vaporabsorption

Vapor absorption refers to the attenuation of electromagnetic radiation as it passes through a vapor composed of gaseous molecules capable of absorbing photons at specific energies. The effect is wavelength-dependent and most pronounced when the radiation coincides with allowed electronic, vibrational, or rotational transitions of the gas-phase molecules. In the infrared, vibrational-rotational transitions produce broad absorption bands; in the ultraviolet and visible, electronic transitions create sharper absorption features.

Quantitative description commonly uses the Beer-Lambert law: A = εlc, where A is absorbance, ε is the molar absorptivity, l is the optical path length, and c is the concentration. In gases, concentration can be related to partial pressure via the ideal gas law, so absorbance scales with number density and the absorption cross-section. Spectral lines are shaped by temperature (Doppler broadening) and pressure (collisional or pressure broadening), and some vapors exhibit continuum absorption at higher pressures or specific wavelength ranges.

Techniques in vapor absorption spectroscopy include ultraviolet-visible (UV-Vis) spectroscopy for gas analysis, infrared spectroscopy and Fourier-transform infrared (FTIR) spectroscopy, as well as specialized methods such as cavity ring-down spectroscopy and tunable diode laser absorption spectroscopy (TDLAS). Data interpretation frequently relies on line-by-line models and spectral databases such as HITRAN or GEISA to provide line positions and strengths.

Applications span atmospheric science (monitoring greenhouse gases, water vapor, ozone), industrial process monitoring, combustion diagnostics, and medical or safety-related gas analysis. Limitations can include spectral congestion from overlapping lines, baseline variations, temperature dependence, and the need for calibration to achieve quantitative results.