  Fourier-transform Infrared (FT-IR) Spectroscopy measurements have been
performed on the three-dimensional dilute antiferromagnet Fe(x)Zn(1-x)F2 with
x=0.99 ~ 0.58 in far infrared (FIR) region. The FIR spectra are analyzed taking
into account the ligand field and the local exchange interaction probability
with J1 ~ J3; |J1|,|J3|<<|J2|, where J1, J2 and J3 are the nearest neighbor,
second nearest neighbor and third nearest neighbor exchange interaction
constants, respectively. The concentration dependence of the FIR spectra at low
temperature is qualitatively well reproduced by our analysis, though some
detailed structure remains unexplained.
