In this award, funded by the Experimental Physical Chemistry Program of the Chemistry Division, Professor Gordon and his students will use lasers to control the relaxation rate of electronically excited organic molecules. Such processes, known as "radiationless transitions," play an essential role in vision, mutagenesis, and carcinogenesis, as well as in many industrial processes. For example, a unique property of the four nucleotide building blocks of DNA is that their conversion of electronic to vibrational energy is faster than fragmentation, allowing these molecules to be stable in the presence of ultraviolet radiation. This property is essential for life to exist on the surface of the earth. The goal of this project is to use ultrashort femtosecond laser pulses either to accelerate or inhibit the rate of such radiationless transitions. By using a combination of theory and experiment, the temporal profile of the pulses will be optimized to achieve the desired goal. This research fits into the larger context of coherent control of chemical reactions, in which light pulses are designed to control the outcome of chemical and physical processes. The research will also provide a deeper understanding of the mechanism of this important class of chemical processes.
Living tissue is constructed from a number of fundamental building blocks, including nucleic acids and proteins, which in turn are composed of simple organic units (e.g., nucleotides and amino acids). In general, organic molecules are unstable when exposed to UV radiation, which can break bonds and induce a large variety of chemical transformations. In view of the fragility of organic molecules in the presence of UV radiation, it is amazing that living organisms evolved before the formation of the stratospheric ozone layer, which today filters out the most harmful components of sunlight. The stability of these biomolecules in the presence of sunlight (i.e., their photostability) is attributed to their ability to convert absorbed energy into harmless heat before fragmentation occurs, which could potentially cause mutations, cancer, and other diseases. This stabilization process is called "internal conversion." The goal of this research is to understand how internal conversion occurs. To do this, we used a combination of experimental and theoretical methods to see if we could control the rate of internal conversion. Learning how to inhibit its rate would allow us to understand its mechanism at a fundamental level. Inhibiting internal conversion could also potentially be useful because if the molecule remains excited long enough to emit light it might be used as a probe for studying reactions in living cells. We chose for this study the pyrazine molecule, which is a simple ring compound containing four carbon atoms and two nitrogen atoms. The molecules were studied in a vacuum chamber, where they do not interact with each other. An ultrashort UV laser pulse (60 femtoseconds long) excited the molecules, and a second laser pulse of a different wavelength subsequently ionized the excited molecules and their fragments. The resulting ions were then detected with a mass spectrometer. The most novel aspect of this experiment was to shape the temporal profile of the first pulse so as to minimize the number of ions produced at a pre-specified time. In other words, we converted the laser pulse into a complex train of pulses designed, using a feedback loop, to accomplish the desired goal of minimizing the ion signal. We found that we could prevent the excited molecules from ionizing for times as long as 1.5 picoseconds. We interpret this finding to mean that the excited molecules were stabilized so that they neither decayed spontaneously nor absorbed light from the second pulse. At longer times the laser did not stabilize the excited molecules; instead the feedback found a way to excite fewer molecules at the same pulse energy. In parallel with the experiments, the group of Paul Brumer at the University of Toronto developed a theory showing how a suitably shaped laser pulse could either accelerate or inhibit the rate of internal conversion by a process called quantum mechanical interference. The idea behind this method is to excite two different states of the molecule simultaneously in such a way that as these states evolve they interfere with each other either to reinforce the reaction rate or cancel it. Using this method they were able to control the internal conversion rate for times as long as 100 femtoseconds. Although the calculations did not reproduce the experimental findings, they provided considerable insight into the quantum mechanical nature of internal conversion.
