分子对飞秒激光场响应的含时密度泛函理论研究

采用含时密度泛函理论方法研究线性分子碳化锂（Li2C2）对飞秒激光场响应的电子-离子动力学行为。在典型的近共振和非共振的激光频率作用下,分别对比分析了分子的共振和非共振电离过程。研究发现：分子在共振频率激光场的作用下发生更强的电离过程,并倾向于发生库伦爆炸,键长的振荡断裂与电离相互促进影响,而分子在较弱的激光场作用下发生单光子电离过程;随着双脉冲时间间隔的增加,离化电子数在一定范围内呈振荡上升趋势,随后趋于常数。     

分子核间距对非时序双电离的影响

利用半经典再散射模型研究了激光强度在一定范围内时分子核间距对非时序双电离过程的影响.分别计算了非时序双电离率、两电子的电离能、两电子的动量相关、Coulomb和激光场的复合势随分子核间距的变化关系.研究表明,分子核间距在1.0—6.0 a.u.范围内时,非时序双电离率和两电子动量和为零的双电离事件数随着分子核间距的增大而增大.当分子核间距继续增大(大于6.0 a.u.)时,非时序双电离率和两电子动量和为零的双电离事件数却减小.     

C_2H_2@C_(60)在飞秒激光作用下的分子动力学模拟

本文用含时密度泛函模拟了体系在外场为强激光场时的非线性光学性.并计算了C_2H_2@C_(60)和孤立C_2H_2分子在飞秒激光中分子的动力学过程,发现体系在脉冲激光作用的时间段内迅速电离,然后随时间演化也会发生持续的电离,这时的电离率不太高.在整个分子动力学过程中,可以观察到C_(60)内的小分子原子发生化学键的断裂和重组的行为,C_(60)结构在此过程中慢慢发生膨胀,最后分子键发生断裂,形成很多单电荷或者多电荷离子.同时发现体系被电离和解离的程度与激光的偏振方向有很大的关系.当激光偏振方向平行于分子轴时,分子发生共振的几率很大,能够产生丰富的碎片离子;垂直于分子轴时,共振的几率几乎为0,仅有母体离子出现.本文还发现当飞秒激光的偏振方向与分子轴平行时,能从氢化分子中有效提取H原子.     

激光诱导等离子体碘甲烷裂解动力学过程

采用色散荧光谱和时间分辨光谱方法,研究了532 nm强激光场诱导等离子体作用下碘甲烷分解动力学过程.通过对所得色散荧光谱归属,确定了碘甲烷分解的主要荧光产物粒子:CH3I 、CH3、CH2、CH、C、H、C 、C2 、I 、I2 ;通过时间分辨光谱分析,讨论了荧光产物粒子的形成动力学机理,归结出了碘甲烷分解所经历的主要过程为"共振多光子激发电→CH3I 解离→库仑爆炸脱氢→电子碰撞激发或电离→次级碰撞电离"的物理过程.所得结果将对其他多原子分子的强激光场作用下激光诱导等离子体分解动力学过程的研究具有参考价值.     

H_2~+在强激光脉冲作用下的电离率和原子核间距的关系

本文利用蒙特卡罗方法模拟电子在激光场以及分子库仑势作用下的经典轨迹,研究了氢分子离子H_2~+的电离率和原子核间距的关系,为电荷共振电离增强现象提供了一种基于电子经典运动的解释.当原子核间距为5—6 a.u.时,H_2~+的电离率显著增大.电子的运动轨迹揭示此时电子先围绕其中一个原子核运动,在逐步获得越来越多的动能后,运动轨迹受到另一个原子核的强烈影响,最后电子逃逸原子核的束缚.原子核之间的库仑势垒和激光调制的库仑势垒的高度差与电离率的大小直接相关.     

分子双电离对激光偏振性的依赖关系

利用经典系综模型研究了氢分子双电离对激光场椭圆率的依赖性.研究发现,氢分子双电离机制对激光的椭偏性有强烈的依赖关系.随着激光场椭圆率的增加,双电离机制由非次序双电离转变为次序双电离.在大椭圆率情况的次序双电离中,电子关联动量分布随椭圆率的灵敏变化显示了电子出射时间对椭圆率的强烈依赖关系.     

超快强激光驱动的原子分子电离

强场电离是超快强激光与物质相互作用时发生的基本物理过程。强场驱动原子分子的电离电子动力学过程发生在一个光学振荡周期以内,是在阿秒时间尺度上研究电子超快动力学的典范。不仅如此,强场驱动下的超短电子束还为探测原子分子的结构及其超快动力学提供了重要的技术手段。文章首先简要阐述了超快强光场中原子分子电离的基本物理图像,在此基础上,介绍了近年来基于强场电离电子开展的超快过程研究的几个例子,最后简要讨论了强场电离研究的未来可能发展方向。     

基于多组态含时Hartree-Fock方法研究电子关联对于H_2分子强场电离的影响

发展了三维的处理双原子分子非微扰电子动力学的多组态含时Hartree-Fock方法,并利用该方法研究了电子关联对于H_2分子强场电离概率的影响.该方法采用能够精确处理双中心库仑势的椭球坐标系,以及减小双电子积分计算量的有限元-离散变量基函数方法.利用多组态含时Hartree-Fock方法计算了H_2分子随分子取向角度变化的XUV光电离结果,并通过与单组态结果的对比研究了电子关联对于单电离和双电离概率的不同影响.研究表明,电子关联对于单电离过程影响很小,而在双电离过程中则发挥了重要作用,导致了电离概率的减小.该方法为进一步研究强场物理过程中的电子关联效应奠定了基础.     

频域图像下的强场非序列电离过程

自20世纪60年代激光发明以来,激光与物质的相互作用就一直成为物理学领域的一个重要研究方向.通过最近几十年激光技术的发展,大大拓展了激光的频率、强度及脉宽范围,使得复杂体系在激光场中的激发、辐射及电离过程得到更精细而深入的研究.本文总结了处理单色和双色激光场中双电子原子非序列电离的频域理论;归纳了碰撞-电离和碰撞-激发-电离两种机理下原子非序列电离在单色和双色激光场中的动量谱分布,并对动量谱上的干涉条纹利用量子通道相干的理论进行了分析;归纳了前向碰撞和背向碰撞在不同激光场条件下对非序列电离的不同贡献,以及高频激光场在非序列电离中所起的作用.     

激光诱导等离子体碘甲烷裂解动力学过程

采用色散荧光谱和时间分辨光谱方法，研究了532nm强激光场诱导等离子体作用下碘甲烷分解动力学过程。通过对所得色散荧光谱归属，确定了碘甲烷分解的主要荧光产物粒子：CH3I+、CH3、CH2、CH、C、H、C+、C2+、I+、I2+；通过时间分辨光谱分析，讨论了荧光产物粒子的形成动力学机理，归结出了碘甲烷分解所经历的主要过程为“共振多光子激发电离→CH3I+解离→库仑爆炸脱氢→电子碰撞激发或电离→次级碰撞电离”的物理过程。所得结果将对其他多原子分子的强激光场作用下激光诱导等离子体分解动力学过程的研究具有参考价值。     

飞秒强激光场中水分子的电离激发

本文运用含时密度泛函理论和分子动力学非绝热耦合的方法, 研究了水分子在不同极化方向的激光场中的电离和动力学行为. 计算结果表明, 对应相同的极化方向, 随着激光强度的增加, 水分子的电离增强; 对于相同强度的激光, 当激光极化方向沿水分子对称轴方向时, 水分子的电离最强, 当激光极化方向垂直水分子对称轴方向时, 水分子电离受到最大程度的抑制. 对水分子偶极矩的研究表明, 当分子处于线性响应区域时, x方向的激光只能激发起x方向的偶极振动而y方向的激光只能激发起y方向的偶极振动. 对水分子的键长和键角的研究表明, 在激光场中水分子的键长变长, 键角变大, 但变化幅度随着激光极化角的增大而减小. 此外, 研究还发现, 虽然在不同极化方向的激光脉冲的驱动下, 水分子OH键的振动频率与激光频率相当, 在脉冲关闭后, 振动频率减小, 但激光场的极化方向对水分子振动模式具有选择性. 关键词： 含时密度泛函理论 分子动力学 水分子 电离     

(1+2+1)双共振多光子电离概率的理论研究

共振增强多光子电离光谱技术已成为研究原子、分子高激发态能级结构的重要方法。运用光和物质相互作用的速率方程理论,推导出四能级物质系统1+2+1双共振增强多光子电离概率的解析表达式,以此为基础,理论模拟了电离概率随激发光强、激光脉冲宽度和碰撞弛豫速率的变化,发现在1+2+1多光子电离机制中,电离概率随光强的增加而增大,继而出现单步、双步激发饱和的现象,直至饱和值1;继续增大光强,电离概率将围绕饱和值1窄幅振荡,振荡幅度随光强增加而增大。随激光脉冲宽度的增大,电离概率从零开始逐渐增大直至饱和值1。而随碰撞弛豫速率的增大电离概率以线性规律减小。     

Theoretical study on non-sequential double ionization of carbon disulfide with different bond lengths in linearly polarizedlaser fields

By using a two-dimensional Monte-Carlo classical ensemble method, we investigate the double ionization(DI) process of the CS_2 molecule with different bond lengths in an 800-nm intense laser field. The double ionization probability presents a "knee" structure with equilibrium internuclear distance R = 2.9245 a.u.(a.u. is short for atomic unit). As the bond length of CS increases, the DI probability is enhanced and the "knee" structure becomes less obvious. In addition,the momentum distribution of double ionized electrons is also investigated, which shows the momentum mostly distributed in the first and third quadrants with equilibrium internuclear distance R = 2.9245 a.u. As the bond length of CS increases,the electron momentum becomes evenly distributed in the four quadrants. Furthermore, the energy distributions and the corresponding trajectories of the double-ionized electrons versus time are also demonstrated, which show that the bond length of CS in the CS_2 molecule plays a key role in the DI process.     

TDDFT study of excitation of water molecules with short laser pulses

We explore the excitation of water molecules subject to short and intense laser pulses in the frame of time-dependent density function theory (TDDFT) at the level of the time-dependent local-density approximation (TDLDA), applied to valence electrons, coupled non-adiabatically to molecular dynamics (MD) of ions. We first study the optical absorption spectra of the water molecule as an observable in the "linear" domain and results are in good agreement with experiments. We then explore the influence of the laser frequency on the excitation. It is found that when the laser frequency is off-resonant or highly above the resonant region, the excitations are weak whereas for the resonant frequency case, the ionization is enhanced and bond lengths are enlarged. Furthermore, a direct coupling of ions with the laser pulse with the off-resonant frequency is found when investigating the OH bond lengths. We finally study the effect of laser intensity on the excitation of H 2 O and it is found that ionization increases when the laser intensity varies from low to high and we observe stable vibrations to Coulomb fragmentation when the ionization is up to typically two more charge units.     

Few cycle dynamics of multiphoton double ionization

In intense field ionization, an electron removed from the atomic core oscillates in the combined fields of the laser and the parent ion. This oscillation forces repeated revivals of its spatial correlation with the bound electrons. The total probability of double ionization depends on the number of returns and therefore on the number of optical periods in the laser pulse. We observed the yield of Ne(2+) relative to Ne(+) with 12 fs pulses to be clearly less compared to 50 fs pulses in qualitative agreement with our theoretical model.     

Influence of laser intensity on the double-resonance multiphoton ionization process of NO molecule

The analytic formula of the ionization efficiency in the process of double resonance enhanced multi-photon ionization （DREMPI） is derived from the dynamic rate equation about the interaction of photon and material. Based on this formula, the ionization efficiency and the laser power index versus laser intensity in the DREMPI process of NO molecule, via A2E and S2E intermediate resonant states, is numerically simulated. It is shown that the ionization efficiency of NO molecule increases with the laser intensity until getting saturation, while the laser power index decreases with the enhancement of the laser intensity and changes to zero at last. The variation of the laser power index with the laser intensity indicates that the ionization efficiency reaches saturation in the one, two, and three excitation steps respectively. It is also found that the narrower the laser pulse duration is, the higher becomes the laser intensity for saturation.     

Theoretical analysis on the efficiency of optical-optical double-color double-resonance multiphoton ionization

Analytic formula of the efficiency of optical-optical double-color double-resonance multi-photon ionization （OODR-MPI） is derived from the dynamic rate equation about the interaction of photon and material. Based on this formula, the influence of characteristic of the pump and probe laser on the ionization efficiency of （1＋2＋1） OODR-MPI process is simulated theoretically. It is shown that the pump laser will affect the ionization efficiency by the number control of the molecules excited to the first resonance state. The ionization efficiency is decided by the probe laser directly. Both of the excited molecules and ionization efficiency increase with the intensity and pulse duration of the laser until saturation. It is also found that the longer the delay time of the probe laser to the pump one is, the lower the ionization efficiency would be. The delay time ought to be smaller than the lifetime of the excited molecule in the practical use of the OODR-MPI technique.     

Autler-Townes Splitting in Photoelectron Spectrum of Three-Level Li2 Molecule in Ultrashort Pulse Laser Fields

The Autler-Townes （AT） splitting in femtosecond photoelectron spectrum of three-level Li2 molecules is theoretically investigated using time-dependent quantum wave packet method. With proper femtosecond laser pulses, three peaks of the AT splitting can be observed in the photoelectron spectrum. The AT splitting stems from rapid Rabi oscillation caused by intense ultrashort laser pluses. The effects of laser parameters on the molecular ionization dynamics are also discussed.     

Controlling vibrational wave packet motion with intense modulated laser fields

Intense, nonresonant laser fields produce Stark shifts that strongly modify the potential energy surfaces of a molecule. A vibrational wave packet can be guided by this Stark shift if the laser field is appropriately modulated during the wave packet motion. We modulated a 70 fs laser pulse with a period on the time scale of the vibrational motion (approximately 10 fs) by mixing the signal and idler of an optical parametric amplifier. We used ionization of H2 or D2 to launch a vibrational wave packet on the ground state of H2(+) or D2(+). If the laser intensity was high as the wave packet reached its outer turning point, the Stark shift allowed the molecule to dissociate through bond softening. On the other hand, if the field was small at this critical time, little dissociation was measured. By changing the modulation period, we achieved control of the dissociation yield with a contrast of 90%.