Collision-partner dependence of energy transfer between the CH a ²Δ and B²∑^- states

We have investigated experimentally the collision-induced electronic energy transfer between the CH A²? and B²? ^- states with the series of partners He, Ar, H₂, N ₂, CO, and CO₂. Single rovibronic states of either of the near-degenerate levels A²?, v = 1, or B²? ^-, 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 A²? and B ²?^- 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 A²? and B ²?^-, most probably through chemical reaction. CO ₂ is notably effective at A²? state vibrational relaxation, possibly through resonant vibrational energy transfer. All the partners cause transfer between CH A²? and B ²?^-. 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 A²?, v = 0 and 1 from B ²?^-, 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 B²?^- to A²? 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.