【作者】 赵新丽；

【导师】 丁大军； 胡湛；

【作者基本信息】 吉林大学 ， 原子与分子物理， 2004， 硕士

【摘要】 分子系统在受到激发后往往会发生不止一种的反应过程，最终产物很多。对分子反应的控制可以使我们得到需要的产物而减少或者消除不需要的产物。化学反应产物的控制在反应动力学领域是一个长期存在的任务。寻找控制反应产物的方法一直是化学研究的核心之一。自激光诞生以后，化学家一直在努力地利用激光的高亮度、高单色性等优点从事激光选键化学的理论与实验研究。然而，大多数分子选键化学的实验均未获成功。在早期化学控制研究中忽略了激光的相干性。最近十几年来，人们提出了大量的主动控制化学反应的策略。其中一种是Brumer and Shapiro在1986年提出的相干控制方案（Coherent Control），利用两束弱的相干激光脉冲通过两不同路径同时激发分子。其原理与杨氏双缝干涉原理类似，建立在量子干涉（quantum interference）机理之上。其基本思想是：如果两个反应通道的初始本征态相同而终态又都淹没在同一组能量简并的连续态中，则可通过控制该叠加态的相应的波函数的边界条件来调节这两个通道的比率，其办法就是调制激发到该连续态的激光的位相。 <WP=64>选择激发路径的方法很多。其中一个是由Shapiro，Hepburn，and Brumer(SHB)首先提出的：一条路径是频率为ω1的三光子激发，另外一条是频率为ω3=3ω1单光子激发。两光束间的位相差为 （1）称为激光位相（laser phase），通过改变位相调制气体的压力即可控制该激光位相，进而控制反应发生的几率。本文以飞行时间质谱手段，对CH3I分子在355nm及其三倍频118nm两束弱的相干激光共同作用下的单分子反应进行实验研究。对三倍频118nm辐射的产生方法与前人的相干控制实验所用的不同，本文采用了混合气体Ar和Xe，用两束光共同解离丙酮分子，通过其母体离子的信号强度变化来监测其产生效率，得到了比单一气体Xe更高的转化效率。实验中采用Ar为位相延迟气体，逐渐改变其压力来调节两束光的位相差，对分子系统电离解离过程进行相干控制，得到了碎片离子CH3+、I+信号随位相调制气体Ar压力变化而成周期性变化的正弦曲线，与Gordon等人得到了可以比较的实验结果，初步实现了CH3I单分子反应的量子相干控制。根据反应控制理论，结合分子能级图，本文提出了在控制过程可能发生的电离解离机制。 （2）随后两中性碎片又吸收多个光子而电离 （3） （4）对于本文信号的调制深度，虽然比较微弱，但是这很容易理解的，因为在光解过程中，存在355nm单光子、双光子吸收，<WP=65>不可避免的产生了不能控制的电离、解离过程。影响调制深度的原因还很多，有待进一步去改善实验条件。通过对CH3I单分子激发动力学过程的相干控制研究，使我们进一步了解了量子相干控制反应的机理及其在反应控制研究领域所起的重要作用，为控制研究过渡到凝聚相、表面、界面以及生物大分子体系奠定了可靠的基础，有着一定的指导作用。对以后的工作还需进一步改善实验条件，从而对更复杂的分子体系的反应进行控制研究。更多还原

【Abstract】 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 <WP=67>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 <WP=68>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.更多还原