Collision-broadening of vibration-inversion-rotation ammonia spectral lines in Q(J)-branch at 6.2 µm by cavity ring-down spectroscopy

Abstract

In this experimental investigation, collision broadening effects on the Q(J)-branch vibration-inversion-rotation spectral lines of ammonia (NH3) within the fundamental ν4 asymmetric bending vibrational band in the 6.2 µm mid-IR region are reported. A continuous-wave external-cavity quantum cascade laser coupled with a high-sensitive cavity ring-down spectrometer was employed to selectively probe 10 transitions in high-resolution, including few inversion doublets of gaseous NH3. Pressure-broadening coefficients, γNH3-Xi (Xi = He, Ar, N2, O2, zero-air) in cm−1 atm−1, characterizing the collision interaction between NH3 and various external perturbing gases, including helium, argon, nitrogen, oxygen, and zero-air, were determined at room temperature (296 K). Mean collision times and optical collision diameters of each collision partner were explored to gain deeper insight into the perturber-induced collision dynamics. This investigation elucidates the intricate intermolecular interactions and collision phenomena induced by various foreign perturbers with NH3, with potential implications for future spectroscopic and atmospheric research of this polyatomic molecule.