Theoretical approach for simulation of femtosecond spectra: New strategies for optimal control of complex systems

Abstract

AbstractWe present the ab initio Wigner distribution approach for simulation of femtosecond pump–probe and pump–dump signals and the new strategy for tailoring the light pulses that allow for optimal control of ultrafast processes. Our ab initio Wigner distribution approach combines the Wigner–Moyal representation of the vibronic density matrix with the ab initio multistate adiabatic and nonadiabatic molecular dynamics “on the fly” involving ground and excited electronic states. This approach allows for accurate simulations of femtosecond signals based on analytic formulation for systems of moderate complexity taking into account all degrees of freedom. For this purpose the following items are needed: (1) temperature‐dependent ground‐state initial conditions; (2) an ensemble of trajectories for the investigation of the dynamics of the system either on the adiabatic electronic excited state or on both the excited and ground states through nonadiabatic coupling; and (3) either the cationic or neutral ground state for the probing step. The goal of optimal control of femtosecond processes is to tailor pump–dump pulses that drive the system via electronically excited state to the desired ground‐state objective with the maximal yield. For the multidimensional systems this is possible only if the connective pathway between initial state and the objective can be ensured and the optimal pathway can be found. We show that these requirements are fulfilled by introducing the concept of the intermediate target in the excited state. Its role is to select the appropriate parts of energy surfaces at the given time delay between the two pulses, allowing us to reach optimally the objective. We illustrate the scope of our approach on nonstoichiometric halide deficient NanFn−1 clusters, which are characterized by strong ionic bonding and one excess electron. The choice of the systems has been made to determine timescales of different processes in the excited states and apply the new strategy for optimal control. This involves the bond breaking leading to (1) periodic geometric rearrangements such as in the case of Na2F or (2) nonadiabatic radiationless decay through conical intersection between the first excited state and the ground state occurring for Na3F2. In the latter case we optimize and analyze the pump and dump pulses to drive the isomerization process in the Na3F2 cluster to the second isomer by suppressing radiationless transition through conical intersection. The shaped pump–dump pulses serve as guidance for the experimental work in progress. Population of one of the two isomers allows us to establish a molecular switch. Such bimodal type of behavior might provide a basic element for information storage. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004