Richard N. Zare of Stanford University is supported by an award from the Chemical Structure, Dynamics and Mechanisms program to pursue studies of the role of collision geometry in chemical reactivity. Professor Zare and his postdoctoral and graduate student colleagues will examine with the most exquisite detail how elementary chemical reactions occur. Prof. Zare and his group will continue their studies of state-to-state reaction dynamics in fundamental chemical systems, seeking to understand how the precise geometry of the collision of two molecules determines the chemical outcome of this interaction. Normally, reagent molecules can be selected in particular rovibrational levels denoted by the vibrational quantum numbers v1, v2, v3, etc. and the rotational angular momentum quantum number J. However, no selection is made of the projection of the rotational angular momentum vector J on the quantization axis. Zare and his colleagues work to prepare reagents and to detect reaction products in selected M states by combining the directionality of polarized radiation for pumping and probing molecular states with the intense electric field from a synchronized nonresonant laser pulse which causes sufficiently large Stark splittings of molecular transitions that individual M states can be accessed. Their goal is to achieve nearly 100 percent transfer of population from a lower to an upper level in an M-state-selective manner, which for reaction dynamics studies would be a first.
Prof. Zare will continue to serve as a mentor to a diverse group of young scientists, and will continue in his wide ranging efforts at educating the public about the importance of science. In this regard, Prof. Zare's service as a host of WONDERFEST and as a member of its board of directors is emblematic. WONDERFEST is a Bay Area celebration of science and attracts over one thousand people a year to learn about and debate upon current topics in science.
Chemical reactions are so easy to perform; they are as simple as striking a match. But understanding what happens as different atoms in molecules rearrange their partners to which they are bonded is a huge challenge. Our research has been devoted to provided such a picture for what many call the simplest chemical reaction, that of a hydrogen atom H colliding with a hydrogen molecule H2 to interchange partners. To make it easier to keep track of the atoms, we use a hydrogen molecule made of heavy hydrogen, called deuterium and symbolized as D2. Thus, we are investigating the reaction H + D2 --> HD + D What supposedly makes this reaction simple, form a theoretical point of view, is that the hydrogen atom is the lightest atom in the periodic table and the HD2 system has only three electrons, so that quantum calculations can be carried out to the highest level of accuracy for any chemical reaction. However, what is simple for the theorist is not simple for the experimentalist. H atoms need to be made in the lab; they cannot be purchased or stored. We do that by using ultraviolet light to break apart the hydrogen bromide molecule HBr, which dissociates into a fast H atom and a more slowly moving Br atom which plays no further role in what we study. Another complication is that molecular hydrogen is a transparent gas so that detecting it in its specific vibrational and rotational levels is another huge challenge. We accomplish this using what is called nonlinear optics in which a laser beam causes molecular hydrogen to go from some specific vibrational-rotational level through an electronically excited bound state and subsequently to ionize by absorbing three photons. We then detect the HD+ molecular ion. We do all this in a molecular beam inside a high vacuum chamber so that we can look at just at the collisions of H with D2, one at a time, and detect the HD products in a manner that is specific to their internal vibrational-rotational states. We are able to measure the speed of the specific HD+ ion from which we can then deduce into what angle the product flew after the reactive collision event. The high vacuum ensures that no subsequent collisions with any species take place before detection. These results are then compared against the best theories available. One of the major findings of our research is that for certain situations we must abandon the simplifying picture of classical motions of particles and instead use a fully quantum description in which the wave nature of the particles lead to interference effects. These are needed to account for the angular distributions of the HD products we observe in specific vibration-rotation states. Such studies help us understand when we are able to make simplifying assumptions about the behavior of the reaction process and when we cannot. Of course, this knowledge is of fundamental significance if we are to truly understand how chemical reactions occur. It might be thought that theory is so advanced that there is no need to perform such difficult experiments. However, our work has repeatedly shown that experimental findings that seem initially quite surprising lead theorists to understand better what actually transpired in the collision process. It might be useful to emphasize that the goal is not simply to see how well theory and experiment compare but also to gain insight into what predictions can be made with confidence for more complex chemical systems.
