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

Abstract

Using pulsed H-atom Lyman-a laser-induced fluorescence spectroscopy along with a photolytic calibration approach, absolute H-atom product quantum yields of Φ_H-b13d = (0.32± 0.04) and Φ_H-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, f_T-b13d = (0.22± 0.03) and f_T-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 σ _b-1,3-d-La = (3.5± 0.2)× 10^-17 cm² and σ _b-1,2-d-La = (4.4± 0.2)× 10^-17 cm² for the previously unknown Lyman-a (121.6 nm) radiation photoabsorption cross sections of buta-1,3- and buta-1,2-diene in the gas-phase were determined.