【作者】 张洪英；

【导师】 陈德应；

【作者基本信息】 哈尔滨工业大学 ， 物理电子学， 2010， 博士

【摘要】 激光感生碰撞（laser-induced collision）是指在两个不同粒子的碰撞过程中,利用激光场与粒子间碰撞的共同作用将其中一个粒子的激发能选择性地转移到另一个粒子的特定能级的一种光学过程。它只有在碰撞和激光场同时存在时才能产生,单独激光或单独碰撞不能产生这种跨越粒子之间的能级跃迁现象。由于激光的参与,使得不同粒子之间通过碰撞传递能量变得更加快速和有效;通过粒子间的碰撞作用,又可以实现直接单光子激发难以完成的跃迁,达到预期的高激发态,形成粒子数反转,获得短波长激光。因此,研究激光感生碰撞过程对于发展短波长激光光源具有非常重要的意义。此外,激发所选粒子的特定靶能级的能力也使其在控制化学反应通道方面具有巨大的潜在应用价值。激光感生碰撞过程包括激光感生碰撞能量转移和激光感生碰撞电荷转移。在激光感生碰撞能量转移的传统研究中,所有的理论模型都假设了在碰撞过程中粒子的相对运动速度保持不变,以往的理论计算也是在此基础上进行的,这很显然不符合实际情况;此外,激光感生碰撞电荷转移的传统实验研究都是在混合金属蒸汽系统中进行,对于气体系统则未见报道。针对这两个问题,本文对这两种重要的激光感生碰撞过程做出了一些创新性的改进和完善,并进行了理论和实验研究。在原子间激光感生碰撞能量转移的理论研究方面,考虑到碰撞过程中原子运动速度的统计分布,对现有的激光感生碰撞能量转移的四能级理论模型进行了完善。根据热平衡状态下的麦克斯韦速度分布函数推导了两原子间相对运动速度的分布函数,并给出了对相对运动速度统计平均的碰撞截面。在此基础上,对以往研究不完善的Eu-Sr、Ba-Sr以及我们新提出的Eu-Sr系统进行了数值计算。这三个系统具有典型的代表性:前两个系统分别满足激光感生碰撞能量转移的两种极限情况,从而可以通过忽略一个中间能级而由四能级系统过渡为三能级系统;而我们提出的Eu-Sr系统则不满足这两个极限条件,因此不能简化为三能级系统,而必须采用四能级模型进行求解。分别计算了三个系统在不同的系统温度和转移激光强度下的激光感生碰撞跃迁几率和碰撞截面,并从准分子势能曲线的角度解释了各个系统的谱线特征。本文对于弱场情况计算得到的碰撞截面谱线形状与以往的结论相同:碰撞截面谱线明显不对称,一侧为非稳态翼,碰撞截面下降非常迅速;另一侧为准稳态翼,在很宽的转移激光失谐范围内都可以得到较大的碰撞截面。但计算表明,原子间相对运动速度的统计分布对碰撞截面的大小有明显影响,因此有必要在计算时加以考虑。在强场下激光感生碰撞能量转移谱表现出了明显区别于弱场情况的特征:（1）强场时谱线的宽度显著变窄,并且谱线形状失去了弱场时的明显不对称性,随着激光强度的提高逐渐趋于对称;（2）谱线的峰值位置明显偏离了共振频率而向着非稳态翼一侧发生偏移,偏移量的大小随着转移激光强度近似成线性增加。（3）峰值碰撞截面随着转移激光强度的增加而增大,并在增大到一定程度时出现了饱和现象。在激光感生碰撞电荷转移的研究方面,本文提出了一个新的单光束Xe⁺-N激光感生碰撞电荷转移系统,这在国内外的研究中尚未见过报道。由于其特殊的能级结构,利用一束⁴40nm激光即可完成从反应初始能态Xe⁺的制备到激光感生碰撞电荷转移产生N⁺的整个过程,这与通常的激光感生碰撞过程需要两束激光完全不同。在理论方面,首先推导了与之相应的激光感生碰撞电荷转移的二能级理论模型,得到了态振幅的运动方程,并给出了碰撞跃迁几率和碰撞截面的表达式。在此基础上,对此Xe⁺-N系统进行了数值计算,得到了不同转移激光强度和失谐量时的碰撞跃迁几率和碰撞截面。对⁴40nm波长转移激光的计算结果表明,在此波长附近激光感生碰撞电荷转移截面随转移激光波长的变化处于水平线性区,并且随着激光强度的增加近似成线性增大,证明了在该系统中实现本文提出的单光束激光感生碰撞电荷转移过程的可行性。在实验上,将分子束技术和飞行时间质谱检测技术相结合,对Xe⁺-N系统的激光感生碰撞电荷转移过程进行了实验研究,据我们所知,迄今为止国内外尚未发表过相关的报道。首先利用⁴40nm染料激光多光子共振电离的方法制备了该系统的初始储能态Xe⁺,并测得了Xe在440nm波长附近的多光子共振电离谱,在不同参数条件下的实验结果表明利用光阑对光束进行模式净化以及适当减小束源压力有利于抑制电离谱的宽度以及避免离子信号出现饱和现象。在此基础上,利用一束⁴40nm染料激光从实验上实现了该系统由Xe⁺产生N⁺的激光感生碰撞电荷转移过程。利用飞行时间质谱检测法对反应的产物离子(Xe⁺、N⁺和N²⁺)进行了探测,并对束源压力、激光波长和强度等参数对离子强度和产额的影响进行了分析。实验结果证实了在440nm波长附近,碰撞截面随激光波长的变化近似恒定,随激光强度的增加近似成线性增大的结论,与理论计算结果相一致,证明了理论计算的合理性。更多还原

【Abstract】 A laser-induced collision is an optical process in which a laser field is used during the collision between two different particles to induce selective energy transfer from a level in one atomic or molecular species to another level in a different species. It can’t occur unless the two mechanisms - collision and radiative interaction - are both present; any of them singly can’t induce this interparticle transition. On one hand, energy transfer between different particles is made more quickly and efficiently because of the participation of the laser field; on the other hand, the effect of interparticle collisions can implement transfers which are difficult for single photon excitation, thus can obtain the expected high excited state and consequently short wavelength laser. Therefore, to study laser-induced collision process is of great significance for development of short wavelength laser sources. In addition, the ability to excite the special target level of the chosen particle permits potential applications in controlling pathways of chemical reactions.Laser induced collisions involve laser-induced collisional energy transfer (LICET) and laser-induced/assisted charge transfer (LICT/LACT). In the traditional researches on LICET, nearly all the theoretical models, based on which former numerical calculations are made, have assumed that the relative speed remains unchanged during the collision process, which obviously does not match the practical situation. Moreover, all of the traditional experimental investigations of LICT/LACT are made for mixed metal vapors, with no gas systems been reported. Aiming at these issues, the two important laser-induced collision processes are innovatively studied both theoretically and experimentally.For the theoretical research of LICET, the existing four-level model of LICET is improved considering the velocity distribution of the atoms during the collision. The distribution function of relative speed between two atoms is derived from the Maxwellian velocity distribution function under thermal equilibrium conditions, with the velocity averaged cross section obtained.On this basis, the incompletely studied Eu-Sr, Ba-Sr systems and the proposed Eu-Sr system are numerically calculated. These three systems are chosen because they are representative in that the former two systems satisfy the two limiting cases of LICET, respectively, thus can be reduced to a three-level system from a four-level one by neglecting one of their intermediate states; while the proposed Eu-Sr system can’t be reduced to a three-level system because it doesn’t satisfy either of the two limiting cases, and must be solved using the four-level model. Laser induced transition probabilities and collision cross sections of the three systems under different temperature and transfer laser intensities are calculated, with the characteristics of their spectrum profile interpreted from the viewpoint of potential curves of the quasimolecules.Our calculations in the weak field obtain the same spectrum profiles as former calculations, in that the spectrums are strongly asymmetrical, i.e. in the antistatic wing on one side of the line peak, the cross section falls rapidly; while in the quasistatic wing on the other side, the cross section is relatively large for a wide range of transfer laser detuning. However, calculations show that the statistic distribution of the relative speed between two atoms has dramatic impacts on the collision cross section, indicating that it is necessary to consider this distribution in the calculations. The LICET spectrum in strong field shows the following features which obviously differs from those in weak field: (a), the spectrum dramatically narrows in the strong field, and the spectral line shape becomes less asymmetric as the laser intensity increases; (b), the peak of the spectrum is shifted from the resonant frequency towards the antistatic region, with the shift increasing approximately linearly with laser intensity; (c), the peak cross section increases laser intensity and shows saturation at a high intensity.For the research of LICT, a novel one-beam Xe~+-N LICT system is proposed in this paper, which has not been reported home and abroad. Because of the special energy structure of this system, only one ~440nm laser beam is needed to complete the whole process from preparation of the initial state Xe~+ to production of N+ through LICT, which totally differs from the usual laser induced collision processes that require two laser frequencies. The corresponding two-level model for LICT is theoretically deduced, with the motion equations for the probability amplitudes obtained, and the expressions of collisional transition probability and cross section offered. On this basis, the LICT process for this Xe~+-N system is numerically calculated, obtaining transition probabilities and cross sections for different transfer laser intensities and detunings. The results for the transfer laser of ~440nm wavelength come to the conclusion that the laser induced collisional cross section is in its horizontally linear region within this wavelength region, and increases approximately linearly with increasing laser intensity, indicating the feasibility of implementing the proposed one-beam LICT process in this system.Combining the molecular beam technique and time-of-flight (TOF) mass spec- trometry, the LICT process in this Xe~+-N system is experimentally sdudied, which is to our knowledge the first report up to now. The initial state Xe~+ of the system is firstly prepared through resonance enhanced multi-photon ionization (REMPI) of atomic Xe by ~440 nm laser, and ionization spectra of Xe are measured. On this basis, the LICT process for this Xe~+-N system is experimentally realized using only one ~440nm dye laser beam. The product ions are detected by using TOF mass spectrometry, with the impacts of source pressure, laser wavelength and intensity on their intensity and yields analyzed. The experimental results indicate that collision cross section remains almost unchanged with laser wavelength and increases approximately linearly with laser intensity in the vicinity of 440nm, which is con- sistent with the calculated results, indicating validity of our theoretical calculations.更多还原