邻碘甲苯分子光解动力学的飞秒时间分辨质谱研究

利用飞秒时间分辨质谱技术研究了邻碘甲苯分子在266 nm激发下的光解动力学. 光解产物碎片通过800 nm强激光场下的多光子电离实现探测. 拟合光解产物C₇H₇自由基和I原子随泵浦-探测延迟时间变化的信号,得到解离时间为38±50 fs,它反映的是266 nm同时激发nσ^*和ππ^*态后C-I键的平均解离时间. 此外,还利用基态碘原子的共振波长298.23 nm作为探测光,通过共振增强多光子电离方法对解离生成的基态碘原子进行了选择性探测. 拟合I⁺随泵浦-探测延迟时间变化的信号,得到解离时间为40±50 fs,这与通过800 nm多光子电离得到的解离时间一致,表明解离生成的主要产物是基态碘原子.     

BrCN分子的电子碰撞离子对解离

离子对解离是一类重要的分子过程,常发生于分子被激发到超激发态. 与光激发的离子对解离实验研究不同,电子碰撞的相关过程研究尚存在实验挑战,特别是在测定其阈值方面. 本文报道了相关的利用单色化电子碰撞分子的实验研究进展. 以BrCN→Br^-+CN⁺离子对解离为例,根据CN⁺离子出现能测定其解离阈值为13.78 eV,同时在16.09 eV获得了CN⁺离子的时间切片速度影像且显示出动量分布的各向异性.     

N,N-二甲基硫代乙酰胺S3(ππ*)态的衰变动力学 (cited: 1)

采用共振拉曼光谱和完全活化空间自洽场方法研究了N,N-二甲基硫代乙酰胺在被激发至S₃(ππ^*)态后的衰减动力学. 指认了紫外吸收光谱和振动光谱. 获得了乙腈、甲醇和水溶剂中不同激发波长下的A带共振拉曼光谱,以探测Franck-Condon区域的结构动力学. 开展了CASSCF计算以确定低能单重激发态和锥形交叉点的电子激发能和优化几何结构. 通过共振拉曼强度分析和CASSCF计算获得了结构参数、A带结构动力学和S₃(ππ^*)态衰减机制. 提出了主要衰减通道为_3,FC(ππ^*)→S₃(ππ^*)/S₁(nπ^*)→_1(nπ^*).     

离子速度成像系统校准及1,4-氯溴丁烷的紫外光解动力学

大气臭氧层破坏越来越严重, 卤代烷烃在太阳紫外线辐射下解离生成破坏臭氧的游离态卤素原子, 是主要元凶之一. 本文选用碘甲烷作为校准分子, 利用离子速度成像技术和共振增强多光子电离技术测得碘甲烷在266 nm紫外光解离下产生的基态碘原子I(²P_3/2)在不同聚焦电压下的离子速度影像, 得到离子速度成像系统的放大系数N=1.13. 并利用该系统研究了1, 4-氯溴丁烷在～ 234 nm紫外辐射下的解离动力学, 分析讨论了解离产生的基态Br (²P_3/2)和激发态Br^* (²P_1/2)的速度和角度分布信息, 揭示了1, 4-氯溴丁烷在～ 234 nm紫外光解离产生基态Br原子和激发态Br^* 原子的通道都是源于C-Br键排斥势能面上的快速解离. 文中通过计算碎片影像角度分布的各项异性参数β值, 得到了生成基态Br(²P_3/2)和激发态Br^* (²P_1/2) 两个解离通道中的平行跃迁和垂直跃迁比例. 另外, 本文还对氯溴甲烷, 1, 2-氯溴乙烷, 1, 3-氯溴丙烷和1, 4-氯溴丁烷在～ 234 nm下的光解动力学进行比较, 分析得到双卤代烷烃分子解离机理对烷基支链长度的依赖关系.     

四氯苯醌与嘧啶类核酸碱基的光化学氢抽取和电子转移反应

采用纳秒时间分辨的激光闪光光解实验技术,研究了四氯苯醌(TCBQ)分子与两种嘧啶核酸碱基(胸腺嘧啶和尿嘧啶)分子在355nm激发下的光化学反应动力学. 355 nm将TCBQ分子布居到激发三重态³TCBQ^*,高反应活性的³TCBQ^*与碱基分子发生两个平行的反应,氢抽取反应和电子转移反应,分别对应生成瞬态光谱上观测到的两个光反应产物TCBQH·和TCBQ·^-. 这两个反应同时生成的碱基自由基和碱基阳     

乙腈-水和乙二醇-水均相溶液中光诱导杜醌激发三重态与色氨酸、酪氨酸的氢原子转移反应

本文用激光闪光光解技术研究了光诱导生物醌杜醌激发三重态(³DQ^*)和色氨酸(Trp)与酪氨酸(Tyr)在乙腈-水(MeCN-H₂O)及乙二醇-水(EG-H₂O)均相溶液中的光化学反应,分析了反应的机理,并基于Stern-Volmer方法测量了反应速率常数. 光解DQ体系可以生成³DQ^*,³DQ^*与Trp、Tyr发生的氢原子转移反应占主导地位. 对于DQ/Trp/MeCN-H₂O和DQ/Trp/EG-H₂O溶液,³DQ^*与Trp反应生成杜醌中性自由基DQH^·、以碳为中心的色氨酸中性自由基Trp^·/NH和以氮为中心的色氨酸中性自由基Trp/N^·. 对于DQ/Tyr/MeCN-H₂O和DQ/Tyr/EG-H₂O溶液,³DQ^*与Tyr反应生成DQH^·和酪氨酸中性自由基Tyr/O^·. ³DQ^*与Trp、Tyr的氢原子转移反应速率常数都在10⁹ L·mol^-1·s^-1量级,反应近似受扩散控制. MeCN/H₂O均相溶液中³DQ^*与Trp、Tyr的反应速率常数要明显高于EG/H₂O均相溶液中的反应速率常数,这与Stokes-Einstein方程定性一致.     

苯基叠氮在光吸收激发态上的结构动力学-共振拉曼和量子力学计算研究

采用共振拉曼光谱学和完全活化空间自洽场方法研究了苯基叠氮被激发到S₂(A')、S₃(A')和S₆(A')光吸收态后的结构动力学. 基于傅立叶变换拉曼、傅立叶变换红外、紫外、密度泛函计算和简正模式分析,指认了紫外吸收光谱和振动光谱. 获得了环己烷、乙腈和甲醇溶剂中273.9、252.7、245.9、228.7、223.1和208.8 nm等不同激发波长下的A、B和C带共振拉曼光谱,以探测Franck-Condon区域的结构动力学. CASSCF计算获得了单重电子激发态能量最低点和势能面交叉点的电子激发能和优化几何结构. 结果表明,苯基叠氮在S₂(A')、S₃(A')和S₆(A')态上的激发态结构动力学各不相同. 与Kasha规则相符,S₂S₁(1)和S₂S₁(2)势能面交叉点在S₂(A')激发态衰变动力学和N7=N8键解离中扮演着重要角色. 提出了两条主要衰减通道：S_2,min→S₀辐射通道和S_2,FC(ππ^*)→S₂(ππ^*)/S₁(nπ^*)→S₁(nπ^*)非辐射通道.     

利用时间切片离子速度成像技术对AlO在193 nm的光解反应动力学研究

本文利用时间切片离子速度成像技术对AlO分子在193 nm下的光解反应动力学进行了研究. 实验通过产物Al⁺的离子速度和角度分布分析，发现了两个光解离反应通道，分别为中性AlO分子的单光子解离生成产物Al(²P_u)+O(³P_g)的通道，和AlO分子吸收两个光子电离产生AlO⁺进而解离生成产物为Al⁺(¹S_g)+O(³P_g)的反应通道. 每一个解离通道包括了AlO(v=0∽2)振动态的贡献，其中中性解离反应通道与离子解离反应通道相比，产物的各向异性参数对AlO的振动态依赖更大.     

密度泛函理论方法研究6H-SiC(0001)表面对氧分子和水分子的吸附

本文通过密度泛函方法计算6H-SiC(0001)表面对氧分子和水分子的吸附. 在6H-SiC(0001)表面上吸附的O₂分子自发地解离成O^*,并被吸收在C与Si原子之间的空位上. 吸附的H₂O自发地分解成OH^*和H^*,它们都被吸附在Si原子的顶部,OH^*进一步可逆地转化为O^*和H^*. H^*可以使Si悬键饱和并改变O^*的吸附类型,并进一步稳定6H-SiC(0001)表面并防止其转变为SiO₂.     

基于激发态运动方程耦合簇的神经网络透热势能矩阵：苯硫酚1πσ*态-介导的光解

本文用最近发展的神经网络拟合方法[Chin. J. Chem. Phys. 34,825 (2021)]构造了一个新的涉及苯硫酚¹πσ^*态-介导光解的¹ππ^*和¹πσ^*态耦合非绝热势能面. 势能面包含了解离过程中的三个关键振动模式,即S-H伸缩、弯曲和扭转振动. 由于单双激发态运动方程耦合簇方法具有简单、效率高、精度高的优点,采用激发态运动方程耦合簇方法计算了苯硫酚激发态¹ππ^*和¹πσ^*的绝热能量. 神经网络拟合绝热激发态S₁和S₂态的均方根误差分别为0.89和1.33 meV,表明神经网络方法具有很高的拟合精度. 在构建非绝热势能面的过程中,仅利用了体系绝热势能,避免了非常耗时的非绝热耦合计算,极大地提高了效率. 为了检测新的非绝热势能面的可靠性,本文进一步展开了苯硫酚光解非绝热动力学模拟. 动力学计算得到的S₁振电态0⁰和3¹的寿命与实验和之前的理论结果均吻合,验证了基于激发态运动方程耦合簇绝热能量构建的非绝热势能面的可靠精确性,并可进一步应用到实际大分子体系中.     

A priori calculation of rate of dissociation of a molecule in an ionic lattice assuming a phonon relaxation process

The rate of dissociation of the rectangular N₄ ^? ion into two diatomic molecules has been calculated in the ionic KN₃ lattice assuming the initial step is the transfer of three lattice phonons to the internal vibrational states of the molecule causing an internal vibrational excitation of the N₄ ^?. The rate of dissociation of the molecule is then calculated using a stochastic random walk approach which enables a determination of the mean time for the molecule to be vibrationally excited to the dissociative state. The results are in order of magnitude agreement with the measured reaction rates at a number of temperatures suggesting the possible role of phonon relaxation processes in unimolecular chemical reactions in the bulk solid state.     

Electron spectroscopy of autoionizing states of nitrogen atoms

During excitation of N₂, NO and N₂O molecules by fast neutral 1¹S helium atoms, highly excited molecular states are formed which yield, through predissociation, autoionizing nitrogen atoms. The energy of electrons released in autoionization was measured and the energy and plausible spectroscopic assignment of autoionizing levels was determined. The excited levels belong to Rydberg series converging to the N⁺(¹D) series limit. Besides molecular states derived by correlation of the final atoms, highly excited molecular parent states are also discussed.     

Self consistent multi-fluid FDTD simulations of a nanosecond high power microwave discharge in air

FDTD simulations of the Maxwell equations are combined with the multi-fluid plasma equations to study the dynamics of a high power microwave discharge in air. The breakdown takes place in a short time of a few nanoseconds and the concentrations of electrons, ions, excited species, and the dissociation products are quickly enhanced. The breakdown time decreases with decreasing of the pressure and the pulse amplitude, while increases with increasing of the pulse width. N${{}_2}^{+}$ and O${}_2^{+}$ are the most important positive ions, whereas O^? is the most populated negative ion. For a single microwave pulse, the electron number density is large up to 1 μs, and the dissociation and excitation continue to increase the small radicals and excited species. Then the electron number density drops and the population of excited species declines. The ozone production becomes important after 1 μs when the three body association of O and O₂ dominates over the dissociation processes. The ozone number density continues to grow up to 5 ms, and then saturates at a value of 10²² m^?3. Quenching of electronically excited nitrogen molecules by O₂ molecules and the subsequent dissociation to atomic oxygen and generation of NO, are found to be important and can play a significant role in the ultrafast gas heating.     

Multiphoton mass spectra of Xe2 molecules in the range of excited Xe*(6p, 5d) atoms

The (2 + 1) photoionization mass spectra of Xe₂ molecules are studied in a supersonic jet upon excitation by laser radiation in the energy range 80321.3–77821 cm^?1, corresponding to the dissociation of the Xe₂ molecule into atoms Xe(¹ S ₀) + Xe*(6p, 5d). Several vibrational progressions are observed, which are attributed to two-photon transitions of Xe₂ from the ground state to the excited states of the O ⁺ _g, 1_g, and 2_g symmetries. Based on the analysis of these progressions, the molecular constants of a number of excited states of Xe₂ are estimated.     

Velocity imaging spectrometer for negative fragment ions: application to dynamics of O2 and N2O ion-pair dissociation

A velocity imaging spectrometer has been developed to observe negative ions from molecular ion-pair dissociation. The imaging spectrometer is equipped with a pair of permanent magnets. The resulting magnetic field prevents efficiently the electrons from reaching the detector, without seriously affecting negative ions’ trajectories. The performance of the imaging spectrometer is demonstrated by the observations of photoelectrons from O₂ and He, and O⁻ from O₂. Application to ion-pair dissociation of O₂ and N₂O is presented, and it proves that the present imaging method provides useful information on the assignments of superexcited states and on the dynamics of ion-pair dissociation.     

Formation of nitric oxide dimers on MgO-supported gold particles

We present density functional theory (DFT) calculations on the formation of nitric oxide dimers (N₂O₂) on Au atoms, dimers and trimers adsorbed on regular O^2 ? sites and neutral oxygen vacancies (F_s sites) of the MgO(100) surface. The study of the N₂O₂ species is of great interest since it has been detected in the NO reduction reaction as an intermediate towards the formation of N₂O. We found that the coupling of a NO molecule with a previously adsorbed one on Au/MgO is energetically favorable on Au₁ and Au₃, but unfavorable on Au₂. The stability of N₂O₂ is in direct relation with the amount of charge taken from the support. Furthermore, one of the N―O bonds can be activated as a result of the attraction between the negatively charged NO dimer and the ionic oxide surface. In fact, for Au₁ anchored on the F_s site a barrierless reaction occurs between N₂O₂ and a third NO molecule, forming adsorbed N₂O and NO₂.     

Nitrogen related excitation processes in a mercury bromide laser discharge

The role of nitrogen as a buffer gas constituent in an avalance discharge mercury bromide laser has been examined, and it is shown that a change over in excitation from electron impact dissociation of HgBr₂ to energy transfer dissociation by excited nitrogen occurs at 1.5%N ₂. This is explanable in terms of excitation cross-sections if considerable pumping occurs from excited states of nitrogen which are higher in energy than the metastableA state. When excitation via nitrogen is dominant, a high dischargeE/N value is needed to avoid coupling of energy to unwanted vibrational excitation.     

Laser isotope separation of carbon by multiple IR photon and subsequent UV excitation of CF3I molecules

A new approach to laser isotope separation is considered. It is based on collisionless multiple photon ir laser excitation and subsequent uv laser dissociation of vibrationally excited molecules. TEA CO₂ and excimer XeF, XeCl lasers are used for ir excitation and uv dissociation, respectively. The products of photolysis (C₂F₆) are enriched with¹²C.     

The primary stages of the charge carrier photogeneration in C60 films studied by the 100-fs laser pulse pump-probe method

The primary stages of photoinduced processes are studied in thin C₆₀ films by the femtosecond laser pump-probe method. The films were excited by 100-fs laser pulses with photon energies above (wavelengths 345 and 367 nm) and below (645 nm) the mobility threshold, the fraction of excited molecules being no more than several percent. Upon probing in the spectral range from 400 to 1100 nm, several regions with substantially different decay kinetics were observed in the difference spectrum, which is caused by the simultaneous presence of several relaxing components. The appearance of the 465-and 500-nm bleaching bands in the difference spectrum upon excitation by photons with energies both above and below the mobility threshold, which are typical for electroabsorption spectra, suggests that charge carriers are produced in both these cases. The observed dependence of relaxation on the oxygen amount in the sample volume suggests that during excitation both charged (electrons and holes) and neutral (excited molecules) components are produced. The fraction of charged components is greater upon excitation into the fundamental band. The appearance of the 500-nm absorption band delayed by 10^?13–10^?14 s, the delay being increased in the presence of oxygen, was attributed to the formation of excited anions due to the capture of electrons by C₆₀ molecules. It is concluded that upon excitation of the films by photons with the energy below the mobility threshold, charge carriers are produced due to two-photon absorption rather than due to singlet-singlet annihilation. When the films are excited by photons above the mobility threshold, the primary charge carriers are produced by direct optical excitation.     

On the dissociation of the ozone dication

A mechanism for the dissociation of the ozone dication formed by double ionisation of neutral O₃ is determined by ab initio calculations. The dication ground singlet state is found to have a linear equilibrium geometry that is stable with respect to dissociation to O⁺ and O₂⁺. However, at the Franck-Condon zone for formation of O₃²⁺ from the neutral molecule the singlet potential energy surface intersects with a dissociative triplet state. We propose that crossing to this dissociative triplet state can account for the absence of any long-lived O₃²⁺ ions in the electron-impact mass spectrum of ozone. Further calculations of the kinetic energy release for the fragmentation of O₃²⁺ to O⁺+O₂⁺ indicate that considerable vibrational excitation may be present in the O₂⁺ ion.