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

这项提案概述了一项为期三年的计划，以开发一种紧凑的相干X射线源，适用于以大分子分辨率对活体、组织和细胞进行三维成像。成像将使用X射线显微全息技术完成。X射线将通过在紫外线(UV)中用强烈的超短(飞秒)激光脉冲激发动态团或分子来产生。由此产生的团簇中原子的多光子电离产生了电子-离子等离子体，该等离子体与激光脉冲的振荡辐射场强烈相互作用。由于场诱导有质动力，电子的集体运动给了电子足够的能量，以碰撞的方式从团簇中的离子中喷射出内壳层电子。当外壳层电子填满“空心”原子中的空位内壳态时，就会发射出X射线光子。以前的研究表明，如果相互作用的激光脉冲既是强的(即短而高能量的)又是相干的(即受相位控制的)，那么这种非线性X射线产生过程的效率就会提高。我们描述了一种全固态、高功率的UV飞秒激光系统，该系统被设计成最适合于激发原子团簇来产生X射线。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到320 nm之间，并将使用已经在近红外(~800 nm)波长范围内很好地建立起来的飞秒激光技术。除了在X射线产生过程中的应用外，我们还希望这种相干UV飞秒脉冲的来源在光谱生物学研究中有很多应用，例如，酶反应和其他蛋白质络合物中水溶剂化动力学的时间分辨。我们打算使用UV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过程中的染色体动力学。我们实验室已经开发的微全息技术将能够重建三维图像，这与当今任何其他具有类似大分子分辨率的成像技术形成了鲜明对比。利用波长为-L的X射线，这种全息技术还可以在不需要结晶的情况下，使生物分子结构三维可视化。我们建议通过研究亲和素-生物素复合体来探索这种可能性。这种基于激光的技术的发展有可能取代昂贵的同步辐射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.