Spectrum NIR Optical constants spectrum of the $2\nu$ mode of $^{12}CO$ (normalized to 100%) in solid solution in $\beta$-$N_2$ crystal at 43 K Spectrum NIR Optical constants spectrum of the $2\nu$ mode of $^{12}CO$ (normalized to 100%) in solid solution in $\beta$-$N_2$ crystal at 41 K Spectrum NIR Optical constants spectrum of the $2\nu$ mode of $^{12}CO$ (normalized to 100%) in solid solution in $\beta$-$N_2$ crystal at 38 K Spectrum NIR Optical constants spectrum of the $2\nu$ mode of $^{12}CO$ (normalized to 100%) in solid solution in $\beta$-$N_2$ crystal at 36.5 K