The generation of low-frequency radiation from a short pulse (∼100 fs) laser with mJ energy incident on a metal surface is investigated. The electrons within the metal surface absorb energy from the laser pulse, increasing in temperature to a few electron volts and resulting in some at the high-energy tail of the distribution to overcome the work function barrier. Emission of these electrons from the surface contributes to a surface current which sources secondary radiation emission. The Richardson–Dushman emission picture breaks down due to Coulombic effects and a modified emission model is presented. Previous precedence exists for modified thermionic emission models for laser–metal interactions of a similar nature, such as the one presented by Riffe et al. [J. Opt. Soc. Am. B 10, 1424–1435 (1993)] upon which we elaborate. Surface currents generated by such a mechanism are modeled with a particle-in-cell (PIC) simulation together with a Monte Carlo treatment of electron–neutral collisions with air molecules. The modified emission model, together with the PIC model, provides a numerical basis from which the radiated spectra are estimated due to the time-varying currents emitted from the surface of the metal. Experiments and numerical simulations for air pressure between 0.1 and 1 atm show qualitative agreement as to the pressure dependence of the measured signal energy from the secondary radiation, with E measured ∝ P air − α for some α near but less than 1. Quantitative agreement between experiments and simulations is improved by including the effect of an additional contribution to secondary emission energy arising from inverse-bremsstrahlung collisions within a nm-scale vapor layer.

中文翻译：

---

来自激光-金属相互作用的动态鞘层形成和亚太赫兹辐射

研究了短脉冲（~100 fs）激光产生的低频辐射，mJ 能量入射到金属表面。金属表面内的电子从激光脉冲中吸收能量，温度升高到几电子伏特，导致一些在分布的高能尾部以克服功函数势垒。这些电子从表面的发射有助于产生二次辐射发射的表面电流。由于库仑效应，Richardson-Dushman 发射图分解，并提出了修改后的发射模型。对于类似性质的激光-金属相互作用的改进热电子发射模型，先前存在优先权，例如 Riffe 等人提出的模型。[J. 选择。社会。是。B 10, 1424–1435 (1993)] 我们对此进行了详细说明。由这种机制产生的表面电流通过粒子内粒子 (PIC) 模拟以及电子 - 中性与空气分子碰撞的蒙特卡罗处理进行建模。修改后的发射模型与 PIC 模型一起提供了一个数值基础，根据从金属表面发射的随时间变化的电流来估计辐射光谱。0.1 到 1 个大气压之间的气压的实验和数值模拟表明，对于来自二次辐射的测量信号能量的压力依赖性，具有定性的一致性，对于某些接近但小于 1 的 α，测量的 E ∝ P air − α。