Quantum Control

Vibrational Control

Chemical reactions are generally considered as a transit on a potential energy surface. The reaction begins with reactants and progresses along the reaction coordinate over a reaction barrier to the products. Molecular catalysts alter chemical reaction rates by applying forces which modify the reaction barrier.

Figure 1: Catalyst modify potential energy surfaces by exerting multi-polar electric forces. Laser fields can be used to modify potential energy surfaces in a similar way.

As electrical forces underlie all of chemistry, such barrier manipulation is also possible by application of the electromagnetic field of a laser. Quantum Control can be viewed as chemistry where light is used as a photonic reagent: the photon is added as a component of the reaction. Here, however, we use the nonresonant dynamic Stark effect to shape a potential energy surface as if it were a photonic catalyst: the photons shape potential energy surface in a process called Dynamic Stark Control (DSC). The notion of a photonic catalyst is even stronger if one considers that there is no net change in catalyst photon number: like a chemical catalyst, it is not consumed.

To experimentally demonstrate DSC, it was applied to an important class of photochemical reactions: non-adiabatic processes. These processes, such as internal conversion or inter-system crossing, entail charge rearrangements that occur along a reaction path at the intersections of potential energy surfaces and act as triggers of the ensuing chemistry. Non-adiabatic processes are of paramount importance in the biological mechanisms of vision and photosynthesis and underlie the photochemistry of almost all polyatomic molecules . Three light pulses are used: 1) A visible pulse photo-excites iodinemonobromide (IBr). 2) An infrared NRDSE pulse shapes the avoided crossing. 3) An independent UV probe beam interrogates the iodine fragments.

By carefully timing the NRDSE pulse, and appropriate distortion of the potential energy surface can be made and the exit channel of the reaction can be controlled. Importantly, control is achieved over the neutral molecule. The target molecule is not ionized, fragmented, or excited to a new electronic state.

Figure 2: Veloity map imaging is used to measure the kinetic energy distrubution of the iodine fragments. By modifying the delay of the infrared control field, the branching ratios of the reaction can be altered.

Figure 3: Experimental iodine velocity distributions showing the two exit channels as a function of control pulse time delay. At each time delay Δt, the distribution is measured as in Fig. 2. By changing the control pulse delay, the branching fraction at the nonadiabatic crossing can be drastically altered. The smaller radius (velocity) corresponds to Br*; the higher, to Br. At early and late delays, the field-free branching ratio is observed, demonstrating the reversible nature of the DSC interaction.