Wavelength-dependent Photodissociation Dynamics of Benzaldehyde

利用飞秒泵浦-探测方法结合飞行时间质谱研究了苯甲醛在不同泵浦波长260、271、284和287 nm下的超快动力学. 在S₂区(λ<284 nm),受激分子首先经过内转换过程到S₁高振动态,实验观察到这个过程随着S₂态分子振动能量的增加而变快,可能源于高振动能级密度导致S₂与S₁之间内转换加快. 双指数拟合的第二个过程(约600 fs)认为可能是由于内转换后在S₁态发生的振动     

飞秒瞬态吸收光谱技术研究反式对氨基偶氮苯的超快激发态动力学 (cited: 1)

利用飞秒瞬态吸收光谱技术研究了反式对氨基偶氮苯(AAB)溶解在乙醇中的超快激发态动力学．当利用400 nm光激发分子到S₂态后,由S_{211102412相似文献}   

N-乙基吡咯S1态超快动力学：飞秒时间分辨光电子成像研究

N-乙基吡咯是吡咯分子的一个乙基取代衍生物,它的激发态衰变动力学目前为止很少被研究. 本文利用飞秒时间分辨光电子成像的实验方法研究N-乙基吡咯分子S₁态的衰变动力学. 实验采用241.9和237.7 nm的泵浦激发波长. 在241.9 nm激发下,得到5.0±0.7 ps,66.4±15.6 ps和1.3±0.1 ns三个寿命常数. 在237.7 nm激发下,得到2.1±0.1 ps和13.1±1.2 ps两个寿命常数. 所有寿命常数都归属为S₁态的振动态. 本文并对不同S₁振动态的弛豫机理进行了讨论.     

紫外光解动力学DNCO+hv→D+NCO: 两个竞争解离通道

本文利用D原子里德堡态时间飞行谱研究了DNCO分子在波长200∽235 nm范围的光解动力学. 实验测量了产物的平动能分布和空间角分布. 在210∽235 nm光解离下,观测到接近统计分布和各向同性的产物,该产物有可能来自从S₁势能面内转换到S₀势能面,然后在S₀势能面上解离. 在更短的解离波长下,除了统计分布的产物,另外一种分布的产物出现在高平动能的地方,具有很高的各向异性,该产物来自从S₁势能面上的直接解离. 相比较HNCO的解离结果,DNCO直接解离通道出现在更高的激发能量. 通过对NCO产物内能态的归属,发现NCO产物主要是弯曲振动激发和适当的伸缩振动激}.     

基于供体调节的含硼热活化延迟蓝色荧光材料的理论研究

基于含硼热活化延迟荧光化合物p-AC-DBNA (a),通过在供体吖啶部分添加两个氮原子,设计了一系列新的热活化延迟荧光分子b1∽b4. 使用密度泛函理论和含时密度泛函理论计算了化合物a和b1∽b4的前线轨道能级、发射光谱、单重态-三重态能隙、反向系间窜越速率常数. 计算结果表明,与分子a相比,新设计分子的最大发射波长显著蓝移了47∽125 nm. 其中,b1和b3分子发射深蓝色荧光,而b2和b4分子发射浅蓝色荧光. 此外,发现了a、b2和b4分子中的反向系间窜越过程不仅可以通过从T₁态到S₁态的跃迁来实现,而且也可能通过从T₂态到S₁态的跃迁来实现. b1和b3分子中的反向系间窜越过程主要通过T₂→S₁过程的热激子方式发生. 重要的是,计算得到的b2和b4分子中T₁→S₁的反向系间窜越速率常数值是a分子的两到三倍,这表明b2和b4分子热活化延迟荧光性能更显著. 研究结果不仅提供了两种有前景的硼基热活化延迟荧光候选化合物(b2和b4),而且为蓝色OLED材料的设计提供了理论依据.     

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.     

3-二甲氨基-2-甲基丙烯醛S2激发态结构动力学 (cited: 3)

采用共振拉曼光谱和完全活性自洽场理论计算研究了3-二甲氨基-2-甲基丙烯醛(DMAMP)光激发到S₂(ππ^*)态后的光物理性能．在B3LYP/6-311++G(d,p)水平计算确定了DMAMP与其三种异构体之间的基态异构化能垒,指认了振动光谱．采用涵盖紫外强吸收带的激光波长,获得了DMAMP在环己烷、乙腈和甲醇溶剂中的A-带共振拉曼光谱,含时密度泛函方法计算确定了该光谱中基频的相对强度,发现振动-电子耦合发生在S₂(ππ^*)态的Franck-Condon区域．CASSCF计算方法确定低单重和三重激发态、势能面锥形交叉点和系间窜跃点的激发能．共振拉曼光谱强度模式分析和CASSCF计算获得了DMAMP的A-带短时结构动力学和其后的衰变动力学表明,C1=O6和C2=C3之间的瞬时去共轭效应发生在S₂(ππ^*)态的Franck-Condon区域,激发态电荷重分布机制表明,C3和二甲氨基之间以及C1和C2之间的共轭增强效应发生在波包离开Franck-Condon区域后．C1=O6和C2=C3之间的去共轭效应使得-C3=N(CH₃)₂沿着C2-C3键旋转更加容易,C1-C2之间以及C3和N(CH₃)的共轭增强效应使得绕C1-C2和C3-N5旋转变得比较困难．这些表明DMAMP初始结构动力学沿着CI-1(S₂/S₀)交叉点展开,而沿CI-2(S₂/S₀)和CI-3(S₂/S₀)交叉点展开的几率可以忽略．提出了DMAMP分子受光激发从S_2,FC(ππ^*)经由各锥形交叉点和各系间窜跃点回到S₀或T_1,min的两个衰变通道.     

利用时间分辨的光电子影像技术研究2,6-二甲基吡啶分子锥形交叉超快动力学

利用时间分辨的飞秒光电子影像技术结合时间分辨的质谱技术,研究了2,6-二甲基吡啶分子锥形交叉超快动力学过程. 2,6-二甲基吡啶分子吸收266 nm泵浦光从基态跃迁至S₂态(π-π^*). 母体离子时间变化曲线包含两个指数函数,一个是时间常数为635 fs的快速组分,另一个是时间常数为4.37 ps的慢速组分. 通过时间分辨光电子影像得到的时间依赖的光电子角度分布和能量分辨的光电子谱分布提供了S₂态演变的动力学信息. 简言之,快速组分反映了通     

锶原子三重态谱线的观测及在光钟中的应用

实验中通过互组跃迁689 nm激光抽运形成三重态最低能态原子布居,引入688 nm激光改变三重态最低能态间的原子布居,利用抽运光与探测光空间分离的方法观测碱土金属锶原子的三重态能态间跃迁(5s6s)³S₁ → (5s5p)³P_j(j=0,1,2)的吸收谱线,对应三条跃迁线的激光波段为679 nm, 688 nm和707 nm.探测三重态原子跃迁谱线可以用于锶原子冷却中再抽运光707 nm和679 nm激光频率的直接锁定,相比于通常利用的腔传递技术,可以把再抽运光频率锁定在原子跃迁谱线上,有利于提高锶原子冷却中俘获原子数目的长期稳定性.     

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.     

乙醛光解动力学的离子速度成像

利用时间切片离子速度成像技术在275～321 nm能量范围内重新研究了乙醛自由基通道CH₃+HCO的光解动力学. 通过共振增强多光子电离的方法探测甲基碎片. 对甲基的伞形振动基态和激发态(v₂=0和1)进行了影像探测. 乙醛通过T₁电子态系间窜越到S₁电子态的解离产物具有很高的动能释放和很低的内能激发,碎片的振动能和转动能随激发能量的增加而增加. 乙醛T₁电子态的势垒高度经测量高于基电子态3.881±0.006 eV.     

超短脉冲激光场中间二氯苯的激发态动力学

利用飞秒时间分辨的飞行时间质谱技术研究了间二氯苯的激发态动力学.间二氯苯分子吸收一个200 nm或者267 nm的光子被抽运到激发态,随后再吸收多个800 nm的光子被电离.实验获得了电离产生的离子质谱信号及其随抽运探测激光延迟时间的变化曲线.在200 nm时,分子被抽运到激发态(π,π*),可观察到三个相互竞争的解离通道的寿命:内转换到排斥态(n,σ*)或者(π,σ*)并发生快速解离,其寿命约(0.15±0.01)ps;内转换到基态的高振动态,能量在基态"热"振动态间弛豫的寿命约为(4.94±0.08)ps;系间窜越到相邻的三重态从而发生预解离过程,其寿命约为(110.09±4.33)ps.在267 nm时,分子被抽运到第一激发态的低振动态,可观察到一个长寿命(约(1.06±0.05)ns)的系间窜越过程.除此之外,在碎片离子信号中还观察到了激发态与基态的高振动态之间的内转换过程.     

Indole photodissociation in the gas phase

Kinetic characteristics of the growth in delayed luminescence intensity of indole in the gas phase have been measured with excitation of the molecules in the spectral region 250–287 nm. The luminescence build-up kinetics show two components with characteristic lifetimes of about 1 and 10 μs that are associated with the formation of free radicals due to N–H bond dissociation. Two basic mechanisms for photodissociation of indole in the gas phase have been identified based on an analysis of the dependence of the kinetic characteristics for these components on the excitation radiation wavelength/intensity and the vapor pressures of indole and foreign gases. The fast component results from the dissociation of an N–H bond of vibrationally excited molecules in the ground state that is populated through internal conversion. The slow component is associated with the generation of free radicals because of annihilation of two triplet indole molecules. A dependence of the quantum yield of intersystem crossing on the excitation wavelength in the spectral region 260–287 nm has been found. It is shown that the fluorescence quantum yield drop with a decrease of the excitation radiation wavelength is caused by an increase in the internal conversion quantum yield to the electronic ground state.     

EPR of photo-excited triplet states: A personal account

A sketch is presented of the path that has led from Zavoisky’s pioneering experiments to modern investigations by electron paramagnetic resonance (EPR) of the phosphorescent (S = 1) triplet state of polyatomic molecules or ions. The group-theoretical method first introduced by Wigner in his analysis of the multiplets of atomic spectroscopy, likewise provides a key for understanding the zero-field splitting and selection rules for radiative decay of the phosphorescent triplet state. Examples to illustrate the progress made through EPR experiments are selected from three fields. (i) Conformational instability on excitation. Both the zero-field splitting and the electron spin density distribution provide unique fingerprints of a triplet state’s geometry — structural information of a kind that is nonexistent for singlet states! Illustrations are provided by benzene C₆H₆ and fullerene C₆₀. (ii) The optical pumping cycle. The spin selectivity of singlet-to-triplet intersystem crossing and radiative decay of the individual spin components of the triplet state is discussed. In practice this selectivity is put to advantage by performing EPR on triplet states in zero-field by means of optical detection. In turn, such experiments have led to a detailed insight into the spin-orbit coupling mechanisms responsible for the spin selectivity of the above processes. The high sensitivity attainable with optical detection has recently culminated in EPR experiments on single molecules. (iii) Quantum interference. In a triplet state of low symmetry two of the spin sublevels may decay to the ground state by the emission of photons of a common polarization (i.e., out of plane for an aromatic hydrocarbon). In such a situation quantum interference between the two decay channels can be induced by an appropriate preparation of the excited state. An example is shown where flash-excitation in the singlet manifold followed by rapid intersystem crossing causes theS = 1 spin angular momentum to be created in a spin state which is not an eigenstate of the zero-field splitting tensor. This nonstationary character of the initial triplet state, which reflects the spin-orbit coupling pathway, is observed through the detection of a spontaneous microwave signal following the 25 ps laser flash.     

Singlet and Triplet State Properties and Singlet Oxygen Yield of an Amino-Acyl-Quinoxalinone Yellow Dye

Amino-acyl-quinoxalinone yellow dyes are cyclised analogues of the yellow azomethine dyes developed for, and still used in, silver halide colour photography. Unlike image azomethine dyes, which are rapidly deactivated in their excited states by torsion about the azomethine bond, amino-acyl-quinoxalinone dyes have an interesting photophysics because torsion is not possible due to their cyclised structure. We report results from studies on singlet and triplet state properties, and singlet oxygen yields, of the yellow dye, 7-diethylamino-3-(2,2-dimethyl-propionyl)-5-methyl-1-phenyl-1H-quinoxalin-2-one, in polar and nonpolar solvents. The dye photophysics is characterised by a weak fluorescence, with a solvent dependent emission yield (Φ_F?≈?0.002–0.004), and short singlet state lifetime (τ_expt?≈?20–50 ps), both increasing by a factor of ≈2 in going from polar acetonitrile to non-polar dioxane as solvent. DFT ZINDO calculations show a transition involving significant electron transfer from the diethyl-amino group into the carbonyl region of the molecule. In solution, in the presence of oxygen, the triplet state decays almost exclusively by oxygen quenching, and singlet oxygen is produced in high yield (Φ_??≈?0.5–0.55). The triplet state absorbs across the 450–750 nm region with maxima around 480 and 650 nm, and moderate molar absorption coefficients (ca. 6000–8000 M^?1 cm^?1). In a glass at 77 K, triplet decay gives a red phosphorescence, with λ_max?≈?640–650 nm, and a ?≈?0.25 s lifetime. If singlet oxygen yields are a good indication of triplet yields, then internal conversion and intersystem crossing occur with roughly equal efficiency.

     

B-type delayed fluorescence of rubreneperoxide in solution.

B-type of delayed fluorescence was observed for the first time for rubreneperoxide. Rubreneperoxide molecules were excited in a two step process. In the first step an excited singlet S₁ is created, which undergoes intersystem crossing to T₁; then T-T absorption creates an excited triplet rubreneperoxide molecule, which returns to the first excited singlet level by intersystem crossing. The recreated first excited singlet of rubreneperoxide decays back to the ground state by emitting B-type of delayed fluorescence.     

The Excited Triplet State of Azoalkanes: Electron Spin Polarization and Magnetic Field Effects During Triplet-Sensitized Photolysis of trans-Azocumene in Solution

The triplet-sensitized photodecomposition of azocumene into nitrogen and cumyl radicals is investigated by time-resolved electron paramagnetic resonance and absorption spectroscopy. The radicals are found to be created spin polarized with a yield depending on the strength of the applied magnetic field. The phenomenon arises because in triplet azocumene, the decay into radicals competes with a fast triplet-sublevel selective intersystem crossing back to the azocumene ground state. The size of the initial spin polarization of the radicals and the magnetic field effect on their yield are determined in solvents of different viscosities. Data analysis yields rate constants for the intersystem crossing and the cleavage reaction of triplet azocumene as well as its zero-field splitting D _ZFS. At room temperature in nonpolar solvents, the most probable values are: k _x ?=?k _y ?=?1.2?×?10¹¹?s^?? and k _z ?=?1.9?×?10¹⁰?s^?? for the intersystem crossing from the energetically lower and upper triplet substates, respectively, k _p ?=?1.6?×?10⁹?s^?? for the cleavage reaction and for the zero-field splitting D _ZFS?=???.4?×?10^10?s^?? (0.18?cm^??).     

High Resolution B-Type Delayed Fluorescence of Dibenzanthracene in PMMA

B-Type delayed fluorescence of 1,2,3,4-dibenzanthracene in PMMA was experimentally observed for the first time. Dibenzanthracene molecules were exerted in a two-step process. In the first step, an excited singlet S₁ is created, which undergoes intersystem crossing to T₁ then T-T absorption creates an excited triplet dibenzanthracene molecule, which returns to the first exerted singlet level by intersystem crossing. The recreated first excited singlet of dibenzanthracene decays back to the ground state by emitting this new type of delayed fluorescence.     

The lowest triplet state of Zn porphin

The dynamics of populating and depopulating the phosphorescent triplet state of Zn porphin in an n-octane matrix is investigated by microwave induced delayed phosphorescence experiments at 1·2K. Values for the absolute rates of decay of the three spin levels are given and also the relative steady-state populations, radiative decay rates and probabilities for entry through intersystem crossing. Further, it is established that the phosphorescence is inplane polarized. The results are compatible with the decay modes expected for a lowest vibronic state that deviates from D ₄ symmetry by combined action of the Jahn-Teller effect and crystal field anisotropy.