Fate of isolated CH(B2Σ−,v=0,J) states in inelastic collisions with CO

The effects of energy-level resonance on collision-induced electronic energy transfer: CD (A 2Δ ↔ B 2Σ−) coupling

Pulsed, time- and wavelength-resolved laser-induced fluorescence spectroscopy has been used to measure rate constants for collision-induced electronic energy transfer (EET) between the A (2)Delta and B (2)Sigma(-) states of the CD radical. EET rate constants in the exothermic direction from B (2)Sigma(-), v = 0 to the unresolved A (2)Delta, v = 0 and 1 levels span the range 0.1-2.4 x 10(-11) cm(3) s(-1) at room temperature (ca. 295 K) for the partners He, Ar, N(2), CO and CO(2). H(2) was also investigated, but was unsuitable for further study because of its rapid isotope exchange with CD(X (2)Pi). As expected, only CO results in a significant rate of removal on any distinct, unobserved channel, presumed to be chemical reaction. The efficient A (2)Delta, v = 1 --> 0 vibrational relaxation by CO(2) observed previously for CH was not found for CD. Despite the significant differences in their detailed rovibronic level structures, the overall efficiency of EET in CD was found to be very similar to that for CH. The positive correlation in a Parmenter-Seaver plot appears to confirm a role for long-range attractive forces in the EET process. However, the detailed deviations from this overall trend found reproducibly for CD and CH suggests that partner-specific interactions are also important.

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 A(2)Delta and B(2)Sigma(-) states with the series of partners He, Ar, H(2), N(2), CO, and CO(2). Single rovibronic states of either of the near-degenerate levels A(2)Delta, v = 1, or B(2)Sigma(-), 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 A(2)Delta and B(2)Sigma(-) 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 H(2) are efficient at total removal of CH A(2)Delta and B(2)Sigma(-), most probably through chemical reaction. CO(2) is notably effective at A(2)Delta state vibrational relaxation, possibly through resonant vibrational energy transfer. All the partners cause transfer between CH A(2)Delta and B(2)Sigma(-). 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 A(2)Delta, v = 0 and 1 from B(2)Sigma(-), 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 B(2)Sigma(-) to A(2)Delta 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.