應用閾值光電子-光離子符合技術(shù)研究若干分子電離解離動力學
發(fā)布時間：2018-08-18 09:54
【摘要】：本論文中,我們采用了兩套閾值光電子-光離子符合速度成像裝置,結(jié)合同步輻射光電離技術(shù),分別開展了氯乙烯,氟甲烷以及碳酸二甲酯分子的光電離-解離動力學研究。兩臺光電子-光離子符合速度成像譜儀分別工作于合肥同步輻射光源和瑞士光源。合肥光源的符合譜儀是我們實驗室自主搭建的,可同時對電子和離子進行速度聚焦,并通過推斥電場放大電子圖像,從而降低熱電子污染,提高光電子的能量分辨,該譜儀在符合離子速度聚焦影像的能量分辨E/Δ E優(yōu)于3%,有助于我們測量碎片離子的平動能釋放及角分布。瑞士光源的符合譜儀(iPEPICO)只對光電子實現(xiàn)速度聚焦成像,離子則采用經(jīng)典的Wiley McLaren飛行時間質(zhì)譜設計,其閾值電子能量分辨可達3 meV,非常適合于測量小分子有機物的異構(gòu)化和解離過程,能夠得到精確的碎片離子出現(xiàn)勢和反應焓。針對氯乙烯分子的電離-解離過程,我們開展了內(nèi)能態(tài)選擇的離子解離動力學研究。在11.0-14.2 eV范圍內(nèi),通過光電離直接得到A2A'、B2A"和C2A'電子態(tài)的氯乙烯離子。在該范圍內(nèi),我們只觀測到了 C2H3+一種碎片離子。通過對不同能態(tài)氯乙烯離子解離產(chǎn)生的C2H3+碎片進行三維時間切片的離子速度聚焦成像,我們直接測量了碎片的平動能分布和角度分布。處于B2A"電子態(tài)的氯乙烯離子解離時,總的平動能分布可以分為統(tǒng)計解離(玻爾茲曼分布)和非統(tǒng)計解離(高斯分布)兩部分;而當激發(fā)光能量超過C2A'電子態(tài)后,又增加了一個具有較低平動能釋放的非統(tǒng)計解離途徑。結(jié)合時間依賴的密度泛函理論計算的氯乙烯離子C-C1斷鍵勢能面,我們給出了處于B2A"和C2A'電子態(tài)的氯乙烯離子解離機理。類似的,我們在12.40-19.90 eV能量范圍內(nèi)對內(nèi)能選擇的氟甲烷離子C-H和C-F鍵解離動力學進行了研究。在較低的能量(如X2E電子態(tài))下,CH3F+是唯一的碎片離子,而在A2A1和B2E離子態(tài)時,CH3+是主要的解離產(chǎn)物。氟甲烷離子X2E態(tài)的解離表現(xiàn)為各向同性,且總的動能釋放為典型的玻爾茲曼分布,這與氟甲烷離子脫氫為預解離過程的性質(zhì)一致。A2A1和B2E電子態(tài)下,解離產(chǎn)物CH3+的角分布為垂直解離。結(jié)合理論計算的氟甲烷離子脫氫和脫氟勢能曲線,我們可以得到其各電子態(tài)的解離機理:CH2F+是從X2E態(tài)的Jahn-Teller分裂的12A'態(tài)解離產(chǎn)生,而在A2A1態(tài)下直接快速C-F斷鍵解離,B2E態(tài)的氟甲烷離子則主要通過內(nèi)轉(zhuǎn)換到A2A1態(tài)后再解離。在10.3-12.5 eV真空紫外光作用下,碳酸二甲酯發(fā)生典型的解離電離過程。在電離能附近,其閾值光電子譜存在著清晰的振動結(jié)構(gòu)。通過Franck-Condon擬合,我們確定這些振動來自于0-C-0彎曲振動和C=0伸縮模式的結(jié)合,在此基礎(chǔ)上,我們得到了碳酸二甲酯的電離能為10.47±0.01eV。隨著能量的增加,更多的異構(gòu)和解離通道逐漸出現(xiàn)。通過擬合Breakdown曲線,我們得到了主要解離通道的0 K出現(xiàn)勢,理論計算的解離路徑也很好地解釋了實驗觀察的主要產(chǎn)物。在此基礎(chǔ)上,我們進一步得到了碳酸二甲酯的生成焓ΔTHOK(DMC(g))=-548.3 ±1.5 kJ/mol,碳酸二甲酯陽離子的生成焓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..
【學位授予單位】：中國科學技術(shù)大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：O621;O644.1
本文編號：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..
【學位授予單位】：中國科學技術(shù)大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：O621;O644.1
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相關(guān)博士學位論文 前1條
1 吳向坤;應用閾值光電子-光離子符合技術(shù)研究若干分子電離解離動力學[D];中國科學技術(shù)大學;2017年
，本文編號：2189111
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