Ultraviolet photolysis of adenine:: Dissociation via the 1πσ* 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 >lambda(phot >)214 nm are reported. TKER spectra obtained at lambda(phot)> 233 nm display broad, isotropic profiles that peak at low TKER (similar to 1800 cm(-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 lambda(phot)<= 233 nm display additional fast structure, which is attributed to N-9-H bond fission on the (1)pi sigma(*) 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 (1)pi pi(*) excited states (the L-1(a) and B-1(b) states) at lambda(phot)similar to 230 nm, both of which dissociate to yield H atoms together with ground state adeninyl fragments by radiationless transfer through conical intersections with the (1)pi sigma(*) 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.