Ultraviolet photolysis of adenine: Dissociation via the π1σ* state

High resolution total kinetic energy release (TKER) spectra of the H atom fragments resulting from photodissociation of jet-cooled adenine molecules at 17 wavelengths in the range 280>λphot>214nm are reported. TKER spectra obtained at λphot>233nm display broad, isotropic profiles that peak at low TKER (∼1800cm−1) and are largely insensitive to the choice of excitation wavelength. The bulk of these products is attributed to unintended multiphoton dissociation processes. TKER spectra recorded at λphot⩽233nm display additional fast structure, which is attributed to N9–H bond fission on the π1σ* potential energy surface (PES). Analysis of the kinetic energies and recoil anisotropies of the H atoms responsible for the fast structure suggests excitation to two π1π* excited states (the La1 and Bb1 states) at λphot∼230nm, both of which dissociate to yield H atoms together with ground state adeninyl fragments by radiationless transfer through conical intersections with the π1σ* PES. Parallels with the photochemistry exhibited by other, smaller heteroaromatics (pyrrole, imidazole, phenol, etc.) are highlighted, as are inconsistencies between the present conclusions and those reached in two other recent studies of excited state adenine molecules.