脉冲宽度0.1 - 10 fs的XFEL单粒子成像实验中电子损伤的理论研究
Theoretical study of electronic damage in single particle imaging experiments at XFELs for pulse durations 0.1 - 10 fs
摘要：
X射线自由电子激光（XFEL）可以允许采用单粒子成像（SPI）方法来确定不形成稳定晶体的大分子的结构。10 fs及以下的超短脉冲可以超过库仑爆炸的完全衰变，并使核运动引起的辐射损伤最小化，但电子损伤仍然存在。对电子损伤的主要贡献来自样品中产生的等离子体，其强烈依赖于俄歇电离的量。由于俄歇过程具有飞秒量级的特征时间尺度，因此可以预期，对于阿秒脉冲，其贡献将显著减少。在这里，我们研究的SPI在脉冲持续时间从0.1 fs到10 fs和在一个大范围的XFEL注量的电子损伤的影响，以确定最佳条件下的生物样品的成像。我们分析了不同的电子激发过程的贡献，发现在高于$10^{13}$ - $10^{15}$光子/ $μ$ m $^2$（取决于光子能量和脉冲持续时间）的能量密度下，衍射信号饱和并且不再增加。通过将脉冲持续时间从10 fs减少到1 fs，可以获得信号的显著增益。与1 fs脉冲相比，持续时间低于1 fs的脉冲在散射信号中不给出显著增益。我们还研究了康普顿散射对SPI的限制。
英文摘要：
X-ray free-electron lasers (XFELs) may allow to employ the single particle imaging (SPI) method to determine the structure of macromolecules that do not form stable crystals. Ultrashort pulses of 10 fs and less allow to outrun complete disintegration by Coulomb explosion and minimize radiation damage due to nuclear motion, but electronic damage is still present. The major contribution to the electronic damage comes from the plasma generated in the sample that is strongly dependent on the amount of Auger ionization. Since the Auger process has a characteristic time scale on the order of femtoseconds, one may expect that its contribution will be significantly reduced for attosecond pulses. Here, we study the effect of electronic damage on the SPI at pulse durations from 0.1 fs to 10 fs and in a large range of XFEL fluences to determine optimal conditions for imaging of biological samples. We analyzed the contribution of different electronic excitation processes and found that at fluences higher than $10^{13}$-$10^{15}$ photons/$μ$m$^2$ (depending on the photon energy and pulse duration) the diffracted signal saturates and does not increase further. A significant gain in the signal is obtained by reducing the pulse duration from 10 fs to 1 fs. Pulses below 1 fs duration do not give a significant gain in the scattering signal in comparison with 1 fs pulses. We also study the limits imposed on SPI by Compton scattering.
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