Rb and Cs broadening of the Na resonance lines

The broadening of the Na resonance lines, due to collisions with Rb and with Cs, has been measured with the use of the normalized fluorescence-intensity method of Chatham, Lewis, and Gallagher. Several Na resonance-radiation diffusion and absorption corrections were necessary in the Rb case, due to an unavoidable excess of Na density in the Rb vapor. The line-broadening rate coefficients $k_1$ and $k_2$ for the $D_1$ and $D_2$ resonance lines are $k_1\left(\mathrm{Rb}\right)=(6.2\mathrm{}\pm 0.4)\times {10}^{-9\mathrm{}}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{-1\mathrm{}}$, $k_2\left(\mathrm{Rb}\right)=(5.5\mathrm{}\pm 0.6)\times {10}^{-9\mathrm{}}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{-1\mathrm{}}$, $k_1\left(\mathrm{Cs}\right)=(6.9\mathrm{}\pm 1.0)\times {10}^{-9\mathrm{}}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{-1\mathrm{}}$, and $k_2\left(\mathrm{Cs}\right)=(5.5\mathrm{}\pm 0.9)\times {10}^{-9\mathrm{}}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{-1\mathrm{}}$ at $T\simeq 300$° C. The leading ($C_6{R}^{-6\mathrm{}}$) dipole-dipole long-range dispersion forces for the Na-Rb and Na-Cs interactions are calculated, and are used in the impact-theory formula to obtain theoretical line-broadening rate coefficients. These are in poor agreement with the measurements, indicating that as suggested by Vadla, the higher-order ($C_8{R}^{-8\mathrm{}}$) dispersion terms are also important at the very long range responsible for this line broadening.