应用阈值光电子-光离子符合技术研究若干分子电离解离动力学
发布时间：2018-08-18 09:54
【摘要】：本论文中,我们采用了两套阈值光电子-光离子符合速度成像装置,结合同步辐射光电离技术,分别开展了氯乙烯,氟甲烷以及碳酸二甲酯分子的光电离-解离动力学研究。两台光电子-光离子符合速度成像谱仪分别工作于合肥同步辐射光源和瑞士光源。合肥光源的符合谱仪是我们实验室自主搭建的,可同时对电子和离子进行速度聚焦,并通过推斥电场放大电子图像,从而降低热电子污染,提高光电子的能量分辨,该谱仪在符合离子速度聚焦影像的能量分辨E/Δ E优于3%,有助于我们测量碎片离子的平动能释放及角分布。瑞士光源的符合谱仪(iPEPICO)只对光电子实现速度聚焦成像,离子则采用经典的Wiley McLaren飞行时间质谱设计,其阈值电子能量分辨可达3 meV,非常适合于测量小分子有机物的异构化和解离过程,能够得到精确的碎片离子出现势和反应焓。针对氯乙烯分子的电离-解离过程,我们开展了内能态选择的离子解离动力学研究。在11.0-14.2 eV范围内,通过光电离直接得到A2A'、B2A"和C2A'电子态的氯乙烯离子。在该范围内,我们只观测到了 C2H3+一种碎片离子。通过对不同能态氯乙烯离子解离产生的C2H3+碎片进行三维时间切片的离子速度聚焦成像,我们直接测量了碎片的平动能分布和角度分布。处于B2A"电子态的氯乙烯离子解离时,总的平动能分布可以分为统计解离(玻尔兹曼分布)和非统计解离(高斯分布)两部分;而当激发光能量超过C2A'电子态后,又增加了一个具有较低平动能释放的非统计解离途径。结合时间依赖的密度泛函理论计算的氯乙烯离子C-C1断键势能面,我们给出了处于B2A"和C2A'电子态的氯乙烯离子解离机理。类似的,我们在12.40-19.90 eV能量范围内对内能选择的氟甲烷离子C-H和C-F键解离动力学进行了研究。在较低的能量(如X2E电子态)下,CH3F+是唯一的碎片离子,而在A2A1和B2E离子态时,CH3+是主要的解离产物。氟甲烷离子X2E态的解离表现为各向同性,且总的动能释放为典型的玻尔兹曼分布,这与氟甲烷离子脱氢为预解离过程的性质一致。A2A1和B2E电子态下,解离产物CH3+的角分布为垂直解离。结合理论计算的氟甲烷离子脱氢和脱氟势能曲线,我们可以得到其各电子态的解离机理:CH2F+是从X2E态的Jahn-Teller分裂的12A'态解离产生,而在A2A1态下直接快速C-F断键解离,B2E态的氟甲烷离子则主要通过内转换到A2A1态后再解离。在10.3-12.5 eV真空紫外光作用下,碳酸二甲酯发生典型的解离电离过程。在电离能附近,其阈值光电子谱存在着清晰的振动结构。通过Franck-Condon拟合,我们确定这些振动来自于0-C-0弯曲振动和C=0伸缩模式的结合,在此基础上,我们得到了碳酸二甲酯的电离能为10.47±0.01eV。随着能量的增加,更多的异构和解离通道逐渐出现。通过拟合Breakdown曲线,我们得到了主要解离通道的0 K出现势,理论计算的解离路径也很好地解释了实验观察的主要产物。在此基础上,我们进一步得到了碳酸二甲酯的生成焓ΔTHOK(DMC(g))=-548.3 ±1.5 kJ/mol,碳酸二甲酯阳离子的生成焓ΔTHOK(DMC+(g))=461.9±1.8kJ/mol。
[Abstract]:In this paper, two threshold photoelectron-photoionic coincidence velocity imaging devices are used to study the photoionization-dissociation kinetics of vinyl chloride, fluoromethane and dimethyl carbonate molecules. Two photoelectron-photoionic coincidence velocity imaging spectrometers are used in Hefei synchrotron radiation. The coincidence spectrometer of Hefei Light Source is built by our laboratory. It can focus both electrons and ions at the same time, and magnify the electronic image by repulsive electric field, so as to reduce hot electron pollution and improve the energy resolution of photoelectrons. The Swiss light source coincidence spectrometer (iPEPICO) is only used for velocity focusing imaging of photoelectrons, while the classical Wiley McLaren time-of-flight mass spectrometry design is used for ions. The threshold electron energy resolution is up to 3 meV, which is very suitable for measuring the isomerization and decomposition of small molecular organic compounds. For the ionization-dissociation process of vinyl chloride molecule, we have studied the ionization-dissociation kinetics of internal energy state selection. In the range of 11.0-14.2 eV, the electrons of A2A', B2A'and C2A'can be obtained directly by photoionization. A fragment ion of C2H3+ was obtained. By focusing on the fragments of C2H3+ produced by dissociation of different energetic vinyl chloride ions, the translational energy distribution and angular distribution of the fragments were measured directly. Boltzmann distribution and non-statistical dissociation (Gauss distribution) are two parts, and a non-statistical dissociation path with lower translational energy is added when the excitation energy exceeds the C2A'electronic state. Similarly, the dissociation kinetics of C-H and C-F bonds of fluoromethane ions with internal energy selections were studied in the range of 12.40-19.90 eV. At lower energies (e.g. X2E electronic states), CH3F+ is the only fragment ion, while CH3+ is the main dissociation product in A2A1 and B2E states. The dissociation of E state is isotropic and the total kinetic energy release is a typical Boltzmann distribution, which is consistent with the fact that dehydrogenation of fluoromethane ions is a pre-dissociation process. The angular distribution of dissociation product CH3+ is vertical dissociation in the electronic states A2A1 and B2E. The dissociation mechanism of the electronic states is that CH2F+ is produced from the 12A'state of Jahn-Teller splitting in the X2E state, and the C-F bond breaking dissociation in the A2A1 state is rapid and direct. The fluoromethane ions in the B2E state are mainly transferred to the A2A1 state and then dissociated. Under the vacuum ultraviolet light of 10.3-12.5 eV, the typical dissociation process of dimethyl carbonate occurs. By Franck-Condon fitting, we determined that these vibrations originated from the combination of the 0-C-0 bending vibration and the C=0 stretching mode. On this basis, we obtained the ionization energy of dimethyl carbonate (DMC) at 10.47+0.01eV. With the increase of energy, more isomerization and dissociation channels were found. By fitting the Breakdown curves, we get the 0 K potential of the main dissociation channels, and the theoretical dissociation paths explain the main products well. On this basis, we further obtain the enthalpy of formation of dimethyl carbonate THOK (DMC (g))= - 548.3 (- 1.5 kJ/mol), the cation formation of dimethyl carbonate. The enthalpy of formation is THOK (DMC+ (g)) =461.9 + 1.8kJ/mol..
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O621;O644.1
本文编号：2189111
[Abstract]:In this paper, two threshold photoelectron-photoionic coincidence velocity imaging devices are used to study the photoionization-dissociation kinetics of vinyl chloride, fluoromethane and dimethyl carbonate molecules. Two photoelectron-photoionic coincidence velocity imaging spectrometers are used in Hefei synchrotron radiation. The coincidence spectrometer of Hefei Light Source is built by our laboratory. It can focus both electrons and ions at the same time, and magnify the electronic image by repulsive electric field, so as to reduce hot electron pollution and improve the energy resolution of photoelectrons. The Swiss light source coincidence spectrometer (iPEPICO) is only used for velocity focusing imaging of photoelectrons, while the classical Wiley McLaren time-of-flight mass spectrometry design is used for ions. The threshold electron energy resolution is up to 3 meV, which is very suitable for measuring the isomerization and decomposition of small molecular organic compounds. For the ionization-dissociation process of vinyl chloride molecule, we have studied the ionization-dissociation kinetics of internal energy state selection. In the range of 11.0-14.2 eV, the electrons of A2A', B2A'and C2A'can be obtained directly by photoionization. A fragment ion of C2H3+ was obtained. By focusing on the fragments of C2H3+ produced by dissociation of different energetic vinyl chloride ions, the translational energy distribution and angular distribution of the fragments were measured directly. Boltzmann distribution and non-statistical dissociation (Gauss distribution) are two parts, and a non-statistical dissociation path with lower translational energy is added when the excitation energy exceeds the C2A'electronic state. Similarly, the dissociation kinetics of C-H and C-F bonds of fluoromethane ions with internal energy selections were studied in the range of 12.40-19.90 eV. At lower energies (e.g. X2E electronic states), CH3F+ is the only fragment ion, while CH3+ is the main dissociation product in A2A1 and B2E states. The dissociation of E state is isotropic and the total kinetic energy release is a typical Boltzmann distribution, which is consistent with the fact that dehydrogenation of fluoromethane ions is a pre-dissociation process. The angular distribution of dissociation product CH3+ is vertical dissociation in the electronic states A2A1 and B2E. The dissociation mechanism of the electronic states is that CH2F+ is produced from the 12A'state of Jahn-Teller splitting in the X2E state, and the C-F bond breaking dissociation in the A2A1 state is rapid and direct. The fluoromethane ions in the B2E state are mainly transferred to the A2A1 state and then dissociated. Under the vacuum ultraviolet light of 10.3-12.5 eV, the typical dissociation process of dimethyl carbonate occurs. By Franck-Condon fitting, we determined that these vibrations originated from the combination of the 0-C-0 bending vibration and the C=0 stretching mode. On this basis, we obtained the ionization energy of dimethyl carbonate (DMC) at 10.47+0.01eV. With the increase of energy, more isomerization and dissociation channels were found. By fitting the Breakdown curves, we get the 0 K potential of the main dissociation channels, and the theoretical dissociation paths explain the main products well. On this basis, we further obtain the enthalpy of formation of dimethyl carbonate THOK (DMC (g))= - 548.3 (- 1.5 kJ/mol), the cation formation of dimethyl carbonate. The enthalpy of formation is THOK (DMC+ (g)) =461.9 + 1.8kJ/mol..
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O621;O644.1
【相似文献】
相关博士学位论文 前1条
1 吴向坤;应用阈值光电子-光离子符合技术研究若干分子电离解离动力学[D];中国科学技术大学;2017年
，本文编号：2189111
本文链接：https://www.wllwen.com/kejilunwen/huaxue/2189111.html
教材专著
