Fragmentation dynamics of condensed phase thymine by low-energy (10-200 eV) heavy-ion impact

We report measurements of the formation and desorption of ionic fragments induced by very low-energy (10-200 eV) Ar(+) irradiation of thymine (T) films, deposited on a polycrystalline Pt substrate. A multitude of dissociation channels is observed, among which the major cation species are identified as HNCH(+), HNC(3)H(4) (+), C(3)H(3) (+), OCNH(2) (+), T-OCN](+), T-OCNH(2)](+), T-O](+), and T+H](+) and the major anions as H(-), O(-), CN(-),and OCN(-). Cation fragment desorption appears at much lower threshold energies (near 15 eV) than anion fragment desorption, where the latter depends strongly on the film thickness. It is proposed that anion fragment formation and desorption results from projectile impact-induced excitation of either (1) a neutral thymine molecule, followed by fragmentation and charge exchange between the energetic neutral fragment and the substrate (or film) and/or (2) a deprotonated monoanionic thymine molecule to a dissociative state, followed by a unimolecular fragmentation of the excited thymine anion. The H(-) and O(-) fragment formations may have a further contribution from dipolar dissociation, e.g., formation of electronically excited neutral thymine, followed by dissociation into O(-)+T-O](+), due to their reduced sensitivity to the film thickness. Positive-ion fragment desorption exhibits no significant dependence on film thickness before the emergence of surface charging, and originates from a kinetically assisted charge-transfer excitation. The results suggest that the potential energy of the incident ion plays a significant role in lowering the threshold energy of kinetic fragmentation of thymine. Measurements of the time-dependent film degradation yields for 100-eV Ar(+) suggest a quantum efficiency for degradation of about six thymine molecules per incident ion.