Investigation of the collisional energy transfer between excited-state NO X2Π1/2 (v = 3, 4) and CO via CARS spectroscopy

Abstract

Excited-state NO was pumped to the vibrational excited states NO X²Π_1/2 (v = 3, J = 7.5, f)(E = 5544 cm⁻¹) and X²Π_1/2 (v = 4, J = 7.5, f)(E = 7335 cm⁻¹) using stimulated Raman pumping (SRP) to investigate the energy transfer process during collisions between excited-state NO and CO. Coherent anti-Stokes Raman scattering (CARS) confirmed NO excitation to the X²Π_1/2 state (v = 3, 4, J = 7.5, f). Time-resolved CARS analysis of NO vibrational levels before and after collision revealed that the relaxation process is dominated by single-quantum relaxation with Δv = 1, accompanied by two-quantum relaxation with Δv = 2. Analysis of the post-collision CO molecule's scanned CARS spectrum revealed a number distribution ratio ξ = 1.71 between the v = 1 and v = 2 energy levels. This indicates that approximately 37% of CO molecules occupy the v = 2 level after collision, while 63% reside at the v = 1 level. Further analysis indicates that the lower excitation energy (v = 3) not only enables post-collision CO molecules to occupy more rotational levels but also yields stronger signal intensities across all levels, with an energy transfer efficiency 1.59 times that of the higher excitation energy (v = 4). At room temperature, the collision depopulation rate of NO molecules was obtained from semi-logarithmic plots of time-resolved CARS spectra. Combined with the Stern–Volmer equation, the collision transfer rate coefficient k_v between excited-state NO and CO under different excitation energies was determined: k_v=3(CO) = (6.77 ± 0.36) × 10⁻¹⁴ cm³ s⁻¹, k_v=3(NO) = (2.47 ± 0.13) × 10⁻¹⁴ cm³ s⁻¹; k_v=4(CO) = (4.49 ± 0.84) × 10⁻¹⁴ cm³ s⁻¹, k_v=4(NO) = (1.85 ± 0.45) × 10⁻¹⁴ cm³ s⁻¹, revealing that the collision relaxation rate coefficient decreases with increasing excitation energy. By controlling the sample cell temperature, it was observed that the collision relaxation rate coefficient significantly increases with rising temperature.