Positronium emission from mesoporous silica studied by laser-enhanced time-of-flight spectroscopy
Positronium emission from mesoporous silica studied by laser-enhanced time-of-flight spectroscopy
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DOI：
10.1088/1367-2630/17/4/043059
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发表时间：
2015-04
影响因子：
3.3
通讯作者：
A. Deller;B. Cooper;T. Wall;D. Cassidy
中科院分区：
文献类型：
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作者：
A. Deller;B. Cooper;T. Wall;D. Cassidy
近年来，使用介孔二氧化硅膜生产和研究电子偶素（Ps）原子变得越来越重要，提供了真空中游离Ps的强大来源，可用于各种实验，包括精密光谱学和反氢的生产。介孔材料冷却和限制Ps的能力也被用来进行Ps-Ps散射和Ps2分子形成的测量，这种方法提供了制造足够密集和冷的Ps系综以实现Ps玻色-爱因斯坦凝聚的可能性。因此，有很大的兴趣在研究的动力学Ps原子内这样的介孔结构，以及它们的形态如何影响Ps冷却，扩散和发射到真空中。现在已经确定，Ps原子最初是在这种材料的大部分中产生的，随后以1 eV的能量喷射到内部空隙中，然后通过数十万次壁碰撞迅速冷却。该过程可导致热化至环境样品温度，但当Ps德布罗意波长接近限制性中孔的尺寸时将被阻止。在这一点上，通过孔隙网络的扩散只能通过隧道以慢得多的速率进行。一个重要的问题是，Ps原子冷却并逃逸到真空中需要多长时间？在这个过程中，使用激光增强的正电子素飞行时间光谱进行直接测量，我们表明，冷却到量子限制制度的薄膜中约5 nm直径的孔几乎是在5 ns内完成，并进入真空的发射需要100 ns时，入射正电子束能量为5 keV。所观察到的Ps发射时间对正电子注入能量的依赖性支持量子限制Ps不采样所有可用的孔体积的想法，而是限于中孔网络的子集。
The use of mesoporous silica films for the production and study of positronium (Ps) atoms has become increasingly important in recent years, providing a robust source of free Ps in vacuum that may be used for a wide variety of experiments, including precision spectroscopy and the production of antihydrogen. The ability of mesoporous materials to cool and confine Ps has also been utilized to conduct measurements of Ps–Ps scattering and Ps2 molecule formation, and this approach offers the possibility of making a sufficiently dense and cold Ps ensemble to realize a Ps Bose–Einstein condensate. As a result there is great interest in studying the dynamics of Ps atoms inside such mesoporous structures, and how their morphology affects Ps cooling, diffusion and emission into vacuum. It is now well established that Ps atoms are initially created in the bulk of such materials and are subsequently ejected into the internal voids with energies of the order of 1 eV, whereupon they rapidly cool via hundreds of thousands of wall collisions. This process can lead to thermalisation to the ambient sample temperature, but will be arrested when the Ps deBroglie wavelength approaches the size of the confining mesopores. At this point diffusion through the pore network can only proceed via tunneling, at a much slower rate. An important question then becomes, how long does it take for the Ps atoms to cool and escape into vacuum? In a direct measurement of this process, conducted using laser-enhanced positronium time-of-flight spectroscopy, we show that cooling to the quantum confinement regime in a film with approximately 5 nm diameter pores is nearly complete within 5 ns, and that emission into vacuum takes ∼10 ns when the incident positron beam energy is 5 keV. The observed dependence of the Ps emission time on the positron implantation energy supports the idea that quantum confined Ps does not sample all of the available pore volume, but rather is limited to a subset of the mesoporous network.