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描述为两个反应物分子A+B之间的相互作用，导致两个新的产物分子C+D的形成。这个过程涉及化学键的断裂和形成，使产物具有与反应物固有的不同性质。鉴定产物和反应物分子的一种方法是使用振动光谱。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，由于不同分子中的键可能以非常不同的频率振动，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，需要同时具有宽带和高分辨率的振动吸收光谱。然而，知道反应的时间表也是有用的。进一步假设这个反应与另一个反应，如A+B-&E竞争，那么不仅要知道分子A和B消失的速度，而且要知道C、D和E出现的速度。从以上假设的化学反应中，我们认识到了解参与反应的分子(反应物和产物)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，并结合监测化学反应动力学所需的时间分辨率。这项拟议的研究使用了光学物理界开发的一种名为腔增强直接频率梳状光谱的技术，并将其应用于一种基本有趣的自由基-自由基反应。这里，频率梳激光器是激发分子振动所必需的红外辐射的来源。它是一种宽带光源，因此可以在很宽的光谱范围(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，数以千计的频谱狭窄的“梳齿”组成了这个宽带信号源，每个都有一个已知的和可控制的频率。这使得它既具有宽带又具有高分辨率，满足了能够根据分子的振动指纹进行光谱识别的标准。光源穿过反应室，在那里发生化学反应(在所提出的实验中，最初的目标反应是自由基-自由基反应CH2SH+NO)。参与这个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”)衰减通过反应单元的红外光的量。在这项拟议的研究中，这种光源的“梳齿”被分散在一台红外敏感相机上，给出了气室中所含物质的高分辨率振动吸收光谱。该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)的时间尺度。这是一种用于研究化学反应动力学和动力学的独特技术，其中关于化学反应的大量细节包含在这一高分辨率、时间分辨率的光谱中。

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.