Mid- and far-infrared fine-structure line sensitivities to hypothetical   variability of the fine-structure constant
  Sensitivity coefficients to temporal variation of the fine-structure constant alpha for transitions between the fine-structure (FS) sub-levels of the ground states of C I, Si I, S I, Ti I, Fe I, N II, Fe II, O III, S III, Ar III, Fe III, Mg V, Ca V, Na VI, Fe VI, Mg VII, Si VII, Ca VII, Fe VII, and Si IX are calculated. These transitions lie in the mid- and far-infrared regions and can be observed in spectra of high-redshift quasars and infrared bright galaxies with active galactic nuclei. Using FS transitions to study alpha-variation over cosmological timescale allows to improve the limit on $|\Delta\alpha/\alpha|$ by several times as compared to contemporaneous optical observations ($|\Delta\alpha/\alpha| < 10^{-5}$), and to suppress considerably systematic errors of the radial velocity measurements caused by the Doppler noise. Moreover, the far infrared lines can be observed at redshifts z > 10, far beyond the range accessible to optical observations (z < 4). We have derived a simple analytical expression which relates the FS intervals and the sensitivity of the FS transitions to the change of alpha. 
M. G. Kozlov, S. G. Porsev, S. A. Levshakov, D. Reimers, and P. Molaro