Ultraviolet photochemistry of buta-1,3- and buta-1,2-dienes: Laser spectroscopic absolute hydrogen atom quantum yield and translational energy distribution measurements

Using pulsed H-atom Lyman-α laser-induced fluorescence spectroscopy along with a photolytic calibration approach, absolute H-atom product quantum yields of φH-b13d=(0.32±0.04) and φH-b12d=(0.36±0.04) were measured under collision-free conditions for the 193 nm gas-phase laser flash photolysis of buta-1,3- and buta-1,2-diene at room temperature, which demonstrate that nascent H-atom formation is of comparable importance for both parent molecules. Comparison of the available energy fraction, fT-b13d=(0.22±0.03) and fT-b12d=(0.13±0.01), released as H+C4H5 product translational energy with results of impulsive and statistical energy partitioning modeling calculations indicates that for both, buta-1,3- and buta-1,2-diene, H-atom formation is preceded by internal conversion to the respective electronic ground state (S0) potential energy surfaces. In addition, values of σb-1,3-d-Lα=(3.5±0.2)x10-17 cm2 and σb-1,2-d-Lα=(4.4±0.2)x10-17 cm2 for the previously unknown Lyman-α (121.6 nm) radiation photoabsorption cross sections of buta-1,3- and buta-1,2-diene in the gas-phase were determined.