Tuneable Femtosecond Laser Sources for Time-Resolved Ultrafast Spectroscopy

分子和材料中电子的超快动态运动是光合作用、视觉和太阳能转换(光伏)的基本过程的基础。电子电荷转移必然涉及电子运动与振动运动的耦合。这些非绝热(非Born-Oppenheimer)过程发生在fS的10‘S时间尺度上，在电子激发的分子态中无处不在。例如电荷转移、异构化和光化学反应。尽管有这种基本的重要性，超快电子非绝热动力学仍然是物理化学中最具挑战性的问题之一。在NRC工作期间(1992-2014)，Stolow博士开创了飞秒时间分辨光电子能谱(TRPES)的强大技术，该技术允许解开电子激发分子中耦合的电子-振动动力学。该方法基于用可调谐的飞秒泵浦脉冲共振激发分子，然后用延迟的可调谐飞秒探测脉冲光致电离演化的激发态。由此得到的能量-角-时间分辨光电子能谱明显地包含了关于激发态电子过程及其非绝热耦合的丰富细节。虽然Stolow的项目最初设在NRC，但在2021年秋季完全过渡到渥太华大学。因此，需要NSERC支持的可调谐飞秒激光器，而渥太华大学不存在这种激光器。Stolow的NSERC支持的TRPES技术要求在UV或VUV范围内产生两个独立可调的飞秒泵浦和探测脉冲，并具有足够的脉冲能量。每个研究中的分子都有特定的共振跃迁，因此必须将泵浦激光调谐到该分子。同样，由于电离势也不同，因此也需要可独立调谐的飞秒探测激光脉冲。因此，该RTI请求是针对两个独立但同步的光转换TOPAS光学参数放大器，其提供可广泛调谐的飞秒脉冲。其中之一必须具有较高的能量输出，以便允许效率较低的飞秒VUV脉冲的产生。这一RTI技术得益于安装在渥太华大学的现有飞秒Ti：Sa(即，不可调谐的800 nm)泵浦激光器和光学台。所要求的TOPAS系统对于斯托洛博士在渥太华大学的TRPES研究计划的继续至关重要：如果没有广泛调谐飞秒激光脉冲的能力，这个TRPES研究计划就不能有效地继续下去。申请的TOPAS单元将用于每个TRPES实验(每月80小时)，并将在非线性光学和超快光谱学方面的HQP培训中发挥核心作用：它们将被所有学生常规使用。

The ultrafast dynamical motions of electrons within molecules and materials underlies the fundamental processes of photosynthesis, vision and solar energy conversion (photovoltaics). Electronic charge transfer necessarily involve the coupling of electronic with vibrational motions. These non-adiabatic (non-Born-Oppenheimer) processes, which take place on the time scales of 10's of fs, are ubiquitous in electronically excited molecular states. Examples include charge transfer, isomerization and photochemical reaction. Despite this fundamental importance, ultrafast electronically non-adiabatic dynamics remains one of the most challenging problems in Physical Chemistry. While employed at the NRC (1992-2014), Dr. Stolow pioneered the powerful technique of fs Time-Resolved Photoelectron Spectroscopy (TRPES) which permits the disentangling of coupled electronic-vibrational dynamics in electronically excited molecules. The method is based on resonantly exciting a molecule with a tuneable fs pump pulse and then photoionizing the evolving excited state with a delayed tuneable fs probe pulse. The resulting energy-angle-time resolved photoelectron spectrum demonstrably contains a wealth of details about excited state electronic processes and their non-adiabatic coupling. Although originally based at the NRC, Stolow's program fully transitioned to uOttawa in Fall 2021. Hence, the need for NSERC-supported tuneable fs lasers which do not exist at uOttawa. Stolow's NSERC-supported TRPES technique requires that two independently tuneable fs pump and probe pulses in the UV or VUV ranges be generated with sufficient pulse energy. Each molecule under study has specific resonant transitions and the pump laser must therefore be tuned to that molecule. Likewise, since Ionization Potentials also vary, an independently tunable fs probe laser pulse is also required. This RTI request is therefore for two independent but synchronized `Light Conversion' TOPAS Optical Parametric Amplifiers which provide broadly tuneable fs pulses. One of these must have higher energy output in order to permit the less efficient generation of fs VUV pulses. This RTI benefits from an existing femtosecond Ti:Sa (i.e. non-tuneable, 800 nm) pump laser and optical table installed at uOttawa. The requested TOPAS systems are essential for the continuation of Dr. Stolow's TRPES research program at uOttawa: without the ability to broadly tune fs laser pulses, this TRPES research program cannot effectively continue. The requested TOPAS units will be used in each and every TRPES experiment (> 80 hours/month) and will play a central role in the training of HQPs in nonlinear optics and ultrafast spectroscopy: they will be routinely used by all students.