We report on the first far–IR detection of H _ { 2 } D ^ { + } , using the Infrared Space Observatory , in the line of sight toward Sgr B2 in the galactic center .
The transition at \lambda =126.853 \mu m connecting the ground level of o-H _ { 2 } D ^ { + } , 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 H _ { 2 } O , OH and CH at \sim 179 , 119 and 149 \mu m respectively .
For the physical conditions of the different absorbing clouds the H _ { 2 } D ^ { + } column density ranges from 2 to 5 \times 10 ^ { 13 } cm ^ { -2 } , i.e. , near an order of magnitude below the upper limits obtained from ground based submillimeter telescopes .
The derived H _ { 2 } D ^ { + } abundance is of a few 10 ^ { -10 } , which agrees with chemical models predictions for a gas at a kinetic temperature of \simeq 20 K .