Knowledge Bank

Electronic quenching rate constants for collisions of OH $(A2\Sigma +,v\prime =0)$ with a variety of collision partners were measured for individual rotational levels using laser-induced fluorescence. Colliders studied to date are $H2O,O2,N2,H2,D2,CH4,D2O,N2O,SF6,CF4$ and $CCl4$. The large rotational energy spacing, along with the generally rapid electronic quenching of OH, permitted the extraction of these rotation-state-specific rate constants from the pressure dependence (0-100 mtorr) of the temporal evolution of the total fluorescence. There exists a significant decrease (up to 60% for $N\prime =7$ compared to $N\prime =0$) in electronic quenching with increasing rotational level for most collision partners, even though the absolute magnitude of the rate constant varies more than a factor of 30 over the set. $SF6$ and $CF4$ are inefficient electronic quenchers but efficient at rotational energy transfer, masking any rotational level effect. The substitution of $D2$ for $H2$ and $D2O$ for $H2O$, thereby changing the vibrational and rotational structure of the quencher, caused a negligible change in the quenching cross sections. Collisions of $OD(A2\Sigma +,v\prime =0)$ with $D2O$ show similar effects. This effect was first observed by McDermid and Laudenslager for $N2$ and $O2\cdot 1$ Its persistence throughout the numerous collision partners suggests that electronic energy transfer in this system, where long range forces are $important,2$ is sensitive to dynamical effects caused by the internal state of the OH which is quenched in the collision.