环己酮分子在飞秒泵浦探测下的电离解离过程研究
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摘要
本文围绕多原子分子在单色飞秒激光场中的电离解离进行了研究,通过飞秒泵浦-探测技术试图理解多原子分子电离解离复杂过程。为了进行适当的数值模拟,本文采用了对电离解离过程进行简单分类,归纳为两种基本情况,即中间态直接激发到终态后发生解离和中间态弛豫后激发到终态发生解离。在此基础上建立了数值模拟方法,对泵浦-探测过程产物时间变化进行了模拟计算。实验上使用800 nm飞秒激光泵浦-探测和质谱方法,测量了环己酮(C_6H_(10)O,cyclohexanone)碎片离子强度随泵浦-探测延迟时间变化的规律,并利用建立的模拟方法对随泵浦-探测延迟时间到变化趋势基本相同的两个碎片C_2H_3~+和C_3H_3~+进行了拟合。拟合获得的这两个碎片离子的弛豫时间相同,结果表明它们来自于同一个中间态。本文同时对环己酮分子的碎片化机制和以及C_2H_2~+、C_2H_3~+、C_3H_3~+、C_3H_5~+和C_3H_6~+这几个碎片产生通道进行了分析,认为正电荷中心转移和自由基诱导H转移对环己酮解离起到主要的推动作用。此外,本文还研究了在纳秒355nm和它的三倍频118nm光场中一些多原子分子电离解离过程的相干控制,分别得到了碘代甲烷的碎片产物CH_3~+和I~+,以及丙酮的碎片产物H~+和CH_3~+的调制信号,计算了它们的调制深度。结果表明,通过上述分子的光解离过程相干控制,能够增加特定产物而减少或消除其余的产物,进而实现分子反应的控制。
Resent years, the dynamics molecular ionization / dissociation in intense laser field (I >10~(14) W/cm~2) with ultrashort pulse (<10~(-13)s) 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 (C_6H_(10)O) 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 ofτ_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/cm~2) 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 C_2H_3~+ 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 C_2H_3~+ and C_3H_3~+ 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 C_2H_3~+ and C_3H_3~+ have the same step, that is, a cleavage of bond induced by the charge center of parent ion. And C_2H_2~+ comes through the loss of H from C_2H_3~+. Induced by the radical center, the parent ion dissociates into C_5H_(10)~+, and then this fragment dissociates into C_3H_6~+. C_3H_5~+ comes through the loss of H from C_3H_6~+.
In addition, coherent control experiments of two polyatomic molecules, CH3I and CH_3COCH_3, 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, CH_3~+ 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 CH_3I 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 CH_3COCH_3, the fragments CH_3~~+和H~~+, 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, CH_3~+ 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 CH_3~+. 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.
Resent years, the dynamics molecular ionization / dissociation in intense laser field (I >10~(14) W/cm~2) with ultrashort pulse (<10~(-13)s) 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 (C_6H_(10)O) 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 ofτ_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/cm~2) 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 C_2H_3~+ 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 C_2H_3~+ and C_3H_3~+ 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 C_2H_3~+ and C_3H_3~+ have the same step, that is, a cleavage of bond induced by the charge center of parent ion. And C_2H_2~+ comes through the loss of H from C_2H_3~+. Induced by the radical center, the parent ion dissociates into C_5H_(10)~+, and then this fragment dissociates into C_3H_6~+. C_3H_5~+ comes through the loss of H from C_3H_6~+.
In addition, coherent control experiments of two polyatomic molecules, CH3I and CH_3COCH_3, 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, CH_3~+ 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 CH_3I 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 CH_3COCH_3, the fragments CH_3~~+和H~~+, 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, CH_3~+ 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 CH_3~+. 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.
引文
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