Elucidating Λ-doublet splittings and rotational quantum number-dependent collisional broadenings in 2∏1/2 and 2∏3/2 spin-split sub-bands of NO at 5.2 µm

Abstract

High-resolution ro-vibrational spectroscopic features of nitric oxide (NO) are particularly interesting because of the unpaired electron and non-zero nuclear spin of the nitrogen atom. Here, ultra-sensitive cavity ring-down spectroscopy (CRDS) coupled with an external-cavity quantum cascade laser (EC-QCL) radiation source was employed to measure the rotationally-resolved fine structure Λ-doublet splittings between the parity doublet e and f components in the (2∏1/2 - 2∏1/2) and (2∏3/2 - 2∏3/2) allowed sub-bands of the v = 1 ← 0 fundamental vibrational band of NO molecule near 5.2 µm. Subsequently, we determined several principal spectroscopic parameters such as the Λ-doubling constants, vibrational transition dipole moments and the Herman-Wallis coefficients for both e and f Λ-doublet components by probing various R-branch rotational lines (J = 0.5 to 23.5) of the studied spin-split sub-bands of NO associated with spin–orbit interaction. In addition, we performed the pressure broadening effect on the Λ-doublet splittings in collision with three vital perturbing gases at room temperature (296 K) and accurately determined the pressure broadening coefficients, γi (e,f) in cm-1atm−1 [(i = He, Ar and Zero air (mainly N2 + O2)] along with their dependence on the rotational quantum number (J). We observed the pronounced collision-induced rotational quantum effect and the result of rotationally inelastic collision in the system for each collision partner. All these measured high-resolution new spectroscopic parameters over 33 ro-vibrational transitions via an EC-QCL based CRDS method will help significantly in interpreting fundamental molecular properties of this diatomic NO molecule.