中红外激光场中分子高次谐波生成的单轨道和多轨道效应

采用非微扰电动力学的方法研究了中红外激光场驱动分子产生的高次谐波能谱,并分析了不同分子轨道的贡献.研究发现,单个分子轨道产生的高次谐波能谱呈现出分子结构导致的干涉极小值.长脉冲中干涉极小值的位置随着激光强度增加向高能端移动,短脉冲中其位置固定.这种差别是由于不同脉宽的激光场中电子抖动动能的变化引起的.干涉极小值的最大移动量等于激光场中电子的抖动能.多个分子轨道辐射的谐波会产生干涉,这种干涉效应主要体现在两个方面:一是掩盖了单个轨道谐波谱的干涉极小值,从而使得总能谱中极小值不明显;二是通过总能谱中干涉结构的变化,体现出单轨道谐波谱中干涉极小值前后的相位突变.本研究解释了一系列的实验观测并预言了有待观测的现象.

High-order harmonics generated from single and multiple molecular orbits in mid-infrared laser fields

High-order harmonics generated from aligned molecules are studied by a nonperturbative QED theory and the effect of the multiple molecular orbits is included.The harmonic spectra generated from single molecular orbit exhibit an interference minimum which is induced by the molecular structure.The location of the spectral minimum shifts with the laser intensity in long laser pulses,but is fixed in ultrashort laser pulses.This difference is owed to the quiver motion of the electron in the laser pulses.The maximal shift of the spectral minimum equals to the increment of the ponderomotive energy and depends linearly on the laser intensity.The interference between the harmonics generated from multiple molecular orbits has two principal effects:one is obscuring the deep minima in the overall harmonic spectrum,the other is manifesting the phase jump in the harmonics generated from single molecular orbit.