Knowledge Bank

The formyl radical, HCO, is an important combustion intermediate that has been studied by a variety of spectroscopic techniques. Despite this, the ground-state potential energy surface and vibrational structure near the dissociation barrier are not fully understood. We use stimulated emission pumping (SEP), via either two-color resonant four-wave mixing (RFWM) or fluorescence depletion, to observe a wide range of vibrational levels in the $X~2A\prime$ state of HCO and DCO. The combination of the two techniques provides determinations of energies and widths of vibrational states above and below the threshold for dissociation into H+CO. Spectra of $X~2A\prime$ state C-O stretch, bend and combination levels obtained by pumping the $B~2A\prime -X~2A\prime$ electronic origin confirm the applicability of RFWM-SEP and yield new results. Spectra involving the C-H stretch in the $X~2A\prime$ state, obtained by pumping one quanta of C-H stretch in the $B~2A\prime$ state, provide new experimental data showing the enhancement in dissociation rate upon C-H excitation. The C-H stretch and bending levels show interesting mode specific dissociation rates that are compared to recent theoretical calculations. In contrast to HCO, only a small spectroscopic data set exisits for DCO. Results for DCO are among the first experimental data to probe its ground-state vibrational structure. Both sets of results will be helpful in a more rigorous determination of the potential energy surface for the $X~2A\prime$ state.