Relaxation behavior of vibrationally excited N2(X1Σg + 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 population evolution 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 × 10−14 cm3s−1 and 1.04 × 10−14 cm3s−1 at 300 K, and 2.57 × 10−14 cm3s−1 and 0.54 × 10−14 cm3s−1 at 320 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 distribution of H2 was observed at molar ratios of 0.3, 0.4, 0.5, 0.6 and 0.8, respectively. The ro-vibrational population distribution of H2 after collision with N2 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.