Figure 1.

(a) Chemical structure of uracil. (b) Schematic of the femtosecond excited state relaxation mechanisms. (c) Carbonyl stretch vibrations: frequencies and relative intensities of the carbonyl marker bands in the electronic ground state S₀ (ν̃_C=O7 = 2012 cm⁻¹, ν̃_C=O8 = 1971 cm⁻¹), and the excited states S₁ (n_O8π^*; ν̃_C=O7 = 2059 cm⁻¹, ν̃_C=8 = 1706 cm⁻¹) and S₂ (π_Oπ^*; ν̃_C=O7 = 2028 cm⁻¹, ν̃_C=O8 = 1929 cm⁻¹).