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 ${\nu }_4$ 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, ${\gamma }_{N{H}_3-X_i}$ ($X_i$ = He, Ar, N_2, O_2, 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.