THEORY OF NEUTRON-SCATTERING BY EXCITONS

Using neutron sources giving particles with energies of a few electron volts it is possible that the dispersion of exciton energy bands could be measured directly. Estimates of the cross sections for creation of excitons of both the Frenkel and Wannier type are given. The former give values similar to those for spin wave or electronic transitions but the electronic form factor makes these very small if the momentum transfer Q lies outside the first Brillouin zone. For Wannier excitons the cross section is always reduced by a factor giving the probability that the electron-hole pair are confined to the same atom. At large Q a process via the creation of virtual phonons gives a larger cross section is always reduced by a factor giving the probability that the electron-hole pair are confined to the same atom. At large Q a process via the creation of virtual phonons gives a larger cross section since it avoids the electronic form factor.