Time-Resolved Spectroscopic Study of Diatomic Molecular Sodium

该项目将测量分子的“激发态寿命”，即分子在吸收光后保持高能态的时间。这个生命周期的特征时间标度是由自然规律决定的，是分子的基本性质之一。通常是几十纳秒（十亿分之一秒）。在这个项目中，钠分子的寿命是通过一种叫做“时间分辨光谱”的技术，使用非常精确的脉冲激光来测量的。当激光脉冲打开时，一个高速电子秒表被打开，以记录光子吸收和发射之间的时间间隔。获得的数据在天体物理学（确定空间中的分子类型及其形成方式）、等离子体物理学（确定非常热的材料的温度）和激光物理学（帮助预测哪些材料可以用来制造更好的激光器）等领域都有应用。本项目着重于使用时间分辨光谱技术对直接荧光衰减的钠分子的激发态离子对状态的辐射寿命测量（用于各种旋转量子数）进行实验研究。钠分子可以表现出奇特的行为，因为它们的结构和辐射性质随着核间距的变化而发生强烈的变化。这是由于离子对特性的影响，使分子势能曲线形成双阱。最近，钠分子的一些离子对态的辐射寿命已经被计算和绘制出来。这提供了基准值，可以在实验室中测试最近理论方法的可靠性。激发态寿命的测量在理解分子势能、分子电子结构和相关数量（如跃迁概率、振子强度和谱线强度）之间的相互作用方面起着重要作用。实验方法是在热管烘箱中制造钠分子，并通过两步激光双共振激发使其达到离子对状态。将测量分子荧光检测、解离分子的原子荧光检测（与分子辐射时间尺度不同的过程）和预解离分子的原子荧光偏振。如果检测到解离分子，极化测量将提供预解离过程的非绝热动力学的详细信息。因此，时间分辨光谱技术为研究分子内振动能量分布和分子内原子运动动力学提供了工具。

This project will measure the "excited state lifetime" of a molecule, which is the amount of time a molecule remains in a high energy state after absorbing light. The characteristic timescale of this lifetime is determined by the laws of nature, and is one of the fundamental properties of the molecule. It is typically a few tens of nanoseconds (billionths of a second). The lifetime of sodium molecules is measured in this project using very precisely pulsed lasers in a technique called "time-resolved spectroscopy." When the lasers are pulsed on, a high-speed electronic stopwatch is switched on to record the amount of time that lapses between the absorption and the emission of the photon. The data obtained has applications in fields such as astrophysics (determining what kind of molecules are in space and how they formed), plasma physics (determining the temperatures of very hot materials), and laser physics (helping to predict what materials could be used to make better lasers). This project focuses on experimental studies of radiative lifetime measurements (for a variety of rotational quantum numbers) of the excited ion-pair state of sodium molecules from direct fluorescence decay using a time-resolved spectroscopic technique. Sodium molecules can exhibit exotic behavior since their structural and radiative properties vary strongly as a function of internuclear distance. This is due to the effect of the ion pair character, which causes the formation of double well in the molecular potential energy curves. Recently, the radiative lifetimes of some of the ion-pair states of sodium molecules have been calculated and mapped out. This provides benchmark values against which the reliability of recent theoretical approaches can be tested in the laboratory. Measurements of excited state lifetimes play an important role in understanding the interplay between molecular potential energies, molecular electronic structure, and related quantities such as transition probabilities, oscillator strengths, and line intensities. The experimental approach is to create sodium molecules in a heatpipe oven and excite them by a two-step laser double resonance excitation to reach the ion-pair state. Molecular fluorescence detection, atomic fluorescence detection from dissociating molecules (a process which happens on different time scale than molecular radiative timescale), and polarization of atomic fluorescence from predissociating molecule will be measured. If a dissociated molecule is detected, polarization measurement will provide detail information about the nonadiabatic dynamics of the predissociation process. Thus, the time-resolved spectroscopic technique provides a tool for studying the intramolecular vibrational energy distribution in molecules and the dynamics of atomic motion in a molecule.