Chemi-ionization of mercury atoms: Potential curves and estimates of the total ionization cross sections

Potential curves for Hg(6s6p)+Hg(6s6p) are calculated in the LambdaS representation and transformed to the spin-orbit representation using empirical spin-orbit coupling constants. With these potential curves, total chemi-ionization cross sections are estimated using a black-sphere approximation for the short-range interaction. Long-range barriers, found in some of the potential curves, reduce the thermal-energy cross sections. Because these barrier heights are rather uncertain, the cross sections are also calculated with their suppression. The result, for Hg(P-3(1))+Hg(P-3(0)) collisions at 300 K, of 30-125 Angstrom(2), overlaps the range of experimental measurements. For the only other pair considered in previous experiments, Hg(P-3(0))+Hg(P-3(0)), chemi-ionization was found to be energetically allowed but dynamically improbable. An upper limit, already lower than the posited measured value, was established, and the actual cross section is expected to be considerably smaller; this finding provides additional evidence that the reactants were misidentified in the experiment. The cross section for the statistically dominant metastable atoms Hg(P-3(2))+Hg(P-3(2)), 8-11 Angstrom(2), is also relatively small. The chemi-ionization cross sections obtained for the other triplet reactants are 33-80 Angstrom(2) for Hg(P-3(1))+Hg(P-3(1)), 17-31 Angstrom(2) for Hg(P-3(2))+Hg(P-3(0)), and 15-22 Angstrom(2) for Hg(P-3(2))+Hg(P-3(1)). The chemi-ionization cross sections for the singlet atom are large, 116-226 A(2) for Hg(P-1(1))+Hg(P-3(0)), 161-285 Angstrom(2) for Hg(P-1(1))+Hg(P-3(1)), 105-293 Angstrom(2) for Hg(P-1(1))+Hg(P-3(2)), and 132-397 Angstrom(2) for Hg(P-1(1))+Hg(P-1(1)). Though we do not yet have the autoionization widths required for quantitative distinction, the relative shapes of the neutral and ion potential curves suggest that most of the chemi-ionization will be of the associative, instead of the Penning type, even when the latter is energetically allowed.