Femtosecond photo-ionization of nucleic acid bases: Electronic lifetimes and electron yields

Femtosecond spectroscopy of aqueous solutions of the four nucleic acid bases adenine, cytosine, thymine and uracil is presented with probing from 282 nm to 750 nm. After two-step excitation at 282 nm part of the molecules returns to the ground state allowing for the determination of the lifetime of the S₁ state yielding values of τ₁≃1 ps for the four bases. The remainder photo-ionize with an ionization yield of 0.7 for adenine, twice as large as for the other bases. The first intermediate is assigned to a (base⁺:e^-)-pair with a lifetime of 10 ps for adenine, four times longer than for the other solutions.