Multiple quantum jumps in vibrational energy transfer of iodine in the B 3Πu+ state along a supersonic beam expansion

An argon ion laser (514.5 nm) excites the I₂ molecules to B ³Π${}_{0_u^{+}}$ (v′=43) state at a certain point in the supersonic He and H₂ jet, and the fluorescence spectrum is observed downstream. An extremely large vibrational energy relaxation of electronically excited iodine molecules in the expanding supersonic beam has been observed over the levels of Δv=−10–2. The vibrational population distribution along the jet flow was analyzed with the rate equation. The Δv dependence of rate constant in a single collision, k_v→v+Δv, is assumed to be a simple function as k_1→0 α${}^{\text{|Δv|−1}}$ v(v−1) . . . (v +Δv+1), for Δv<0 and k_1→0 α${}^{\text{|Δv|−1}}$ v(v+1) . . . (v +Δv−1) exp (−ΔE/k_BT_eff), for Δv>0, where ΔE is the energy gain by vibrational activation, T_eff is the effective temperature and α is the fitting parameter for multiple quantum jumps, and K_1→0, α and T_eff have been determined to be 1.70±0.05×10⁻¹²(cm³/mol s), 0.0065 and 19 K, 3.2±0.1×10⁻¹²(cm³/mol s), 0.009 and 30 K for He and H₂, respectively. It was found that vibrational multiple quantum jumps in a single collision contributed to the relaxation process and the relaxation rate constant seemed to be almost constant along the expanding flow.