Long-range forces between two excited mercury atoms and associative ionization

The long-range quadrupole-quadrupole $\left(\sim R^{-5}\right)$ and leading dispersion $\left(\sim R^{-6}\right)$ interactions between all pairs of excited $\text{Hg}\left(6s6p\right)$ ${{}^{3}P}_0$, ${{}^{3}P}_1$, ${{}^{3}P}_2$, and ${{}^{3}P}_1$ atoms are determined. The quadrupole moments are calculated using the ab initio relativistic configuration-interaction method coupled with many-body perturbation theory. The van der Waals coefficients are approximated using previously calculated static polarizabilities and expressions for the dispersion energy that are validated with similar systems. The long-range interactions are critical for associative ionization in thermal and cold collisions, and are found to be quite different for different pairs of interacting states. Based on this knowledge and the short-range parts of previously calculated potential curves, improved estimates of the chemi-ionization cross sections are obtained.

Figure 1

Associative ionization cross sections for collisions of two mercury atoms in the indicated states. The solid curves take into account the long-range and short-range potentials [Eq. (18)]. The dashed curves take into account only the long-range potentials [Eq. (26)]. The cross section for $\text{Hg}\left({{}^{3}P}_0\right)+\text{Hg}\left({{}^{3}P}_0\right)$ is shown as a dotted curve for a special case (see text).