Cytosine exhibits a short S₁ excited-state lifetime upon UV photoexcitation. We have measured the S₀ → S₁ vibronic spectrum [1] of supersonic jet-cooled cytosine. All observed vibronic excitations are overtone and combination bands of the low-frequency out-of-plane ν₁ (butterfly), ν₂ (boat) and ν₃ (H-N-C⁶-H twist) vibrations. The lifetimes of the individual vibronic states are determined from the Lorentzian broadening of the band contours: At the vibrationless level, the lifetime is 44 picoseconds.
Increasing the vibrational energy in the S₁ state rapidly decreases the lifetime to below <1 ps above 600 cm^-1 vibrational energy [2]. This implies that the out-of-plane vibrations ν₁, ν₂ and ν₃ are the promoting modes for the S₁ → S₀ internal conversion in cytosine [2].

This work was carried out by Simon Lobsiger, Maria Trachsel and Hans-Martin Frey in the group of Prof. Samuel Leutwyler.

References:

1. S. Lobsiger, S. Leutwyler;
   "The Adiabatic Ionization Energy and Triplet T₁ Energy of Jet-Cooled Keto-Amino Cytosine"
   J. Phys. Chem. Lett., 3, 3576-3580, (2012); doi:10.1021/jz301719h.
2. S. Lobsiger, M. A. Trachsel, H.-M. Frey, S. Leutwyler;
   "Excited-State Structure and Dynamics of Keto–Amino Cytosine: The ¹ππ* State Is Nonplanar and Its Radiationless Decay Is Not Ultrafast"
   J. Phys. Chem. B, 117, 6106–6115, (2013); doi:10.1021/jp401881b.