## 3 Experimental Setup  
A schematic of the setup, which comprises the generation, broadening and characterization of UV pulses, is shown in Figure 3. Tunable visible pulses were generated from a single-stage noncollinear parametric amplifier (NOPA) pumped by a 1-kHz Ti:Sa amplifier system (CPA 2001; Clark MXR) and optimally compressed by a pair of fused silica Brewster prisms. UV pulses with a typical pulse length of 25 fs were generated by frequency doubling or sum frequency mixing the NOPA pulses with the CPA fundamental in BBO crystals with a thickness of 53 \(\upmu\)m. For the frequency doubling we typically obtain \(<1\)\(\upmu\)J which corresponds to an efficiency of \(<15\%\). For the sum frequency mixing both beams were collinearly overlapped and the CPA fundamental was focused with a fused silica lens to match the focal size of the beam of the visible pulse in the BBO. Typical energies of 4 \(\upmu\)J are achieved with a visible to UV efficiency of 50 \(\%\).  
To introduce a variable positive chirp the UV pulses pass through a set of UV antireflection coated movable fused silica wedges (apex angel 45\({}^{\circ}\)) with counter oriented apexes [24]. To obtain a clean and spatially symmetric beam profile, which is advantageous for SPM, we introduce an aperture accepting a minimal loss of energy. The UV beam is then focused with the f = 150 mm antireflection coated fused silica lens L1 into a few mm thick crystal which spectrally broadens the pulses due to SPM. The position of the crystal relative to the soft focus of the UV beam can be adjusted to obtain maximum broadening without impairing the spatial mode due to self-focusing inside the crystal. A second fused silica lens L2 (f = 100 mm), that can be translated along the beam axis, recollimates the spectrally broadened UV pulse. This not only accounts for the change in divergence caused by the partial self-focusing in the crystal but also adjusts the collimation of the beam such that the specific requirements for pulse shaping with an acousto-optic programmable dispersive filter (AOPDF) are met [1]. Specifically, the intermediate beam waist of the broadened UV pulse after the AOPDF has to be located one focusing length in front of the focusing element which is used to focus the beam into the interaction region, e.g., the sample.  
The compression of the positively chirped broadened UV pulses is achieved by either a second UV prism compressor or a combination of a UV prism compressor and an AOPDF (DAZZLER(tm) model T-UV-250-400; Fastlite) [25] for higher order chirp correction. The prisms are UV antireflection coated and have an apex angle of 45\({}^{\circ}\). The throughput of the UV PC is roughly 93%. In the PC-AOPDF combination the precompression with the UV PC permits a larger bandwidth of the AOPDF because the acoustic wave is less chirped and thus less elongated within the shaper crystal.  
To measure the pulse lengths obtained in the simple setup where only a UV-PC is used to compress the broadened pulses, we employ a newly developed UV autocorrelator [26] based on two photon absorption in a 173 \(\upmu\)m thick BBO crystal. To check if the higher order chirp contributions of the UV-broadened pulses can be compensated for by the PC-AOPDF combination, we characterized the spectral phase with a ZAP-SPIDER [27].