Carbon dioxide absorption spectrum
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Carbon Dioxide Absorption Spectrum: Key Wavelengths and Bands
The absorption spectrum of carbon dioxide (CO₂) covers a wide range of wavelengths, from the far-infrared through the visible and into the vacuum ultraviolet. In the infrared region, CO₂ exhibits strong absorption bands due to vibrational-rotational transitions, which are important for atmospheric and planetary studies. Notably, the 4.3-micron (2395–2216 cm⁻¹) region contains about twenty identified transitions, including bands from isotopologues like ¹²C¹⁶O¹⁷O, and about 90% of the observed lines in this region have been interpreted and assigned to specific molecular transitions .
In the far-infrared (20–110 cm⁻¹), CO₂ shows absorption bands attributed to collision-induced dipole moments, with a prominent band centered at 50 cm⁻¹, especially under high pressure . The wings of CO₂ absorption bands in the 790–910 cm⁻¹ range are influenced by self-broadening and broadening by nitrogen, with temperature-dependent absorption coefficients that align well with experimental data .
Temperature Effects on CO₂ Absorption Spectrum
Temperature has a significant impact on the CO₂ absorption spectrum. As temperature increases, the absorption spectrum in the vacuum ultraviolet (230–355 nm) shifts to longer wavelengths (red shift), extending absorption above 300 nm at temperatures as high as 2273 K. This shift is due to the relative positions and shapes of the excited and ground state potential curves, making CO₂ absorb more in the blue end of the solar spectrum at high temperatures .
Comprehensive studies of CO₂ absorption and emission spectra across 1–25 microns and temperatures from 220 to 2500 K have provided detailed data on line intensities, half-widths, and spectral transmission functions. These findings are crucial for modeling radiative heat exchange in planetary atmospheres and high-temperature environments Moskalenko2017Moskalenko2017.
Ultraviolet and Vacuum Ultraviolet Absorption Features
In the ultraviolet and vacuum ultraviolet regions, CO₂ displays strong resonance bands between 1306 Å and 700 Å, with a particularly large absorption cross section at 923 Å. A continuum with superimposed bands starts at about 860 Å, corresponding to the first ionization potential, and becomes smoother with a broad maximum at 550 Å .
Quantum mechanical calculations have shown that in the 120–160 nm range, the CO₂ absorption spectrum consists of low and high energy bands, with vibronic interactions (such as Renner-Teller coupling and conical intersections) shaping the band structure. The high energy band is associated with pseudorotational motion, while irregular peaks in the low energy band are linked to specific molecular configurations .
For pressurized and supercritical CO₂, the lowest absorption band transition energy remains stable with increasing density, but the vibrational structure becomes less pronounced. This is likely due to perturbations in electronic state potential energy surfaces caused by close molecular proximity, rather than just collisional broadening .
Solid and Condensed Phase CO₂ Absorption
In the condensed phase, solid CO₂ exhibits second-order infrared absorption features involving combinations of intermolecular vibrations and Fermi doublet components. Theoretical models, such as the rigid ion model and two-phonon infrared absorption theory, provide reasonable agreement with observed spectra, helping to interpret the complex features seen at low temperatures .
Conclusion
The absorption spectrum of carbon dioxide is complex and highly dependent on wavelength, temperature, pressure, and phase. Key absorption bands in the infrared, ultraviolet, and vacuum ultraviolet regions are well-characterized, with temperature and density playing significant roles in shifting and shaping the spectrum. These findings are essential for applications in atmospheric science, planetary studies, and industrial monitoring.
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