Observationally , both the 3.4 { \mu m } aliphatic hydrocarbon C–H stretching absorption feature and the 9.7 { \mu m } amorphous silicate Si–O stretching absorption feature show considerable variations from the local diffuse interstellar medium ( ISM ) to Galactic center ( GC ) : both the ratio of the visual extinction ( A _ { V } ) to the 9.7 { \mu m } Si–O optical depth ( \Delta \tau _ { 9.7 { \mu m } } ) and the ratio of A _ { V } to the 3.4 { \mu m } C–H optical depth ( \Delta \tau _ { 3.4 { \mu m } } ) of the solar neighborhood local diffuse ISM are about twice as much as that of the GC .
In this work , we try to explain these variations in terms of a porous dust model consisting of a mixture of amorphous silicate , carbonaceous organic refractory dust ( as well as water ice for the GC dust ) .