Ultrafast photoprotection mechanisms and the extraterrestrial survivability of adenine
Evans, Nicholas Lee
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Electronic excited state relaxation dynamics of ammonia and adenine have been investigated by ultrafast time-resolved photoelectron and photoion spectroscopy. The ammonia investigation has determined that relaxation of 02 = 0 - 5 levels of NH3 (A~) yields low kinetic energy H-atom dissociation products within 150 fs of initial excitation. Quantum tunneling through the NH3 (A~) state energy barrier is observed as a lengthened dissociation timescale for the 02 = 1 level and a high kinetic energy feature in the H-atom spectrum of the 02 = 0 and 2 levels. Measurement of adenine's photoelectron spectrum has determined that the S2( *)!S1(n *)!S relaxation time decreases from 1032 to 700 fs as excitation energies increase from 4.7 to 6.2 eV. Participation of the S3( *) state is indicated for excitation energies above 5.2 eV. A separate investigation of the interstellar survivability of adenine has shown that previously reported lifetimes are decreased, up to 30%, when the charged particle component of the Galactic Cosmic Radiation is considered. This result does not exclude the survival of adenine in small icy grains over the lifetime of star forming molecular clouds.