Observing ATR Spectra Below the Critical Angle

Our conversation with Thomas Mayerhofer has explored the fundamental concept of the evanescent field and how it impacts attenuated total reflectance (ATR) spectroscopy. Mayerhöfer earned his chemistry diploma from the University of Regensburg in 1996 and a PhD in physical chemistry from Friedrich Schiller University Jena in 1999, completing his habilitation in 2006.¹ Since 2007, he has worked at the Leibniz Institute of Photonic Technology, advancing infrared (IR) spectroscopy through wave optics and dispersion theory.¹ His recent work focuses on ATR theory and complex-valued chemometrics. He has authored over 120 peer-reviewed papers and a book on wave optics in spectroscopy.¹

The paper published in Applied Spectroscopy that formed the basis of this interview reexamined the role of the evanescent field in attenuated total reflection (ATR) spectroscopy, showing that it vanishes when the rarer medium is absorbing.² The findings redefine the critical angle under absorption and conclude that evanescent waves play little to no significant role.²

In this interview clip, Mayerhofer discusses the importance of considering the structure of a sample in wave optics, particularly in the context of an optical model. He explains that for a homogeneous sample, only the transmitted light is significant as there are no more interfaces to reflect light. However, for a thin layer with a thickness similar to the penetration depth, light can still detect changes, and some light can return to the detector. Mayerhofer also discussed that even if total reflection is no longer present below the critical angle, total reflection can still occur at the interface between air and the sample, which complicates the analysis.

This interview is the fourth 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. The third part of our conversation examined the transition from transmission spectroscopy to ATR spectroscopy, highlighting the importance of sample thickness for signal detection.

References

1. Mayerhofer, T.; Popp, J. Complex-Valued Chemometrics for Composition Analysis. Spectroscopy 2025, 40 (6), 16–21. DOI: 10.56530/spectroscopy.wn4265d4
2. 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