Revealing the Mechanisms of Diffuse Reflectance IR

Diffuse reflectance is a well-known sampling technique in mid-infrared (mid-IR) and near-IR spectroscopy. Despite its significance, however, the underlying mechanism of the technique is not well understood-particularly in mid-IR diffuse reflectance. Eric B. Brauns, an Associate Professor at the University of Idaho, has developed an instrument capable of studying the mechanism, using time-resolved measurements. Brauns won the 2015 Applied Spectroscopy William F. Meggers Award for this work (1). He recently spoke with Spectroscopy about his award-winning paper and what it means for the field.

In your award-winning paper (1), you described an instrument capable of studying diffuse reflection of mid-infrared (mid-IR) photons from powdered KBr samples with varying amounts of carbon black on ultrafast time scales. How does this instrumental setup enable you to perform time-resolved measurements?

Brauns: My instrument uses a method called “upconversion” to achieve ultrafast time resolution. This is an optical gating technique where a pulsed laser beam is divided into two parts. One part is used to irradiate the sample to generate the diffusely reflected light while the other-called the “gate”-is passed through a variable optical delay. Both the diffusely reflected light and the gate pulse are overlapped in a nonlinear crystal. When both are simultaneously present in the crystal, frequency mixing occurs and the sum frequency is produced. By scanning the optical delay of the gate, I can map the intensity profile of the signal. Since the delay can be varied in sub-femtosecond increments, the time resolution is determined by the width of the gate pulse-100 fs in my setup. An additional benefit of this approach is that the sum frequency propagates in a different direction than either the gate or the signal. This makes the technique very sensitive because the light that is detected is effectively background-free.

What are the advantages of this system compared to traditional mid-IR instruments?

Brauns: To be honest, there really isn’t any comparison. As far as I know, nobody has tried to measure time-resolved IR diffuse reflection-especially on ultrafast time-scales. When I first began working on this, I was actually surprised by how little we know about IR diffuse reflection in general.

Your research states that diffusely reflected mid-IR photons fall into two distinct categories, with some photons traveling long pathlengths and the majority traversing a much shorter distance. Was this a surprising finding? How did this discovery impact your research and the mid-IR field overall?

Brauns: At the time, I can’t say that I was surprised because I really had no expectations. Diffuse reflection was a totally new research area for me. While I was confident that I would see something interesting, I didn’t have any preconceived notions as to what that might be. However, once I did the experiments and began trying to make sense of the data, I was surprised by just how interesting the results were. The fact that there are two distinct pathlengths rather than a continuous distribution is quite unexpected. What’s even more surprising is that the fraction of photons that travels short pathlengths is so much greater than the fraction that travels longer pathlengths.

While there is a great deal yet to learn about the underlying mechanism of IR diffuse reflection, my results still have immediate practical value. Diffuse reflection spectrometry has been used for decades to obtain spectra of solids and turbid media. What my results show is that the overwhelming majority of the diffusely reflected light that reaches the detector has only penetrated the first few hundred microns of the sample. Researchers who use diffuse reflection will benefit tremendously from this knowledge.

What is your group currently working on?

Brauns: I have a number of rods in the fire right now. One is that I’m trying to better understand the diffuse reflection results. The approach that I took in the paper to analyze the data was largely phenomenological. It provided a way to quantify the data and describe what was happening, but it didn’t tell me anything about why. As a physical chemist, this is what I really want to know. I’m also continuing my work on RNA folding kinetics using time-resolved IR spectroscopy. This has been the main thrust of my research since my days as a postdoc. In addition to research, I recently began writing a physical chemistry textbook that will be published in 2016.

What are the next steps in your research?

Brauns: In terms of diffuse reflection, there are two experiments that are next in line. One is to look at the angular dependence. In other words, I’d like to do a study where I vary the angle of the incident laser pulse as well as the angle at which the diffusely reflected light is collected. I’d be interested to see what-if any-affect this has on the results. The other is to look at the wavelength dependence, which is particularly relevant in the context of diffuse reflection spectrometry. However, this will require that I modify the instrument considerably. My current setup uses broadband femtosecond pulses, which prevents me from obtaining wavelength-resolved data. To look at the wavelength dependence, I’ll have to use narrower bandwidth picosecond laser pulses. Although this will sacrifice some of the time resolution, this is unavoidable if I want to obtain a complete picture of IR diffuse reflection.

Reference

(1) E.B. Brauns, Appl. Spectrosc. 68(1), DOI: 10.1366/13-07258 (2014).