Collision-Partner Dependence of Energy Transfer between the CH A²Δ and B²Σ^- States

We have investigated experimentally the collision-induced electronic energy transfer between the CH A²Δ and B²Σ^- states with the series of partners He, Ar, H₂, N₂, CO, and CO₂. Single rovibronic states of either of the near-degenerate levels A²Δ, v = 1, or B²Σ^-, v = 0, were prepared by laser excitation. Collisional transfer processes were monitored by detecting dispersed, time-resolved fluorescence from the initial and product states. The microscopic rate constants for vibronically resolved transfer between the A²Δ and B²Σ^- states, vibrational relaxation within the A state, and total removal to unobserved final products were determined for each partner. In line with previous work, we find that only CO and H₂ are efficient at total removal of CH A²Δ and B²Σ^-, most probably through chemical reaction. CO₂ is notably effective at A²Δ state vibrational relaxation, possibly through resonant vibrational energy transfer. All the partners cause transfer between CH A²Δ and B²Σ^-. An important new observation is that their efficiencies are well correlated with the strength of long-range attractive forces, as revealed through a positive correlation of the Parmenter−Seaver type. The vibronic branching to A²Δ, v = 0 and 1 from B²Σ^-, v = 0 is found to be significantly collision-partner-dependent and not well predicted by energy gap scaling laws. We do not find any enhanced effectiveness in B²Σ^- to A²Δ coupling for those partners which form strongly bound intermediates, suggesting that this specific electronic channel is controlled by different regions of the potential energy surfaces.