The Key Mechanism in Attenuated Total Reflection Spectroscopy

A recent study published in the journal Applied Spectroscopy takes a look at the fundamental assumptions behind attenuated total reflection (ATR) spectroscopy, particularly the role of the evanescent field in ATR spectroscopy.¹ Traditionally considered essential for ATR measurements, the evanescent field is shown, through wave optics, to vanish when the rarer medium is absorbing. In such cases, attenuation of total reflection is instead caused by transmission into the absorbing medium, challenging long-standing interpretations.¹

The study, which was conducted by Thomas Mayerhofer and Jürgen Popp of the Leibniz Institute of Photonic Technology, demonstrates that ATR system behavior varies smoothly with both the angle of incidence and the imaginary component of the dielectric function, rather than exhibiting sharp transitions.^1,2 By comparing electric field distributions and spectra across semi-infinite media and finite-thickness layers, the researchers show that ATR spectra can still be recorded below the critical angle, even without an evanescent field.¹ They also highlighted the additional effects, including tunneling and frustrated total reflection, particularly when a vacuum layer overlies a weakly absorbing medium.¹

Based on the findings of the study, the critical angle in absorbing systems is redefined as the point where the real and imaginary components of the perpendicular wavevector are equal. Overall, the study concludes that deviations from total reflection are primarily driven by transmission effects, not evanescent waves.¹ Mayerhofer’s broader work focuses on advancing infrared (IR) spectroscopy through wave optics, dispersion theory, and complex-valued chemometrics.¹

In this interview segment, Mayerhofer discusses his extensive work in spectroscopy, particularly the Beer–Lambert law, which he discovered was an approximation rather than a fundamental law. This realization led him to re-evaluate spectroscopic methods, including ATR spectroscopy.

This interview is the second 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.

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