Coherent Control Of The Ionization And Dissociation Processes Of Several Molecules And Clusters

To control a chemical reaction is always the goal that physicists and chemists seek after. A lot of theoretical and experimental methods on reaction control have been formed throughout many previous works for many years. Coherent control is one of these methods in which reaction control is performed by exploring the quantum coherent characters of reactant system. The central idea of coherent control is to let excited molecules to evolve coherently by using coherent radiation, to actively control the dynamic processes of the reaction and to make the reaction to go through a desired channel by changing the characters of the radiation field interacting with molecules. Coherent control methods, developed, can be devided into two types, one is frequency domain coherent control (such as, phase coherent control method, Stimulated Transition Induced by Raman Adiabatic Passage (STIRAP) method, and so on), the other is time domain coherent control (such as, pump-dump method, open-loop schemes by using femtosecond pulse shaping, designing phases and close-loop schemes by using femtosecond pulse shaping and genetic algorithms and so on).In this thesis, coherent optimal control methods of nanosecond phase coherent control in frequency domain and femtosecond pulse shaping coherent control in time domain are developed and used for investigating the optimal control research on several molecules, such as acetone, carbon disulfide, butanone, and cyclopentanone.In frequency domain, we design and manufacture a set of the devices for phase coherent control, which include a frequency-tripled cell for generating 118nm and 177nm deep-UV radiation from 355nm and 532nm nanosecond laser by gas medium of Xe or Hg vapor, and a phase modulator for adjusting the phase difference between the fundamental and the third harmonic and using the interference between two independent excting paths, as shown Brumer and Shapiro schemes. The phase coherent control researches for carbon disulfide (CS₂) and butanone molecules (CH₃COC₂H₅) molecules are implemented. The main reseach results obtained are: (1) The modulating the yields of the ionized and dissociated products, CS₂⁺ and CS⁺ from CS₂ molecule and CH₃CO⁺ and CH₃⁺ from CH₃COC₂H₅ molecules are realized. The results indicated that the phase coherent control method is effective in the dissociation and ionization processes of CS₂ and CH₃COC₂H₅. The experiments also found that there are no phase lag between CH₃CO⁺ and CH₃⁺ product channels in CH₃COC₂H₅ molecule and between CS₂⁺ and CS⁺ product channels in CS₂ molecule, which makes it impossible to control their branching ratios. (2) We find that the focusing condition can affect the branching ratio of reactive products in acetone and its clusters experiments radiated by 355nm and 118nm laser. This can act as a simple method for controlling a reaction.In time domain, we build a femtosecond pulse shaping setup that central part is a spatial light modulator (SLM). We choose two strategies: shaping a fs pulse after amplifying it and amplifying a fs pulse after shaping it. The optimal controls on the photoionized and photodissociated products of NO₂, CH₃I, CH₃COCH₃, CH₃COC₂H₅ and C₅H₈O are investigated by combining this fs pulsed shaper method and genetic algorithm or by open-loop method. The main reseach results obtained are: (1) The pulse shapes obtained with the goal of optimizing parent ions of NO₂ and acetone are similar to that of transform limited pulse. In other words, shorter duration and higher peak intensity benefit the ionization process of these molecules. This result indicates that in current experimental laser intensity range (4 x10¹³W/cm²), the process, in which these molecules are ionized into parent ions, is mainly multiphoton ionization. (2) The yields of the fragment ion NO⁺ from NO₂ molecule is optimized using GA optimal control in the photo induced process of molecule NO₂ by 800nm fs laser. By analyzing the optimal pulse shape, it is indicated that the fragment ion NO⁺ comes from the further dissociation of parent ion NO₂⁺, i.e. dissociation ionization in fs laser field. (3) The optimal control of branching ratio I⁺/CH₃I⁺ is realized in the photo induced process of molecule CH₃I by 800nm fs laser. By analyzing the optimal pulse shape, it is indicated that the fragment ion I⁺ may come from atomic fragment ionization after dissociation from neutral molecule CH₃I. (4) The GA optimal control of the yield of fragment ion CH₃⁺ is realized in the photoreaction process of molecule acetone. By combining the results from the open loop experiment and analyzing the optimal pulse shape, it is indicated that the optimal pulse consists of sub-pulses with equal intervals which controls the evolution of wavepack in excited state of acetone, and promotes the cleavage of the C-C bond, and further increases the yields of the fragment ion CH₃⁺. (5) The GA optimal control with the goals of maximizing the ratio CH₃CO⁺/CH₃⁺ and CH₃⁺/CH₃CO⁺ are achieved in the photoreaction process of acetone. From the temporal structure of the optimal pulse shape and combining the results from the chirped phase open loop experiment, the optimizing mechanism is analyzed. The results indicate that the duration and peak intensity of optimal pulse have different promoting and depressing effects to different reactive channels, and hence realize the controls of branching ratio of products. (6) The GA optimal control with the goals of maximizing the ratio C₂H₃⁺/ C₅H₈O⁺, C₂H4⁺/ C₅H₈O⁺ and C₂H₃⁺/ C₂H4⁺ are achieved in the photoreaction process of cyclopentanone.These optimal control results in frequency and time domain experiments in the present study indicate that phase coherent control and close-loop optimal control with combining fs pulse shaper and genetic algorithms are effective coherent control methods for these photodynamic processes of the molecules under study.We also study the characters of dissociation and ionization products from the butanone clusters and CS₂ clusters created by supersonic expansion beams and irradiated by a VUV and visible laser beam, empioying coherent control apparatus. The results give as follows. (1) The peak intensities of the butanone ion clusters and the CS₂ ion clusters in TOF mass spectrua show an exponential decrease with increasing the cluster size in general. (2) There are mainly the parent cluster ions (CS₂)n⁺ (n=1⁸). For butanone, the parent cluster ions can not be observed except single parent molecule ion. Instead, several series of protonated and fragmental butanone cluster ions are observed in mass spectra, mainly the cluster fragment ions (CH₃COC₂H₅)_nCH₃CO⁺. (3) The peak intensity of the butanone clusters (CH₃COC₂H₅)_nCH₃CO⁺ is not monotonic decrease with increasing the cluster size, an oscillation from n=4 to 8 are found in the intensity of these clusters. This phenomenon is different from the observation in a previous study on acetone clusters, in which the intensities of (CH₃COCH₃)_nCH₃CO⁺(n=1¹1) showed a monotonic decrease with the cluster size. (4) The stable structures and energies of neutral butanone clusters (CH₃COC₂H₅)_n+1 (n+1= 2-7) are calculated using the density functioinal theory. Neutral clusters (CH₃COC₂H₅)_n+1 (n+1=5 and n+1=7) show higher stability over the adjacent even number clusters according to the binding energies and HOMO-LUMO gaps obtained. By analysizing possible reaction channels, it is indicated that the intensity variation of (CH₃COC₂H₅)_nCH₃CO⁺ reflects the character of the stability of neutral clusters (CH₃COC₂H₅)_n+1. These results are useful for deeply understanding of the structure, stability and the photoinduced processes of these weak bonding molecular cluster systems,.