Infrared Spectroradiometry of Lithium Benzoate from 21 to 235 THz

TL;DR

This study addresses how thermal radiation from lithium benzoate encodes temperature-dependent vibrational populations by measuring infrared emission across 21–235 THz as the sample is heated from 313 to 553 K, using the Boltzmann population relation $\frac{N_{\text{ex}}}{N_{\text{gr}}} = \exp\left[-\frac{E_{\text{ex}}-E_{\text{gr}}}{k_{\text{B}}T}\right]$ to motivate the analysis. It employs infrared spectroradiometry with a JASCO FT-IR system and a liquid-nitrogen-cooled MCT detector, implementing a relative-emission method to isolate sample emission from background. A key finding is that near-IR emission features sharpen with temperature and overall emission exhibits more peaks than the corresponding absorption spectrum, consistent with a cascade-like thermal excitation and spontaneous emission mechanism. The work demonstrates that emission spectra reveal temperature-dependent vibrational dynamics not captured by absorption alone, suggesting a framework for deeper peak attribution and mechanism validation in molecular systems.

Abstract

This paper presents an extensive survey of the thermal radiation properties of lithium benzoate. We heated the sample from 313 to 553 K, just below the melting point, while performing an infrared spectroradiometry with an FT-IR spectrometer from 21 to 235 THz (700-7800 cm$^{-1}$). We have provided a detailed analysis of the infrared spectrum data and a comparison of the absorption spectrum of the same sample. It turned out that the recorded spectra are not only different from ordinary absorption spectra but also carry substantial information about the temperature dependence of the population of vibrationally excited states. We conclude by proposing a hypothesis on the thermal excitation mechanism of vibrational energy levels of molecules consistent with the distinct characteristics of the obtained infrared emission spectra.

Figures (4)

• Figure 1: A schematic drawing of the infrared emission measurement system.
• Figure 2: (a) Temperature dependence of the relative IR emission spectra of lithium benzoate powder. (b) An enlargement of (a) into the region 24-36 THz (800-1190 cm$^{-1}$). (c) The degree of shifts of all numbered peaks in (b) relative to those at the lowest temperature.
• Figure 3: The amount of shift of the ninth peak in Fig.\ref{['fig:relative_emission']}-(b) relative to that at the lowest temperature.
• Figure 4: Emission and absorption spectrum of lithium benzoate. The black line corresponds to the emission spectrum at 313 K, while the red line represents the absorption spectrum at 300 K.