Context : Aims : Disk-averaged infrared spectra of Neptune between 1.8 and 13 \mu m , obtained by the AKARI Infrared Camera ( IRC ) in May 2007 , have been analysed to ( a ) determine the globally-averaged stratospheric temperature structure ; ( b ) derive the abundances of stratospheric hydrocarbons ; and ( c ) detect fluorescent emission from CO at 4.7 \mu m . Methods : Mid-infrared spectra ( SG1 and SG2 channels of AKARI/IRC ) , with spectral resolutions of 47 and 34 respectively , were modelled using a line-by-line radiative transfer code to determine the temperature structure between 1-1000 \mu bar and the abundances of CH _ { 4 } , CH _ { 3 } D and higher-order hydrocarbons . A full non-LTE radiative model was then used to determine the best fitting CO profile to reproduce the fluorescent emission observed at 4.7 \mu m in the NG channel ( with a spectral resolution of 135 ) . Results : The globally-averaged stratospheric temperature structure is quasi-isothermal between 1-1000 \mu bar , which suggests little variation in global stratospheric conditions since studies by the Infrared Space Observatory a decade earlier . The derived CH _ { 4 } mole fraction of ( 9.0 \pm 3.0 ) \times 10 ^ { -4 } at 50 mbar , decreasing to ( 0.9 \pm 0.3 ) \times 10 ^ { -4 } at 1 \mu bar , is larger than that expected if the tropopause at 56 K acts as an efficient cold trap , but consistent with the hypothesis that CH _ { 4 } leaking through the warm south polar tropopause ( 62-66 K ) is globally redistributed by stratospheric motion . The ratio of D/H in CH _ { 4 } of 3.0 \pm 1.0 \times 10 ^ { -4 } supports the conclusion that Neptune is enriched in deuterium relative to the other giant planets . We determine a mole fraction of ethane of ( 8.5 \pm 2.1 ) \times 10 ^ { -7 } at 0.3 mbar , consistent with previous studies , and a mole fraction of ethylene of 5.0 _ { -2.1 } ^ { +1.8 } \times 10 ^ { -7 } at 2.8 \mu bar . An emission peak at 4.7 \mu m is interpreted as a fluorescent emission of CO , and requires a vertical distribution with both external and internal sources of CO . Finally , comparisons to previous L-band studies indicate significant variability of Neptune ’ s flux densities in the 3.5-4.1 \mu m range , related to changes in solar energy deposition . Conclusions :