Controlling Chemical Reactions with Laser Pulses — "Quantum Control of Molecular Vibrational Motion"

Michihiko Sugawara (Senior Assistant Professor, Department of Chemistry)

What image comes to mind when you think of a chemical reaction? Perhaps you picture scenes of heating and stirring in glassware like test tubes and flasks, or phenomena like heat generation and color changes. Even these familiar chemical reactions, which we can see with our own eyes and feel with our skin, can be interpreted at the atomic and molecular level—that is, from a microscopic perspective—as "a phenomenon in which the atoms that make up molecules are rearranged." The yield of products in thermal reactions, such as those that typically proceed in a solution at room temperature, is determined based on statistical mechanics. According to this theory, stable substances are inevitably produced in greater quantities than unstable ones. However, there may be cases where the more unstable product is the one that is needed. What can be done in such situations?

Let's return to our initial definition of a chemical reaction from a microscopic perspective. Some might imagine that it could be achieved by directly manipulating the atoms that constitute a molecule. If we had a pair of tweezers small enough to grasp atoms, we might be able to forcibly create new chemical substances by plucking an atom from one place and attaching it to another. Unfortunately, no such convenient tweezers exist. This is where laser pulses (laser light irradiated for a short duration) come in, acting as a substitute for those tweezers. The characteristic properties of laser pulses include the following:

[Property 1] It is light with a defined phase (coherent light).

[Property 2] A large amount of energy can be injected in a very short time (femtoseconds = 10^-15 seconds).

[Property 1] is suitable for changing the vibrational state of a molecule into a state that evolves over time. Meanwhile, [Property 2] is extremely useful for controlling chemical reactions because it allows us to specify the timing of energy injection from the laser on the very short timescale at which reactions are said to occur. Recent lasers are becoming increasingly high-powered and short-pulsed, and it is also becoming easier to shape the laser pulses themselves (e.g., their center frequency, pulse width, and pulse shape). Therefore, to control chemical reactions by making a selective "incision" on specific bonds within a molecule through irradiation with a laser field, a theory for skillfully designing laser pulses is required. My research involves designing optimal laser fields to promote a desired reaction by combining detailed quantum mechanical analysis of molecular vibrations with optimal control theory.

Figure 1 shows the result of designing a laser field to selectively break only the OH bond in deuterated water. By irradiating the molecule with a "control laser field" that has a complex time evolution, as shown in the theoretically designed figure, a vibrational wave function (wave packet) is generated in which only the vibrations in the OH stretching direction are excited, making the OH bond easier to break.