Geometrical Optimization Approach to Isomerization: Models and Limitations

Absolute control over the quantum yield of a photodissociation reaction mediated by nonadiabatic couplings

Control of molecular reaction dynamics with laser pulses has been developed in the last decades. Among the different magnitudes whose control has been actively pursued, the branching ratio between different product channels constitutes the clearest signature of quantum control. In polyatomic molecules, the dynamics in the excited state is quagmired by non-adiabatic couplings, which are not directly affected by the laser, making control over the branching ratio a very demanding challenge. Here we present a control scheme for the CH3I photodissociation in the A band, that modifies the quantum yield of the two fragmentation channels of the process. The scheme relies on the optimized preparation of an initial superposition of vibrational states in the ground potential, which further interfere upon the excitation with a broad pump pulse. This interference can suppress any of the channels, regardless of its dominance, and can be achieved over the whole spectral range of the A band. Furthermore, it can be accomplished without strong fields or direct intervention during the dynamics in the excited states: the whole control is predetermined from the outset. The present work thus opens the possibility of extensive and universal control of the channel branching ratio in complex photodissociation processes.

Impact of Early Coherences on the Control of Ultrafast Photodissociation Reactions

By coherent control, the yield of photodissociation reactions can be maximized, starting in a suitable superposition of vibrational states. In ultrafast processes, the interfering pathways are born from the early vibrational coherences in the ground electronic potential. We interpret their effect from a purely classical picture, in which the correlation between the initial position and momentum helps to synchronize the vibrational dynamics at the Franck-Condon window when the pulse is at its maximum intensity. In the quantum domain, we show that this localization in time and space is mediated by dynamic squeezing of the wave packet.