Ionization/Dissociation Of Cyclohexanone With Femtosecond Pump-Probe Technique

Resent years, the dynamics molecular ionization / dissociation in intense laser field (I >10¹⁴ W/cm²) with ultrashort pulse (<10^-13s) duration has been explored widely. Femtosecond (fs) pump-probe technique has been proved to be a useful tool for understanding the dynamics of molecular ionization / dissociation.In this thesis, a qualitative analysis method on the observation of ionization / dissociation of polyatomic molecule by means of fs pump-probe method is presented. As an example, we have attempted to illustrate the mechanisms of the different fragments of cyclohexanone (C₆H₁₀O) ionization / dissociation from our measurement in fs pump-probe experiments. According to the feature of moleclular ionization/dissociation in a fs laser field, multiphoton ionization or even field ionization is dominant within one laser pulse duration. We consider that the processes of moleclular ionization/dissociation can be classified into two cases, as the following:Case 1: If the relaxation with a time constant ofÏ„₁ is relatively large, the molecular intermediate (i), produced by the first laser pulse from the ground-state molecule (g), can be excited further by the second laser pulse with a certain delay time to a final state (f), and then the molecule dissociates from the f state into the product D⁺ that is observed experimentally.Case 2: If the intermediate relaxes fast into another intermediate (j), further excitation by the second laser pulse will be from the j state to the final state f which dissociates finally into the product D⁺ observed. The probability of the observed product D⁺ in each case is related with the excitation and relaxation of the intermediate.Based on this consideration, we propose a numerical simulation model. Using this model we simulate and calculate the variation of the fragments with the delay time between pump and probe beams in cyclohexanone (C6H10O) ionization / dissociation in a fs pump-probe experiment.To get the ionization/dissociation mechanism of cylcohexanone under different laser intensity (～10^{144 W/cm2} and different delay time (-2～3 ps), with the aim of time-of-flight mass spectra, we carry out a series of experimental measurements. Here, the laser wavelength is 800 nm and pulse duration is 90 fs. We analyze the experimental results with the method proposed above. Under different laser intensities, a given fragment comes from different ionization/dissociation source. As far as C₂H₃⁺ is concerned, when the laser intensity is weak, case 1 is dominant. With the increase of laser intensity, case 2 makes a more and more important role. At last, when it is very intense, case 2 is overwhelming. Meanwhile, under different laser intensities, even in the same ionization/dissociation case, the time constant of relaxation of i is changeable. Our numerical simulation curves are in good agreement with the experimental data. The time constants of the relaxation obtained for C₂H₃⁺ and C₃H₃⁺ are roughly the same at a given laser intensity. This similarity indicates that these two fragments most probably come from a same precursor, whose pathways can be complicated (e.g., throughαbond breakage, H transfer, and several sequential dissociating, etc.).The production process of some fragments is also analyzed. The production of fragments is provoked by the carbonyl, positive charge center and free radical center of cyclohexanone parent ion and the transmission of H atom. The main possible dissociating channels of some fragments are presented. The production of C₂H₃⁺ and C₃H₃⁺ have the same step, that is, a cleavage of bond induced by the charge center of parent ion. And C₂H₂⁺ comes through the loss of H from C₂H₃⁺. Induced by the radical center, the parent ion dissociates into C₅H₁₀⁺, and then this fragment dissociates into C₃H₆⁺. C₃H₅⁺ comes through the loss of H from C₃H₆⁺.In addition, coherent control experiments of two polyatomic molecules, CH3I and CH₃COCH₃, in a ns laser field with 355 nm and 118 nm, are also performed. The principle of coherent control experiment is to control the probability of reaction through the coherence of two channels which are competitive. The initial state of reactant is made by two laser beams, and the final state is controlled through varing their relative phase and amplitude. We obtain the modulating curves for some fragments, CH₃⁺ and I ⁺ from CH3I, H⁺ and CH3⁺ from CH3COCH3, and calculate their modulation depthes.We show that the mechanism of ionization / dissociation of two moleculars, CH3I, and CH3COCH3, as following:1. It is possible for CH₃I to occur two dissociation channels:(1)Then, these two neutral fragments absorb photons again, and ionize therefore:That is, it is a process of ionization of neutral fragments。(2)The parent molecule ionizes directly, then the the parent ion obsorbs photons and dissociates:That is, it is a parent ion ladder.2. As for CH₃COCH₃, the fragments CH₃^<sup>+å’ŒH^<sup>+, whose yields are modulated, come from the following processes:There is a three-photon process in this mechanism, in which the excitation with 3ω1 can be finished throughω3 excitation. Based on this process, CH₃⁺ is produced and its yield is modulated. From the modulation curves, it is obvious that the variation of H⁺ with the modulating gas (Ar) pressure is as same as that of CH₃⁺. Therefore, it is possible that H⁺ is attribute to the dissociation of CH3⁺.These results show that the yield of some given product can be increased and some can be reduced or eliminated, thus the control for the molecular reaction is realized.