Multireference configuration interaction based electronic Floquet states for molecules in an intense radiation field: Theory and application to Li ₂ ⁺

A multireference configuration interaction (CI) method which includes single and double excitations based description of adiabatic Floquet states for the electronic structure of a molecule in an intense laser field is introduced. Using a variant of a recently introduced configuration state function (CSF) based Table-CI methodology, it is shown that the multiple states of several irreducible representations required for a good description of low-lying Floquet states can be obtained using modifications of computational molecular electronic structure techniques. In particular, formulas for all components of the transition dipole moment matrix elements within the CSF-based Table-CI method are derived and presented. Moreover, the flexibility of the recently introduced macroconfiguration description of model and external configuration spaces is shown to lead to multiple potential energy surfaces of sufficiently uniform quality to allow construction of useful Floquet states. The formalism and computer programs developed are demonstrated on Li ₂ ⁺ in a 0.9×10 ¹² W/cm ² field. In analogy with Na ₂ ⁺, the 1,2 ²Σ _g ⁺, 1,2 ²Σ _u ⁺, 1 ²Π _g, and 1 ²Π _u states are of relevance, although the pattern of couplings is shown to be more complex. A hitherto unnoticed metastable state, which correlates asymptotically with 2 ²Σ _u ⁺, is described.