Rotational energy transfer in the Na₂b³Π _u state: Propensity rules for transitions between hyperfine components

The changes in the hyperfine quantum number (F′) that accompany collision-induced ΔJ, ΔΩ, and Δ(e/f-parity) transitions in the Na₂ b ³Π_u,-state have been studied by sub-Doppler, cw, perturbation-facilitated optical-optical double resonance (PFOODR) spectroscopy. The Na₂ is contained in a heat pipe oven at ∼1 Torr and the primary collision partner is Na (3s ²S). The PUMP laser selectively excites a single b ³Π_0u v′=12, J′=43e, s or 44e, a rotational level, the parent level. All F′ hyperfine components of the parent J′ level are directly populated by the PUMP laser, but with different velocity projections relative to the laser propagation direction. Thus each parent hf component is labeled by its longitudinal velocity. As the PROBE laser is scanned through various 2 ³Π_Ωg v=2, J←b ³Π_Ω′u v′= 12, J′ transitions, sub-Doppler hyperfine structure can be resolved on each parent and daughter rotational line in the PFOODR fluorescence excitation spectrum. The collisional propensity rule ΔF=ΔJ is obeyed for (s, a permutation symmetry conserving) ΔJ′=0, ΔΩ′=0, +1, and +2 and ΔJ′ = ±1, ±2, ΔΩ′=0 collision-induced transitions. No systematic exploration of the parent-J′, Ω′ dependence of the ΔF propensity was undertaken; in particular, the present study was restricted to the high-J limit where the Na₂/Na collisions are not sudden relative to the rotational (half) period and where J≫I. The ΔF=ΔJ hyperfine propensity rule observed for high-J levels of the Na₂ b ³Π_u state is consistent with previous theoretical predictions of a ΔF=0 propensity for collision-induced ΔJ=0 transitions between Λ-doublet components of the OH X ²Π state and a ΔF=ΔJ propensity for collision-induced transitions between CaBr X ²Σ ⁺ rotational levels.