Control of ionization and dissociation by optical pulse trains

Ever since the first lasers were built over 40 years ago, chemists and physicists have been attempting to exploit them as tools for controlling the outcome of chemical reactions. Over the last decade this dream has become a reality. The most successful approaches have employed learning algorithms to shape femtosecond laser pulses; however, in these experiments, the laser light effectively learns for itself what pulse shape is required to generate a specific product and it is not always easy to unravel the underlying physics of the control process. In this theoretical investigation we unravel the mechanism of ionisation/dissociation control in the prototypical H-2 molecule. We track the excited state molecular dynamics from the moment of interaction with the laser field to ionization and dissociation, and determine how sequences of carefully tuned laser pulses are able to change the ionization/dissociation branching ratio.