There is an increasing interest in the photoinduced dynamics in the low frequency, ω, regime. The multiphoton absorptions by molecules in strong laser fields depend on the polarization of the laser and on the molecular structure. 
The unique properties of the interaction of atoms and molecules with lasers in the low frequency regime imply new concepts and directions in strong-field light-matter interactions. We represent a perturbational approach for the calculations of the quasi-energy spectrum in the low frequency regime, that avoids the construction of the Floquet operator with extremely large number of Floquet channels. The zero-order Hamiltonian in our perturbational approach is the adiabatic Hamiltonian where the atoms/molecules are exposed to a dc electric field rather than to ac-field. The second order perturbation correction terms are obtained when -i\hbar ω(∂/∂τ) serves as a perturbation and τ is a dimensionless variable. The second order adiabatic perturbation scheme is found to be an excellent approach for calculating the ac-field Floquet solutions in our test case studies of a simple one-dimensional time-periodic model Hamiltonian. It is straightforward to implement the perturbation approach presented for calculating atomic and molecular energy shifts (positions) due to the interaction with low frequency ac-fields using high-level electronic structure methods. This is enabled since standard quantum chemistry packages allow the calculations of atomic and molecular energy shifts due to the interaction with dc-fields. These energy shifts are functions of the laser parameters (low frequency, intensity and polarization).