  A strong emission line at 2.8935 mu discovered by Rubin et al. (2001) in an
ISO SWS02 spectrum of the Orion Nebula is identified as the 4p - 4s multiplet
of the O I triplets. Line formation is due to de-excitation cascades following
UV-pumping of high triplet terms and occurs in the O I zone immediately behind
the Hydrogen ionization front. This cascade mechanism also accounts for
permitted O I triplet lines in the optical spectrum (Grandi 1975). An escape
probability treatment of the O I cascades accounts for the strength of the 2.89
mu line and suggests interesting diagnostic possibilities for the optical
lines.
