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题名	强激光场中库仑势对原子多光子电离的影响
作者	王懿
学位类别	博士
答辩日期	2010
授予单位	中国科学院上海光学精密机械研究所
导师	张敬涛
关键词	多光子电离 光电子角分布 准能量 两能级模型 Freeman共振
其他题名	Influence of Coulomb potential on multiphoton ionization of atoms in intense laser fields
中文摘要	多光子电离是强激光场原子物理研究中一种基本而重要的非线性效应，其中原子核对电子的库仑引力起了非常重要的影响，特别是近共振跃迁的时候，库仑引力起到了决定性作用。因此，研究库仑势在原子多光子电离过程中的影响可以加深人们对强场物理过程的认识，并在诸多方面，例如标定激光强度，探测原子能级结构等，都有应用前景。 本论文研究了库仑势对强激光场中原子多光子电离的影响，主要包括：电子偶素离解时的光电子角分布特性，库仑势对原子多光子电离中光电子角分布喷射结构的影响，发展两能级模型用于研究Freeman共振，首次精确求解两能级模型的准能量和波函数。本论文取得了一些创新性的研究成果，具体如下： 1，采用非微扰散射理论研究强激光场中电子偶素离解时的光电子角分布。由于电子偶素中电子出射的动量大小依赖于出射方向，破坏了空间对称性，光电子角分布显示出不同于原子四对称的二对称分布。在一定的激光强度下，只有前向而没有后向的喷射结构。分析了不对称性产生的机制，将之归因于广义贝塞尔函数自变量的不对称性。该研究对慢正电子技术和表面物理探测具有潜在应用价值。 2，采用不同方法研究光电子角分布喷射结构数目的变化，并采用Coulomb-Volkov态研究库仑势对光电子角分布的影响。研究发现：在强场近似下，不同处理方法得到的光电子角分布类似，喷射结构的数目相同，但相对大小有所变化。考虑到库仑势的影响时，光电子角分布有了明显变化，但喷射结构的数目随吸收光子数的变化规律不变。本研究进一步证明了喷射结构的数目和光电子角动量没有关系，多吸收1个光子，喷射结构数目可能增加1个，3个或者其它奇数个，也可能减少1个。 3，从Bloch方程出发，精确求解两能级模型的准能量和波函数。波函数表示成只依赖于准能量的递推关系式，准能量由无限阶三对角行列式的反余旋函数表示。本文发展了一系列数学计算方法，精确计算无限阶三对角行列式的准确值，得到了前16位准确的有效数字，从而得到准能量的精确值。高精度求解三对角矩阵使该方法能适用于更高的激光强度。两能级模型精确解的计算用于解释Freeman共振等实验现象，并为下一步研究三能级原子模型以至N能级原子模型奠定了基础。 4，将两能级模型的计算结果用于研究Freeman共振现象，得到共振时激光峰值强度的精确值，发现不同强度对应不同共振峰的出现，共振峰的位置对应相应的Rydberg态且不随强度增加而变化，更深的Rydberg态需要更高的激光强度激发等等。这些特点与实验观测一致。 该方法可以用于物理测定激光强度，也能探测原子能级结构。 5，研究了能级在不同强度不同频率激光场中的移动情况，发现能级间隔存在两种变化方式。当初始能级间隔小于单个光子能量时，能级间隔随着激光强度的增加先减小后增加，并会发生零光子共振这一新的物理现象。反之，能级间隔先增加后减小，则会发生二次Freeman共振，光电子能谱上共振峰从低能向高能依次出现，这和一般的Freeman共振相反。零光子共振和二次Freeman共振有助于认识Freeman共振发生的物理机制。
英文摘要	Multiphoton ionization is one of the most important nonlinear effects in intense-laser atomic physics, in which the Coulomb force of the parent core plays a crucial role, especially in near-resonance case. The study on the influence of Coulomb potential in multiphoton ionization of atoms can deepen the understand on intense-laser atomic physics and can be applied in many fields such as measurement of laser peak intensity and atomic Rydberg states. In this thesis we study the influence of Coulomb potential on multiphoton ionization of atoms and focus on the Freeman resonance. Based on the exact mathematics treatment, we develop a two-level atom model to deal with the Freeman resonance, and perform high-precision calculations of the quasienergy and wave function for the first time. The main results are as follows: 1, We calculate the photoelectron angular distributions (PADs) of positronium based on a nonperturbative scattering theory. The momentum of electron and positron depends on the emission direction. The ionization rate along the laser propagation is not equal to that against the laser propagation, i.e. the PADs are inversion asymmetric, which becomes notable for higher kinetic energy of electrons. We analysis the origin of asymmetry, and attribute the asymmetry to that of variable of generalized Bessel function. 2, We apply different methods to study the number of jet-like structure in PADs, and use Coulomb-Volkov state to study the influence of the Coulomb potential to PADs. Our previous result shows that the number of jets on one side of PADs may increase by one, three, or other odd numbers and may decrease by one when one more photon is absorbed. Our study verifies this statement. Within the strong-field approximation, good agreement is obtained between two quite different treatments. By taking a Coulomb–Volkov state as the continuum state of photoelectrons, the PADs change greatly but the predicted phenomena still hold. 3, We develop an algebraic method to solve the quasienergy and wave function of a two-level atom model. Many mathematical difficulties, including solving infinite number of infinite determinants, are conquered. A data table is presented. With this table, one can easily obtain all quasienergies of driven two-level atom with given laser intensity, frequency and original energy spacing. This method can be applied to the calculation of Freeman resonances in photoelectron energy spectra. 4, We apply the above method to Freeman resonance, and obtain the following features: (1) Different non-occupied Rydberg states resonate at different laser-beam intensities. (2) The Freeman resonance peak corresponding to a particular Rydberg state has a fixed position in the photoelectron spectrum. It becomes prominent as the peak laser intensity further increases. (3) To trigger the Freeman resonance of a Rydberg state with a deeper binding energy, a higher peak laser intensity is needed. When the peak laser intensity increases further, the Freeman resonance of another Rydberg state with a deeper binding energy may occur. These features agree with experimental observations. 4, We calculate the energy spacing shift of a two-level atom at different laser intensities and frequencies. When the original energy level spacing is less than one photon energy, the energy level spacing decreases firstly and then increases with the increase of laser intensity, then zero-photon Freeman resonance occurs; otherwise, energy level spacing increases firstly and decreases, and a twice Freeman resonance occurs.
语种	中文
内容类型	学位论文
源URL	[http://ir.siom.ac.cn/handle/181231/15619]
专题	上海光学精密机械研究所_学位论文
推荐引用方式 GB/T 7714	王懿. 强激光场中库仑势对原子多光子电离的影响[D]. 中国科学院上海光学精密机械研究所. 2010.

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