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We present an experimental study of the absorption, between 40 and 640cm$^{-1}$, by CO$_2$, CH$_4$ and H$_2$ gases as well as by H$_2$+CO$_2$ andCH$_4$+CO$_2$ mixtures at room temperature. A Fourier transform spectrometerassociated to a multi-pass cell, whose optics were adjusted to obtain a 152 mpathlength, were used to record transmission spectra at total pressures up toabout 0.98 bar. These measurements provide information concerning thecollision-induced absorption (CIA) bands as well as about the wing of theCO$_2$ 15 $\mu$m band. Our results for the CIAs of pure gases are, withinuncertainties, in agreement with previous determinations, validating ourexperimental and data analysis procedures. We then consider the CIAs byH$_2$+CO$_2$ and CH$_4$+CO$_2$ and the low frequency wing of the pure CO$_2$ 15$\mu$m band, for which there are, to our knowledge, no previous measurements.We confirm experimentally the theoretical prediction of Wordsworth et al. 2017that the H$_2$+CO$_2$ and CH$_4$+CO$_2$ CIAs are significantly stronger in the50-550 cm$^{-1}$ region than those of H$_2$+N$_2$ and CH$_4$+N$_2$,respectively. However, we find that the shape and the strength of theserecorded CIAs differ from the aforementioned predictions. For the pure CO$_2$line-wings, we show that both the $\chi$-factor deduced from measurements near4 $\mu$m and a line-mixing model very well describe the observed stronglysub-Lorentzian behavior in the 500-600 cm$^{-1}$ region. These experimentalresults open renewed perspectives for studies of the past climate of Mars andextrasolar analogues.

Far infrared measurements of absorptions by CH4+CO2 and H2+CO2 mixtures and implications for greenhouse warming on early Mars