Design of Optimal Laser Pulses for Controlling Molecular Processes

Abstract

Control of molecular dynamics using external laser has achieved significant progress in recent years. Optimal control theory (OCT) has emerged as a powerful tool for laser desiging. In this work, we have used time dependent quantum mechanics along with OCT to design pulses for controlling processes, such as state-selective population transfer and dissociation in molecular systems in their ground electronic state. The generation of the highly excited vibrational states in a specified bond is an interesting problem in relation to intramolecular vibrational relaxation, bond dissociation and molecular quantum computing. We have treated the interaction of the molecule with the laser within the dipole approximation. The time dependent quantum mechanical propagation is achieved using the split-operator form of evolution operator together with Fast Fourier Transforms (FFT) and grid methods. The detailed analysis of laser pulses are carried out in terms of time dependent variation of electric field and its corresponding frequency spectrum. We have obtained fields for vibrational and rovibrational control in diatomic system (HF) using conjugate gradient method. Pulses are also designed to control nuclear dynamics in the fragments located in biomolecules, using simplified but realistic models. Optimized laser give nearly complete population transfer to the preselected molecular state at the end of pulse duration. The possibility of using Genetic Algorithm has also been investigated. This procedure allows us to select limited laser parameters to vary that correspond to those actually available to the experimentalist. Thus, it provides good control over pulse parameters, allowing optimized pulses with simple time and frequency structures. We have applied this method for selective state-to-state transitions in HF molecule and selective dissociation in HF molecule and the Fe CO bond in the CO-heme complex. It is shown that pulses designed in this way have simple time and frequency structures, and are experimentally feasible.