Relaxation behavior of vibrationally excited N₂(X¹Σ_g ⁺ v″ = 6) collisions with H₂

Abstract

Relaxation behavior of vibrationally excited N2 (X1Σg+ v′′ = 6) induced by collisions with H2 has been investigated using coherent anti-Stokes Raman spectroscopy (CARS). The total pressure of the N2-H2 mixture was 500 Torr, and the molar ratios of H2 were 0.3, 0.4, 0.5, 0.6 and 0.8, respectively. The v′′ = 6 vibrational state of the electronic ground-state manifold X1Σg+ of N2 was selectively excited by overtone pumping, and the evolution of the population was monitored using CARS spectroscopy. The collisional deactivation rate coefficients of the excited state N2 (v′′ = 6) with H2 and N2 are approximately 2.59×10-14cm3s-1 and 1.04×10-14 cm3s-1 at300 K, and 2.57 ×10-14 cm3s-1 and 0.54 ×10-14 cm3s-1 at320 K, respectively. The relaxation rate coefficient of the N2-H2 collision was approximately 2.5 and 5 times that of the self-relaxation rate coefficient. The experimental results show that the population densities of the (1,2), (2,2), (3,5), and (3,6) levels of H2 have a maximum at 320 K, while the population densities of (2,3) and (2,4) show little change with increasing temperature. Simultaneously, the time-resolved CARS profiles of the vibrational levels v = 6,5,4 by preparing v = 6 of N2 also indicated that a near-resonant multi-quantum relaxation process occurred between N2-H2. The collision-induced population transfer of H2 was observed at molar ratios of 0.3, 0.4, 0.5, 0.6 and 0.8, respectively. The vibrational population distribution of H2 molecules after collision with N2 molecules is given by the CARS signal intensity ratio, and the population of hydrogen molecules at v = 2,3 vibrational states also provides strong experimental evidence for energy near-resonance collisions between N2-H2.