The rotational relaxation of ro-vibrationally excited H2(1,11) in collision with N2

Abstract

The collisional energy relaxation behavior of the H₂(v = 1, J = 11) molecule in an H₂-N₂ mixture was investigated at 300 K using a coherent anti-Stokes Raman spectroscopy technique, focusing on its near-resonant rotation-vibration dynamics. The pressure of the H₂-N₂ mixture is maintained at 500 Torr, while the molar ratios of N₂ molecules are, respectively, adjusted to 0.2, 0.4, 0.5, 0.6, and 0.7. The H₂(1,11) molecule is selectively excited through a stimulated Raman pumping technique, and the time-resolved CARS signal is observed from the J ≤ 11 rotational level of H₂ molecules following collisions with N₂. The results indicate that the rotational relaxation of H₂(1,11) molecule involves both the multi-quantum relaxation process triggered by H₂-H₂ collisions and the single-quantum relaxation process via H₂-N₂ collisions. The rate coefficient for self-relaxation of H₂-H₂ collisions is (1.35 ± 0.14) × 10^–14 cm³s⁻¹, whereas the relaxation rate coefficient for H₂-N₂ collisions is (2.77 ± 0.27) × 10^–14 cm³s⁻¹. As the molar ratio of N₂ increases, the continuous single-quantum relaxation of H₂(1,11) molecules with J = 11 → 9 → 7 gradually becomes more pronounced. The rotational temperature of H₂ with various N₂ molar ratios is determined by the relative populations of a rotational Boltzmann distribution. When the molar ratio of N₂ is small, the rotation temperature decreases at a faster rate primarily due to the rapid multi-quantum relaxation resulting from near-resonant collisions between H₂-H₂.