnetic dipole transitions and are in agreement
with the exciton-magnon transitions observed in the quasi one-dimensional
Heisenberg antiferromagnet KCuF3. The calculated magnon density of states in
combination with a structure dependent factor identifies the observed optical
magnon sideband to originate from transverse magnon modes.
