# Ultraviolet Transient Absorption Spectrometer with Sub-20-fs Time Resolution
## 4 Experimental Results  
Figure 4a reports a sequence of UV pump pulses generated either by SHG or by SFG of the NOPA output, which cover a wavelength range from 260 to 370 nm (4.75 to 3.35 eV). The figure also plots the spectral phases of the pulses, retrieved by 2 DSI, which are essentially flat: the corresponding temporal intensity profiles, shown as solid lines in Figure 4b-d, reveal sub-20-fs pulses, with a FWHM duration very close to that of the corresponding TL pulses (with intensity profiles shown as dashed lines): 16.2 fs for the 270-nm pulse, 18.6 fs for the 290-nm pulse and 8.4 fs for the 340-nm pulse. Figure 5 shows spectra of the WLC generated in the different materials pumping with the FW or with the SH, which -as previously discussed- cover different wavelength ranges. For each spectrum we calculated the wavelength dependent root-mean-square (rms) of fluctuations between consecutive pulses as:  
\[\varepsilon_{rms}\big{(}\lambda_{pr}\big{)}\ =\ \frac{\sqrt{\sum_{i=1}^{N}\big{[}S_{i+1} \big{(}\lambda_{pr}\big{)}-S_{i}\big{(}\lambda_{pr}\big{)}\big{]}^{2}}}{S\big{(} \lambda_{pr}\big{)}}\]  
where \(S_{i}(\lambda_{pr})\) is the signal measured by the spectrometer at a given probe wavelength for the \(i\)-th laser shot and \(\overline{S\big{(}\lambda_{pr}\big{)}}\) is the average probe signal. The wavelength dependent rms fluctuations of the WLC measured for N = 300 consecutive pulse pairs are reported in Figure 5 as shaded areas. One can see that the WLC generated in sapphire has the best noise properties, with fluctuations below 0.2% over most of the detection range; the WLC generated in CaF\({}_{2}\) driven by the FW has slightly higher fluctuations but adds a very significant spectral interval from 330 to 450 nm; finally, the WLC generated in CaF\({}_{2}\) driven by the SH displays higher but still acceptable fluctuations which are lower than 0.4% over most of the detection range. Figure 4: Upper panel: Spectra of UV pulses centered at 270 nm, 290 nm and 345 nm and corresponding spectral phases obtained using the modified 2 DSI technique described in Section 3.2. Lower panel: corresponding retrieved pulse temporal profiles together with the TL profiles (dashed black lines). The overall stability of the WLC is quite good: in fact, a 0.2% rms fluctuation corresponds to a noise \(<\)10\({}^{-4}\) for the \(\Delta\)T/T signal when averaged over 500 consecutive pulse pairs, which correspond to 1 second measurement time. This high stability of the probe is crucial for TA measurements with UV pump pulses; in water solvent, in fact, two-photon absorption of the pump can produce hydrated electrons [80], which display broad and long-lived absorption spectra in the visible [81]. To avoid or minimize such spurious signals, the pump fluence should be kept as low as possible, thus calling for a high sensitivity of the TA setup. In the following we present a few examples of ultrafast TA spectroscopy results using our high time resolution apparatus. As exemplary molecular system we use pyrene (C\({}_{16}\)H\({}_{10}\), chemical structure shown in Figure 6), a polycyclic aromatic hydrocarbon consisting of four benzene rings. Pyrene purchased from Sigma-Aldrich was used as received, dissolved in methanol, and flown in a 200-\(\upmu\)m-thick cuvette with 200-\(\upmu\)m-thick entrance and exit windows. The absorption spectrum of pyrene, reported in Figure 6, displays an intense band peaking at 3.7 eV (334 nm), with well-resolved vibronic replicas at 3.9 eV (319 nm) and 4.1 eV (306 nm), followed by a second band peaking at 4.6 eV (270 nm) with a well visible replica at 5 eV (250 nm). According to the well-known electronic structure of pyrene, the ground state has A\({}_{\mathrm{g}}\) symmetry (S\({}_{0}\) or 1\({}^{1}\)A\({}_{\mathrm{g}}\) state), while the first excited state has B\({}_{\mathrm{3u}}\) symmetry(S\({}_{1}\) or 1\({}^{1}\)B\({}_{\mathrm{3u}}\) state); however, the oscillator strength of the S\({}_{0}\)\(\rightarrow\)S\({}_{1}\) transition is very weak so that it is essentially optically forbidden [82].