Two-magnon scattering and the spin-phonon interaction beyond the adiabatic approximation

We consider a model of Raman scattering for a two-dimensional {ital S}=1/2 Heisenberg antiferromagnet which includes a {ital dynamical} spin-phonon interaction. We observe a broadening of the line shape due to increased coupling with excited high-energy spin states. Our results are close to a model of random, static exchange interactions, introduced in this context by Haas {ital et} {ital al}. [J. Appl. Phys. {bold 75}, 6340 (1994)], which, when extended to large numbers of spins, explains experiments in the parent insulating compounds of high-{ital T}{sub {ital c}} superconduc- tors. {copyright} {ital 1996 The American Physical Society.}