Strong-field dipole resonance. I. Limiting analytical cases

We investigate population dynamics in N-level systems driven beyond the linear regime by a strong external field, which couples to the system through an operator with nonzero diagonal elements. As concrete example we consider the case of dipolar molecular systems. We identify limiting cases of the Hamiltonian leading to wavefunctions that can be written in terms of ordinary exponentials, and focus on the limits of slowly and rapidly varying fields of arbitrary strength. For rapidly varying fields we prove for arbitrary $N$ that the population dynamics is independent of the sign of the projection of the field onto the dipole coupling. In the opposite limit of slowly varying fields the population of the target level is optimized by a dipole resonance condition. As a result population transfer is maximized for one sign of the field and suppressed for the other one, so that a switch based on flopping the field polarization can be devised. For significant sign dependence the resonance linewidth with respect to the field strength is small. In the intermediate regime of moderate field variation, the integral of lowest order in the coupling can be rewritten as a sum of terms resembling the two limiting cases, plus correction terms for N>2, so that a less pronounced sign-dependence still exists.Comment: 34 pages, 1 figur

Femtochemistry – Theory

The objective of this project part is to develop methods and models for the investigation of processes induced by the interaction of molecules with laser pulses, and the microscopic control of molecular processes. These concepts are then applied in the simulation and interpretation of current experiments. The research in the 2nd period focused on (i) the complete high-level ab initio treatment of small systems, (ii) the dynamics and control of molecular model systems, including the role of the carrier envelope phase, and (iii) the development and assessment of approximate methods and their application to larger molecular systems. (i) The work comprises the determination of highly accurate molecular energy surfaces in electronically excited states including nonadiabatic coupling vectors, the discussion of conical intersections and nonadiabatic dynamics. Significant breakthrough has been achieved in several ways. The analytic computation of energy gradients and coupling vectors has been implemented in the distribution version of our public domain COLUMBUS program system. ‘On-the-fly ’ non-adiabatic dynamics could be performed for the first time with highlevel quantum chemical methods and novel views of standard photodynamic processe