The mid-infrared spin-wave spectrum of antiferromagnetic YBa2Cu3O6.0 w
as determined by infrared transmission and reflection measurements (k
parallel to c) at T = 1OK. Excitation of single magnons of the optical
branch was observed at E(op) = 178.0 meV. Two further peaks at 346 me
V (approximate to 1.94E(op)) and 470 meV (approximate to 2.6E(op)) bot
h belong to the two-magnon spectrum. Linear spin-wave theory is in goo
d agreement with the measured two-magnon spectrum, and allows to deter
mine the exchange constant J to be about 120 meV, whereas the intrabil
ayer coupling J(12) is approximately 0.5J.