Journal
CHEMPHYSCHEM
Volume 21, Issue 8, Pages 707-711Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/cphc.202000018
Keywords
Absorbance; Beer's law; dispersion; index of refraction; vibrational spectroscopy
Funding
- Federal Ministry of Education and Research, Germany Funding Source: Medline
- German Science Foundation Funding Source: Medline
- Thüringer Ministerium für Wirtschaft Funding Source: Medline
- European Union's Horizon 2020 research and innovation programme [780240] Funding Source: Medline
- Thüringer Aufbaubank Funding Source: Medline
- Wissenschaft und Digitale Gesellschaft Funding Source: Medline
- Carl-Zeiss Foundation Funding Source: Medline
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Beer's empiric law states that absorbance is linearly proportional to the concentration. Based on electromagnetic theory, an approximately linear dependence can only be confirmed for comparably weak oscillators. For stronger oscillators the proportionality constant, the molar attenuation coefficient, is modulated by the inverse index of refraction, which is itself a function of concentration. For comparably weak oscillators, the index of refraction function depends, like absorbance, linearly on concentration. For stronger oscillators, this linearity is lost, except at wavenumbers considerably lower than the oscillator position. In these transparency regions, linearity between the change of the index of refraction and concentration is preserved to a high degree. This can be shown with help of the Kramers-Kronig relations which connect the integrated absorbance to the index of refraction change at lower wavenumbers than the corresponding band. This finding builds the foundation not only for refractive index sensing, but also for new interferometric approaches in IR spectroscopy, which allow measuring the complex index of refraction function.
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