With the use of a surface-scattering machine, layers of 27 +/- 4 nm diborane (B2H6) frosts were irradiated at 10 K and 10(-10) Torr with energetic electrons. The electrons induce a unimolecular decomposition of the diborane molecules and lead to boron-hydrogen bond rupture processes. Here, we report the first infrared spectroscopic detection of the monobridged diboranyl radical (B2H5, C-2v) via the nu(8) mode at 1033 cm(-1). The infrared assignment of the B2H5 isomer was verified by conducting experiments with diborane-d(6) (B2D6). Under identical experimental conditions, the monobridged B2D5 isomer was observed via the nu(6)(1154 cm(-1)), nu(8)(823 cm(-1)), and nu(5)(1307 cm(-1)) absorptions. These data can be utilized in future spectroscopic studies of chemical vapor deposition processes to allow an identification of the monobridged diboranyl isomer in real time via infrared spectroscopy.