## 2 Experimental Section  
Figure 1A shows a block diagram of the setup for the pump-probe technique to generate Raman sidebands. A fundamental beam of a Ti:sapphire laser (800 nm, 35 fs, 1 kHz, 6 mJ, Solstice Ace, Spectra-Physics, Santa Clara, CA, USA) was separated into two parts (ratio, 2:1) by a beam splitter (1 mm thick, FEMTO OPTICS) composed of a dichroic mirror. One part (4 W) of the beam was used as the pump source for an optical parametric amplifier (TOPAS Prime, Spectra-Physics) to generate a near-infrared beam (1200 nm, 470 mW, ca. 40 fs), the polarization of which was rotated horizontally (p-wave) using a half-wave plate (WPZA-200-1/2-W 1200, CASTECH, Fuzhou, China). The output of OPA and the remaining fundamental beam (800 nm, 664 mW, ca. 40 fs) in the OPA were combined using a dichroic mirror (HR at 740-860 nm, LAYERTEC, Mellingen, Germany) for use as a two-color pump beam. The remaining part of the fundamental beam from the Ti:sapphire laser (2 W) was passed through an optical delay consisting of a series of dichroic mirrors (107978, LAYERTEC) mounted on the translational stage (TSD-401C, SIGMA KOKI), and a telescope consisting of fused silica lens (focal lengths, 40 and \(-\)25 cm, EKSMA OPTICS, Vilnius, Lithuania). The laser beam was introduced into an optical system for third harmonic generation, which consisted of a BBO crystal (\(\theta\) = 29.2\({}^{\circ}\), \(\diameter\) = 0\({}^{\circ}\), thickness 0.2 mm, CASTON, Jin'an, China) for second harmonic generation, a time plate made of a BBO crystal (\(\theta\) = 29.2\({}^{\circ}\), thickness 3 mm, NEWLIGHT, Huntington Beach, USA) for adjustment of the optical delay between the fundamental and second harmonic pulses, a dual-wave plate (WPD05-H800-F400-SP, NEWLIGHT) for the adjustment of the polarization of the beams (half-wave plate at 800 nm and one-wave plate at 400 nm), and a BBO crystal (Type I, \(\theta\) = 29.2\({}^{\circ}\), \(\diameter\) = 0\({}^{\circ}\), thickness 0.1 mm, CASTON) for the third harmonic generation (267 nm, 165 mW, ca. 60 fs, conversion efficiency 45%). A fused silica plate (1-5 mm thick) was inserted so as to positively chirp the pulse occasionally. The pump beam (800 and 1200 nm) and the probe beam (267 nm) were combined with each other using a dichroic mirror (LWP-laser mirror 2005500, thickness 2 mm, LASER COMPONENTS, Olching, Germany) and was focused into a Raman cell (CaF\({}_{2}\) windows, thickness 0.5 mm) containing a hydrogen gas (0.7 atm), using a UV-enhanced aluminum concave mirror with a focal length of 50 cm (TCAE-30C05-1000, SIGMA KOKI, Tokyo, Japan), and a flat UV-enhanced aluminum mirror (TFAE-25.4C05-10, SIGMA KOKI). The spectrum of the Raman emission irradiating on a diffuser plate (silica, DFSQ1-30C02-240, SIGMA KOKI) was measured using a spectrometer coupled with an optical fiber (Maya2000 Pro, 200-1100 nm or 190-410 nm, OCEAN OPTICS, Largo, FL, USA). The time delay between the pump and probe pulses was adjusted to ca. 6 ps, which was  
optimized for the generation of the Raman sidebands. The laser power was measured using a power meter (S310C, PM100D, THORLABS, Newton, NJ, USA). Figure 1B shows a block diagram of the SD FROG system used for evaluating the phase and the temporal profile of the Raman sidebands. The three-color beam emitting at 240, 267, and 300 nm, i.e., the anti-Stokes, the probe, and the Stokes beam, respectively, was collimated using a concave aluminum mirror, and was reflected by the flat aluminum mirror. The beam was reflected by three dichroic mirrors (DIMQ-25C01-10-Rp241-Tp267/1200-NOAR, DIMQ-25C01-10-Rp267-Tp300/1200-NOAR, DIMQ-25C01-10-Rp300-Tp200/1200-NOAR, SIGMA KOKI), and the resulting three beams were reflected by means of three pairs of aluminum roof mirrors mounted on translational stages with precisions of 0.5 \(\upmu\)m (TAM-401 SWP, SIGNA KOKI), and were recombined with the dichroic mirrors.