飞秒时间分辨的光电子影像对3-甲基吡啶分子超快动力学研究(英文)

利用飞秒时间分辨的光电子影像技术结合时间分辨的质谱技术,研究了3-甲基吡啶分子激发态的超快过程.实时观察到了3-甲基吡啶分子S2态向S1态高振动能级的超快内转换过程,该内转换的时间大约为910fs.二次布居的S1态主要通过内转换衰减到基态S0,该内转换的时间尺度为2.77 ps.光电子能谱分布和光电子角分布显示,S2态和S1态在电离的过程中跟3p里德堡态发生偶然共振.本次实验中还用400 nm两个光子吸收的方法布居了3-甲基吡啶的3s里德堡态.研究表明,3s里德堡态的寿命为62 fs,并主要通过内转换快速衰减到基态.     

飞秒时间分辨的光电子影像技术研究2-氯吡啶超快内转换动力学

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-氯吡啶分子激发态的超快过程. 实时观察到了2-氯吡啶分子第二激发态(S₂)向第一激发态(S₁)高振动能级的的超快内转换过程,该内转换的时间常数为(162±5)fs. 实验结果表明, 通过S₂/S₀的锥形交叉衰减到基态的衰减通道也是退布居的重要通道, 其时间尺度为(5.5±0.3) ps.     

2-甲基呋喃分子激发态超快非绝热动力学（英文）

利用飞秒时间分辨的光电子影像技术研究了2-甲基呋喃分子激发态超快非绝热动力学。2-甲基呋喃分子吸收两个400 nm的光子后同时被激发到n=3的里德堡态S1[~1A′′(π3s)]、~1A′(π3p_x)、~1A″(π3p_y)、~1A″(π3p_z)和价电子态~1A′(ππ*),之后被两个800 nm的光子电离。通过母体离子产率随泵浦-探测延迟时间的变化曲线测得这些里德堡态与价电子态的平均寿命为50 fs。通过解析光电子能谱中n=3的里德堡态与价电子态所对应的组分峰的相对演化特征,观测到了这些激发态之间的内转换过程,并且揭示了价电子态~1A′(ππ*)在内转换过程中扮演的重要"纽带"作用。里德堡态与价电子态之间的混合,形成势能面间的锥形交叉,导致了如此超快的内转换过程。     

间甲基苯甲醚分子旋转异构体的质量分辨阈值电离光谱

间甲基苯甲醚分子有顺式和反式两个转动异构体. 利用单光共振双光子电离技术和质量分辨阈值电离技术,研究了间甲基苯甲醚分子顺反异构体的基态到第一电子激发态（S₁←S₀）的跃迁和阈值电离. 得到顺式、反式间甲基苯甲醚分子S₁态的激发能（E₁）分别为（36049±2）和（36117±2）cm^-1,绝热电离能（I_p）分别为（64859±5）和（65110±5）cm^-1. 结合从头算法和密度泛函理论的量子化学计算,解释了顺式、反式间甲基苯甲醚分子E₁和I_p存在差异的原因,并且对S₁态和离子基态D0态出现的谱峰进行了标识. 间甲基苯甲醚分子顺反异构体在S₁态和D₀态的活性振动主要是甲基转动、面内环的运动和与取代基相关的弯曲振动. 间甲基苯甲醚分子的S₁态振动光谱、D₀态离子光谱以及理论计算均表明这两个转动异构体在D₀态的几何构型与S₁态的中性几何构型相比有较大改变,取代基与取代基、取代基与苯环间的相互作用强度高低次序为：S₀<S₁<D₀.     

飞秒时间分辨的光电子影像研究氯化苄分子内转换动力学(英文)

结合时间分辨的飞秒光电子影像(TRPEI)技术和时间分辨的质谱技术,研究了氯化苄(BzCl)分子内转换动力学过程.从光电子影像中获得了光电子动能分布和角度分布.氯化苄分子吸收两个400nm的光子后从基态跃迁到S4态和S2态.获得的母体离子随泵浦-探测时间延迟变化的曲线可以用两个指数函数进行拟合,包括一个时间常数为50fs的快速组分和一个时间常数为910fs的慢速组分.通过分析光电子动能分布随延迟时间的变化,我们认为分子被激发到S4态后在很短的时间内与S2态发生耦合迅速弛豫到S2态,然后再经内转换(IC)弛豫到S1态.最初布居的激发态分子经过内转换弛豫到S1态的时间尺度为50fs.910fs的慢速时间组分反映了分子弛豫到S1态后,经内转换向基态S0的弛豫.光电子角度分布的各向异性参数从零时刻的0.87增加到25fs时的0.94,然后逐渐减小到190fs时刻的0.59的现象,也反映了氯化苄分子从S4态耦合到S2态,然后内转换到S1态的动力学过程.     

锌卟啉的轴向配位对其激发态光物理性质的影响

采用稳态吸收光谱、荧光光谱及皮秒时间分辨荧光光谱技术研究了轴向配体4-N,N-二甲氨基吡啶(DMAP)对5,10,15,20-四(对甲氧基苯基)锌卟啉(Zn(p-OCH₃)TPP)的第一单重激发态(S₁态)和第二单重激发态(S₂态)的荧光特性的影响, 观测到源于S₁态高振动能级的热荧光, 并发现轴向配体的引入导致卟啉分子: (1) S₁态热荧光寿命缩短, 相对含量增加; (2) S₁态荧光寿命缩短. 对轴向配体引起的上述电子激发态性质变化的机理进行了探讨.     

环丙酮光解离和热异构机理的从头算研究

运用精确的量子化学计算方法CASSCF, B3LYP和MP2, 结合cc-pVDZ基组, 优化了环丙酮的基态和激发态势能面上的驻点结构, 计算了它们的相对能量. 在此基础上, 深入探讨了环丙酮光解离反应的机理. 在292～365 nm波长的光的激发下, 环丙酮被激发至S₁态, 最可能的初始过程是α C—C键断裂. 我们的理论研究发现, 在α C—C键断裂途径上, 存在基态和第一激发势能面的交叉点, 它在随后的反应过程中起着重要作用. 一方面可形成单态双自由基中间体, 然后发生另一个C—C键的断裂, 生成基态产物一氧化碳和乙烯. 另一方面, 经过S₁/S₀交叉点可以回到热的基态. 在这种情况下, 体系具有足够的能量, 克服基态途径上的势垒, 生成同样的基态产物乙烯和一氧化碳. 此外, 还对环丙酮基态异构化反应进行了理论研究.     

飞秒时间分辨的光电子谱对苯S2态的超快动力学研究

结合飞秒时间分辨的质谱技术与时间分辨的光电子影像技术对苯S₂激发态的超快动力学进行了研究.苯分子吸收两个400 nm的光子被激发到S₂态,之后再用一个267 nm的光子对其进行探测.获得的母体离子产率随泵浦探测时间延迟的变化曲线包含了两个不同的时间寿命组分.第一个时间寿命组分(90 ± 1) fs被归纳为S₂态到S₁/S₀态的内转换过程;第二个时间寿命组分(5.0 ± 0.2) ps被归纳为S₁态的衰减过程.实验中观察到的第二个寿命组分小于早前的研究结果,这表明了在S₁态的衰减过程中还可能存在其他的过程.从时间分辨的光电子影像提取得到的光电子能谱中发现了一个新的失活过程,该过程被归结为激发态S₁的振动态与“热”三重态T₃之间的系间交叉过程.     

丙烯酸分子的激发态超快预解离动力学

利用飞秒泵浦-探测技术结合飞行时间质谱(TOF-MS),研究了丙烯酸分子被200nm泵浦光激发到第二电子激发态(S2)后的超快预解离动力学.采集了母体离子和碎片离子的时间分辨质谱信号,并利用动力学方程对时间分辨离子质谱信号进行拟合和分析,揭示了预解离通道的存在.布居在S2激发态的分子通过快速的内转换弛豫到第一电子激发态(S1),时间常数为210fs,随后再经内转换从S1态弛豫到基态(S0)的高振动态,时间常数为1.49ps.分子最终在基态高振动态势能面上发生C-C键和C-O键的断裂,分别解离生成H2C=CH和HOCO、H2C=CHCO和OH中性碎片,对应的预解离时间常数分别约为4和3ps.碎片离子的产生有两个途径,分别来自于母体离子的解离和基态高振动态势能面上中性碎片的电离.     

氯苯分子第一激发态超快动力学

利用时间分辨飞秒光电子影像技术结合时间分辨质谱技术, 研究了氯苯分子第一激发态的超快过程. 266.7 nm单光子将氯苯分子激发至第一激发态. 母体离子时间变化曲线包括了不同的双指数曲线. 一个是时间常数为(152±3) fs的快速组分, 另一个是时间常数为(749±21) ps的慢速组分. 通过时间分辨的光电子影像得到了时间分辨的光电子动能分布和角度分布. 时间常数为(152±3) fs的快速组分反映了第一激发态内部的能量转移过程, 这个过程归属为氯苯分子第一激发态耗散型振动驰豫过程. 时间常数为(749±21) ps的慢速组分反映了第一激发态的慢速内转换过程. 另外, 实验实时观察到典型的非对称陀螺分子(氯苯)激发态的非绝热准直和转动退相干现象. 并推算出第一次转动恢复时间为205.8 ps (C类型)和359.3 ps (J类型).     

Ultrafast Dynamics Through Conical Intersections in 2,6-dimethylpyridine Studied with Time-resolved Photoelectron Imaging

The ultrafast dynamics through conical intersections in 2,6-dimethylpyridine has been stud-ied by femtosecond time-resolved photoelectron imaging coupled with time-resolved mass spectroscopy. Upon absorption of 266 nm pump laser, 2,6-dimethylpyridine is excited to the S₂ state with a ππ^* character from S₀state. The time evolution of the parent ion sig-nals consists of two exponential decays. One is a fast component on a timescale of 635 fs and the other is a slow component with a timescale of 4.37 ps. Time-dependent photo-electron angular distributions and energy-resolved photoelectron spectroscopy are extracted from time-resolved photoelectron imaging and provide the evolutive information of S₂ state. In brief, the ultrafast component is a population transfer from S₂ to S₁ through the S₂/S₁ conical intersections, the slow component is attributed to simultaneous IC from the S₂ state and the higher vibrational levels of S₁ state to S₀ state, which involves the coupling of S₂/S₀ and S₁/S₀ conical intersections. Additionally, the observed ultrafast S₂→S₁ transition occurs only with an 18% branching ratio.     

2-甲基呋喃分子激发态超快非绝热动力学

Excited-state dynamics of 2-methyl furan has been studied by femtosecond time-resolved photoelectron imaging. The molecule 2-methyl furan was simultaneously excited to the n=3 Rydberg series of S₁[¹A"(π3s)], ¹A'(π3p_x), ¹A"(π3p_y) and ¹A"(π3p_z) and the valence state of ¹A'(ππ^*) by two 400 nm photons and subsequently probed by two 800 nm photons. The average lifetime of the Rydberg series and the valence state was measured to be on the time scale of 50 fs by the time-dependent ion yield of the parent ion. Ultrafast internal conversions among these excited states were observed and extracted from the time-dependences of the photoelectron kinetic energy components of these excited states in the photoelectron kinetic energy spectra. Furthermore, it is identified that the ¹A'(ππ^*) state might play an important role in internal conversions among these excited states. The Rydberg-valence mixings, which result in numerous conical intersections, act as the driving force to accomplish such ultrafast internal conversions.     

Femtosecond photoelectron imaging of transient electronic states and Rydberg atom emission from electronically excited he droplets

Ultrafast relaxation of electronically excited pure He droplets is investigated by femtosecond time-resolved photoelectron imaging. Droplets are excited by extreme ultraviolet (EUV) pulses with photon energies below 24 eV. Excited states and relaxation products are probed by ionization with an infrared (IR) pulse with 1.6 eV photon energy. An initially excited droplet state decays on a time scale of 220 fs, leading predominantly to the emission of unaligned 1s3d Rydberg atoms. In a second relaxation channel, electronically aligned 1s4p Rydberg atoms are emitted from the droplet within less than 120 fs. The experimental results are described within a model that approximates electronically excited droplet states by localized, atomic Rydberg states perturbed by the local droplet environment in which the atom is embedded. The model suggests that, below 24 eV, EUV excitation preferentially leads to states that are localized in the surface region of the droplet. Electronically aligned 1s4p Rydberg atoms are expected to originate from excitations in the outermost surface regions, while nonaligned 1s3d Rydberg atoms emerge from a deeper surface region with higher local densities. The model is used to simulate the He droplet EUV absorption spectrum in good agreement with previously reported fluorescence excitation measurements.     

Spectroscopy and femtosecond dynamics of the ring opening reaction of 1,3-cyclohexadiene

The early stages of the ring opening reaction of 1,3-cyclohexadiene to form its isomer 1,3,5-hexatriene, upon excitation to the ultrashort-lived 1 1B2 state, were explored. A series of one-color two-photon ionization/photoelectron spectra reveal a prominent vibrational progression with a frequency of 1350 cm(-1), which is interpreted in a dynamical picture as resulting from the ultrafast wave packet dynamics associated with the ring opening reaction. Photoionization in two-color three-photon and one-color four-photon ionization schemes show an ionization pathway via the same ultrashort-lived 1 1B2 state, and in addition, a series of Rydberg states with quantum defects of 0.93, 0.76, and 0.15, respectively. Using those Rydberg states as probes for the reaction dynamics in a time-resolved pump-probe experiment provides a direct observation of the elusive 2 1A1 state that has been implicated as an intermediate step between the initially excited 1 1B2 state and the ground electronic state. The rise and decay times for the 2 1A1 state were found to be 55 and 84 fs, respectively.     

Ultrafast dynamics in thiophene investigated by femtosecond pump probe photoelectron spectroscopy and theory

A hybrid of a time-of-flight mass spectrometer and a time-of-flight "magnetic-bottle type" photoelectron (PE) spectrometer is used for fs pump-probe investigations of the excited state dynamics of thiophene. A resonant two-photon ionization spectrum of the onset of the excited states has been recorded with a tunable UV laser of 190 fs pulse width. With the pump laser set to the first intense transition we find by UV probe ionization first a small time shift of the maxima in the PE spectrum and then a fast decay to a low constant intensity level. The fitted time constants are 80+/-10 fs, and 25+/-10 fs, respectively. Theoretical calculations show that upon geometry relaxation the electronic state order changes and conical intersections between excited states exist. We use the vertical state order S1, S2, S3 to define the terms S1, S2, and S3 for the characterization of the electron configuration of these states. On the basis of our theoretical result we discuss the electronic state order in the UV spectra and identify in the photoelectron spectrum the origin of the first cation excited state D1. The fast excited state dynamics agrees best with a vibrational dynamics in the photo-excited S1 (80+/-10 fs) and an ultrafast decay via a conical intersection, presumably a ring opening to the S3 state (25+/-10 fs). The subsequently observed weak constant signal is taken as an indication, that in the gas phase the ring-closure to S0 is slower than 50 ps. An ultrafast equilibrium between S1 and S2 before ring opening is not supported by our data.     

Excited states in electron-transfer reaction products: ultrafast relaxation dynamics of an isolated acceptor radical anion

The spectroscopy and ultrafast relaxation dynamics of excited states of the radical anion of a representative charge-transfer acceptor molecule, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, have been studied in the gas phase using time-resolved photoelectron spectroscopy. The photoelectron spectra reveal that at least two anion excited states are bound. Time-resolved studies show that both excited states are very short-lived and internally convert to the anion ground state, with the lower energy state relaxing within 200 fs and a near-threshold valence-excited state relaxing on a 60 fs time scale. These excited states, and in particular the valence-excited state, present efficient pathways for electron-transfer reactions in the highly exergonic inverted region which commonly displays rates exceeding predictions from electron-transfer theory.     

A Time‐Dependent Picture of the Ultrafast Deactivation of keto‐Cytosine Including Three‐State Conical Intersections

Using mixed quantum–classical dynamics, the lowest part of the UV absorption spectrum and the first deactivation steps of keto‐cytosine have been investigated. The spectrum shows several strong peaks, which mainly come from the S₁ and S₂ states, with minor contributions from the S₃. The semiclassical trajectories, launched from these three states, clearly indicate that at least four states are involved in the relaxation of keto‐cytosine to the ground state. Non‐adiabatic transfer between the ππ* and nπ* excited states and deactivation via three‐state conical intersections is observed in the very early stage of the dynamics. In less than 100 fs, a large amount of population is deactivated to the ground state via several mechanisms; some population remains trapped in the S₂ state. The latter two events can be connected to the fs and ps transients observed experimentally.     

Ultrafast non-adiabatic dynamics of methyl substituted ethylenes: the π3s Rydberg state

Excited state unimolecular reactions of some polyenes exhibit localization of their dynamics at a single ethylenic double bond. Here we present studies of the fundamental photophysical processes in the ethylene unit itself. Combined femtosecond time-resolved photoelectron spectroscopy (TRPES) and ab initio quantum chemical calculations was applied to the study of excited state dynamics in cis-butene, trans-butene, trimethylethylene, and tetramethylethylene, following initial excitation to their respective π3s Rydberg states. The wavelength dependence of the π3s Rydberg state dynamics of tetramethylethylene was investigated in more detail. The π3s Rydberg to ππ(?) valence state decay rate varies greatly with substituent: the 1,2-di- and tri-methyl substituted ethylenes (cis-butene, trans-butene, and trimethylethylene) show an ultrafast decay (～20 fs), whereas the fully methylated tetramethylethylene shows a decay rate of 2 to 4 orders of magnitude slower. These observations are rationalized in terms of topographical trends in the relevant potential energy surfaces, as found from ab initio calculations: (1) the barrier between the π3s state and the ππ? state increases with increasing methylation, and (2) the π3s∕ππ? minimum energy conical intersection displaces monotonically away from the π3s Franck-Condon region with increasing methylation. The use of systematic methylation in combination with TRPES and ab initio computation is emerging as an important tool in discerning the excited state dynamics of unsaturated hydrocarbons.