Observations with the Short Wavelength Spectrometer ( SWS ) onboard the Infrared Space Observatory ( ISO ) have led to the first detection of the methyl radical { CH _ { 3 } } in the interstellar medium . The \nu _ { 2 } Q - branch at 16.5 \mu m and the R ( 0 ) line at 16.0 \mu m have been unambiguously detected toward the Galactic center SgrA ^ { * } . The analysis of the measured bands gives a column density of ( 8.0 \pm 2.4 ) \times 10 ^ { 14 } cm ^ { -2 } and an excitation temperature of ( 17 \pm 2 ) K. Gaseous { CO } at a similarly low excitation temperature and { C _ { 2 } H _ { 2 } } are detected for the same line of sight . Using constraints on the { H _ { 2 } } column density obtained from { C ^ { 18 } O } and visual extinction , the inferred { CH _ { 3 } } abundance is ( 1.3 { { +2.2 } \atop { -0.7 } } ) \times 10 ^ { -8 } . The chemically related { CH _ { 4 } } molecule is not detected , but the pure rotational lines of { CH } are seen with the Long Wavelength Spectrometer ( LWS ) . The absolute abundances and the { CH _ { 3 } / CH _ { 4 } } and { CH _ { 3 } / CH } ratios are inconsistent with published pure gas-phase models of dense clouds . The data require a mix of diffuse and translucent clouds with different densities and extinctions , and/or the development of translucent models in which gas-grain chemistry , freeze-out and reactions of { H } with polycyclic aromatic hydrocarbons and solid aliphatic material are included .