The Infra-Red Spectrum and Molecular Constants of HF and DF

New high dispersion measurements on the infra-red absorption spectrum of HF and DF are reported. Discussion of the experimental details and analysis is given. Observed frequencies of the spectral lines, as well as observed and calculated constants, are tabulated. The principal constants for HF are ${\ensuremath{\omega}}_{e}={4138.52}_{2}$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${x}_{e}{\ensuremath{\omega}}_{e}={90.06}_{9}$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${y}_{e}{\ensuremath{\omega}}_{e}=0.980$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${z}_{e}{\ensuremath{\omega}}_{e}=0.025$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${B}_{e}={20.939}_{0}$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${\ensuremath{\alpha}}_{e}={0.770}_{5}$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${\ensuremath{\gamma}}_{e}={0.005}_{0}$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${D}_{e}={0.002}_{2}$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The principal constants for DF are ${\ensuremath{\omega}}_{e}={3000.35}_{8}$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${x}_{e}{\ensuremath{\omega}}_{e}={47.33}_{6}$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${y}_{e}{\ensuremath{\omega}}_{e}=0.373$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${z}_{e}{\ensuremath{\omega}}_{e}={0.006}_{8}$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${B}_{e}={11.004}_{5}$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${\ensuremath{\alpha}}_{e}={0.293}_{6}$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${\ensuremath{\gamma}}_{e}={0.001}_{4}$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${D}_{e}={0.0006}_{5}$ ${\mathrm{cm}}^{\ensuremath{-}1}$.