A Compact Laser Excited Coherent X-ray Source for Macromolecular Imaging

该提案概述了一个为期三年的计划，以开发一种紧凑的相干x射线源，适用于具有大分子分辨率的生物体，组织和细胞的三维成像。成像是用x射线微全息技术完成的。x射线将通过在紫外（UV）中使用强烈的超短（飞秒）激光脉冲激发原子簇或分子而产生。团簇中原子的多光子电离产生电子-离子等离子体，与激光脉冲的振荡辐射场强烈相互作用。由于场致自重动势，电子的集体运动使电子有足够的能量从团簇中的离子中碰撞出一个内壳电子。当外层电子填充空的“空心”原子的内壳层态时，就会发射x射线光子。先前的研究表明，如果相互作用的激光脉冲既强（即短而有能量）又相干（即相位控制），则这种非线性x射线产生过程的效率会提高。我们描述了一种全固态，高功率紫外飞秒激光系统，该系统被设计为最适合用于产生x射线的原子团簇的激发。该激光系统将基于新型固体紫外激光材料Ce掺杂LiSrAlF6，其工作波长在280至320nm之间，并将采用飞秒激光技术，该技术已经在近红外波长（~ 800nm）建立。除了在x射线产生过程中使用外，我们期望这种相干紫外飞秒脉冲源在光谱生物学研究中找到许多应用，例如酶反应和其他蛋白质复合物中水溶剂化动力学的时间分辨率。我们打算利用紫外激光系统产生的相干x射线源来成像有丝分裂和减数分裂期间的染色体动力学。我们实验室已经开发的微全息技术将能够重建三维图像，与目前任何其他具有类似分子分辨率的成像技术相比。使用波长为- 1的x射线，这种全息技术还可以在不需要结晶的情况下以三维方式可视化生物分子的结构。我们建议通过研究亲和素-生物素复合物来探索这种可能性。这种基于激光的技术的发展有可能取代昂贵的同步加速器x射线源，用于生物科学的成像。

This proposal outlines a three year program to develop a compact coherent X-ray source that is suitable for three-dimensional imaging of living organisms, tissue and cells with macromolecular resolution. The imaging is to be accomplished using X-ray rnicroholographic techniques. The X-rays will be produced by exciting atornic clusters or molecules with an intense ultrashort (femtosecond) laser pulse in the ultraviolet (UV). The resulting multiphoton ionization of the atoms in the clusters produces an electron-ion plasma which interacts strongly with the oscillating radiative field of the laser pulse. The collective motion of the electrons, due to the field-induced ponderomotive force, gives the electrons sufficient energy to collisionally eject an inner-shell electron from an ion in the cluster. A X-ray photon is emitted when an outer shell electron fills the vacant inner shell state in the "hollow" atom. Previous investigations have shown that the efficiency of this nonlinear X-ray generation process is increased if the interacting laser pulse is both intense (i.e. short and energetic) and coherent (i.e. phase controlled). We describe an all solid-state, high-power UV femtosecond laser system that is designed to be optimally suited for the excitation of atomic clusters for the production of X-rays. The laser system will be based on the new solid-state UV laser material Ce doped LiSrAlF6 which operates between 280 and 320nm, and will employ femtosecond laser technology that is already well established at wavelengths in the near infrared (~ 800nm). In addition to its use in the X-ray generation process, we expect this source of coherent UV femtosecond pulses to find many applications in spectroscopic biological research, e.g. the temporal resolution of the water solvation dynamics in enzyme reactions and other protein complexes. We intend to use the coherent X-ray source produced using the UV laser system to image chromosome dynamics during mitosis and meio sis. The microholographic techniques that have already been developed in our laboratories will enable three dimensional images to be reconstructed, in contrast to any other imaging technology available today with comparable marcomolecular resolution. Using X-rays with a wavelength of -l, this holographic technique should also allow the structure of biological molecules to be visualized in three dimensions without the need for crystallization. We propose to explore this possibility by investigating the avidin-biotin complex. The development of this laser-based technology has the potential to replace expensive synchrotron X-ray sources for imaging in the biological sciences.