OBSERVATION OF $\tilde{B}$-$\tilde{X}$ FLUORESCENCE EXCITATION SPECTRUM OF FORMYL RADICAL

$^{1}$ R. Vasudev and R. N. Zare, J. Chem. Phys. 53, 5267 (1982).Author Institution: Department of Chemistry and George R. Harrison Spectroscopy Laboratory, Massachusetts Institute of TechnologySeveral vibrational bands belonging to the $\bar{B}-\bar{X}$ electronic system) of formyl radical (HCO) have been observed in fluorescence excitation in the $38,750-38,340 cm^{-1}$ region. The formyl radical was formed by photodissociation of acetaldehyde at 308nm. Surprisingly, the lifetime for the lowest few $\bar{B}$-state vibrational levels is 10-50ns. In contrast, the lifetimes of the first few $\bar{A}$-state vibrational levels are approximately $30ps.^{1}$ We have recorded high resolution spectra (FWHM $0.1 cm^{-1}$) of the origin band and assigned the rotational transitions as belonging to a type-A and type-B hybrid band. The proof that these sub-bands are from HCO comes from ground state combination differences and term value plots. Analysis of the high resolution spectra has also allowed us to determine the upper state rotational constants $(A'=15.8 \pm 0.5 cm-1, B'=1.193 \pm 0.02 cm^{-1}, C= 1.106 \pm 002 cm^{-1})$ and band origin $(38,695.48 cm^{-1})$ for the $\bar{B} - \bar{X} O^{0}_{0}$ band. We are currently recording SEP spectra which will verify our rotational assignments, as well as fluorescence excitation spectra of DCO, to more accurately determine the geometry of the B-state