Knowledge Bank

The electronic excitation spectrum of the OH-CO reactant complex in the OH A-X (1,0) spectral region has been investigated using a fluorescence depletion technique. This technique combines infrared overtone excitation of OH-CO at $1.4\mu m$ with ultraviolet excitation of OH-CO between 275 and 300 nm, which induces a fluorescence signal. When the IR and UV transitions originate from a common ground state, the IR excitation reduces the ground state population and causes a depletion in the UV laser induced fluorescence signal. Scanning only the IR laser yields a rotationally resolved infrared spectrum of the pure OH overtone band of OH-CO at $6941.8cm-1$, while tuning only the UV laser results in a broad and relatively unstructured electronic spectrum for OH-CO spanning from 33500 to $36000cm-1$. The electronic spectrum peaks around $34600cm-1$ and exhibits a secondary shoulder near $35300cm-1$. The breadth and position of the electronic spectrum reflect the Franck-Condon window on the $OHA2\Sigma +(v\prime =1)+CO$ excited state potential. The lack of observable structure in the electronic spectrum is attributed to extensive homogeneous line broadening, most likely arising from rapid electronic quenching of OH $A2\Sigma +$ by the CO partner. The electronic excitation spectrum observed for the OH-CO will also be compared with analogous spectra obtained for complexes of OH with other partners.