Solids irradiated by energetic photons can be eroded in two modes, depending on the radiation intensity. High average, low-peak power sources, e.g., synchrotron radiation and high-order harmonics, induce desorption of the material at a low etch rate. In contrast, high-peak-power radiation from extreme ultraviolet and x-ray lasers usually causes a massive removal of the material even by a single shot. In this contribution, an effective material erosion is reported in PMMA exposed to multiple accumulated pulses generated by the free-electron x-ray-laser Linac Coherent Light Source (LCLS, tuned at a photon energy of 830 eV in this study, operated in Menlo Park at Stanford, CA, USA) at a fluence below the single-pulse ablation threshold. The effect is caused by polymer-chain scissions initiated by single photons carrying enough energy to break the $\mathrm{C}—\mathrm{C}$ bounds. High efficiency of the erosion is supposed to occur due to a correlation of the single-photon effects. The subthreshold damage exhibits a nonlinear dose dependence resulting from a competition between chain scissions and cross-linking processes. The cross-linking is proven by Raman spectroscopy of the irradiated polymer. Two theoretical models of the x-ray free-electron-laser-induced erosion are suggested, which provide an excellent agreement with the experimental results.

中文翻译：

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X射线自由电子激光器多次发射引起的有机聚合物的亚阈值腐蚀

高能光子照射的固体可以根据辐射强度以两种模式腐蚀。高平均，低峰值电源（例如同步加速器辐射和高阶谐波）会导致材料以低蚀刻速率解吸。相比之下，来自极紫外和X射线激光的高峰值功率辐射通常会导致材料的大量去除，即使是单次射击也是如此。在这项贡献中，据报道，PMMA暴露于由自由电子X射线激光直线加速器相干光源（LCLS，在本研究中以830 eV的光子能调节）产生的多个累积脉冲的情况下，发生了有效的材料侵蚀。能量密度低于单脉冲消融阈值的美国加利福尼亚州斯坦福公园。这种影响是由携带足够能量以破坏光子的单光子引发的聚合物链断裂引起的。$\mathrm{C}—\mathrm{C}$界限。推测由于单光子效应的相关性，发生了高效率的腐蚀。亚阈值损伤表现出非线性的剂量依赖性，这是由链断裂和交联过程之间的竞争引起的。交联通过辐射聚合物的拉曼光谱证明。提出了两种由X射线自由电子激光诱导的腐蚀的理论模型，这些模型与实验结果非常吻合。