Fluorescence Property and its Magnetic Field Dependence Across the Photodissociation Threshold of CH3CHO and CD3CDO in a Supersonic Jet

Fluorescence characteristics of jet-cooled acetaldehyde at zero field is confirmed to change from the small molecule behavior to the statistical limit behavior in CH3CHO and to the intermediate case in CD3CDO, as the excitation energy increases across the dissociation threshold located below 320 nm. The excitation energy dependence both of the intensity and of the lifetime of the slow fluorescence shows that the dissociation rate becomes faster abruptly with increasing excess energy above the threshold, and the excess energy dependence above the threshold seems to be more drastic in CH3CHO than in CD3CDO. External magnetic field as well as the dissociation in the triplet state plays a role to increase the level density of the triplet state coupled to S1. On excitation below the dissociation threshold, the magnetic quenching of fluorescence becomes more efficient with increasing excess energy and the efficiency of the quenching is larger in CD3CDO than that in CH3CHO by a factor of about 2. On excitation above the threshold, however, the efficiency becomes lower with increasing excitation energy. Thus, the magnetic field effects on fluorescence also change drastically across the dissociation threshold.