Extracting pure absorbance spectra in infrared microspectroscopy by modeling absorption bands as Fano resonances.

Alex J Schofield,Carol J Hirschmugl,Reinhold Blümel,Rozalia Lukacs,Achim Kohler

doi:10.1063/1.5085207

Alex J Schofield, Carol J Hirschmugl + Show 3 more

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https://doi.org/10.1063/1.5085207

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Abstract

Midinfrared absorbance spectra obtained from spatially inhomogeneous and finite samples often contain scattering effects characterized by derivative-like bands with shifted peak positions. Such features may be interpreted and accurately modeled by Fano theory when the imaginary part of the complex dielectric function is small and Lorentzian in nature-as is the case for many biological media. Furthermore, by fitting Fano line shapes to isolated absorbance bands, recovery of the peak position and pure absorption strength can be obtained with high accuracy. Additionally, for small and optically soft spherical scatterers, recovery of one or the other of constant refractive index or radius (given approximate knowledge of the other) is possible.

Highlights

Distorted spectral peaks are often encountered in FTIR absorbance spectra— noticeable in the Amide-I band, where a derivative-like line shape may be observed.1–3 Previously, it has been shown that these derivative features occur due to a nonlinear mixing between the absorbance and dispersive spectra on top of a smooth-varying broadband baseline.2–5 In such cases, identification of chemical bands in a scattering sample may be deceptive as the distorted bands frequently suffer from a shift in the peak position.2,3,5 Identification and correction of such distortions are critical for FTIR spectroscopy if one seeks a quantitative understanding of the sample under investigation.A
It is reasonable that the derivative line shape that arises from electromagnetic interference can be understood as interference between a broadband continuum of states and a discrete absorption band as it occurs in Fano theory
In the case of Mie scattering, the broadband continuum of states can be attributed to the dielectric interactions that produce the constant refractive index

Summary

Introduction

Distorted spectral peaks are often encountered in FTIR absorbance spectra— noticeable in the Amide-I band, where a derivative-like line shape may be observed.. It has been shown that these derivative features occur due to a nonlinear mixing between the absorbance and dispersive spectra on top of a smooth-varying broadband baseline.. It has been shown that these derivative features occur due to a nonlinear mixing between the absorbance and dispersive spectra on top of a smooth-varying broadband baseline.2–5 In such cases, identification of chemical bands in a scattering sample may be deceptive as the distorted bands frequently suffer from a shift in the peak position.. Identification and correction of such distortions are critical for FTIR spectroscopy if one seeks a quantitative understanding of the sample under investigation

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