Electronic structure and photoionization cross sections of Li2, Na2, and K2

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Abstract
We have calculated the photoionization cross sections and oscillator strengths for the strongest bound–bound transitions of Li2, Na2, and K2 using multiple scattering theory. The three dimers strongly resemble the corresponding isolated atoms. There are, however, significant molecular effects. In Li2 the photoionization cross sections are split in a manner consistent with a cylindrical perturbation. There is no Cooper minimum in the photoionization cross section in analogy with the atom. In Na2 and K2 there is a very large enhancement of the cross section over the atomic one within a few volts of threshold. This enhancement has been observed experimentally. For K2 our cross sections are in good agreement with experiment while for Na2 they are too low. The Cooper minimum in Na2 is broadened considerably over atomic Na because the matrix elements for the σ and π channels pass through zero at different photon energies. We find no Cooper minimum in K2. We also do not find molecular shape resonances of the type found for N2 and CO. It is possible that atomic peaks observed in our cross sections may evolve into molecular resonances in the heavier dimers.