离子速度成像方法研究溴代环己烷的紫外光解动力学

利用二维离子速度成像方法对C6H11Br分子在234 nm附近的光解动力学行为进行了研究. 通过(2+1)共振增强多光子电离探测了光解产物Br*(2P1/2)和Br(2P3/2), 得到它们的相对量子产率. 从光解产物Br*(2P1/2)和Br(2P3/2)的速度图像得到了能量和角度分布. 结果表明, Br*原子主要来自于S1态的直接解离, 而Br则绝大部分是从S2态向T3态的系间交叉跃迁得到, 并导致了两种解离通道能量分布的差别. 实验发现C6H11Br分子解离过程中大部分能量都转化为内能, 但与其它长链溴代烷烃分子相比, 可资用能更多地被分配到平动能中, 结合软反冲模型分析了这种能量分配跟环烷基的构象和稳定性的关系.     

离子速度成像方法研究碘代正戊烷的紫外光解动力学

利用离子速度成像方法对n-C5H11I分子在266和277 nm下的光解动力学进行了研究. 实验分析了I*(5p 2P1/2)和I(5p 2P3/2)的离子影像, 得到其相应速度、角度分布和相对量子产率, 并根据相对量子产率和角度分布计算了不同解离通道的比例. 实验发现n-C5H11I的3Q0和1Q1态之间存在较强的耦合效应, 并且随着波长的减小, 这种非绝热耦合作用有递增的趋势. 由离子影像(I*和I)的角度分布结果发现, 在同一解离激光波长下I*的各向异性参数β值比I的β值小, 其中I*主要由3Q0直接解离产生, 而I绝大多数是由分子先跃迁到3Q0再经过3Q0→1Q1的非绝热耦合产生.     

CCl3Br光解动力学的离子速度影象研究

为建立高分辨的离子速度影象装置,进行了“离子透镜”的精确设计,得到了“将离子源空间分布中速度相同而位置不同的离子聚焦在同一点上”的效果．同时在新建立的装置上研究了CCl3Br在267肿附近的光解反应产物Br(^2P3／2)和Br(^2P1／2)的速度和角度的分布．利用实验得到的各向异性参数β(Br)=-0．48,β(Br)=1．44,讨论和分析了分子可能的解离通道．     

离子速度影像法研究n-C7H15Br分子光解反应动力学

利用离子速度成像方法, 研究n-C7H15Br分子在231～239 nm范围内几个波长处的光解离动力学. 通过同一束激光经(2+1)共振多光子电离(REMPI)过程探测光解碎片Br(2P3/2)和Br*(2P1/2), 得到了不同激光波长处的离子速度分布图像, 从而获得C7H15Br光解产物的能量分配和角度分布. 结合各向异性参数和量子产率, 计算了n-C7H15Br分子在234 nm波长下不同解离通道的比例. 实验表明光解产物的能量分配可以用冲击模型中的软碰撞模型来解释. 实验还发现, 各向异性参数β(Br*)的值对光波长变化很敏感, 这是由电子激发态的绝热和非绝热过程决定的.     

正丙醇和异丙醇的紫外光解动力学

利用高里德堡态氢原子飞行时间(HRTOF)探测技术,研究了正丙醇和异丙醇的紫外光解动力学过程.在193.3 nm光辐射下,O-H键快速断裂过程构成主要的氢原子生成通道.伴随O-H键的碎裂,相当大的一部分能量转换成氢原子及其相应碎片的平动能(正丙醇〈fv〉=0.76; 异丙醇〈fv〉=0.78).氢原子碎片具有各向异性的角度分布;其角分布异向因子β分别为-0.79(正丙醇)和-0.77(异丙醇).研究结果表明,吸收1个193.3 nm光子后,丙醇分子跃迁到一个寿命很短的电子激发态;沿着O-H反应坐标,该激发态势能面是排斥的,因而O-H键快速断裂.此外,还得到了丙醇的O-H键离解能: (432±2)kJ/mol(正丙醇)和(433±2)kJ/mol(异丙醇).     

二溴代烷烃的紫外光解动力学研究

利用飞行时间质谱装置研究了234和267nm激光作用下二溴甲烷、二溴乙烷、二溴丙烷和二溴丁烷分子的光解离过程．研究表明二溴代烷烃分子在紫外激光的作用下主要是断裂C—Br键解离出一个Br原子,并且存在两种可能的布居：基态Br(^2P3/2^0)和激发态Br^*(^2P1／2^0)．通过共振增强多光子电离技术探测两种光解产物布居的分支比．对比得到了分子构型对称性不同的二溴代烷烃的分支比,提出了两种假设的光解离模型．     

离子速度成像方法研究C8H17Br分子的光解动力学

用离子速度成像方法, 研究了长链C8H17Br分子在234 nm激光下的光解过程. 通过2+1共振增强多光子电离探测了两种光解产物Br*(2P1/2)和Br(2P3/2), 得到了它们的相对量子产率. 从光解产物Br*(2P1/2)和Br(2P3/2)的速度图像得到了能量和角度分布. 并根据相对量子产率和角度分布, 计算了不同解离通道的比例. 实验发现C8H17Br分子解离过程中大部分能量都转化为内能, 该能量分配可以较好地用软反冲模型来解释, 并分析了这种能量分配跟烷基大小的关系.     

离子速度成像在阈值光电子-光离子符合测量中的应用和改进

在阈值光电子-光离子符合质谱中, 通过增加离子检测端的电极板, 并调整相对电压形成渐增的加速电场, 获得了很好的离子速度聚焦效果. 通过这种电场, 使得具有较大平动能的离子团沿飞行方向逐渐膨胀的同时, 在垂直于飞行方向受到电场的束缚, 最终在探测器的表面实现速度聚焦, 形成缩小的图像(即速度成像的放大倍率N<1), 从而确保谱仪能够同时获得较高的平动能分辨率和质量分辨率. 应用这种缩小的速度聚焦电场, 我们研究了振动态选择的O₂ (B²Σ_g^-)离子解离动力学, 获得了碎片离子O在阈值光电子-光离子符合条件下的三维时间切片图像, 并通过比较两个解离通道所产生的O不同的平动能分布, 证实了这种离子透镜电场在较宽的离子平动能范围内能够始终保持良好的速度聚焦效果.     

二溴甲烷的同步辐射光电离研究

采用５０～２００ｎｍ同步辐射光，对ＣＨ２Ｂｒ２的光电离过程进行了研究，根据测定的母体离子及其碎片离子出现势，得到二溴甲烷的绝热电离势为１０．２３±０．０１ｅＶ，并获得了离子的生成焓，解离能及键能等热力学数据，分析了碎片离子的光解离电离通道。     

Ion-Velocity Map Imaging Study of Photodissociation Dynamics of Acetaldehyde

The photodissociation dynamics of acetaldehyde in the radical channel CH3＋HCO has been reinvestigated using time-sliced velocity map imaging technique in the photolysis wavelength range of 275-321 nm. The CH3 fragments have been probed via （2＋1） resonance-enhanced multiphoton ionization. Images are measured for CH3 formed in the ground and excited states （v2=0 and 1） of the umbrella vibrational mode. For acetaldehyde dissociation on T1 state after intersystem crossing from S1 state, the products are formed with high translational energy release and low internal excitation. The rotational and vibrational energy of both fragments increases with increasing photodissociation energy. The triplet barrier height is estimated at 3.8814-0.006 eV above the ground state of acetaldehyde.     

氯碘甲烷在A带的光解动力学

利用离子速度影像方法结合共振增强多光子电离(REMPI)技术研究了氯碘甲烷在A带的光解机理. 从266和277 nm的I*(5p 2P1/2)和I(5p 2P3/2)离子速度影像获得了碎片的平动能分布和角度分布. I和I*的平动能分布呈单高斯型, 可用软自由基近似来解释. I和I*是在排斥的势能面上直接解离产生的. 实验得到的各向异性参数β证实分子受激发后主要产生3Q0态, 并且3Q0和1Q1态之间存在非绝热转移. 波长越短, 这种非绝热转移越强. 在235 nm附近, Cl和Cl*各向同性的离子影像说明氯原子来自于CH2ICl的二次解离过程, 即CH2ICl先解离产生CH2Cl自由基, 自由基再解离产生氯原子.     

Photodissociation Dynamics of Dichlorodifluoromethane (CF2Cl2) around 235 nm using Time-Sliced Velocity Map Imaging Technology

Photodissociation dynamics of dichlorodifluoromethane (CF₂Cl₂) around 235 nm has been studied using the time-sliced velocity map imaging technology in combination with the resonance enhanced multi-photon ionization technology. By measuring the raw images of chlorine atoms which are formed via one-photon dissociation of CF₂Cl₂, the speed and angular distributions can be directly obtained. The speed distribution of excited-state chlorine atoms consists of high translation energy (E_T) and low E_T components, which are related to direct dissociation on ³Q₀ state and predissociation on the ground state induced by internal conversion, respectively. The speed distribution of ground-state chlorine atoms also consists of high E_T and low E_T components which are related to predissociation between ³Q₀ and ¹Q₁ states and predissociation on the ground state induced by internal conversion, respectively. Radical dissociation channel is confirmed, nevertheless, secondary dissociation and three-body dissociation channels are excluded.     

Slow energy transfer between vibrational models of CH2Cl2, CD2Cl2, CH2ClBr and CH2Br2

Inefficient vibrational energy exchange between the lowest vibrational mode and the higher lying vibrational modes of CH₂Cl₂, CD₂Cl₂, CH₂ClBr and CH₂Br₂ was investigated by ultrasonic absorption experiments. Breathing sphere theory is used to interpret the data available for VV and VR, T transfer in methylene halides.     

Ion-Neutral Photofragment Coincidence Imaging of Photodissociation Dynamics of Ionic Species

The recently constructed cryogenic cylindrical ion trap velocity map imaging spectrometer (CIT-VMI) has been upgraded for coincidence imaging of both ionic and neutral photofragments from photodissociation of ionic species. The prepared ions are cooled down in a home-made cryogenic cylindrical ion trap and then extracted for photodissociation experiments. With the newly designed electric fields for extraction and acceleration, the ion beam can be accelerated to more than 4500 eV, which is necessary for velocity imaging of the neutral photofragments by using the position-sensitive imaging detector. The setup has been tested by the 355 nm photodissociation dynamics of the argon dimer cation (Ar\begin{document}$_2$\end{document}\begin{document}$^+$\end{document}). From the recorded experimental images of both neutral Ar and ionic Ar\begin{document}$^+$\end{document} fragments, we interpret velocity resolutions of \begin{document}$\Delta v/v$\end{document}\begin{document}$\approx$\end{document}4.6% for neutral fragments, and \begin{document}$\Delta v/v$\end{document}\begin{document}$\approx$\end{document}1.5% for ionic fragments, respectively.     

Infrared Photodissociation Spectroscopic and Theoretical Study of [Co(CO2)n]+ Clusters

The mass-selected infrared photodissociation (IRPD) spectroscopy was utilized to investigate the interactions of cationic cobalt with carbon dioxide molecules. Quantum chemical calculations were performed on the [Co(CO₂)_n]⁺ clusters to identify the structures of the low-lying isomers and to assign the observed spectral features. All the [Co(CO₂)_n]⁺(n=2-6) clusters studied here show resonances near the CO₂ asymmetric stretch of free CO₂ molecule. Experimental and calculated results indicate that the CO₂ molecules are weakly bound to the Co⁺ cations in an end-on con guration via a charge-quadrupole electrostatic interaction. The present IRPD spectra of [Co(CO₂)_n]⁺ clusters have been compared to those of Ar-tagged species ([Co(CO₂)_n]⁺-Ar), which would provide insights into the tagging effect of rare gas on the weakly-bounded clusters.     

Using Ion-velocity Map Imaging Technique to Study Photodissociation of 2-Bromopentane

The photodissociation dynamics of 2-bromopentane at -234 nm has been investigated by utilizing ion-velocity map imaging technique. The mapped images of Br（2P3/2） （denoted as Br） and Br（2p1/2） （denoted as Br＊） fragments were analyzed by means of the speed and angular distributions, respectively. The speed distributions can be fitted with two Gaussian components which are correlated to the two independent reaction paths on the excited po- tential energy surfaces （PES）. The high-energy component is from the prompt dissociation along the C-Br stretching mode, while the low-energy one is related to the dissociation from the coupling of the C-Br stretching and bending modes. Relative quantum yield is measured to be 0.892 for Br in the photodissociation of 2-bromopentane at 234 nm. Combining the anisotropy parameter with the relative quantum yield of Br and Br fragments, the contributions of the excited 3Q0, 3Q1, and 1Q1 states to the products Br and Br were derived. The effect of alkyl branching on the mechanism of photodissociation was discussed by comparing the photodissociation processes of four isomers of bromopentane.     

Global topological approach to highly excited vibration: a case study of H2O, CH2Br2 and CD2Br2

A global topological approach based on the algebraic coset representation to study highly excited molecular vibration is proposed. The algorithm allows us to elucidate ample highly excited vibrational dynamics from a global viewpoint. Topics for highly excited states studied include (1) global topological structures, (2) global symmetric and antisymmetric characters, (3) resonance overlaps and chaotic motion, (4) action transfer coefficients and (5) survival probabilities denoting the decaying of the action stored in a mode, with sample calculations for H₂O, CH₂Br₂ and CD₂Br₂.