Nonadiabatic Dynamics in the UV Photodissociation of Ethyl Radical via the òA' (3s) Rydberg State

The photodissociation dynamics of jet-cooled ethyl radical (C₂H₅) via the A²A′(3s) state are studied in the wavelength region of 230–260 nm using the high-n Rydberg H-atom time-of-flight (TOF) technique. The H + C₂H₄ product channels are examined using the H-atom TOF spectra and photofragment translational spectroscopy. A prompt H + C₂H₄(X¹A_g) product channel is characterized by a repulsive translational energy release, anisotropic product angular distribution, and partially resolved vibrational state distribution of the C₂H₄(X¹A_g) product. This fast dissociation is initiated from the 3s Rydberg state and proceeds via a H-bridged configuration to a conical intersection and then directly to the H + C₂H₄(X¹A_g) products. A statistical-like H + C₂H₄(X¹A_g) product channel via unimolecular dissociation of the hot electronic ground-state ethyl (X²A′) after internal conversion via the same conical intersection from the 3s Rydberg state is also examined, showing a modest translational energy release and isotropic angular distribution. An adiabatic H + excited triplet C₂H₄(ã³B_1u) product channel (a minor channel) is identified by energy-dependent product angular distribution, showing a small translational energy release, anisotropic angular distribution, and significant internal excitation in the C₂H₄(ã³B_1u) product. The branching ratio between the non-statistical repulsive product channel and the statistical product channel varies significantly with the photolysis excitation energy, indicating intriguing nonadiabatic dynamics and product bifurcation via the conical interaction.