# Ultraviolet Transient Absorption Spectrometer with Sub-20-fs Time Resolution
## 3 Experimental Setup
### Broadband TA Spectrometer  
Figure 3a shows the experimental setup of the high time resolution broadband TA spectrometer in the UV. Most of the laser energy is used for the UV pump pulse generation, according to the scheme reported in Figure 1. A flip mirror is used to deviate the pump pulse to the 2 DSI setup displayed in Figure 2, for its accurate characterization before the measurement. To take into account the effect of air dispersion, which is not negligible at UV wavelengths, the path-length leading to the nonlinear crystal in the 2 DSI is made equal to the path to the sample. For measurements performed in a flow cell, a 200 \(\upmu\)m-thick compensating fused silica window, identical to the entrance window of the cell, is placed in front of the 2 DSI. A 35-\(\upmu\)J fraction of the laser output is sent to a computer-controlled delay line, which serves the dual purpose of synchronizing the pump and probe pulses and varying their delay, which is followed by the WLC generation stage. Figure 3b,c show the experimental setups used for WLC generation driven by the FW or by the SH. In both cases the energy for WLC generation is adjusted to \(\sim\)1 \(\upmu\)J. The driving pulse is focused by a lens with focal length f\({}_{1}=75\) mm and the WLC is recollimated  
Figure 2: Modified scheme of the 2 DSI technique used for the characterization of the UV pulses. by a spherical mirror with focal length f\({}_{2}\) = 50 mm used under the smallest possible incidence angle (\(\theta\) = 10\({}^{\circ}\)), followed by a pick-up mirror. An iris and a variable attenuator on the driving pulse path are used to vary the numerical aperture and the power of the beam; these parameters, together with the position of the nonlinear material in the focus, controlled by a precision translation stage, are finely tuned to optimize the stability of the generated WLC. Optionally, the WLC can be driven by the SH of Ti:sapphire, generated in a 1-mm-thick Type I BBO crystal (\(\theta\) = 29\({}^{\circ}\)). Different setups and materials are used to cover the visible and UV ranges: a 2-mm-thick sapphire driven by the FW to cover the 440-710 nm range, a 3-mm-thick CaF\({}_{2}\) driven by the FW to cover the 330-650m range and the same plate driven by the SH to cover the- UV (270-370 nm) range. For the case of CaF\({}_{2}\), to avoid optical damage, a two-axis translation stage (Zaber Technologies, Vancouver, Canada, model T-LS28M) is used to slowly scan the plane in a plane perpendicular to the propagation direction [79]. The pump and the probe are separately and non-collinearly focused on the sample at approximately 5\({}^{\circ}\) by two UV enhanced aluminum spherical mirrors with focal lengths f\({}_{\text{pump}}\) = 250 mm and f\({}_{\text{probe}}\) = 200 mm, to match their focal spots. The transmitted probe, selected by an iris, is focused on the entrance slit of a spectrometer (SP2300 Princeton Instruments, Acton, MA, USA) equipped with a CCD detector, with the spectral resolution of the order of 0.5 nm, and a read-out electronics (Stresing Entwicklungsburo, Berlin, Germany) capable of single-shot readout of the probe spectrum at the full 1-kHz laser repetition rate [7]. The pump is modulated by a mechanical chopper at 500 Hz and the differential transmission (\(\Delta\)T/T) spectrum as a function of probe wavelength \(\lambda_{\text{pr}}\) and pump-probe delay \(\tau\) is calculated as:  
\[\frac{\Delta\mathbf{T}}{T}(\lambda_{\mathbf{pr}},\mathbf{\tau})\ =\ \frac{T_{\text{ON}}( \lambda_{\mathbf{pr}},\mathbf{\tau})-T_{\text{OFF}}(\lambda_{\mathbf{pr}},\mathbf{\tau})}{T_{ \text{OFF}}(\lambda_{\mathbf{pr}},\mathbf{\tau})}\]  
Please note that since consecutive laser pulses have the highest degree of correlation, the single-shot readout of the probe spectrum synchronous to the laser repetition rate contributes to the improvement of the S/N ratio. In addition, the spectral resolution of the spectrometer makes it  
Figure 3: (**a**) Experimental setup of the broadband UV TA spectrometer.