Mid-Infrared Frequency Comb Lasers for Chemical Kinetics: Applying Physics Technologies to Kinetics, Dynamics, and Molecular Spectroscopy

一个简单的化学反应可以被描述为两个反应物分子A + B之间的相互作用，这导致两个新的产物分子C + d的形成。这个过程涉及到化学键的断裂和形成，从而赋予产物与反应物本质上不同的性质。识别生成物和反应物分子的一种方法是使用振动光谱学。分子中的每个键都以特定的频率振动，使得一个分子（如分子a）的振动吸收光谱与另一个分子（如分子B、C或D）的振动吸收光谱不同，就像识别该分子的“指纹”。然而，由于不同分子中的化学键可以以不同的频率振动，因此很难同时看到A + B -> C + D反应中所有分子的指纹。要做到这一点，就需要同时具备宽带和高分辨率的振动吸收光谱。然而，知道反应的时间标度也是有用的。进一步假设这个反应与另一个反应竞争，比如A + B -> E。那么不仅要知道分子A和B消失的速度，还要知道分子C、D和E出现的速度。从上述假设的化学反应中，我们认识到了解参与反应的分子（反应物和生成物）的身份以及它们消失或出现的速度是很重要的。因此，必须同时使用宽带（宽光谱宽度）和高光谱分辨率技术，并结合必要的时间分辨率来监测化学反应的动力学。这项提议的研究使用了一种由光学物理社区开发的技术，称为腔增强直接频率梳光谱，并将其应用于一个基本有趣的自由基-自由基反应。在这里，频率梳激光器是激发分子振动所必需的红外辐射源。它是一个宽带源，因此它可以在宽光谱区域（3 - 3.5微米）内激发一系列不同的分子振动。然而，它的独特之处在于，成千上万个频谱狭窄的“梳齿”组成了这个宽带源，每一个都有一个已知的可控频率。这使得它既宽频又高分辨率，满足了根据分子的振动指纹光谱识别分子的标准。该光源通过反应池，在反应池中发生化学反应（在本实验中，初始目标反应为自由基-自由基反应CH2SH + NO）。参与该反应的一些分子吸收红外辐射，以分子吸收的特定频率（“梳齿”）衰减通过反应细胞的红外光量。在提出的研究中，该光源的“梳状齿”在空间上分散到红外敏感相机上，给出了气体细胞中所含物质的高分辨率振动吸收光谱。相机在反应发生时拍摄图像，产生作为反应时间函数的振动吸收光谱，从而同时识别和绘制参与化学反应的分子出现（和消失）的时间标度。这是一种独特的技术，用于研究化学反应的动力学和动力学，其中关于化学反应的大量细节包含在这种高分辨率，时间分辨光谱中。

A simple chemical reaction could be described as an interaction between two reactant molecules, A + B, which leads to the formation of two new product molecules, C + D. This process involves the breaking and making of chemical bonds, giving the products inherently different properties than the reactants. One way to identify the product and reactant molecules is by using vibrational spectroscopy. Each bond in a molecule vibrates at a specific frequency, making the vibrational absorption spectrum of one molecule (such as molecule A) different than another molecule (such as molecules B, C, or D), like a "fingerprint" identifying that molecule. However, because bonds in different molecules could vibrate at vastly different frequencies, it is hard to view the fingerprints of all of the molecules in the A + B -> C + D reaction at once. To do so, a simultaneously broadband and high resolution vibrational absorption spectrum would be needed. However, it would also be useful to know the timescale for the reaction. Suppose further that this reaction was competing with another reaction, like A + B -> E. It is then not only important to know the rate at which molecules A and B disappeared, but also the rate at which C, D, and E appeared. From the above hypothetical chemical reactions, we realize that it is important to know both the identity of molecules involved in a reaction (reactants and products) as well as the rate at which they disappear or appear. Thus, it is essential to use a simultaneously broadband (wide spectral width) and high spectral resolution technique, combined with the time resolution necessary to monitor the kinetics of the chemical reactions. The proposed research uses a technique developed by the optical physics community called cavity-enhanced direct frequency comb spectroscopy and applies it to a fundamentally interesting radical-radical reaction. Here, a frequency comb laser is the source of the infrared radiation necessary to excite molecular vibrations. It is a broadband source, so it can excite a range of different molecular vibrations within a wide spectral region (3 - 3.5 microns). It is unique, though, in that thousands of spectrally narrow "comb teeth" make up this broadband source, each with a known and controllable frequency. This makes it both broadband and high resolution, meeting the criteria for being able to spectrally identify molecules based on their vibrational fingerprints. This light source is passed through a reaction cell, where a chemical reaction takes place (in the proposed experiment, the initial target reaction is the radical-radical reaction CH2SH + NO). Some of the molecules involved in this reaction absorb the infrared radiation, attenuating the amount of infrared light passing through the reaction cell at the specific frequencies ("comb teeth") that the molecules absorbed. In the proposed research, the "comb teeth" of this light source are spatially dispersed onto an infrared sensitive camera, giving a high resolution vibrational absorption spectrum of what is contained in the gas cell. The camera takes images as the reaction occurs, yielding vibrational absorption spectra as a function of reaction time, thus simultaneously identifying and mapping the timescale of the appearance (and disappearance) of molecules involved in the chemical reaction. This is a unique technique to be applied to studying the kinetics and dynamics of chemical reactions, where a significant amount of detail about a chemical reaction is contained in this high resolution, time-resolved spectrum.