转动分辨的NH3X→C′→Ã跃迁的离子凹陷光谱

NH_3Ã(~1A_2~″)是一个快速预解离态, 以往的光谱研究从未得到它的转动分辨光谱。我们用两个脉冲染料激光器研究了NH_3C′→X的3+1多光子电离光谱和NH_3X→C′→Ã的折叠光学双共振多光子电离光谱, 称为“离子凹陷光谱”。对前者观察到了较高分辨的转动光谱, 得出C′v_2~′=O的高精度转动常数。通过谱线宽度, 得到了C′v_2~′=0,1,2能级的预解离寿命。从离子凹陷光谱上, 首次观察到了C′→Ã跃迁的0-0, 1-1, 2-2等带的转动分辨光谱。由双共振选择定则简化的光谱使漫散至～50 cm~(-1)线宽的转动线得以分开, 由此得到Ãv_2~″=0,1,2能级的转动常数和各谱带的带原点。在消除功率加宽的情况下, 测出Ãv_2~′=1能级的寿命。这种方法对研究分子的快速预解离态有一定的普遍意义。     

转动分辨的NH_3→′→跃迁的离子凹陷光谱

NH_3(~1A_2~″)是一个快速预解离态,以往的光谱研究从未得到它的转动分辨光谱。我们用两个脉冲染料激光器研究了NH_3′→的3 1多光子电离光谱和NH_3→′→的折叠光学双共振多光子电离光谱,称为“离子凹陷光谱”。对前者观察到了较高分辨的转动光谱,得出′v_2~′=O的高精度转动常数。通过谱线宽度,得到了′v_2~′=0,1,2能级的预解离寿命。从离子凹陷光谱上,首次观察到了′→跃迁的0-0,1-1,2-2等带的转动分辨光谱。由双共振选择定则简化的光谱使漫散至～50cm~(-1)线宽的转动线得以分开,由此得到v_2~″=0,1,2能级的转动常数和各谱带的带原点。在消除功率加宽的情况下,测出v_2~′=1能级的寿命。这种方法对研究分子的快速预解离态有一定的普遍意义。     

VA族元素氢化物的紫外光解和多光子电离Ⅲ. AsH3分子的紫外激光光解和多光子电离

通过193 nm的ArF激光光解AsH_3, 首次在420-650 nm之间观察到了AsH_2(Ã ~2A_1→X ~2B_1)的发射光谱。经过认证之后, 获得了v=19928+868v′_2-3v′_2~2-(989v″_2-6v″_2~2)的经验公式, 并辨认了七个谱带系。AsH_2(Ã ~2A_1)的辐射寿命是(130±20)ns。此外, 还观察到四条As(5s→4p)的原子发射谱线和多光子电离信号。     

VA族元素氢化物的紫外光解和多光子电离Ⅰ. NH3分子紫外激光光解

在光强为10~6 W cm~(-2)的ArF激光作用下, NH_3分子被光解为NH(A~(3Π))先后吸收两个193 nm光予, 经过NH_3(Ã ~1A_2″)和NH_2( Ã ~2A_1)两个中间物的过程。NH(A~(3Π)→X~(3∑~-))的发射光谱表明, 该产物具有7700 K的转动高温。此系NH_2在激发过程中剧烈的变角效应所致。我们利用高功率的紫外激光器及时间分辩的测量仪器, 系统地研究了这些分子的光谱能级, 反应、光解机理和电离过程等, 显然有重要的意义。     

563～660 nm波段SO2+的PHOFEX连续谱

在超声分子束条件下,利用380.85 nm的电离激光使SO2分子经由[3+1]共振增强多光子电离(REMPI)产生纯净的SO2+(X 2A1)分子离子,用另一束解离激光在可见光波长区(563～660 nm)扫描获得了光解碎片SO+的激发(PHOFEX)谱.从563～660 nm波长区SO+的无结构连续谱以及SO2+解离的效率随波长增加而减少的实验事实,提供了SO2+(E,D,C)电子态附近存在α2A2对称性排斥态的证据,分析了产生SO+的[1+1]光解机理:(1)SO2+(X2A1)首先经由单光子激发到达B2B2中间态的密集能级区;(2)吸收另一个光子到达SO2+(E,D,C)电子态附近的α2A2排斥态,经由α2A2排斥态产生了到SO+(X2∏)+O(3Pg)的直接解离.     

VA族元素氢化物的紫外光解和多光子电离——Ⅲ.AsH_3分子的紫外激光光解和多光子电离

通过193nm的ArF激光光解AsH_3,首次在420-650nm之间观察到了AsH_2(■2A_1→X 2B_1)的发射光谱。经过认证之后,获得了v=19928+868v′_2-3v′_2~2-(989v″_2-6v″_2~2)的经验公式,并辨认了七个谱带系。AsH_2(■2A_1)的辐射寿命是(130±20)ns。此外,还观察到四条As(5s→4p)的原子发射谱线和多光子电离信号。     

乙烯基A2A″电子态的振转分析

通过193 nm光解丁烯酮分子产生乙烯基自由基(•C2H3). 经射流冷却后, 以另一束可调谐激光光解•C2H3, 生成的氢原子碎片经共振增强多光子电离(REMPI)过程, 记录氢离子信号随光解波长变化, 得到21180~21320 cm-1范围内乙烯基 A2A″(µ′5,6,8=1)←X2A′(µ″=0)跃迁的振转光谱. 结合量化计算和光谱拟合, 对该段光谱进行了细致的振转分析, 确定了各振动谱带位置, 识别了其中主要的转动跃迁. 由光谱拟合得到各振动能级的预解离寿命, 讨论了其与振动模式及激发转动量子数的依赖关系, 证实了理论预测的乙烯基A2A″电子态的面内解离机制.     

乙烯基~2A″电子态的振转分析

通过193nm光解丁烯酮分子产生乙烯基自由基(·C2H3).经射流冷却后,以另一束可调谐激光光解·C2H3,生成的氢原子碎片经共振增强多光子电离(REMPI)过程,记录氢离子信号随光解波长变化,得到21180￣21320cm-1范围内乙烯基A!2A″(!′5,6,8=1)←X!2A′(!″=0)跃迁的振转光谱.结合量化计算和光谱拟合,对该段光谱进行了细致的振转分析,确定了各振动谱带位置,识别了其中主要的转动跃迁.由光谱拟合得到各振动能级的预解离寿命,讨论了其与振动模式及激发转动量子数的依赖关系,证实了理论预测的乙烯基A!2A″电子态的面内解离机制.     

一氧化碳的双共振多光子电离研究

用两束脉冲染料激光进行了CO的光学双共振四光子电离, 获得了转动分辩的CO A~(1Π)←X~(1Σ~+)和B~(1Σ~+)←A~(1Π)跃迁的光谱。观察到了在此2+1光子相继吸收中特有的选择定则。动力学研究表明CO基态到A~(1Π)态的双光子吸收是速率决定步骤, 其吸收截面约为1×10~(-49) cm~4 s。还测定了A~(1Π)态的单态转动传能截面。对于宇称指数e守恒、△J×±1的过程的截面约为0.05 nm; 随△J值增大而减小。对相同的△J, e正负变号的过程的截面总是小于e不变过程的截面。     

VA族元素氢化物的紫外光解和多光子电离Ⅱ. PH3的多光子电离光谱

用染料激光器在波长为300-640 nm范围内扫描, 观察到PH_3的一系列多光子电离光谱。其中351-398 nm的近二十条谱带, 其能级间隔约263 cm~(-1), 拟归属于通过Ã态的(2+1)电离谱。在460-495 nm的弱谱, 428-452 nm的六条谱带和385-398 nm之间的四条谱带, 则分别认证为相应于经历了B, C和D态的(3+1)MPI光谱。求得B态的项值T_0≤60729 cm~(-1), D态的T_0≤75567 cm~(-1), 而D态的反演振动频率ω_2为510 cm~(-1).     

在超音速F-F2-IF与NH3气流中产生IF(B3Π0+)的研究

研究了在超音速F-F_2-IF气流与NH_3气流中通过化学反应与碰撞传能产生IF(B~(3Π_0~+))的过程。用光学多通道分析仪在450-750 nm范围内记录到IF(B~(3Π_0~+)→X~(1∑~+))辐射的33条光谱线。提出了产生IF(B~(3Π_0~+))的机制。     

6-N,N-二甲基腺嘌呤激发态结构动力学的共振拉曼光谱

采用共振拉曼光谱技术和密度泛函理论方法研究了6-N,N-二甲基腺嘌呤(DMA)的A带和B带电子激发和Franck-Condon 区域结构动力学. π_H→π_L^*跃迁是A带吸收的主体, 其振子强度约占整个A带吸收的79%.由弥散轨道参与的n→Ryd 和π_H→Ryd 跃迁在B带跃迁中扮演重要角色, 其振子强度约占B带吸收的62%,而在A带吸收中占主导的π_H→π_L^*跃迁的振子强度在B带吸收中仅占33%. 嘌呤环变形伸缩+C8H/N9H面内弯曲振动ν₂₃和五元环变形伸缩+C8H弯曲振动ν₁₃的基频、泛频和合频占据了A带共振拉曼光谱强度的绝大部分, 说明¹π_Hπ_L^*激发态结构动力学主要沿嘌呤环的变形伸缩振动, N9H/C8H/C2H弯曲振动等反应坐标展开, 而ν₁₀, ν₂₉, ν₂₁, ν₂₆和ν₄₀的基频、泛频和合频占据了B带共振拉曼光谱强度的主体部分, 它们决定了B带激发态的结构动力学. A带共振拉曼光谱中ν₂₆和ν₁₂被认为与1nπ*/1ππ*势能面锥型交叉有关. B带共振拉曼光谱中ν₂₁的激活与¹ππ^*/¹πσ_N9H^*势能面锥型交叉相关.     

Quantum state-selected photodissociation dynamics of H2O: two-photon dissociation via the C̃ electronic state

The photodissociation dynamics of H(2)O via the C? state by two-photon excitation has been investigated using the H atom Rydberg tagging time-of-flight technique. The rotational resolved action spectrum of the C?←X? transition band has been measured. The line widths show a pronounced dependence on the parent rotational excitation in the C? state. The quantum state resolved OH product translational energy distributions and angular distributions have also been obtained. By carefully simulating these distributions, quantum state distributions of the OH product as well as the state-resolved angular anisotropy parameters were determined. The experimental results confirm the variation of two competitive predissociation pathways. A heterogeneous predissociation channel is mediated by rotational coupling to the B??(1)A(1) state associated with the a-axis (k(a)(')), and a homogeneous pathway arises from purely electronic coupling to the A??(1)B(1) state. We have also obtained the branching ratios of the OH(X) and OH(A) products, and related these to the C?→A? and C?→B? pathways. The branching ratios display a strong k(a)(') dependence.     

DFT studies on key intermediates in thiamin catalysis

The diphosphate ester (ThDP) of thiamin (vitamin B₁) is an important cofactor of enzymes within the carbohydrate metabolism. From experiments of site‐specific variants and nuclear magnetic resonance (NMR) studies, it is known that the protonation of the N1′ atom is a significant step in the coenzyme activation by the enzymatic environment. Therefore, we have performed density functional theory (DFT) calculations on the B3LYP/6‐31G* level of N1′H and N1′CH₃ thiamin as model systems to study the protonation and methylation effect on the structure and the electronic properties of the 4′‐amino group. The relaxed rotational barriers related to the C4′‐4′N bond are correlated with findings of ¹H NMR studies and proton/deuterium exchange experiments. Moreover, the effect of N1′ protonation was studied in more detail on the hydroxyethyl‐thiamin carbanion (HETh^?), a key intermediate during catalysis of some ThDP‐dependent enzymes. The relaxed rotational barriers related to the C2? C2α bond and the reaction coordinates of the proton transfer 4′N? H→C2α of HETh^? and N1′H‐HETh^? show that they are significantly determined by the protonation at N1′ of HETh^?. The influence of the apoenzyme environment on the active coenzyme conformation is modeled in a very simple way. The characteristic torsion angles Φ_T and Φ_P are considered to be restricted in terms of their values in the corresponding enzyme as well as free optimization parameters. Frequency calculations were performed to characterize the minima and transition state structures, respectively. The applicability of the DFT method was checked by comparing calculations on the MP2‐HF‐SCF/6‐31G* level. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Lifetimes of C-2 in rotational levels of the B̃2Σ+u state in the gas phase

Lifetimes of C^-₂ in rotational levels of the B?²Σ⁺_u:ν′ = 0, ν′ = 1 states have been measured. C^-₂ was produced from bromoacetylene and rare-gas metastables and the B?²Σ⁺_u—X?²Σ⁺_g transition was laser excited. The lifetimes are constant within a vibrational level, 77 = 8 ns for ν′= 0 and 73 = 7 ns for ν′ = 1. The oscillator strength f_νo = 0.044 ± 0.004.     

Internal energy disposal in the chemiluminescent reaction CD + NO → ND(A) + CO and the kinetic isotope effect

The vibrational and rotational energy disposal for ND(A) from the CD + NO reaction was measured in a flowing afterglow. The initial vibrational and rotational distributions of ND(A) were obtained from a spectral simulation. The initial vibrational distribution was (0.51 ± 0.05)_{ν′ = 0}: (0.26 ± 0.05)_{ν′ = 1}: (0.16 ± 0.05)_{ν′ = 2}: (0.07 ± 0.05)_{ν′ = 3}. The rotational temperatures in ν′ = 0, 1, 2 and 3 levels were 4500 ± 500, 4000 ± 500, 4000 ± 500 and 4000 ± 500 K, respectively. The fractions of the available energy deposited into the vibration, 〈fv〉 and rotation, 〈f_R〉, were 0.21 and 0.36, respectively. The results for ND(A) were compared with those for NH(A) from the CH + NO reaction reported previously and the reaction dynamics was discussed on the basis of the observed isotope effect on the energy state distributions. The kinetic isotope effect k_H/k_D in the CH, CD + NO reactions was measured to be 1.84 ± 0.23. The experimental result was compared with a theoretical calculation using transition-state theory.