The absorption of infrared and visible radiation from stellar emission spectra by the negatively charged hydrogen ions H$^{ -}$ is considered. We derive the explicit formulas for the bound-free absorption coefficient $a_{ \nu}$ for the negatively charged hydrogen ion ${ }^{ \infty}$H$^{ -}$ and its isotopes ${ }^{ 1}$H$^{ -}$ (protium) and ${ }^{ 2}$H$^{ -}$ (deuterium or D$^{ -}$). The bound-free absorption coefficient $a_{ \nu}$ is used to evaluate the actual absorption of infrared and visible radiation by the H$^{ -}$ ion in photospheres of many cold stars with surface temperatures $T_s \le$ 8,250 $K$.
Categories: astro-ph.SR physics.atom-ph