微波弹性散射多光子电离中的电子计数
Counting the electrons in a multiphoton ionization by elastic scattering of microwaves
摘要：
激光诱导等离子体已经发现了许多应用，包括等离子体辅助燃烧，燃烧诊断，激光诱导击穿光谱，光探测和测距技术（LIDAR），微波制导，可重构等离子体天线等。多光子电离（MPI）是高能激光与物质相互作用的基本第一步，是重要的理解等离子体形成的机制。自50多年前MPI被发现以来，人们曾多次尝试通过直接实验来确定这一过程的基本物理常数，即MPI的光电离率和截面，但直到今天还没有可靠的数据，文献中流传的数值往往达到2-3个数量级。这是由于无法对MPI产生的等离子体电子数进行绝对测量，这导致了不确定性，有时，该领域不同研究人员使用的MPI横截面值之间存在矛盾。在这里，我们报告的第一个直接测量的绝对等离子体电子数产生的MPI的空气，随后，我们精确地确定电离率和截面的八光子电离的氧分子由800 nm的光子$σ_8=(3.32{\pm}0.3)*10^{-130} W^{-8}m^{16}s^{-1}$。方法是基于绝对测量的电子数创建MPI使用弹性散射的微波等离子体体积在瑞利政权，并建立了一个通用的方法来直接测量和制表的MPI过程中的各种气体和光子能量的基本常数。
英文摘要：
Laser induced plasmas have found numerous applications including plasma-assisted combustion, combustion diagnostics, laser induced breakdown spectroscopy, light detection and ranging techniques (LIDAR), microwave guiding, reconfigurable plasma antennae etc. Multiphoton ionization (MPI) is a fundamental first step in high-energy laser-matter interaction and is important for understanding of the mechanism of plasma formation. With the discovery of MPI more than 50 years ago, there were numerous attempts to determine basic physical constants of this process in the direct experiments, namely photoionization rates and cross-sections of the MPI, however, no reliable data is available until today and spread in the literature values often reaches 2-3 orders of magnitude. This is due to inability to conduct absolute measurements of plasma electron numbers generated by MPI which leads to uncertainties and, sometimes, contradictions between the MPI cross-section values utilized by different researchers across the field. Here we report first direct measurement of absolute plasma electron numbers generated at MPI of air and subsequently we precisely determine ionization rate and cross-section of eight-photon ionization of oxygen molecule by 800 nm photons $σ_8=(3.32{\pm}0.3)*10^{-130} W^{-8}m^{16}s^{-1}$. Method is based on the absolute measurement of electron number created by MPI using elastic scattering of microwaves off the plasma volume in Rayleigh regime and establishes a general approach to directly measure and tabulate basic constants of the MPI process for various gases and photon energies.
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