## 2 Experimental Section  
Our main optical setup to achieve tunable FSRS in the UV range uses a portion of the fundamental pulse (FP) output from a Ti:sapphire-based fs laser regenerative amplifier (Legend Elite-USP-1K-HE, Coherent, Inc.) seeded by a mode-locked Ti:sapphire oscillator (Mantis-5, Coherent). The FP of \(\sim\)1 W at 800 nm center wavelength with 35 fs time duration (full-width-half-maximum, or fwhm) and 1 kHz repetition rate is split into two parts with a 9:1 ratio to pump the home-built second harmonic bandwidth compressor and the broadband up-converted multicolor array setup, respectively (see Figure 1). In the second harmonic bandwidth compressor section, the input beam is separated evenly into two arms, which go through reflective grating and cylindrical lens pairs and are stretched from fs to ps pulses with opposite chirps tuned to have the same magnitude [20]. After recombining the two arms at a 1-mm-thick Type-I BBO crystal (\(\theta\)= 29.2\({}^{\circ}\)) we achieve the chirp-free narrowband ps pulse centered at \(\sim\)402 nm as a result of the chirp elimination effect [21]. The time duration of the second harmonic pulse is characterized by the optical Kerr effect (OKE) measurement with another 400 nm pulse from second harmonic generation of the FP followed by prism compression (40 fs, acting as the gate pulse), yielding a temporal profile with \(\sim\)1.45 ps fwhm (Figure 2). To obtain the spectral width, we disperse the pulse with a 1200 grooves/mm, 500 nm blaze ruled reflective grating and image onto a CCD camera. After wavelength calibration with a mercury argon source (HG-1, Ocean Optics) across the UV/Vis range, the fwhm of the picosecond 402 nm pulse is measured to be \(\sim\)11 cm\({}^{-1}\). This represents a time-bandwidth product of \(\sim\)15.9 ps\(\cdot\)cm\({}^{-1}\) that is close to the Fourier-transform limit of a Gaussian-profile pulse (\(\sim\)14.7 ps\(\cdot\)cm\({}^{-1}\)), indicating that the 402 nm pulse is largely chirp-free and can be used as the narrowband Raman pump pulse in FSRS. In the broadband up-converted multicolor array setup, an FP and an SCWL (alternatively referred to as WL) pulse are loosely focused onto a 0.1-mm-thick BBO crystal to generate multiple sidebands via SFG-based cascaded four-wave mixing [17]. The crossing angle between the two incident fs pulses is \(\sim\)6\({}^{\circ}\) to achieve balance between conversion efficiency and spatial separation of nascent sidebands [15, 22, 23]. The phase-matching condition of the BBO crystal is set to favor SFG. The first sideband on either the FP (S\({}_{\text{+1}}\)) or WL (S-1) side is selected with an iris diaphragm and used as the Raman probe (Figure 1). The pulse-to-pulse intensity stability is within 5%, which can be effectively averaged out by repeatedly collecting FSRS signals over several minutes (see below). Notably, the highly nonlinear pulse generation does not incur substantial intensity noise because we use low pump power to generate laser sidebands in the background-free directions (see Figure 1) so the interference effect with fundamental incident pulses is much reduced. Furthermore, the bandwidth of S\({}_{\text{+1}}\) and S-1 is measured to be _ca_. 1400 and 1700 cm\({}^{-1}\), respectively, which supports the self-compression of these CFWM-induced sidebands to fs pulses [15, 17]. To potentially achieve transform-limited pulses, further compression with accurate chirp compensation is needed [24, 25]. The selected BUMA sideband and the SHBC output pulse are focused onto a 1-mm-thick quartz sample cell by an \(f\)= 12 cm off-axis parabolic mirror to avoid introducing additional chirps (e.g., if we use a focusing lens instead). Both incident beams pass through the sample solution containing 15 mM LD390 laser dye in methanol (Figure 1). The Raman pump is then blocked while the Raman probe pulse carrying the stimulated Raman scattering signal is re-collimated and focused into the spectrograph with a 1200 grooves/mm, 500 nm blaze ruled reflective grating.