多光子电离伴随的碎片化过程的速率方程分析

提出一个描述多原子分子多光子激发、电离和离子再被激发过程的布局速率方程模型，并且得到电离效率和离子体系吸收的平均能量＜E＞的代数解．通过计算这两个量与光强的关系，分析了不同电离机制下，多原子分子多光子电离实验中伴随的碎片化过程的控制性问题．结果表明只有采用1＋1电离方案，可以通过控制电离激光的强度来实现“软电离”和“硬解离”．采用3＋1电离，即使在单离子条件下，离子的＜E＞已高达20－30eV，远超过离子的解离阈值，特别是增加光强时，＜E＞的增加速度比电离效率增加得快，因此一般条件下不能让分子有效电离而不使其离子解离．上面的分析可以圆满解释呋喃等分子在3＋1电离时观察不到母体离子这个实验事实．     

CCl2自由基与H2O分子反应动力学研究

用213 nm激光光解CCl4产生CCl2自由基，用LP LIF技术测定了室温下基态CCl2自由基与H2O分子的反应速率常数为(5.45±0.95)×10－14 cm3•molecule－1•s－1.在G2MP2理论水平上计算了CCl2＋H2O反应的最低单重态势能面，揭示了插入与加成 消除两种反应机理，得到了三个可能的产物通道:HCl＋HClCO、HCl＋trans ClCOH以及HCl＋cis ClCOH.并用RRKM TST和传统过渡态理论计算了这三个通道的分支比及其温度效应.结果说明在低温下（273 K），插入机理的产物通道的分支比远大于加成 消除机理的产物通道， HCl＋HClCO是主要产物，分支比为77.4％，其次是HCl＋cis ClCOH，分支比为22.6％.而在高温下（3000 K），加成 消除机理的反应通道大于插入机理， HCl＋trans ClCOH分支比为82.3％.     

Ca+-叔丁胺络合物的激光诱导反应

使用反射式飞行时间质谱仪, 研究了Ca⁺-叔丁胺络合物在激光诱导下的反应. 得到了反应的光解谱和作为波长函数的光解行为光谱以及各反应通道的分支比. 反应有两个通道, Ca⁺-与分子的解离通道和生成产物Ca⁺-NH₂的反应通道, Ca⁺-是主要产物, 而且在整个激光扫描的范围都存在, 并且在530～595 nm波段是唯一的产物. 反应的光解行为光谱显示出明显的无结构的峰, 分别对应于络合物的跃迁. 结合反应通道的分支比以及量化计算, 对这些峰进行了指认, 并初步探讨了反应的动力学机理.     

间-吡啶基紫外光化学中的氢原子解离通道

利用高里德堡态氢原子飞行时间探测技术, 在224～248 nm激发波长研究了间-吡啶基紫外光化学中的氢原子解离通道的动力学过程．氢原子光解碎片产率谱显示在234 nm附近有较宽的吸收．产物的平动能释放较小;在224～248 nm激发波长区间平均〈 fT〉是0.12～0.19．产物的平动能分布显示产物是H＋HC≡C-CH=CH-C≡N,H 3,4-吡啶和H 2,3-吡啶,以H HC≡C-CH=CH-C≡N为主要的氢原子生成通道．氢原子碎片具有各向同性的角度分布．研究结果表明,在紫外电子态激发以后,间-吡啶基经过内转换到电子基态,再经由单分子解离到H HC≡C-CH=CH-C≡N,H 3,4-吡啶和H 2,3-吡啶产物．间-吡啶基的紫外光解机理和以前报道过的邻-吡啶基的紫外光解机理相似．     

2-溴噻吩和3-溴噻吩在267nm的C—Br键解离机理

利用离子速度影像技术,研究了2-溴噻吩和3-溴噻吩两种同分异构体在267 nm激光作用下的C-Br键解离机理,获得了光解产物Br(2P3/2)和Br*(2p1/2)的能量和角度分布,分析了两异构分子在267 nm的C-Br键解离通道.对于2-溴噻吩和3-溴噻吩,产物Br来源于三个通道:(i)从单重激发态系间窜跃到排斥的三重激发态的快速预解离;(ii)单重激发态内转化到高振动基态的热解离;(iii)母体分子多光子电离后的解离.2-溴噻吩的产物Br*具有类似的产生机制;但对于3-溴噻吩,从激发态内转换到高振动基态发生热解离成为产物Br*的主导通道,而来自激发三重态的快速预解离通道则几乎消失.定量地给出了各个通道的相对贡献、能量分配及各向异性分布信息.实验发现,随着溴原子在噻吩上取代位置远离硫原子,来自通道(i)和(ii)产物之间的比例明显减小,相应的各向异性分布有变弱趋势.     

氯甲烷分子在13至17 eV激发能量范围内的电离解离

利用阈值光电子-光离子符合飞行时间质谱研究了氯甲烷分子在13至17eV激发能量范围内的光电离和光解离动力学.在此能量范围内,电离产生的CH3Cl+离子处于A2A1和B2E电子激发态.两电子态均为完全解离态,可生成CH3+和CH2Cl+碎片离子,其中CH3+是最主要的解离产物.拟合CH3+离子的符合飞行时间质谱峰形,可以得到CH3Cl+离子解离过程中释放的平动能分布,结果显示CH3Cl+离子A2A1态解离生成CH+3的过程接近直接解离机理,而B2E态的解离过程则具有统计解离的特征.此外,结合理论计算的势能面信息,我们推测在A2A1态出现的CH2Cl+碎片离子来源于CH3Cl分子自电离产生高振动激发的CH3Cl+(X2E)离子统计解离过程.     

2-溴噻吩和3-溴噻吩在267 nm的C-Br键解离机理

利用离子速度影像技术, 研究了2-溴噻吩和3-溴噻吩两种同分异构体在267 nm激光作用下的C—Br键解离机理, 获得了光解产物Br(2P3/2)和Br*(2P1/2)的能量和角度分布, 分析了两异构分子在267 nm 的C—Br键解离通道. 对于2-溴噻吩和3-溴噻吩, 产物Br来源于三个通道: (i) 从单重激发态系间窜跃到排斥的三重激发态的快速预解离; (ii)单重激发态内转化到高振动基态的热解离; (iii) 母体分子多光子电离后的解离. 2-溴噻吩的产物Br*具有类似的产生机制; 但对于3-溴噻吩, 从激发态内转换到高振动基态发生热解离成为产物Br*的主导通道, 而来自激发三重态的快速预解离通道则几乎消失. 定量地给出了各个通道的相对贡献、能量分配及各向异性分布信息. 实验发现, 随着溴原子在噻吩上取代位置远离硫原子, 来自通道(i)和(ii)产物之间的比例明显减小, 相应的各向异性分布有变弱趋势.     

噻吩光解反应机理的理论研究

使用密度泛函理论(DFT)中的B3LYP方法, 采用6-31G**和6-31＋＋G**基组, 对噻吩的光解反应进行了理论研究. 对照实验结果, 我们研究了五个光解通道, 包括生成C₄H₄＋S, C₂H₂＋C₂H₂S和CS＋C₃H₄的三个闭壳层分子解离通道与生成HCS＋C₃H₃和HS＋C₄H₃的自由基解离通道. 各个可能的反应通道的产物碎片的具体形式得到了确认. 研究发现在基态生成C₂H₂＋C₂H₂S和在最低三态生成C₄H₄＋S的反应从能量上考虑最为有利, 而实验上观测到的主要产物C₂H₂＋C₂H₂S主要是在基态上产生的. 通过对比实验结果与计算结果, 我们认为噻吩光解反应机理与所用激发光波长有关.     

CH3NO2与HE(23S)、Ne(3P0.2)的解离激发反应

利用分子束和化学发光技术，在单次碰撞条件下，首次研究了亚稳态原子He（23S）、Ne（3P0．2）与CH3NO2的解离激发反应，探测到反应的激发态产物（CH（A）、CH（B）、CH（C）的化学发光，在He（23S）／CH3NO2反应中同时探测到H（Balmer）的发射．利用He（23S）＋N2→N2＋（B）＋He＋e－作参考反应，测定了反应He（23S）／CH3NO2产生的CH的A－X，B－X，C－X以及H原子的发射速率常数．利用化学发光光谱的计算机模拟，求得了激发态产物CH（A）的初生态振动布居和转动温度．结合相空间理论对解离过程CH（A）的形成通道进行了讨论，认为CH（A）的形成是经由中间体CH3*的二体解离过程．     

量子态选择的分子离子解离动力学：阈值光电子-光离子符合成像技术的应用

阈值光电子一光离子符合速度成像技术通过对光电子和符合的光离子同时进行速度聚焦控制,大幅提高了电子和离子的收集效率和离子平动能分辨率,成为开展气相分子光电离和光电离．解离动力学研究的有效工具．利用该技术,我们精确地测量了分子的电离能、离子出现势等重要参数,并且开展了若干具有量子态或内能选择的离子解离动力学研究,描绘了相关势能面存在浅势阱等重要动力学特征,讨论了不同振动态和电子态的激发对解离机理和产物通道的重要影响．     

Bond-selective photodissociation of partially deuterated ammonia molecules: photodissociations of vibrationally excited NHD(2) in the 5nu(NH) state and NH(2)D in the 5nu(ND) state

Ultraviolet photodissociation of NHD(2) excited to the fourth overtone state of the NH stretching mode (5nu(NH)) and NH(2)D excited to that of the ND stretching mode (5nu(ND)) has been investigated by using a crossed laser and molecular beams method. Branching ratio between the NH and ND bond dissociations has been determined by utilizing a (2+1) resonance enhanced multiphoton ionization scheme of H and D atoms. For the photolysis of NHD(2) in the 5nu(NH) state, the NH dissociation cross section is 5.1+/-1.4 times as large as the ND dissociation cross section per bond. On the other hand, for the photolysis of NH(2)D in the 5nu(ND) state, the ratio of the NH dissociation cross section per bond to the ND dissociation cross section decreases to 0.68+/-0.16. In comparison with the branching ratios for the photolysis of vibrationally unexcited NH(2)D and NHD(2), the present results indicate that the excitation of the NH stretching mode enhances the NH dissociation with ca. two times larger NH/ND branching ratio, whereas the excitation of the ND stretching mode results in the preferential ND dissociation with ca. 3-4 times larger ND/NH branching ratio than that for the vibrational ground states. The mechanism of the bond-selective enhancement has been discussed in terms of the energetics and dynamics of wave packet.     

Superexcited state reconstruction of HCl using photoelectron and photoion imaging

The velocity-map imaging technique was used to record photoelectron and photofragment ion images of HCl following two-photon excitation of the E Sigma(+)(0+), V 1Sigma(+)(0+) (nu=9,10,11) states and subsequent ionization. The images allowed us to determine the branching ratios between autoionization and dissociation channels for the different intermediate states. These branching ratios can be explained on the basis of intermediate state electron configurations, since the configuration largely prohibits direct ionization in a one-electron process, and competition between autoionization and dissociation into H* (n=2)+Cl and H+Cl*(4s,4p,3d) is observed. From a fit to the vibrationally resolved photoelectron spectrum of HCl+ it is apparent that a single superexcited state acts as a gateway to autoionization and dissociation into H+Cl*(4s). Potential reconstruction of the superexcited state to autoionization was undertaken and from a comparison of different autoionization models it appears most likely that the gateway state is a purely repulsive and low-n Rydberg state with a (4Pi) ion core.     

Vibrational mode and collision energy effects on reaction of H2CO+ with C2H2: charge state competition and the role of Franck-Condon factors in endoergic charge transfer

The effects of collision energy (E(col)) and six different H(2)CO(+) vibrational states on the title reaction have been studied over the center-of-mass E(col) range from 0.1 to 2.6 eV, including measurements of product ion recoil velocity distributions. Ab initio and Rice-Ramsperger-Kassel-Marcus calculations were used to examine the properties of complexes and transition states that might be important in mediating the reaction. Reaction is largely direct, despite the presence of multiple deep wells on the potential surface. Five product channels are observed, with a total reaction cross section at the collision limit. The competition among the major H(2) (+) transfer, hydrogen transfer, and proton transfer channels is strongly affected by E(col) and H(2)CO(+) vibrational excitation, providing insight into the factors that control competition and charge state "unmixing" during product separation. One of the more interesting results is that endoergic charge transfer appears to be controlled by Franck-Condon factors, implying that it occurs at large inter-reactant separations, contrary to the expectation that endoergic reactions should require intimate collisions to drive the necessary energy conversion.     

Vibrationally mediated photodissociation of C2H4+

We report the vibrationally mediated photodissociation dynamics of C2H4+ excited through the B2Ag state. Vibrational state-selected ions were prepared by two-photon resonant, three-photon ionization of ethylene via (pi, 3s) and (pi, 3p) Rydberg intermediate states in the wavelength range 298-349 nm. Absorption of a fourth photon led to dissociation of the cation, and images of the product ions C2H3+ and C2H2+ were simultaneously recorded using reflectron multimass velocity map imaging. Analysis of the multimass images yielded, with high precision, both the total translational energy distributions for the two dissociation channels and the branching between them as a function of excitation energy. The dissociation of ions that were initially prepared with torsional excitation exceeding the barrier to planarity in the cation ground state consistently gave enhanced branching to the H elimination channel. The results are discussed in terms of the influence of the initial state preparation on the competition between the internal conversion to the ground state and to the first excited state.     

Vibrationally mediated photodissociation of ethylene cation by reflectron multimass velocity map imaging

A new imaging technique, reflectron multimass velocity map ion imaging, is used to study the vibrationally mediated photodissociation dynamics in the ethylene cation. The cation ground electronic state is prepared in specific vibrational levels by two-photon resonant, three-photon ionization via vibronic bands of (pi, nf) Rydberg states in the vicinity of the ionization potential of ethylene, then photodissociated through the (B 2A(g)) excited state. We simultaneously record spatially resolved images of parent C2H4+ ions as well as photofragment C2H3+ and C2H2+ ions originating in dissociation from the vibronic excitations in two distinct bands, 7f 4(0)2 and 8f 0(0)0, at roughly the same total energy. By analyzing the images, we directly obtain the total translation energy distributions for the two dissociation channels and the branching between them. The results show that there exist differences for competitive dissociation pathways between H and H2 elimination from C2H4+ depending on the vibronic preparation used, i.e., on the vibrational excitation in the ground state of the cation prior to photodissociation. Our findings are discussed in terms of the possible influence of the torsional excitation on competition between direct dissociation, isomerization, and radiationless transitions through conical intersections among the numerous electronic states that participate in the dissociation.     

Two-photon dissociation of vibrationally excited H2+. Complex quasi-vibrational energy and inhomogeneous differential equation approaches

We present two practical theoretical methods — the complex quasi-vibrational energy and the inhomogeneous differential equation approaches — for numerical computation of multiphoton dissociation cross sections. The methods are applied to the study of the two-photon dissociation of H₂⁺ (1sσg). The cross sections are small for low-lying vibrational states but increase very rapidly with increasing vibrational quantum number, suggesting that experimentally accessible powerful lasers can be used to probe the highly excited vibrational states of the ground electronic state of a homonuclear diatomic molecule.     

Dissociative recombination study of Na+ (D2O) in a storage ring

The dissociative recombination of Na(+)(D(2)O) ion has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The cross section has been measured as a function of center-of-mass energy ranging from 1 meV to 0.1 eV and found to have an E(-1.37) dependence. The rate coefficient has been deduced to be (2.3+/-0.32)x10(-7)(T(e)/300)(-0.95+/-0.01) cm(3) s(-1) for T(e)=50-1000 K. The branching ratios have been measured at 0 eV. Of the four energetically accessible dissociation channels, three channels are found to occur although the channel that breaks the weak Na(+)-D(2)O bond is by far dominant.     

Vibrational mode and collision energy effects on reaction of H2CO+ with C2D4

We report the effects of collision energy (Ecol) and five different H2CO+ vibrational modes on the reaction of H2CO+ with C2D4 over the center-of-mass E(col) range from 0.1 to 2.1 eV. Properties of various complexes and transition states were also examined computationally. Seven product channels are observed. Charge transfer (CT) has the largest cross section over the entire energy range, substantially exceeding the hard sphere cross section at high energies. Competing with CT are six channels involving transfer of one or more hydrogen atoms or protons and one involving formation of propanal, followed by hydrogen elimination. Despite the existence of multiple deep wells on the potential surface, all reactions go by direct mechanisms, except at the lowest collision energies, where short-lived complexes appear to be important. Statistical complex decay appears adequate to account for the product branching at low collision energies, however, even at the lowest energies, the vibrational effects are counter to statistical expectations. The pattern of Ecol and vibrational mode effects provide insight into factors that control reaction and interchannel competition.     

Theoretical study of the benzyl+O2 reaction: kinetics, mechanism, and product branching ratios

Ab initio calculations at the level of CBS-QB3 theory have been performed to investigate the potential energy surface for the reaction of benzyl radical with molecular oxygen. The reaction is shown to proceed with an exothermic barrierless addition of O2 to the benzyl radical to form benzylperoxy radical (2). The benzylperoxy radical was found to have three dissociation channels, giving benzaldehyde (4) and OH radical through the four-centered transition states (channel B), giving benzyl hydroperoxide (5) through the six-centered transition states (channel C), and giving O2-adduct (8) through the four-centered transition states (channel D), in addition to the backward reaction forming benzyl radical and O2 (channel E). The master equation analysis suggested that the rate constant for the backward reaction (E) of C6H5CH2OO-->C6H5CH2+O2 was several orders of magnitude higher that those for the product dissociation channels (B-D) for temperatures 300-1500 K and pressures 0.1-10 atm; therefore, it was also suggested that the dissociation of benzylperoxy radicals proceeded with the partial equilibrium between the benzyl+O2 and benzylperoxy radicals. The rate constants for product channels B-D were also calculated, and it was found that the rate constant for each dissociation reaction pathway was higher in the order of channel D>channel C>channel B for all temperature and pressure ranges. The rate constants for the reaction of benzyl+O2 were computed from the equilibrium constant and from the predicted rate constant for the backward reaction (E). Finally, the product branching ratios forming CH2O molecules and OH radicals formed by the reaction of benzyl+O2 were also calculated using the stationary state approximation for each reaction intermediate.