极紫外阿秒脉冲在高次谐波产生过程中引起的非偶极效应

当红外强激光和极紫外(XUV)阿秒脉冲共同作用于原子分子时，电离出去的电子通常会吸收和辐射激光光子而发生能量扩展.讨论了由于XUV阿秒脉冲的短波长与扩展后的电子波包尺度可相比拟时在高次谐波产生过程中引起的非偶极效应.采用H⁺₂作为模型分子，并把分子轴置于激光场的传播方向，通过解二维含时薛定谔方程并比较考虑非偶极效应和采用偶极近似两种方法计算得到的结果，两者相比，前者的谐波强度降低，谐波频率向低级次稍有移动，电子能谱的能带内出现了更多的光电子峰.在相同的光电子能量处，两种方法计算得到的信号强度相差2—5倍.并且这种非偶极效应随着红外基频光光强的增大而增强，随阿秒脉冲波长的增大而减弱. 关键词： 非偶极效应 光场空间不均匀性 阿秒脉冲 高次谐波产生

Nondipole effects in high-order harmonic generation induced by extreme ultraviolet attosecond pulse

When intense infrared laser and extreme ultraviolet (XUV) attosecond pulse interact with an atom or molecule, the released electrons generally suffer a spread of energies due to the absorption and emission of laser photons. The nondipole effects in the high-order harmonic generation caused by the short wavelength of XUV attosecond pulse which is comparable to the scale of spreading electron wavepacket are discussed in this paper. The H⁺₂ ion as a model molecule is assumed to be aligned along the propagation direction of the laser field. By solving the two-dimensional time-dependent Schrdinger equation and comparing the results taking account of nondipole effects with those under the dipole approximation, we find that intensity of the former harmonic yield is lower than that of the latter with a shift of the frequency towards lower order harmonics, and that more photoelectron peaks appear in the energy band of the photoelectron spectrum and the difference of their signal intensity at the same photoelectron energy is 2—5-fold. The results of numerical simulation also show that the nondipole effects are enhanced with the infrared laser field strength increasing and are weakened with the wavelength of attosecond pulse increasing.