Beyond the Evanescent Field: A New Perspective on ATR Spectroscopy

In fifteen words or less, explain the significance of this contribution (Novel Aspect).: Wave optics shows that no evanescent field exists in ATR spectroscopy when the sample absorbs.

Abstract Text:

When Harrick and Hansen established the foundations of ATR spectroscopy, they rightly based them on wave optics, specifically Fresnel’s equations.^{1, 2} These are complex, and unlike transmission absorbance, the negative decadic logarithm of reflectance (“ATR absorbance”) does not show an obvious linear dependence on thickness or concentration. Still, a linear approximation can be obtained by expanding the ATR absorbance, though its validity depends on the IRE and angle of incidence and is often not well justified.

This approximation allows the introduction of an “effective thickness,” which, to first order, is independent of the absorption index and includes a term commonly linked to the evanescent field’s penetration depth. This has contributed to the emphasis on the evanescent field in ATR spectroscopy.

However, wave optics predicts that the evanescent field vanishes if the less dense medium is absorbing; attenuation then arises from transmission into this medium. This is illustrated by comparing electric field maps for systems with a gap between IRE and sample above and below the critical angle. In cases like weakly absorbing media beneath a vacuum gap, tunneling and frustrated total reflection occur, but no evanescent field exists.³

 

 

REFERENCES

1. N.J. Harrick, F.K. du Pré. “Effective Thickness of Bulk Materials and of Thin Films for Internal Reflection Spectroscopy”. Appl. Opt. 1966. 5(11): 1739-1743. doi:10.1364/AO.5.001739.

2. W.N. Hansen. “Expanded formulas for attenuated total reflection and the derivation of absorption rules for single and multiple ATR spectrometer cells”. Spectrochim. Acta. 1965. 21(4): 815-833. doi:http://dx.doi.org/10.1016/0371-1951(65)80039-X.

3. T.G. Mayerhöfer, Popp, J. “Understanding the role of the evanescent field in ATR Spectroscopy”. submitted. 2025.