Understanding the Light–Matter Interaction in Attenuated Total Reflectance Experiments

Attenuated total reflectance (ATR) spectroscopy is often used in conjunction with infrared (IR) spectroscopy techniques, and the main purpose of ATR spectroscopy is to allow for direct analysis of samples without the need for sample preparation.¹ Recently, researcher Thomas Mayerhofer and scientific director Jürgen Popp of the Leibniz Institute of Photonic Technology explored the fundamentals of ATR spectroscopy, investigating the foundational assumptions and focusing on the traditionally central role of the evanescent field.^1,2

In their study, the researchers demonstrated that the evanescent field disappears when the rarer medium is absorbing using wave optics.¹ In these situations, attenuation of total reflection arises not from evanescent wave interaction, but from transmission into the absorbing medium, challenging long-standing interpretations of ATR mechanisms.¹

By analyzing electric field distributions and spectral responses in both semi-infinite media and finite layers, the researchers demonstrated that ATR spectra can still be obtained below the critical angle, even in the absence of an evanescent field. They also identify additional phenomena, such as tunneling and frustrated total reflection, particularly in systems involving a vacuum layer over a weakly absorbing medium.¹

In this video clip, Mayerhofer discusses the transition from transmission spectroscopy to ATR spectroscopy, highlighting the importance of sample thickness for signal detection. Mayerhofer explains that with ATR spectroscopy, the interaction at the sample–crystal interface is crucial, especially for absorbing samples. Unlike transmission spectroscopy, only the light reflected at the interface is detected, not the light absorbed by the sample. This new perspective emphasizes the significance of the interface in ATR spectroscopy.

This interview is the third part of a five-part interview with Mayerhofer. The first part of our conversation focused on defining what the evanescent field is and how it relates to ATR spectroscopy. The second part of our conversation with Mayerhofer focused on what led him to re-examine several spectroscopic methods, including ATR spectroscopy.

References

1. Mayerhofer, T.; Popp, J. Understanding the Role of the Evanescent Field in Attenuated Total Reflection (ATR) Spectroscopy. Appl. Spectrosc. 2026, 80 (2), 125–132. DOI: 10.1177/00037028251358400
2. Mayerhofer, T.; Popp, J. Complex-Valued Chemometrics for Composition Analysis. Spectroscopy 2025, 40 (6), 16–21. DOI: 10.56530/spectroscopy.wn4265d4