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We present a method for recovering the intrinsic (extinction-corrected)luminosity of the 11.2 micron PAH band in galaxy spectra. Using 105 high S/NSpitzer/IRS spectra of star-forming galaxies, we show that the equivalent widthratio of the 12.7 and 11.2 micron PAH bands is independent on the opticaldepth, with small dispersion of ~5% indicative of a nearly constant intrinsicflux ratio R_int = (f_12.7/f_11.2)_int = 0.377 +/- 0.020. Conversely, theobserved flux ratio, R_obs = (f_12.7/f_11.2)_obs strongly correlates with thesilicate strength (S_sil) confirming that differences in R_obs reflectvariation in the optical depth. The relation between R_obs and S_sil reproducespredictions for the Galactic Centre extinction law but disagrees with otherlaws. We calibrate the total extinction affecting the 11.2 micron PAH fromR_obs, which we apply to another sample of 215 galaxies with accuratemeasurements of the total infrared luminosity (L_IR) to investigate the impactof extinction on L_11.2/L_IR. Correlation between L_11.2/L_IR and R_obsindependently on L_IR suggests that increased extinction explains the wellknown decrease in the average L_11.2/L_IR at high L_IR. Theextinction-corrected L_11.2 is proportional to L_IR in the range L_IR/L_sun =10^9--10^13. These results consolidate L_11.2 as a robust tracer of starformation in galaxies.

Extinction in the 11.2 micron PAH band and the low L_11.2/L_IR in ULIRGs