Control of the photodissociation of CsCl

A femtosecond UV laser pulse is used to resonantly excite CsCl molecules from the ionically bound ground state to the first excited repulsive state. The excitation leads to the dissociation of CsCl. After a certain time delay a visible (VIS) femtosecond laser pulse interrupts the dissociation process by resonantly de-exciting the molecule back to the ground state. According to the Tannor–Rice control scheme, the fraction of dissociated CsCl molecules is controlled by changing the delay time between the two fs laser pulses. The processes involved are investigated theoretically and experimentally. Based on the results, a self-learning system has been realized, which is able to control the dissociation without any a priori knowledge of the molecule. Received: 2 December 1999 / Published online: 24 July 2000     

Dissociation of water buried under ice on Pt(111)

Water on Pt(111) is generally thought to be nondissociative. However, by adsorbing a thick ice film [>150 monolayers (ML)], substantial (approximately 0.16 to 1 ML) dissociation of the "buried water" occurs for T>151 K. New temperature-programmed desorption peaks signal the dissociation (after careful isothermal predesorption of the overlying ice films). The buried water likely dissociates via the elevated temperatures and/or solvation changes experienced under the ice. Dissociation charges the growing ice film (up to +9 V) due to trapping of approximately 0.007 ML H3O+ at the vacuum-ice interface.     

飞秒泵浦-探测实验数据分析

陈述了几种获取飞秒泵浦 探测实验数据中的光解动力学信息的解析物理模型 .其中单分量和双分量模型用来解释母体分子的单通道和双通道解离过程 ,另一个单分量模型用来解释碎片分子的解离或去激发过程 .所有模型都结合泵浦 探测实验数据予以解释     

二氯亚砜分子三体解离研究

本文利用飞秒时间分辨的质谱技术,研究了二氯亚砜分子在235 nm泵浦下的超快光解过程.实验记录了母体离子产率随泵浦-探测延迟时间的变化趋势,测得235 nm泵浦下的初始激发态寿命为166 fs.需要注意的是,实验中不仅观察到了二氯亚砜分子逐步解离过程还观察到了协同解离过程.     

Chirping-dependent ionization and dissociation of methane in an intense femtosecond laser field

We studied the ionization and dissociation of polyatomic molecule methane in an intense femtosecond laserfield with wavelength of 810 nm and intensities ranging from 1.4×10~(14)to 2.6×10~(15) W/cm~2 by massspeetroscopy.Abundant fragment inos were observed in addition to the strong parent ion.The effect offrequency chirp was investigated and it was found that the negatively chirped pulses dramatically enhancedthe dissociation probability,which might be used to control the dissociation pathways.     

Impact of ice structure on ultrafast electron dynamics in D2O clusters on Cu(111)

The structure of D2O clusters on a Cu(111) surface and the femtosecond dynamics of photoexcited excess electrons are investigated by low-temperature scanning tunneling microscopy and two-photon photoemission spectroscopy. Two types of amorphous ice clusters, porous and compact, which exhibit characteristic differences in electron dynamics, are identified. By titration with Xe we show that in both structures solvated electrons preferentially bind on the cluster surface.     

Ultrafast dynamics of electron localization and solvation in ice layers on Cu(111)

The femtosecond dynamics of localization and solvation of photoinjected electrons in ultrathin layers of amorphous solid H2O and D2O have been studied by time- and angle-resolved two-photon-photoelectron spectroscopy. After electron transfer from the metal substrate into the conduction band of ice, the excess electron localizes within the first 100 fs in a state at 2.9 eV above E(F), which is further stabilized by 300 meV on a time scale of 0.5-1 ps due to molecular rearrangements in the adlayer. A pronounced change of the solvation dynamics at a coverage of approximately 2 bilayers is attributed to different rigidity of the solvation shell in the bulk and near the surface of ice.     

飞秒激光场中OCS分子的里德堡态激发

本文研究了800nm飞秒强激光场下OCS分子的里德堡态激发过程. 实验不仅观测到强激光场中的中性母体分子的里德堡态激发,而且观测到大量的中性里德堡态碎片. 我们测量了里德堡态激发产率随激光强度及椭偏率的变化,并与强场电离解离进行了比较. 分析表明,飞秒激光场下中性里德堡态碎片的产生与强场多次电离密切相关. 此外,我们还讨论了中性里德堡态碎片对激光椭偏率依赖的内在原因.     

吡啶团簇的多光子电离和离解研究

利用飞秒激光和飞行时间质谱结合从头计算对吡啶团簇的多光子电离和离解进行了研究. 给出了吡啶团簇的稳定结构和簇内的质子转移过程.     

H2+在阿秒以及双色飞秒激光脉冲中解离时电子位置的相干控制

通过求解含时薛定谔方程, 提出了利用三束激光脉冲控制H₂⁺解离以及解离过程中电子位置的方案. 第一束阿秒激光脉冲将H₂⁺从1sσ_g激发至2pσ_u, 在H₂⁺的解离过程中, 引入两束波长分别为800 nm 与400 nm 的飞秒激光脉冲控制电子在分子内部的运动. 通过改变两束激光脉冲的绝对相位, H₂⁺解离后电子的位置可以得到有效控制(最大有86%的概率使得电子附着在某一个原子核上). 现有的激光技术条件可以在实验上实现这一理论方案. 关键词： 相干控制 电子位置 不对称性参数 2⁺')" href="#">H₂⁺     

Effects of cosmic rays on atmospheric chlorofluorocarbon dissociation and ozone depletion

Data from satellite, balloon, and ground-station measurements show that ozone loss is strongly correlated with cosmic-ray ionization-rate variations with altitude, latitude, and time. Moreover, our laboratory data indicate that the dissociation induced by cosmic rays for CF(2)Cl(2) and CFCl(3) on ice surfaces in the polar stratosphere at an altitude of approximately 15 km is quite efficient, with estimated rates of 4.3 x 10(-5) and 3.6 x 10(-4) s(-1), respectively. These findings suggest that dissociation of chlorofluorocarbons by capture of electrons produced by cosmic rays and localized in polar stratospheric cloud ice may play a significant role in causing the ozone hole.     

Coherent control of fragmentation of methyl iodide by shaped femtosecond pulse train

Coherent control of fragmentation of CH_3I using shaped femtosecond pulse train is investigated.The dissociation processes can be modulated by changing the separation of the shaped pulse train, and the yield of I~+under the irradiation of the optimal pulse is significantly increased compared with that using the transform-limited pulse.We discuss the control mechanism of dissociation processes with coherent interference in time domain.A three-pulse control model is proposed to explain the counterintuitive experimental results.     

Dissociation of CF<Subscript>2</Subscript>HCl molecules by intense radiation from a femtosecond laser in the near-IR range

It is found that CF ₂HCl molecules dissociate under irradiation by high-intensity pulses of femtosecond radiation (λ=1.8 μm) in the vicinity of resonance with the frequency of overtone vibrational transitions in the C-H bond. It is shown that the dissociation products substantially differ from the products of dissociation of CF₂HCl molecules from the ground and excited electronic states under the action of IR and UV nanosecond pulses.     

Above-Threshold Dissociation of HD+ in Femtosecond Laser Field

The above-threshold dissociation （ATD） of the HD＋ molecular ion in femtosecond laser field is investigated theoretically. The energy-dependent distribution of the dissociated fragments is calculated using an asymptoticflow expression in the momentum space. The calculations show that the ATD of HD＋ is sensitive to the initial vibrational level of ground electronic state. Multiphoton ATDs can be observed in the dissociation processes. The dynamics phenomena are interpreted by using the concept of light-dressed potential.     

Cosmic-ray-driven electron-induced reactions of halogenated molecules adsorbed on ice surfaces: Implications for atmospheric ozone depletion and global climate change

The cosmic-ray-driven electron-induced reaction of halogenated molecules adsorbed on ice surfaces has been proposed as a new mechanism for the formation of the polar ozone hole. Here, experimental findings of dissociative electron transfer reactions of halogenated molecules on ice surfaces in electron stimulated desorption, electron trapping and femtosecond time-resolved laser spectroscopic measurements are reviewed. This is followed by a review of the evidence from recent satellite observations of this new mechanism for the Antarctic ozone hole, and all other possible physical mechanisms are discussed. Moreover, new observations of the 11-year cyclic variations of both polar ozone loss and stratospheric cooling and the seasonal variations of CFCs and CH₄ in the polar stratosphere are presented, and quantitative predictions of the Antarctic ozone hole in the future are given. Finally, a new observation of the effects of CFCs and cosmic-ray-driven ozone depletion on global climate change is also presented and discussed.     

超短脉冲激光产生等离子体通道寿命研究

利用强飞秒激光在大气中产生等离子体通道内带电粒子的动力学模型,综合考虑二体吸附、三体吸附、离解和复合等因素,分析后续激光退吸附作用对等离子体通道寿命的影响,给出有效延长通道寿命的后续激光控制参量。计算表明,选择不同的激光注入形式和注入时间,对通道寿命的延长有着不同的效果,适当优化的激光可以延长通道寿命达25 s以上。     

The dissociation pathways of N2+ in intense femtosecond laser fields

Using a neutral N2 beam as target,this paper studies the dissociation of N2+ in intense femtosecond laser fields(45 fs,～1×10 16 W/cm 2) at the laser wavelength of 800 nm based on the time-of-flight mass spectra of N + fragment ions.The angular distributions of N+ and the laser power dependence of N + yielded from different dissociation pathways show that the dissociation mechanisms mainly proceed through the couplings between the metastable states(A,B and C) and the upper excited states of N2+.A coupling model of light-dressed potential energy curves of N+2 is used to interpret the kinetic energy release of N+.     

Atomically resolved images of I(h) ice single crystals in the solid phase

The morphology and crystal structure of nanoparticles of ice were examined by high-resolution transmission electron microscopy. Two different crystal structures were found and unambiguously assigned to hexagonal (I(h)) and cubic (I(c)) ice crystals. Direct observation of oxygen columns clearly revealed the hexagonal packing of water molecules. Electron energy-loss spectroscopy was used to monitor the electronic excitation in ice, suggesting possible dissociation of water molecules. Dynamic process of phase transition between I(h) and I(c) phases of individual ice nanoparticles under electron beam irradiation was also monitored by in situ transmission electron diffractometry.     

The role of excitons and substrate temperature in low-energy (5–50 eV) electron-stimulated dissociation of amorphous D2O ice

We have studied the interaction of low-energy (5–50 eV) electrons with nanoscale (10 ML) ice films by probing the yields and quantum-state distributions of the neutral dissociation products using laser resonance-enhanced multiphoton ionization spectroscopy. In particular, we have observed the electron-stimulated desorption (ESD) of D (²S), O (³P₂) and O (¹D₂) from amorphous D₂O films. These products are observed at threshold energies (relative to the vacuum level) between 6.5–7 eV and desorb with low kinetic energies (60–85 meV) which are independent of the incident electron energy. We associate the ESD of atomic fragments from ice with dissociation of Frenkel-type excitons of 4a₁ character which are near the bottom of the ice conduction band. These excitons are created either directly or via electron-ion recombination. Changing the surface temperature from 88 to 145 K results in an increase in the thermal component of the time-of-flight (kinetic energy) distributions and an overall increase in the neutral fragment yield. We suggest that the change in neutral yield with substrate temperature results from a combination of: (1) increased electron-ion recombination; (2) exciton transport to the near-surface region; and (3) dissociation followed by inelastic scattering and desorption.     

Fragmentation of CO in Femtosecond Laser Fields

Fragmentation of CO in a linearly polarized femtosecond laser field within the intensity order of 10^14 W.cm^-2 at 820nm is investigated experimentally by using velocity mapping technique. According to the observed kinetic energy and angular distributions of different charged fragment ions, fragmentation channels of CO are proposed. The angular distributions provide helpful information for assigning the dissociation channels.