  We report on the first far-IR detection of H2D+, using the Infrared Space
Observatory, in the line of sight toward Sgr B2 in the galactic center. The
transition at 126.853 um connecting the ground level of o-H2D+, 1_1,1 with the
the 2_1,2 level at 113 K, is observed in absorption against the continuum
emission of the cold dust of the source. The line is broad, with a total
absorption covering 350 km s^-1, i.e., similar to that observed in the
fundamental transitions of H2O, OH and CH at ~179, 119 and 149 um respectively.
For the physical conditions of the different absorbing clouds the H2D+ column
density ranges from 2 to 5x10^13 cm^-2, i.e., near an order of magnitude below
the upper limits obtained from ground based submillimeter telescopes. The
derived H2D+ abundance is of a few 10^-10, which agrees with chemical models
predictions for a gas at a kinetic temperature of ~20K.

Astrophysics