# Ultraviolet Transient Absorption Spectrometer with Sub-20-fs Time Resolution
## 1 Introduction  
Since the development of mode-locked laser sources [1], ultrafast optical spectroscopy has provided invaluable insight into the light-triggered dynamical processes in many different systems of interest for physics, materials science and (bio)chemistry [2; 3]. While several techniques with growing level of complexity and sophistication have been developed [4; 5], transient absorption (TA) still remains a very powerful and versatile approach, delivering a rich information content with a comparatively simple experimental setup. TA typically works in a stroboscopic fashion, using two synchronized pulses, the pump and the probe. The pump pulse is resonant with a transition of the system under study, triggering a photoinduced process whose time course is followed by measuring the absorption change of the time-delayed probe pulse, which should ideally be as broadband as possible in order to deliver the maximum amount of spectroscopic information on the system [6; 7; 8; 9]. The time resolution of TA spectroscopy is determined by the so-called instrumental response function (IRF). In TA spectrometers with relatively narrow-band (>100 fs duration) transform-limited (TL) pump and probe pulses, the IRF is the cross-correlation of the intensity profiles of pump and probe pulses [10]. When dealing with broadband probe pulses, it can be shown that, independently from their chirp, the temporal resolution is determined by their TL duration, provided that the probe pulses are detected through a narrow-band filter [11; 12]. The ability to generate pump pulses with duration of just a few optical cycles [1; 13] and probe pulses with ultrabroad bandwidth has pushed the time resolution in the visible and near-infrared (NIR) ranges to the sub-10-fs domain, allowing the observation of primary light-induced processes in molecules and solids, which govern important biochemical processes such as photosynthesis [14] and vision [15]. The ultraviolet (UV) wavelength range is of great interest for ultrafast spectroscopic investigations due to several reasons. First of all, the UV range is resonant with electronic transitions of many small molecules with fairly simple excited state energy level structure, whose photoinduced dynamics can be accurately modeled using ab initio computational approaches; ultrafast TA spectroscopy in the UV can thus be used to benchmark the accuracy of such methods [16; 17]. In addition, the UV matches the absorption spectrum of many biomolecules, which present cyclic aromatic rings with strongly allowed \(\pi\)\(\rightarrow\)\(\pi^{\bullet}\) transitions. Examples are the nucleobases (adenine, cytosine, guanine, thymine, and uracil) and their multimers up to the double-stranded DNA helix, as well as the aromatic amino acids (tryptophan, phenylalanine, and tyrosine), all of which have strong absorption bands in the 250-300 nm wavelength range (\(\approx\)4-5 eV). While the main biological functions of these biomolecules are not directly related to their interaction with light, still the study of their ultrafast optical response is very important. In DNA the absorbed UV photon energy could initiate a variety of photoreactions, involving substantial rearrangements of the molecular structure, that corrupt the information encoded in the base sequence. Nevertheless, the quantum yield of such photoproducts is remarkably low, because DNA manages to dissipate the absorbed energy very efficiently, mainly through harmless ultrafast non-radiative decay processes [18; 19; 20; 21]. This self-protecting property of DNA is not just an interesting feature of its photophysics, but it is an essential requirement for the very existence and the replication of life, which relies on the stability of the hereditary genetic information.