Abstract
Acetaldehyde supersonically cooled was excited to the S 1 state with a UV laser in the wavelength range of 300-319 nm. Fragment HCO was detected by laser-induced fluorescence at the transition of ̄B - ̄X O 0 0 . The appearance rate of HCO was detected by probing the Q R 0 (10) transition. The formation threshold of HCO is determined to be 317.5 nm from the onset of the measured appearance rate. No vibrational promoting mode was found to produce HCO. Emission decay curves of excited acetaldehyde were recorded in the same wavelength region. The time constant of the slow component of acetaldehyde emission starts to decrease at an excitation wavelength 317.5 nm near the formation threshold of HCO. The decay rate of this slow component is approximately equal to the appearance rate of HCO at the same wavelength. Strong interaction between the S 1 and T 1 states in acetaldehyde is implied by these experimental results.