Ultrafast Quantum Control in Molecules

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The wide-ranging properties of substances around us (their different colors and smells, their taste, flammability, solubility, feel, toxicity) are tied closely to different physical arrangements of the atoms that define the molecules of the substance. When sunlight bleaches a shirt or aspirin relieves a headache, changes in molecular structure are responsible. Physicists model these arrangements of atoms in molecules as locations in a contour map called an "energy landscape" or a "potential energy surface." The stable atomic arrangements (the atomic arrangement for colored dye molecules, for example) are located in the valleys of the energy landscape. Chemical change, such as the change required to sun-bleach clothing dye, is a journey of the atomic positions from one valley to another. This NSF research project proposes a new way to guide the molecular journey by the use of laser light. Light doesn't move the atoms, but it can do something that is even better: it can change the shape of the energy landscape. Just like a bridge built over a gorge or a tunnel built through a mountain can reduce the difficulty of a journey over land, the new light-induced energy landscape can have shortcuts that encourage particular chemical changes. This project examines light-induced shortcuts at naturally occurring cross-roads in the energy landscapes. The new idea here is that laser light near these crossings, which are called "conical intersections," can employ quantum interference to guide the molecule. The remarkable prediction is that by changing the properties of the laser light, the molecule will switch from one path at the crossroads to the other. This basic scientific understanding about how light can encourage or inhibit chemical change could lead to applications such as light-activated pharmaceuticals.This project will investigate quantum control of trajectories nearby conical intersections (CIs), brought about by interference between the non-Born-Oppenheimer (NBO) couplings and laser-induced dipole couplings. The systems chosen for study are dissociation of singly ionized water and isomerization of doubly ionized acetylene. These are selected because they are hydrogen-rich small molecules with large NBO couplings and strong dipole couplings. The atomic motion will be launched by ionization of neutral water or acetylene using a high harmonics source of tunable vacuum ultraviolet radiation. The control field will be an ultrafast infrared laser pulse. The relative orientation of these NBO and dipole vector couplings can be inferred by mapping the momentum of dissociating fragments, or by Coulomb explosion imaging using a second intense infrared laser pulse following the reaction. The goal is to demonstrate control of the output channel via interference between NBO and dipole vector couplings, as revealed by the dependence of the dissociation or isomerization yield on the polarization, field strength, and frequency of the coupling field.