A new method for studying laser and electron interactions for a wide range of atomic targets - collision studies in an optical enhancement cavity

物理学中需要理解的最基本的过程之一是原子如何通过与不同粒子（如电子）的碰撞而被激发。这一过程发生在许多领域，从照明的生产、激光的发展、对气候变化具有重要意义的电离层和大气过程、闪电放电、恒星和行星中的天体物理过程、原子和分子的光谱学以及所有使用电力的工业。在基础层面上理解这些过程是至关重要的，这样新技术才能被开发出来，这样我们才能应用我们的知识来进一步理解气候变化和宇宙结构。这些过程的最详细信息是通过实验确定受激原子在电子非弹性散射后的“形状”而获得的。我们可以通过研究原子松弛回基态时发出的光来做到这一点。这些研究的结果作为电子散射角的函数，然后与复杂量子理论的预测进行比较。曼彻斯特最近开发的一项新技术使我们能够在所有散射角上测量原子的形状——这在以前是不可能的任务。这些实验使用一个特殊设计的磁场来引导电子进出相互作用区域，这样所有的角度都可以被访问。在碰撞之前，激光束使原子处于激发态，因此电子散射的能量比碰撞前更多（超弹性散射）。然后我们检测这些高能电子作为激光束特性的函数。因此，实验有效地逆转了时间——而不是从一个电子开始，然后从被激发的原子中寻找光子，我们从一个激光光子开始，然后寻找出现的电子！通过这样做，实验产生数据的速度比使用标准技术快数千倍（因为激光束总是向同一个方向发送）。通过采用这些方法，我们可以非常精确地确定原子的形状，以便与理论（目前正在美国和澳大利亚发展）进行比较。位于曼彻斯特的仪器是目前世界上最精密的超弹性散射光谱仪，并产生了前所未有的数据。我们希望通过结合一种光学技术来显著扩展这些研究，这种技术使我们能够激发比目前可能的更多的目标（与目前可以激发的目标相比，最多可访问25个新目标）。为了实现这一点，我们将在相互作用区域周围放置高反射率的镜子，作为光谱仪内部光的“存储”。这种“光学腔体”允许反射镜之间的激光功率比直接从激光发出的功率增加多达50倍。我们将首次利用这种新技术利用紫外激光辐射制备原子。然后，超弹性散射的电子将从被激发的原子中被探测到，这些原子将包括锌、银和金目标。这对新的照明技术（目前正在考虑用锌代替荧光灯和紫外线灯中对环境有害的汞）和正在开发的描述这些复杂原子的新量子理论都很有意义。作为该计划的一部分，我们还将开发一种新型的外部倍频腔，可以同时从一个激光束激发两个激光频率。这对于具有超精细结构的原子（包括金和银）的激发是必要的，其中核自旋分裂基态的能级。我们将在这里开发的新型倍腔将在激光和原子物理的不同领域得到广泛的应用，光谱仪内部光学腔增强的开发和实施也将得到广泛的应用。

One of the most fundamental processes to be understood in physics is how atoms are excited by collision with different particles such as electrons. This process occurs in many areas from the production of lighting, the development of lasers, ionospheric and atmospheric processes of importance to climate change, lightning discharges, astrophysical processes as in stars and planets, the spectroscopy of atoms and molecules and in all industries using electricity. It is essential to understand these processes at a fundamental level so new technologies can be developed, and so we can apply our knowledge to further understanding of climate change and the structure of the universe.The most detailed information on these processes is obtained by experimentally determining the 'shape' of an excited atom following inelastic scattering of an electron. We can do this by studying the light emitted from the atom as it relaxes back to the ground state. The result of these studies as a function of the electron scattering angle are then compared to predictions from sophisticated quantum theories. A new technique recently developed in Manchester allows us to measure the shape of the atom over ALL scattering angles - a task that has been impossible previously. These experiments use a specially designed magnetic field to steer electrons to and from the interaction region so all angles can be accessed. A laser beam prepares the atoms in an excited state prior to the collision, and so the electrons scatter with more energy than they had prior to the collision (super-elastic scattering). We then detect these higher energy electrons as a function of properties of the laser beam. The experiments therefore effectively reverse time - instead of starting with an electron and then looking for a photon from the excited atom, we start with a laser photon and then look for the emerging electron! By doing this, the experiments produce data thousands of times faster than using standard techniques (since the laser beam is always sent in the same direction). By adopting these methods, we can very precisely determine the shape of the atom for comparison to theory (now being developed in the USA and Australia).The apparatus in Manchester is now the most sophisticated super-elastic scattering spectrometer in the world, and has produced data never seen before. We wish to significantly extend these studies here, by incorporating an optical technique which allows us to excite many more targets than is currently possible (up to 25 new targets will be accessible compared to those which can be excited at present). To facilitate this, we will place high reflectivity mirrors around the interaction region to act as a 'storage' of light inside the spectrometer. This 'optical cavity' allows the laser power between the mirrors to be increased by up to 50 times compared to that directly from the laser. We will use this new technique to prepare atoms using UV laser radiation for the first time. Super-elastically scattered electrons will then be detected from the excited atoms, which will include zinc, silver and gold targets. These are of interest for new lighting technologies (which are currently considering using zinc as a replacement for the environmentally toxic mercury in fluorescent and UV lights), and for comparison to new quantum theories being developed to describe these complex atoms.As part of this programme we will also develop a new type of external doubling cavity that can excite two laser frequencies simultaneously from the one laser beam. This is necessary for excitation of atoms with hyperfine structure (including gold and silver), where the nuclear spin splits the energy level of the ground state. The new type of doubling cavity we will develop here will have application in a wide range of different areas of laser and atomic physics, as will the development and implementation of the optical cavity enhancement inside the spectrometer.