# Ultraviolet Transient Absorption Spectrometer with Sub-20-fs Time Resolution
## 5 Conclusions  
In this paper, we described a TA spectroscopy system in the UV spectral range, for the study of the ultrafast optical response of biomolecules. The setup, which is driven by a 100-fs regeneratively amplified Ti:sapphire laser, combines sub-20-fs UV pump pulses with broadband white-light-continuum probe pulses. The ultrashort UV pump pulses are generated by frequency up-conversion (SHG or SFG) of a visible ultrashort NOPA output, and are tunable in the 260-370 nm range. Direct frequency-doubling of the ultrashort visible light delivers broadband UV pulses with carrier wavelength from 260 to 290 nm. SFG between the NOPA pulses and a fraction of the laser fundamental wavelength produces pulses with bandwidth in the 320-370 nm range. After suitable  
Figure 11: (**a**) Dynamics at 385 nm (main panel) and residuals (inset) after subtraction of a multi-exponential fit; (**b**) Fourier transform of the residual from panel (**a**); (**c**) The Fourier intensity of the oscillatory component as a function of the probe wavelength.  
Figure 10: TA dynamics of pyrene following excitation at 270 nm, at UV (**a**) and visible (**b**) probe wavelengths.  
compression stages, the pulse duration ranges from 8.4 fs (for the 340 nm pulse) to 18.6 fs (for the 290 nm pulse).  
The broadband probe pulses are obtained by white-light continuum generation in sapphire or CaF\({}_{2}\) plates, and cover the spectral ranges from 270 to 710 nm with energy fluctuations of 0.2-0.4% rms, corresponding to a noise \(<\)10\({}^{-4}\) for the \(\Delta\)T/T signal averaged over 1 s. The ultrashort duration of the pump pulses and the broad bandwidth of the probe pulses determine the extremely high temporal resolution of the pump-probe measurement.  
The TA spectroscopy system was applied to study ultrafast processes in pyrene in methanol. After excitation of the bright optical transitions S\({}_{0}\)\(\rightarrow\) S\({}_{2}\) (occurring at 3.7 eV) and S\({}_{0}\)\(\rightarrow\)S\({}_{3}\) (at 4.6 eV), the system undergoes ultrafast internal conversion from either S\({}_{2}\) or S\({}_{3}\) to the dark S\({}_{1}\) state. Thanks to the ultrashort duration of the pump pulses and the ultrabroad bandwidth of the probe pulses, we could successfully track the internal conversion process, which occurs on hundreds of femtoseconds timescale. In addition, the temporal resolution allowed resolving an impulsively excited molecular vibration, with 85-fs period, which survives the internal conversion process, pointing to a vibrationally coherent photochemistry in pyrene.  
The tunability of the pump pulses in the UV spectral range, the sub-20-fs temporal resolution and the broad spectral coverage of the probe contribute to make this TA system a unique, powerful, and versatile tool for the study of many biomolecules, which exhibit absorption in the 250-350 nm wavelength range. This system will allow tracking fundamental biochemical processes such as the photoproduction mechanisms in DNA and the secondary structure of proteins based on the response of aromatic amino acids.  
Investigation, R.B.-V. and L.G.; Methodology, G.C. and C.M.  
The authors acknowledge support by the H2020 Grant Agreement number 765266 (LightDyNAmics), European Research Council Advanced Grant STRATUS (ERC-2011-AdG No. 291198) and the Marie Curie actions (FP7-PEOPLE-IEF-2012).  
The authors declare no conflict of interest.