  Infrared transmission measurements on the $S=1$ antiferromagnetic chain
compound NENP in applied magnetic fields show a sharp absorption line at the
field-shifted Haldane gap. This violates a wave-vector selection rule of the
Hamiltonian normally used for NENP, as the gap excitations occur at the
Brillouin zone boundary. We argue that the crystal structure admits terms which
can explain the absorption lines. In addition, in an applied field, staggered
orientations of the g-tensors produce a staggered magnetic field. This can
explain the observation of a finite gap at all applied fields.

Condensed Matter