Broadening of the SodiumDLines by Atomic Hydrogen. An Analysis in Terms of the NaH Molecular Potentials

The interatomic forces between sodium and hydrogen atoms which are responsible for the broadening of the sodium $D$ lines are discussed in terms of calculated interatomic potential curves for the NaH molecule. The importance of including overlap interactions and of considering both upper and lower states of the transitions is emphasized. A calculation of the line broadening using a molecular model for the pair of colliding atoms with the correct adiabatic limits yields damping parameters of 8.3 × ${10}^{-9\mathrm{}}$ rad ${\sec }^{-1\mathrm{}}$ with an estimated accuracy of 5-10% under the conditions of the solar photosphere and fits well with the observed wings of the absorption line profiles. The temperature dependence of the broadening is found to be approximately $T^{0.42}$, and the width/shift ratios are of the order of 30:1 for both resonance lines. Cross sections for transitions induced between the fine-structure states ${}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ are estimated, and the value ${\sigma }_{\frac{1}{2}\to \frac{3}{2}}=(70\mathrm{}\pm 10)\pi a_0^{}{}_{}{}^2$ at a velocity of 1.28 × ${10}^6$ cm ${\sec }^{-1\mathrm{}}$ is in agreement with previous calculations. Ground-state spin-exchange cross sections are estimated for hydrogen-sodium collisions as $22\pi a_0^2$ in the same velocity range.