Coherent Laser Control Over The Unimolecular Reaction Of CH₃I

When the molecular system is excited, there are always some kinds of reaction pathway and many different products. The control of the molecular reaction may maximize the yield of a desired compound while reducing the yields of unwanted by-products. Control over the outcome of a chemical reaction has been a long-standing goal in the field of reaction dynamics. Seeking the method of the control of the reaction has been one of the major goal of the chemical studying field. With the advent of lasers, some experiments tried to use its high intensity, high monochromaticity, short pulse duration to perform the theory and experiments of the laser selective chemistry. These efforts were generally unsuccessful, although a few recent studies on molecules have shown some success. In the earlier studies, people omited the phase coherent of the laser. In recent years, a number of strategies have been proposed to achieve more active control of chemical reactions. One of these methods, first proposed by Brumer and Shapiro, uses tow weak laser pulses to excite the molecule simultaneously by two distinct optical paths. The approach is based on the principle of quantum-mechanical interference, as an analog of Young's two-slit experiment. This principle states that the simultaneous excitation of a molecule by two distinct excitation paths connecting the same initial and final states, by adjusting the relative phases of the two beams it is possible to alter the combination in such a way to enhance one product channel at the expense of the other.There are various ways of selecting the excitation paths. One of these, proposed by Shapiro, Hepburn and Brumer, consists of excitation by three photons of frequencyω3 along the first path, and excitation by one photon of frequency along the second path. In this case the phase difference is given by: (1)as the "laser phase", by experimentally varying it is possible to modulate the transition probability. In this article, Coherent laser control are performed to the unimolecular reaction of CH3I molecule under the weak coherent mixtures of 355nm and its third harmonic 118nm. The third harmonic 118nm is produced in a mixture gas medium of Xe and Ar, distinct from others in the coherent control. It is necessary to optimize the efficiency of THG. This procedure is typically performed with acetone by monitoring the m/z=58 signal, which is maximized to give the optimum SPI signal. it gives higher efficiency then pure Xe. The relative phase difference between the two beams was continuously varied by decrease the pressure of gas Ar, so control over the photodissociation of molecular system. We have observed the CH3+ and I+ ions signals as functions of Ar pressure in the phase-tuning cell. It is changing sinusoidally,the result is comparable with Gordon's. We initially realize the coherent control of CH3I molecular reaction. From Coherent Control theory, and the CH3I molecular energy graph, we give the dissociation and ionization mechanisms of the controlling process. That is, (2)followed by (3) (4)The final point to be discussed is the modulation depth of the fragment signals. It is very weak, but this is readily understood, because in the process of photodissociation, some other dissociation\ionization uncontrolled happened at one-photon or two-photon level. There are many factors caused modulation loss, we need improve the condition of experience.Through the study of coherent control of CH3I reaction, we understand furthermore about the Coherent Control Theory, and the importance of its application to the field of chemical reaction. On the other hand, the principal finding of my dissertation will the guidance of the future work.