ESR studies of the excitation spectrum of Haldane spin systems are pre
sented. We performed temperature-dependent high-held and high-frequenc
y ESR measurements up to 21T and 1042GHz on the compound NENP [Ni(C2H8
N2)(2)NO2ClO4]. Based on extensive experimental investigations we prop
ose to distinguish between different classes of resonances: first, the
transitions between excited states at the boundary of the Brillouin z
one (q = pi) below the critical field; second, the high-frequency tran
sitions from the ground state (q = 0) above the critical field and thi
rd, the low-frequency resonances at q = 0 in the whole field range. Ne
w results are presented in the high-held phase, which exhibit an appre
ciable temperature dependence of the resonance field and at low temper
atures show the existence of multiple high-frequency excitation branch
es. In the low-frequency regime we also observe an additional ground s
tate excitation.