氟利昂F113分子在飞秒激光作用下的多光子电离解离动力学

大气臭氧层因吸收太阳紫外光, 是人类必不可少的保护伞. 氟利昂在太阳光辐射下解离生成破坏臭氧的游离态氯原子, 是破坏大气臭氧层的主要元凶之一. 本文利用飞行时间质谱技术和离子速度成像技术研究了氟利昂F113(三氟三氯乙烷)分子在800 nm 飞秒光作用下的多光子电离解离动力学. 利用飞行时间质谱探测技术, 得到了三氟三氯乙烷在该波长飞秒激光作用下发生多光子电离解离产生的碎片质谱. 通过荷质比对碎片质谱进行了详细的标定和分析. 在质谱上未发现母体离子, 所有观察到的离子都是由于激光脉冲作用下产生的碎片. 三个最主要的碎片离子是CFCl₂⁺, CF₂Cl⁺, C₂F₃Cl₂⁺. 通过飞行时间质谱标定, 发现并归属了多个解离通道. 三个主要的解离机理分别为: 1) C-Cl键断裂直接生产氯自由基的通道C₂F₃Cl₃⁺→C₂F₃Cl₂⁺+Cl; 2) C--C键断裂C₂F₃Cl₃⁺→CFCl₂⁺+CF₂Cl; 3) C--C键断裂C₂F₃Cl₃⁺→CF₂Cl⁺+CFCl₂. 利用离子速度成像技术对这三个主要通道产生的碎片离子进行成像, 得到了C₂F₃Cl₂⁺, CFCl₂⁺和CF₂Cl⁺离子的速度影像. 由C--Cl键断裂产生的碎片离子C₂F₃Cl₂^{+}的速度分布由两个高斯分布曲线拟合, 而由C--C键断裂产生的碎片离子CFCl₂⁺和CF₂Cl⁺可以用一个高斯曲线拟合. 通过影像分析得到了解离碎片的平动能分布和角向分布各向异性参数等详尽的动力学信息. 结合高精度密度泛函理论计算对解离动力学进行了进一步的分析和讨论.深入认识氟利昂的解离动力学可为进一步控制破坏臭氧层提供理论参考和实验依据.     

Transient localized electron dynamics simulation during femtosecond laser tunnel ionization of diamond

A real-time and real-space time-dependent density-functional theory (TDDFT) is applied to simulate the nonlinear electron–photon interactions during femtosecond laser processing of diamond when tunnel ionization dominates. The transient localized electron dynamics including the electron excitation, energy absorption and electron density evolution are described in this Letter. In addition, the relationships among average absorbed energy, Keldysh parameter and laser intensity are revealed when the laser frequency is fixed.     

Intracavity ionization and pulse formation in femtosecond enhancement cavities

We experimentally and numerically investigate the intracavity ionization of a dilute gas target by an ultrashort pulse inside a femtosecond enhancement cavity. Numerical simulations detail how the dynamic ionization of the gas target limits the achievable peak intensity of the evolving intracavity pulse beyond that of linear cavity losses, setting a constraint on the strength of the nonlinear interaction that can be sustained in such optical cavities. Experimental measurements combined with numerical simulations predict ionization levels in a femtosecond enhancement cavity for the first time. We demonstrate how the resonant response of the femtosecond enhancement cavity can itself be used as a sensitive probe of optical nonlinearities at high intensities.     

First-principles electron dynamics control simulation of diamond under femtosecond laser pulse train irradiation

A real-time and real-space time-dependent density functional is applied to simulate the nonlinear electron-photon interactions during shaped femtosecond laser pulse train ablation of diamond. Effects of the key pulse train parameters such as the pulse separation, spatial/temporal pulse energy distribution and pulse number per train on the electron excitation and energy absorption are discussed. The calculations show that photon-electron interactions and transient localized electron dynamics can be controlled including photon absorption, electron excitation, electron density, and free electron distribution by the ultrafast laser pulse train.     

Size distribution control of metal nanoparticles using femtosecond laser pulse train: a molecular dynamics simulation

Microscopic mechanisms and optimization of metal nanoparticle size distribution control using femtosecond laser pulse trains are studied by molecular dynamics simulations combined with the two-temperature model. Various pulse train designs, including subpulse numbers, separations, and energy distributions are compared, which demonstrate that the minimal mean nanoparticle sizes are achieved at the maximal subpulse numbers with uniform energy distributions. Femtosecond laser pulse trains significantly alter the film thermodynamical properties, adjust the film phase change mechanisms, and hence control the nanoparticle size distributions. As subpulse numbers and separations increase, alternation of film thermodynamical properties suppresses phase explosion, favors critical point phase separation, and significantly reduces mean nanoparticle size distributions. Correspondingly, the relative ratio of two phase change mechanisms causes two distinct nanoparticle size control regimes, where phase explosion leads to strong nanoparticle size control, and increasing ratio of critical point phase separation leads to gentle nanoparticles size control.     

Multielectron ionization of CdSe quantum dots in intense femtosecond ultraviolet light

Multielectron ionization of colloidal CdSe quantum dots under intense femtosecond UV excitation has been studied. By directly probing the absorption from the ionized electron, quantitative measurements of the yield and dynamics of the ionization have been made as a function of excitation fluence and variations of size and potential structure of quantum dots. The results have been explained by an ionization mechanism involving resonant two-photon absorption.     

Numerical simulation of the dynamics of the spin of an electron and polarized nuclei at a quantum dot

The dynamics of the spin of a localized electron at a hyperfine interaction with a large number of initially polarized nuclei with an inhomogeneous coupling constant has been calculated by the diagonalization method. In the absence of an external magnetic field, the decoherence of the spin at large times is incomplete in view of a finite number of nuclei. The asymptotic behavior of the amplitude of vibrations is in agreement with a previously reported analytical expression.     

Dissipative quantum dynamics in a multiwell system

We investigate the dynamics of a quantum particle moving in a tight-binding lattice and coupled to a heat bath environment. Using the Feynman-Vernon influence functional method, we obtain an exact series representation in powers of the tunneling matrix for the generating functional of moments of the probability distribution which is valid for arbitrary temperatures and linear dissipation. We prove that the Einstein relation between the linear mobility and the diffusion coefficient holds to any order of the expansion for Ohmic, and for a restricted region of super-Ohmic dissipation. We also compute in the Ohmic case the mobility in certain regions of the parameter space. In particular, we find that the low temperature correction to the zero temperature mobility behaves asT ², and we also determine the prefactor. Finally, the exact solution of the dynamics for any times, temperatures and bias is presented for a particular value of the damping strength in the case of strict Ohmic dissipation.     

飞秒啁啾脉冲放大系统中折叠反射式望远镜对脉冲波前的影响

推导出飞秒激光啁啾脉冲放大系统中使用的折叠反射式望远镜对脉冲波前的影响. 分析和计 算了波前倾斜与反射式望远镜参数之间的关系.结果发现：当脉冲宽度小于100 fs时，波前 倾斜对光束质量的影响不可忽略，并提出了抑制这种脉冲波前倾斜的有效方法. 关键词： 飞秒激光啁啾脉冲放大 波前倾斜 反射式望远镜     

On the confinement potential formation in a two-electron quantum dot

A model of a quantum dot for two interacting electrons is proposed and analyzed. The properties of the ambient determining the form of the confinement potential for electrons are simulated using the electrostatic field of the image charge. Analytic expressions for the eigenvalues of each subsystem are derived taking into account the external magnetic field and using the representation of the system Hamiltonian as the sum of the Hamiltonians of the center of mass and of relative motion on the basis of the method of oscillator representation [M. Dineykhan and G. V. Efimov, Element. Chast. At. Yadra 26, 651 (1995); M. Dineykhan, G. V. Efimov, G. Ganbold, and S. N. Nedelko, Oscillator Representation in Quantum Physics, Lecture Notes in Physics, Vol. 26, Springer, Berlin (1995)]. The relative motion of electrons is responsible for a confinement potential which differs from the parabolic confinement potential and is a function of the electron effective mass as well as the characteristics of the image charge.     

Intersubband optical absorptions of a two-electron quantum ring

In this Letter, the optical properties of a quantum ring with two electrons are studied. Its effective-mass Hamiltonian matrix was diagonalized with numerical methods, followed by the calculations of a number of optical quantities. We have found that the intersubband optical absorptions strongly depends on the ring radius, electron-electron interaction, and the incident optical intensity. We also found that the spin-singlet states are more sensitive to ring radius than the spin-triplet states.     

Generation of femtosecond radiation at 211 nm by femtosecond pulse upconversion in the field of a picosecond pulse

We show that, in the case of sum-frequency mixing, one can alleviate group-velocity mismatch between IR and UV pulses by choosing different pulse widths, thus extending the interaction length of ultrashort pulses within nonlinear crystals. By fifth-harmonic generation with a Nd:glass laser, we demonstrate efficient frequency upconversion of 195-fs 264-nm pulses under the envelope of 0.9-ps 1055-nm pulses in beta-barium borate crystal, yielding <270-fs pulses with energy of up to 110muJ at 211 nm.     

Numerical simulation of space-charge dynamics in a gyrotron trap

The particle-in-cell (PIC) method is used to simulate the self-consistent accumulation and bunching of space charge in the trap of a gyrotron electron-optical system. It is shown that it is possible to generate charge bunches that oscillate along the direction of the magnetic field. The dependence of the characteristics of these oscillations on the magnitude of the electron current into the trap is determined, along with the effect of the accumulated charge on the velocity distribution of electrons in the current passing through the magnetic mirror. Satisfactory agreement with the experimental data is obtained. Zh. Tekh. Fiz. 67, 98–101 (September 1997)     

Mean-field dynamics of a quantum dot-microcavity system

Mean-field evolution equations for the exciton and photon populations and polarizations (Bloch–Lamb equations) are written and numerically solved in order to describe the dynamics of electronic states in a quantum dot coupled to the photon field of a microcavity. The equations account for phase space filling effects and Coulomb interactions among carriers, and include also (in a phenomenological way) incoherent pumping of the quantum dot, photon losses through the microcavity mirrors, and electron–hole population decay due to spontaneous emission of the dot. When the dot may support more than one electron–hole pair, asymptotic oscillatory states, with periods between 0.5 and 1.5 ps, are found almost for any values of the system parameters.     

Time-resolved quantum dynamics of double ionization in strong laser fields

Quantum calculations of a (1+1)-dimensional model for double ionization in strong laser fields are used to trace the time evolution from the ground state through ionization and rescattering to the two-electron escape. The subspace of symmetric escape, a prime characteristic of nonsequential double ionization, remains accessible by a judicious choice of 1D coordinates for the electrons. The time-resolved ionization fluxes show the onset of single and double ionization, the sequence of events during the pulse, and the influences of pulse duration and reveal the relative importance of sequential and nonsequential double ionization, even when ionization takes place during the same field cycle.     

Single-shot chirped-pulse spectral interferometry used to measure the femtosecond ionization dynamics of air

A novel interferometry technique is presented by which, in one shot, one can measure phase changes with a resolution of tens of femtoseconds while extending the measurement over picoseconds or even longer. The method is based on spectral (frequency-domain) interferometry with a pair of linearly chirped pules as probes. With this technique we obtained single-shot measurements of the rapid phase changes induced by optical field ionization of air. This allowed us to calculate the time profile of the electron density created by an intense short laser pulse.     

Energy spectra and quantum crystallization in two-electron quantum dots in a magnetic field

Quantum crystallization of electrons in a quantum dot (QD) subjected to an external magnetic field is considered. Two-electron QDs with two-dimensional (2D) parabolic confining potential in an external transverse magnetic field are calculated. The Hamiltonian is numerically diagonalized in the basis of one-particle functions to find the energy spectra and wave functions for the relative motion of electrons with inclusion of electron-electron interaction for a broad range of the confining-potential steepness (α) and external magnetic fields (B). The region of the external parameters (α, B) within which a gradual transition to quantum crystalline order occurs is numerically determined. In contrast to a 2D unbounded system, a magnetic field acts nonmonotonically on “crystallization” in a quantum dot with several electrons because of a competition between two effects taking place with increasing B, namely, decreasing spread of the electron wave functions and increasing effective steepness of the confining potential, which reduces the average separation between electrons. Fiz. Tverd. Tela (St. Petersburg) 40, 1753–1759 (September 1998)     

Numerical simulation of quantum directional couplers

 A numerical analysis of a quantum directional coupler based on Π-shaped electron waveguides is presented with use of the scattering-matrix method. After the optimization of the device parameters, uniform output for the two output ports and high directivity are obtained within a wide range of the electron momenta. The electron transfer in the device is found more efficient than that in the previously proposed structures. The study of the shape-dependence of transmission for the device shows that the device structure with smooth boundaries exhibits a much better performance. Received: 4 July 1996/Accepted: 9 September 1996